On Sunday 15 January 2012 03:50:48 [email protected] wrote: > ----- Original Message ----- > From: "Agua Das" <[email protected]> > To: [email protected] > Sent: Saturday, January 14, 2012 12:43:29 AM > Subject: Re: [Stoves] Inverted top lit updraught > > The ejector conveys the gas and secondary air to the forge. This > allows access to the molten metal without the need for a flue. > I can heat bronze to 2150 F in 20 to 40 minutes ( as fast as propane). > Using 1 to 3 lb wood per pound of metal melted.
We haven't had quite the discussion I had hoped for about comparing conventional down draught with inverted down draught and I'll try and add a few thoughts below but first the dasifier: http://www.cd3wd.com/cd3wd_40/EMGroups/WasteWatts/DASIFIER.htm I have been fascinated by this device since it was first described because of the apparently high temperatures that can be achieved[1]. I hope everyone understands the principle of the device, it uses a small compressor which injects high pressure air into the tube between the gasifier and the burner, this compressor is a luxury that most people wouldn't consider for a stove and the ejection it forces is lossy compared with a mechanical fan moving the same amount of air. Its advantage is not having moving parts in a hot dirty environment. This air is directed such that it acts as an ejector forming a depression on the gasifier side and entrains 3 other air streams. The largest amount is the secondary air that oxidises the offgases from the gasifier sections to form the flame that can melt bronze in the furnace section. Its the dual gasifier part that is relevant to my earlier questions. The fuel falls through the upper section under gravity and air is sucked by the depression at the ejector down with, and through, the fuel where it is pyrolysed and gasified at the throat at the bottom of the chamber. This throat is crucial to DD gasifiers as it allows for the gradual shrinkage of the chunks of wood as they descent into the first oxidation zone and gradually shrink. This zone has to be hot enough to cause any CO2 to be reduced to CO when it meets hot char, the gases leaving this zone should be mostly nitrogen from the primary air and CO and Hydrogen. Most DD gasifiers would add sufficient air at this stage ( often by blowing through pipes ( tuyeres) into the throat region) to gasify all the char so only ash falls below the throat. The design and configuration of this section is what bugs most wood gasifiers. If we invert this top its action is what forms a tlud and if the primary air is delivered such that it is just sufficient to heat the pyrolysis zone then pyrolysis offgas is driven off, a small amount of char is burnt in the process, if the fuel is dry, and a char residue remains. The essential difference in the first half of the dasifier is the fuel moves down to the pyrolisis area and in the char making tlud the pyrolysis front moves down through the fuel I think the off gas composition should be about the same, given the same limited primary air. If so then the particulates from the secondary flame should be about the same with primary air controlled TLUD or Down Draught. The DD offers continuous refueling and as long as the air path is not constricted by extra fuel then emissions should be constant and the cooking surface convenient, the TLUD suffers from limited means to refuel ( or does it ? ;-)) and it has a height problem. With a conventional DD the char is all consumed and only charry ash falls through the grate but Aqua Das doesn't attempt to gasifiy the char at the throat using the stratified approach of all the primary air descending through the fuel or adding air via tuyeres, he allows the char to fall into a second gasifier that is a simple updraught char gasifier, this needs again to operate at high temperature, the third part of the entrained air is sucked up into the hot char, turns to CO2 and reaches a high temperature before it is reduced to CO in the hot char and low oxygen environment. The output from this is mostly CO and N2. the two offgas streams ( N2 H2 and CO from the downdraught section and N2 and CO from the updraught section) combine in the area below the throat and exit the gasifier via the tube with the ejector. In practice not all the wood is perfectly gasified and there will be steam, methane CO2 and tars going to the burner. Mass flow considerations mean that all the heat released in the gasifier and the heat from the secondary flame less losses to the gasifier walls will be distributed in the flame and thence the flue gases and it looks like Aqua Das gets above the melting point of bronze, say 1000C. The flame must be much higher than this to allow for heat transfer and losses. In fact I think with dry wood about 1600C is achievable but in practice we know wood cannot melt iron ( 1500 C and a bit lower for iron with high carbon content). One of the benefits of gasification is in reducing the need for excess air necessary in a conventional fire. The gasification uses controlled primary air to produce hot simple gases that will easily burn and if the output is first cooled ( recycling the heat) then the combustion gases can be premixed before the flame. I'm not sure what is happening in the dasifier regarding pre mixing but I suspect the gases meet the secondary air above their spontaneous ignition temperature and secondary combustion starts before much premixing can occur, the ejector causes enough turbulence to keep everything mixed in the flame. Add too much secondary air and the heat released in the flame has to be spread over a larger mass flow and the temperature drops, the same happens if the wood is wet, the water robs the flame temperature with both its mass flow and its latent heat of vaporisation. Getting back to my question and Alex's recent posting on that ingenious method of inserting a burner into a conventianal stove without cutting the body about; the problem with DD is that unprocessed wood does not flow, the grate or throat areas need agitation. Which is why Alex uses pellets ( an expensive, pre processed fuel) and I don't think the air controlled DD to produce a clean flame can compete with TLUD without a means to constantly remove char from the bottom. That was too much finger bashing and probably too long to read but thoughts anyone? Further musings: [1] it should be just about possible to melt steel with a wood flame if the wood is perfectly dry, in practice it doesn't happen and iron was not produced until charcoal was used, the reason is simple; whilst the charcoal only has <50% of the heat of the raw wood it was produced from, it is dry nearly pure carbon with about 30MJ per kg available to be released on full oxidation. C+O2+4N2=>4N2+ CO2+30MJ heat per kg of C is a low massflow reaction and heats of over 2000C are achievable. Consider a propane torch flame, when premixed with the correct amount of air the flame is two blue flames, a bright blue inner flame surrounded by a paler blue enveloping flame, the hottest part is at the tip of the inner cone where nearly all of the premixed oxygen is consumed burning hydrogen and some carbon to H2O and CO/CO2, remaining CO is then burned in the outer enveloping flame by oxygen that has survive the first flame and oxygen that diffuses in from the surrounding air. Can we consider doing something similar with the dasifier, using the high mass flow ~1600C pyrolysis offgas flame to form a hot envelope around the lower heat capacity but higher temperature CO+N2 flame? Without destroying the burner pipe from the heat? AJH _______________________________________________ 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://www.bioenergylists.org/
