Clean Air Program -- Assesment of the Safety, Health, Environmental and System Risks of Alternative Fuel
Clean Air Program
Summary Assesment of the Safety, Health, Environmental and System Risks of Alternative Fuel
PREFACE
National goals for both energy security and clean air have resulted in heightened interest in theuse of alternative motor fuels (AMFs) in the transportation market. The growth of interest inalternative fuels has expanded not only the numbers of alternative fuel vehicles, but also the listof viable alternative transportation fuels.
Thus, an increasing number of transit fleets and other fleet owners are operating vehicles onalternative fuels often with a minimum of technical guidance related to the possible safety oroperational impacts on traditional fleet operations, including fueling, inspecting, and cleaningvehicles, as well as performing the light and heavy maintenance activities necessary to keep thefleet in operation.
Moreover, the buildings or facilities used for storing, loading, and maintaining alternative fuelvehicles form an important portion of a fleet operation. Here, the experience with fire andbuilding codes is not yet complete. This situation requires additional care on the part of theowners of these facilities to recognize all hazards associated with the use of alternative fuelvehicles and to ensure that these hazards are properly addressed in the design and operation ofthe facility.
Experience has shown that not all local community and regulatory groups view the use ofalternative fuels as a purely positive option. Transit properties and others who propose the use ofalternative fuels need to deal not only with the perceptions of fire and building code officials whogrant approvals, but also with the perceptions and concerns of community and neighborhoodorganizations. The concerns of these groups are not limited to fleet operations, but may alsoinclude the production of the alternative fuel and the transportation of the fuel to the point of use.
In view of the diversity of these safety concerns, as well as the number of possible hazards, acomprehensive and systematic program is needed to recognize and organize the existingknowledge about the health, safety, and environmental hazards of alternative fuels and to identifywhere additional study is needed. The objective of this report is assist the Volpe Center, FTAand DOE in providing information on these issues to the transit and fleet operator communitywhile avoiding a commitment to or bias against any given fuel or point of view.
This report presents the results of a research effort undertaken for the Volpe NationalTransportation Systems Center. This work was funded jointly by the U.S. Department ofTransportation, Federal Transit Administration Office of Engineering and the U.S.Department of Energy, Alternative Fuels Utilization and Analysis Division. The interest,insight and advice of David Knapton of the Volpe National Transportation Systems Center,John Russell of the U.S. Department of Energy, and Tony Yen and Steven Sill of theFederal Transit Administration are gratefully acknowledged.
TABLE OF CONTENTS
LIST OF FIGURES
LIST OF TABLES
EXECUTIVE SUMMARY
LIST OF ACRONYMS
- INTRODUCTION
1.1 Background
1.2 Objectives and Scope
- PREPARATION AND ORGANIZATION OF REPORT
2.1 Information Sources
2.2 Organization of Report
- PRODUCTION, BULK TRANSPORT, AND BULK STORAGE OF ALTERNATIVE FUELS
3.1 Introduction
3.2 Methodology
3.3 Issues Associated with Bulk Transport and Storage of Alternative Fuels
3.3.1 Methanol/Methanol Blends
3.3.2 Ethanol/Ethanol Blends
3.3.3 Compressed Natural Gas
3.3.4 Liquefied Natural Gas
3.3.5 Propane
3.3.6 Biodiesel
3.3.7 Hydrogen
3.3.8 Electricity
3.4 Assessment of Alternative Fuel - Bulk Transport, Transfer, and Fleet Storage Safety Risks
3.4.1 Introduction
3.4.2 Assessment of Relative Potential for Spills and Leaks
3.4.3 Assessment of Safety Hazards
3.4.4 Assessment of Health Hazards
3.4.5 Assessment of Environmental Hazards
- USE OF ALTERNATIVE FUELS BY VEHICLE FLEETS
4.1 Introduction
4.2 Objectives and Scope
4.2.1 Fuels Included
4.2.2 Hazardous Properties Included
4.2.3 Accident Events Included
4.3 Summary List of Alternative Fuel Hazards for Vehicle Fleet Operations
4.3.1 Overview of Alternative Fuel Hazards
4.3.2 Safety Hazards Considered
4.4 Summary List of Alternative Fuel Hazards
4.5 Alternative Fuel Safety Case Studies
4.5.1 Methanol Vehicle Fire
4.5.2 LNG Bus Explosion
4.5.3 High Pressure CNG Fittings as Projectiles
4.5.4 Propane Tank Damage
4.5.5 Pressure Relief Device (PRD) Failure on CNG Bus
4.5.6 CNG Cascade Relief Valve Failure
4.5.7 Static Electricity Ignition of Venting CNG
4.5.8 CNG Bus Drive-Away and Fire
4.5.9 Propane Leak from Faulty Installation
- APPENDIX A. SOURCES FOR ALTERNATIVE FUEL SAFETY
LIST OF FIGURES
- Flash Point Temperatures for Liquid AMFs
- Fuel Volatility--Reid Vapor Pressure (@ 38C)
- Autoignition Temperature
- Flammability Limits Range
- Relative Heat Release Rate for Liquid Pool Fires
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LIST OF TABLES
- Relative Potential for Spills During Transport
- Relative Potential for Leaks During Transport
- Relative Potential for Spills During Unloading
- Relative Potential for Leaks During Unloading
- Relative Potential for Spills During Fleet Storage
- Relative Potential for Leaks During Fleet Storage
- (A-H). Compressed Natural Gas (CNG)
- (A-H). Liquefied Natural Gas (LNG)
- (A-H). Propane
- (A-H). Methanol
- (A-H). Ethanol
- (A-H). Biodiesel
- (A-H). Hydrogen
- (A-H). Electricity
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EXECUTIVE SUMMARY
A. BACKGROUND
National goals for energy security and clean air have resulted in a heightened interest in theuse of alternative transportation fuels. This growing interest in alternative fuels has led toboth an increase in the number of alternative fuel vehicles, and to an expansion in the list ofcandidate alternative fuels.
This summary assessment consists of two parts. The first part considers the hazardsassociated with the bulk transport and storage of alternative fuels. The second part considersthe hazards associated with the operation, fueling, and maintenance of alternative-fuel vehiclefleets. The report does not cover estimating the hazard probability or calculating the overallrisk.
Both sections of the hazard assessment discussion include information on the followingalternative fuels:
1. Compressed Natural Gas (CNG)
2. Liquefied (LNG)
3. Propane
4. Methanol and methanol blends
5. Ethanol and ethanol blends
6. Biodiesel blends In this analysis biodiesel fuel is considered to be a mixture of 10-30 percent of a vegetable oilester, such as methyl soyate, and conventional diesel fuel.
7. Hydrogen
8. Electricity
B. PRODUCTION, TRANSPORT AND BULK STORAGE HAZARDS
The types of hazards which may be encountered are categorized as follows:
Safety Issues, including fire hazards and other hazards
Health Issues, including fuel toxicity
Environmental Issues, including effects of fuel spills.
Highlights of this analysis follow.
Fire Hazards
Since all fuels burn, they constitute fire hazards to a greater or lesser degree. However,fuels vary widely in the degree of flammability. Of the many combustion-related propertiesof substance, fuel flammability limits and pool burn rate are especially relevant to a safetyhazard analysis.
Fuel Flammability Limits
Flammability limits are a basic measure of flammability. Flammability limits are the rangeof composition over which mixtures of fuel and air will burn. At an ambient temperature of
22C, natural gas in the form of CNG or LNG has the widest flammability limits. Due toincreased volatility at higher temperatures, the alcohols, methanol and ethanol have extendedflammability limits at elevated temperatures (60C). Biodiesel fuel is below its flashpoint at22C and shows a flammable range only at elevated temperatures.
Fuel Pool Burn Rate
If liquid fuels spill and ignite, the pool burn rate is a measure of the rate at which a givensize spill will burn and release heat. Since fuels burn only when they are in gaseous form,the pool burn rate tends to be limited by the rate of vaporization. Thus, the pool burn ratesfor the alcohols, which have relatively high heats of vaporization, are lower than those forhydrocarbon fuels like gasoline or propane. Note too, that the gaseous fuels hydrogen andcompressed natural gas can have very high heat release rates since the burn rate for thesefuels is not limited by the need to first vaporize a liquid.
Health Hazards
In addition to fire hazards, the use of alternative fuels can present health hazards. For mostfuel health effects, inhalation of fuel vapors is the most likely exposure route. The thresholdlimit value for the health effects of fuel vapors is a measure of fuel toxicity. The limits forall fuels except LNG vapor (considered to be nearly pure methane), and hydrogen are basedon toxic effects. The limit values for these fuels are based on the lower flammability limitand the premise that inhalation of a flammable mixture of fuel and air constitutes a healthhazard. In the case of hydrogen and natural gas, excessive exposure can also result inasphyxiation. However, approximately 140,000 ppm (14 percent) of an inert gas would berequired to lower the oxygen concentration of air to less than the 18 percent, the limit for abreathable atmosphere.
Methanol and methanol blends are the most toxic AMFs for inhalation-exposure with athreshold limit value - time weight average (TLV-TWA) concentration value of 200 ppm. By comparison, the next lowest TLV-TWA concentration value for an AMF includes ethanol1,000 ppm, followed by natural gas at a value of 10,500 ppm. In addition, there is anOSHA-set personnel exposure time limit (PEL) of 1,000 ppm for propane.
Environmental Hazards
The spill or leak of an AMF is not likely to result in any long term environmental damage. A review of the potential environmental hazards for each AMF, that is not gaseous at normaltemperatures and pressures, shows that all of the liquid AMFs are biodegradable over areasonably short period of time (i.e., a period of several months or less). The major concernis that the liquid AMF should be prevented from entering into any waterway or drainagesystem. Aside from any consideration of aquatic toxicity, there is actually a potentialfire/explosion safety hazard situation created when a flammable or combustible liquid entersa waterway where there are covered sections where vapors can accumulate. This problem isparticularly acute for the alcohols (methanol and ethanol) since they are soluble in water. Once such alcohol AMFs have mixed with water there is no simple and low cost method forseparating them out.
C. FLEET USE HAZARDS
This portion of the work was structured around a summary list of safety, fire, and healthhazards for each alternative fuel in fleet use. In each instance, the assessment of theconsequences of the hazards and of the state of knowledge concerning the hazards is basedon a comparison with diesel or gasoline fuel as currently used by fleet operators and transitagencies.
To construct the summary list of hazards associated with the fleet use of alternative fuels, thefollowing eight hazardous properties are included:
(a) Flammability
(b) Corrosivity
(c) Toxicity (including asphyxiation)
(d) High pressure
(e) High temperature
(f) Cryogenic temperature
(g) Mechanical energy (includes energy stored as potential or kinetic energy)
(h) Electrical energy
The existence of these hazardous properties and their associated hazards is not sufficient tocause an accident. Some event is necessary before the hazard and the hazard consequencesare realized.
The application of the eight hazardous properties to the eight alternative fuels produces anumber of hazards. The more significant hazards for each fuel are:
CNG - Important hazardous properties and hazards for CNG include:
Flammability hazard -- fire or explosion from ignition of gas leaks. Such gasleaks can occur from fuel dispenser or fuel system damage, use of impropercomponents, or poor overall design. High pressure natural gas leaks canignite from static electricity. Several such cases have already occurred, someresulting in the loss of the vehicle.
