Energy content of gas calculation 1

[cevans]

I'd like to know how to calculate the energy content in BTU/cubic feet of a gas if we are burning it in an engine to produce a known output of power (13 kW) at 1400 rpm.

 

[TD2K]

The best way would be to get a gas compositional analyses and then calculate the heating value from that.

You can estimate it from your engine output but you would need to have an idea of your engine's efficiency and the fuel flowrate to back calculate the heating value.

 

[cevans]

We have the volumetric capacity and speed of the engine to calculate flowrate; as for engine efficiency, a standard assumption for efficiency would be sufficient for this calculation.

We are doing a compositional analysis, but for the time being we would like to have a rough estimate of the energy content of our gas. Any suggestions on how this might be performed?

Thanks!

Chris

 

[pablo02]

I would suspect that your gas supplier is is running a gas chromatagraph on the system somewhere -- it should spit out the heating value (do you want high or low heating value?) -- you might call and ask...

also, this value will swing depending upon the supply of gas and how well the "goodies" are stripped out... the heating values have dropped significantly in our territory over the past 10 years: from an average of 990 to 1000 Btu/CF down to 910 to 930 Btu/CF... it may have dropped from those values - I have checked in about 2 years..

 

[cevans]

Thanks Pablo; as this gas is for use in IC engines, we're interested in the LHV, but I assume we will be able to get both once the gas analysis is completed.

The gas isn't being supplied; we are generating our own low-BTU gas from municipal solid waste and we wish to determine the actual energy content of the gas.

From reading the power output of the engine at a known shaft speed we should be able to calc the energy content; I'm just wondering if anyone knows a procedure for performing this calculation.

Thanks for your help,

Chris

 

[TD2K]

Well, if you want a rough estimate, I'd try this.

You know your number of cylinders, piston throw and rpm so you know the volume per time the pistons sweep out. I assume this is a 4 stroke engine so 1/4 would be the volume swept out by the pistons when air/fuel is added.

Off the top of my head, I don't know what the volumetric efficiency of a engine is. A compression ratio of 8.5:1 would imply a top cylinder clearance of about 12%. Try 80% for the volumetric efficiency and that gives you the fuel volumetric flow rate (which would be close to atmospheric pressure).

I don't have a feel for what a typical engine efficiency in this size would be, 20%? If you have any documentation on it, fuel consumption is typically given as BTU/bhp*hr and you can back calculate an efficiency from that. The info I have are for much larger engines and I doubt this one will be as efficient.

Given those two figure, fuel flow rate and input power, you can estimate the heating value. Be interesting to see what you get.

 

[TD2K]

I looked at some data I had for a 3 MW diesel engine driven generator. Not too comparible I know.

Specific fuel consumption was 187.5 grams/kW*hr which works out to 43%, better than I would have estimated. Your engine is much smaller than this and I suspect engine efficiency is going to be correspondingly lower.

 

[ElwoodGreen]

Can you measure the methane content? We run 5 2.1 MW engine-generators on landfill gas here. Due to the constantly changing conditions, we use a Landtec GEM-500 to determine methane, carbon dioxide, oxygen and "balance gas" percentages every few hours. Typically, we average 58% methane. It is my understanding that digester gas usually exceeds 60% methane.

100% methane contains about 1010 Btu/scf higher heating value. Therefore, 60% methane fuel contains about 606 Btu/scf. We had lab analysis of our fuel (ASTM D3588) in the past that showed the lower heating value to be 0.9006*HHV. That results in about 546 Btu/scf for 60% methane fuel.

You really need an analysis of your fuel. Then you need a field instrument to periodically check methane. The GEM-500 was expensive--over $5000. I know there are other instruments that would be less costly, but we needed to use the same type as that used by the landfill environmental technician. I have noticed that the lab analysis typically shows about 3% less methane than the field instrument. I don't know why, since we calibrate to a bottle mixure.

Hope this helps.