# BEST PRACTICES TO AVOID LNG

FUELING STATION VENTING LOSSES

Charles A. Powars June 2 5 , 2010

Prepared for: Brookhaven National Laboratory

P.O. Box 5000 Upton, NY 11973 - 5000 Contract No. 157688 Prepared by: St. Croix Research 5535 Fern Dr. San Jose, CA 95124 - 6311 

# S . Croix ResearchTABLE OF CONTENTS

Section Page 1 Introduction 1 1.1 Objective 1 1.2 The Magnitude of the Venting Problem 3 1.3 Issues and Challenges 4 1.4 Information Sources, LNG Quantity Units, and Report Organization 6 1.4.1 Prior Work 6 1.4.2 Information Sources and Citation s 6 1.4.3 Patents and Commercial Products 6 1.4.4 LNG Quantity Units 6 1.4.5 Report Organization 7 2 LNG Infrastructure Requirements Summary 9 2.1 Overview 9 2.2 LNG Saturation, Properties, and Weathering 9 2.3 LNG Tank Basic Characteristics 11 2.4 Natural Gas Engine Fuel Pressure Requiremen ts 14 2.5 LNG Vehicle Fuel System Design Options 14 2.6 LNG Tank Trucks 18 2.7 Station Utilization 19 2.8 Temporary and Mobile LNG Fueling Facilities 20 2.9 Weights and Measures Certification 21 2.10 Applicable Codes and Standards 22 3 LNG and LCNG Fueling Station Design and Operati on E ffects on Venting 23 3.1 Overview 23 3.2 Tank - Truck - to - Station Transfer 23 3.2.1 Pressure Transfer Offloading 23 3.2.2 Offloading Using Dedicated Pump 25 3.2.3 Offloading Using Dispensing Pump 28 3.3 LNG and LCNG Station Design and Operation 28 3.3.1 Station Sizing 30 3.3.2 Dispensing Pump Opt ions 31 3.3.3 LNG Fuel “Conditioning” Options 33 3.3.4 Fueling Stations Without LNG “Conditioning” Equipment 353.3.5 Stations Capable of Refueling All Types of LNG Vehicles 36

3.3.6 LNG Fuel Dispenser Cool - Down Options 38 3.3.7 Station LNG Storage Tank Pressure Management 38 3.3.8 LCNG Fuel ing stations 39 3.4 LNG Vehicle Operations 41 4 Options for Recovering Vapor Generated at LNG Fueling Stations 45 4.1 Overview 45 4.2 Reliquefaction of Boil - Off Vapor 46 4.3 Utilization of Boil - Off Vapor for On - the - Fly Conditioning 47 4.4 Flaring of Boil - Off V apor 4 8 4.5 Sell Boil - Off Vapor to Local Natural Gas Utility 49 4.6 Utilize Boil - Off Vapor to Supply Facility N atural Gas Requirements 49 4.7 Use Boil - Off Vapor to Generate Electric Power to be Sold to Local Electric Utility 50 4.8 Use Boil - Off Vapor to Generate Electric Power for On - Site Use 50 4.9 Compress Boil - Off Vapor for Use for CNG Vehicle Fueling 51 5 Emissions Implications of LNG Vapor Atmospheric Venting 53 5.1 Overview 53 5.2 Greenhouse Gas (GHG) Emissions 53 5.3 Criteria Pollutant Emissions 55 6 Conclusions and Recommendations 57 6.1 Conclusions 57 6.2 Bes t Practices Recommendations 57 6.3 Research and Development Recommendations 60 References 63 Appendix A

– LNG Tank Pressure, Temperature, Ullage Fraction, Heat Leak, Hold Time, and Venting Rate Relationships A - 1 Appendix B

– Methane Phase Diagram B - 11 Section 1 INTRODUCTION 1.1 Objective The objective of this document is to recomm end liquefied natural gas (LNG) fueling station design and operation best practices to avoid or minimize LNG vapor venting. Atmospheric venting of natural gas vapor is undes irable because it negatively affects economics, increases greenhouse gas (GHG) emissions, and creates a safety hazard in some situations. Figure 1 is a highly simplified illustration of some potential causes of vapor venting associated with LNG fueling st ation operations. Tank truck delivery of LNG to a fueling station can result in venting from the station and/or from the tank truck itself. In some circumstances, LNG vehicle refueling involves processes that increase the likelihood that vapor will need to be vented from the station or vehicle. Therefore, this document considers pot ential LNG vapor generation and venting associated with three closely relat ed aspects of the overall LNG transportation fuel infrastructure:

 Tank truck delivery of LNG to a fueli ng station

 LNG fueling station operation

 LNG vehicle operation and refueling Figure 2 shows examples of LNG vapor venting from each of these infrastructure elements

. The white plume usually associated with LNG vapor venting is actually atmospheric water va por, which is condensed as it is entrained and cooled by the much colder LNG vapor. The extent of this plume depends strongly on the ambient humidity and temperature, and so the magnitude of LNG vapor flow cannot be judged from the plume size. This docume nt also addresses best practices to avoid venting from LCNG fueling stations. LCNG stations receive and store LNG, which is pumped to a high pressure, vaporized, and dispensed to refuel compressed natural gas (CNG) vehicles. Some LCNG stations are equipped to refuel both LNG and CNG vehicles. The LNG fueling station illustrated in Figure 1 is but one example configuration . There are in fact many different LNG and LCNG station design options, the details of which affect potential venting. There are also var ious types of LNG vehicle tanks and fuel systems, which also affect venting. Finally, there are optional LNG tank truck offloading connections and procedures. These options and th eir effects on vapor generation and venti ng are discussed in this report

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Figure 1. Highly simplified illustration of potential LNG venting losses associated with LNG fueling station operation

including tank truck delivery and vehicle refueling. Blowdown of truck following pressure build to transfer LNG to station (particularly if station has no offloading pump) Use of back pressure regulator to maintain convenient saturation pressure in storage tank

Storage tank PRD opens (e.g., due to inadequate station utilization or too much vapor injected into storage tank) Blowdown of storage tank (e.g., to facilitate delivery truck - to station LNG transfer)

Vehicle tank PRD opens (e.g., due to infrequent truc k use, reduced hold time associated with high Sat. P refueling, tank soft vacuum)3

Figure 2. LNG vapor v enting from a fueling station (A), tank truck (B), and v ehicle (C) 1.2 The Magnitude of the Venting Problem The societal benefits of using natural gas as a transportation fuel are broadly recognized and well documented. LNG is used primarily as a fuel for heav y - duty vehicles, which would otherwise consume diesel fuel. This natural gas fuel application is especially beneficial with respect to criteria pollutants such as NO x and particulate matter, because it is more straightforward to minimize these emissions fr om natural gas engines than from diesel engines. These LNG vehicles also produce less GHG emissions, primarily because the carbon content of natural gas is much less than that of diesel fuel. The total emissions (especially total GHG emissions) associated with a particular transportation fuel are often compared on a “well - to - wheels” basis. In this context, any LNG fueling station vapor venting contributes to the well - to - wheels GHG emissions of LNG - fueled vehicles, and this contribution is exacerbated by the fact that the CO 2 GHG equivalence of methane is approximately 2 5 . The GHG emissions implications of LNG fueling station vapor venting is discussed in more quantitative detail in Section 5 . While the GHG implications of LNG fu eling station vapor venting ar e important to consider as use of LNG as a transportation fuel increases, the fact is that this source of GHG emissions is relatively insignificant at the present time. This is demonstrated by the comparison summarized in Table 1, which was provided b y Gla dstein, Neandross and Associates (Ref erence 1). This very approximate estimate indicates that the 

### A B C Image courtesy Jerry Wiens4

methane currently vented from LNG fueling station and vehicle operations in California are at least two orders - of - magnitude less than the methane released fro m dairy cows in California. Cows in California* 2,264,000 cows Annual methane emissions per cow (pounds)* 350 Lb /cow Annual metric tons of methane emitted by cows in CA 360,000 MT Approximate daily LNG fuel demand in CA from the LNG vehicle fuel market 160,000 gal Approximate annual LNG fuel demand in CA from the LNG vehicle fuel market 58,400,000 gal Approximate annual LNG fuel demand in CA from the LNG vehicle fuel market 93,500 MT Assumed LNG vehicle fuel market maximum venting loss percentage 1.0 % Rough estimate of venting losses associated with the LNG vehicle operations in CA 935 MT Ratio of venting from CA LNG vehicle operations to methane emissions from cows 0.26% * Data from Ref erence 2 Table 1. The current contribution of methane venti ng from LNG vehicle refueling and

operation is relatively small, as indicated by this very approximate comparison provided by Gladstein, Neandross and Associates (Ref erence 1). 1.3 Issues and Challenges Venting from LNG vehicles and fueling stations is due pr imarily to the simple fact that LNG must be maintained at cryoge nic temperatures (approximately - 260°F to - 170°F , depending on pressure and composition) in order to stay in th e liquid phase. For this reason, LNG fueling stations utilize highly insulated st orage tanks and plumbing systems to minimize heat leaks that may eventually cause venting. However, even when good cryogenic design practices are followed, certain situations tend to increase the potential for vapor venting from LNG fueling stations. The o verarching issue is simply station utilization. If a substantial number of LNG vehicles are refueled at the station and these vehicles have typical heavy - duty vehicle fuel consumption rates, then LNG tank truck deliveries will be reasonably frequent. Regul ar additions of substant ial quantities of colder (i.e., lower saturation pressure 1 ) LNG ensures that the storage tank pressure remains well below the pressure relief device (PRD) pressure, and venting does not occur. Moreover, if the LNG vehicles’ fuel con sumption is significant, vehicle fuel tank pressure increases are not large, and there is seldom any need to transfer vapor from the vehicle to the station. On the other hand, if the utilization is low or sporadic, and/or the supported LNG vehicles consum e very little fuel, the potential for vapor venting is much higher. For this reason, LNG is sometimes referred to as a “use it or lose it” fuel.

1 Graphs showing the quantitative relations among LNG satura tion pressure, temperature, density, and other properties are contained in Appendix B. The analytical methods and assumptions employed to generate these graphs are explained in Appendix A.5

In addition to utilization, certain other issues and situations tend to exacerbate the potential for venting f rom LNG fueling stations and vehicles. First and foremost among these is the fact that most current heavy - duty natural gas vehicles have spark ignition natural gas (SING) engines, which require fuel supply pressures near 100 psig and have no on - board fuel pump (fuel supply pressure requirements are discussed in more detail in section 2 .3). The LNG delivered by tank trucks typically has a saturation pressure in the 10 to 30 psig range, and heat transfer during the truck - to - station transfer process increases this saturation pressure somewhat. The saturation pressure in the station storage tank immediately after a delivery depends on the delivered LNG saturation pressure, the state of the residual LNG in the storage tank prior to the delivery, and the delivered - to - residual LNG mixture ratio . In most circumstances, this pressure is substantially less than 100 psig. If LNG vehicles with SING engines are refueled with this LNG, they will “run out of gas” because the pressure in the vehicle fuel tank is less than th e required engine fuel supply pressure. The most common current approach for addressing this problem is to “condition” the LNG in the station storage tank to increase its saturation pressure so that it exceeds the engine fuel supply pressure requirement a fter it is dispensed into vehicle fuel tanks. This conditioning is accomplished by adding heat to the freshly delivered LNG by circulating it through a heat exchanger (which is sometimes called a vaporizer) located at the station. Because th is reduces the difference between the saturation pressure of the LNG in the station storage tank and that tank’s pressure relief device (PRD) opening pressure, the station tank’s time - to - vent is decreased, which exacerbates the station venting potential. The LNG conditio ning process is discu ssed in more detail in Section 3 .3. Besides increasing the station cost and venting potential, this conditioning practice has another liability. Not all heavy - duty natural gas vehicle engines require LNG fuel tank pressures near 100 p sig. In particular, the Westport ISX - G high - pressure direct injection (HPDI) engine s have LNG fuel systems tha t include a pump (see Sections 2.3 and 2 .4). It is disadvantageous to refuel these vehicles with high saturation pressure LNG for two reasons: Fir st, this decreases the quantity of LNG that the fuel tank can hold, because the higher - pressure LNG has a lower density (Appendix B). Second, this shortens the fuel tank time - to - vent, because the pressure rise to the PRD pressure is less. Therefore, the co nditioned LNG currently dispensed by most LNG stations is not optimally suited for vehicles with HPDI engines. As will be discussed in Section 3 .3, a few stations are equipped to dispense two “grades” of LNG in order to optimally refuel all types of LNG ve hicles. LNG fuel ing station venting is also affected by other aspects of the LNG vehicle fuel system design as well as the vehicle operating cycle. For example, if some combination of these factors cause the vehicle LNG fuel tank pressure to be quite high when it enters the station for refueling, vapor may have to be transferred from the vehicle fuel tank to the station before LNG can be dispensed into the vehicle fuel tank. Specific fueling station and vehicle conditions that may necessitate vapor return a re discussed in Sections 2 and 3

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Finally, another issue that affects potential vapor venting is LNG fueling station economics. Simply put, LNG stations can incorporate well - understood design features and equ ipment selections that minimize or eliminate vap or generation and venting, but stations without these particular design features and equipment selections may be less expensive. This tradeoff is sometimes an awkward issue for projects involving a very small number of LNG vehicles, particularly if a dedic ated station with cost proportional to total vehicle cost is anticipated. The likelihood of vapor venting may be greater in these situations due to low station utilization as well as station budget limitations. 1.4 Information Sources , LNG Quantity Units, and Report Organization 1.4.1 Prior Work Many other studies have addressed aspects of LNG vehicle and fueling station vapor generation and venting, and this report draws heavily from that prior work. References 3 through 27 are examples of prior reports, articles, and presentations that contain material related to the subject of this report. 1.4.2 Information Sources and Citations The information in this report is based on prior work (as noted in section 1.4.1), analyses consistent with the principles summarized in Appen dix A, observation s of LNG deliveries and refueling operations at dozens of LNG fueling stations, and interviews with LNG station operators, designers, and equipment suppliers. With reg ard to these interviews, candid information was usually provided based on the anticipa tion of confidentiality, and so these information sources are not cited by name in this report. Photographs used in this report are credited with regard to their source. Photographs with no credit noted were taken by and are the property of the report author. 1.4.3 Patents and Commercial Products Many patents describe inventions involving LNG vehicles and fueling stations with features intended to reduce or eliminate vapor generation and venting losses. Specific patents are not cited or discussed in this report because to do so might be misconstrued as an opinion regarding the patent validity, and this would also be unfair to patents that were not cited. Similarly, any reference to specific companies or products is for illustration purposes only an d is not intended to be an endorsement or recommendation. 1.4.4 LNG Quantity Units Liquid transportation fuels such as gasoline and diesel fuel are traditionally quantified in terms of gallons (or liters in most non - US countries), which is a volume measurement

. For cryogenic liquids such as LNG, which are usually transported, stored, and dispensed at or near saturation, the liquid density depends strongly on temperature and pressure, and so the mass in a gallon at one condition may be substantially different f rom the mass in a gallon at another condition. For example, for high - purity LNG, a7

gallon at 20 psig weighs 3.38 Lbm, but a gallon at 120 psig weighs 3.03 Lbm (i.e.,10.4% less) 2

. For this reason, LNG supply contracts and pricing is usually based on fuel m ass rather than volume. However, most people ha v e a strong preference to discuss LNG vehicle fuel quantities in terms of gallons. An approach that minimizes the ambiguity of this unit of measurement is to define a “LNG gallon” as the mass of fuel in one ga llon of all methane LNG saturated at one atmosphere of absolute pressure (14.7 psia), which is 3.53 Lbm. This convention is used in this report so that, regardless of pressure and temperature, o ne “LNG gallon” means 3.53 pounds of LNG

. 1.4.5 Report Organization Section 2 summarizes various technical aspects of the LNG vehic le fueling infrastructure that affect vapor generation and potential venting. With this background established, Section 3 discusses the effects of different design and operation alternatives o n venting associated with LNG tank truck deliveries, refueling station operations, and vehicle fuel system types. Options for avoiding venting by recovering and productively using LNG vapor are reviewed in Section 4. Section 5 discusses the emissions impli cations, both GHG and criteria pollutants, of LNG vapor atmospheric venting. Best practices for avoiding LNG vapor venting are summarized in Section 6, which also includes some recommendations regarding new technology directions

. This report includes two a ppendices. Appendix A summarizes the basic equations and contains some useful graphs pertaining to the LNG storage tank heat leaks, pressure and temperature rise rates, ullage fractions, hold times, and venting rates. For reference, Appendix B contains gra phs of selected thermodynamic properties (pressure, temperature, density, enthalpy) of methane at and near the liquid - vapor mixture region of the phase diagram.

2 Graphs of LNG density are in Appendix B.89 Section 2 LNG INFRASTRUCTURE REQUIREMENTS SUMMARY 2.1 Overview Vapor generation and potential venting from LNG fueling stations depends on various technical issues and requirements associated with the overall LNG transportation fuel infrastructure. These are briefly reviewed in the following subsections in order to provide a basis for subsequent d iscussions of design options and operating practices that prevent or minimize vapor venting. 2.2 LNG Saturation , Properties , and Weathering LNG fueling station storage tanks and vehicle tanks contain a mixture of liquid and vapor. The space occupied by the vap or is called the ullage. When the liquid and vapor phases are in equilibrium (i.e., at the same pressure and temperature), the mixture is said to be saturated. Although both phases have the same pressure and temperature, the liquid is denser (e.g., pounds per cubic foot) than the vapor, and the vapor has a higher internal energy (e.g., Btu per pound) than the liquid. As the pressure and temperature increases, these differences between the phases decrease. At the critical pressure (6 73 psi a for methane) and temperature ( - 11 7 ° F for methane) , the liquid and vapor phases have the same density ( 10.1 Lbm/ft 3 for methane). At higher pressures and temperatures, there is no distinction between the liquid and vapor phases, and the fluid i s said to be supercritical. L NG usually consists of a very high percentage of methane, a much smaller percentage of heavier hydrocarbons (e.g., ethane, propane, butane), and very small quantities of other constituents (e.g., nitrogen). Because of differences in their vapor pressures, the heavier hydrocarbon constituents are more concentrated in the liquid phase than the vapor phase. Thus, when vapor is vented, the remaining LNG becomes more “enriched” with respect to these heavier hydrocarbons. This process is referred to as “weatheri ng.” In rare cases (mainly for older heavy - duty natural gas engines that don’t have closed loop electronic controls), the fuel composition changes associated with weathering can result in engine operation problems, and so weathering can be regarded as anot her undesirable consequence of venting. LNG weathering, natural gas composition effects on engine performance, and issues associated with government agency fuel composition specifications are discussed in more detail in various of the listed references. In this document as in many other similar reports, LNG is usually assumed to be 100% methane with regard to physical properties. There are two reasons for this : First, most LNG currently sold as a transportation fuel is contractually required to be high - puri ty10

(e.g., 99%) methane 3 , Second, if constituents other than methane are included, calculation of LNG properties involves more complex multi - component, multi - phase analyses, and the results cannot be shown in simple - to - understand graphs. For reference, Fig ure 3 shows the temperature - pressure relation for saturated LNG that is 100% methan e. Methane is a superheated vapor at temperatures above the saturation line and a subcooled (or compressed) liquid at temperatures below the saturation line. Figure 4 shows the density variation with pressure of saturated methane vapor and liquid. Appendix B contains more details regarding LNG properties.

