Yes, Greg and all, this is the first lesson in good combustion practices: keep your fire hot and turbulent. And firebricks provide both, thermal insulation and capacity, even more so when the bricks or fire cement itself is insulated at the cool side by convenient fibers or bubble clay. And yes, they wear off and even melt in good fires but they are still the best low budget stuff I know. The radiation effect of a dome shaped brick hearth on combustion quality should not be underestimated imho. Together with the offset twin 2ndary air injets i find it extremely useful in any device with batch loading and burning. It helps to keep good oxidizing conditions over periods of low power combustion, this is important if you want to baffle for overnight heating and are not keen on flue pipe sweeping...
RolfGreg Manning <[email protected]> escribió:Everyone has had a great contribution to this thread. I build water type district heating systems, in my dealings with design, I have found that down-draft units can be direct steel for only the upper portion of the firebox, and using common fire-brick for lining the lower 1/3 and floor of the firebox works very well, also, the secondary burn chamber is totally fire-brick as well. Dealing with up-draft units, adding a fire-brick lined "smoke shelf" that is prior to any heat transfer areas has worked well at keeping a clean burn. As well, with up-draft units, building the fire ON TOP of the load, instead of underneath keeps the start-up very clean, as well as moderating the entire process naturally (batch process once the water mass is up to temperature). IMO (In My Opinion) a good stove burns long and complete, something very hard to do with most traditional stove designs. Lining the firebox with refractory (even 1-1/4 splits) and using areas after the smoke shelf for heat transfer is the cleanest, best, (and most costly) method of using wood as a heating source. Using the areas beside or even below a traditional stove as a heat transfer area, only requires a "start-up bypass door" that opens to the chimney (bypassing the transfer area) to get natural convection going eliminating the need for electrically driven induced draft or forced draft systems. Greg Manning, Canadian Gasifier On Sun, Jan 5, 2014 at 3:01 PM, Jason <[email protected]> wrote: Ken, I find these sorts of exchanges useful to sharpen my own thinking on these issues. "I suppose that firebricks are a simple capacitive thermal mass, to prevent the outer casing of the traditional cast iron stove from seeing the worst effects of thermal cycling, and to prevent excessive surface temperature." - Steel and cast iron also have capacitance, what is different is the heat transfer coefficient. Ceramics are much much slower to absorb heat so your fire gets hotter quicker. The most polluting part of the burn cycle in a (traditional) stove is after the introduction of new fuel so using ceramics to help ramp up the temperature quickly will help a great deal. My system naturally ramps up the temperature quickly so ceramics are not required. In comparison to the absorption rate the emissivity rates of ceramic and iron are much closer. "What is the problem with pyrolysis occurring too early? Is it simply because fuel is pyrolysing in the wrong place, and there is no means to transfer the pyrolysis gases to the combustion chamber, or is the problem tar generation in the fuel magazine?" The problem is driving off all the volatiles too early so the gas mix is largely carbon monoxide, which is much harder to combust, so while a glowing bed of coals might seen great there will be a lot of wasted, but unseen, energy going up the flue. Catalytic combustors work by bringing down the combustion temperature of carbon monoxide. I did work this out this until I had some instrumentation on it at the Decathlon. "My motivation for design is a more efficient woodstove, which radiates more heat into the room in which it's located - say the living room, plus provides adequate hot water via a heat exchanger to provide heating for some additional rooms and hot water." ......"Traditional stoves generally lose a lot of heat straight up the chimney. Whilst this generates draft, it is a major cause of inefficiency." Taking heat from the top of the flue is a much better solution as at that point you are capturing waste heat rather than heat needed for combustion. Not easy to do though. "The nominal 8kW stove I have at the moment fails to produce much radiant heat, and I am sure that the simple heat-exchanger tank at the back of the combustion chamber seriously effects the combustion temperatures resulting in more emissions and poor, inefficient combustion. For this reason I believe that the only way to control emissions and combustion temperatures, is to first gasify the wood fuel and then burn the wood gas at high temperature with preheated secondary air." The simplest solution I can see for you is to put ceramics between the heat exchanger tank and the combustion chamber. Bring your combustion efficiency up and it will improve the whole system. Pre-heated secondary