Thanks for that Mike, the furnace temperature will be between 1000C and 1200C the further addition of air causes the furnace to burn very hot and hence the efficiency and low emissions. I was hoping to try and maintain the mid point on the low bandwidth sensor but understand that it has very little range, I expect it wont hurt if it hunts slowly from too rich to too lean as long as I don't go too far in lean direction at least. For start-up I was going to move slowly to a set point which will err on the side of rich until I get to a predetermined temp and then try to do some smart things. I have some of the MAX6675's on order to monitor the exhaust temp. I am going to use a PC for the controller with a number of PIC USB devices to control servos and monitor temp and fuel/air mix. There will be separate fail safes as well as I don't wont to overheat 700l of water! I will be aiming for a 90C running temp when it is working. regards Lee McLaren -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Michael Rigby-Jones Sent: Thursday, 6 May 2004 6:35 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: ] stoichiometric values for a wood gasifying furnace >-----Original Message----- >From: Lee McLaren [mailto:lmclaren@TASNET.NET] >Sent: 06 May 2004 09:18 >To: PICLIST@MITVMA.MIT.EDU >Subject: [OT:] stoichiometric values for a wood gasifying furnace > > >I know this sounds WAY off topic but there is a PIC (or 3) in >this honest! > >I am designing and building a gasifying wood furnace, part of >the design requires the introduction of air after the wood has >been turned into a gas so that the gas burns completely. I am >going to use a butterfly valve moved by a servo to control the >air mix but need to measure the fuel / air ratio to ensure >that the mix is correct. I have been doing some research I >would like to confirm or correct my conclusions. > >Conclusions: >I need to adjust my air / fuel mixture to stoichiometric for >clean efficient burning (without eating the firebox!). >Stoichiometric for a wood furnace will read the same value on >a automotive 'Exhaust Gas Oxygen sensor' as it would for >Stoichiometric for a correctly tuned car. > >I expect when things are working correctly that the furnace >will burn with a blue flame but I need to build the controller >to mix the air correctly. I have found some interesting >information on wide band sensors but I need a ball park to >know if even these are going to work. I am trying to use a >standard automotive exhaust gas oxygen sensor with careful >consideration to where it is installed so it does not melt. >They are a lot cheaper than the alternative. > >Please be gentle, I know a lot more about electronics than I >do about wood gasification. A standard oxygen sensor has a very non linear output that is only good for indicating too much oxygen, or not enough oxygen as you probably know. The sesnor should be mounted as close to the source of the exhaust gasses as possible, both to heat the sensor to working temperature and reduce lag. I don't know what the typical exhaust gas temperature of your furnace would be, but I suspect lower than the EGT of a typical gas/petrol powered engine. Typicaly an oxygen sensor needs to beheated to at least 300 degrees C (600F) to start operating, and normal operating temperature is closer to 750-800 degrees C (around 1400F). You will have to derive a control scheme based on the lag of the exhaust gasses reaching the sensor, and also on the lag of the furnaces response to opening or closing the butterly valve. The output of the sensor will probably just need to be integrated, I suspect that any propotional term will cause problems given the non-linearity of the sensor. You will also need to be able to operate the system in an open loop mode until the sensor is heated up and operating properly, although I don't know what the best method of detecting this point would be. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. 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