[Stoves] why does charcoal produce more CO?

Mon Feb 5 13:56:32 CST 2018

Dear Crispin,
thanks for your comments. I have several Chinese journal papers on NO reduction by char. Do you mean this group  (Li Dong, Shiqui Gao and Guanwen Xu, NO Reduction over Biomass Char in the Combustion Process, Energy & Fuels 24, 446-450, 2010)? The authors are from Chinese Academy of Sciences.

As I  wrote 16.10.2017 to the list, we studied NO formation and reduction from char (and from fuel bed). It was mainly in connection with grate combustion in an European project “Reduction of Nitrogen Oxide Emissions from Wood Chip Grate Furnaces (Low-NOx Wood Chip Project; Contract JOR3-CT96-0059)” but we had also another national research programme focusing on stoves or fireplaces “TULISIJA” (=fireplace).

NO is formed in parallel with oxidation of char from a single particle. Part of NO diffuses into the same particle and reacts to N2. When the particle size decreases with combustion, the conversion of char-N to NO increases. This is due to the fact that the rate of escape of NO increases with decreasing particle size and there remain less possibilities for the reduction of NO with char in the same particle it is formed. This applies to single particles and can be explained with a model (Saastamoinen, J., and Hämäläinen, J., Release of nitrogen from wood. International Energy Agency (IEA) - Workshop:”NOx/N2O Formation and Destruction in Fluidized Bed Combustors”, 16.5.1999, Savannah, USA, 16 p.). If someone is interested, I can send a copy.

In a fuel bed, NO formed from a single particle can also be reduced to N2 by other surrounding char particles. For example, in fluidized bed batch combustion of char, the conversion of char N to N2 increases with increasing batch size, because with large batch size there are more particles acting as reducing material. The same applies to char particles in a fixed bed. NO is already partly reduced in the particle it is formed but it can further effectively be reduced in other particles later along the gas flow.

Jaakko

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Crispin Pemberton-Pigott
Sent: maanantaina 29. tammikuuta 2018 17.58
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] why does charcoal produce more CO?

Dear Jaakko

This is fascinating. Thanks so much for sending such a detained report.

I recommend, based on recent information and a visit to the Chinese Academy of Sciences in Beijing, that you contact Prof Xue from the Beijing University of Chemical Technology. He has been modeling the behaviours you describe and one additional product (not mentioned below) is that NO produced from fuel N can be reduced to N2 and O2 if the bed has certain characteristics. It is an interesting assumption in the US stove tests that the heat available from fuel is for all fuel N to be ‘burned to NO2.” Now, with an understanding of how to do it, it can rather be converted to the molecular form N2.

They built a stove with several optimisations in mind which I hope to test at CAU shortly. There are two extant copies and they look really good. They are low pressure boilers (water heaters for homes).

If you can’t locate Prof Xue contact me and I will find an intermediary.

Regards
Crispin

Dear stovers,

the question of CO release from char bed is interesting. We developed models for the combustion of a char bed [1] and CO [2].

[1] Saastamoinen, J., Huttunen; M., Kilpinen, P., Kjäldman, L., Oravainen; H, and Boström, S., Emission formation during wood log combustion in fireplaces - Part II: Char combustion stage. Progress in Computational Fluid Dynamics 6 (4/5), 209-216, 2006.
[2] Saastamoinen, J.J, Kilpinen, P.T, and Norström, T.N., New simplified rate equation for gas-phase oxidation of CO at combustion. Energy & Fuels, Vol. 14, No 6, pp. 1156-1160, 2000.

Input values in the model are the particle size, bed thickness, inlet air rate and air inlet temperature (and inlet air composition, H2O etc.). Equations describing the particle and gas temperatures and the gas composition (species O2, CO, CO2, H2O, H2, NO) as function of location (distance above the air inlet) are solved. Oxygen reacts with the char particles producing CO and CO2 in exothermic reaction. There is some literature on the product ratio CO2/CO on the char surface. Endothermic gasification reactions, where H2O and CO2 react with char take also place. CO may be burning in the gas depending on the temperature. All the reaction rates (kinetics) depend on the temperature.

Initially one has to assume some temperature for the char bed. Then iterative calculations are needed using the equations of the model, which lead to actual temperature and species distributions as function of the distance from the air inlet. Interestingly two solutions for the same problem were found depending on the initial guess. If one assumes relative low temperature (for example 700 oC), the iterative calculations lead to one stable solution for the temperature and species distributions. If one assumed higher initial temperature, iterative calculations lead to another solution at higher temperature level.  After some consideration, it was reasoned that both solutions could be existing. Assuming low temperature, CO produced at char surface did not burn in the gas and the temperature level remained low but assuming higher temperature it did burn producing heat keeping the bed at higher temperature level and sustaining burning of CO. Actually, there are three stable solutions. Assuming initial temperature 20 oC, there is no burning.

So there are some reasons for high CO emissions:

  *   The fuel bed is disturbed so that temperature deceases (for example too high air rate cools it). Then it may be difficult to get CO again burning. Preheating of combustion air could promote burning of CO. Burning of CO is shown by the blue flame.
  *   In the end stage usually, the temperature becomes too low to sustain burning of CO. I usually burn thin sticks in the end stage in my fireplace on the char bed, so that much of CO from the larger char particles is burnt with the volatile flame from the sticks.
  *   In a thick char bed, CO may burn inside the bed producing CO2 and rising the temperature. However, CO2 may again react with char upper in the bed producing CO.
  *   Burning rate of CO depends on the H2O level even in the simple reaction CO+O2/2=CO2 there is no H2O. In reality the reaction scheme is more complicated, where radicals formed from H2O promote the reaction rate.  If there is no H2O in the air, the burning rate of CO becomes very low. Usually there is some H2O in the air, but in dry weather conditions, CO release may become higher.

Jaakko

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Andrew Heggie
Sent: perjantaina 26. tammikuuta 2018 20.45
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org<mailto:stoves at lists.bioenergylists.org>>
Subject: Re: [Stoves] why does charcoal produce more CO?

On 26 January 2018 at 01:31, Darpan Das <darpandasiitb at gmail.com<mailto:darpandasiitb at gmail.com>> wrote:
[https://mailtrack.io/trace/mail/e69f256f0f88a1aa36e8809aa9d03574740386e1.png?u=1788967]

Thanks Andrew.

I was also wondering if the decrease in bound oxygen content would have any role to play in the increase in co emissions. My ultimate analysis results for wood and charcoal show that charcoal has negligible O/C content. This probably is because of the pyrolysis process for wood has already removed the volatiles. Now the charcoal is more energy densified containing most of it part as carbon.

Can this less O present in the solid fuel matrix be attributed as a reason for formation of high CO?

Darpan

Crispin has replied to this.

By the time the fire is reduced to a glowing bed of coals there will be little oxygen bearing compounds left.

In the early stages of pyrolysis CO is given off and this is  a result of the decomposition of wood, so yes the CO is a product from the decomposition of oxygen beariing compounds in the wood.

The reduction of CO2 by carbon to CO only takes place at higher temperatures by which time the char  is nearly all carbon with no oxygen bearing compounds remaining.

If the firebed is thin and able to radiate its heat freely so it looks like "black" heat the conditions for making CO from char don't exist and the char should oxidise directly to CO2, as the bed gets deeper and the heat cannot escape then the conditions  are there.

I cannot imagine what -56C is like +40C was bad enough

Andrew in balmy England
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