-350 -300 -250 -200 -150 -100 -50 0 -100 0 100 200 300 400 500 600 700 Pressure ( psig )Temperature(oF) VAPOR REGION

LIQUID REGION Saturation Line

Atmospheric pressure Critical point

Figure 3. The temperature vs. pressure curve for saturated liquid and vapor methane.

3 In many cases, th is hi gh - purity LNG requirement appears to be based more on legacy contract language than actual natural gas engine fuel composition specifications and realistic weathering concerns . Some experts are of the opinion that relaxing unnecessarily rigid LNG contract composition specifications would enable a slight LNG transportation fuel cost savings.11 0 100 200 300 400 500 600 700 800 900 1,000

0 5 10 15 20 25 30 Density ( Lbm / ft 3 )Pressure(psig)

LIQUID REGION LIQUID + VAPOR REGION VAPOR REGION

Saturation Line

SUPERCRITICAL REGION Example isotherm (-180 °F)

Figure 4. The density - pressu re relation for saturated liquid and vapor methane.

2.3 LNG Tank Basic Characteristics Like other cryogenic liquids, LNG must be stored in well - insulated containers in order to minimize heat transfer from the environment, the rate of pressure and temperature increase, the likelihood of venting, and the vent gas flow rate if venting does occur. LNG fueling station storage tanks, LNG tank truck tanks, and LNG vehicle fuel tanks (such as those shown in Figure 5) are all vacuum jacketed. Other design details and construction features are somewhat different. The vacuum jacket, which is an evacuated space between the inner tank and outer shell, minimizes convection heat transfer. The system that supports the inner tank within the outer shell as well as the plumbing to the inner tank is designed to minimize conduction heat transfer. For LNG vehicle fuel tanks, multi - layer insulation (MLI) is placed between the inner and outer tanks to minimize radiation heat transfer. MLI consists of layers of highly reflective (i.e. , low emissivity) f oil - like material separated by a spacer material such as silk or nylon netting. Most larger cryogenic vessels used to store LNG at fueling stations or on tank - truck trailers use highly reflective granular materials such as Perlite in the vacuum jacket. Details regarding cryogenic vessel insulation systems and other design features are available in cryogenic textbooks such as Ref erence 28 . More information specific to LNG fueling station and vehicle fuel tanks is contained in Ref erences 7, 11, 13, 16, 19, 20, and 25.12

Figure 5. LNG fueling station tanks (A), vehicle fuel tanks (B), and tank - truck tanks (C)

are vacuum jacketed to minimize heat leaks. Safety, codes, and common sense require LNG tanks of all ty pes to be equipped with multiple pressure relief devices (PRDs). The PRD purpose is to open and relieve the tank pressure before it reaches a potentially dangerous level (which depends on the tank construction details such as the inner tank diameter, thick ness , and material). For most currently used LNG vehicle fuel tanks, the first PRD opens at 230 psig. LNG fueling station tanks are available with various maximum allowable working pressures (MAWPs), but 175 psig is a common MAWP for these tanks, and the f irst PRD is usually set at the MAWP. The tank heat leak characteristics are usually specified in terms of the normal evaporation rate (NER) and sometimes the hold time. The NER is the rate of vapor withdrawal that maintains the tank saturation pressure at a constant level. NER is usually expressed as a percent per day. In addition to the heat leak rate, the NER depends on the particular cryogen and the pressure at which the vapor is withdrawn. Similarly, in addition to the heat leak rate, the hold time dep ends on the cryogen, the initial pressure and ullage fraction, and the ending (usually PRD) pressure. These details are not always specified, and so interpretation of NER and hold time is somewhat ambiguous in these cases. The NER, hold time, and heat leak are all interdependent. For a given tank, cryogen, fill pressure and ullage fraction, and PRD pressure, specification of any one of these 

### A B Ima ge from Stirling Corp. press release 5/11/08

C13

enables calculation of the other two (subject to a few assumptions 4 ). For example, Figure 6 shows the theoretical pre ssure, vent rate, and ullage fraction history for a 115 - gallon LNG vehicle fuel tank with a 5 - day hold time and a 230 - psig PRD that is filled with LNG saturated at 1 20 psig and no fuel is consumed (e.g., vehicle is not driven). For this tank and these cond itions, the heat leak that results in a 5 - day hold time is approximately 18 Watts. Figure 6 shows that the pressure rises steadily to 230 psig on day five , at which time the PRD opens and vapor vents at approximately 2.5 equivalent LNG gallons/day until al l the liquid is vaporized by day 39 . This analysis is addressed in more detail in Appendix A, which includes a graph showing that if the same tank i s filled with LNG saturated at 20 psig, the hold time is increased to 13 days.

0 50 100 150 200 250

0 5 10 15 20 25 30 35 40 45 Elapsed Time (days)SaturationPrassure(psig)&Ullage(%)

0 1 2 3 4 5VentingRate(LNGgallonsequivalent/day) (230 psi PRD) Pressure

Ullage % 5 days (brief increase in venting as liquid is exhausted and tank warms to ambient temperature)

Venting rate

Figure 6. Hypothetical 115 - gallon LNG vehicle fuel tank with 230 - psig PRD and 5 - day

hold time when filled with 1 20 - psig saturation pressure LNG: pressure rise rate, venting rate, and ullage fraction variation if vehicle is not driven after tank filling. If LNG tank venting must occu r, it is much safer (and also more effective) to vent vapor than liquid. Therefore, good practice as well as most codes require that, when LNG tanks are filled, adequate ullage should remain so that there is still a vapor space (i.e., ullage > 0) if and wh en the saturation pressure increases to the PRD opening pressure. If the ullage fraction decreases to zero before the PRD opens, the tank is said to be liquid full, which is a situation that should be avoided. Any heat leak subsequent to a liquid full cond ition produces an extremely rapid pressure rise rate.

4 The main assumption is that the tank contents are at a uniform pressure and temperature, which is usually the case for smaller tanks but is less applicable for large s tationary tanks in which stratification may occur.14

For LNG in a closed container with no withdrawal, the ullage fraction decreases as the saturation pressure and temperature increase for most (but not all) conditions. The ullage fraction that should be provided when a tank is filled can be calculated from LNG’s properties and mass and energy conservation equations. For the example LNG tank considered in Figure 6, a 9 % ullage is needed if it is filled with 120 - psig saturation pressure LNG (although a min imum of 10% is required by the applicable SAE Recommended Practice, Ref erence 29 ), and a 18% ullage is needed if it is filled with 20 - psig saturation pressure LNG . Most LNG vehicle fuel tanks have an automatic ullage - control feature to ensure that the tank can’t be over - filled with liquid. Appendix A contains considerable more detail regarding, and useful graphs illustrating, the relationships among LNG tank heat leak rate, pressure, temperature, ullage fraction, hold time, and venting rate. 2.4 Natural Gas En gine Fuel Pressure Requirements A factor that affects LNG fueling station designs and potential venting is the fuel supply pressures required by various natural gas engines. Because essentially all current LNG fueled natural gas vehicles are heavy - duty veh icles, consideration here is confined to heavy - duty natural gas engines. Table 2 lists some relevant heavy - duty natural gas engines, their manufacturer - specified minimum engine fuel supply pressure requirement, the manufacturers’ estimate of the pressure d rop from the LNG fuel tank to the engine 5 (if available), the consequent LNG fuel tank pressure requirement (if there is no pump), and the information source. 2.5 LNG Vehicle Fuel System D esign Options A variety of on - vehicle LNG fuel system design strategies are described in Ref erence 19 . Some of these have been demonstrated and a few have been used on significant numbers of light - duty vehicles in the 1960s through 1980s and heavy - duty vehicles (primarily in the 1990s through the present). However, nearly all currently operating heavy - duty LNG vehicles employ one of two types of LNG fuel systems: A. Most LNG vehicles have SING engines (Table 2) that require fuel supply pressures near 100 psig. These vehicles generally employ LNG fuel systems that have no pump or o ther pressure - building features. They require the saturation pressure of the LNG in the fuel tank to exceed the required engine fuel supply pressure plus the maximum anticipated pressure drop from the tank to the engine. Fueling stations supporting these v ehicles must be capable of dispensing LNG with saturation pressure s approximately 6 equal to those listed in column 5 of Table 2.

5 The pressure drop from the LNG fuel tank to the engine is associated with flow through the fuel lines, vaporizer, valves, and (in some cases) regulators and/or filters. This pressure drop is highly depe ndent on the particular application and operating conditions. 6 The exact dispensed LNG saturation pressure requirement depends on other factors such as heat transfer associated with the dispensing process and t he quantity and state of the LNG in the vehic le tank at the beginning of refueling

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B. LNG vehicles with HPDI engines require an engine fuel supply pressure of at least 4,000 psig. An on - vehicle fuel system with an in - tank LNG pump is employed to provide LNG at this pressure (which is actually a supercritical state, see Figure 4). At the present time, only one engine of this type is commercially ava ilable (the Westport ISX - G, Reference 30 ), and the engine manufac turer also provides the special LNG tank and fuel system required to support the HPDI engine. As will be discussed subsequently in more detail, vehicles with pump - equipped HPDI engines can be refueled with LNG at almost any saturation pressure. However, as summarized in Sections 2.2 and 2.3 and detailed in Appendix A, refueling with lower saturation pressure LNG will provide a greater mass of fuel in the tank and a longer hold time. Natural gas engine Remarks (1) Min. engine fuel supply pressure Tank - to - en gine

delta - pressure recommendation Min. LNG

fuel tank pressure Source(s) Cummins Westport Inc. (CWI) ISL - G Currently

EPA/CARB certified and available SING engine 75 psig 30 to 40 psi 105 to 115 psig Cummins Westport Tech. Bulletin 15.11.209, “Natural Gas Fuel System LNG” Westport ISX - G Currently

EPA/CARB certified and available HPDI engine N/A (2) N/A (2) N/A (2) Westport technical literature Clean Air Partners Caterpillar C - 12 Not currently

EPA/CARB certified or available, but still in use PING engine 98 psig (3) 128 to 130 psig Clean Air

Partners C - 12 engine specifications Other CWI, DDC, Deere, and Mack SING engines Most of these are

not currently EPA/CARB certified or available, but some are still in use ~60 psig to

~100 psig (3) ~90 psig to ~140 ps ig Manufacturers’

technical literature and Refs. 12, 16, 19

Notes: 1. SING = spark ignition natural gas, HPDI = high pressure direct injection, PING = pilot ignition natural gas 2. The ISX - G HPDI engine uses a pump - in - tank LNG fuel system, which has no minimum LNG fuel saturation pressure requirement 3. The tank - to - engine minimum delta - pressure recommendation for these applications is assumed to be the same as the CWI ISL - G recommendation Table 2. Heavy - duty natural gas engine fuel supply pressure requirements.16

Figures 7 A and B are simplified generic diagrams of previously listed LNG vehicle fuel systems types A and B . The discussion here focuses on features that may affect venting. Mo re comprehensive explanations of these systems are available in Ref erence 19 a s well as manufa cturers’ literature (e.g., References 31 and 32 ). Both types of LNG fuel tanks normally employ a single - line “vapor collapse” top - filling procedure. This process relies on t he fact that, most of the time, the interaction of incoming liquid with the residual ullage vapor tends to condense much of that vapor to liquid so that the tank pressure doesn’t increase to the maximum station pump discharge or PRD pressure during the fill. Occasionally, this doesn’t work, and this is usually because the initial tank saturation pressure and liquid/vapor ratio is such that, before or during filling, the station pump discharge pressure (or recirculation valve opening pressure) is inadequate to dispense LNG into the tank. In these cases, a two - line filling p rocedure must be used. The second line is connected to the tank vapor return receptacle shown in Figures 7 A and B, and vehicle tank ullage vapor is returned to the station before and/or during filling 7

. Vapor returned to the fueling station can increase t he station venting likelihood, which will be discussed in Section 3.3. Occasionally, vehicle tank ullage vapor is simpl y vented to the atmosphere (by opening the vent valve shown in Figures 7 A and B) in order to reduce the tank pressure and enable or expe dite refueling. This is obviously a bad practice. An important feature of the “saturated fuel” type of LNG vehicle fuel system illustrated in Figure 7 A is the economizer valve and circuit. This circuit enables the tank pressure to be reduced any time the vehicle is driven. If and when the tank pressure exceeds a set point, the economizer valve opens so that the engine draws vapor from the ullage. The ratio of vapor - to - liquid withdrawal is increased if the liquid line is equipped with a check valve that pro vides a small additional pressure drop. Because energy is being withdrawn from the tank at a substantial rate when the economizer valve is open, the saturation pressure of the remaining liquid and vapor decreases. When the pressure decreases to the econom izer valve set point, the valve closes so that only liquid is withdrawn. Current versions of the “pumped” type of LNG vehicle fuel system illustrated in Figure 7 B do not have an economizer circuit or equivalent capability to withdraw vapor to supply fuel to the engine. Studies ( References 33 and 34 ) have demonstrated the feasibility of pumps that can withdraw liquid - vapor mixture s , but these substantially complicate the design and increase the cost of the overall system. It can be argued that, because pum ped systems can be refueled with very low saturation pressure LNG (which provides a greater pressure rise to the PRD pressure), they have less need for an economizer circuit. It should be emphasized that there are many other LNG vehicle fuel system design concepts. The discussion here is focused on the two in common use at this time and their design features potentially affecting venting.

7 Alternatively, for some LNG fueling station designs, the vapor can be returned to the station through the filling li ne before LNG dispensing starts

.17

A. “Saturated fuel” type of LNG vehicle fuel system depends on the LNG saturation pressure to supply fuel to the engi ne.

Fuel pickup line (with optional low Δ P check - valve) Top fill line Vacuum jacketed

tank Primary PRV Secondary PRV

Economizer regulator/valve Vapor vent Refueling Fuel to vaporizer and engine Excess flow

check - valve Vapor line

B. “Pumped” type of LNG vehicle fuel system.

Vacuum jacketed tank Top fill line Primary PRV Secondary PRV Vapor vent Fuel to vaporizer, accumulator, and engine Refueling Liquid drain line Pump drive (e.g., hydraulic)

Vapor line

Cryogenic pump Figure 7. Simplified generic diagrams of the two types of currently used LNG vehicle

fuel tanks and systems (components such as liquid level sensors and ullage control features are not shown).18

2.6 LNG Tank Trucks LNG is usually delivered to LNG fueling stations in tank trucks. Most LNG tank trucks are actually tractor - trailers such as the one shown in Figure 8. A Class 8 tractor pulls a semi - trailer consisting of a vacuum - insulated LNG tank, fluid transfer p lumbing, and related components. LNG semi - trailers typically have a gross capacity of 10,000 to 13,000 gallons. The maximum LNG payload is slightly less due to ullage requirements.

Figure 8. LNG tractor - trailer tank truck. Most LNG semi - traile rs use a pressure - building circuit instead of a pump to offload LNG. This circuit vaporizes a small quantity of LNG in an ambient heat exchanger underneath the tank (which is visible in Figure 8) to create a temporary “false head 8

. ” The pressure - build nece ssary to transfer LNG to the station storage tank depends on the station design and the saturation states of the LNG in the station and storage tanks. As will be discussed in S ection 3.2, nearly all currently operating LNG fueling stations are equipped wit h LNG tank truck offloading pumps. Usually, this is a dedicated high capacity pump, but some stations use the same pump for vehicle refueling and tank truck offloading. LNG tank semi - trailer design and construction features and operating procedures are reg ulated by the U.S. Department of Transportation , which classifies LNG as a hazardous material. The procedures used to transfer LNG from a tank truck to a fueling station can affect potential venting from the truck and the station. These procedures are disc ussed in Section 3.2.

8 The additional u llage vapor subjects the liquid to a pressure gr eater than it s saturation pressure. Image courtesy Clean Energy Fuels Corp.19

2.7 Station Utilization Station utilization and an appropriate balance of station utilization, station storage capacity, and LNG delivery schedules are major factors affecting excess vapor generation and potential venting. Station utiliza tion is usually quantified in terms of average LNG gallons per day, and it can be estimated as the number of vehicles refueled at the station times their average fuel consumption rate. It is more straightforward to avoid venting from stations with a high u tilization, but LNG stations with medium to low utilization can also be designed and operated to avoid atmospheric venting. In addition to utilization, various other factors also affect LNG fueling station design, operation, and potential venting. Many of these factors involve economics. For example, due to the economics of scale, larger LNG storage tanks are less costly per gallon than smaller ones. However, station storage tank sizing is often affected by site issues as well as economics. Most LNG station s have one or more tanks with approximately 15,000 gallon capacities, and some have larger tanks up to approximately 30,000 gallons. It is more cost effective to accept full rather than partial tank truck deliveries, and this is possible with some flexibil ity with a 15,000 gallon tank. Figure 9 shows the predicted storage tank saturation pressure and liquid content histories for two hypothetical low - utilization LNG stations with 15,000 - gallon storage tanks and 10,000 LNG gallon tank truck deliveries. One s tation dispenses 1,000 LNG gallons per day and the other station dispenses 5 00 LNG gallons per day (e.g., 5 vehicles, each consuming an average of 100 LNG gallons per day). This hypothetical case assumes that the LNG is conditioned to 120 psig saturation p ressure after each delivery 9 , and the ullage is adequate to avoid a liquid full condition if no product is withdrawn. Details pertaining to assumptions such as the tank heat leak and the dispensing schedule are provided in Appendix A. Figure 9 indicates th at the 1,000 LNG gallon per day consumption case will of course require a 10,000 LNG gallon delivery every 10 days, at which time the storage tank saturation pressure will have increased to approximately 142 psig. However, the very - low 500 LNG gallon per d ay consumption case is predicted to cause the storage tank saturation pressure to increase to the assumed 175 - psig PRD pressure by day 15, i.e., before the tank is able to accept a 10,000 LNG gallon delivery. Therefore, this hypothetical station would need a smaller storage tank and more frequent deliveries of less than 10,000 LNG gallons (or some type of active vapor management) to avoid atmospheric venting. LNG vehicle project developers and LNG station designers/installers can balance almost any anticipa ted current and future station utilization with site conditions and other requirements to develop design concepts for LNG fueling stations with no atmospheric venting.