Toxicity hazard natural gas can accumulate in enclosed spaces. Theodorant may not provide sufficient warning of the actual gas concentration.
High pressure hazard fuel tank explosion, missile damage from failure orimproper assembly or disassembly of fuel system components. Flailing of fuelhoses and fuel lines.
Mechanical energy hazard natural gas compressors have rotating and/orreciprocating parts moving it high speeds. Failure of such equipment couldlead to missile damage from fragments.
LNG - Important hazardous properties and hazards for LNG include:
Flammability hazard fire or explosion from ignition of leaks of fuel. Non-odorized fuel gas increases the hazard. Note that the design base forcryogenic fuel system components is still relatively small.
Toxicity hazard asphyxiation from exposure to non-odorized fuel gas.
High pressure hazard while LNG storage pressures are not as high as thosefor CNG, they are still significant. Also, trapped liquid fuel can produceextremely high pressures upon warming and vaporization.
Cryogenic hazards LNG presents several hazards associated with thecryogenic property of the fuel:
Personal injury may occur from exposure to cold fuel or fuel vapors. This is especially true if proper personal protective gear is not worn.
Structural failure can occur due to stress from contraction of structuralmembers exposed to cold fuel or fuel vapors.
Structural failure can also occur due to embrittlement of materialsexposed to cold fuel or fuel vapors.
Propane - Important hazardous properties and hazards for propane include:
Flammability hazard propane gas can collect in low spaces; large propanevapor clouds can detonate.
Toxicity hazard propane gas can collect in low spaces and thereforedisplace the air necessary for breathing.
Methanol and Methanol Blends - Important hazardous properties and hazards for methanol and methanol blends include:
Flammability hazard vapors in fuel tanks are within the flammable rangefor typical ambient temperatures.
Flammability hazard the flames from methanol fires are not as luminous asthose from other hydrocarbons. While this serves to limit fire injury anddamage, it can also make initial detection of methanol fires more difficult.
Corrosivity hazard being a polar liquid, methanol is slightly acidic and cancorrode some active metals.
Ethanol and Ethanol Blends - Important hazardous properties and hazards forethanol and ethanol blends include:
Flammability hazard vapors in fuel tanks are within the flammable rangefor typical ambient temperatures.
Corrosivity hazard being a polar liquid, ethanol is slightly acidic and cancorrode some active metals.
Toxicity hazard ingestion of a fuel billed as food-based, but which must bedenatured, i.e., made poisonous.
Biodiesel - Important hazardous properties and hazards for the biodiesel component of biodiesel fuel blends include:
Corrosivity hazard elastomer or polymer component failure due to thecomposition difference between biodiesel fuel and gasoline or conventionaldiesel fuel is a type of corrosivity hazard.
Toxicity hazard ingestion of a fuel which has been billed as non-toxic, butwhich is generally an ester of a fatty acid and methanol. If ingested themethanol component is released. In primates (including humans) this cancause toxic effects.
Hydrogen - Important hazardous properties and hazards for hydrogen include:
Flammability hazard fire or explosion from ignition (especially staticignition) of gas releases or gas leaks. Note that hydrogen fuel is a non-odorized flammable gas.
Corrosivity hazard hydrogen embrittlement of certain materials represents atype of corrosivity hazard associated with hydrogen.
High pressure hazard fuel tank explosion, missile damage from failure orimproper assembly or disassembly of hydrogen fuel system parts.
Electricity - Important hazardous properties and hazards for electricity include:
Flammability hazard fire caused by electrical malfunctions, such as shortcircuits.
Corrosivity, toxicity, or high temperature hazard from contact with batteryelectrolyte.
Electrical energy hazard electric shock.
D. CONCLUDING REMARKS
No fuel is free from hazards. Although some fuel hazards are obvious, a systematicconsideration of hazardous properties and hazards can identify hazards which may have beenoverlooked. Hazards differ for various alternative fuels. This implies that:
Modifications of equipment and procedures will be required for eachalternative fuel.
No alternative fuel will be a "drop in" replacement for the status quo.
The full report from this study provides a framework for organizing information aboutadditional hazardous properties and hazards. However, a risk assessment, includinginformation about hazard probabilities and hazard consequences, can support conclusionsabout the safety ranking of various fuels, fuel systems, fueling equipment, and overallstrategies for using alternative fuels.
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LIST OF ACRONYMS
| ACGIH |
American Conference of Governmental Industrial Hygienists |
| AMF |
Alternative motor fuel |
| API |
American Petroleum Institute |
| BLEVE |
Boiling Liquid Expanding Vapor Explosion |
| C |
Celsius |
| CAP |
Clean Air Program |
| CARB |
California Air Resources Board |
| CNG |
Compressed natural gas |
| CO |
Carbon monoxide |
| DOE |
Department of Energy |
| EMI |
Electromagnetic interference |
| EPA |
Environmental Protection Agency |
| F |
Fahrenheit |
| FTA |
Federal Transit Administration |
| kPa |
Kilo Pascals (1 psia = 6.9 kPa) |
| LNG |
Liquefied natural gas |
| LPG |
Liquefied petroleum gas |
| MPa |
Mega Pascals |
| M-100 |
Neat (100 percent) methanol |
| M-85 |
Mixture of 85 percent methanol and 15 percent gasoline |
| NFPA |
National Fire Protection Association |
| NIOSH |
National Institutes of Occupational Safety and Health |
| NOx |
Nitrogen oxides |
| OEM |
Original equipment manufacturer |
| OSHA |
Occupational Safety and Health Administration |
| PEL |
Personal exposure limit |
| PRD |
Pressure relief device |
| psi |
Pounds per square inch |
| psig |
Pounds per square inch gage |
| RFG |
Reformulated gasoline |
| RLM |
Refrigerated liquid methane |
| RMP |
Risk Management Plan |
| RPT |
Rapid-phase transition |
| RVP |
Reid vapor pressure |
| SCRTD |
Southern California Rapid Transit District |
| STEL |
Short term exposure limit |
| TLV |
Threshold limit value |
| TWA |
Time-weighted average |
| VNTSC |
Volpe National Transportation Systems Center |
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1. INTRODUCTION
1.1 BACKGROUND
The national goals for both energy security and clean air have resulted in heightened interestin the use of alternative motor fuels (AMFs) in the transportation market. The EnergyPolicy Act of 1992 (EPACT) contains specific requirements for fleet use of alternative fuels. In a number of regions of the country, primarily where air quality is an issue, state and localclean air initiatives and fuel mandates have been enacted for certain vehicle classes. Thesemandates will have consequences for a number of transit and other fleets that must complywith local, state, and federal regulations while continuing to provide the highest qualitytransit programs and other services in their areas.
Other government programs have sought to encourage the use of alternative fuels throughgrants and awards for alternative fuel demonstration programs. For example, as part of itsClean Air Program (CAP), the Federal Transit Administration (FTA) has awarded grants foralternative fuel demonstration programs. The Department of Energy, through the NationalRenewable Energy Laboratory has also funded a number of alternative fuel demonstrationprograms, such as the comprehensive CleanFleet program involving Federal Expressmedium-duty delivery trucks.
Growth of interest in alternative fuels has expanded not only the number of alternative fuelvehicles, but also the list of viable alternative transportation fuels. In recognition of theincreasing need to more fully understand critical aspects of the candidate AMFs, the FTAand the Volpe National Transportation Systems Center (VNTSC) have established a programthat addresses the safety hazards and operational issues associated with the use of alternativefuels by vehicle fleet operators.
This effort to supply additional information concerning the safety hazard implications of allAMFs is timely. An increasing number of transit fleets and other fleet owners are operatingvehicles on alternative fuels often with a minimum of technical guidance related to thepossible safety or operational impacts on their facilities, as well as those related to theproduction, transport, and bulk storage of alternative fuels that support these demonstrations.
The environmental, safety hazard, and health aspects analysis of AMFs have become morecomplex in recent years. Several developments have contributed to this complexity. Thefirst development is the increasing number of candidate alternative fuels. For example, atfirst, methanol was the only alternative fuel being seriously considered for transit use. Theearly commitment by Detroit Diesel Corporation to provide a methanol fueled-engine fortransit use contributed to this emphasis. However, natural gas engine development soonfollowed, with the natural gas being stored in compressed form.
The roster of alternative fuels used in transit has now expanded to include methanol andmethanol blends (M-100 and M-85), ethanol and ethanol blends (E-95 and E-85), compressednatural gas (CNG), propane (LPG), liquefied natural gas (LNG), bio-diesel, and electric batteries, with additional interest in reformulated gasoline and advanced diesel, fuel cells,and even hydrogen as fuels for transit and other fleets.
The second development is the realization that some previous safety analyses haveconcentrated on only a portion of the total transit or fleet operation. Transit properties andfleet operators must consider the entire path from the fuel supplier all the way to the vehiclefuel tank. Also, fleet operations involve not only operating alternative fuel vehicles inrevenue service, but also fueling, inspecting, cleaning, washing, and performing the light andheavy maintenance activities necessary to keep the fleet in operation.
The buildings or facilities used for storing, loading, maintaining, and sometimes fueling,alternative fuel vehicles form an important portion of a fleet operation. Here, thedevelopment of fire and building codes is not yet complete. This requires additional care onthe part of the designers and owners of these facilities to consider all hazards associated withthe use of alternative fuel vehicles and to ensure that these hazards are properly addressed inthe plans for and the operation of the facility.
The third development, which adds to the complexity of alternative fuel use, is therecognition that more hazards must be considered than the traditional "Will it burn orexplode?" examination of fuel issues. The use of compressed gases raises issues concerninghigh fuel system pressures. LNG has the potential to cause blindness if splashed in the face. Methanol and denatured ethanol are toxic to humans. Ethanol fuel raises the issue ofdiversion for non-authorized use. Several fuels demand a further scrutiny of the need forpersonal protective gear.
Lastly, the experience of some transit properties and private fleet operators has shown thatnot all local community and regulatory groups view the use of alternative fuels as a purelypositive option. Opposition from neighborhood groups has already caused alternative fuelplans in several cities to be changed or curtailed. Transit properties and others who proposethe use of alternative fuels need to deal not only with the perceptions of fire and buildingcode officials who grant approvals, but also with the perceptions and concerns of communityand neighborhood organizations. The concerns of these groups are not limited to fleetoperations, but may also include the production of the alternative fuel and the transportationof the fuel to the point of use. It is important that the fleet operator recognize at thebeginning of a conversion to alternative fuels the types of safety issues that will need to beaddressed to satisfy these constituencies.
In view of the diversity of these safety concerns, as well as the number of possible hazards,a comprehensive and systematic program is needed to recognize and organize the existingknowledge about the health, safety, and environmental hazards of alternative fuels and toidentify where additional study is needed.
The existence of special safety concerns does not mean that alternative fuels are inherentlymore dangerous than conventional fuels, but does emphasize that forethought, goodengineering, and thorough training are requisites for the safe and successful use ofalternative fuels. Programs in which alternative fuels are used while all other aspects of thefleet operations remain unchanged are apt to have difficulties.