air is overated, good design can make it unnecessary. I have a prototype stove built and tested in October that I believe would be very close to the capacity and dimensions of your stove. I did four sequential loads of 45% moisture content wood without any visible smoke from the stack. The test was duplicating NZS 4012/4013 compliance testing as best I could (stove on scales + testing and weighing the fuel and its input times) and this test was witnessed by a Justice of the Peace using a ringlemann card and the subsequent report endorsed by him. "Some heat could be recuperated for secondary air pre-heating, using a simple concentric heat exchanger made from twin-wall fluepipe." Check out the Mulciber from the Wood Stove Decathlon. They still had issues with carbon monoxide though, too much heat too early. "A good stove should be easy to light, be easy to load, easy to clean out ash. Additionally it should have a convenient batch burn time, and the ability to control the heat (turn down), without too much loss of efficiency. The stove should be capable of handling the predominant fuel type (say split logs) without additional fuel preparation." ......"These are the features that I consider necessary to meet customer expectations." You can have any two of those. Lol. Bringing it back to ceramics, my retrofitted stove in the Wood Stove Decathlon beat all the EPA certified stoves for efficiency, so in the 83-84% region but I don't have the official report on that yet from Brookhaven National Laboratory. The curve of diminishing return comes to mind. I did not use any ceramics. I also achieved a zero emission (particulates and carbon monoxide) test run that was supposed to take a 15 minute sample but had to be terminated at the 12 minute mark because the gas analyser overheated. Regards Jason On Mon, Jan 6, 2014 at 12:11 AM, Ken Boak <[email protected]> wrote: Jason, Thanks for the interesting comments. I suppose that firebricks are a simple capacitive thermal mass, to prevent the outer casing of the traditional cast iron stove from seeing the worst effects of thermal cycling, and to prevent excessive surface temperature. Taking this capacitive idea to the max, I guess is the masonry stove, which is all thermal mass intended to absorb and slowly release the heat from a brief but intense fire. I have magnetite bricks left over from an electric storage heater (common in the UK). My intention was to experiment with these for heat retention. What is the problem with pyrolysis occurring too early? Is it simply because fuel is pyrolysing in the wrong place, and there is no means to transfer the pyrolysis gases to the combustion chamber, or is the problem tar generation in the fuel magazine? My motivation for design is a more efficient woodstove, which radiates more heat into the room in which it's located - say the living room, plus provides adequate hot water via a heat exchanger to provide heating for some additional rooms and hot water. The nominal 8kW stove I have at the moment fails to produce much radiant heat, and I am sure that the simple heat-exchanger tank at the back of the combustion chamber seriously effects the combustion temperatures resulting in more emissions and poor, inefficient combustion. For this reason I believe that the only way to control emissions and combustion temperatures, is to first gasify the wood fuel and then burn the wood gas at high temperature with preheated secondary air. Traditional stoves generally lose a lot of heat straight up the chimney. Whilst this generates draft, it is a major cause of inefficiency. Some heat could be recuperated for secondary air pre-heating, using a simple concentric heat exchanger made from twin-wall fluepipe. A good stove should be easy to light, be easy to load, easy to clean out ash. Additionally it should have a convenient batch burn time, and the ability to control the heat (turn down), without too much loss of efficiency. The stove should be capable of handling the predominant fuel type (say split logs) without additional fuel preparation. There may be good reason to have the stove non-reliant on electrical power, relying on natural draft and thermosyphoning for it's normal operation. These are the features that I consider necessary to meet customer expectations. Having intensively run my existing stove for around 14 hours per day for the last 16 days, as the primary source of heat over the festive holiday period, I am tolerating its less than ideal performance, but am now certain that there must be a better design. regards Ken _______________________________________________ Gasification 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/gasification_lists.bioenergylists.org for more Gasifiers, News and Information see our web site: http://gasifiers.bioenergylists.org/ _______________________________________________ Gasification 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/gasification_lists.bioenergylists.org for more Gasifiers, News and Information see our web site: http://gasifiers.bioenergylists.org/
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