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-off Emissions 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 of liquid natural gas (LNG) boil-off emissions, a spreadsheet (LNGANG Venting Calcs.xls) was adapted from another version by Dr. Michael Tschantz of MeadWestVaco. 

The calculations in the spreadsheet include two separate processes that occur in the tank during a 

venting incident. The first process is the expansion of the gas as the pressure in the tank decreases from 

the venting pressure to the pressure that the relief valve closes. The second process is accounting for the 

lower boiling temperature of the 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) of the venting is the highest and the frequency of the venting events are the lowest when the LNG 

tank is full. When the LNG tank is on the emptier side, the frequency of venting events increases but the 

magnitude of emissions 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-off occurs 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 of venting on a hot summer day versus a winter day, nor does it attempt to assess what the LNG 

venting volume and frequency would be if the 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 of Charles A. Powars, St. Croix Research, "Best Practices to Avoid LNG Fueling Station 
Venting 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 of the adsorbent for natural gas (ANG) control material for a potential 

canister for the 115 gallon tank example vehicle boil-off calculation, is calculated on the "Breathing 

Life" worksheet as well. 

All of the densities, heats of vaporization, liquid heat capacity, etc were referenced from Perry's 

Chemical Engineering Handbook, 71

h Edition. It was assumed that the heat load of the vaporized 

methane in the headspace was negligible. 

2