[Stoves] Thick wood and MC in micro-gasifiers was Re: Smoke-free biomass pellet fueled stove

Wed Nov 14 08:36:05 CST 2012

Dear Jaakko

This is very helpful and I am happy to share in this conversation!

>the energy from char combustion goes partly to evaporate the moisture by
radiation down and partly to heat the gas and it is obtained as usable
energy for cooking.  

Understood.

>The velocity of the propagation of the ignition is slower with high
moisture and the available time for cooking becomes then longer. 

Rather obvious, yes.

>However, above some limiting moisture, the ignition front does not
propagate at all. 

Exactly when will be fuel dependent and would change if there were primary
air preheating, however I agree. I have tried it both ways. I found that the
surface to volume ratio of the fuel is a factor. Hard wood needs to be split
smaller (etc).

>So there seems to be some optimum moisture giving maximum total amount
energy for cooking, if no value is given for the residual char (and one is
not interested in the heating power but only in total energy). 

I was not specifically looking for the maximum total energy though of course
I have an interest in it. I am trying to track down the 'extra' energy that
lies between the missing char (burning wet fuel) and the energy needed to
dry the fuel. I was going to make a chart to post here showing the amount of
extra energy involved but have been travelling.

>Water gas shift reaction takes palace in gas CO+H2O = CO2+H2 and it does
not have effect on char. H2 is oxidized to H2O rapidly if O2 is present.

This is what several people have been saying and I can report, with some
mysteriousness, that in the coal fires ignited with wood that we tested so
many of in Mongolia, there is something taking place that produces 'far too
much' H2 during ignition. The simple explanation is contradicted by the test
measurements.

To put numbers on it, we often reach 2,000 ppm H2 in the stack when there is
plenty of O2 available, a healthy fire burning (wood under coal) and the
coal is igniting just fine. The H2(EF) which means the undiluted hydrogen
concentration reaches >15,000 ppm. It is very strange to watch a burning
coal+wood fire inside an enclosed stove that cannot ignite the hydrogen
emerging from that same fire. 

One of the metrics we watch is the ratio of CO:CO2 as you would expect,
however we also watch the CO:H2 ratio. Both gases indicate poor combustion.
There is nearly always some H2 in the stack and when it goes high, it
becomes interesting. Even very late in a coke/charcoal fire there is some H2
present. Ditto for H2S and SO2 which move opposite, in concert

Every conventional explanation is that there is no water gas shift reaction
taking place, even though the conditions for it are present. This does not
add up. The reaction was deliberately created in gas plants, why should it
not take place when conditions are right in a stove?

Most interesting that so many people are sure the reaction is not taking
place. Is the H2 coming from direct outgassing from the non-carbon portion
of the wood and the coal volatiles?

>However char is oxidised by gasification reactions H2O + C = H2 + CO and
CO2 +C = CO. The rate of these is much lower than reaction with oxygen, but
however at about temperatures  [above?] 850°C they become influential. 

That also happens to be the temperature at which we can find a consistent
ignition of CO (in free space, not on the surface of the char. Also
interesting...

>The reactions are endothermic, which adjusts the extent of gasification
(high gasification rate results in low temperature decreasing the
gasification  rate). There is always some H2O present in the gas from
ignition front since wood contains hydrogen and combustion air also contains
some H2O. Char is also consumed by CO2 produced in the ignition front. So
gasification of char takes place also for combustion products from pyrolysis
of dry wood.  

Understood.

>So the main reason for "missing char" for moist wood as Tom Reed and AJH
have explained is  that in addition to the part going down to dry the fuel,
a considerable part is going up (and obtained  as usable energy) and the
cooking time is longer. 

I like the explanation of the particle size being a factor. It means that by
carefully choosing a particle size one can manipulate the burn so as to
reduce or increase the amount of char created.

It is going to emerge soon as a major stove testing issue that the char
remaining cannot be considered 'unburned fuel' in a char making stove. This
greatly affects the stoves rating on fuel consumption which at present is
quite misleading. The importance is that someone who is making a stove that
maximises char production will find the penalty on the fuel consumption
number to be so great in certain markets (where there is no interest in the
char) they should change the combustion parameters to reduce the char to
nearly zero. If this can be done without adding moisture, that is a huge
bonus. If the moisture is itself a contributor to the removal of the char
(being charred on the outside and damp on the inside as you suggest) then
there is a more difficult problem to solve.

My attempts to burn all the char while creating gas have been reasonably
successful so I called the device a semi-gasifier because it is able to
switch back and forth between the char making and char-burning modes. That
name may not be technically correct but names are not as important as
function.

In very brief, the fuel consumed by a stove is the amount of raw fuel it
takes to perform some task assuming that the task is repeated, i.e. one of a
series of identical burns. The thermal efficiency overall is the task (work
done) divided by the heat potential of that quantity of raw fuel. When
applied to ordinary stoves the answer is pretty much the same as any water
boiling test, for example. However for char producing stoves where the char
mass is a significant fraction of the initial fuel, the fuel consumption and
thermal efficiency are significantly changed, often dramatically so. They
more or less match the real numbers observed in the field during, say, an
uncontrolled cooking test (UCT). The problem is at present the calculation
methods do not consider that stove would every produce large amounts of
charcoal so the calculated results are (erroneously) far from the real
values.

Being able to switch a char making gasifier into a whole fuel consuming one
will be an important feature for the different markets.

Thanks for your observations

Crispin

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