Crispin, Thanks for the clarification. As for energy content analysis, the US Forest Service puts out a handy fuel value calculator that cover various materials at various prices, energy densities and moisture contents. Personally, I am now burning Vermont softwood pellets with a BTU content of around 8,800 BTUs per pound.
In general, we presume that pyrolysis will require almost twice as much feedstock as a similar combustion device if equivalent thermal output is desired. This is why I ask the question concerning the value of the charcoal/Biochar produced. If it valuable enough, then it more than offsets the extra feedstock used. And of course this "value" depends on the value of the feedstock as well. The lower the cost of the feedstock, the lower the value of the charcoal has to be in order to make economic sense. I know of a case where farmers make charcoal in their stoves which is then collected by the stove suppliers. I believe it is bought back. Part of the charcoal is returned to the farmers for cooking and part is returned, mixed with compost, as a soil amendment for the kitchen gardens. As the farmers experience for themselves the value of the charcoal enriched soil amendment, ie better yields, they start to ask for more soil amendment and less charcoal for cooking. Interesting what direct personal experience can accomplish. So in the end, it is all about the total value proposition of the entire system. There are many in the US who argue that well made biochar is worth a good deal more than the thermal energy produced in its creation. Hence they have no economic problem with simply flaring off the pyrolytic gases produced as they make their biochar. Personally, I prefer to make full use of both the gas and the carbon. As for the value of the biochar, I know of one producer in California who sells all he can make, a lot, for over $1,500 per ton. He offered to buy all I could make for $600 per ton, no questions asked. I am sure conditions in Central Java are rather different, but this is a most interesting conversation. Thanks, Jock On Apr 9, 2013, at 1:18 AM, Crispin Pemberton-Pigott <[email protected]> wrote: > 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 > > _______________________________________________ > Stoves mailing list > > to Send a Message to the list, use the email address > [email protected] > > to UNSUBSCRIBE or Change your List Settings use the web page > http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org > > for more Biomass Cooking Stoves, News and Information see our web site: > http://stoves.bioenergylists.org/ >
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