LNG Tank System Analysis

Over the past 20 years it has been demonstrated that natural gas in its liquid form (LNG), is the most cost effective method of storing, transporting and distributing this domestic energy resource for end-use as a transportation fuel. However, if history is any guide it will take at lease 50 years to develop the necessary, infrastructure to facilitate its end use in private vehicles.

LNG Truck Figure 1

The down side to LNG, that must be considered, is that its application and end use may be limited, for the foreseeable future, to vehicles that are operated on a daily basis, from a central terminal, with a duty cycle range of 600 to 800 miles between refueling. There are over 4.6 million heavy-duty trucks and buses, operating an average of 340 days per year, that constitute this “niche market” for LNG. (Ref. Fig. 1)

The design, installation and operation of the vehicular fuel tanks, dispensers and fueling equipment must be compatible with the following fleet characteristics and operational duty cycle if LNG is to be accepted as a viable, cost effective substitute for petroleum based fuels.

 
 
Fleet Size 40 to 200 Vehicles
LNG Fuel Use 1,600 to 18,000 GPD
Vehicle Range 200 to 500 MPD

Vehicle Fuel Tank

Unfortunately, there are two major issues that must still be addressed, if natural gas, in its liquid state, is to become a viable alternative to petroleum based fuels. First, it is imperative that potential users be made aware of vehicle fuel tank & fueling facility compatibility issues. Secondly, users need to prepare “performance based” procurement specifications for the vehicle fuel tanks, fueling equipment and require full “code compliance” by all suppliers.

There is worldwide consensus, in the user community, that the recurring problems of variations in vehicle fuel tank pressure, fuel quality (density) and the weathering of LNG in the vehicle fuel tank are due to the inherent high “heat leak” of the conventional industrial cryogenic containers being used. These containers, which constitute over 95% of the LNG fuel tank market, were originally designed for the shipping and handling of flammable and non-flammable industrial cryogens, not for vehicular or mobile applications.

Unlike the passive fuels, gasoline, diesel, compressed natural gas or propane, LNG is an active fuel, continually changing state as heat enters the liquid fuel through the vehicle fuel tank’s suspension system, insulating materials and the tank vacuum. This system “heat leak” has a dramatic effect on the pressure, temperature and density relationships of the LNG. The resulting phase-change behavior in the LNG makes it very difficult to control the vehicle fuel tank pressure and maintain a consistent fuel quality for delivery to the engine within the specified air/fuel ratios.

Problems Due to "Tank Heat Leak"

FUEL WEATHERING

LNG Vehicle TanksFigure 2

The weathering of LNG occurs when an economizer valve (regulator) is used on the vehicle fuel tank to allow, “boil off gas” (BOG), pure gaseous methane vapor, to be withdrawn to reduce the pressure in the tank. The frequency and duration of the vapor withdrawal is a function of the amount of heat entering the LNG. The line from this economizer valve is connected to the liquid fuel delivery line and the combined vapor/liquid flow is then routed to the LNG vaporizer (heat exchanger). It is important to note that all conventional “industrial gas” cryogenic containers are equipped with a pressure management device of this type. (Ref. Fig. 2)

FUEL COMPOSITION

The weathering process increases the content of the heavier hydrocarbons, most notably, ethane as the pure methane gas is withdrawn from the vehicle fuel tank to reduce tank pressure. Weathering became a major problem in the mid 1990’s with the increased use of “industrial gas” cryogenic tanks in LNG fueled vehicles. Amid increasing reports of engine damage, due to increased ethane accumulation in the fuel tank, it was decided that the weathering problem could be resolved by stripping out the ethane and other hydrocarbons, thereby increasing the methane content of the LNG to 99%.

LLNG Gas CompositionFigure 3

NOTE: This decision was made, to accommodate the poor thermal performance of conventional, industrial gas tanks and added over $ 0.08 a gallon to the cost of the refined LMG. In addition, the removal of the heavy hydrocarbons reduced the energy content of the fuel from 1047 btu’s/SCF to 972 btu’s/SCF.

FUEL DENSITY - PRESSURE VARIATION

LNG DELIVERY PRESSURE VARIATIONFigure 4

The actuation of the economizer valve also has an adverse impact on the density (quality) of the fuel that is delivered to the engine. The vapor/liquid flow is now a mixture of pure gaseous methane and LNG from the tank to the vaporizer.

Flow and pressure testing on LNG vehicle fuel tanks, conducted for the Department of Energy (DOE), under Contract No. DE-FCO2-01CH11084, in 1998 provided the data shown in Figure 4. The testing confirmed that, “at the 5% vapor flow level” the methane gas component in the fuel delivery line could occupy as much as 70% of the total fuel line volume.

FUEL DELIVERY - PRESSURE VARIATION

With the economizer valve open the fuel flow to the vaporizer is primarily gaseous methane with “slugs” of LNG entrained. The result is a fuel being delivered to the vaporizer, with variations in both pressure and density. The magnitude of the variations in pressure, at the first stage regulator, on the engine, is shown in the “data logger” graph, Figure 5. As the graph shows, the drop in fuel tank pressure can be in the 40 to 60 PSI range.

The economizer valve on most conventional “industrial gas”, LNG fuel tanks is set at either 80 or 100 PSI, depending on the type of engine, spark ignited or diesel.

LNG DELIVERY PRESSURE VARIATIONFigure 5

International LNG Market

According to reports from the Japan Gas Association, Australia, Europe and Korea, LNG with a methane content of 99 % is not available and is therefore, not a viable option since all natural gas imported or liquefied in these countries has a methane content of 89% to 94%, with the ethane content as high as 7%. Therefore, weathering of LNG, in the vehicle fuel tank, will not be an option in these markets.

LNG Vehicle Fuel Tank Technology Comparison

LNG PROPERTIES CRYOGENIC FUELS INC INDUSTRIAL GAS
Weathering: No Yes
Fuel Delivery: Liquid Only Vapor/Liquid
Fuel Density: Constant Variable
Pressure Control: ± 5 psi ± 40 psi
Ullage Control : Yes No
Standby Time: 12 Days @ 90 % Full 3 Days @ 90 % Full
Heat Leak: 11btu/hr @ 90 F 40 to 60 btu/hr@ 90 F
Vent Rate: .055 lbs/hr @ 160 psi .31 lbs/hr @ 230 psi
Pressure Rise: 0.3 psi/hr 1.76 psi/hr
Economizer Valve: No Yes