With Respect to Mositure How Are Anthracite Btus Calculated?

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lsayre
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Post Sun. May. 08, 2011 8:13 am

I often see figures in the general vicinity of 13,250 BTU's per pound for anthracite, but then I also see moisture figures hovering somewhere around 6%. Does the 13,250 (give or take about 250 BTU''s per lb.) figure account for the inherent moisture, or is the real nominal BTU content of anthracite closer to 13,250 * 0.94 ~= 12,450 BTU's/lb. (with this latter figure accounting for the nominal 6% moisture).


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VigIIPeaBurner
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Post Sun. May. 08, 2011 9:18 am

This is not an accurate answer to your question but the formula you post, 13,250 * 0.94, is based on weight percent subtracted from the BTU/carbon content. Apples to oranges as it were. I'm get from your question is how many BTU are consumed by the 6% water content in you example. The equation would have to be normalized to BTU. Here goes my shot ...
  • A BTU is the amount of heat it takes to raise one Lb of water 1*F
  • Given that definition, raising the temperature of the water from 60* F to 212*F, boiling temperature @ST&P, it would require ~1,200 BTU to do this.
  • If a Lb of coal at 60*F has 6% moisture content then it would take 6% of the 1,200 BTU to boil the 1 Lb of water off: .06 x 1,200 BTU = 72 BTU
  • In your example that would leave 13,178 BTU heat content remaining in the Lb of coal after the 72 BTU is used by boiling the 6% moisture off.
I'm not representing that this is the actual laboratory procedure to analyze the effect of the moisture. It's pretty close but I'm just trying to convert apples to apples.

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lsayre
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Post Sun. May. 08, 2011 9:23 am

Your logic is sound, but the fact remains that you do not have a pound of anthracite. What you really have when you purchase a pound of coal is 0.94 lbs. Can it be therefore that the devastation of the water upon the BTU's actually applies a double whammy upon the bottom line BTU's.

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Post Sun. May. 08, 2011 9:31 am

You do have a Lb of anthracite: the analysis of that Lb of coal is that it has 13,250 BTU and 6% moisture . Put another way, that Lb of coal is not 100% carbon but consists of the components represented in the lab's analysis of which 6% is water.

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lsayre
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Post Sun. May. 08, 2011 9:35 am

Ah, I understand now! Then on a 100% dry basis anthracite must be closer to 14,100 or so BTU's.

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Post Sun. May. 08, 2011 9:45 am

You could be right. It would depend on the actual analysis procedure and how it's to be reported. Your assumption of 14,000 BTU would be correct if the BTU content is expresses as total available BTU content. Hopefully someone in the know will chime in, otherwise we're just theorizing about how they actually determine the reported BTU content.

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Post Sun. May. 08, 2011 10:25 am

VigIIPeaBurner wrote:This is not an accurate answer to your question but the formula you post, 13,250 * 0.94, is based on weight percent subtracted from the BTU/carbon content. Apples to oranges as it were. I'm get from your question is how many BTU are consumed by the 6% water content in you example. The equation would have to be normalized to BTU. Here goes my shot ...
  • A BTU is the amount of heat it takes to raise one Lb of water 1*F
  • Given that definition, raising the temperature of the water from 60* F to 212*F, boiling temperature @ST&P, it would require ~1,200 BTU to do this.
  • If a Lb of coal at 60*F has 6% moisture content then it would take 6% of the 1,200 BTU to boil the 1 Lb of water off: .06 x 1,200 BTU = 72 BTU
  • In your example that would leave 13,178 BTU heat content remaining in the Lb of coal after the 72 BTU is used by boiling the 6% moisture off.
I'm not representing that this is the actual laboratory procedure to analyze the effect of the moisture. It's pretty close but I'm just trying to convert apples to apples.
I think the 1200 BTU estimate is for a gallon, not a pound. For a pound, by definition the BTU requirement would be (212-60=) 152 btu's.