1.2 OBJECTIVES AND SCOPE
This study is intended to provide a systematic assessment of the safety hazards of AMFsfrom
a fleet operations perspective. It is narrowly focused on the hazards associated with moving
the fuel from the point of production to the point of use (bulk transport), the process of
transferring the fuel from the transport vehicle, and on-site storage at the fleet operator's
facility. The types of hazards that may be encountered during bulk transport, transfer, and
storage generation have been categorized as follows:
Safety Issues
- Fire Hazards
- Other Hazards
Health Issues
- Fuel Toxicity - inhalation/skin exposure
Environmental Issues
- Effects of spills
Six candidate fleet motor fuels received primary consideration during the assessment process.
These fuels and the automotive engines that are specifically designed to use the fuel have
been the subject of extensive research and development. The fuels are:
Compressed Natural Gas (CNG)
Liquefied Natural Gas (LNG)
Propane
Methanol and Methanol Blends (M-85, etc.)
Ethanol and Ethanol Blends (E-85, etc.)
Biodiesel
Hydrogen
Electricity
Hydrogen-fueled vehicles, including those using a fuel cell-electric drive, are just beingintroduced into actual operations on a prototype/demonstration basis. Battery-poweredvehicles have received increased attention in recent years, including a number of applicationsinvolving battery electric transit buses.
The overall objective of this report is to organize, analyze, and present existing information
about the potential hazards of the AMFs selected for this study. The specific focus is on the
hazards associated with potential leaks and spills of the AMFs in the bulk transport,
unloading, fleet storage processes, and fleet operations.
It should be noted that all of the potential hazards considered in this report are "acute"
hazards, i.e., immediate- or short-term hazards. Long-term ("chronic") hazards have notbeen
addressed.
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2. PREPARATION AND ORGANIZATION OF REPORT
2.1 INFORMATION SOURCES
The major sources of information used to conduct the assessment of safety, health, andenvironmental hazards associated with each AMF come from the following:
Recent key reports that cover one or more of the hazard assessment issues.
Information gathered through contacts and interviews with industry officials,trade groups, and government agencies.
The key references used to acquire information are provided at the end of this report inReferences - Section Three.
The following agencies and organizations were contacted for information on AMFs:
U.S. Department of Energy
U.S. Environmental Protection Agency
U.S. Department of Transportation
Gas Research Institute
National Hydrogen Association
National Soydiesel Development Board
Massachusetts Division of Energy Resources
New York State Energy Research and Development Authority
Boston Gas Company
Boston Edison -- Travelectric Services Corp.
Commonwealth Gas Company
2.2 ORGANIZATION OF REPORT
This report is composed of two main sections reflecting the two project tasks. The firstsection, "Production, Bulk Transport, and Bulk Storage of Alternative Fuels," focuses on thehazards associated with moving the fuel from the point of production to the point of use atthe fleet operators facility. The second section, "Use of Alternative Fuels by VehicleFleets," focuses on the operation, fueling, and maintenance of alternative fuel vehicles. Bothsections include discussion of the following fuels:
Compressed Natural Gas (CNG)
Liquified Natural Gas (LNG)
Propane
Methanol and methanol blends
Ethanol and ethanol blends
Biodiesel
Hydrogen
Electricity
Within the first section, the report is organized around a discussion of the properties, safetyissues, health issues, and environmental issues applicable to each alternative fuel, withsections on methodology, an analysis of issues, and a summary assessment of risks. Thesafety issues considered include:
General properties affecting fire hazards
Fire hazards during transport
Fire hazards during unloading to fleet storage
Fire hazards during fleet storage
Other hazards (e.g., high pressure, low temperature)
Within the second section, the report is organized around a summary list of hazards of eachalternative fuel. An introductory discussion considers the types of hazards considered andthe distinctions between hazardous fuel properties, hazards, and risks. The summary list ofhazards follows. It is accompanied by a selection of actual case histories which serve toillustrate various hazards in the summary list of hazards.
For the summary list of hazards of alternative fuels, the following hazardous properties areconsidered:
1. Flammability
2. Corrosivity
3. Toxicity (including asphyxiation)
4. High pressure
5. High temperature
6. Cryogenic temperature
7. Mechanical energy
8. Electrical energy
Although this document intends to be a comprehensive list of safety hazards, it is not a riskassessment in which the risk associated with the use of various alternative fuels are ranked orcompared. The definitions on the following page will help clarify these terms as used in thisreport.
Two separate sections of source material are included. Appendix A, titled "Sources forAlternative Fuel Safety Information" provides a bibliography, by categories, which givesbasic information for readers. Specific references in the text of the report are given in"References Section 3" and "References Section 4."
DEFINITIONS
An accident is a general term for an unplanned event with undesirable consequences.
A hazardous property (or hazardous condition) is a physical or chemical property of a substanceor situation that has the potential to cause harm. For example, a substance may be flammable orit may be contained under a high pressure.
A hazard is the combination of a hazardous property with an outcome that can cause damage orharm to people, property, or the environment. For example, a material which is flammable mayignite and result in a fire. Or a material at high pressure may release that pressure quickly,resulting in an explosion. Thus, it is common to speak of "fire hazards" or "explosion hazards"or to discuss the hazard of fire or the hazard of explosion.
A hazard event (or initiating event, or just event) is an occurrence involving equipment failure,human action or external cause that results in a hazard. For example, the ignition of a flammablematerial can cause a fire, while the rupture of a pressure vessel can result in an explosion.
The hazard probability is the chance that the hazard will occur. The hazard probability may bethought of as the combination of a hazardous property with the probability of one or moreinitiating events. For example, the probability of a fire may depend on the probability that a fuelspill could occur coupled with the probability that an ignition source is available. Hazardprobability may be expressed in purely numerical terms, such as the number of expected eventsper year or by using other qualitative or quantitative scales.
The severity of a hazard is a measure of the possible consequences of that hazard in terms ofproperty damage or the amount of injury. For example, the severity of a fire hazard may beranked by the dollar value of the property which may be destroyed. Other qualitative orquantitative scales of severity may also be used. A given hazard may have many possibleconsequences, so the severity of a hazard often depends on the hazard scenario. For example,for a given type of fuel, the fire hazard severity may be greater if the amount of fuel is greater,or if the equipment configuration allows it to burn more rapidly. Or, the severity of an electricalshock hazard is usually greater if the voltage is greater.
Risk is the combination of a hazard, a hazard probability, and a severity. For example, the riskof a vehicle fire is a combination of (a) the hazard the vehicle burning, the hazard probability (b) the chance of this event occurring, and (c) the severity of the damage the amount ofdamage to the vehicle and/or the extent of injury to the occupants.
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3. PRODUCTION, BULK TRANSPORT, AND BULK STORAGE OF ALTERNATIVE FUELS
3.1 INTRODUCTION
This section provides a detailed description of each AMF of interest, along with a discussionof its special characteristics that affect safety, health, and the environment. Each AMF ispresented separately using the following format:
General Description
(A brief summary of production sources and the general characteristics of the fuel.)
Safety Issues
(a) General Properties Affecting Fire Hazards
(b) Fire Hazards During Transport
(c) Fire Hazards During Unloading to Fleet Storage
(d) Fire Hazards During Fleet Storage
(e) Other Hazards (e.g., high pressure, low temperature)
Health Issues
Environmental Issues
The order of presentation of the AMFs is as follows:
Methanol/Methanol Blends
Ethanol/Ethanol Blends
Compressed Natural Gas
Liquefied Natural Gas
Propane
Biodiesel
Hydrogen
Electricity
3.2 METHODOLOGY
It was apparent after a number of the key reports and reference documents had been collectedthat the amount of information available is very extensive. In order to provide acomprehensive and understandable assessment, the methodology used to extract informationwas based on setting up a specific framework along the following lines:
General properties of the AMF that affect fire hazards
Potential fire hazards during bulk transport
Potential fire hazards during unloading to fleet storage
Potential fire hazards during fleet storage
Other safety hazards, particularly high pressure and low (cryogenic) temperatures thataffect personnel safety
Toxicity of the fuel based on inhalation, skin contact, and ingestion
Environmental effects of spills on land or water
This same framework is used for the presentation on each AMF in Section 3.3 Analysisof Issues. The information in this section represents a synthesis of the specific safety andhealth concerns derived from a relatively large number of documents.
Section 3.4 Summary Assessment of Risk provides a summary assessment of thesafety, health, and environmental issues on a comparative basis. This assessment is intendedto provide a broader understanding of the relative ranking of each AMF with regard to:
the relative potential for an AMF leak or spill during bulk transport and storageoperations; and
the relative consequences of an AMF leak or spill in the context of safety, health, andenvironmental impacts.
3.3 ISSUES ASSOCIATED WITH BULK TRANSPORT AND STORAGE OF
ALTERNATIVE FUELS
3.3.1 Methanol/Methanol Blends
General Description
Methanol or methyl alcohol is a clear colorless liquid that can be made from a variety ofsources including coal and natural gas. All methanol used commercially in the United Statesis manufactured from natural gas because this is by far the most economical feedstock.
Often, methanol fuel is designated M-100 to identify it as essentially 100% pure methanol. A popular methanol blend composed of 85% methanol and 15% unleaded gasoline isdesignated as M-85. The addition of 15 percent unleaded gasoline increases both the flameluminosity and the fuel volatility. The latter effect both increases the cold starting capabilityand also generally makes the vapors present in fuel tank ullage spaces too rich to beflammable.
Typically, M-85 is considered as an alternative fuel for light and medium duty gasoline(spark ignition) engine applications whereas M-100 is typically used in heavy duty diesel(compression ignition) engine applications. M-85 is also used in the flexible fuel vehicle(FFV) application where such vehicles can operate on any mixture in proportions of M-85and conventional unleaded gasoline.
3.3.1.1 Safety Issues
(a) General Properties Affecting Fire Hazards
The physical properties of methanol that affect fire hazards include its volatility, flash pointtemperature, range of flammability limits, autoignition temperature, and electricalconductivity. There are other properties of importance that affect the consequences orpotential damage associated with a methanol (or any alternative fuel) fire. These include theburn rate of liquid pools, the heating value of the fuel, flame temperature, and thermalradiation emitted from the fire.
Section 3.3 of this report provides a relative comparison of the physical characteristics ofeach alternative fuel that affects the safety, health, or environmental effects associated withits use. In this section, the major physical characteristics that differentiate the hazardsassociated with each fuel are summarized.
One general physical characteristic that differentiates methanol from other fuels is itscorrosive characteristics. Methanol is incompatible with several types of materials normallyused in petroleum storage and transfer systems, including aluminum, magnesium, rubberizedcomponents, and some other types of gasket and sealing materials.1 Therefore it is necessaryto take special precautions to ensure that methanol is transported or stored in containers andtransfer lines that have been specifically selected for that purpose.
The other significant difference between methanol and other AMFs is that it is considered tobe more toxic. However, exposure limits for inhalation of methanol vapor are only slightlylower than those for gasoline (200 ppm threshold limit value [TLV] for methanol vapor; 300ppm for gasoline vapor).2 Since gasoline is much more volatile than methanol, it is likelythat more gasoline vapors will be generated for an equivalent spill volume and therefore aremore likely to be hazardous to the persons exposed.
NFPA 325M -- Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids,1991 Edition provides a Health Hazard Rating that provides an assessment of exposure risksfor fire fighters. Methanol, along with natural gas, gasoline, and propane, has a hazarddegree of 1, which is a material that, on exposure, would cause irritation, but only minorresidual injury and is considered as only slightly hazardous to health. All of the other AMFshave a hazard degree of 0 which means that under fire conditions, they offer no hazardbeyond that of ordinary combustible material.
