Dear Crispin, I was just thinking it would be a simple set up that would take an average of the gases collected at a single point and then see how well that correlates with the findings of gas readings taken directly. Sloshing water acts as a filter. Just a double check of the results you are surprised to see.
Regards Frank Frank Shields Control Laboratories; Inc. 42 Hangar Way Watsonville, CA 95076 (831) 724-5422 tel (831) 724-3188 fax [email protected] www.controllabs.com From: Stoves [mailto:[email protected]] On Behalf Of [email protected] Sent: Wednesday, September 18, 2013 1:06 PM To: [email protected] Subject: Re: [Stoves] Charcoal stove design Dear Frank A combustion analyser does what you suggest in a continuous manner at a typical flow of 1 to 1.5 litres per minute. The only draw back is a delay between the sample entering the metal pipe and the reading on the gas detection cell. With filters along the way (introducing a smoothing effect on the concentration) it can take a while - perhaps 1 minute. When moving the sample pipe around the gases can be determined at each location. Remember that gasses will react instead the sampling system so it is not 100% representative of reality. Precisely what is happening in the fire isn't critical. It is really complex. Something like 450 reactions are involved. Regards Crispin heading south >>Q10>>> From: Frank Shields Sent: Thursday, September 19, 2013 03:44 To: 'Discussion of biomass cooking stoves' Reply To: Discussion of biomass cooking stoves Subject: Re: [Stoves] Charcoal stove design Dear Crispin and Saastamoinen, With the GEK I work with I have control of the air and a good long bed of char to work with. But what is the goal of these small household stoves? Perhaps with natural draft? I would think a surface reaction where the C > CO > CO2 all at the surface below the pot. No real secondary and all the heat produced close to the same location. I would think that would require a narrow blast of air. Suggestion: Take a section of plastic pipe that holds about 4 Liters. Cap the top and bottom. The bottom cap has a ½” pipe fitted with a ball valve. The top cap has a ½” pipe coming out. That top pipe is connected to flex tubing and that connected to a stainless tube to draw in gas. Open the ball valve and set 4L pipe in a water bath such that water enters and fills large pipe. Close valve and remove from water bath. Place the stainless tube at exact location you want to sample the gas (say under the pot). Open the ball valve to let the water drain and gas is drawn into the large pipe. The rate and volume is the same as the water draining. Leave a couple inches of water in the large pipe and close the ball valve. Shake to mix the gases. Place large pipe into the water bath, open the valve, so water filling and the stove gas exits the top where you can measure it using your tester. You might add a small amount of acid to the water to keep the CO2 from dissolving. Or add a known amount of NaOH to capture the CO2 (CO?) for analysis leaving the other gases for the tester. With this setup a lot of the variables are controlled like air flow and temperature. You get an average or set your tester just below the input to get real time. Regards Frank Frank Shields Control Laboratories; Inc. 42 Hangar Way Watsonville, CA 95076 (831) 724-5422 tel (831) 724-3188 fax [email protected] www.controllabs.com From: Stoves [mailto:[email protected]] On Behalf Of Saastamoinen Jaakko Sent: Wednesday, September 18, 2013 2:37 AM To: Discussion of biomass cooking stoves Subject: Re: [Stoves] Charcoal stove design Dear Frank and Crispin, the main DIRECT product in the reaction of carbon with oxygen is CO but also some CO2 is directly formed. CO that is formed can be oxidized to CO2 (CO+½O2=CO2) at high enough temperatures in the vicinity of carbon particle or in later stage in the gas flow. So there are different zones along the gas flow in carbon particle layer when air (or gas) is flowing upwards through it: I) Zone where exothermic reaction C+½O2=CO takes place (but some CO2 is also formed). Here the gas temperature is rather low because it is close to the inlet and the gas has not heated up enough. So CO is not burning well. II) Zone where the gas temperature becomes high enough so that also the exothermic reaction CO+½O2 =CO2 (enhanced by H2O) takes place in the gas. This leads to even higher local temperature along the gas flow so that exothermic reactions (C+½O2=CO and CO+½O2=CO2) take place at even higher rate. Here also the endothermic reaction CO2+C=2CO takes place because the char temperature is high enough. This rrwaction adjust the temperature level preventing it to increase very high. Then at the location, where all oxygen is consumed, the gas temperature and CO2 concentration reach the maximum values. III) After that the carbon reacts with CO2 producing carbon monoxide in endothermic gasification reaction CO2+C=2CO. The gas is cooled due to this endothermic reaction. If the layer is very thick, the gas is cooled to a temperature at which the reaction rate C+CO2=2CO becomes very low. So some conclusions: 1. For a thin layer, there is only zone I producing CO. 2. For thicker layer (zone II), the exit gas contains much CO2 and some CO. The exit gas is hot and CO may be burned introducing (preheated) secondary air. The exit gas is hottest if the thickness of the layer is just in the intermediate transfer regime from zone II to III. 3. For a thick layer, the gas contains much CO and some CO2. Its temperature is low and it is difficult to burn CO without highly preheated secondary air. The reaction CO+½O2=CO2 can take place also in the other direction. This reverse reaction (dissociation) of carbon dioxide is not significant at temperatures <1800 K. Regards Jaakko From: Stoves [mailto:[email protected]] On Behalf Of Frank Shields Sent: 18. syyskuuta 2013 0:55 To: 'Discussion of biomass cooking stoves' Subject: Re: [Stoves] Charcoal stove design Dear Crispin, So the CO2 > CO is endothermic. But still needs carbon. As I see it the only difference is the temperature changing as the gases move around the pot. Wondering if the reading could be affected by temperature? Perhaps amount of gas entering the instrument or something? Interesting. Could hydrogen react with one of the oxygen in CO2 making water leaving CO? Frank From: Stoves [mailto:[email protected]] On Behalf Of Crispin Pemberton-Pigott Sent: Tuesday, September 17, 2013 2:01 PM To: 'Discussion of biomass cooking stoves' Subject: Re: [Stoves] Charcoal stove design My conclusion is the CO splits endothermically. It does not happen in stoves with high EA. Crispin Dear Crispin, <snip> I found that directly above the middle of a good stove (under the pot) there is zero CO – I was amazed. But nearer the edges there is more. [Frank >] How is that possible? I thought going from CO2 > CO could only occur be in a bed of char. This is very strange. Regards Frank
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