9 Note that, based on the definition of “LNG gallon” in Section 1.4.4, one LNG gallon at 120 psig saturation pressure occupies a volume larger than one gallon.20 0 20 40 60 80 100 120 140 160 180

0 2 4 6 8 10 12 14 16 18 Elapsed Time ( days )SaturationPressure(psig)

0 5,000 10,000 15,000 20,000 25,000 30,000LNGinTank("LNGgallons") 1,000 LNG gal/day dispensed during 4-hr window 500 LNG gal/day dispensed during 2-hr window (175 psi PRD)

Conditioning to 120 psig 10,000 gal added to tank

For this scenario, LNG storage tank refilled every 10 days, and cycle repeats For this scenario, LNG storage tank starts venting in ~15 days

Figure 9. Effect of LNG fueling station utilization on 15,000 - gallon storage tank

satu ration pressure and LNG content variation. 1,000 LNG gallon/day utilization would enable 10,000 LNG gallon tank truck deliveries every 10 days, but 500 LNG gallon/day utilization would result in PRD venting before storage tank was empty enough to receive a 10,000 LNG gallon delivery. See Appendix A for assumption details. 2.8 Temporary and Mobile LNG Fueling Facilities The primary focus of this report is on permanent LNG vehicle fueling stations . However, it is important to acknowledge the use of temporary and mobile LNG fueling facilities and the fact that they can involve special vapor management and venting issues. Specific examples of these facilities include:

 QRS ®

– A truck - transportable skid - mounted temporary LNG fueling station consisting of a 6,000 - gall on vacuum - jacketed tank, sump - submerged LNG pump, and associated conditioning, metering, and dispensing equipment. Developed by MVE, which was subsequently acquired by Chart Industries.

 TVAC ®

– A 5,300 - gallon gross capacity vacuum - jacketed cryogenic tank in an easily transported intermodal container frame. Developed by Cryenco, which was subsequently acquired by Chart Industries. Frequently used as a temporary station21

LNG storage tank in combination with a Cryenco - developed LNG pump, metering, and dispens ing skid.

 ERVING

– A trailer - mounted mobile LNG and LCNG fueling facility consisting of a TVAC tank (see above), LNG pump, vaporizer, CNG buffer vessel, c ontrols, dispensing hoses, Etc. E RVING units assembled by Jack B. Kelley. Inc. (using mostly Cryenco equipment) were used to refuel LNG and CNG buses at the 1997 Atlanta Olympics and various other NGV projects.

 ORCA ®

– A Chart Industries trailer or truck chassis - mounted mobile LNG fueling facility consisting of a 3,500 - gallon vacuum - jacketed tank, submerg ed pump, fuel conditioning vaporizer, flow meter, dispenser, and other equipment needed to refuel LNG vehicles. Venting is sometimes more frequent from temporary and mobile LNG fueling facilities 10 , primarily because they are often employed to support LNG v ehicle start - up and demonstration projects with low utilization, sporadic refueling schedules, and extended periods with no vehicle refuelings. Venting can be particularly severe from temporary or mobile LNG fueling facilities that employ pumps that must b e cooled in order to prime (see Section 3.3 for more detailed discussion of LNG pumps). In addition to the above temporary and mobile LNG fueling facility examples, LNG tank trucks have occasionally been jury - rigged to enable LNG vehicles to be refueled di rectly from the tank truck. The code compliance of this practice is arguable, and it is usually accompanied by substantial vapor venting. 2.9 Weights and Measures Certification Weights and measures certification of LNG fueling stations involves many technical issues, and only those issues related to vapor management and venting are discussed here. The main factor connecting accurate and certifiable LNG vehicle fuel metering with vapor generation is the fact that commonly used flow metering equipment cannot acc urately measure the mass flow rates of two - phase fluid mixtures 11

. Ensuring that only liquid flows through the metering system restricts the station design with respect to features such as pre - dispensing cool - down and recirculation processes. In addition, d ispensers that utilize what are basically volumetric flow meters must have temperature compensation systems in order to accurately measure LNG mass flow. A few early 10 For example, Reference 15 , which is a NREL report on the R aley’s LNG truck project initial (1997 - 1998) experience, states, “The temporary fueling station required a significant amount of natural gas to be vented to the atmosphere...” . Venting was substantially reduced when the temporary station was replaced by a pe rmanent station, and more recent temporary facilities have design features that reduce venting. 11 In theory, commonly used coriolis meters can measure mass flow rate independent of the fluid phase, but field experience has shown that they cannot accurately measure the flow rate of natural gas liquid vapor mixtures.22

generation LNG fueling stations metered both the LNG dispensed into vehicle tanks and vapo r returned from the vehicle (e.g., Reference 5 ). However, few if any currently operating LNG fueling stations, even those that have received weights and measures certification, measure any vapor that is returned from the vehicle fuel tank to the station, o r just vented to the atmosphere, during refueling 12

. Even though weights and measures certifiable LNG station designs and dispensing equipment are available, not all stations are certified. This is primarily because many LNG stations are dedicated to suppo rt a specific heavy - duty LNG vehicle fleet (e.g., refuse collection, transit) and never used to refuel other vehicles. Since these stations don’t sell fuel, they are not required to be weights and measures certified . Moreover, fleet operators purchase LNG on a mass basis 13 , and accurate mass - balance analysis to reconcile fuel delivered with fuel dispensed is not usually regarded as a high priority. In these cases, precise measurements of dispensed LNG is also not a high priority, and so it is not possible to quantify venting losses. This is the main reason why this document doesn’t report a quantitative measurement of overall LNG transportation fuel venting losses. Uncertainties regarding fuel venting also sometimes thwart attempts to make accurate diesel vs . LNG vehic le fuel consumption comparisons. For example, one such effort (Ref erence 15 ) reported, “Losses from vent filling the LNG trucks may also have affected the in - use fuel economy results.” Other researchers (who asked not to be identified in this re port) reported an interesting result based on their analysis of what they believe to be accurate and equivalent LNG and CNG heavy - duty vehicle fuel consumption data: the CNG vehicles appear to have a slightly lower average fuel consumption (in equivalent u nits, such as pounds per mile) than comparable LNG vehicles. These researchers speculate that unaccounted - for venting losses from the LNG vehicles is the most likely reason for this otherwise - puzzling result. 2.10 Applicable Codes and Standards Various regulati ons, codes, standards, and recommended practices (“codes and standards”) that apply to LNG fueling stations, vehicles, and tank trucks specify design and operation requirements related to vapor generation and venting. Because these codes and standards are listed and explained in many available publications (e.g., Ref erences 7, 8, 20, and 21 ), these lists and explanations are not repeated here. Applicable codes and standards are occasionally revised, and up - to - date lists are maintained by web sites such as R ef erences 35 and 36.

12 Note that, because of the substantial difference between natural gas vapor and liquid densities (Figure 4 and Appendix B), the mass of any offloaded vapor is usually quite small r elative to the mass of onloaded liquid. 13 Usually, the LNG tank truck is weighed before and after delivering fuel, and the customer is billed based on the weight difference.23 Section 3 LNG AND LCNG FUELING STATION DESIGN AND OPERATION EFFECTS ON VENTING 3.1 Overview The following subsections sequentially address vapor generation and venting issues associated with tank - truck - to - station LNG transfer, L NG and LCNG fueling station operations, and LNG vehicle refueling and operation. The venting implications of different design options and operating practices are discussed. These discussions presume an understanding of the basic LNG transportation fuel technical requirements reviewed in Section 2 as well as the LNG physical characteristics summarized in Appendices A and B. In many cases, LNG vaporization and venting from one infrastructure element may have been caused by design features or operations associated w ith another infrastructure element (e.g., stations with poorly designed LNG delivery receipt systems can provoke LNG tank truck venting, stations that refuel with unnecessarily high saturation pressure LNG can cause premature LNG vehicle venting). T herefor e, the order in which these topics are discussed in the following subsections is somewhat arbitrary. 3.2 Tank - Truck - to - Station LNG Transfer The basic characteristics of LNG tank trucks were reviewed in Section 2.6. The process by which tank trucks t ransfer LN G to fueling stations and associated vapo r generation and venting issues are discussed here. This transfer process depends on the LNG fueling station design and equipment. Therefore, the tank - truck - to - station transfer process is discussed separately for th ree categories of stations in Sections 3.2.1, 3.2.2, and 3.2.3. However, a few LNG tank truck venting issues are common to all LNG station types. One such issue is the need to usually purge the truck - to - station transfer hose of air before transferring LNG from the truck to the station and emptying the hose of LNG afterward. Because it is impractical to put this air plus LNG and vapor mixture anywhere else, it usually escapes to the atmosphere (e.g., Figure 10). Based on typical transfer hose sizes, vapor eq uivalent to very approximately 5 LNG gallons is typically lost due to hose purging and emptying, which is 0.05% of a 10,000 LNG gallon delivery. 3.2.1 Pressure Transfer Offloading Some LNG fueling stations built in the 1990s or earlier relied on pressure transfe r to transfer LNG from the tank truck to the station storage tank. These stations h ave no dedicated LNG tank truck offloading pu mp, and they are not designed to use the24

dispensing pump for tank truck offloading. This type of station is a very small percent age of all the LNG fueling stations in operation as of 2010.

Figure 10. A small quantity vapor associated with LNG tank truck transfer hose purging and emptying is usually lost to the atmosphere.

Three sources of venting may be associated with LNG delivery to a station requiring pressure transfer: 1. Blow - down of the station storage tank pressure before the LNG delivery 2. Blow - down of the truck tank pressure after the LNG delivery 3. Transfer hose purging and emptying before and after the LNG delivery Blow - down of the LNG station storage tank is required if it s pressure is too high for a pressure transfer from the tank truck. This might be due to one or both of the following reasons: First, if the station utilization is low and significant time has elapsed since the last delivery, the LNG storage tank saturation pressure and temperature may have increased as discussed in Section 2.3. Second, if the station “conditions” LNG in order to refuel vehicles with high fuel supply pressure requirements (Sections 2.4 and 2.5), the storage tank pressur e is likely to exceed 100 psig. For example, 70 psig is a common MAWP and PRD pressure for LNG tank trucks (e.g., Reference 37 ). Therefore, even with a pressure build, a tank truck with this MAWP cannot pressure transfer L NG into a st ation storage tank at 100 psig, and so blow - down of the station tank to a lower pressure is necessary. The LNG tank blow - down process involves substantial venting. This is because the liquid and vapor are assumed to be initially in equilibrium, so that as the pressure is reduced, more liquid promptly boils (“flashes”), which adds to the mass of vapor that25

must be vented 14 . As a purely hypothetical example, consider a 15,000 - gallon LNG storage tank that is initially 10% full of liquid saturated at 1 00 psig that is vented until it s pressure is 60 psig (i.e., slightly less than the maximum pressure - build capability of a 70 - psig MAWP tank truck). Figure A - 6 in Appendix A indicates that this process w ould involve a vented loss of approximately 170 LNG gallons equivalent

. Pressure transfer requires the LNG tank truck to use the pressure - build circuit (described in Section 2.6) to generate the pressure differential needed to transfer the LNG from the tank tru ck to the station storage tank. While there i s no vapor loss associated with the pressure build itself, this process is often followed by vapor venting after the LNG transfer to reduce the pressure in the trailer tank (e.g., Figure 2 - B). The tank truck operator’s decision to vent (“blow down”) the ta nk is based on consideration of factors such as the tank pressure, quantity of remaining LNG, and future schedule plans 15

. The quantity of vapor released to reduce that truck tank pressure depends on many factors such as the starting and ending pressures, the quantity of liquid in the tank, and the extent to which the liquid in the tank may have approached saturation at the tank pressure. For example, the initial pressure - build pressure may change as the transfer pr oceeds because the liquid level in the tru ck tank drops and perhaps the station tank pressure decreases (e.g., if the station tank is top filled with colder liquid that condenses the ullage vapor). An LNG tank truck venting after an initial pressure build from 20 to 60 psig to transfer 10,000 gall ons could release approximately 20 to 150 LNG gallons equivalent, depending on these various factors. Therefore, the total vapor loss associated with the previously mentioned pressure transfer example might be as much as 2% to 3% of the total LNG transferr ed to the station. In theory, a vapor line could be employed to connect the tank truck and station tank ullage spaces prior to the tank truck pressure build in order to utilize some of the station tank vapor, which would otherwise be vented, to provide par t of the tank truck pressuri zation. It is unclear how often this procedure is used as part of pressure transfer s , but an analogous process is often used to support pumped transfer tank truck offloading (Sections 3.2.2 and 3.2.3). 3.2.2 Offloading Using Dedicate d Pump A tank - truck - to - station LNG transfer method that minimizes potential venting and expedites the transfer process is for the station to have and use a separate and dedicated LNG tank truck offloading pump. “Separate and dedicated” means the pump is se parate from the LNG dispensing pump and it is dedicated to tank truck offloading. This type of station configuration was sketched in Figure 1, and an example separate LNG transfer pump in operation is shown in Figure 11.

14 This illustrates that blowing down (i.e., venting) a cryogenic tank is not the exact opposite of pressurizing (e.g., via a pressure - build circuit) a cryogenic tank. 15 DOT regulations stipulate that on - highway venting of LNG vapor is a reportable vapor release.26

Figure 11. Separate and dedic ated cryogenic pump for tank - truck - to - station LNG transfer.

Because high flow capacity at relatively low pressure differential is desired, cryogenic centrifugal pumps are used for dedicated tank - truck - to - station LNG transfer. A few stations utilize su mp - submerged pumps, but most do not (e.g., Figure 11) . The tradeoff between ambient or submerged pumps for LNG tank truck offloading is subtle. LNG pumps that are at or near ambient temperature generate substantial vapor as they cool down (management of th is vapor is discussed subsequently). This is avoided with sump - submerged pumps. However, even the low level of heat transfer into an insulated sump continuously affects the LNG in the station storage tank by slig htly increasing it s saturation pressure, tem perature, and venting potential. Also, because the LNG sump is at approximately the same saturation pressure as the station storage tank (which is likely to be more than 100 psig), even a sump - submerged pump requires some cool down in order to pump the low er saturation pressure and temperature LNG from the tank truck

. Equipment availability and costs are also considerations affecting th is choice. A factor that strongly affects vapor venting during tank - truck - to - station pumped LNG transfer is the use of one or two fluid transfer lines (cryogenic hoses) between the tank truck and the station (Figure 12). In either case, one line is obviously for flowing liquid from the truck tank to the station. The purpose of the second line can be explained as follows: The station LNG storage tank is at a higher pressure than the truck tank and the transfer pump suction and discharge before the pump is primed. Consequently, the vapor generated to cool down and prime the pump cannot be routed to the station tank ullage, and s o (unless two - line transfer is employed), it is usually vented to the atmosphere. However, in two - line transfer, the second line flows this vapor back to the truck tank ullage.27

Figure 12. Tank - truck - to - station LNG transfer using one liquid line (A) and a vapor line

in addition to the liquid line to minimize venting (B). In the two - line transfer method, the vapor line connects the station transfer pump discharge to the truck tank ullage. This opens a circulation path that enables LNG to flow to the pump and vapor from the pump to flow back to the tank truck. This facilitates cooling and priming of the pump. When the pump is primed and discharging liquid (which is indicated by an abrupt discharge pressure increase), the operato r actuates a valve that closes the return path so that LNG flows directly to the station storage tank. In most applications, the pressure differential associated with the truck tank liquid level and the pump elevation is inadequate to flow enough liquid to prime the pump in a rea sonable time. In these cases, the discharge side of the offloading pump is sometimes briefly vented to the atmosphere to increase the flow of LNG to the pump and expedite priming. Depending on the station design and tank truck offloading procedure, when tw o - line transfer is used, this brief vapor venting can be from the fueling station vent or from the LNG tank truck ullage vent. Priming of the transfer pump is also expedited if the pump is at an elevation significantly lower than that of the tank truck. Th is is the case for LNG fueling stations in which the pump and storage tank are in a below - grade containment 16 pit (i.e., as an alternative to at - grade installations that have containment walls). As explained here, two - line tank - truck - to - station LNG transfer using a separate and dedicated pump is generally the best approach for quickly offloading tank trucks with

16 Applicable codes, standards, and regulations require LNG fueling station storage tanks to be within an impounding area (e.g., a wall, pit, or combination thereof) adequate to contain the tank contents in case of a spill. 

### A B Image courtesy Gladstein, Neandross & Associates28

minimum venting. However, even this technique usually results in a small amount of vapor vented to the atmosphere. 3.2.3 Offloading Using Dispensing Pump Some LNG fueling stations are designed to use the dispensing pump (i.e., the pump that transfers LNG from the station to vehicles being refueled, which will be discussed in section 3.3) to offload LNG deliveries from tank trucks. These are typically statio ns with relatively low utilization that are not inconvenienced by a few hours of refueling nonavail ability during tank truck LNG deliveries. For example, a few LNG stations dedicated to refueling refuse collection truck fleets (which typically refuel over a set time period each day when LNG deliveries are not scheduled) are of this type. These stations have plumbing a nd valves that enable the dispensing pump (which is almost always submerged in a LNG sump) to be switched from station - tank - to - vehicle transfe r to tank - truck - to - station - tank transfer. The primary advantage of this approach is lower station capital cost because only one pump is required. The primary disadvantage is the substantially longer time required to offload LNG tank trucks (and, if require d, to condition the newly delivered LNG to a higher saturation pressure) because the sump submerged pumps used for dispensing typically have much less flow capacity than separate and dedicated tank truck offloading pumps (Section 3.2.2). While configurati ons and equipment for stations with separate and dedicated LNG tank truck offloading pumps are different from stations that simply use the dispensing pump for tank truck offloading , the issues affecting vapor generation and venting during LNG deliveries ar e similar. Single - line transfer of LNG from a tank truck to the station storage tank is possible for either type of station, but this usually results in significant vapor venting to the atmosphere. Two - line transfer (as discussed in Section 3.2.2) is the p referred practice, because this usually involves less vapor venting. I n situations for which the trailer tank is at a high pressure after transferring LNG to a fueling station, drivers sometimes vent some of the trailer tank ullage vapor to reduce this pre ssure prior to highway transport. Whenever possible, these situations should be avoided. 3.3 LNG and LCNG Station Design and Operation There are many options pertaining to the design and operation of LNG and LCNG fueling stations. The following subsections dis cuss some of these options with emphasis on vapor generation and potential venting implications. For reference, Figure 13 is a simplified schematic of an example LNG fueling station configuration. This example has the following basic features:

 One above - gr ound vertical LNG storage tank (e.g., 15,000 - gallon gross capacity)

 Above - grade containment/impoundment wall29

 LNG “conditioning” circuit (which includes a forced - air heat exchanger)

 Sump - submerged centrifugal dispensing pump

 One LNG dispenser with one refue ling line and one vapor - return line

 Separate and dedicated LNG tank truck offloading pump (centrifugal, not in sump)

 Tank truck offloading panel with two - line (liquid and vapor) connection receptacles

Figure 13. Example LNG fu eling station simplified schematic. For simplicity, many station features (e.g., instrumentation, controls, leak detection, fire detection) and plumbing details are not shown in Figure 13. Also, while this sketch is representative of some currently operati ng LNG stations, other configurations are possible and may be more suitable for particular LNG vehicle fleet circumstances. VJ LNG dispensing hose & nozzle

