Yes Crispin is correct that is another stovers myth is that biochar saves trees. Energy lost in carbon buried is carbon borrowed from another source.
It's the lignin fraction in biomass (trees, grasses roots etc) that is not readily decomposed when added to soils. Lignin can be considered the most important relatively stable C fraction in plants that is not readily decompsoed. Phytoliths in grasses are also an important carbon source for forming soils. The grassland soils of the world (chernozems etc) are formed as a result of years of deposition of phytoliths We can grow grasses and pelletize them and create both improved soils and lots of clean bunring high yielding fuel. http://en.wikipedia.org/wiki/Phytolith Stovers on this list are creating myths (and even videos) that include there is lots of wood to burn yet in Africa and biochar saves trees. Roger -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Crispin Pemberton-Pigott Sent: Wednesday, September 15, 2010 9:58 AM To: [email protected]; 'Discussion of biomass cooking stoves' Subject: Re: [Stoves] Stoves Digest, Vol 51, Issue 7 Dear Rajan Your message and the one from Otto both make me reach for the calculator because some things are not adding up. Perhaps you can help me understand the energy content here. Otto could do the same if he likes. >>> The real powers of biochar producing stoves are: >>> 1) lower emissions >>> 2) eliminate the need to cut down trees for fuel >> Really? How does that figure? Assuming the energy needed is >> ?>>constant and the fuel source is constant, the need for fuel will increase >>if biochar is sequestered since biochar has a carbon content which can >>be used to provide energy. >These stoves operate at a high level of thermal efficiency > - which means they consume less fuel for the same task. Well......any stove burning any fuel can be improved to reduce fuel consumption so it is not confined to biochar producing stoves. If the point is that improved stoves reduce fuel, fuel can be saved. The challenge to those saying 'biochar stoves will not increase fuel harvesting' is to show that the efficiency gain from the device exceeds the obvious need to increase efficiency is the current amount of fuel is going to be consumed and char is going to be left over. Another way of saying that is you can't reduce the amount of fuel harvested (in kg) AND produce char unless you have an increase in thermal efficiency that is greater than the energy content of the char you are not burning, but burying in the ground. The numbers do not look too good. One thing working against the argument is that stoves are getting better and better. To want to have all that char being left over at the end means leaving a considerable percentage of the energy not used. Let's put some numbers on it: Suppose branches of trees (as one suggested) have an energy content of 16 MJ/kg considering their moisture content. About 40% of that mass is Carbon. The proposal is that 25% of the final mass will be char. OK. The heat content of that 25% will be about 29.5 MJ/kg. So taking 1.0 initial kg of fuel at 16 MJ will give 29.5/4 = 7.375 MJ of energy not used (in the form of char at the end). That leaves 16-7.375 = 8.625 MJ of heat yielded from the fuel. That is 54% of the energy that used to be yielded. Before, all 16 MJ was used and the efficiency was perhaps 15% in an open fire, 30% in an improved stove. To accomplish the same cooking with only 54% of the heat, the thermal efficiency would have to be 15/0.54 to 30/0.54 which translates to 28% and 56% respectively. Are these numbers realistic? >Also, only the pyrolysis gases ( which contain around 2/3rd of the >total energy content of the fuel ) are used for cooking. This 2/3 number is confusing me. 2/3 of 16 MJ is 10.66. I can't see that amount of heat coming out of the fuel and still having 25% char. Biomass is about 50% carbon when it is bone dry. As the moisture is considered at realistic levels (15-20%) it reduces of course. The problem for the biochar producer is that all the moisture has to be dealt with during the pyrolysis burn, leaving dry char at the end. That is just a fact of life. The heat lost getting rid of the moisture is subtracted from the pyrolysis gas heat, or there will be no gas heat. If one includes in the pyrolysis gases only the non-carbon energy content, the heat produced by dry (average) hardwood will be a miserable (18.4 - (29.5/2)) = 3.95 MJ per kilo 'used'. So it goes without saying that some of the carbon is being burned in the process and the char yield will be less than 50%. >When only biomass residues (many of them available as waste >materials) are used for fuel, we avoid cutting of trees. This is also true but applies to any stove that burns the whole fuel instead of making char. If one used biomass residues and did not make char, even less would be required and the biomass residue supply would stretch to far more homes. As the general argument is being made that 'burying biochar is good' one should also consider the possibility that some of the biomass residue could be used in fuel burning stoves (no char production) and the rest could be properly charcoaled in a kiln optimised for char production (without using any of the heat created doing so). Also one could consider using the heat produced. It seems numbers are being tossed out without real calculations to support them. For example, creating chips and pellets used a great deal of energy that comes from (usually) liquid fuelled machines. It is hard to claim to be saving energy while not counting all the energy consumed. >When the resulting charcoal ( which contain only around 1/3rd of the >total energy content of the fuel ) is put back into the soil, the whole >process becomes carbon-negative. This means we are practically removing >carbon from the atmosphere. At 25% yield the char contains 46% of the energy in the fuel, one could easily say 'half'. If the moisture content is high, it is half. We can't talk about 'absolutely dry' fuel because no one burns absolutely dry fuel. This is not a theoretical exercise. It is being proposed that real people burning real fuels change their stoves to create char which they will bury. Question 1 : at what moisture level of initial fuel does a 25% char output contains 1/2 the initial heat content? Question 2 : at what moisture content of initial fuel does a 25% char output contains ALL the initial heat content? (theoretically speaking) >In case the financial constraint is acute and if it is possible to sell >the lumpy part of the charcoal ... Chipped and pelleted fuel and grassy fuels and chopped little branches do not produce lumpy charcoal to sell. The general proposal is that we burn crummy fuels that are presently ignored. It is pretty unlikely that anyone is going to chop wood into sizes that will make lump charcoal. Charcoal is usually made from giant pieces and broken afterwards. I think the biochar people are happy to put powder into the ground (please correct me if it has to be lumps) so there does seem to be a match between poor fuels and biochar. There is not such a good match for the claims of thermal efficiency and economics, though. >Even then ( assuming the buyer is burning the charcoal ), the whole >process is carbon-neutral ( since only residues are used ). This means >there is no net addition of carbon to the atmosphere. Biomass is already carbon-neutral. >The real advantage of these stoves is that they can burn a variety of >waste biomass materials in a very efficient way. Yes it is. And we don't need to say any more than that, do we? If we can burn, as Otto points out, a much wider variety of fuels that are presently ignored, there is more fuel available. It is a hard reality that is some communities they are not short of fuel and fuel saving is not a big priority. In other there are seasonal shortages. In Harare, for example, people burn 6 to 8 different fuels depending on the season (Hancock, D, 1986). In rural Mozambique people burn 5 or 6 fuels depending on the season and whether or not it is raining (Cook, C, 2007). In places where there is an absolute shortage of any fuel at all like Darfur (Abdelnour, S, 2009) it is going to be really hard to convince people to gather more fuel than absolutely necessary. Getting them to collect enough to produce a high energy carbonaceous byproduct to bury .... not going to happen. Any miracle stove that has such a high heat transfer efficiency that it can produce char without increasing the total fuel input, in Darfur, is going to compete with a similarly efficient miracle stove that burns the char and needs only 1/2 the fuel. I am wondering, given all this char burying enthusiasm, if someone has compared the productivity of the soil when the whole plant is sequestered, compared with the char that could be produced from it (25% of the same initial mass). It seems to me the Permaculture and composting people have looked at this pretty carefully. How do their numbers compare with biochar numbers? 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