One other general property of methanol is the low flame luminosity of a pure (M-100)methanol fire. This makes it difficult to see the fire or even estimate its size, particularly ifit occurs in bright daylight. The methanol blends (M-85) have increased visibility becausethe burning of the gasoline fraction produces some luminance.3
One other property of interest is the relative vapor density of methanol compared to air; at1.11, methanol vapor is heavier than air. Therefore the vapor will tend to accumulate atground level or in low-lying areas such as maintenance pits.4 If the methanol vapor is not quickly dissipated through adequate ventilation, it will linger in the low-lying areas creatingan increasing opportunity for exposure to an ignition source and a subsequent fire.
The addition of unleaded gasoline to methanol to create M-85 can improve the cold startingcapabilities and increase the flame luminosity of the fuel. With regard to some of the keycharacteristics noted above, the presence of the gasoline can be expected to reduce thecorrosivity of the M-85 compared to M-100, but it will also increase the toxic healthhazards.3
(b) Fire Hazards During Transport
The bulk transport of methanol is usually done by a standard petroleum products tanker truckwhich carries approximately 10,000 gallons of fuel. From a fire hazard perspective, there islittle discernible difference in the bulk transport of methanol compared to gasoline or diesel. There is no reason to expect that methanol transportation, in general, will be any moresubject to leaks or spills than conventional gasoline or diesel transport. However, onespecific issue that must be considered is the possible use of materials that may not bemethanol compatible in the tanker truck. This could become a problem if there is a long-term exposure of methanol to seals and gaskets that may deteriorate and become subject toleaks.
One physical characteristic of methanol that is an important fire hazard consideration duringboth transport and storage is the combination of vapor pressure and flammability limits. ForM-100, vapor/air mixtures are potentially flammable at volume concentrations ranging from6.7 to 36 percent. In a fuel or storage tank, a methanol liquid temperature between 10C to43C (approximately 50F to 110F) at standard atmospheric pressure will create aflammable vapor/air mixture.4 Therefore any ullage space in a container or storage tank thatis vented to the atmosphere will contain flammable vapor-air mixtures at normal ambienttemperatures found in transport and storage operations.
This condition is different from the ullage space in a gasoline container or storage tank wherethe vapor concentration will be above the flammable limits range at normal temperature andpressure (i.e., too "rich"). In the case of diesel fuel, which is much less volatile thanmethanol, the vapor/air mixture in the headspace will generally be below the flammablelimits (i.e., too "lean") at normal ambient temperatures.
Therefore, with methanol, it is extremely important to ensure that there are strong safeguardsagainst any ignition sources inside the tank and that any vent lines or other openings haveflame arrestors. Any fill lines must extend below the liquid methanol surface to provide aseal between any external ignition sources and the methanol/air vapor.
The transport of M-85, assuming that it is not blended on-site at the fleet operators facility,mitigates some of the problems noted above for M-100. In general, M-85 is quite similar togasoline in its flammability characteristics because the fuel vapor is composed primarily ofgasoline.3 Under normal circumstances, the headspace in the container or storage volumewill contain a vapor/air mixture that is above the flammability limits concentration range,i.e., too rich to burn.
(c) Fire Hazards During Unloading to Fleet Storage
The transfer of methanol from the bulk transport tanker truck to fleet storage must take intoaccount the fact that any vapor/air mixture that leaks during the transfer operation will createa flammable volume. In addition, any methanol spill will quickly vaporize and formflammable vapor/air mixtures. For this reason, it is essential that all hose connectors havemechanical locking features, vapor recovery devices be in place between the tanker truck andthe fuel storage tank, and that grounding devices be provided to prevent static electricaldischarges from taking place. As noted earlier, any vent lines should have spark arrestorsand the fill line should extend to the bottom of the storage tank.
(d) Fire Hazards During Fleet Storage
Methanol fuel is typically stored in an underground tank that is sized to meet the needs offleet operations. The installation must be designed to use methanol compatible materials toavoid long term degradation and leaks. Fuel storage tanks designed for diesel or gasolineuse may not be methanol compatible.
The fire hazards associated with M-100 storage will be greater than for diesel fuel storagebecause it is a much more volatile fuel. A spill or leak of M-100 will create a much largervolume of flammable vapor/air mixture than an equivalent diesel spill. However, the firehazards associated with methanol storage should be approximately the same as, or lowerthan, with gasoline storage. Gasoline is more volatile than methanol; however, the potentialrange of flammability limits for M-100 is much greater (6.7% to 36%) than for gasoline(1.4% to 7.6%). This means that, considering an equivalent spill or leak (volume) of fuel,there will be an increased probability that the methanol/air vapor will come in contact withan external ignition source when compared to gasoline.
It should be noted that the range of flammability limits for most AMFs are highly dependentupon the maximum temperature of the fuel. For example, if M-100 is only exposed to amaximum temperature of 22C (70F) it is only possible to reach a maximum volumeconcentration of approximately 13% methanol based on its equilibrium vapor pressure at
22C and at atmospheric pressure. Therefore, the actual range of flammability limits formethanol may not be greater than the range for other AMFs.
The use of M-85 is primarily considered as an AMF for light and medium duty gasolineengines; therefore, it is appropriate to consider the fire hazards as being comparable to thatof gasoline. In fact the volatility and flammability limits of M-85 are very similar to thosefor gasoline because the fuel vapors from the blend are composed primarily of gasoline. Therefore, all of the precautions that are normally associated with gasoline storage must beobserved. These are primarily those that are designed to minimize the presence of anyexternal ignition sources. In addition, the presence of methanol requires that the storage tankinstallation must be methanol compatible.
3.3.1.2 Health Issues
Exposure to methanol can occur through inhalation of vapor, or through ingestion or skincontact with the liquid fuel. The toxic effects of methanol are the same regardless of themeans of exposure. Considering the fact that methanol is quite volatile, it is most likely thatthe typical route for exposure is through inhalation of methanol vapors.
Among the AMFs considered in this study, methanol vapor is considered the most toxic forinhalation exposure. The measure of fuel toxicity is the threshold limit value (TLV) forvapor exposure and it can be expressed in terms of either a time-weighted average (TWA)for an eight-hour workday or a 40-hour week, or as a short term exposure limit (STEL)expressing the maximum concentration allowable for a 15-minute exposure. For methanolvapor, the TLV-TWA value is 200 ppm, while the TLV-STEL value is 250 ppm.2 OtherAMF vapors have toxicity (TLV-TWA) concentration values that are at least five timeshigher. As noted earlier, none of the AMFs are considered to be serious health hazards bythe NFPA based on potential exposure during fire fighting activities.
Interestingly, conventional gasoline has a TLV-TWA close to that of methanol (300 ppmversus 200 ppm) and it is more volatile. Therefore, the toxic exposure risks with both ofthese fuels are likely to be similar. Diesel fuel vapors are apparently much more toxic thaneither methanol or gasoline since the TLV-TWA value for kerosene (as a proxy for dieselfuel) is only 14 ppm.2 Fortunately, diesel fuel is relatively non-volatile at normal ambienttemperature, therefore vapor exposure is not a significant issue.
The health issues with M-85 are similar to M-100. Considering the relative vapor toxicityand volatility of both methanol and gasoline, M-85 must be considered in the same healthhazards category as M-100.
Personnel involved in the bulk transport and storage of both M-85 and M-100 must beprotected from exposure through proper design of tanks and transfer lines, selection ofmethanol compatible materials, use of personnel protection equipment, and proper training toavoid accidental exposure. Something as simple as a drain line for a fuel filter or a transferhose for emptying fuel tanks can help to reduce exposure for the personnel working on theequipment.
3.3.1.3 Environmental Issues
The major environmental issues of concern with all liquid AMFs is a fuel spill, particularly aspill that reaches a sewer or drainage system. The release of flammable liquids into a sewersystem is prohibited by NFPA-30 -- Flammable and Combustible Liquids Code. One of thephysical properties of methanol that affects fuel spills is its water solubility. Normally, fuelhandling facilities that have an emergency drain connecting to a sewer will have a separatoror clarifier to ensure that the fuel (gasoline or diesel) will not reach the sewer. Thisapproach will not work with methanol since it is soluble in water and will pass directlythrough the separator. Methods for separating methanol from water exist but they are quitecomplex and costly. Therefore, the best approach is to ensure that any spills in a facility are absolutely prevented from entering any drain through the use of impoundment systems tocontain the entire volume of any potential above ground spill. In a bulk transport situationthere is obviously no way to provide such assurance for any type of liquid AMF.
Fortunately, methanol is quite volatile so that it will not persist for a long period of timewhen exposed to the environment. Methanol also biodegrades quickly.
3.3.2 Ethanol/Ethanol Blends
General Description
Ethanol is produced by the fermentation of plant sugars. Typically, it is produced in theUnited States from corn and other grain products, while some imported ethanol is producedfrom sugar cane. Like methanol, ethanol is a pure organic substance whose physical andchemical properties are invariant, unlike some other AMFs such as natural gas or propanewhich are mixtures of different hydrocarbon molecules with no standard or averagecomposition.
Pure or neat ethanol (E-100) is rarely used for transportation applications because of theconcern about intentional ingestion. In fact, ethanol for commercial or industrial use isalways denatured (small amount of toxic substance added) to avoid the federal alcoholicbeverage tax. Therefore, it is unlikely that ingestion would be a serious problem. For heavyduty diesel (compression ignition) engine applications, such as transit buses, two ethanolblends have been used:
Ethanol E-95, composed of 95 percent ethanol and 5 percent unleaded gasoline.
Ethanol E-93, composed of 93 percent ethanol, 5 percent methanol, and 2 percentkerosene.
Both blends have been used in Detroit Diesel heavy duty engines similar to the 23:1 highcompression ratio engines developed for methanol. For light and medium duty gasoline(spark ignition) engine applications, the typical ethanol blend is 85% ethanol and 15%unleaded gasoline. This fuel is similar to M-85; therefore, it can be used in flexible fuelvehicles which can ignite any mixture composition of E-85 and unleaded gasoline.
3.3.2.1 Safety Issues
(a) General Properties Affecting Fire Hazards
The general properties of ethanol (C2 H5 OH) are relatively similar to those of methanol(CH3 OH). With respect to fire hazards, ethanol is less volatile than methanol (the Reidvapor pressure of ethanol is less than half that of methanol) and the range of flammabilitylimits is smaller. On this basis alone, ethanol is safer than methanol. However, as pointedout above, there are relatively few situations where the ethanol will be in a pure form since itis usually used as either E-95 or E-85. With both ethanol and methanol blends, any fuel vapors will contain a substantial percentage of gasoline, therefore there would be very littledifference in the flammability characteristics of the two fuels.3
There are other general physical characteristics of pure ethanol that are important from asafety perspective. While ethanol is less corrosive to metals, gaskets, and seals thanmethanol, it is still necessary to make sure that any container, transfer lines, and fittings aremade from materials that are ethanol compatible. Ethanol vapor is much heavier than air(much more so than methanol) so that any vapor from a leak will move downwards andcollect in low lying areas where it may linger as a flammable vapor/air mixture unless thereis adequate ventilation. Fortunately ethanol, similar to gasoline, has a relatively low odorthreshold such that personnel in the vicinity of a leak of E-100 or any blend should be able torapidly detect it. As noted in Reference 2, there is considerable variation in the reportedodor threshold data for various AMFs, particularly ethanol and methanol. Therefore, thedetection of a leak of any AMF by odor is subject to a number of variables.