Dispensing meter & controls Vehicle ta nk vapor hose (as needed) LNG

refueling dispenser LNG bulk conditioning circuit & heat exchanger

Vent valve Pressure relief valve "Auto - vent" pressure regulator

VJ LNG storage tank Vapor return to tank truck (during pump priming)

Dedicated tank tru ck offloading pump Liquid line Recirculation and vapor return line Sump - submerged dispensing and conditioning pump Containment wall30

3.3.1 Station Sizing LNG fueling station sizing refers to the quantification of various related specification and design parameters. Key p arameters include: the total LNG storage capacity (and number of storage tanks), the number of refueling dispensers, and the LNG dispensing flow rates (total and per dispenser). These sizing parameters are driven primarily by the anticipated station utili zation, which was discussed in Section 2.7. The station utilization expressed as average LNG gallons per day dispensed is the main parameter affecting the total storage capacity choice as well as the LNG tank truck delivery frequency. The number of vehicle s that must be refueled in a given time period (e.g., the extent to which LNG dispensing is highly “peaked” 17 ), typical per - vehicle dispensed LNG quantities, and how quickly vehicles must be refueled are all factors affecting choices such as the number of d ispensers and dispensing pump flow rate capacities. LNG vehicle fleets with a substantial and reasonably constant fuel utilization (e.g., >3,000 LNG gallons per day) present the most straightforward circumstances with regard to fueling station sizing. If these fleets have well - designed and similar LNG vehicles (e.g., all requiring LNG at the same saturation pressure) that are constantly operating, their refueling stations can be designed and operated to produce almost no atmospheric venting of LNG vapor 18

. Stations with much less utilization (e.g., <1,000 LNG gallons per day), or highly sporadic utilization, are more likely to experience vapor venting. Specification developers and designers of low - utilization stations are faced with various dilemmas. The ma in dilemma pertains to the choice of LNG storage tank capacity and tank truck delivery frequency. For example, a 15,000 - gallon storage tank will enable infrequent deliveries of full tank truck loads of approximately 10,000 gallons, but the tank saturation pressure will increase substantially between deliveries. This may result in venting if the PRD pressure is reached, or it may provoke the very bad practice of using a back - pressure regulator to constantly “auto - vent” (which will be discussed subsequently). Anoth er alternative is to specify a smaller LNG storage tank (such as a 5,000 - gallon intermodal tank discussed in Section 2.8) . In this case, the station cannot accept full tank truck loads, and delivery schedules must be more frequent and precise. Additi onally, budgets for low - utilization stations are often such as to make it more difficult to include design features and equipment that minimize vapor generation and venting. Perhaps the most challenging circumstance for LNG station specification developme nt and design is presented by fleets that want a LNG station that will initially support three or four “demonstration” LNG vehicles but will accommodate growth to fifty o r more vehicles. A large - capacity station with very low initial utilization is likely to be

17 Heavy - duty LNG vehicle fleets such as most transit buses and refuse trucks typically queue up to be refueled in sequence (usually in the late afternoon or early evening), which results in a highly peaked station utilization profile. 18 A few LNG equivalent gallons of vapor venting associated with tank truck delivery transfer hose purging and emptying is essentially unavoidab le, as discussed in Section 3.2.)31

problematic with regard to storage tank saturation pressure increases and potential venting between deliveries (e.g., as illustrated in Figure 9). The best approach for addressing these difficult requirements may be one of the active vapor managemen t optio ns to be discussed in Section 4

. 3.3.2 Dispensing Pump Options An important LNG fueling station component that can significantly affect vapor generation and venting is the pump (or other means) used to transfer LNG from the station storage tank to the veh icle fuel tank. The various station - to - vehicle transfer methods that have been used include:

 Pump submerged in separate sump

 External (i.e., not submerged in LNG) pump

 Pump submerged in LNG storage tank

 Pressure transfer

 Gravity transfer Most currently ope rating LNG stations use a pump submerged in a LNG sump that is connected to the main LNG storage tank(s). Before discussing the details of this configuration, it is useful to briefly review other approaches from a historical and lessons - learned perspective

. Various early - generation LNG stations employed external uninsulated centrifugal pumps because this approach is simple and inexpensive. A well - documented example is the station supporting the Roadway Express LNG truck project in 1991 through 1993 (Referen ce 5 ). External pumps generate substantial vapor associated with the cool down process needed to prime the pump. Most cryogenic pumps cannot function unless they are receiving a slightly subcooled liquid at their suction 19

. Therefore, external pumps are gen erally installed underneath LN G storage tank s . Cool - down is accomplished by flowing LNG into the pump , which boils as it is heated by the pump structure. External pump priming can be expedited by building pressure in the LNG tank (if it is equipped with a pressure - build circuit). Vapor generated by the cool - down process can be vented to the atmosphere or recirculated back to the tank ullage (which adds substantial heat and shortens the tank hold time )

. Cool - down times and vented vapor quantities are highly situation - specific, but cool - down times as long as 20 minutes have been reported (Reference 1 2 ). A few stations have used cryogenic pumps submerged in the main LNG storage tank(s). An early example is the LNG station installed in Yosemite National Park in the 1970s (Reference 3 ), which used a centr ifugal pump. M ore recent example s are the Orange

19 In technical terms, these pumps require a slightly positive net positive suction head (NPSH). If supplied with vapor or a saturated liquid - vapor mixture, they will cavitate.32

County Transportation Authority (OCTA) LNG stations in Garden Grove and Anaheim , which used underground tanks with reciprocating pumps. This approach requires addi tional protrusions into the vacuum - jacketed storage tanks to facilitate pump operation and maintenance, which increases heat transfer and decreases hold time. Pressure transfer (i.e., no pump) has also been applied to transfer LNG from the station storage tank to the vehicle fuel tank. An example is the Greater Austin Transportation Corporation LNG station installed in 1992 (Reference 38 ). This station employed a combination of heat exchangers, compressors, intermediate LNG tanks, and CNG vessels to manage vapor and minimize venting. LNG tank trucks that are jury - rigged to directly refuel LNG vehicles also employ pressure transfer. The possibility of gravity transfer has been acknowledged ( e.g., Reference 7 ), and Reference 8 lists a few early - generation sm all - capacity LNG stations as using a combination of gravity and pressure transfer, but no stations are known to have used only gravity transfer. An obvious problem is the substantial LNG storage tank elevation required to produce adequate pressure differen tials and flow rates. Use of centrifugal pumps in a LNG sump that is separate from but connected to the LNG storage tank(s) for station - to - vehicle transfer dates from the early 1970s (e.g., the Atlanta Gas Light LNG station used a sump - submerged pump in 19 73 , Reference 7 ). This approach has been relatively successful, and it is used in nearly all r ecently installed LNG stations. Figure 13 illustrated the basic sump - submerged pump configuration. Figure 14 shows an example sump - submerged pump installation at the Clean Energy LNG station near the Ports of Long Beach and Los Ang eles , California. Even though the sum p - submerged centrifugal pump has emerged as the current method - of - choice for station - to - vehicle LNG transfer, this approach is not without liabiliti es. The sump and associated LNG circulation increases the heat input to the LNG storage tank. This is inconsequential for stations with high utilization and frequent LNG deliveries, but it can create vapor management challenges for stations with low or spo radic utilization. Another liability derives from the fact that the centrifugal cryogenic pumps typically used in this application have a limited pressure - ratio capability. This occasionally requires venting of high - pressure vehicle tanks before LNG can b e pumped into the tank (this issue will be discussed in more detail in Section 3.3.4). Reciprocating cryogenic pumps, which have much higher pressure - ratio capability than centrifugal pumps, do not have this liability, and they have been used occasionally for station - to - vehicle LNG transfer (e.g., the previously noted OCTA LNG stations with in - tank reciprocating pumps ). However, reciprocating pump drives are not generally sump - submerged, and so their use in this application involves a more complicated insta llation and more heat transfer to the stored LNG.33 Figure 14. Example sump submerged pump installation at a LNG fueling station. The vertical grey cylinder is the vacuum jacketed sump containing a centrifugal pump submerged in LNG.

3.3.3 LNG Fuel “C onditioning” Options A controversial LNG vehicle a nd fueling infrastructure issue , and one that significantly affects heat addition, vapor generation, and potential ventin g, pertains to the best way to provide the fuel supply pressure required by most SING - type and some PING - type heavy - duty natural gas engines. This pressure is typically approximately 75 psig or more for current - generation engines of this type (Table 2). A variety of approaches for addressing this requirement have been implemented or sugges ted. Approaches that require no special LNG fueling station equipment include in - tank fuel pumps (which have seen limited application for SING engines, Reference 9 ) and vehicle fuel systems with pressure - building circuits ( References 39, 40, and 41 ). Appr oaches that require special station equipment include “bulk conditioning” and “saturation - on - the - fly”. These are discussed in detail in this section. The great majority of LNG fueling stations installed within the last decade employ bulk conditioning to i ncrease the saturation pressure of the LNG in the station storage tanks. If the LNG dispensed into vehicle fuel tanks has a saturation pressure greater than the engine fuel supply pressure requirement plus the maximum pressure drop associated with the vehi cle fuel system, no on - vehicle fuel pump or other pressure - building equipment is required 20

. Bulk conditioning is accomplished by circulati ng the LNG from

20 In rare circumstances involving continuous h igh - load operation and well - insulated tanks, the energy removal associated with the LNG outflow decreases the saturation pressure in the vehicle fuel tank. In these cases, the vehicle should be refueled with even higher saturation pressure to avoid “runnin g out of gas”.34

the storage tank through a vaporizer and back to the storage tank until the resulting heat addition ra ises the saturation pressure to the desired level. This is typically done immediately a fter a LNG tank truck delivery. During conditioning, the d ispensing pump is almost always used to circulate LNG through the vaporizer by closing and opening valves as sk etched in Figure 13. Examples of the two types of vaporizers used at LNG stations, ambient natural - draft and forced - draft, are shown in Figures 15 A and B, respectively.

Figure 15. Bulk conditioning of LNG in fueling station storage tanks i s accomplished by

circulating the LNG through a natural - draft (A) or forced - draft (B) ambient vaporizer. Saturation - on - the - fly conditioning is accomplished by heating the LNG as it is being dispensed, e.g., by the electric heater in the cylindrical vessel above the LNG dispenser (C) at the OCTA Garden Grove station. Because it is relatively simple and inexpensive in terms of station and vehicle equipment, bulk conditioning has evolved as the current method - of - choice for addressing the needs of LNG vehicle s with fuel supply pressure requirements in the 80 to 130 psig range. The primary disadvantage is that by transferring heat to the LNG to raise its saturation pressure, hold times are decreased and venting potential is increased with respect to both the st ation storage tank and vehicle fuel tank. Also, as will be discussed in Section 3.3.4, bulk - conditioned LNG is less suitable for refueling vehicles capable of being refueled with low saturation pressure LNG. Saturation - on - the - fly conditioning involves addi ng heat to the LNG as it flows from the station storage tank to the vehicle fuel tank. The heat addition is controlled so that the resulting saturation state increase ensures that the dispensed LNG has the desired 

### A B 

### C35

saturation pressure. For saturation - on - the - fly systems that have been installed to date, the heat addition is provided by electric resistance elements in a heat exchanger vessel located near the LNG dispenser (Figure 15 C) or by mixing vapor with the LNG being dispensed (which will be discussed in Section 4.3)

. The main advantage of saturation - on - the - fly is that it allows the LNG in the station storage tank(s) to be maintained at the lowest possible saturation pressure, which corresponds to the maximum density (Figure 4) and the longest hold time. In principle, saturation - on - the - fly can also enable modulation of the saturation state of the LNG as it is being dispensed so that the initial “slug” of LNG is colder and therefore more effective in condensing (“collapsing”) the vehicle tank ullage vapor. In some cases, this would eliminate the need to remove vapor from the vehicle tank (as will be discussed in Section 3.4 ). Ho we ver, no saturation - on - the - fly conditioning systems with this type of control have been installed. The main disadvantage of electr ically heated saturation - on - the - fly conditioning is cost, primarily operating cost (e.g., for the electric power, which can be substantial). Capital costs (e.g., for heaters, power supplies, heat - exchanger vessel, controls) are also greater than bulk condi tioning equipment costs. Experience with vapor - mixing saturation - on - the - fly technology is inadequate to articulate overall advantages and disadvantages. Due primarily to the incremental costs, relatively few LNG fueling stations with saturation - on - the - fly conditioning have been built. 3.3.4 F ueling Stations Without LNG “Conditioning” Equipment As discussed in Section 3.3.3, special equipment must be added to fueling stations that must dispense conditioned LNG for vehicles requiring LNG saturated at pressures near 100 psi. Stations with bulk conditioning systems need a vaporizer and associated plumbing and controls. Usually, vehicles cannot be refueled while conditioning is in process, the storage tank hold time is decreased, and the potential for atmospheric venti ng is increased. Stations with saturation - on - the - fly conditioning require special heat exchangers and incur higher operating costs for electric power. If dispensing of conditioned LNG to vehicles isn’t required, the fueling station design is simplified, st ation capital and operating costs are less, and the potential for venting is reduced. Three circumstances for which conditioned LNG is not required are: 1. Current

– Heavy - duty vehicles with Westport ISX - G HPDI natural gas engines (Section 2.5 and Table 2) ha ve fuel systems that include a high - pressure pump, and so they do not require conditioned LNG. It is preferable to refuel these vehicles with low - saturation - pressure LNG (to maximize the mass of onloaded fuel as well as the tank hold time), but they can al so be refueled with conditioned LNG. At the present time, this is the only commonly used heavy duty natural gas engine that doesn’t require refueling with conditioned LNG. 2. Previous

– In the 1960s through the mid - 1990s, m any LNG vehicles were light or medi um - duty vehicles with gasoline engines that had been converted to36

natural gas fueling by replacing the carburetor with a natural gas mixer 21 . These engines usually required fuel supply pressures ranging from approximately 15 psig (if naturally aspirated) to 30 psig (if turbocharged), which were easily accommodated by LNG dispensed without intentionally increasing the saturation pressure . In the 1990s, more than 100 transit buses were powered by Detroit Diesel Corporation 6V - 92TA PING engines, which required fuel supply pressures in the 300 - 400 psi range (Reference 42 ). These buses had LNG fuel tanks with pumps similar to the system sketched in Figure 7 B. This type of pumped LNG fuel system was also used on a LNG - fueled bus with a Cummins L - 10 SING engine tha t had a much lower fuel supply pressure requirement (Reference 9 ). While these 1960s through 1990s LNG vehicles didn’t require conditioned LNG, almost none of these vehicles are still operating, and the natural gas engine technologies of this era are now obsolete. 3. Hypothetical

– Various alternatives to LNG conditioning have been suggested for addressing the fuel supply pressure requirements of current - generation SING (and some PING) engines. These include on - vehicle low - pressure pumps, various on - vehicle p ressure - building strategies, (e.g., as reviewed in Reference 19 ), and a few unpublished proprietary concepts. Successful implementation of these technologies would simplify fueling station construction, increase hold times in station and vehicle fuel tanks , and potentially reduce vapor venting. While none of these technologies are commonly used at this time, their potential benefits may motivate future development and commercialization if LNG use as a transportation fuel continues its current growth rate. 3.3.5 S tations Capable of Refueling All Types of LNG Vehicles Planners of LNG fueling stations that must be capable of refueling all types of LNG vehicles (e.g., vehicles with the variety of fuel supply pressure requirements listed in Table 2) have two basic choi ces:

 Option 1

– Dispense conditioned LNG (i.e., saturation pressure ~120 psig) to all vehicles

 Option 2

– Dispense LNG at multiple saturation pressures that are best suited to each vehicle being refueled (e.g., ~120 psig for vehicles with Cummins Westport ISL - G engines and ~30 psig for vehicles with Westport ISX - G engines) The disadvantage of Option 1 is that some vehicles that don’t need conditioned LNG will be refueled with high - saturation - pressure LNG, which, as previously discussed, reduces the mass of fuel that can be dispensed to the vehicle, decreases the vehicle tank hold time, and increases the likelihood of venting. This is, however, the lowest - cost option, and most LNG fueling stations are of this type.

21 Reference 7 contains a listing of LNG vehicle projects in this time period.37

At the present time, only two LNG station s h ave Option 2 capability: The Clean Energy LNG station in Carson, California , and the nearby Clean Energy Ports of Long Beach and Los Angeles LNG station. These stations have separate storage tanks and dispensers for conditioned and unconditioned LNG (Figur e 16). Therefore, from one perspective, there are essentially two stations at each site. Clean Energy refers to them as the “ Blue Station”, which dispenses unconditioned LNG at a < - 235°F & <35 psig saturation state, and the “Green Station”, which dispenses conditioned LNG at saturation states in the - 195 to - 207°F & 85 to 120 psig range (Reference 27 ).

Figure 16. The Clean Energy Ports of Long Beach and Los Angeles fueling station (A)

dispenses LNG at two saturation pressures: “Blue” LNG (B) has a low saturation pressure for vehicles with engines such as the Westport ISX - G, and “Green” LNG (C) has a higher saturation pressure for vehicles with engines such as the Cummins Westport ISL - G. Because it requires duplicate storage tanks, dispense rs, and other equipment, the “two grades of LNG” Option 2 strategy is much less cost effective for LNG stations with low utilization than stations with high utilization. In theory, LNG fueling stations could utilize saturation - on - the - fly technology (Sectio n 3.3.3) to provide Option 2 capability, but no stations of this type have been built to date. 

