Greetings all, In a mail to Crispin, Ron wrote : RWL3.4 >I have gathered from the comments of John Davies, that coal was always pretty bad until one had turned the coal to coke.
CPP: Quite incorrect. The cleanliness of the burn depends heavily on the availability of Hydrogen in the early fire. Later, retained heat can be used to keep the coke burning. RWL3.4.2: John Davies replied two days ago on this point - I thought agreeing with my comment. Maybe we are talking about the traditional approaches to combustion vs TLUDs and BLDDs? John? ***** Clarification is needed here. If the traditional way of dumping large amounts of coal, or even wood onto a relatively clean burning wood or coke fire is carried out, we will indeed create a very smoky fire. This is due to the flaming gasses being cooled before complete combustion. Now as the new fuel is heated from bottom to top, the pyrolysis gasses are cooled adding to the smoke. At this point I have to agree with Ron that introduction of a fan blower will in fact increase the temperature of the wood fire, and also force some air through the burning wood, to greatly aid total combustion, before the gasses are cooled by the coal above. Now comes the problem ! As the coal above is heated layer by layer, and pyrolysis increases, the fan power would have to be increased to cope with the increased gas production, otherwise it will cooled by the layers above and make smoke. In order to keep the status quo, ever increasing forced air flow and combustion will be taking place resulting in blast furnace heat output, until the whole bed is turned into coke. Heat outputs that would soon destroy a domestic metal stove, and be far in excess of requirements. And when the coke burns down, the whole process is repeated, with the whole system swinging madly. Of course if small amounts of new coal were added regularly, this effect would be greatly reduced. But even this is not ideal. If 1 piece of coal it put on top of of a hot coke bed, it flares up as the pyrolysis increases, and then dies down as it forms coke, making it impossible to control the correct volume of air at all times. Now remember that The above is all about the traditional fire, where a fan can help with smoke reduction, but needs constant operator attention. There has to be a better way ? *********** Ideal combustion of Coal. Unlike wood, which gives off easy burning pyrolysis gas, consisting of Hydrogen and alcohol type vapours, containing some oxygen, coal gives off gasses ranging from hydrogen, methane and right through the spectrum right up to heavy oils and tars. these heavy fractions will not burn until heated to above their thermal decomposition temperature, which causes thermal cracking to Hydrogen, lighter fractions and carbon. ( soot ) In the process poly-cyclic aromatic compounds are formed, which are highly carcogenic, and if allowed to escape into the air, cause great health problems. This is why it is imperative that coal combustion is taken to absolute completeness. A fair measure of this is the very low Carbon Monoxide emissions that Crispin talks about with BLDD and CROSS DRAFT SYSTEMS. IDEAL CONTINUOUS COMBUSTION PROCESS. 1. Now imagine a circular updraft coke fire burning at a relatively high temperature, say 800 to 1000 C, with a secondary burn consuming any CO produced, everything is very hot and complete combustion is achieved. Now a source of coke is required to continuously feed the fire as the coke is consumed. 2. Now imagine Placing coal around this coke bed in a vertical anulus column 3 layers wide, with continual slow addition to the outside, pushing continuously toward the coke bed. Heat from the coke bed will radiate into these coal layers and the following will happen : The outer third layer will be receiving gentle heat, heating it up to the pyrolysis temperature and driving off the moisture. The 2nd ( middle ) Layer will be heating at a greater rate and pyrolysing to coke. The inner layer, now warm coke will be heating at an even greater rate, up to ignition temperature. As the coke is consumed, it is continuously being replaced by new coke formed in the outer 3 layers. 3. One common denominator exists, a constant and controlled process is occurring, in each of the 4 vertical layers of the process. The coke core will be giving off gasses containing CO2 and CO at a very high temperature. The first outer layer will be giving off the last of the pyrolysis gas. and cracking heavy oils, with some air, as it approaches red hot temperature. The middle outer layer will be giving off large volumes of pyrolysis gas, with some entrained air. The outer layer will be giving off water vapour, steam and some air which is slipping through the system. We now have 4 sources of gas leaving the top of the bed, ALL AT A CONSTANT RATE, mixing together in a secondary burner, at a combined temperature well above the required temperature for complete combustion, and containing heated air. Any shortage of air can be added at this stage, or the system can be tweaked to allow more air slippage, In this process a lesser or greater degree of gasification will be taking place, "you be the judge." FANS are not needed, as the gentle and constant draft from the chimney is sufficient to drive the process to completion. with constant heat output. This concludes a perfect theoretical coal combustion system. It is neither practical or attainable, in the stoves sense. IMPROVED STOVES This is where the BLDD or CROSS DRAFT STOVE fills the gap. These stoves emulate the system described above. The TLUD does not do it so well, due to the batch nature of the burn, and the two distinct processes witnessed during the burn, namely the pyrolysis stage, and the coke burn, each requiring different air control. The two distinct processes can be merged to proceed together to a large degree, reducing the problems , but this is a very fine balancing trick. Burning coal in this stove is best limited to shallower beds, with a short burning time. ( up to 4 hours ) However it can work with beds of up to 500mm deep, with a burn time of up to 12 hours, but this is pushing the boundaries of the system, where better results can be obtained using a BLDD OR cross draft system. Next time I will discuss my experiences of burning coal in the TLUD stove. I hope that these words are of interest, and clear up any confusion with coal combustion. ******** Crispin, I am very pleased to hear of the progress with the new stove testing lab in UB, And improvements to the SETAR facility, and testing protocols being developed around these facilities. Keep up the good work. John Davies Secunda, South Africa.
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