(b) Fire Hazards During Transport
The bulk transport of pure ethanol or ethanol blends by tanker truck will be subject to thesame types of hazards as other bulk transportation of petroleum products. As long as thetanker truck container, lines, and fittings are constructed from ethanol compatible materials,there would be no reason to expect an increased rate of leaks or spills when compared to theequivalent volume of gasoline or diesel fuel transported.
As with M-100, the bulk transport and storage of E-100 will involve an ullage spacevapor/air mixture that is in the flammable range at volume concentrations from 3.3 to 19%,corresponding to ethanol tank temperatures between 4C and 46C (approx. 40-115F).4Therefore, stringent precautions have to be taken to avoid the possibility of ignition sourcesinside any container or tank containing E-100.
Ethanol blends, typically E-85, that are transported will exhibit volatility and flammabilitycharacteristics that are very similar to gasoline because the fuel vapors will be composedprimarily of gasoline. As with methanol blends, the headspace vapor/air mixture for E-85will be above the flammability limits concentration range.
(c) Fire Hazards During Unloading to Storage
The transfer of E-100 from bulk transport truck to fleet storage must take into account thevolatility and flammability of any leaked or spilled fuel. The following precautions arenecessary:
hose connections with mechanical locking fasteners;
vapor recovery devices; and
grounding devices to prevent static electric discharge.
The unloading of E-100 and ethanol blends must be accomplished at the same level of safetystandards as used for gasoline. These standards are spelled out in NFPA30--Flammable andCombustible Liquids Code and NFPA30A--Automotive and Marine Service Station Code. These codes address fueling facility, storage, and handling requirements for all flammableand combustible liquids including both M-100 and E-100. It is of interest to note that theNFPA classification for gasoline, M-100, and E-100 is exactly the same (Class IB flammableliquids defined as those having closed-cup flash points below 23C and having a boiling pointat or above 38C). This is an example of the need to consider the spectrum of fire hazardproperties when considering AMFs because as discussed above, the ullage space hazardsalone make the transport and transfer of E-100 (and M-100) an increased fire hazard riskwhen compared to the blended fuels and gasoline.
(d) Fire Hazards During Fleet Storage
Ethanol fuel storage requires the selection of materials that will not degrade over the longterm. Fuel tanks designed for diesel or gasoline use may not be ethanol compatible.
The safety precautions that must be taken with ethanol storage are similar to those formethanol and include:
Positive prevention of ignition sources entering the storage space by providingsuch devices as spark arrestors in vent pipes, properly sized ground straps, and fillpipes extending to the bottom of the tank; and
Prohibiting the placement of any pumps or other equipment within the storagetank that can create an ignition source.
All of the above requirements for the prevention of ignition sources, leaks and spills, andadequate provision for handling any leakage of spills when storing or handling ethanol (andany other NFPA-designated flammable or combustible liquids) are spelled out in great detailin the applicable NFPA codes. For example, typical ignition sources identified in NFPA30include:
open flames
lightning
hot surfaces
radiant heat
smoking
cutting and welding
spontaneous ignition
frictional heat or sparks
static electricity
electrical sparks
stray currents
ovens, furnaces, heatingequipment
Therefore, there is a very substantial base of experience in handling and storage of suchflammable liquid AMFs, such as E-100, E-85, M-100, and M-85. The experience has beencodified into the NFPA codes which are used by local regulatory authorities (or alternatively,the Uniform Fire Code which is used more often in the Western part of the U.S.). On thepresumption that these codes are followed by the agencies involved in the bulk transport and storage of AMFs, in cooperation with local fire authorities, there is no reason to expect agreater incidence of fires in ethanol (or other AMF) storage situations then for a comparablenumber of gasoline storage facilities.
3.3.2.2 Health Issues
Ethanol is less toxic than methanol. The threshold limit value-time weighted average (TLV-TWA) concentration for ethanol vapor is 1,000 ppm compared to 200 ppm for methanol. Extensive skin exposure to ethanol can cause redness and irritation. Concern aboutintentional ingestion of ethanol by employees is mitigated by the fact that alcohols intendedfor industrial use must be denatured in order to avoid the federal alcoholic beverage tax. Denatured alcohol is ethanol that contains a small amount of a toxic substance such asmethanol or gasoline, which cannot be removed easily by chemical or physical means. However, ethanol fuels have been widely advertised as food-based, so there may beconfusion among some users concerning the denatured status of fuel ethanol.
3.3.2.3 Environmental Issues
The major environmental concern with ethanol is the same as for methanol; since it is watersoluble, it is necessary to take stringent precautions in order to ensure that any ethanol spilldoes not reach a sewer or drainage system. These same precautions cannot be assured forthe bulk fuel transport situation.
3.3.3 Compressed Natural Gas
3.3.3.1 General Description
Natural gas has been used as a vehicle fuel in the United States for several decades. Becauseof the residential and industrial use of natural gas, the industry has its own distributionsystem and supply network that is much more extensive than for any other liquid or gaseousAMF. The issues of bulk transport and storage are completely different from most of theother AMFs which are typically transported to fleet storage via tanker truck, unless thenatural gas has been liquefied. (Liquefied Natural Gas [LNG] is presented in the nextsection.)
The typical fuel system for natural gas vehicles is one with highly compressed (typically 20to 25 MPa or 3,000 to 3,600 psi) gas stored in high pressure cylinders on the vehicle. Thecontainment of natural gas at such high pressures requires very strong storage tanks whichare both heavy and relatively costly. This distinguishing feature of CNG is the one that hasthe most impact on safety issues.
CNG is generally produced on-site at a fleet fueling facility using compressors fed from anearby natural gas pipeline in conjunction with some limited high pressure on-site storage. For example, with very large fleets, the preferred approach will involve direct fast fill from the compressor where the compressor flow rate is sufficient to fill a vehicle tank in less than10 minutes. In order to accomplish this filling effectively, an intermediate high pressurestorage tank with a volume of 3 to 4 times the vehicle fuel tank capacity is required.5 Forslow fill (overnight), there is no need for a large storage tank, a small buffer tank issufficient.
3.3.3.2 Safety Issues
(a) General Properties Affecting Fire Hazards
Natural gas is a mixture of gases comprised primarily of methane with small amounts ofethane, propane, and butane. These heavier hydrocarbons (i.e., ethane, propane, and butane)tend to reduce the octane rating of natural gas. Therefore, the actual composition of thenatural gas plays an important role in the performance of fleet vehicles. For the purposes ofdiscussion in this report, the physical properties are based on the properties of the principalcomponent, methane, unless otherwise specifically noted. The typical range of methane forpipeline natural gas in various parts of the country is from approximately 80% to 95%. TheCalifornia Air Resources Board (CARB) has adopted specifications for natural gas as avehicular fuel which require that the methane content be greater than 88%. Even with thistype of specification, there is still considerable variation possible in the general physicalproperties of natural gas.
The physical properties of natural gas that affect safety include the autoignition temperatureand the flammability limits range. The autoignition temperature (also known as ignitiontemperature) is the lowest temperature at which a substance will ignite through heat alone,without an additional spark or flame. The ignition temperature of natural gas varies withfuel composition, but it is always lower than that of pure methane. The estimated ignitiontemperature of natural gas is in the range from 450-500C. The flammability limits rangefor natural gas is approximately 5% to 15% volume concentration.
More importantly, the leakage of compressed natural gas will immediately form a largegas/air mixture volume that is in the flammable range within a portion of the immediate areaaround the leak. A unit volume of CNG at 25 MPa psi will expand by approximately 200times when released to the atmosphere. The ignition energy required is very small forvirtually all of the AMF vapor/air mixtures being considered (in the range fromapproximately 0.15 to 0.30 millijoules).2 Therefore, the existence of a CNG leak creates anincreased probability of exposure to a stray ignition source such as a static electric sparkwhen compared to the leakage of an equivalent mass of an AMF that is expelled in a liquidform and vaporizes over a period of time.
Natural gas is colorless, tasteless, and relatively nontoxic. An odorant is added in suchamounts to make the odor noticeable at 1/5 of the lower flammability limit of 5%. Thus, theodor threshold for CNG is approximately 10,000 ppm. Therefore, personnel in the vicinityof a natural gas leak will be able to detect the presence well before the gas has reached theflammable limit in the area adjacent to the person.
The most unique physical characteristic of CNG does not derive from the physical propertiesof methane, but from the fact that the gas is stored at an extremely high pressure for use as avehicular fuel. The presence of material stored and transferred at pressures that far exceedthe normal experience of most fleet operations personnel raises the standard of precautionand training required. Inadvertent opening of valves or loosening of fittings containing highpressure natural gas will not only lead to creation of a fire hazard, but can also result in thehigh velocity ejection of metal parts or fragments that could be lethal to nearby personnel.
The existence of the high pressure methane gas also leads to thermodynamic expansion considerations which have not been addressed thoroughly in prior studies of CNG safety. The rapid expansion of methane gas from a high pressure cylinder or transfer line leak toatmospheric pressure will inevitably result in a significant cooling effect which will result ina vapor cloud of very cold and dense gas. Conventional practice has been to assume that anyleak of CNG will rise immediately due to the fact that methane at normal temperatures islighter than air. Consequently, safety design practices have been focused on ceilingventilation and detection of methane vapors. In fact, it is highly likely that any significantleakage from storage tanks and transfer lines will migrate down and fill in low lying areas asit is moved about by any wind or circulatory effects. Ultimately, the methane will warm upand rise (assuming a flammable mixture has not come into contact with an ignition source),but it is extremely difficult to estimate the time involved and the configuration of theflammable methane/air mixture during that time period.
(b) Fire Hazards During Transport
In most cases, the only "transport" issue involves the connection from the existing naturalgas pipeline to the fleet operators compressor station. The local gas utility will typicallywork with the fleet operator to provide an underground supply delivering pipeline qualitynatural gas at pressures ranging from 5 to 50 psig. While this is a much lower pressure,there is still a significant potential for a massive gas release if there is some unauthorizeddigging or trenching at the connection line resulting in a line break, or in the event of an on-site accident resulting in a line rupture at the connection to the compressor station. Onenecessary provision is a rapid and positive means of shutting off the supply flow from thepipeline in the event of any type of leak in the supply line.
In some cases, natural gas is delivered to the fleet user in compressed form by means of atruck trailer containing compressed gas. This type of gas delivery may be used on apermanent basis for small users who cannot justify the cost of a compressor station, or on atemporary basis to users whose compressor station is unavailable.
In this case, issues arise concerning the crashworthiness of the trailer: while the gas cylindersthemselves are robust, the valves and associated piping may be vulnerable. Also, it ispossible that the tanks might be exposed to a gasoline- or diesel-fueled fire should the tractortrailer truck be involved in a traffic accident.
The use of the CNG delivery trailer also requires that flexible connections be made andbroken in the course of each delivery. Experience shows that extra vigilance is necessary during truck loading and unloading because of the making and breaking of connections,possibility of leaking connections, possibility of truck movement when connected, etc.
