[Stoves] Alternative to charcoal

[Crispin Pemberton-Pigott]

Dear Jock

 

>The numbers I have for wood pellets suggest around 8,000 BTUs per pound. 

 

There is a pretty comprehensive list of heat values at the back of a WBT spreadsheet. 12,000 would be a a bit about right for charcoal. The heat value of char produced in a TLUD or a fire has not been very well studied. Jim Jetter’s Aug 2012 paper has some values.

 

The heat value of wood is only what people are going to get from it, not what it would be if it were dried (which also applies to charcoal).

 

I am in Central Java at the moment and the charcoal in the lab has been sitting around for a while. The average moisture content is 8.8% according the lab lady Julianna. The heat content is 26.7 MJ/kg which is slightly below the 12,000 BTU mark.

 

The local wood even after months of drying is about 15-16% moisture. With a heat content of about 19.2 MJ/kg dry at 16% it is 15.7 MJ/kg. Lots of woods have a lower heat value than that and many people use damp wood – no doubt about it. It bubbles and dribbles continuously.  So the reality is that what people put into their stoves is often below ½ the heat value of charcoal per kg. Further, the char produced by a TLUD is expected to be 0% moisture if it is used within a day or so it will pay to keep an eye on what exactly is being claimed. Fresh charcoal has a very low moisture level. Most fuelwood does not. That is the comparison I am thinking of. A direct comparison between the average fuelwood as used and locally made charcoal as used is ≈15.5 v.s. 29.5 MJ/kg.

 

If I put 1 kg of 15% moisture wattle (15.9 MJ/kg) into a TLUD stove and create 20% char (20% of the moist mass) the net heat provided by the fire is 10 MJ/kg. The heat available from the char is still 29.5 so the total is the difference the bit between: 15.9-10 = 5.9 MJ.

 

If someone wants to switch from wood fuel to a ‘cleaner’ TLUD and that TLUD is not 1.5 times as efficient in transferring heat, their raw fuel consumption will increase.

 

Regards

Crispin

 

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