OFFICE OF AND RADIATION June 9, 2015 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY NATIONAL VEHICLE AND FUEL EMISSIONS LABORATORY 2565 PLYMOUTH ROAD ANN ARBOR , Ml 48105-2498

AIR

MEMORANDUM SUBJECT: FROM: TO : Liquid Natural Gas Boil-offEmissions Calculations Constance Hart, Assessment and Standards Division Greenhouse Gas Emissions Standards and Fuel Efficiency Standards for Medium- and Ileavy-Duty Engines and Vehicles- Phase 2- Docket EPA-HQ-OAR-2014-0827 To determine an estimate ofliquid natural gas (LNG) boil-offemissions, a spreadsheet (LNGANG Venting Calcs
.xls) was adapted from another version by Dr. Michael Tschantz ofMeadWestVaco. The calculations in the spreadsheet include two separate processes that occur in the tank during a venting incident. The first process is the expansion ofthe gas as the pressure in the tank decreases from the venting pressure to the pressure that the reliefvalve closes
. The second process is accounting for the lower boiling temperature ofthe LNG as the pressure in the tank decreases, which is associated with additional gas release from the liquid
. The spreadsheet shows that the magnitude (mass per venting event) ofthe venting is the highest and the frequency ofthe venting events are the lowest when the LNG tank is full. When the LNG tank is on the emptier side, the frequency ofventing events increases but the magnitude ofemissions from each venting event decreases. Overall, the average venting rate, based on the assumption that heat input into the LNG tank is constant, is constant over time. When the pressure valve opens and the boil-offoccurs the LNG is cooled therefore keeping the liquid saturated in the tank until the heat builds for the next iteration. This spreadsheet does not attempt to differentiate between the amount ofventing on a hot summer day versus a winter day, nor does it attempt to assess what the LNG venting volume and frequency would be ifthe LNG storage tank experienced some deterioration in insulating qualities. Calculations for a single venting event are found on the worksheet "Cales"
. Inputs are low tank pressure in AS, high tank pressure at which the tank will vent in B5 , tank volume in C5 and the fill % of the tank in 05. The next step is to perform a goal seek by setting the final H,L (enthalpy and liquid level in btu) in cell 05 to the value for initial H, L (enthalpy in btu) shown in E5 by changing the% Tank final fill in F5. Repeating this calculation over and over generates the curves found on the " Breathing Life" worksheet. An example situation for a heavy duty LNG vehicle with a 115 gallon tank was taken from the appendix ofCharles A. Powars, St. Croix Research, "Best Practices to Avoid LNG Fueling StationVenting Losses", June 25 , 2010 report, figure A-5. The heat leak per day from this example is 18
. 1 Watts, and is input on the "Breathing Life" worksheet in cell Fl39. An estimated amount ofthe adsorbent for natural gas (ANG) control material for a potential canister for the 115 gallon tank example vehicle boil-offcalculation, is calculated on the " Breathing Life" worksheet as well. All ofthe densities, heats ofvaporization, liquid heat capacity, etc were referenced from Perry' s Chemical Engineering Handbook, 7 1h Edition. It was assumed that the heat load ofthe vaporized methane in the headspace was negligible.

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