Strategies for Fueling a Vehicle Equipped to Operate on LNG
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
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)
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)
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)
SUB-COOLING: One of the major benefits of the two-line fill procedure is the "sub-cooling" effect on the LNG. During the fueling operation the combined, bulk tank/pump, transfer pressures induce flash boiling of the liquid, thus sub-cooling the LNG to a lower saturation pressure and increasing the density-energy content of the fuel. In addition, the flash boiling provides further assurance that the LNG will remain a single-phase liquid during the fueling of the vehicle tanks.
FILL PRESSURE & FILL RATE: The two-line fill procedure is performed at transfer pressures of 60 to 120 psi with a maximum fill rate of 20 GPM. By comparison, nominal diesel fill rates, for heavy-duty truck fleets are in the 12 to 16 GPM range in captive fleet operations. This fill pressure range will accommodate operating pressures for spark ignited natural gas engines (45 to 60 psi) and compression ignition (45 to 80 psi) dual fuel, diesel/LNG, engine retrofit kits.
VEHICLE FUEL TANK PRESSURE CONTROL: With a two-line fill procedure the specified tank operating pressure may be established during the fueling process by means of a "back-pressure" regulator installed in the vent return line within the dispenser. This regulator is normally preset to match the fuel delivery pressure required at the first stage of the primary regulator on the engine. The initial operating pressure in the vehicle fuel tank may then be maintained, totally independent of the LNG's saturation pressure since there are no temperature, density gradients at the gas-liquid interface in the fuel tank that could create a pressure collapse due to vehicle motion.
FACILITY COOLDOWN & FILL TERMINATION: The premature venting and loss of methane gaseous vapors during the fueling of LNG vehicles is not acceptable for both environmental and economic considerations. The major reasons that venting occurs are in the overfill of the vehicle fuel tanks, the heat generated in conditioning LNG and the conductive heat leak associated with the use of submersible pumps. The use of "liquid sensors" with a two-line fill procedure makes it possible to precisely control and terminate fueling when the vehicle tank is filled to its maximum capacity. Liquid sensors are also used to assure "liquid only" conditions in the cool-down process of the non-submersible pump, and all liquid lines, including the fill and vent disconnects.
SYSTEM COMPATIBILITY: The two-line fill procedure is compatible with all "industrial gas" LNG fuel tanks and fueling disconnects currently available. This has been successfully demonstrated on tanks and disconnects provided by major manufacturers and documented by the fleet operators.
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.
To prevent the collapse of pressure in their vehicle fuel tanks the industrial gas fueling facility providerâ€™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
HEATING & CONDITIONING: The major problem with the single line fill procedure is that although the warm, gaseous vapors in the tank are condensed during the fill, the residual heat is not removed and the result is "thermal stratification". Vehicle motion or acceleration will mix these stratified zones and the end result is, a rapid and significant pressure decay. The solution, to heat the LNG and increase its saturation pressure not only adds substantial costs to the LNG fueling facility, but also reduces the vehicleâ€™s range by over 20%.
FILL PRESSURE & FILL RATE: In order to condense the warm gas in the vehicle fuel tank and collapse the "pressure head", the single line fill procedure requires pressures of 120 to 180 psi and a fill rate of 50 GPM. This requirement adds costs and complexity to the fueling facility, requires high pressure, high flow disconnects and increases the possibility of additional methane vent losses and lower fueling system reliability.
FILL TERMINATION & FACILITY COOLDOWN: The single line fill relies on "pressure equalization" to terminate the fueling operation. A pressure transducer or a flow meter signal is used to indicate tank full conditions as the tank pressure equalizes with the fill pressure from the station dispenser. Historically, the problem has been that, depending on the amount of residual LNG and pressure in the vehicle tank, the pressure could and would equalize with the tank being only partially filled. The end result was an increase in vehicle tow backs and higher operating costs.
COMPATIBILITY: The use of the single line fill procedure in LNG fueling facilities is limited to LNG fuel tanks equipped with a "spray bar" fill line and a maximum allowable working pressure (MAWP) of 230 psi. Low pressure, light weight, LNG fuel tanks would not compatible.
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|
|LNG PROPERTIES||CRYOGENIC FUELS INC
TWO LINE FILL
|CONVENTIONAL INDUSTRIAL GAS
SINGLE LINE FILL
|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|