[Stoves] combustion of char

Sat Feb 23 10:14:19 CST 2013

Dear Harold and Other Oxygen Breathers

The uptake of oxygen by coke (which is after all, a form of biomass) is surely taking place in exactly the same manner as described below.

While testing the performance of lignite burning stoves at the SEET Laboratory in Mongolia about 2 years ago, Prof Lodoysamba and I did some very long tests that included a complete burnout to see what would happen.  The experimental method was this: run the stove through the usual ignition and refuelling cycle as prescribed by the social science team. Then leave the recording instruments on long after the standard “90% of fuel burned” time window. The stove was left burning overnight and the recording instruments tracked the gases, six temperatures and the mass.

When the fire became very small and extinguished there was eventually no detectable change in mass. As the instruments were left on for many hours after that a very surprising thing happened: the stove gained weight and continued to do so for some time.

It will take some time out dig out the tests in question but the effect was noticed immediately we processed the data. What, after all, was the final burned-out mass of the fuel?

For some stoves, that answer is the initial fuel minus everything that burns, leaving the ash. For other stoves, there is some char left (in this case coke, which is charred coal, which is compressed biomass). The coke might be in the ash in small pieces, or it is on the edges of the fuel bed where it got cold and never burned completely. We tested one stove that left considerable unburned material because it was so poorly designed that the fire simply went out after a while leaving more than 20% of the burnable fuel mass unburned.

At the time we considered that the ceramic components of the stove (usually bricks and clay) were absorbing moisture from the air; that in the heating cycle the stove components first lost moisture and then regained it when they could, tapping any humidity in the air passing through the stove. This is a reproducible effect and is a consideration for testers using a mass-based and chemically balanced emissions measurement method. All ceramic (clay) stoves have to be watched careful for the loss or gain of moisture during the test.

However this observation that the char may be collecting oxygen following the termination of the ‘fire’ as we know it is most interesting and may explain, in certain cases and partially, the increase of mass by the stove.

I have no doubt that there is a concurrent ceramic effect, that the stove gains weight, but it appears the fuel is also going to gain mass. The implications for determining the moisture content of fuel is obvious. Perhaps the moisture measurements of char remaining, charcoal and coke should be conducted in a nitrogen atmosphere, or in a low pressure environment, or…?

We should take some char, especially high temperature char, and place it on a microbalance for a few days to observe the uptake of O2. If this can be done in a sealed, water vapour-free, high O2 box the effect could be separated from the absorption of moisture (which also occurs).

Regards

Crispin

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Response from Harold Annegarn

I make reference not to char, but to coal. Perhaps the same will apply to freshly prepared char surfaces.

The following information arises from the phenomenon of spontaneous combustion in underground coal and in coal slag heaps. 

Exposure of freshly broken coal surfaces, or in situ coal that has been in water filled voids that is subsequently exposed to air (oxygen) (as occurs when previously mined areas by bord & pillar extraction is exposed by strip mining) results in a chemical reaction known as oxysorption ==> oxygen reacts with carbon/carbonaceous matter at the surface of the coal in an exothermic reaction that results in the oxygen being absorbed into the chemical structure of the coal without necessarily being emitted as CO.. This oxysorption can occur at room temperatures and does not require elevated temperatures or a spark to start. 

Thereafter, basic physics applies: if thermal energy is released from a chemical reaction, that heat is either conducted, convected or radiated away. If the rate of production is faster than the rate of dissipation, then the temperature will rise in the zone where the oxysorption is taking place. The balance between conduction, convection and radiation may change as the local temperature increases; likewise the reaction rate will increase. There must be some convection possible, otherwise the available oxygen will be consumed and the process will stop, If the local convection is large, then the surface remains cool and the reaction proceeds without large temperature rise. If the convection is limited, local temperature will increase, and eventually may reach the point where volatile matter that is driven off reaches ignition point, i.e. spontaneous combustion has commenced. 

It is not a matter of whether spon com occurs, it is just a matter of when. For this reason, modern practise of stockpiling coal discards and fines requires continual compaction of the surface to limit air ingress, and final sealing of surfaces with a meter or more of soil.

Although I I have not read anything about oxysorption on char, it is logical that similar processes are occurring, The char is a freshly prepared surface with high specific surface area due to the voids created by driving off volatile material, and with many dangling bonds at these surfaces. Not for nothing is this material also known as "activated charcoal" with good properties for absorbing ions e.g. from water for water purification.

.

To test this it would be possible to char wood in a dry nitrogen or helium stream, although this would result in a different surface chemistry than if the same material is passed through the same heating cycle in the presence of oxygen. If the material is then cooled, and placed on a balance with automatic recording, the the change of weight with time can be recorded. If this is done in a closed system, with dry air passed over, then one could isolate the effect of oxygen absorption from the absorption of H2O from the atmosphere if this was done in open laboratory air with variable relative humidity.

In the case of the spontaneous combustion accident described by Frank Shields, it appears that the raising of the temperature to just over 100 degrees C assisted in creating a heat generation rate and oxygen supply to the char samples that allowed the oxysorption to proceed to the point at which full ignition commenced - yes, he is lucky to have a lab still!

I have copied this message and thread to Glenn Stracher, an expert on spontaneous combustion on coal, who has a much deeper understanding of these matters than the qualitative handwaving that I have given above.

Best regards

Harold

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