### A

B C38

3.3.6 LNG Fuel Dispenser Cool - Down Options LNG vehicle fuel tanks are refueled most effectively if the initial quantity (“slug”) of fuel dispensed into the tank is liq uid and not vapor. If a substantial slug of vapor is initially dispensed into the vehicle tank, this can cause a pressure rise that terminates refueling (because the pressure exceeds the dispensing pump capability or approaches the tank PRD pressure ), and it usually requires vapor offloading or venting before refueling can proceed. The potential for the initial slug of dispensed fuel to be mostly vapor is due to heat leaks to the LNG in the plumbing in and near the dispenser between refueling episodes (i.e ., when there is no flow), especially if the time increments between refuelings are long. This heat transfer essentially boils some of the LNG in the plumbing. If the station has no features designed to avoid this situation, the pump drives this vapor into the vehicle fuel tank at the beginning of the next refueling. LNG fueling station designs incorporate a variety of features intended to avoid or minimize this situation. The most basic technique is simply to recirculate LNG through the dispenser (as can b e visualized from Figure 13) to cool down most of the affected plumbing. Some stations have metering elements that are submerged in a small LNG sump in the dispenser . These stations don’t require LNG recirculation prior to dispensing, but a low level of co nstant recirculation is required to offset heat leaks and maintain a constant level of LNG in the metering sump. With either of these approaches, the recirculation process adds heat to the station storage tank 22 , which shortens its hold time and may increas e the likelihood of atmospheric venting. Some LNG fueling station designs seek to minimize the volume of vapor generated between refuelings by locating the sump with the dispensing pump very close to the dispenser (which in these cases is usually a dispen sing panel in the containment wall), so that little or no recirculation is needed prior to refueling vehicles. The heat addition and/or vapor management challenge associated with the need to cool down the dispenser prior to vehicle refueling is much less for stations with high utilization than those with low utilization (i.e., those with sporadic or lengthy periods between vehicle refuelings). 3.3.7 Station LNG Storage Tank Pressure Management For LNG fueling stations with adequate utilization and no excessive h eat or vapor injection into the storage tank(s), the saturation pressure and temperature of the LNG in the storage tank(s) fluctuates very little between LNG tank truck deliveries. However, for stations with low utilization (as discussed in Section 2.7), t he saturation pressure of the LNG in the storage tank can increase substantially between deliveries. If the station utilization is extremely low, the saturation pressure can reach the storage tank PRD pressure before the LNG level in th e tank is low enough to accept another full tank truck

22 In a few stations, the vapor from the cool - down process is vented to the atmosphere (which is obviously an inappropriate practice) instead of being returned to the LNG storage tank.39

delivery (as illustrated in Figure 9). Even for stations with higher utilization, the saturation storage tank pressure can increase substantially between deliveries if the tank receives significant vapor input from vehicl e fuel tank vapor offloading, LNG transfer pump priming, many dispenser cool - down recirculation cycles, e tc. A given storage tank saturation pressure increase is of course a more challenging issue for stations with bulk conditioning that have already incre ased the saturation pressure of the LNG in the storage tank to 100 psig or more. Stations that experience substantial LNG saturation pressure increases sometimes address this problem by using a back - pressure regulator (as shown in Figure 13) to constantly vent vapor to the atmosphere at a rate so as to maintain the storage tank pressure at a constant and convenient level (e.g., 130 psig). This inappropriate practice is occasionally referred to as an “auto vent” system or simply “riding the regulator.” A mo re appropriate approach is to address and mitigate the basic cause of the substantial saturation pressure increase or to manage the vapor by one of the means to be discussed in Section 4. 3.3.8 LCNG Fueling Stations As noted in Section 1, L CNG fueling stations are stations that receive and store LNG (just like LNG stations), but the y dispense CNG instead of or in addition to LNG. In LCNG stations: 1. The LNG is pumped to a high pressure (e.g., ~4,500 psi), 2. Vaporized and warmed to near - ambient temperatures (in a vap orizer similar to those shown in Figure 15 A and B), 3. In most cases, an odorant is added to the gas after it is vaporized (because LNG contains no odorant s ; they are incompatible with the liquefaction process , and so any odorants in the feedgas are removed by the pretreatment system), 4. The gas is stored in pressure vessels (either a buffer tank or a sequenced cascade), and 5. The high - pressure gas is dispensed to refuel CNG vehicles. Figure 17 shows an example of a LCNG fueling station. This particular example i s selected simply because most of the principal components are visible in the photograph. LCNG stations are often combined with LNG stations so that CNG and LNG are dispensed at the same station. These are usually referred to as LNG/LCNG stations, but the re isn’t uniform agreement regarding this nomenclature. Combining of LCNG and LNG stations takes advantage of obvious economies because they can share the same LNG storage vessels and other components. Also, combining LCNG and LNG refueling enables certain vapor management options, which will be discussed in Section 4.40

Figure 17

. The Santa Cruz Metropolitan Transit District (“METRO”) LCNG fueling

station is an example of this type of facility. METRO chose LCNG primarily because convenien t access to an adequate - capacity natural gas pipeline was unavailable. The tradeoff between conventional gas - pipeline - supplied CNG stations and LCNG stations is subtle and situation dependent. One advantage of LCNG stations is that LNG pumps are less expen sive and consume substantially less power than equivalent flow capacity gas compressors. On the other hand, there are no vapor management or venting issues associated with pipeline - supplied CNG stations. More detailed information regarding LCNG station des ign features, advantages, and disadvantages is available in References 13, 14, 17, 22, 26, and 43

. LCNG fueling stations have some, but not all, of the same vapor management issues as LNG fueling stations. For example, due to LNG storage tank heat leak eff ects (Section 2.3) , the tank capacity, average fuel dispensing rate, and tank truck delivery frequencies 15,000 - g allon LNG storage tank

Vaporizers Pumps in containment pit High - pressure gas storage vesse ls CNG dispensers41

should be such that the tank saturation pressure never increases to the PRD pressure. This is usually less of a challenge with LCNG stations than LNG s tations, because there is no need to condition the LNG and there are fewer sources of vapor that need to be accommodated by the LNG in the storage tank. On the other hand, some of the earlier LCNG station installations experienced a substantial quantity o f vapor generation and vapor venting associated with pump priming. This is because reciprocating cryogenic pumps are required in order to produce the pressure ratios necessary for CNG service, these pumps generally have greater NPSH (Section 3.3.2) require ments than centrifugal pumps, and they are not usually submerged in a sump. More recent LCNG stations have adopted a practice that essentially solves this problem. LNG from the storage tank flows first to a sump submerged centrifugal pump, which provides a more - than - adequate NPSH to a high pressure reciprocating pump. The centrifugal pump functions as an inducer for the reciprocating pump so that it primes promptly with minimal vapor generation. Figure 18 illustrates this arrangement of multiple centrifugal and reciprocating pumps at a relatively large - capacity LCNG fueling station. This arrangement is particularly cost effective at stations with combined LCNG and LNG refueling capabilities, because the same sump - submerged centrifugal pump(s) can be used for LNG dispensing and as a LCNG pump inducer.

Figure 18. These pumps at the Omnitrans San Bernardino LCNG station illustrate the practice of using sump - submerged centrifugal pumps as inducers for high - pressure reciprocating pumps.

3.4 LNG Vehicle Oper ations Vapor can be vented directly from LNG vehicles (e.g., Figure 2 C), and/or certain vehicle fuel tank and system situations can increase the likelihood of venting from the fueling station. Sump - submerged centrifugal pumps High - pressure reciprocating pumps42

Venting from LNG vehicles occurs when the saturation pressure of the LNG in the fuel tank exceeds the PRD pressure. Codes and standards (Section 2.9) require LNG vehicle fuel tanks to have two PRDs (Figure 7). The primary PRD is usually set at the tank MAWP and the secondary PRD is usually set at 1.5 times the MAWP. The primary PRD is connected to a discharge tube located so that any vented vapor is directed upward and away from likely personnel locations. Venting from LNG vehicles should not occur if the vehicle is driven regularly and its equipment is operating pro perly. If the vehicle is not regularly driven (e.g., parked for many days), normal heat leaks can cause the LNG saturation pressure to increase to the PRD pressure and venting will occur as described in Section 2.3 and illustrated in Figure 6. The LNG fuel tank saturation pressure variation between refuelings depends on many factors including the tank heat leak characteristics, the driving cycle, the saturation pressure and ullage fraction in the tank immediately after refueling, and whether or not it is eq uipped with an economizer valve (or equivalent device). An occasional LNG vehicle problem that substantially reduces fuel tank hold time and increases venting likelihood is a partial loss of vacuum, which is usually referred to as a “soft vacuum”. Sweatin g or frosting on the tank exterior (which may be evident before venting) are likely indications of a soft vacuum condition. LNG vehicles that are venting or sho w evidence of a soft vacuum should be taken out of service. They should be parked outdoors in a safe location and inspected and/or tested to diagnose if the fuel tank insulation has been compromised. If a soft vacuum condition is confirmed, the fuel tank should be defueled (if possible) and repaired or replaced. LNG vehicles can affect fueling stati on venting if and when vapor is transferred from the vehicle fuel tank to the station storage tank. This may occur in either of two related circumstances: 1. The pressure in the vehicle fuel tank at the beginning of refueling exceeds the station dispensed LNG maximum pressure so that no fuel is pumped into the vehicle tank . As discussed in Section 3.3.2, this may be because the dispensing pump pressure ratio capability is such that it cannot “buck” the initial pressure in the vehicle tank 23

. Or, for some statio ns, this may be because a dispenser recirculation valve opens at a pressure less than the initial vehicle tank pressure. In most cases, this condit ion is detected by the refueler who attaches the vapor hose (Figure 13) to the fuel tank vapor receptacle and removes vapor until the pressure decreases so that refueling can proceed. Reducing the vehicle tank pressure by simply opening the vehicle fuel tank vapor valve and venting vapor to the atmosphere is an inappropriate practice. 2. In some situations (which ar e affected by factors including the vehicle fuel tank pressure and ullage fraction prior to refueling, the dispensed LNG saturation state, and the station dispenser cool - down system design), the inflowing LNG

23 In some designs and situations, the vapor can actually flow “backward” through the main refueling hose

.43

does not promptly condense (“collapse”) the veh icle fuel tank vapor, and the vehicle tank pressure increases toward the PRD pressure (or beyond the dispensing pump capability). In this case, as in circumstance #1, vapor must be removed from the vehicle fuel tank and returned to the station before refue ling can proceed. Again, reducing the vehicle tank pressure by venting the vapor to the atmosphere is a bad practice. The above descriptions apply to almost all currently used LNG vehicle fuel systems. Some systems used in the past employed two - hose refuel ing couplings, and some vapor was automatically returned from the vehicle fuel tank to the station storage tank as part of the refueling process (e.g., References 7, 16, 19, 2 3) . Few if any of these systems are in use at this time. Closely related circumst ances #1 and #2 described above both inject heat (in the form of vapor) into the fueling station LNG storage tank, which increases the LNG saturation pressure and temperature. This is the mechanism by which certain LNG vehicle fuel tank and system situatio ns increase the likelihood of atmospheric venting of vapor from the fueling station.4445 Section 4 OPTIONS FOR RECOVERING VAPOR GENERATED AT LNG FUELING STATIONS 4.1 Overview Th ere are two basic ways to mitigate atmospheric venting at LNG fueling stations: 1) design and operate the station so that no vapor is released to the atmosphere, or 2) recover and use excess vapor for some useful purpose. Previous sections of this report have discussed LN G station design principles and operating practices that avoid or minimize vapor generation and venting. For situations where it is technically or economically impractical to eliminate all vapor releases, there are various options for recovering and using the otherwise - vented vapor. These options are discussed in the following subsections. Any LNG fueling station vapor ma nagement strategy that involves continuous or frequent removal of vapor from the storage tank will cause the composition of the LNG dispensed to vehicles to be different from that delivered to the stat ion by tank trucks. In most cases, this has no practical implications because most LNG currently delivered to LNG fueling stations is of very high purity (e.g., ~99% methane, Section 2.2) and the vapor removed is likely to be a very small fraction of the L NG dispensed. However, in rare cases for which the LNG has a substantial percentage of heavier hydrocarbons and/or the vapor removal rate is a large fraction of the LNG dispensing rate, the LNG in the storage tank will become enriched (Section 2.2 and Appe ndix B) and the fuel dispensed to vehicles may not comply with engine manufacturers’ fuel composition specifications 24 . Any plans to remove and utilize LNG vapors should include consideration of potential composition change implications. It should be noted that technologies for utilizing vapors generated from LNG fueling stations can be regarded as a subset of the more general problem of recovering and productively using methane vapors from LNG tanker ships, LNG processing and peakshaving plants, gas and oi l well operations, gas compressor leakage, landfills, and various agricultural activities. Because these sources produce methane releases many orders of magnitude greater than those from LNG fueling stations, they have been the subject of various internati onal symposia (e.g., Reference 44 ) , Environmental Protection Agency programs (e.g., Reference s 45, 46, and 47 ) , and hundreds of special studies

. Even though there is significant overlap with methane - release - mitigation technologies applicable to these sourc es, the focus here is confined to LNG fueling station vapor utilization.

24 In extremely rare cases for which the LNG contains significant nitrogen, the vapor phase will have a much higher nitrogen conc entration and may not be suitable for some of the boil - off vapor utilization options discussed in this section.46

It should also be noted that there have been a few previous studies directed specifically at LNG fueling station vent gas utilization. Reference 48 evaluated alternatives for one part icular station that was anticipated to have low initial utilization (the Barstow, California, LNG/LCNG station). Other studies that have addressed aspects of LNG fueling station vapor utilization include References 5, 7 , 8 , 16, and 25. Options for recoveri ng and utilizing otherwise - vented vapor (which is often referred to as boil - off vapor) from LNG fueling stations are listed in Table 3. These options are discussed in the following subsections with emphasis on prior application experience and what was lear ned from that experience. Reliquefaction of boil - off vapor Utilize boil - off vapor for on - the - fly LNG fuel conditioning Flaring of boil - off vapor Sell boil - off vapor to local natural gas utility Utilize boil - off vapor to supply facility natural gas req uirements Use boil - off vapor to generate electric power to be sold to local electric utility Use boil - off vapor to generate electric power for on - site utilization Compress boil - off vapor for use for CNG vehicle fueling Table 3. Options for recovering a nd utilizing boil - off vapors from LNG fueling stations. 4.2 Reliquefaction of Boil - Off Vapor Numerous studies have considered reliquefaction of LNG fueling station boil - off vapor, but there has been only one known attempt to apply this technology, and this app lication was unsuccessful. No currently operating LNG stations have a reliquefaction capability

. The reliquefaction concept consists of processing excess vapors (which might otherwise be vented to the atmosphere) through a liquefier to condense the vapor to the liquid phase and return the liquid to the LNG storage tank. Almost any liquef action or refrigeration cycle (e.g ., as reviewed in cryogenics textbooks such as Reference 28 ) can be employed. However, even for very large LNG fueling stations, the vapor generation rate is relatively small, and so the liquefier should be cost effective for low - throughput or frequent - on/off applications. Studies that have considered reliquefaction include References 8, 10, 16, 25, and 48

. While most studies considered reli quefaction based on liquefaction or refrigeration cycles such as Joule - Thomson, Stirling, and Gifford - McMahon, t he previously noted Kellogg study ( Reference 8 ) concep tualized two somewhat different strategies: LNG47

vapor reliquefaction by heat transfer to e vaporating liquid nitrogen (with inert nitrogen vapor vented to the atmosphere), and integration of the reliquefaction system with an on - the - fly fuel conditioning system ( heat transferred from compressed boil - off vapor to increase the saturation pressure o f the dispensed LNG condenses the vapor , which is similar to the concept considered in Section 4.3 ). Neither of these concepts have been applied at actual LNG fueling stations. Reference 48 considered a simple reliquefaction system utilizing a compressor, heat exchanger, and expansion valve for the Barstow LNG station. Based on an approximate economic analysis, it was concluded that reliquefaction would be much less cost effective than other vapor management options. The only known non - confidential applica tion of boil - off vapor reliquefaction at an actual LNG fueling station was at the Orange County Transportation Authority (OCTA) station in Garden Grove , California. The procurement specifications for this station required a “recondenser system” to manage v apor and avoid atmospheric venting. A reliquefaction system that utilized two 200 - Watt CVI Gifford - McMahon refrigerators (Reference 16 ) was initially applied to address this requirement. This system did not perform consistent with expectations (Reference 4 9 ), and details regarding this experience were not documented. Natural gas fueled electric generators (which are discussed in Sections 4.6 and 4.7) were subsequently installed at the OCTA LNG stations to accommodate worst - case vapor management episodes. Al though there have not yet been any successful applications of reliquefaction for LNG fueling station excess vapor management, several recent small - scale liquefier technology advances may contribute to making this option more viable in the future. One examp le is the natural gas liquefier technology developed by the Gas Technology Institute (GTI), which utilizes a relatively inexpensive screw compressor and a proven mixed - refrigerant cycle to provide a small - capacity liquefier with low capital and operating c osts (Reference 5 0 and 5 1 ). A prototype 1,000 LNG gallon per day GTI liquefier is shown in Figure 19 A. There have also been recent advances in boil - off gas reliquefiers for LNG carrier ships , which is an application analogous to (but with much higher thr oughput than) LNG fueling station vapor reliquefaction. For example, the Hamworthy Company has success fully applied this technology (R eferences 5 2 and 5 3 ). Figure 19 B shows a 600 LNG gallon per day laboratory liquefier cooperatively developed by Hamworthy and SIMTEF (a large Norwegian research organization). 4.3 Utilization of Boil - Off Vapor for On - the - Fly LNG Conditioning A small LNG vapor stream can be mixed with a larger stream of LNG liquid to produce a LNG liquid stream saturated at a higher pressure. In principle, this process could be utilized as part of an on - the - fly LNG conditioning system (see Section 3.3.3) that would consume some of the station boil - off vapor while producing LNG at the saturation pressure required by the vehicles being refueled. How ever, such a system would require extra equipment (e.g., compressor, mixing chamber, sensors) as well as precise control. Also, because the desired vapor removal rate and dispensed fuel saturation pressure wouldn’t ordinarily be balanced, such a system wou ld probably have to be combined with another LNG conditioning technology (e.g., electric heaters).48

Figure 19. Recent advances in economical small - scale liquefaction technology may be

applicable to LNG fueling station boil - off vapor reliquefaction. Examples include this prototype of a GTI liquefier that utilizes a mixed - refrigerant cycle and a low - cost screw compressor (A) and this 600 LNG gallon per day laboratory liquefier develop by SINTEF in cooperation with Hamworthy (B). A process similar to th is has been developed and demonstrated by the Idaho National Laboratory (INL , previously referred to as INEEL ). The INL process achieves on - the - fly LNG conditioning (Section 3.3.3) by combining controlled amounts of vapor with the flowing LNG in a mixer lo cated just upstream of the LNG dispenser. In the standard embodiment of this system, the vapor is derived from LNG that is routed through a vaporizer (Reference s 43 and 5 4 ). However, the vapor mixed with the LNG could also be drawn from the LNG storage tan k ullage (perhaps with the aid of a compressor), in which case the system could provide both on - the - fly conditioning and excess vapor management . This technology (with the vapor from a LNG vaporizer and not from the storage tank ullage) has be en employed at INL and Southwest Transportation Agency (Caruthers, Califo rnia) LNG/LCNG fueling stations

. 4.4 Flaring of Boil - Off Vapor In a 1996 study of LNG vehicle fueling technologies, the M. W. Kellogg Company (which is now part of the KBR Company) discussed and diag ramed the possibility of flaring (i.e., burning in an elevated incinerator) LNG fueling station boil - off vapors (Reference 8 ). This study pointed out that flaring is inexpensive and that releasing carbon dioxide and water instead of methane reduces GHG emi ssions. While flaring is a common practice in oil and gas well operations, and at some landfills, there is general agreement that this is an inappropriate vapor management option for LNG fueling stations. 

