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It should be noted that the computer program expresses properties based on standard volume basis by dividing the appropriate ideal quantities by the gas compressibility. Such division does not produce real gas heating value, for example, but only allows calculation of custody transfer rates on energy basis using the real gas flow rate rather than the ideal gas flow rate. The HHV and LHV values for combustion calculations are recovered by multiplying the standard volume basis values by the base compressibility. Thus, for the above composition, the ideal HHV is 1036.6 Btu/cf (38,626.4 kJ/m3) and the ideal LHV is 934.9 Btu/cf (34,836.8 kJ/m3), based on a LHV/HHV ratio of 0.9019.

The computer program recalculates the density and certain other parameters to any reference conditions specified. For natural gas vehicle (NGV) applications, one could easily compute the density values at EPA's reference conditions of 20°C and 101.325 kilopascal (kPa), which is approximately 14.6969 psia. By resetting the default values, the relative density, compressibility, HHV, LHV, and Wobbe Index can be recalculated to international standard pressure (101.325 kPa) and temperature (15°C) reference conditions with quantities in customary units referred to A.G.A. standard reference conditions. Density will be calculated at any pressure and temperature specified.

Alternative programming methods may use American Society for Testing and Materials (ASTM) D 3588, Standard Practice for Calculating Heat Value, Compressibility Factor, and Relative Density (Specific Gravity) of Gaseous Fuels. While the compressibility factor calculation is not as rigorous as that in the A.G.A. program, the results are quite adequate and computed data are applicable to all common types of utility gaseous fuels on the basis of HHV and LHV. Although the base conditions used are 14.696 psia (101.325 kPa) and 60°F (15.6°C), calculation procedures for other conditions are given as well.

As previously mentioned, the LHV and HHV of pipeline gas, as well as the LHV/HHV ratio, vary depending on the mass percent of hydrogen and the amount of inert gas. As the proportions of propane and other heavier constituents increase, the energy density of the gas increases. Whereas the LHV of methane (CH4) is 90.03% of its HHV, the LHV of ethane (C2H6) is 91.47% and that of propane (C3H8) is 92.00% of HHV. Reducing the amount of non-hydrocarbons, of course, will increase the energy density of the gas.

Table 5-4 shows sample analyses of natural gas from several U.S. fields. In these samples, the methane constituent ranges from 83.4 to 93.3% by volume. Notice that the heating value, in Btu/cf, and specific gravity (relative to air) are lowest for Sample 3, which features the highest methane content. This is to be expected due to the absence of heavier, higher energy density constituents such as ethane. A comparison of Sample 5 with Samples 1 and 2 reveals a significant difference in heating value, even though the methane content and specific gravity is almost identical. This is also to be expected since the non-methane constituents of Samples 1 and 2 are primarily ethane (about 15% by volume), while Sample 5 features more than 8% nitrogen, which does not contribute to heating value.

Given the varying energy density levels, natural gas sales today are conducted using therm as the base unit. A therm is defined by the U.S. Secretary of Commerce as "a natural gas energy unit equal to 105,480,400 Joules" (Federal Register Vol. 33, No. 146, July 27, 1968). Since natural gas transactions are based on higher heating value, quantities expressed in therms always have a connotation that is not apparent when Btu is used.

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