(c) Fire Hazards During Transfer to Fleet Storage
In the case of CNG, the process involves the compression of the natural gas to the desiredpressure (approximately 25 MPa, 3600 psi) and transfer to the storage tank systems. Thereare various approaches that can be used for the CNG storage depending upon whether a fastfill (i.e., approximately 9,000 SCF of gas transferred to a vehicle in less than 10 minutes) ora slow fill (many hours or overnight) approach is used. In either case, however, there issome limited storage involved at pressures from 20 or 25 MPa (slow fill) up to 35 MPa forfast fill operations.
Pipeline natural gas contains small amounts of nitrogen, carbon dioxide, hydrogen sulfide,and helium. The quantity of these contaminant gases can vary from zero to a few percentdepending upon the source and seasonal effects. More importantly, the pipeline gas cancontain water vapor in amounts up to 112 mg/m3 (7 lbs. per million cubic feet) of gas.
The carbon dioxide and hydrogen sulfide components of natural gas, in the presence ofwater, can be corrosive to carbon steel. The corrosive effect is increased by pressure. Sincethe pressure considered in CNG vehicle applications is so high, there is a real concern aboutexcessive corrosion leading to the sudden explosive rupture of a container. NFPA 52 --Compressed Natural Gas (CNG) Vehicular Fuel Systems, 1992 Edition provides that the gasquality in any pressurized system components handling CNG comply with the followingspecification:
H2S and soluble sulfides partial pressure 0.35 kPa, max
Water vapor 112 mg/m3 (7.0 lb./MMSCF), max
CO2 partial pressure 48 kPa, max
O2 0.5 volume %, max
The NFPA committee involved in developing the standard relied on field experience andresearch which led them to believe that if the water content is limited as specified above, thepotential for corrosion problems is not a major concern. It should be noted that a watervapor content of 112 mg/m3 amounts to a very small concentration of water vapors;therefore, natural gas at or below this level is quite dry. The federal government has taken amore conservative position due to the corrosion failure of a cylinder comprising one ofseveral in a tube trailer in 1978. As a result, U.S. DOT has specified the composition ofCNG being transported in interstate commerce. The limits for the corrosive components arevery low, including an upper limit for water vapor set at 8 milligrams per cubic meter ofgas.
The existence of this potential problem with the corrosive properties of natural gas makes itnecessary to dry and treat the gas before high pressure storage whenever such corrosiveconstituents are in place. NFPA 52 also states that cast iron, plastic, galvanized aluminum, and copper alloys exceeding 70% copper are not approved for CNG service because thesematerials lack the necessary strength or resistance to corrosion required for CNG service.
In addition to the NFPA standard, the Society of Automotive Engineers has established SAEJ1616 Recommended Practice for Compressed Natural Gas Vehicle Fuel with provisionsintended to protect the interior of the fuel container, as well as other fuel systemcomponents, from corrosion.6
All of the above serves to point out that there is a substantial level of care which must betaken in the design and operation of high pressure CNG storage systems in order to avoidleaks or ruptures. In the event of a leak or rupture, the CNG fuel flow rate out of thestorage tank or piping can be very high, and any ensuing fire (or explosion) will be likely tohave a very high heat release rate. Compounding this problem is the difficulty of shuttingoff the CNG leak and extinguishing the fire.
(d) Fire Hazards During Storage
The amount of CNG that has to be stored at the fleet operator's facility is a function of thefill technique. For fast fill, the CNG storage volume should be at least 3 times (often up to4 times) the individual fleet vehicle fuel tank volume. For a typical 40-foot bus, the fueltanks would require approximately 250 kg. of CNG. This would mean a buffer storagecapacity of approximately 750 to 1,000 kg. Compared to other AMFs, this storage volumeis fairly small, thereby reducing the total potential fire and explosion impact of a massiverupture of the storage tank.
A slow fill system would have a much smaller buffer storage system because thecompression system would typically be sized to handle the maximum number of vehicles tobe fueled on an overnight basis.
In the unlikely event that a fleet operator decided to fast fill from a mobile CNG tube trailertruck, the amount of CNG stored on-site would increase substantially. If more than onetrailer were present on the site, the total amount of CNG would be in the order of 6,000 kg(13,000 lb). The Environmental Protection Agency has recently (Federal Register, January31, 1994, pp. 4478-4499) issued a Final Rule promulgating a list of regulated substances andthresholds required under Section 112(r) of the Clean Air Act, as amended. Methane is onthe list of regulated flammable substances with a threshold quantity of 4550 kg (10,000 lb). A facility storing more than this threshold amount is subject to the development andsubmission of a Risk Management Plan (RMP) which includes a hazard assessment, aprevention program, and an emergency response program. The RMP requirement is in therulemaking process currently; the proposed rule was published on October 20, 1993 (58 FR54190).
This requirement is much more applicable to the storage of LNG, hydrogen, and propanewhere there is more likely to be more than 4550 kg (10,000 lb.) stored at a facility. Thisthreshold quantity can easily be exceeded for AMFs used in medium to large fleetoperations.
3.3.3.3 Health Issues
The principal constituents of natural gas, methane, ethane, and propane, are not consideredto be toxic. The American Conference of Governmental Industrial Hygienists (ACGIH)considers those gases as simple asphyxiants, which are a health risk simply because they candisplace oxygen in a closed environment. The Occupational Safety and HealthAdministration (OSHA) has set a time-weighted average (TWA) personal exposure limit(PEL) of 1,000 ppm for propane. A number of minor constituents of natural gas haveACGIH-listed threshold limit values (TLVs), including butane - 800 ppm, pentane - 600ppm, hexane - 50 ppm, and heptane - 400 ppm. The effective TLV for an average naturalgas composition, considering all of these limits, is about 10,500 ppm.3
The odor threshold of odorized natural gas is about 10,000 ppm. Therefore, it is unlikelythat personnel will be unknowingly exposed to the TLV concentration since they can detect itby odor.
3.3.3.4 Environmental Issues
There are no significant environmental hazards associated with the accidental discharge ofCNG.
3.3.4 Liquefied Natural Gas
3.3.4.1 General Description
Liquefied natural gas (LNG) is produced by cooling natural gas and purifying it to a desiredmethane content. The typical methane content is approximately 95% for the conventionalLNG produced at a peak shaving plant. Peak shaving involves the liquefaction of natural gasby utility companies during periods of low gas demand (summer) with subsequentregasification during peak demand (winter). It is relatively easy to remove the non-methaneconstituents of natural gas during liquefaction. Therefore, it has been possible for LNGsuppliers to provide a highly purified form of LNG known as Refrigerated Liquid Methane(RLM) which is approximately 99% methane.
The primary advantage of LNG compared to CNG is that it can be stored at a relatively lowpressure (20 to 150 psi) at about one-third the volume and one-third the weight of anequivalent CNG storage tank system. The big disadvantage is the need to deal with thestorage and handling of a cryogenic (-160C, -260F) fluid through the entire process ofbulk transport and transfer to fleet storage.
3.3.4.2 Safety Issues
(a) General Properties Affecting Fire Hazards
Even though the end product of the use of CNG and LNG for vehicular applications isessentially the same, the general properties affecting safety are quite different. On one hand,LNG is a more refined and consistent product with none of the problems associated withcorrosive effects on tank storage associated with water vapor and other contaminants. On theother, the cryogenic temperature makes it extremely difficult or impossible to add anodorant. Therefore, with no natural odor of its own, there is no way for personnel to detectleaks unless the leak is sufficiently large to create a visible condensation cloud or localizedfrost formation. It is essential that methane gas detectors be placed in any area where LNGis being transferred or stored.
The cryogenic temperature associated with LNG systems creates a number of generalizedsafety considerations for bulk transfer and storage. Most importantly, LNG is a fuel thatrequires intensive monitoring and control because of the constant heating of the fuel whichtakes place due to the extreme temperature differential between ambient and LNG fueltemperatures. Even with highly insulated tanks, there will always be a continuous build upof internal pressure and a need to eventually use the fuel vapor or safely vent it to theatmosphere. When transferring LNG, considerable care has to be taken to cool down thetransfer lines in order to avoid excessive amounts of vapor from being formed.
The constant vaporization of the fuel also has an interesting effect on the properties of thefuel, unless it is a highly purified form of LNG, i.e., RLM. The methane in the fuel willboil off before some of the other hydrocarbon components such as propane and butane. Therefore, if LNG is stored over an extensive period of time without withdrawal andreplenishment the methane content will continuously decrease and the actual physicalcharacteristics of the fuel will change to some extent. This is known as "weathering" of thefuel.7
Another consideration is that under low temperatures, many materials undergo changes intheir strength characteristics making them potentially unsafe for their intended use. Forexample, materials such as carbon steel lose ductility at low temperature, and materials suchas rubber and some plastics have a drastically reduced ductility and impact strength such thatthey will shatter when dropped.
As before, many of these potential issues have been identified and addressed in the variouscodes that have been developed by the NFPA and under the Uniform Fire Code. Forexample, the NFPA has the following national standards and codes applicable to LNG:
NFPA 59A -- Standard for Production, Storage, and Handling of LiquefiedNatural Gas
NFPA 57 (draft) -- Standard for Liquefied Natural Gas Vehicular Fuel Systems
(final code expected to be published in 1995)
(b) Fire Hazards During Transport
LNG may either be liquefied on-site or it can be delivered to fleet storage using a standard10,000 gallon LNG tanker truck. In general, only the largest fleet operators would find on-site liquefaction to be advantageous. Typical LNG storage vessels, including those used onthe tanker truck, have the following basic components:
Inner pressure vessel made from nickel steel or aluminum alloys exhibitinghigh strength characteristics under cryogenic temperatures
Several inches of insulation in a vacuum environment between the outer jacketand the inner pressure vessel. Stationary tanks often use finely ground perlitepowder, while portable tanks often use aluminized mylar super-insulation.
outer vessel made of carbon steel and not normally exposed to cryogenictemperatures
control equipment consisting of loading and unloading equipment (piping,valves, gages, pump, etc.) and safety equipment (pressure relief valve, burst disk,gas detectors, safety shut off valves, etc.)
The double walled construction of the LNG tanker truck is inherently more robust than theequivalent tanker truck design for transport of other liquid AMFs. Therefore, the transportof LNG is safer from the perspective of fuel spills resulting from a tank rupture during anaccident. A rupture of the outer vessel would cause the loss of insulation and result in anincreased venting of LNG vapor. While this is of concern, it is relatively minor comparedto the prospect of an LNG spill.
An explosion of an LNG container is a highly unlikely event that is possible only if thepressure relief equipment or system fails completely or if there is some combination of anunusually high vaporization rate (due to loss of insulation) and some obstruction of theventing and pressure relief system preventing adequate vapor flow from the inner pressurevessel with a resultant pressure build up. If the pressure builds up to the point where thevessel bursts, the resulting explosion is known as a BLEVE (boiling liquid expanding vaporexplosion) with the container pieces propelled outward at a very high velocity.7 This is ahighly unlikely event due to the extensive requirements for pressure relief including pressurerelief valves and burst discs that are built into the design codes. (There have been no reportsin the literature reviewed of any BLEVE occurring with LNG.)
In the event that the LNG vessel is ruptured in a transport accident and the LNG is spilled,there will be a high probability of a fire because a flammable natural gas vapor/air mixturewill be formed immediately in the vicinity of the LNG pool. In an accident situation, thereis a high likelihood of ignition sources due to either electrical sparking, hot surface, orpossibly a fuel fire created from the tanker truck engine fuel or other vehicles involved in theaccident. The vapor cloud from an LNG pool will be denser than the ambient air; therefore,it will tend to flow along the ground surface, dispersed by any prevailing winds.
