[Stoves] Burning wet wood

[Crispin Pemberton-Pigott]

Dear Paul O

 

I am not sure why you think I am opposing the preparation of fuels. You have
mentioned it perhaps a dozen times in your last message as if it is
something I oppose. Because I have no idea why you think I oppose fuel
preparation - an essential part of most combustion I will address this
point.

 

Let me set the record straight so you do not waste any more time telling me
that I oppose fuel preparation.

 

I fully agree that fuels need to be prepared. All fuels should be prepared
in some way - either tree branches cut to length for transport or split to
promote faster drying, chopping in certain cases or turning it into pellets
or briquettes for certain applications.

 

I not only fully agree with coal preparation, I have researched the correct
size that the raw coal should be in order to perform well in certain
ultra-low emissions stoves. The answer is a 10 to 15 g pellet with well
rounded edges so that it flows well under gravity without shaking or
vibration. This can be done with high volatiles coal without adding any
binder merely by getting the moisture and pressure of formation correct.
This was researched in South Africa by Prof Horsfall (Shell Coal Chair, Wits
Univ, JHB) and our fellow list member Prof Philip Lloyd. Briquetting raw
coal is an excellent way to deal with the difficult of lowering emissions
from very low priced stoves.

 

What does not work well at all, proven over and over, is making semi-coked
briquettes and trying to promote that to households as a domestic fuel. This
product, which is nearly useless for cooking, is promoted over and over as a
'clean coal' product. Time and again users find it is very hard to light,
requires a much larger fire to remain alight, must be refuelled much sooner
than unprocessed coal and is three times the price required the whole
industry to be subsidised. It does not 'burn cleaner', it makes as much PM
to light and makes more CO. To burn it correctly it has to be reduced to the
same optimal size and manufactured with the volatiles intact so it can burn
properly.

 

>If you say "dirty coal" to someone in the industry, they will understand
exactly what you mean.

They will point to a coal their combustor cannot burn.

Obviously there are coals from the edge of the field that are very difficult
to burn at all. In fact they call it 'burned' coal, meaning deteriorated. I
was involved slightly in the exploration of the Transkei coal fields near
MacClear in the early 80's working with Heinemann whose PhD thesis describes
the field well. The edges of the deposits will not burn in a domestic stove.
That product is suited to >20 kW institutional stoves that are never turned
off. In short, the device is tuned to the fuel.

>Every body in the trade can tell you what a bad coal is, and they usually
have a good idea of what is needed to prepare it into a good coal for a
specific purpose. There are almost no coals coming out of the ground that
must not be prepared.

 

I am 'in the trade' and I think there is far too much 'old boy
understanding' of what a good or bad coal is. The best coal I have even
worked with is from Nalaikh mine: 25% moisture, 50% volatiles, 0.2% Sulphur.
This qualifies in the old school methods as a terrible, dirty, bad coal.

 

It is one of the cleanest burning fuels available - but not when put into a
badly made copy of a Russian wood stove. It is really hard to produce
clinkers with it (high ash fusion temperature) and is very easy to light. It
will work in a TLUD or cross draft or downdraft stove and burn clean enough
to take the PM out of the ambient air. Show me a power station running
'clean good coal' that will do that. The stoves being promoted had lower CO
per MJ than a new Eskom power station, lower PM and of course a much greater
system efficiency than using electricity.

 

>But, Crispin, just about all coal must be prepared. Also there is extensive
blending going on. Combustors generally are not designed to handle
everything.

 

That is correct - combustors are designed to handle particular fuels. Fuels
are prepared for particular combustors. We all know this. When fuels are not
matched to the stove or vice versa, it is not true that the fuels are
'dirty'. That is my point. In the wrong stove, biomass is a 'dirty smoky
fuel'. At a ProBEC conference once there was a lady who was just dead set
against anyone burning wood because it was 'a smoky fuel'.  It did not occur
to her that there were more options than an open fire.

 

>Yes, but please do not tell me the South Africans do not do extensive coal
preparation. 

 

That is why I said nothing about that. I am not sure where you are coming
from. Coal is always prepared, usually by sizing.

 

>Yes, there are power stations designed to burn coal of a 40% ash. But often
they have little choice in doing so. 

