Fuel costs for running an engine on natural gas? 2

[InlineMercury]

I am buying a large generator for my company to provide my shop with 3 phase power. I'm trying to look at the different costs for gasoline, natural gas, and diesel fuel. I can't seem to find much information about natural gas. Can anyone tell me roughly how much natural gas per hour per HP an engine will burn? I don't even know how natural gas is measured...is it cubic feet? How much does a cubic foot of natural gas usually cost?

 

[franzh]

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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[jsummerfield]

Be sure to remove the road tax from the gasoline for the calculations. Obviously natural gas will be much cheaper but you need to assure that you are not comparing apples and potatoes.

John

 

[franzh]

You are absolutely correct. Industrial, agriculture, and off road engines here in the US can purchase fuel pre-tax or with no road tax (dyed diesel etc). Many states have a tax system worked out where the consumer will pre-pay the road-use tax when buying natural gas which has to be calculated when doing an apples to apples comparison.

Someone buying pipeline natural gas for a stationary generator application would not be subject to road use tax but will be liable for any end-use tax imposed by the gas utility entity.

Regards;
Franz

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[harrisj]

InlineMercury:

You haven't said where you are. There is (literally) a world of difference between fuel costs in Denmark, New Zealand, Mongolia, Peru and Tanzania. Many fuel prices are artificial, depending on local taxation, incentives for anti-pollution measures etc.

For this reason, they may change dramatically for no economic or market-driven reason. So a good decision this year may not be a good one for next year (for example, powerboat owners in UK are facing a 400% hike in diesel prices due to EU legislation).

Diesel is however a good bet from a fundamental viewpoint, because of the high system efficiency. 200 g/kWh is achievable - better than LPG or gasoline.

John

 

[dcasto]

Natural gas engines consume anywhere from 9500 BTU/hp-hr down to 6500 BTU/HP-hr. The 9000 would be a 1000 HP unit NA and the 6500 is a 8000HP turbocharged. This is based on LHV which is about 90% of the upper heating value. Natural gas sells for $6.00/MMBTU with out transportation and handling. The transport could be $1.00.

a 100 HP unit would consume 950000 BTU/hr-LHV, 1.056 MMBTU/hr at Upper Heating Value. At $7.00/MMBTU, $7.38/hr.