### A B Image courtesy SINTEF & Hamworthy Image courtesy Gas Technology Institute49

4.5 Sell Boil - Off Vapor to Local Natural Gas Utility An obvious LNG fueling station excess vapor management strategy is to simply inject the gas into the local gas distribution pipeline network and sell it to the natural gas utility, i.e., the local distribution company (LDC). This option is not easily impleme nted, however, and there is only one documented instance of LNG station boil - off gas being routed to the LDC. The challenges involve the required process equipment and contractual arrangements. The process equipment requirements are illustrated, for examp le, by the one documented implementation, which was the Roadway Express LNG truck station in Copley, Ohio, in the early 1990s (Reference 5 ). This project required a special gas compressor, buffer storage vessel, heat exchanger, and gas meter. In other case s, an odorizer may be required to accommodate LDC requirements for gas injected into their distribution network. The major challenge is anticipated to be consummating the legal and contractual arrangements with the LDC to permit and pay for gas received i nto their network. Issues include accounting, pricing, composition, metering and scheduling. Selling gas to the LDC at the distribution network level is an unusual situation, applicable regulations are nonexistent or location dependent, and LDCs are not al ways eager to improvise. Perhaps recent renewable percentage legislation and interest in biomethane will affect this situation. It is significant to note that the previously cited Roadway Express project LNG station, which fed boil - off gas to the nearby Co nsolidated Natural Gas 60 - psig pipeline, was in fact funded and managed by Consolidated Natural Gas (Reference 5 ). 4.6 Utilize Boil - Off Vapor to Supply Facility Natural Gas Requirements An option that may be suitable for LNG fueling stations that are part of a la r ger facility with natural - gas - consuming equipment (e.g., heaters, boilers) is to utilize excess vapor from the fueling station to provide some of the facility natural gas requirements. This strategy avoid s most of the contractual and regulatory challe nges associated with selling natural gas to the LDC, which were discussed in Section 4.5. This option is suitable only for facilities with LNG fueling stations that consume significant quantities of natural gas. The facility natural gas demand schedule sh ould be in phase with the station excess vapor management needs, because any need to store significant quantities of gas would probably make this option uneconomical. Other equipment requirements (e.g., for compression, odorization, metering) depend on spe cific facility situations. While this option avoid s local gas utility contractual challenges, it may encounter some facility management challenges pertaining to odorization requirements, building codes, insurance policy restrictions, e tc. The Grand Canyon LNG/LCNG fueling station is the only station known to be using this type of system at this time. In this application, boil - off gas is used to heat maintenance buildings, which don’t have access to a natural gas pipeline.50

4.7 U se Boil - Off Vapor to Generate Ele ctric Power to be Sold to Local Electric Utility Excess vapor from a LNG fueling station can be supplied to an on - site natural - gas fueled electric generator, and the power can be sold to the local electric utility. In this situation, the station owner/oper ator becomes an Independent Power Producer (IPP) or Non - Utility G enerator (NUG) and must comply with various regulations applicable to these entities. The main equipment item required is a natural - gas - fueled electric generator, i.e., a “genset.” Given the range of LNG station sizes, likely excess vapor fractions, and typical genset heat rates, the genset rating would probably be in the 20 kW to 200 kW range. Gensets using natural gas internal combustion engines (which are usually diesel derivatives) and sm all gas turbines are available in this power range. Depending on how the genset is equipped, additional equipment that may be needed includes a gas compressor, buffer storage vessel, controls, and electrical equipment for utility grid interconnection. The feasibility of this option benefits from the US P ublic Utility Regulatory Policy A ct (PURPA) of 1978, which requires utilities to purchase power from NUGs. With this option, the fueling station would most likely be classified as a parallel operator, which applies to entities that operate a generating facility that simultaneously delivers and receives power while interconnected. Numerous state - and utility - dependent rules and regulations apply to NUGs. For example, in California, the facility application, in terconnection, operation, and metering must comply with “Rule 21,” which is slightly different for each of the electric utilities over which the California Public Utilities Commission (CPUC) has jurisdiction. In addition to securing electric utility approv als, a NUG must also obtain various other permits. In some locations such as California, Authority to Construct and Operating Permits must be obtained from air quality management or air pollution control districts, and this is not always a straightforward process. An economic assessment of this option should consider the avoided electric cost and any reduced demand charges as offsets to the operating cost and amortized capital cost. While using boil - off gas to generate electricity for sale to the local ele ctric utility may be an attractive option in some circumstances, no LNG fueling s tations are known to be IPPs or NUGs at this time. 4.8 Use Boil - Off Vapor to Generate Electric Power for On - Site Use Although no LNG fueling stations are known to be IPPs or NUGs, a few stations have installed gensets fueled with LNG vapor. Two examples are OCTA and Santa Monica Transit (Big Blue Bus) in California. In each of these cases, the gensets are basically backup generators that are not used for routine LNG station boil - of f vapor management. At OCTA, natural - gas - fueled internal combustion engine generators were installed after the reliquefaction system (Section 4.2) was found to be ineffective. The stated intent of51

the OCTA gensets is to avoid substantial atmospheric venti ng of boil - off vapors if no LNG is dispensed into buses for an extended period of time (e.g., in case of a bus drivers’ strike). The Natural - gas - fueled gensets at Big Blue Bus are essentially backup power generators and are not routinely used (nor are they needed) for boil - off vapor management. The use of natural - gas - fueled gensets for LNG fueling station excess vapor management , while producing electric power for facility use without becoming a parallel operator and supplying power to the local grid , appea rs to be feasible. However, no LNG stations are known to be currently operating in this fashion. 4.9 Compress Boil - Off Vapor for Use for CNG Vehicle Fueling An excess vapor recovery and use method that is being applied successfully at a few LCNG and LNG/LCNG f ueling stations consists of compress ing the vapor and dispens ing it to r efuel CNG vehicles. In principle, this method could be applied to LNG fueling stations, but it is more straightforward to implement at LCNG or LNG/LCNG stations that already have disp ensers, high - pressure gas storage vessels, and other equipment needed for CNG vehicle refueling. E quipment needed to implement this option at any station that stores LNG includes:

 A heat exchanger (to cool the vapor so that it meets compressor specificatio ns )

 A gas compressor ( to ~4,500 psig)

 High - pressure cascade or buffer gas storage vessels (already exist at LCNG stations)

 Gas odorization equipment 25 (already exist at LCNG stations)

 A CNG dispenser (already exist at LCNG stations)

 Associated plumbing, co ntrols, e tc. This option also obviously requires CNG vehicles that can be refueled at the station location. Figure 20 shows the heat exchanger and compressor installed at the Santa Cruz Metropolitan Transit District LCNG fueling station to manage excess va por. The quantity of vapor that may need to be managed at any station is a small fraction of the total LNG throughput (for LNG or CNG fueling), and so a relatively low - capacity and low - cost compressor is adequate for this application.

25 In some cases, a waiver from regulations requiring odorized CNG vehicle fuel can be obtained. For example, CNG dispensed into transit buses at the two Omnitrans LCNG stations is not odorized (Reference 22 ).52

Another facility tha t uses this method of LNG boil - off gas management is the LNG/LCNG fueling station in Barstow, California. The Barstow facility planning study (Reference 48 ) identified compression - to - CNG and supplying gas to the LDC network (Section 4.5) as the two most hi ghly ranked vapor management options. After considering the various regulatory hurtles associated with selling gas to the LDC network, Barstow management chose the compression - to - CNG option.

Figure 20. Small - capacity compressor and heat exchanger (to th e left of the compressor) installed at Santa Cruz Metropolitan Transit LCNG station to manage excess vapor and avoid atmospheric venting.53 Section 5 EMISSIONS IMPLICATIONS OF LNG VAPOR ATMOSPHERIC VENTING 5.1 Overview Atmospheric venting of vapor from LNG fueling stations should be av oided or minimized for economic, s afety, and environmental reasons. The negative environmental impacts of LNG vapor venting are discussed here. Greenhouse gas effects are discussed in Section 5.2 and criteria pollutant emi ssio ns are discussed in section 5.3. 5.2 Greenhouse Gas (GHG) Emissions The quantitative effect of atmospheric venting of vapor at LNG fueling stations on overall well - to - wheels LNG fuel cycle emissions can be estimated from results generated using the Greenho use Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model developed at the Argonne National Laboratory (Reference 5 5 ). A GREET version that is well suited to this application is the CA GREET model (Reference 5 6 ), which includes variou s features specific to California applications of alternative transportation fuels. Two publications that document CA GREET predictions of well - to - wheels GHG and criteria pollutant emissions for LNG fueled heavy - duty vehicles are References 57 and 5 8 . The results from these two studies are similar, but they differ slightly with respect to the fuel pathways considered and a few quantitative assumption details (e.g., liquefier efficiencies). The total well - to - wheels (WTW) emissions can be regarded as the wel l - to - tank (WTT) plus the tank - to - wheels (TTW) emissions, i.e.: WTW = WTT + TTW. Absent any venting during vehicle operation, the venting considered here affects only the WTT emissions. GREET analyses account for boil - off and boil - off gas recovery associate d with LNG storage at the liquefaction plant , transportation (e.g., tanker ship), bulk terminal, distribution (e.g., tank truck), and fueling station. The GREET default assumption for fueling station boil - off is 0.1% per day with 80% recovery 26 (which is eq uivalent to 0.02% per day vapor venting). The discussion here is based on the analysis in Reference 5 8 because the results are summarized in a suitably simple fashion and GHG emissions are expressed as grams of CO 2 equivalent per diesel equivalent gallon ( g - CO 2eq /deg). This analysis also includes an especially representative WTW pathway, which is labeled NA - LNG - CE and consists of:

 Natural gas from North American wells,

 Transported via pipeline to California, 26 GREET documentation doesn’ t discuss specific assumptions regarding LNG fueling station boil - off recovery technologies

.54

 Liquefied at the Clean Energy Fuels liquefaction plant in Boron, California,

 Transported via tank trucks to LNG fueling stations near the Ports of Long Beach and Los Angeles, and

 Dispensed into heavy - duty LNG - fueled vehicles (with operating characteristics as detailed in Reference 5 8 ). For this pathway, the GHG emissions calculated by Reference 5 8 are:

WTT = 2,104 g - CO 2eq /dge TTW = 7,954 g - CO 2eq /dge WTW = 10,058 g - CO 2eq /dge The effect of additional atmospheric venting at the LNG fueling station can be considered as a perturbation to these results. Venting at the station will have two effects: 1. The methane escaping to the atmosphere will increase the WTT GHG emissions, and this increase will be affected by the fact that the CO 2 GHG equivalence of methane is 25 27

. 2. If some of the LNG transported to the f ueling station is vented to the atmosphere and not dispensed into vehicles, more LNG must be delivered to the station in order to maintain the same refueling throughput. If the fraction of LNG that is vented at the station is small, these two effects can b e considered independently and then added without introducing a substantial error. For this analysis, we conside r a hypothetical case for which 1% of the LNG delivered to the station is vented to the atmosphere. Therefore, for every 100 g of LNG delivered to the station, 1 g will be vented and 99 g will be dispensed to vehicles. This 1 g of vented methane vapor is equivalent to 25 g of CO 2 emissions. Using the method of Reference 58 and the applicable diesel fuel and LNG heating values to reference this inc rease in GHG emissions to diesel equivalent gallons dispensed to vehicles: dge / CO g 684 LNG g / Btu 4

. 47 dge

/ Btu 104 , 2 LNG g 99 CO g 25 eq 2 eq 2

         

   Therefore, venting 1% of the LNG at the station adds approximately 684 g - CO 2eq /dge to the WTT GHG emissions (effect #1 above), and the lost fuel requires the quantity of fuel delivered to the station to be increased by approximately 1% (effect #2 above). Therefore, the WTT GHG emissions with 1% venting at the station is: dge / CO g 816 , 2 99

. 0 dge

/ CO g 684 dge / CO g 104 , 2 WTT eq 2 eq 2 eq 2

     

27 Different sources regard the CO 2 GHG equivalence of methane to be be tween approximately 20 and 25. Here we use 25 to be consistent with References 57 and 5 8

.55

The TTW GHG emissions are unaffected, and so the WTW GHG emissions with 1% venting at the station is: dge / CO g 770 , 10 dge / CO g 954 , 7 dge / CO g 816 , 2 WTW eq 2 eq 2 eq 2       The increase in WTW G H G emissions associated with 1% of the delivered LNG being vented at the fueling station is: 07

. 1 dge / CO g 058 , 10 dge

/ CO g 770 , 10 eq

2 eq 2 

  Thus, a very small percentage of venting at the LNG fuelin g station increases the WTW GHG emissions of LNG - fueled heavy - duty vehicles (for this NA - LNG - CE pathway) by a percentage that is approximately seven times the vented LNG percentage. 5.3 Criteria Pollutant Emissions Criteria pollutant emissions of interest for transportation systems include reactive organic gases (ROG), carbon monoxide (CO), oxides of nitrogen (NO x ), particulate matter 28 (PM), and sometimes sulfur oxides (SO x ) . To compare the impacts of different fuels and pathways on overall fuel cycle criteria pollutant emissions, the well - to - wheels approach summarized in Section 5.2 is generally employed. Reactive organic compounds, as defined by the California Air Resources Board (ARB) , specifically exclude certain compounds such as methane that have negligibl e photochemical reactivity. The US Environmental Protection Agency (EPA) uses the designation Volatile Organic Compounds (VOC), which is nearly the same thing as ROG . The EPA also excludes photochemically non - reactive compounds such as methane from the def inition of VOC. Therefore, because LNG transportation fuel (and especially LNG vapor) is almost all methane, atmospheric venting of LNG vapor at a fueling station is not a ROG emission. Venting at the station does, however, increase the quantity of LNG tha t must be delivered to the station in order to maintain the same vehicle refueling throughput. Consider the hypothetical case introduced in Section 5.2 for which 1% of the LNG delivered to the fueling station is vented to the atmosphere. In this case, the quantity of LNG delivered to the station must be increased by 1/0.99 in order to maintain the same LNG vehicle refueling throughput. Therefore, vapor venting at the station increases all WTT criteria pollutant emissions by a factor of 1/0.99, but the TTW criteria emissions are unaffected. For both diesel fuel and all credible LNG fuel pathways, the WTT portion of WTW criteria pollutant emissions are quite small relative to the TTW portion. For example,

28 PM is usually subdivided into particle size ranges (e.g., P M 10 , P M 2.5 ), but this distinction is unnecessary for the purpose of this discussion.56

Table 4, which is abstracted from Reference 5 8 , lists the CA - GREE T - predicted WTT, TTW, and WTW criteria pollutant emissions, and the TTW/WTW ratio, for the NA - LNG CE pathway defined in Section 5.2. Urban criteria pollution emissions are tabulated in tons per year for a heavy - duty LNG truck that averages 40,0 00 miles/year, has a 10 - year lifetime, and a fuel economy of 5 miles/dge. Other analysis assumptions are detailed in Reference 58

.

Criteria Pollutant WTT

(tons/yr) TTW

(tons/yr) WTW

(tons/yr) WTT

WTW ROG 0.0002 0.008 0.008 0.024 CO 0.0004 0.048 0.048 0.0 08 NO x 0.0009 0.061 0.062 0.015 PM 2.5 0.00002 0.002 0.002 0.010 Table 4. Calculated criteria pollutant emissions for a heavy - duty LNG truck and a

North American natural gas to California liquefier and fueling station pathway (from Reference 58 ). Table 4 shows that, for this representative scenario, the WTT criteria pollutant emissions (the only ones affected by station venting) are all much less than TTW emissions (which are not affected by station venting). Consider NO x emissions, for example. Based on the numbers in Table 4 , a 1% increase in atmospheric venting at a LNG fueling station would produce an overall WTW increase in NO x emissions of approximately 0.015%. Changes this small should not be interpreted quantitatively because they are beyond the pr ecision of the model and input assumptions. The overall conclusion is that atmospheric venting of vapor at LNG fueling stations can have a substantial effect on WTW GHG emissions, but it has a negligible effect on WTW criteria pollutant emissions.57 Secti on 6 CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusions How much fuel is lost due to atmospheric venting of vapor from LNG fueling stations, LNG tank trucks, and LNG vehicles? The answer is that nobody knows. There have been no documented attempts to directly me asure venting losses. Indirect measurement by subtracting the quantity of LNG dispensed to vehicles from the quantity of LNG delivered to the station has not yielded definitive data because few LNG stations perform this mass balance calculation and because the calculation is strongly affected by “the difference between two big numbers ” inaccuracies. A study conducted in 1995 reported that, “75% of the stations surveyed vent excess vapors to the atmosphere” (Reference 8 ), but the survey provided no informati on regarding the quantity of vapor vented. Also, since 1995, LNG fueling station designs and operating practices have advanced substantially, and average per - station utilization has increased, both of which reduce venting. There ha ve also been suggestions that, when comparing the fuel consumption of otherwise - similar CNG and LNG heavy - duty vehicles, the CNG vehicles appear to have slightly superior fuel economy, and the difference may be due to LNG vapor venting. But this information is anecdotal, undocumen ted, and not validated. Finally, expert opinions regarding the extent of vapor venting associated with the LNG transportation fuel infrastructure range from “negligible” to “substantial.” While the magnitude of LN G fueling station vapor venting is quantita tively uncertain, and it is clear that the GHG effects are infinitesimal relative to dozens of other sources including cow flatulence (Section 1.2), there is general agreement that it i s prudent to design and operate LNG vehicle fueling stations so as to a void or minimize atmospheric venting of vapor. To this end, this report has sought to document what is known about this subject, and some recommendations are summarized in the following subsections. Recommended LNG fueling station design and operating prac tices to avoid or minimize venting are listed in Section 6.2. Some areas of research and development that might result in technologies that help to reduce atmospheric venting of LNG transportation fuel vapors are suggested in section 6.3. 6.2 Best Practice Re commendations 29

 While this is often easier said than done, project developers should ensure that LNG fueling station capacities are well matched with LNG vehicle fleet characteristics so as to minimize heat input accumulation and stored LNG saturation press ure increases that might precipitate atmospheric venting. For example, the LNG storage capacity should be consistent with the anticipated LNG dispensing throughput to 29 These recommendat ions are based on the use of current LNG fueling infrastructure technology.58

enable full LNG tank truck deliveries that are frequent enough to avoid substantial stora ge tank saturation pressure increases between deliveries.

 If the fueling station utilization is anticipated to be low (e.g., less than approximately 2,000 LNG gallons per day, although this threshold can vary substantially depending on specific conditions ), station planners should consider one of the following options:

– If anticipated station utilization is very low and not anticipated to increase: Arrange to refuel vehicles at a nearby LNG station. If this is not possible, reconsider the use of LNG - fueled vehicles.

– If station utilization is low and not anticipated to increase: Install a small capacity permanent station (e.g., skid - mounted) and plan for partial - load LNG tank truck deliveries.

– If station utilization is initially low but anticipated to increas e as more LNG vehicles are deployed: Consider temporary use of a mobile or small skid mounted LNG fueling facility while a permanent LNG station is being planned and installed.

– If initial utilization is low or highly sporadic, and/or future utilization is uncertain: Consider installing a LNG fueling station with one of the excess vapor recovery and utilization options discussed in Section 4.

 In almost all cases, LNG/LCNG fueling station plans should include equipment to compress and dispense excess vapor as CNG, because the incremental cost of this equipment is relatively low and this is a proven effective vapor management method.

