Inline mercury:
Natural Gas is typically sold by the 1,000 cu/ft measure (MCF). This is also called a “Therm”, or 100,000 btu’s.
Typical US grade pipeline natural gas has about 1025 btu’s per cu/ft, but it can range from 950 to 1125, depending on contract, source, season, location, proximity to a gas field and/or refinery, etc.
The gas cost will depend on what your gas utility will sell it for. The higher your use, the lower the cost per therm. Your genset manufacturer should have the fuel consumption usage per KW, based in MCF or therm.
How do you convert the NG usage in comparison to gasoline? One method that seems to hold true is to take a typical gallon of gasoline, at about 125,000 btu’s, divide it by the btu’s per cu/ct of NG (122 cu/ft NG equals one gallon of gasoline in energy value), and that will produce the raw GGE, or gasoline gallon equivalence. There are other factors involved too, such as air displacement by a gaseous fuel, but this will get you in the general area. I like to use 115 btu’s per cu/ft, it works out most of the time. If you are trying to compare diesel consumption, take the same calculations but with diesel fuel, then add 20% NG consumption due to the diesel engine efficiency (the NG engine being throttled).
Lastly, you need to compare apples to apples. Take the high heat value of both fuels, or the low heat value for both fuels, just be consistent. If your raw high heat value of NG is different in your area, adjust your numbers accordingly.
Franz
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