Energy content of a fuel - mixture with H2O

[Frootloops000]

Hey guys...

I am currently in conflict with a colleague on how to properly determine the energy content of a multi-component fuel with a certain % moisture using relative weighted heating values.

The fuel we are dealing with is municipal solid waste (MSW) aka garbage/trash/rubbish etc... We went to a landfill in Zambia, Africa for a couple days and estimated the make up of the waste, breaking it down into several components (polyethylene, polystyrene, cardboard, diapers. aluminum cans etc...), as well as estimating the relative amount each component (by mass) and its moisture content (MC). We then looked up the heating values for each component (kJ/kg) to determine how much potential energy the fuel contains. I understand that generally heating values are for 100% moisture-free material unless otherwise stated.

So... my approach to determine the energy content of the fuel was pretty simple: I created a spreadsheet and evaluated the fuel based on a 1000kg sample, so for example if the MSW contained 5% of paper that would be 50kg of paper per 1000kg of fuel. I calculated the weight of each component and multiplied it by 1-MC (where MC is the percent moisture content) to get the dry weight. I then multiplied the dry weight by the heating value to get the energy contribution of each constituent, which I then took the summation of to get the energy content of the fuel per 1000kg.

I believe this is correct, however my colleague (who has a masters. P.Eng, and 20 years experience) did it a different way which resulted in a significantly different answer. The method he used was to calculate the dry weight of each component (like I did), sum the values together to get the total dry weight per 1000kg of MSW, but then he divided the dry weight of each component by the total to get the % each component makes of the total DRY weight. So for example if there was 100kg of diapers with a MC of 90% and the total dry weight of the fuel was 500kg the diapers would be contributing 2% of the DRY weight. He then multiplied this percent by the heating value for each component and took the sum to be the energy content of the fuel in terms of kJ/kg. We both got results in the same order of magnitude, however significantly different values (my 12,000kJ/kg vs. His 18,000kJ/kg)

Like I said, I believe my method is correct but my colleague is usually right and I want to be sure be for I accuse him of screwing up, so if you guys could provide a sanity check for me that would be great!

(Sorry for any typos, I wrote this on my phone!)

 

[Compositepro]

You are both neglecting the heat that is absorbed by the vaporization of water. You will not be able to recover or use this heat unless you condense the water during heat recovery, which is rarely done for this kind of fuel because of cost and corrosion problems.

As far as I can tell your two methods seem to be essentially the same. You may just have an arithmetic error somewhere. It would certainly help if you used the same example numbers for both calculations. It is called problem solving skills. Start by writing down every step in your calculations so you can follow the logic as well as the actual numbers. This has to be done before you can create a spreadsheet, which merely automates the calculation.

 

[IRstuff]

Your colleague's approach appears to be MORE correct. Your approach makes implicit assumptions about the MC and heat content being the same for each material.

Try the two approaches with:
> heat content 1 and 10
> MC content 0% and 50%
> dry weight 1 and 10

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[FredRosse]

"I understand that generally heating values are for 100% moisture-free material unless otherwise stated."

That is almost never the case for my industry, which burns all kinds of solid, liquid and gaseous fuels.

In combustion and heat value calculations the distinction between High Heat Value (HHV) and Low Heat Value (LHV) must always be addressed, and a given fuel heat value, without sure knowledge if it is the HHV or th LHV, is nearly useless. Understanding the difference is essential before meaningful calculations can be made.

The way the heating values of fuel are determined, only the HHV can be tested for any given fuel, or combination of fuels. The HHV always includes the moisture component if any moisture is present. The LHV can only be found by calculation, there is no test for this property.

It appears to me that the two calculation methods you mention are identical, but, as delineated above, not necessarily correct.