 Plans for LNG tank truck delivery and offloading procedures should be carefully developed and agreed on by the station owner/oper ator, the LNG supply contractor, and the station designer. A key goal of these plans should be minimization of any atmospheric venting of vapor from the station or tank truck during LNG deliveries.

 LNG fueling station designs should include a pump to trans fer LNG from tank trucks to the station storage tank. The pump and transfer system should be designed to minimize any vapor generation and/or station heat input required to prime the transfer pump. The station dispensing pump can be used for tank truck off loading, or a separate dedicated offloading pump can be used if the time required for offloading is a critical factor.

 Pressure transfer alone (i.e., without a pump) should never be used to transfer LNG from a tank truck to a fueling station storage tank.

 Never expedite LNG tank truck deliveries by first blowing down the station storage tank (i.e., venting vapor to the atmosphere) to reduce its pressure.

 LNG station designs and tank truck delivery procedures should employ a two - line (LNG delivery and vapor return to the tank truck ullage) transfer system.59

 LNG station designs and tank truck offloading procedures should be such as to avoid or minimize any required vapor venting from the tank truck after transferring LNG to the station (e.g., due to a pressur e build needed to expedite transfer pump priming).

 Personnel familiar with LNG fueling station operations should participate in LNG vehicle procurement. For LNG vehicles not equipped with fuel pumps, engine fuel supply pressure requirements should be taken into consideration. LNG vehicle producers should ensure that the vehicle LNG fuel tank and system are designed and installed so as to minimize the pressure drop from the fuel tank to the engine.

 LNG fueling station planners and designers should give care ful consideration to the best way to dispense fuel at the saturation conditions required by the vehicles being refueled. If “conditioned” LNG dispensing is required, the choice between bulk and on - the - fly conditioning should consider the latest demonstrate d technologies, reliabilit y, initial and operating costs, a nd effects on potential vapor venting.

 If the LNG fueling station is anticipated to refuel vehicles that do and don’t require conditioned LNG, station planners and designers should carefully consid er the tradeoffs (especially with respect to potential vapor venting) among: 1. Use of saturation - on - the - fly conditioning 2. Use of separate LNG storage tanks and dispensers for conditioned and non conditioned LNG 3. Dispens ing conditioned LNG to all types of LNG v ehicles

 Never manage LNG fueling station storage tank pressure by constantly venting vapor through a back - pressure regulator.

 LNG station designs should ensure that the initial quantity of fuel dispensed into vehicle fuel tanks is liquid phase and at the lowest practical temperature. LNG stations should be planned and designed to minimize heat addition to stored LNG and any vapor generation associated with the dispenser cool - down process.

 LNG station planners and designers should carefully balance the nee d for a dispensing pump with a high flow capacity (e.g., to expedite vehicle refueling and possible bulk conditioning) and/or a high pressure ratio capability (e.g., to ensure that LNG can be pumped into vehicle fuel tanks initially at a high pressure) wit h the need to minimize heat input to the stored LNG.

 LNG vehicle fuel tanks should never be vented to the atmosphere. If vapor must be offloaded to enable refueling, the vapor should be recovered and routed to the station LNG storage tank.60

6.3 Research and De velopment Recommendations

 A research effort to accurately quantify the extent of atmospheric venting associated with the LNG vehicle fuel infrastructure would help to resolve uncertainties regarding whether this is or is not a serious issue.

 Successful and cost - effective alternatives to bulk conditioning as a method for addressing SING (and some DING) engine fuel supply pressure requirements would provide multiple benefits possibly including increased LNG storage capacities, enhanced station flexibility, le ss atmospheric venting, and improved economics. Potential R&D areas in this regard include:

– On - vehicle fuel pressure increasing technologies

– Low - cost on - the - fly conditioning technologies

– New approaches to LNG vehicle fuel systems and/or refueling

 If vehicl es that require refueling with conditioned LNG continue to operate for many years into the future, better strategies to enable one LNG fueling station to economically dispense LNG at the various saturation conditions best suited to different types of LNG v ehicles would be highly beneficial.

 Experience with LNG fueling station excess vapor recovery and utilization methods is minimal and not well documented. A better understanding of the effectiveness and costs of various options (especially those involving sale of natural gas or electricity to local utilities) would benefit planners and designers of future LNG stations. Implementation of trial projects at LNG stations could provide this information.

 Improved technologies in the related areas of net dispensed LNG fuel metering and dispenser cool - down systems (e.g., to ensure that cold liquid is initially dispensed into vehicles with no vapor venting and minimal heat rejected to stored LNG) would significantly improve LNG fueling station functionality.

 LNG fuel ing stations and vehicles could benefit from cost - effective pump/compressor technologies that can receive natural gas (NG) liquid - vapor mixtures at their suction and discharge higher - pressure LNG (saturated or subcooled) or supercritical NG. Such technolog ies, which have seen marginal success in the past, could also enable less atmospheric venting of vapor.

 Transferring LNG from tank trucks to fueling stations may be one area where it is impractical to completely eliminate all atmospheric vapor venting thro ugh prudent use of current equipment. This dilemma might benefit from objective R&D to reconsider the best overall LNG vehicle fuel infrastructure strategy (e.g.: Should the transfer pump be at the fueling station or on the tank truck trailer?).

 The subjec ts of NGV fuel composition requirements, fuel composition effects on NG engine emissions and fuel consumption, CARB CNG composition specifications, LDC NG composition rules, composition specifications in LNG fuel supply contracts,61

and LNG enrichment effect s associated with venting are all intertwined in a bewildering fashion. Any R&D that helps to unravel these issues and facilitate broadly supported NG vehicle fuel composition specifications (at all levels) would be highly beneficial.6263 REFERENCES 1. Personal communication, Erik Neandross, Gladstein, Neandross & Associates, February, 2010. 2. State by State Dairy Cow Emissions ( http://milk.procon.org/view.resource.php?resourceID=001154 ) 3. Gibson, C. , “Some Recent LNG Ve hicle Developments,” Cryogenic Society of America Symposium and exhibition, Los Angeles, CA, Oct. 2 - 4, 1973 (also published in Applications of Cryogenic Technology, Vol. 6, Scholium, Int’nl., Flushing, NY). 4. AGA LNG Information Book , 1981 , Third Printing, C atalog No. X00981, American gas Association, Arlington, VA, 1981. 5. Jayaraman, V., “Fueling Station design: The Roadway Project ,” presented at the LNG as a Transportation Fuel SAE TOPTEC Meeting, Houston Texas, Jan. 25 - 27, 1993. 6. Beale, J., “LNG Fueling Stati on Overview,” presented at the LNG as a Transportation Fuel SAE TOPTEC Meeting, Houston Texas, Jan. 25 - 27, 1993. 7. Powars, C., Moyer, C., and Lowell, D., “LNG Vehicle Technology, Economics, and Safety Assessment,” GRI - 94/0051, Feb., 1994. 8. Midgett, D., “Best Available Practices for LNG Fueling of Fleet Vehicles,” GRI 96/0180, Feb., 1996. 9. Bechtold, R., and Gibbs, J., “The Maryland Mass Transit Administration Demonstration of Liquefied Natural Gas Buses,” EA Engineering, Science, and Technology, Inc. Final Repor t for Contract MTA - 3 - 45 - 1, Oct., 1996. 10. State of Utah Office of Energy Services, Jack B. Kelley, Inc., Bruderly Engineering Services, Inc., and Cryenco sciences, Inc., “Liquefied Natural Gas as a T ransportation Fuel for Heavy - Duty Trucks, Vol. 1” NREL/SR - 54 0 - 23094, Dec., 1997. 11. O’Brien, J., and Siahpush, A., “Investigation of Low - Cost LNG Vehicle Fuel Tank C oncepts,” GRI - 98/0015, Jan., 1998. 12. Trucking Research Institute and Detroit Diesel Corp., “Development of LNG Powered heavy - Duty Trucks in C ommercial Hauli ng.,” NREL/SR - 540 - 25154, Dec., 1998. 13. Wegrzyn, J. , Litzke, W., and Gurevich, M., “Natural G as as a F uel Option for Heavy Vehicles,” SAE 1999 - 01 - 2248 , presented at the SAE Government/Industry Meeting, Washington, D.C., April 26 - 28, 1999.64

14. Moore, J, “Developme nt of a Liquefied to Compressed Natural Gas (LCNG) Fueling Station,” CVI Division of Chart Industries Final Report for Contract DE AC02 - 99CH10984, June, 1999. 15. Candler, K., Norton, P, and Clark, N., “Raley’s LNG Truck fleet: Final Results’” NREL/BR - 540 - 2767 8, May, 2000. 16. Wegrzyn, J., and Gurevich, M., “Liquefied Natural Gas for Trucks and B uses,” SAE 200 - 01 - 2210, presented at the SAE Government/Industry Meeting, Washington, D.C., June 19 - 21, 2000. 17. Blue Fuels Group L.P., “Implementation of Advanced LCNG Fuelin g Infrastructure in Texas Along the I - 35 / NFTA Clean Corridor Project,” Final Report for DOE Contract DE - AC02 - 99CH11012, May, 2001. 18. “Liquefied Natural Gas for Heavy - Duty Transportation,” Arthur D. Little Report FR 01 - 101 for the Gas Technology Institute a nd Brookhaven National Laboratory, May, 2001. 19. “Wiens, J., Powars, C., and Pope, G., “LNG Vehicle Fuel Pressure Strategy Alternatives,” SAE 2001 - 01 - 1919, presented at the International Fuels & Lubricants Meeting, Orlando, Florida, May, 2001. 20. Stephens, D., H erridge, J., and Gifford, M. , “Refer ence Guide for Integration of Natural Gas Vehicle Fuel Systems, “ Battelle report for GRI, DOE, SCAQMD, and Transport Canada, GRI - 02/0013, January, 2002. 21. Candler, K., Gifford, M., and Carpenter, B., ”Resource Guide for Heavy - Duty LNG Vehicles, Infrastructure, and Support Operations,” GRI - 02/0105, March, 2002. 22. Bach, R., “Omnitrans LNG - to - CNG R efueling Station Success,” APTA Bus and Paratransit Conference, Milwaukee, Wisconsin, May 5, 2003. 23. Wegrzyn, J, “Processing, Transpo rtation, and S torage of LNG,” presented at the Natural Gas Vehicle Technology Forum, Albany, New York, Sept. 9 - 10, 2003. 24. “The Transit Bus Niche Market for Alternative Fuels, Module 4: Overview of L iquefied Natural gas as a Transit Bus fuel,” part of the DO E Clean Cities Coordinator Toolkit, prepared by TIAX LLC, Dec., 2003. 25. Chen, Q., Wegrzyn, J., and Prasad, V., “Analysis of Temperature and Pressure Changes in Liquefied Natural Gas (LNG) Cryogenic Tanks,” Cryogenics (Elsevier), Vol. 44, Issue 10, Oct. 2004. 26. Mackey, M. “LNG and Hydrogen Fueling Stations,” presented at the Southern California Clean Vehicle Expo, Ontario, California, October 9 - 11, 2006. 27. Roche, G., “Fueling Infrastructure for Natural Gas Trucks,” presentation at the Faster Freight

– Cleaner Air California 2009 Conference, Long Beach, CA, March 23 - 25, 2009. 28. Timmerhaus, K.D., and Flynn, T.M., Cryogenic Process Engineering , Plenum Press, NY, 1989.65

29. SAE J2343, Recommended Practice for LNG Medium and Heavy - Duty Powered Vehicles , July, 2008, SAE Interna tional, Warrendale, Pennsylvania. 30. Duggal, V., et al, “Development of the High - Pressure, Direct - Injected, Ultra Low NO x Natural Gas Engine: Final Report,” NREL/SR - 540 - 35911, May, 2004. 31. Chart Industries, NexGen Fueling , On - Board Fueling systems ( http://www.n exgenfueling.com/p_onboard.html ) 32. Westport, Cryogenic Storage & Delivery ( http://www.westport.com/business_dev/cryogenic.php ) 33. Powars, C., and Derbidge, T., “LNG Vehicle High - Pressure Fuel System and ‘Cold Energy’ Utilization,” DOE/ER/82468 - 2, March, 2001. 34. H arper, G., and Powars, C., “Advanced LNG Onboard Storage System (ALOSS),” Cummins Westport Inc. Final Report for DOE Contract DE - FC36 - 02CH11138, October, 2003. 35. Clean Vehicle Education F oundation, Natural Gas Vehicle Codes and Standards ( http://www.cleanveh icle.org/technology/image/code.pdf ) 36. Chart Industries, NexGen Fueling, Codes, Standards & Advisories ( http://www.nexgenfueling.com/t_codes.html ) 37. Alloy Custom Products, Inc., 13,000 - gallon LNG transport trailer Model ACP ST - 13 specifications ( http://www.allo ycustomproducts.com/uploads/ST 13%20SALES%20INFO%201 - 2 - 06.pdf ) 38. “MVE’s New LNG Station Design,” LNG Express (Zeus Development Corp.), Vol. III, No. 1, Jan/Feb, 1993. 39. “Improvement in LNG Storage Tanks, Full - Scale Demonstration Testing,” Program Status R eport prepared by Cryogenic Fuels, Inc., for Brookhaven National Laboratory Contract 723418, Nov., 1999. 40. Wilding, B., “Natural Gas Vehicle Program, INEEL,” presented at the Zeus Development Corp. LNG: Prospecting Downstream Markets Conference, Colorado Springs, Colorado, March 23 - 25, 1998. 41. Lenz, D., “Onboard Low - Pressure Fuel Delivery Systems,” presented at the LNG as a Transportation Fuel SAE TOPTEC Meeting, Houston Texas, Jan. 25 - 27, 1993. 42. Miller, S., “LNG Usage in 2 - Stroke Engines,” presented at the LNG as a Transportation Fuel SAE TOPTEC Meeting, Houston Texas, Jan. 25 - 27, 1993. 43. “INL Technologies: LCNG Fueling Station, A ‘Natural’ Choice,” ( http://www.inl.gov/lng/factsheets/fuelingstation.pdf ) 44. Flaring and Venting Reduction & Natural Gas Utilisation Forum, Ams terdam, Dec. 3 - 5, 2008 ( http://www.flaringreductionforum.org/ ) 45. The EPA Natural Gas STAR Program ( http://www.epa.gov/gasstar/ ) 46. The EPA Landfill Methane Outreach Program ( http://www.epa.gov/lmop/ )66

47. The Methane to Markets Partnership , an international program steered by the US EPA ( http://www.methanetomarkets.org/ ) 48. Vapor Management Option S tudy , by Jim Lewis, PTL, which is part of unpublished Barstow, California, LNG fueling station project planning materials provided by Erik Neandross, Gladstein, Nea n dross & A ssociates, Jan., 2010. 49. Personal communication. Dennis Elefante, Orange County Transportation Authority , Garden Grove, California, March, 2002. 50. Kountz, K., et al, “Development of a Small - Scale Natural Gas Liquefier,” GTI 03/0101, April, 2003. 51. GTI Small - Scal e Liquefier Development ( http://media.godashboard.com/gti/1ResearchCap/1_2EnergySyst/LiquefierOvervie wBrief1205new.pdf ) 52. Millang, H., “Possibilities and Barriers with Small - Scale LNG,” presented at the Flaring and Venting Reduction & Natural Gas Utilisation Forum, Amsterdam, Dec. 3 - 5, 2008

. 53. Kunert, S., and Larsen, O., “Small is Beautiful

– Mini LNG Concept,” Hamworthy Gas Systems AS presentation ( http://www.dge - wittenberg.com/vortraege/ Small_is_beautiful_Hamworthy_MiniLNG_Concept_June_2008.pdf ) 54. “LNG

– L/CN G Fueling Station,” charts distributed by The Hanover Company at the Southwest Transportation Agency Caruthers, California, LNG/LCNG station inauguration ceremony, April 1, 2005. 55. Argonne National Laboratory, The Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) Model , March , 2009 ( http://www.transportation.anl.gov/modeling_simulation/GREET/index.html ) 56. TIAX LLC, “Full Fuel Cycle Assessment: Well - to - Wheels Energy Inputs, Emissions, and Water Impacts,” CEC - 600 - 2007 - 004 - REV, Augu st, 2007. 57. California Air Resources Board, “Detailed California - Modified GREET Pathway for Liquefied Natural Gas (LNG) from North America and Remote Natural Gas Sources,” Version 1.0, July, 2009 (http://www.arb.ca.gov/fuels/lcfs/092309lcfs_lng.pdf) 58. Rosenfel d, J., and Jackson, M., “Life - Cycle Cost Estimator and Pollutant Emissions Estimator

– Greenhouse Gas and Criteria Pollutant Estimator,” TIAX LLC report prepared for Westport Innovations Inc., March, 2008. ( http://www.westport.com/pdf/GHG_and_Criteria_Pol lutant_Emissions_Estimator.p df )A - 1 

### Appendix A LN G Tank Pressure, Temperature, Ullage Fraction, Heat Leak, Hold Time, and Venting Rate RelationshipsA - 2

A - 1. Purpose LNG tanks at fuel stations, on LNG - fueled vehicles, and LNG tank trucks, are vacuum ja cketed cryogenic vessels designed to minimize the rate of heat transfer from the environment to the LNG (the “heat leak”). Common cryogenic tank design features are described in many articles and textbooks (e.g., Ref erence A - 1). If no product is withdrawn from or added to the tank, the heat transferred to the LNG increases its internal energy, pressure, and temperature. The mass and volume ratios of the liquid and vapor phases in the tank also shift in subtle ways. The tank volume occupied by the vapor phas e is referred to as the ullage. If and when the ullage decreases to zero, the tank is said to be liquid full, which is a situation that should be avoided for safety reasons. Safe practice , as well as various codes and standards , require s LNG tanks to be e quipped with a pressure relief device (PRD). The PRD must discharge (usually vent) some of the tank contents when the tank pressure increases to no greater than the tank maximum allowable working pressure (MAWP). Because it is much safer to vent vapor than liquid A - 1 (and a liquid - full condition is to be avoided as previously discussed), cryogenic tanks are constructed so that the PRD communicates with the ullage. Two closely related figures - of - merit that depend on the tank’s heat leak characteristics are h old time and normal evaporation rate (NER). Hold time (also called time - to - vent) is the elapsed time between when the tank is filled and when it vents (because the pressure increases to the PRD set point) if no product is withdrawn. There is some ambiguity to published hold times if the filling conditions (i.e., cryogen, saturation state, ullage fraction) are not specified. NER is the rate of product withdrawal that maintains the remaining product at a constant saturation pressure and temperature. NER is al most always expressed as a percent per day (%/day), where the percentage is defined as the vapor mass withdrawn daily relative to the full tank product mass. Published NER numbers can also be ambiguous if the cryogen and withdrawal pressure ( i.e., atmosphe ric or PRD pressure?) are not specified. With certain simplifying but generally applicable assumptions, mass and energy conservation laws can be combined with the cryogen’s thermodynamic properties to calculate important relations involving the tank pressu re, temperature, ullage fraction, heat leak, hold time, and venting rate (or NER). The purpose of this appendix is to summarize the pertinent assumptions and equations and illustrate how these are applie d to calculate various LNG tank characteristics. The more general discussion in this appendix provides a technical basis for the example graphs shown and described in Section 2 of the main text.