When spilled along the ground or any other warm surface, LNG boils quickly and vaporizes. A high volume spill will cause a pool of LNG to accumulate and the boiling rate will decrease from an initial high value to a low value as the ground under the pool cools. Theheat release rate from an LNG pool fire will be approximately 60% greater than that of agasoline pool fire of equivalent size.
(c) Fire Hazards During Transfer to Fleet Storage
The transfer of LNG from a tanker truck to fleet storage is a complex process that involvesthe active participation of both the tanker truck driver and a representative of the fleetoperator. A partial listing of some of the steps involved provides some indication of thesafety precautions that are necessary.7
After the truck is chocked and the engine is shut off, a grounding cable is attachedto the truck to ground any electrostatic discharge.
A flexible liquid transfer hose is attached to the tanker and purged with LNG toremove all air.
A fleet operator representative will open the storage vessel liquid fill line and thedriver will open the trailer's main liquid valve.
The driver will control the pressure in the trailer tank via a pressure building linewhere LNG is vaporized and returned to the tank to maintain a pressuredifferential of at least 15 psi between the tanker and the storage vessel.
The driver will use a mechanical means to maintain a tight connection at the hosecoupler to compensate for differential expansion.
The safety features that are typical of truck storage transfer of LNG include equipmentdesign such as trailer liquid valves that are interlocked with the truck brake system to preventfuel transfer before the truck is properly secured; remote-controlled, redundant liquid valves;storage vessel alarms to prevent overfill; and long drain lines for safety-directing ventedLNG vapor.
The complexity of the fuel transfer arrangement creates the potential for leaks and spillsthrough human error and equipment failure. One of the particular concerns is that the fueltransfer equipment goes through a continuous cycle of cool down to cryogenic temperaturesand warm up to ambient temperature. This type of thermal cooling can create additionalstresses on equipment and sealing devices which could result in decreased reliability overtime.
(d) Fire Hazards During Fleet Storage
LNG storage facility requirements for a total on-site storage capacity of 70,000 gallons orless are defined in the draft NFPA 57 -- Standard for Liquefied Natural Gas (LNG) VehicularFuel Systems. NFPA 59A -- Standard for the Production, Storage, and Handling ofLiquefied Natural Gas (LNG) is applicable to storage volumes above 70,000 gallons. Both ofthese standards address similar issues including siting of the storage tank, provision for spilland leak control, and the basic design of the storage container and LNG transfer equipment.
One of the major provisions at any LNG storage facility is the requirement to provide animpounding area surrounding the container to minimize the possibility of accidental dischargeof LNG from endangering adjoining property on important process equipment and structure,or reaching waterways. This requirement ensures that any size spill at a fleet storage facilitywill be fully contained and the risk of any fire damage will be minimized.
(e) Other Hazards
LNG has a unique safety hazard among the AMFs because of the potential exposure ofpersonnel to cryogenic temperatures. Workers can receive cryogenic burns from direct bodycontact with cryogenic liquids, metals, and cold gas. Exposure to LNG or direct contactwith metal at cryogenic temperatures can damage skin tissue more rapidly than when exposedto vapor. It is also possible for personnel to move away from the cold gas before injury.
The risk of cryogenic burns through accidental exposure can be reduced by the use ofappropriate protective clothing. Depending upon the risk of exposure, this protection canrange from loose fitting fire resistant gloves and full face shields to special extra protectionmulti-layer clothing.
Another unusual hazard associated with aged LNG will arise in the unlikely event that thereis a large spill of LNG onto a body of water. This could occur in an accident situationinvolving an LNG transport vehicle container rupture and spill into an adjacent water body. The hazard is known as a rapid-phase transition (RPT) -- in this case a rapid transformationfrom the liquid phase to vapor. If significant vaporization occurs in a short time period, theprocess can, and usually does, resemble an explosion.8
The RPT "explosion" phenomenon for LNG on water has been observed in a number ofsituations and has been studied extensively in both laboratory and large scale tests. Thetemperature of the water and the actual composition of the LNG are important factors indetermining whether an RPT will take place. It should also be noted that RPTs have beenobtained for pure liquefied propane with water temperature in the range of 55C (130F).
3.3.4.3 Health Issues
The principal constituents of natural gas, methane, ethane, and propane, are not consideredto be toxic. The American Conference of Governmental Industrial Hygienists (ACGIH)considers those gases as simple asphyxiants, which are a health risk simply because they candisplace oxygen in a closed environment. The Occupational Safety and HealthAdministration (OSHA) has set a time-weighted average (TWA) personal exposure limit(PEL) of 1,000 ppm for propane. A number of the minor constituents of natural gas haveACGIH listed threshold limit values (TLVs), including butane - 800 ppm, pentane - 600ppm, hexane - 50 ppm, and heptane - 400 ppm. The effective TLV for an average naturalgas composition, considering all of these limits, is about 10,500 ppm.3
Unlike CNG, LNG cannot be odorized; therefore, there is some concern about the ability ofpersonnel to detect TLV concentrations. This is another reason to ensure that methanedetectors are in place wherever personnel may be exposed.
3.3.4.4 Environmental Issues
There are no significant environmental hazards associated with the accidental discharge ofLNG.
3.3.5 Propane
3.3.5.1 General Discussion
Propane, which is otherwise known as liquefied petroleum gas, consists of a mixture ofpropane, propylene, butane, and butene. These gases are referred to as natural gas liquidssince they are present in wellhead natural gas. Liquefaction of these gases will occur bycompressing them to pressures above 800 kPa (120 psi) at room temperature. The termpropane is used in this section to reflect the fact that this AMF is typically composed of morethan 95% propane. The term also reflects industry practice for the gas as a motor fuel.
Approximately 60% of the U.S. propane supply comes from the processing (stripping) ofwellhead natural gas and the remaining 40% is a by-product of petroleum refining. Propanefor use in vehicle fleet operations has to be formulated so that it contains at least 95%propane and contains no more than 2.5% butane and heavier hydrocarbons. ASTMspecifications for propane meeting this requirement include those for commercial propanewhich is suitable for light duty internal combustion engine applications and special dutypropane which is suitable for heavy duty applications.
There is a substantial base of experience with propane as an automotive fuel since it is thethird most heavily used fuel, after gasoline and diesel fuel. It is estimated that there areapproximately 350,000 propane vehicles in operation, with most of them being aftermarketconversions of gasoline vehicles. Historically, propane was used extensively in transit
applications from the 1940s up to 1970. The largest single user was the Chicago TransitAuthority which in 1970 operated 1,400 propane buses, reportedly with a good safetyrecord.5
3.3.5.2 Safety Issues
(a) General Properties Affecting Fire Hazards
Propane is an extremely volatile fuel compared to the other liquid AMFs being considered. The Reid vapor pressure (RVP) of propane is more than an order of magnitude greater thangasoline which is the next most volatile fuel (1400 kPa versus 100 kPa). Propane is storedunder moderate pressure (110 to 150 psi) at ambient temperatures to maintain it in a liquidstate. In the event of an accidental release of propane to the atmosphere, about one-third ofthe liquid flashes to vapor at a temperature of -70F or lower.5 Leaking propane will discharge at a high velocity due to the pressure differential, turning the liquid into anatomized spray with the droplets typically evaporating before they can fall to the ground. Larger spill quantities will form a boiling pool on the ground surface which will cool downand essentially stop active boiling of the pool when the ground surface becomes sufficientlycool. Vaporization will continue until all of the propane evaporates.
Due to the rapid vaporization of propane, the pool burn rate is the highest of all the liquidAMFs considered. As a result, the heat release rate from a propane fire is approximatelytwice that of a gasoline fire for the same liquid spill volume. The flammability limits rangefor propane is similar to that for gasoline. Consequently, when compared to accidental spillsof an equivalent volume of gasoline, propane vapor is more apt to come into contact with anignition source due simply to the much higher volatility of the fuel and the resulting largervolume of flammable propane/air mixture.
Another physical characteristic of interest is that propane vapor is heavier than air so it willdescend from the point of a leak and accumulate and linger in low-lying areas unless there isadequate ventilation.
(b) Fire Hazards During Transport
Propane fuel is typically delivered to fleet storage via tanker trucks with capacities up toapproximately 10,000 gallons. All propane tanker trucks must conform to applicable U.S.DOT regulations regarding Hazardous Materials Regulations and Federal Motor CarrierSafety Regulations. The regulations specify the materials design factors and pressure reliefconsiderations for cargo transport. A major concern is the setting of pressure relief valvesso that the container will not vent propane vapor in the event of an unusually warm day. Allof these containers are typically manufactured from steel and are qualified under the ASMEpressure vessel code. The minimum design pressure for the container is based on the vaporpressure of the propane at 45C (115F). Since the vapor pressure for commercial propaneat that temperature is 243 psig, the design pressure typically is 250 psig with a safety factorof 4:1, for the tank stress calculations and selection of tank construction materials.
These pressure requirements result in a very strong tank container design. The net effect isthat the container for propane on a tanker truck will be much more rugged and resistant torupture from mechanical forces associated with an accident when compared to the transportof other liquid AMFs that are not pressurized, with the exception of the double shell tank forLNG.
On the other hand, the transport of a liquid fuel at moderately high pressure means that thereis an increased probability of fuel leaks at joints and fittings. The piping system includinghoses, along with fittings and valves will all be designed to code requirements for theexpected pressures. But with any piece of equipment that is in frequent use on the road,there is an increased likelihood of eventual wear and vibration that could create theopportunity for small leaks.
(c) Fire Hazards During Unloading to Fleet Storage
Propane is typically transferred from the tanker truck to fleet storage by pumping it from atruck into the storage container. As with any transfer of fuel, this is likely to be the mostpotentially hazardous part of the bulk transport to storage process. The fact that personnelare dealing with pressurized valves and lines, where any human error may result in a seriousdischarge of propane, makes it a point of concern.
Fortunately, propane is odorized so that the presence of a small leak may be detected by thepresence of its odor in the vicinity of any personnel responsible for unloading it. However,as noted earlier, propane vapor will descend and in the absence of any circulating air, it maygo undetected in a low-lying area.
(d) Fire Hazards During Storage
All propane storage containers are constructed according to the appropriate ASME PressureVessel Code. Design pressures are usually on the order of 250 psig with the pressure releasedevices typically set in the vicinity of 375 psig. Normally, underground tank installation isspecified for liquid fuels such as gasoline and diesel, mainly because it eliminates the hazardof fuel spills caused by vehicles running into the tank, and also because it allows more spacefor parking of vehicles. Propane, however, is ordinarily stored in above-ground tanksconstructed of thick guage steel. The tanks are strong enough to be supported by concrete orsteel saddles without deforming. The tanks are then surrounded by heavy upright steel pipesstructurally mounted in concrete to act as a barrier against vehicle intrusion into the tankarea.5
The structural strength of the storage tank and the proper design of all piping, valves, andfittings should provide a high level of protection against any massive leaks. The weakestpoints in any pressurized system like a propane storage system will be at any joints,connections, or fittings where there are always possibilities for developing small leaks overtime. The odorization of propane along with the proper placement of combustible gasdetectors and the natural ventilation in an outdoor area should help to prevent any seriousfire hazard from developing.
