Post
by lsayre » Wed. Oct. 19, 2016 5:48 am
I'm making an initial estimate that the average hydrogen in typical coal/wood volatiles represents about 16% of the volatile component by weight, since data I saw ranged from 13% to 19%.
The regressed BTU equation from the source referenced above:
BTU's/lb. = 198.11*C%+ 620.31*H% + 80.93*S% + 44.95*A% - 5153
Can be thus modified empirically to read:
BTU's/lb. = 198.11*C%+ 620.31*(V%*0.16) + 80.93*S% + 44.95*A% - 5153
And then lastly you must vaporize the water and account for BTU losses therein.
This revision to the equation allows you to use the volatiles percentage (V%) as given for your as delivered anthracite, and not concern yourself with the hydrogen component (since you are accounting for it as 16% of the Volatiles). This may be the best I can do to come up with a simple yet respectably accurate "as delivered" BTU equation for coal. It should work for all types of coal and even for wood.
Perhaps the most interesting thing I have taken from this is that there are not as many BTU's in volatiles as there are in carbon, on a pound for pound, or percent by weight for percent by weight basis. So if you are being told that higher volatiles mean more BTU's you are being told a lie. Higher carbon and lower volatiles means higher overall as delivered BTU's. So much for my initial assumption of equivalence.
I saw some data which indicated the percent carbon in kiln dried hardwood is typically 48%. And kiln dried softwood typically has about 51% carbon. Kiln dried wood also has typically about 37% Volatiles, 8% water, and 1% Ash.
If 8% moisture, 48% carbon, 37% Volatiles, and 1% ash are assumed, when I plug these figures into my revised spreadsheet I get 7,989 BTU's per pound, which is a match to the 8,000 BTU/lb. figure often quoted for premium hardwood pellets. Softwood pellets yield 8,584 BTU's/lb., and mixed pellets would result with ~8,300 BTU's/lb.
Multiply C and V and Ash as listed above for pellets by 0.88 and then bump water up to 20% to simulate well seasoned cut and stacked hardwood and you get 6,240 BTU's/lb., which is again quite close to the accepted value for ideally seasoned mixed hardwoods. Multiply by 0.78 and bump water to 30% and you have 4,817 BTU's/lb.
If there is interest, I will post my revised version of the spreadsheet.
Last edited by
lsayre on Wed. Oct. 19, 2016 5:35 pm, edited 2 times in total.