Dear Ron
1) I would guess that adding an insulating layer on the outside of the convective skirt would be cost effective. Have you (or anyone) seen this tried? I have accidentally tried it using a ceramic pipe as a skirt and sinking the pot into it! Not really a deliberate as much as an exploratory work. The gain is that the skirt is already an insulator. The losses from the aside are less than the losses of the unconstrained gases into the air near the pot. I have seen pot cosies and insulated lid cosies. Because a lot of heat is lost from the top of a lit an insulated layer on top of a lid retains a lot of heat and cheaply too. 2) You mentioned two tests at Alex' location, but only described the home self-feeding stove. I presume the other was the char-producing moving grate greenhouse boiler. Your measurements/thoughts on that system (if you saw it)? I ask because I hope others will start replicating this one. The other working system was a wood chip version of the pellet stove. It has a motor powering a foot that rises and lowers in order to tamp fuel into a combustion chamber. If the fuel builds up, it stops turning until the fuel has burned down. It does not produce char and it has no moving grate. I was a higher power device and operated with a straight pipe extending horizontally from the combustion chamber. The CO/CO2 was frequently zero as there was no detectable CO for periods of time. I found the excess air to be 200% but Alex adjusted it to about 80% and the CO disappeared. The system efficiency was very high even though the heat exchanger was not all that complex. I don’t remember what the number was but it is on the memory of the analyser so I will download then sometime. The unit that has a moving grate is enormous. It can easily produce char simply by running the grate at a speed that does not let all the char burn and it falls off the end. He can make as much as he wants by turning a knob. It is not a trifling system, however. It has operated by a programmable logic controller and cost hundreds of thousands of $. It does burn a very cheap fuel (about 500 tons of it was on site) and it has a marvellous electro-mechanical fuel feeding system that is very effective. It consists of a travelling auger and a four stage conveyor-hopper system that ultimately places the fuel into the burner without letting in much air. This is does by using a sort of paddle wheel like you would see on a simple centrifugal fan. It fits the space closely and lowers batches of chips into the hopper attached to the furnace. If the fire gets into the hopper a water pipe sprays the fuel automatically. One surprising thing about the system is that there is a very small difference between the input and outlet temperature. It is a hot water system that operates at a high flow rate and the entire system is about the same temperature all the time. In terms of char production, as the % of char produced can be changed at will just be changing the grate speed, the formation temperature can also be adjusted by the same method. As the lower temperature char seems not to work all that well perhaps the lower yielding high temperature char will have a practical use all the time. Regards Crispin
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