One of the major safety considerations with the storage of propane is the possibility of apressure buildup in the tank due to external heating from a fire combined with a failure ofthe pressure relief or venting system. The resultant explosion of the tank due tooverpressure would lead to a BLEVE incident. The fact that all of the applicable codes andfederal regulations for container design provide for the placement of pressure relief devices,and the subsequent testing of those devices on a regular basis, leads to the conclusion that thelikelihood of an overpressure leading to a BLEVE is exceedingly small, particularly in afixed storage facility situation. Unlike an accident situation with a transport vehicle where itis possible to roll over and damage the pressure relief and other protective equipment, there is little reason to expect that multiple devices for pressure relief at a stationary facility wouldsimultaneously fail.
(e) Other Hazards
Since propane is stored under pressure during bulk transport and storage operations, there isa potential hazard associated with an inadvertent opening of a fitting or plug which couldbecome a projectile. In addition, when propane expands out of a leak or hole, the rapidvaporization or flashing of the liquid causes the stream to reach temperatures that can causefreeze burns.
When compared to other AMFs, the potential high pressure hazard with propane is much lessthan with CNG (3600 psi vs. 150 psi); and the freeze burn hazard is much less than withLNG, because the propane liquid starts at ambient temperature as it leaves the tank.
3.3.5.3 Health Issues
Since propane for fleet use is a mixture of hydrocarbons, the toxicity of the fuel is difficultto determine. The major constituent, pure propane, is considered to be a simple asphyxiantby the ACGIH and does not have an assigned TLV. The other significant, but muchsmaller, constituent is butane which has a TWA-TLV of 800 ppm. OSHA has set a PEL of1000 ppm for propane, with the requirement that exposure to more than half this levelrequires that a medical monitoring program be instituted. Other than this OSHArequirement, there is no other agency or body that has established an exposure limit forpropane.
It should also be noted that propane has been reported to contain a relatively high level ofradon gas, with radon concentrations in propane that are well above current EPA guidelinesfor radon exposure.9 Since the exposure of personnel to propane will be limited, thepotential exposure to radon gas should not be a serious problem.
3.3.5.4 Environmental Issues
There are no significant environmental issues associated with the spill of propane, since theliquid will quickly vaporize.
3.3.6 Biodiesel
3.3.6.1 General Discussion
Biodiesel is an AMF that is derived from biological sources such as soybean oil, rapeseedoil, other vegetable oils, animal fats, or used cooking oil and fats. The chemical process forcreating biodiesel involves mixing the oil with alcohol in the presence of a chemical catalystsuch as sodium hydroxide. This process produces a "methyl ester" if methanol is used (typically the most common for economic reasons), or an "ethyl ester" if ethanol is used. In either case, the reaction also produces glycerin which is a valuable co-product. Eithermethyl ester or ethyl ester can be used neat (100%) or blended with conventional diesel("petrodiesel") as a fuel for diesel (compression ignition) engines.
Current efforts to commercialize biodiesel in the United States were started by the NationalSoyDiesel Development Board (NSDB) in 1992. The emphasis of their activity is on the useof soybean oil methyl ester (SME) blended with petrodiesel at a 20% volume SME/80%petrodiesel (BD-20) and a 30%/70% blend (BD-30). These blends are believed to offer thebest balance of cost and engine emissions characteristics. NSDB reports that as of thebeginning of 1994, biodiesel had accumulated nearly eight million miles in demonstrationsinvolving more than 1,500 vehicles in fleets across the country, particularly in urban buses.10
Methyl ester made from rapeseed oil (RME) is in widespread use in Europe due to a total ornear-total exemption from fuel taxes in most EC countries. As a result, there is a muchlarger base of operating experience with biodiesel in Europe amounting to several hundredtimes more vehicles and miles than in the U.S.
3.3.6.2 Safety Issues
(a) General Properties Affecting Fire Hazards
Data for the properties of soybean oil methyl ester (SME) indicate that it is a safer fuel thandiesel, which in turn, makes it safer than the other AMFs considered. For example, theflash point for SME is 218C (425F) compared to approximately 73C (160 F) for theaverage No. 2 diesel fuel. It also has an extremely low vapor pressure, less than 1.3 x 10 - 5kPa at 72C. Therefore, when SME is blended with petrodiesel to create BD-20, theresultant flash point for the mixture is 118C, still well above that for the petrodiesel alone.
Past experience with neat (100%) biodiesel has indicated that it is incompatibile to immerse itwith certain rubbers and plastics, but not with metals. Reports indicate that nitrile rubberand polyurethane-based compounds showed unacceptable deterioration while other elastomerssuch as SBR, butadiene, isoprene, hypalon, silicon, and polysulphide were not resistant toneat biodiesel. Acceptable replacement materials include fluorine - rubber (Viton A®) andpolypropylene- and polyethylene-based plastics.10 Therefore, the selection of materials toavoid degradation of seals, fittings, and hoses is important for biodiesel applications.
An unusual physical characteristic of biodiesel that has a fire hazard implication is thepossibility of spontaneous combustion in highly unsaturated materials such as some vegetableoils and methyl ester which oxidize in the air. This is classically known as the "oily rag"problem where the rag is placed in a confined space, such as a pile in the corner, and thereis no way for the generated heat of oxidation to dissipate. The higher temperatureaccelerates the oxidation process giving off even more heat until the pile of rags begins tosmolder and then burn. Since oil-soaked rags or other materials such as filters in typicalpetrodiesel operations are not subject to spontaneous combustion, it will be necessary to alert personnel (e.g., at the fleet operator's fuel storage and maintenance facilities) of the potentialfor spontaneous combustion. This is not a serious problem and can be simply resolved byhaving closed metal cans for storing oil soaked rags and other oily combustible material.
(b) Fire Hazards During Transport
Due to the very low volatility and high flash point temperature of neat biodiesel and blends(BD-20, BD-30), there are no specific fire hazard problems during transport. Any leak orspill is less likely to ignite than diesel or gasoline under equivalent conditions. Biodiesel-compatible materials should be selected to avoid problems of degradation of seals andfittings.
(c) Fire Hazards During Unloading to Storage
There are no specific fire hazards. Unloading equipment should be designed to handlebiodiesel to avoid any possibility of leaks.
(d) Fire Hazards During Fleet Storage
There are no specific fire hazards, other than the potential spontaneous combustion issuenoted above.
3.3.6.3 Health Issues
Because there are essentially no vapors generated at normal transport and storagetemperatures, pure or neat biodiesel can only be considered as a potential health hazard dueto ingestion. Pure biodiesel looks and smells like a food product and could conceivably beingested. If biodiesel were ingested, enzymes in the body would break the ester back into itsoriginal components, e.g., soybean oil and methanol.11 This raises the potential issue ofmethanol toxicity as a potential health hazard associated with biodiesel. Consequently,biodiesel cannot be considered to be non-toxic, as often cited in the promotional literature.
3.3.6.4 Environmental Issues
Biodiesel is considered to be biodegradable based on the chemical nature of the materials. Test data indicates that biodiesel is in the same range as biodegradable soaps and detergents. Therefore there are no significant environmental hazards associated with biodiesel.
3.3.7 Hydrogen
3.3.7.1 General Description
Hydrogen is unique among AMFs because it cannot be produced directly, as in drilling awell for petroleum oil and natural gas. Hydrogen must be extracted chemically fromhydrogen-rich materials such as natural gas, water, coal, or plant matter. A substantialquantity of hydrogen is produced each year in the U.S. -- about 8.5 billion kilograms peryear.
About 95% of the hydrogen in the U.S. is produced by steam reforming, a chemical processthat makes hydrogen from a mixture of water and a hydrocarbon feedstock, such as naturalgas. When steam and methane contained in the natural gas are combined at high pressureand temperature, a chemical reaction converts them into hydrogen and carbon dioxide. Theoverall energy efficiency of the process, i.e., the energy content of the hydrogen produceddivided by the total energy (natural gas and energy used to run the reformer) consumed, isapproximately 65%. Other techniques for producing hydrogen, including off-gas cleanup andelectrolysis, are much more costly.
Over the long term, it may be possible to consider large scale electrolysis (passing anelectrical current through water to split individual water molecules into hydrogen andoxygen) using sunlight on photovoltaic cells as the electrical power source, or some otherrenewable energy source such as wind power. Hydrogen obtained using this approach istermed "solar hydrogen" or "renewable hydrogen."
The actual use of hydrogen in automotive vehicles is limited to experimental and prototypevehicles. A number of prototype vehicles burn hydrogen directly using modified automotiveengines. There are also a number of vehicles that use the hydrogen in a fuel cell to produceelectrical power for electrical motor drives, i.e., a hydrogen powered electric vehicle.
In addition to the direct use of hydrogen there has been a demonstration program involvingblends of up 15 percent in volume of hydrogen added to natural gas to create "hythane." Inthis case, the hydrogen provides up to 5 percent of the energy content of the blend.
3.3.7.2 Safety Issues
(a) General Properties Affecting Fire Hazards
Hydrogen is a difficult fuel to deal with because of its physical properties. One of these wellknown properties is that as a gas its density is very low -- only 1/15th that of air. Therefore,for any practical applications, it is necessary to either compress the hydrogen or liquefy it. The problem with compressed gaseous hydrogen in a fleet vehicle application is the weight ofthe high pressure tanks. It has been estimated that the weight of the compressed hydrogenwill only vary from 1 to 7% of the total weight of the tank. Fortunately, the energy densityof hydrogen is very high so that 1 kg of hydrogen contains approximately 2.5 times moreenergy than 1 kg of natural gas. Therefore, assuming an equivalent engine efficiency, theweight of a vehicle's compressed hydrogen fuel storage system will be similar to that for aCNG fuel storage system. The alternatives to compressed hydrogen tanks on the vehicle include liquefied hydrogen, an on-board converter fueled by methanol to create hydrogen,and storage of hydrogen in metal hydride systems. All of these techniques are the subject ofresearch.12
For bulk distribution of hydrogen, the most common method by far is to liquefy thehydrogen and transport it by truck trailers, barges, or railcars. At atmospheric pressure,liquid hydrogen (known as LH2) boils at -253C (423F), which is only about 20C aboveabsolute zero. The process of hydrogen liquefaction, storage, and distribution is challenging,to say the least. Hydrogen is usually liquefied in a complex multi-stage process that involvesthe use of liquid nitrogen (boiling point of approximately -200C). Special precautions arerequired during liquefaction to maintain the proportions of two types of hydrogen moleculesin order to avoid excessive internal heating and vaporization while the LH2 is beingtransported or in storage. LH2 requires special insulation to maintain liquid conditions aslong as possible.12
The physical property of hydrogen that creates the most significant fire hazard is theextremely wide range of flammability limits, i.e., from 4% to 75% by volume. This rangeis twice that of methanol which has the next widest range. In effect, any release of hydrogeninto the air results in a much larger volume of a flammable mixture than an equivalentamount of any other AMF.
More importantly, the potential for an explosion or detonation of a flammable hydrogen-airmixture is very high. The ignition energy for hydrogen-air mixtures is much lower than forhydrocarbon-air mixtures. Very low energy sparks, such as from a static electric discharge,can lead to ignition; and if the burning gas is even slightly confined, the resulting pressurerise can lead to a detonation.
Among the other physical properties of hydrogen that are of interest is the propensity of thegas to leak more easily than other AMF gases due to the relatively small siz
