[Stoves] Three fuel combustion/pyrolysis papers

Sun May 21 18:23:28 CDT 2017

Dear Andrew

I am supporting this understanding:

" As this hot CO2 rises through the bed it reacts with hot char and  is reduced to CO but this is an endothermic reaction so the gas and char cools . As this producer gas reaction is an equilibrium reaction  once the bed falls below about 800C no more CO is formed. However if the bed was deep enough and the primary air controlled all the CO2 has been reduced so only CO and nitrogen survive."

I have found in hundreds of tests that there is an upper limit to the CO/CO2 ratio at the end of a fire (smouldering) that is well below the ratio which can be reached during a much more powerful fire earlier on. The highest CO/CO2 ratio I have seen was 63% earlier this year trying to burn 8mm wood pellets in the Model 4 coal gasifier. I intend to report that to Paul A because I promised to try it.  The result was terrible: 57 kW!  It is common to see a 30% ratio in a charcoal fire that is starved for secondary air, or an enclosed fire that has been at high power then turned down.

When the temperature drops, as you note, the CO2 level rises and CO drops into the 12-14% range (CO/CO2).

I had not seen so clear a description of the bed conditions that lead to this but what you wrote makes sense. If the CO2 is formed immediately and the region above is not hot enough to split it, the CO level remains in that surprisingly narrow range. This applies to both remnant biomass char and coke if it is a coal fire.  We have extended tests where the char was left to burn until it was completely out, after say, 16 hrs. The ratio remained low the whole time. I expected that it would rise as the combustion conditions 'worsened'. Not so. The O2 reacts with the carbon on the surface to produce CO2.

Regards

Crispin

-----Original Message-----

On 20 May 2017 at 20:39, Paul Anderson <psanders at ilstu.edu<mailto:psanders at ilstu.edu>> wrote:

> 2.  FLAMING pyrolysis.   This is down INSIDE the fuel bed.   It is flaming

> because O2 is present as the pyrolytic gases are released from the

> fuel particle.(actually sort of glowing because the flames are VERY small.

I would still like to see a video of these flames, Perceived wisdom is that this flaming pyrolysis only occurs at a low superficial velocity, from my limited observations most TLUD burns  are the complete combustion of a portion of the nascent char at the pyrolysis front and glow red rather than exhibit a flame. This is related to the business of smouldering see below

> Contrast this with NON-flaming pyrolysis which is pyrolysis as it

> occurs inside of a retort (where there is sufficient heat but no entry of O2).

> Note that this is NOT referring to the flaming of the gases at the top

> of the gasifier (or even further away if through a pipe, etc.)

This is different in that no combustion is taking place in the retort and all the heat is supplied through the retort walls. In TLUD combustion does take place at the pyrolysis front and it is the heat from this which drives the descending pyrolysis front.

>

> 3.  CHAR-gasifification.    (why call this "smouldering"?)    Note that the

> realtively small amount of gases being created are CO and they will

> cause

Well it is neither smopuldering nor gasification in my eyes unless the superficial velocity of the air is high enough to cause gasification.

Rather it is char combustion. Normally this char combustion will produce some CO but mostly CO2 and will glow red. If the fuel has not completed its pyrolysis then the heat from the char burning will volatilise and crack pyrolysis products which in the absence of a flame or temperatures above the auto ignition point of the offgas will rise as smoke. To my mind this is smouldering, the combustion of nascent char without the combustion of pyrolysis offgas.

It is so easy to demonstrate this by taking a smouldering log and raising the local temperature by using a small blowpipe, once the invigorated char combustion reaches the auto ignition temperature of the offgas the offgas flares.

In the char  burning case if the superficial velocity of the primary air is increased and the fire bed is deep enough (Tom Reed suggested

20 particle diameters deep) then the initial reaction at the bottom of the heap is to convert most of the char to CO2 and the temperature rises to 2000C, As this hot CO2 rises through the bed it reacts with hot char and  is reduced to CO but this is an endothermic reaction so the gas and char cools . As this producer gas reaction is an equilibrium reaction  once the bed falls below about 800C no more CO is formed. However if the bed was deep enough and the primary air controlled all the CO2 has been reduced so only CO and nitrogen survive.

Andrew

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