The thing that's been left out of these figures is the "latent heat of vaporization" required to evaporate the water at 212 deg. I believe that is about 970 btu/lb, so the total would be around 1120 btu/lb.

Mike

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lsayre
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Post Sun. May. 08, 2011 10:31 am

Pacowy wrote: I think the 1200 BTU estimate is for a gallon, not a pound. For a pound, by definition the BTU requirement would be (212-60=) 152 btu's.

The thing that's been left out of these figures is the "latent heat of vaporization" required to evaporate the water at 212 deg. I believe that is about 970 btu/lb, so the total would be around 1120 btu/lb.

Mike
So where does that leave the "real world" BTU's/lb. for anthracite?


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Post Sun. May. 08, 2011 11:01 am

lsayre wrote: So where does that leave the "real world" BTU's/lb. for anthracite?
It's my understanding that the moisture content of the coal is taken into consideration in the BTUs calculation for the specific coal - so you don't add or subtract anything for the % moisture in the coal.

dj

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Post Sun. May. 08, 2011 11:09 am

I haven't looked at very many lab analyses, but my understanding is that the btu/lb for anthracite (and other fuels) may be reported as "higher heating values (HHV)" that do not take into account the heat loss associated with water vapor, or "lower heating values (LHV)" that do take that loss into account. Maybe some members who have greater familiarity with lab analyses will chip in on this.

Mike

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Post Sun. May. 08, 2011 11:18 am

Penn Keystone has an analysis on their website that states the BTU's for their anthracite at 14,876 per pound (dry and ash free). The same coal is then listed as having 13,451 BTU's per pound. I guess this means that the typical figures we see hovering at or around 13,250 BTU's per pound do seem to take both the moisture and the non-heat contributing ash content into consideration somehow. Therefore it appears that 13,250 BTU's per pound (or there abouts) is a pretty good figure for real world BTU's.

I have also seen the technical term "MAF" used in rating the BTU content of coal, and I assume that "MAF" means "mositure and ash free". Surprisingly, bituminous coal often has a higher MAF rating for BTU's than does anthracite.

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Post Sun. May. 08, 2011 11:45 am

I think the 13,451 number, for which the testing basis is not stated, results from a mathematical adjustment to "put the ash back in" to the 14,876 number -
i.e., 14,876 x (1-.0958 [ash content]) is exactly equal to 13,451. The 13,451 therefore still assumes the coal is dry, so the "real world" yield from burning the coal as received would be around 12,300 btu/lb after the water is taken into account.

Mike

P.S. Source of numbers is http://penncoal.com/wst_page6.html .

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dlj
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Post Sun. May. 08, 2011 11:49 am

lsayre wrote: Surprisingly, bituminous coal often has a higher MAF rating for BTU's than does anthracite.
Most bituminous coals have a higher BTU rank than anthracite - see ASTM D 388

I'll see if I can post a scanned copy of a page from a book that may have some interesting info on it...(today this web site is being hard for me to post to...)

dj

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lsayre
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Post Sun. May. 08, 2011 11:50 am

Pacowy wrote:I think the 13,451 number, for which the testing basis is not stated, results from a mathematical adjustment to "put the ash back in" to the 14,876 number -
i.e., 14,876 x (1-.0958 [ash content]) is exactly equal to 13,451. The 13,451 therefore still assumes the coal is dry, so the "real world" yield from burning the coal as received would be around 12,300 btu/lb after the water is taken into account.

Mike

P.S. Source of numbers is http://penncoal.com/wst_page6.html .
Mike, that is what I was afraid of. Then if you assume roughly 82% for stove or boiler efficiency, the final figure is about 10,000 usable (recoverable) BTU's per pound of anthracite.

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dlj
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Post Sun. May. 08, 2011 11:53 am

Here, take a look at this table (if I can post it...)

dj
coal_quality2.jpg


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