[Stoves] char information book by

[rongretlarson at comcast.net]

Lists: 

I spent yesterday reading (skimming about half) a 200 page book entitled: "The Mastery and Uses of Fire in Antiquity" by a J. E (Ned) Rehder. It was much more on the combustion of char for metals working - some but not much on production of char. I thought Mr. Rehder had done a nice job on some aspects related to char and so tried to reach him - but I found Mr. Rehder, who worked at the University of Toronto is recently deceased. 

The book is unusual in being written for an archaeological audience - but from a quite technical perspective. Among some interesting statistics are: 

a. He gives maximum stochiometric temperatures (called AFT for adiabatic flame temperature) for the combustion of char (1940 C, p 169) vs coke (2070 C, p 171), but shows even higher AFT values for wood if exceptionally dry. I am surprised by these numbers. 
He drops the AFT for one type of wood by 11 degrees C for each 1% moisture and 8 degrees C for each 1% excess air. (p27) 

b. He introduces a "space velocity" term that I have not seen used for our type of stoves/furnaces/kilns. He says typically 10-15 meters/minute in antiquity, now up to 40-50 meters/minute for modern apparatus that still keeps the combustibles in place. This is a measure of the power output - as zero "space velocity" gives zero power output. I don't see a way to use this in our typical stove development - but it could be very helpful in well equipped labs and when using fans/blowers. 

c. He discusses the combustion of char in terms of particle diameters with emphasis on two types of combustion. In the first type (2-3 particle diameters), CO2 is produced, but then that is converted to CO (over 15 diameters - but not more) - while dropping temperature, because this is an endothermic conversion. Doesn't continue forever in deep beds because of the temperature drop. Again, something we should know - but not important in char-making stoves (which he doesn't discuss at all). 


d. His appendices 2 and 3 give the equations needed to design pressure drops and air flow areas. But in his (metallurgy) business, this is mostly done in this book with "tuyeres" - air supplies that I recall seeing only in fan/blower systems. There are also (in Appendix 3) forms for natural draft. I might come back to this if I ever get back to char-making stove design - as these appendices are backed up by numerous citations (mostly anthropological). This all relates to "antiquity", but that is probably good for our simple stove and char-making. I am sure changes need to be made for TLUDs and (especially) TLODs. I hope that someone can report on any comparisons between their experimental results and anything coming out of these (or similar) theoretical treatments. 

e. There are numerous places where he shows how inefficient char is in supplying heat - compared to wood. This is NOT a plug for char over wood. 

f. There are some interesting computations on how much wood (and land) was needed to make carbon to process various metals (the most for copper - about 200 kg of wood for 1 kg of copper - in Chapter 13). Chapter 14 talks about deforestation due to this need for wood - and shows that is a fallacy. Deforestation has occurred - but not for making char (at least in antiquity - I think deforestation is occurring today in many African countries - becasue of the huge changes in population in many countries). 

3 I found one place where the book could be obtained in electronic form for only a few bucks - but I had no difficulty getting this through inter-library loan. It is definitely of more value for anyone refining ores rather than cooking, but the book could be helpful to some on these lists. 

4. If anyone else has had exposure to this book, I'd like to hear of anything else I should have caught. 

Ron 

Ron 


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