A - 1 It is also more effective to vent vapor. Because the enthalpy of saturated vapor is greater than that of saturated liquid, removal of a given mass of vapor decreases the tank p ressure and temperature more than removal of the same quantity of liquid.A - 3

A - 2. Assumptions The LNG is all methane

.

This assumption simplifies calculations because it avoids highly comp osition dependent multi - component multi - phase calculations. It produces results that are accurately applicable to most currently used LNG transportation fuels. Implications of LNG components other than methane are discussed in Appendix B. When a tank cont ains both liquid and vapor phases, they are in thermodynamic equilibrium, i.e., both phases are saturated

.

If there is negligible stratification (see below), this is a reasonable assumption if the tank contents have “settled” for a period of time, but it is not applicable to tanks that have recently received a pressure - build to create a “false head.” The liquid in the tank is not significantly stratified, i.e., all the liquid has the same temperature and density

.

This is a reasonable assumption for smaller tanks but not for large tanks that have been left undisturbed for a substantial time (see Ref erence A - 1 for details)

. Methane’s equation of state and thermodynamic properties are accurately modeled by relations published by the National Institute of Stand ards and Technology (NIST)

.

See Ref erence A - 2

. A - 3. Equations The equations needed to relate LNG tank pressure, temperature, ullage fraction, heat leak, hold time, and venting rate are: conservation of mass and energy, a tank volume equation, and the meth ane’s thermodynamic properties as noted in Section A - 2.

T v L 

### V V V : vapor and liquid by occupied volume Tank h

m Q dt dU :

on conservati Energy m dt dM :

on conservati Mass

     

  A - 4 

### k tan total denotes T phase

vapor denotes v phase

liquid denotes L : Subscripts time t density ρ energy

ernal int unit u k tan of out flowing ) liquid or vapor ( fluid of enthalpy unit h volume V k tan o int transfer heat of rate Q k tan of out rate flow mass m u V ρ u V ρ k tan in energy ernal int total U V ρ V ρ k tan in mass fluid total M : Where

v v v L L L v v L L



###             

 

Subject to the assumptions listed in Section A - 2, these equations can be combined with methane’s equation of s tate and thermodynamic properties to cal culate various relations among pressure, temperature, ullage fraction, heat leak, and venting rates for vessels containing natural gas liquid and vapor. The specific mathematical manipulations are straightforward and will not be detailed here. These equati ons can be combined to express the pressure rise rate (and ullage fraction change) of a tank with a known heat leak rate and no outflow or inflow, for example. Or, the vapor outflow rate can be calculated for a tank venting at a constant pressure. This rel ation can be applied to calculate the heat leak rate from a tank normal evaporation rate (NER) specification. This venting or “boil - off” rate is sometimes estimated as the heat leak rate divided by the fluid heat - of - vaporization at the venting pressure (e. g., Ref erence A - 3): 

### )

h h ( / Q m L

v 

###     Examination of the prior equations shows that this relation is an approximation obtained by assuming that the vent rate is equal to the liquid vaporization rate (i.e., neglecting the mass accumulation in the incr easing ullage space) and assuming that the internal energy and enthalpy of the liquid phase are equal. Both of these effects are usually small, and the error associated with this approximate equation is usually les s than 1%.A - 5

A - 4. LNG Quantitative Measure ment and “LNG Gallons” Liquid transportation fuels such as gasoline and diesel fuel are traditionally quantified in terms of gallons (or liters in most non - US countries), which is a volume measurement. For cryogenic liquids such as LNG, which are usually t ransported, stored, and dispensed at or near saturation, the liquid density depends strongly on temperature and pressure, and so the mass in a gallon at one condition may be substantially different from the mass in a gallon at another condition. For exampl e, for high - purity LNG, a gallon at 20 psig weighs 3.38 Lbm, but a gallon at 120 psig weighs 3.03 Lbm A - 2 (i.e.,10.4% less). For this reason, LNG supply contracts and pricing is usually based on fuel mass rather than volume. However, there is still a stron g preference to discuss LNG vehicle fuel quantities in terms of gallons. An approach that minimizes the ambiguity of this unit of measurement is to define a “LNG gallon” as the mass of fuel in one gallon of all methane LNG saturated at one atmosphere of ab solute pressure (14.7 psia), which is 3.53 Lbm. We accept this convention so that, in this document, regardless of pressure and temperature: One “LNG gallon” means 3.53 pounds of LNG

A - 5. Example LNG Property and Quantity Graphs The following figures are graphs that illustrate example relationships among LNG properties based on the assumptions discussed in Section A - 2 and equations listed in Section A - 3. These examples are selected to be applicable to issues of interest for LNG fueling stations. The pe rtinent application, assumptions, and implications are summarized in the captions beneath each graph.

A - 2 Graphs of LNG density are in Appendix B.A - 6 0 20 40 60 80 100 120 140 160 180 200

0 10 20 30 40 50 60 70 Elapsed Time ( days )SaturationPressure(psig) A: LNG tank heat leak only

B: LNG tank heat leak and LNG dispensing (including heat from pump & sump)

(Initial saturation pressure = 20 psig) ( 175 psig PRD)

C: LNG tank heat leak and LNG dispensing (including heat from pump & sump) and vapor withdrawal (e.g., for CNG refueling) (venting at 64 days)

(all liquid consumed at 24 days)

(all liquid consumed at 23 days)

Figure A - 1. Illustration of the saturation pressure rise of LNG stored in a 15,000 - gallon

LNG tank at a station with low utilization. Three differe nt liquid and vapor withdrawal schedules are shown. For all cases, key assumptions are: LNG is 100% methane, all tank contents are saturated at the same pressure and temperature, tank is filled with 12,295 gallons of LNG (= 43,347 pounds of LNG) at an init ial saturation pressure of 20 psig (which provides a 14.5% ullage, which is the minimum ullage to ensure that the tank is not liquid - full at the assumed 175 - psig PRD pressure). A. No liquid or vapor withdrawal; heat leak = 35,000 Btu/day (which corresponds to NER = 0.3%/day of methane at 1 Atm.) B. Time - averaged LNG dispensing rate = 500 gallons/day = 1,763 Lbm/day (e.g., refueling 5 vehicles/day X 100 LNG gal/vehicle), heat leak = 35,000 Btu/day (as in A) plus 10,000 Btu/day associated with LNG pump and sump ope ration. C. Time - average LNG dispensing rate = 500 gallons/day = 1,763 Lbm/day (as in B), heat leak = 35,000 Btu/day plus 10,000 Btu/day associated with LNG pump and sump operation (as in B), plus vapor withdrawal (e.g., compressed into storage vessels for CNG vehicle refueling) of 4,158 SCF/day (which is 176 Lbm/day, i.e., 10% of LNG withdrawal).A - 7 0 20 40 60 80 100 120 140 160 180 200

0 5 10 15 20 25 30 35 Elapsed Time ( days )SaturationPressure(psig) ( 175 psig PRD)

(Initial saturation pressure = 120 psig) B: LNG tank heat leak and LNG dispensing (including heat from pump & sump) A: LNG tank heat leak only

C: LNG tank heat leak and LNG dispensing (including heat from pump & sump) and vapor withdrawal (e.g., for CNG refueling) (venting at 16 days) (venting at 14 days)

(all liquid consumed at 23 days)

Figure A - 2

. Illustration of the saturation pressure rise of LNG stored in a 15,000 - gallon

LNG tank at a station with low utilization. Three different liquid and vap or withdrawal schedules are shown. For all cases, key assumptions are: LNG is 100% methane, all tank contents are saturated at the same pressure and temperature, tank is filled with 12,295 gallons of LNG (= 43,347 pounds of LNG) at an initial saturation pr essure of 20 psig (which provides a 14.5% ullage, which is the minimum ullage to ensure that the tank is not liquid - full at the assumed 175 - psig PRD pressure), and tank contents are then “conditioned” to a saturation pressure of 120 psig. A. No liquid or vapo r withdrawal; heat leak = 35,000 Btu/day (which corresponds to NER = 0.3%/day of methane at 1 Atm.) B. Time - average LNG dispensing rate = 500 gallons/day = 1,763 Lbm/day (e.g., refueling 5 vehicles/day X 100 LNG gal/vehicle), heat leak = 35,000 Btu/day (as in A) plus 10,000 Btu/day associated with LNG pump and sump operation. C. Time - average LNG dispensing rate = 500 gallons/day = 1,763 Lbm/day (as in B), heat leak = 35,000 Btu/day plus 10,000 Btu/day associated with LNG pump and sump operation (as in B), plus va por withdrawal (e.g., compressed into storage vessels for CNG vehicle refueling) of 4,158 SCF/day (which 176 Lbm/day, i.e., 10% of LNG withdrawal).A - 8 0 20 40 60 80 100 120 140 160

0 2 4 6 8 10 12 Elapsed Time ( days )SaturationPressure(psig)

0 5,000 10,000 15,000 20,000 25,000LNGinTank("LNGgallons") 1,000 LNG gallons dispensed between 5 PM and 9 PM each day (10-day cycle repeats)

10,000 LNG gallons added to storage tank every 10 days (12 noon - 1 PM) LNG in storage tank conditioned to 120 psig (1 PM - 3 PM)

Saturation pressure

Tank contents

Figure A - 3

. Hypothetical example LNG fueling station with 15,000 gallon storage tank,

showing the saturati on pressure and LNG content variation between LNG deliveries. Conditions and assumptions:

 Station utilization: 1,000 LNG gallons dispensed each day between 5 PM and 9 PM (e.g., 10 vehicles refueled daily with 100 gallons)

 Tank truck deliveries: 10,000 LNG gallons (saturated at 20 psig) transferred to station between 12 noon and 1 PM every 10 days

 Storage tank contains 2,564 LNG gallons (saturated at 143 psig) just prior to deliveries, so that ullage fraction is approximately 10% after delivery

 Between 1 PM and 3 PM following each delivery, the LNG is conditioned to raise its saturation pressure to 120 psig

 Tank heat leak = 35,000 Btu/day (which corresponds to NER = 0.3%/day of methane at 1 Atm) plus 10,000 Btu/day associated with LNG pump and sump operation

 LNG is 100% methane and all tank contents are saturated at the same pressure and temperatureA - 9 0 20 40 60 80 100 120 140 160 180

0 2 4 6 8 10 12 14 16 18 Elapsed Time ( days )SaturationPressure(psig)

0 5,000 10,000 15,000 20,000 25,000 30,000LNGinTank("LNGgallons") 1,000 LNG gal/day dispensed during 4-hr window 500 LNG gal/day dispensed during 2-hr window (175 psi PRD)

Conditioning to 120 psig 10,000 gal added to tank

For this scenario, LNG storage tank refilled every 10 days, and cycle repeats For this scenario, LNG storage tank starts venting in ~15 days

Figure A - 4

. Effect of LNG fueling station utilization on 15,000 - gallon storage tank

saturation pressure and LNG content variation. Key assumptions are identic al to those listed for Figure A - 3. The 1,000 LNG gallon/day utilization scenario results in a 143 - psig saturation pressure and 2,564 - gallon LNG residual just prior to a tank truck delivering 10,000 gallons every 10 days (as in Figure A - 3). The 500 LNG gall ons/day utilization scenario (e.g., 5 vehicles refueled in 2 hours every day with 100 gallons) would result in the tank saturation pressure reaching the assumed 175 - psig PRD pressure in 15 days with 5,064 gallons remaining in the storage tank. This low - uti lization scenario would require a smaller storage tank with more frequent deliveries of lower LNG quantities (or some form of active vapor management) to avoid atmospheric venting.A - 10 0 50 100 150 200 250

0 5 10 15 20 25 30 35 40 45 50 Elapsed Time (days)SaturationPrassure(psig)&Ullage(%)

0 1 2 3 4 5VentingRate(LNGgallonsequivalent/day) (230 psi PRD) Pressure

Ullage % 5 days (brief increase in venting as liquid is exhausted and tank warms to ambient temperature)

Venting rate

0 50 100 150 200 250

0 5 10 15 20 25 30 35 40 45 50 Elapsed Time (days)SaturationPrassure(psig)&Ullage(%)

0 1 2 3 4 5VentingRate(LNGgallonsequivalent/day) (230 psi PRD) Pressure

Ullage % 13.4 days (brief increase in venting as liquid is exhausted and tank warms to ambient temperature)

Venting rate

Figure A - 5 . Pressure rise rate, time - to - vent, venting rate, and u llage fraction variation for

hypothetical 115 - gallon LNG vehicle fuel tank with a 230 - psig PRD that is left dormant (e.g., vehicle not driven). The top graph is for this tank and a 5 - day hold time and a 10% ullage (minimums required by SAE J2343, Reference A - 4) when filled with LNG saturated at 120 psig. This hold time implies a heat leak of 1,485 Btu/day = 18.1 Watts. If no fuel is used, after 5 days, this tank will vent at 8.7 Lbm/day (= 2.5 LNG equivalent gallons / day) until it is empty (assuming ideal PRD behavior). The bottom graph is for the same tank filled with LNG saturated at 20 psig, which requires a 18 % ullage in order to avoid a liquid - full condition at the PRD pressure. This fill condition would provide a hold time of approximately 13 days.A - 11 0 5 10 15 20 25 30 35 40 45 50

0 50 100 150 200 250 Starting and Ending Saturation Pressure ( psig )ProductLoss(%ofstartingliquidmass) Solid curves denote tank initially 10% full Broken curves denote tank initially 50% full

Figure A - 6

. Product loss associated with vapor venting to reduce the saturation

pressure in an LNG tank of any capacity containing both liquid and vapor phases (sometimes referred to as tank “burping”). Example illustrating quantitative use:

 Problem: A 5 0% full 15,000 - gallon LNG tank is vented to reduce its saturation pressure from 200 psig to 100 psig. How much vapor must be vented?

 Solution: Select the broken (50% full) curve that originates from 200 psig. At 100 psig on the horizontal scale, read 14% o n the vertical scale. The vapor mass vented is 0.14 x 21.24 Lbm/ft 3 (saturated liquid density at 200 psig, e.g., from Appendix B) x 0.5 x 15,000 gal ÷ 7.48 gal/ft 3 = 2,980 Lbm. And 2,980 Lbm ÷ 3.53 Lbm/LNG gallon = 844 LNG gallons.

 Answer: 14% of the tank contents must be vented. This is equivalent to 844 LNG gallons.A - 12 76 78 80 82 84 86 88 90 92 94 96 98 100

0 20 40 60 80 100 120 140 160 180 200 220 240 260 Pressure Relief Valve Setting ( psig )MaximumLiquidPercentage(=100-UllagePercentage)020406080100200120140160180 Tank Saturation Pressure After Filling ( psig )

Figure A - 7 . LNG tank maximum liquid filling percentage in order to avoid a liquid - full

condition if no liquid or vapor is withdrawn and the saturation pressure increases to the PRD p ressure. Note that this maximum liquid filling percentage is 100

- the ullage percentage. Example (illustrated by broken - line arrows):

 Problem: A LNG tank with a 230 - psig PRD is to be filled so that its settled saturation pressure is 100 psig after fillin g, and substantial time may lapse before any LNG is withdrawn. To what level can the tank be filled?

 Answer: As indicated by the arrows, the tank should be filled so that LNG occupies no more than 89.7% of its total volume (i.e., A 10.3% vapor ullage). If it is filled with a higher percentage of liquid, it will become liquid - full (0% ullage) before the saturation pressure increases to the PRD pressure. This shortens the hold time and produces potentially dangerous situation.A - 13 0 0.2 0.4 0.6 0.8 1 1.2

0 10 20 30 40 50 60 70 80 90 100 % Liquid by VolumeHeatLeak/(PressureRiseRatexVolume)[Watt-hour/psi-gallon] Saturation pressure:

10 20 30 40 50 100 150 200 230 5 psig

Tank contents follow these curves during pressure build-up

Figure A - 8

. Generalized grap hical representation of the relations among various

quantities of interest for any LNG tank containing a saturated methane liquid and vapor mixture and for which nothing is added or withdrawn. For example, this graph shows the saturation pressure rise rate (and therefore, hold time) for any tank with a known heat leak, volume, and fill condition. It also shows the ullage change with increasing saturation pressure.A - 14

References to Appendix A A - 1 Timmerhaus, K., and Flynn, T. , Cryogenic Process Engineering , Plenum Press, New York A - 2 F riend, D., Ely, J. , and Ingham, H., National Bureau of Standards Technical Note No. 1325, 1989 (see also: same authors, “Thermal Properties of Methane,” J. Phys. Chem., Ref. Data, Vol.18, No. 2, 1989, pp. 583 - 638) A - 3 Chen, Q. , Wegrzyn , J., and Prasad, V., “Analysis of Temperature and Pressure Changes in Liquefied Natural Gas (LNG) Cryogenic Tanks,” Cryogenics, Vol. 44, Issue 10, October 2004, pp. 701 - 709 A - 4 SAE J2343, Recommended Practice for LNG Medium and Heavy - Duty Powered Vehicles , Ju ly , 2008 , SAE International, Warrendale, PAB - 1 

### Appendix B METHANE PHASE DIAGRAMB - 2

This appendix contains graphs illustrating various aspects of methane’s phase diagram. All graphs show the saturated liquid and saturated vapor curves (i.e., corresponding to the temperatures and pressures at which the phases coexist in equilibrium). Quantitative values are based on the thermodynamic property data in Ref erence B - 1.

-350 -300 -250 -200 -150 -100 -50 0 -100 0 100 200 300 400 500 600 700 Pressure ( psig )Temperature(oF) VAPOR REGION

LIQUID REGION Saturation Line

Atmospheric pressure Critical pointB - 3 0 100 200 300 400 500 600 700 800 900 1,000

0 5 10 15 20 25 30 Density ( Lbm / ft 3 )Pressure(psig)

LIQUID REGION LIQUID + VAPOR REGION VAPOR REGION

Saturation Line

SUPERCRITICAL REGION Example isotherm (-180 °F)

-350 -300 -250 -200 -150 -100 -50 0 0 5 10 15 20 25 30 Density ( Lbm / ft 3 )Temperature(oF) VAPOR

LIQUID + VAPOR REGION LIQUID REGION Saturation Line SUPERCRITICAL REGION Example isobar (200 psig)B - 4 -100 0 100 200 300 400 500 600 700 800

-200 -150 -100 -50 0 50 100 150 Enthalpy & Internal Energy ( Btu/Lbm )Pressure(psig) Enthalpy Internal energy

VAPOR

LIQUID Saturation lines

Reference to Appendix B B - 1 Friend, D., Ely, J. , and Ingham, H., National Bureau of Standards Technical Note No. 1325, 1989 (see also: same authors, “Thermal Properties of Methane,” J. Phys. Chem., Ref. Data, Vol.18, No. 2, 1989, pp. 583 - 638)