Strategies for Fueling a Vehicle Equipped to Operate on LNG

Vehicle Fueling

There are two competing strategies for fueling a vehicle equipped to operate on LNG. It is the basic difference in the "fill procedure", that is imposed on the fleet operator by the vehicle fuel tank supplier, that has the greatest impact on (1) tank operating pressure, (2) fuel density, (3) vehicle range and most importantly, (4) the capital, operating and maintenance costs of the LNG fueling facility. A brief review of the two competing strategies is presented in this analysis.

Two Line Fill Procedure

Figure 1 - LNG HelicopterFigure 1

Prior to 1996 the two-line, vent recovery, fill procedure was common practice and was used in all of the early (1967-1984) demonstration projects involving the experimental use of LNG as a vehicular fuel. These programs included light duty vehicles operated by a number of Public Utility Companies, the General Services Administration (GSA), the State of California and the United States Army in both light duty ground vehicles and the US Army, TH-55 Trainer Helicopter. (Ref. Figure 1)

Figure 2 - LNG CarFigure 2

The two-line, vent fill procedure is performed with both the fill (liquid) line and the vapor (vent) line connected to the LNG vehicle fuel tank. A cryogenic pump is normally used to "boost" the fill pressure in order to accommodate the fill rates specified by the tank manufacturer. A simple pressure transfer may also be used if electrical power is not available. The normal station "fill pressure" is therefore, a combination of the pump pressure and the preset operating pressure in the fueling facility storage tank. (Ref. Figure 2)

Figure 3 - LNG VanFigure 3

With both lines connected the vent gas, exiting from the ullage space of the vehicle tank may be maintained at the same saturation temperature, -245°F, as the LNG being transferred into the tank. This "fill procedure" eliminates temperature stratification at the gas/liquid interface inside the vehicle fuel tank by removing the warmer gas and, in effect, sub-cooling the residual LNG during the fueling process. The vent gas is recovered to the fueling facility bulk storage tank and amounts to approximately, 2 %, or approximately two (2) gallons for every 100 gallons of LNG that is transferred. (Ref. Figure 3)

Operational Benefits

Figure 4 - LNG single line fill ProcedureFigure 4

Single Line Fill Procedure

The single line fill procedure is standard practice in the "industrial gas sector" and is used to transfer flammable and non-flammable cryogens, such as nitrogen, oxygen or carbon dioxide into cryogenic containers designed specifically for stationary applications. In 1996 the single line fill procedure was promoted, by the "industrial gas" sector, as a solution to meet the, supposed need, for higher operating pressures of 80 to 100 psi, for the first generation, dedicated natural gas engines. In fact, all spark ignited natural gas engines available at that time operated at 45 to 60 psi (Ref. Figure 4)

However, in the dynamic environment of vehicular operations the pressure will collapse as the cold liquid is sloshed and mixed with the warmer, stratified gas, in the ullage space of the tank. In the initial phase of several early fleet operations using this fill procedure the warmer gas was condensed, and due to vehicle motion and the sloshing effect, the tank pressure would drop to the "saturated pressure" of the transferred LNG, or about 18 psig, (-245° F) and the vehicle could not be operated.

Figure 2 - LNG CarFigure 5

To prevent the collapse of pressure in their vehicle fuel tanks the industrial gas fueling facility provider’s solution was to "pre-condition" the liquid by adding low watt density heat to the LNG in order to increase its saturation pressure to 100 psig, (-200° F). The 100 psig saturation pressure was selected to accommodate the pressure variations that occur when the economizer valve on their vehicle fuel tank is actuated.

The down side to this "fill strategy" is a reduction in vehicle range since the density/energy content of the LNG is reduced by over 22% at the higher temperature, lower density conditions of the fuel. Furthermore, heating the LNG requires additional control valves, heat exchangers, submersible pumps and the associated instrumentation. The end result is a substantial increase in station capital, operating and maintenance costs. (Ref. Figure 5)

Problems & Disadvantages

The following data are from the Pressure Enthalpy Chart for Methane, published by the National Institute of Standards and Technology (NIST). This data is provided, as reference for users in understanding the phase change relationships that occur in LNG with respect to enthalpy, temperature, density, and pressure.

LNG Enthalpy vs Temperature vs Density vs Pressure
Temperature: -260°F -240°F -220°F -200°F
Saturation Pressure: 0 psig 18 psig 50 psig 100 psig
Density: 26.5 lbs/ft3 25.5 lbs/ft3 24.3 lbs/ft3 21.6 lbs/ft3
Enthalpy: 122 btu/lb 138 btu/lb 157 btu/lb 174 btu/lb