Fuel consumption calculations

<kg/t / XXXX = kgmol/t>

So XXXX is the molecular weight of the gas in kg/kmol. OK so far, though clearly this number needs to be established from the composition of the fuel gas. Does the supplier present this information?

<m^3/t x 41.3 = MJ/t>

So the calorific value of the gas is 41.3MJ/m³. OK, though this will vary with the composition of the gas, its temperature and its pressure at the flowmeter. Note that 1kmol of gas occupies 22.4m³ at Standard Temperature and Pressure. Be very careful about the temperature and pressure the gas is supplied at through the volumetric flowmeter(s) that is(are) used as this will have a significant effect on any billing calculation. Other than that, the thinking looks good from here.

Gas suppliers often quote the calorific value of the gas supplied on their billing statements. One could use that figure as an input to the calculation and cross-check the bill.

No comment is possible on the "funny algebra" the other financial person used without its being presented.

Hi SWSlak,

Thanks for the comment. kgmol value of the gas is 18.2 just as a FYI.

The Funny algebra was trying to see if the units correspond, by cancelling out over and under the line. But she left out things like the 1000 multiplications, thus 1000 x kg did not cancel out a tonne, but only 1 x kg. So she confused us all.

Thanks for the effort.

Hello T4T

You say everything is on basis of volume of inputs/tonne metal produced, but start your calc from mass of inputs.

But apart from that, as you say, you have cost of gas per GJ, and object is gas cost/tonne metal produced. Presumably you know either m³ of gas (more likely) or GJ per tonne metal produced. You're there in either case. If you know GJ per tonne cost per tonne is immediate. If you know m³ per tonne, you have gas CV 41.3 MJ/m³ = 41.3/1000 GJ/m³ so you can easily get GJ per tonne. This assumes CV = 41.3 MJ per m³ as measured by your meter, which is likely to be approx standard Sm³, 15° or 20° C (in US or Europe) and 1 atm, but it doesn't make a big difference.

You don't need gas MW unless you want to calculate tonnes gas. Density = MW/24 kg/Sm³. 22.414 is molar volume in Nm³, i.e. 0° C, 1 atm.

From the MW 18.2, it looks to me like 41.3 MJ/m³ is gross CV, i.e. assumes condensation of the water vapour produced. But this doesn't affect the calcs.

Cheers.....Codey

Hi Codey,

Thanks for your input, you are right about the measurement as well as the standard (which is (Normal) Nm^3 and not (standard) Sm^3 in our case) but it amount s to absically the same thing. And yes is based on 25°C at 1 atm or such.

Have a good one.

What exactly is your question ? Do you want to calculate Fuel used /ton of metal produced? There are 2 methods :

1 . Easy - install Gas Flow Meter(if Gas Burners) and this will record Gas m3/Ton of product produced.

2. Hard Way - undertaking detail algebra like yours. First find out Gas pressure/temperature/line size/type burner/make burners/capacity KW each burners/ore composition/inlet temp. & outlet temp./temperature required/time taken etc.etc. Go thru series of calculation & physical data collection etc., then come out with a figure based on estimate & assumption.

Depending on the setup of your plant and its importance, a Gas Flow Meter (density compensated) of US$8000/ Furnace is a worthwhile investment. The Flowmeter will not keep indicating hourly/daily/monthly consumption and can be taken directly into your office computers for printout Gas/Ton/hr or Total Gas/total tonnage/month etc., but the flow meter will also indicate when excess Gas Vs Ton of product is going on due to burner inefficiency(fouling of burners) to take corrective action.

Believe me the Flow Meter pays back on the investment less than 6 months.