 

Correct. They design burners to go with the fuel. Until then, the fuel is
'bad' - is that correct? Then it becomes 'good'?

 

>>Imagine trying to design a biomass stove that was tuned to each type of
fuel that happened to be available.oh wait.that is exactly what is happening
on this list! What a surprise, again. Is that not exactly what you are
doing?

>Not at all. That is precisely what I am opposing. 

 

What exactly are you opposing? It is not at all clear. Are you opposing
stoves that can burn unprocessed fuels? Are you opposing stoves that only
burn one fuel?

 

>Crispin, I challenge you to put coconut powder or fine sawdust into the
best TLUD that you can design. 

 

Why would I try to burn fine sawdust in a TLUD? There is nothing magical
about a TLUD. There are plenty of bad TLUD's. Fine sawdust burns perfectly
well in a blown burner and there is a very fine ceramics factory in Malawi
that uses such a burner - two of them actually. Just because a fuel doesn't
burn well in a TLUD does not mean the fuel is 'bad' or 'dirty'.

 

>Then tell me if air is going to flow up through this fine biomass in a
uniform manner. 

 

It is not going to, that is why a TLUD is not suitable for burning that
fuel. Use something more appropriate.

 

>Tell me that there will be minimal CO2 present in the outgoing syngas. 

 

Why should we have to produce syngas? Is this based on the idea that only a
TLUD gasifier can burn cleanly??

 

>Also the device you design has to be small and easy to use. It cannot
occupy an entire corner of your kitchen, and no part of it should be
situated outdoors. Go for it.

 

That's silly. I will choose my own design criteria, thank you very much. The
challenge is to burn a fuel properly in a way convenient to the user. 

 

>However, if we pelletize the coconut dusk or the wood shaving, air flows up
through it in a uniform manner, and we have an incredibly simple reactor
that weighs less than 1.2 kg. It can be in continuous operation for up to
1.5 hours.

 

That is one solution. Go for it. See if people want to buy it. If they do,
you have a winner. If it tests well, I will promote it. Testing will include
user acceptance, emissions and durability, cost and controllability.

 

>Ok, then show me a TLUD operating on loose coconut dust or fine sawdust. 

 

Why would I bother trying to burn in in a TLUD? They are batch loaded. They
are hard to control, they can't really be turned off.  A TLUD is very
finicky about the fuel - in fact the fuel can be considered a significant
part of the stove. Fuel preparation is so onerous that when we tried late
last year to talk about 'improved stoves' in Central Java, the group of
women immediately, loudly and without prompting, told us firmly that we
should not bring to them any stove that required extensive fuel preparation,
"especially chopping wood into small pieces". They were simply not willing
to do that, they said.

 

It seems someone had been there before us.

 

>There should be relatively little CO2 in the syngas

 

Why make syngas? Why not just burn the fuel? This gasifier thing is getting
out of hand. We are not trying to supply town gas we are trying to cook. If
you don't separate the fire from the fuel many problems are eliminated.
Close-coupled semi-gasifiers avoid all sorts of problems for this reason.

 

>It should also make a uniform biochar. 

 

What on earth for? Having collected all the fuel and dried it, with effort,
and handled it into the stove, why would I not burn it? The economic
argument about char in the soil may well be shown, in future, to be valid.
Until then it is arm waving. If char helps soil, make a reactor that is
smokeless and make it at scale. Cheaply, locally, and apply it without
transporting to the kitchen and back. In your special circumstances, which
are hardly universal, you have a local market - go for it. Don't advertise
it for people for whom it is completely inappropriate.

 

>>We do not focus enough on how to design good stoves and hope that
preparing the fuel will compensate for our collective ignorance. 

>No, but we can make things infinitely complicated and expensive if we do
not prepare fuels correctly. 

 

So, prepare the fuels. Nothing wrong with that if people are willing to do
it.

 

>Ok, then put Mongolian coal, of a 30% moisture content and of all shapes
and sizes, into a TLUD, and tell me if you are going to get a beautiful
flame with no CO2 formation in the synas. 

 

You are mixing two things: a decent burn and gas production. No Mongolian is
trying to make syngas in a TLUD. I doubt they know what the term 'TLUD'
means, 99% of the TLUD users. It is just a stove. It burns "Mongolian coal,
of a 30% moisture content and of all shapes and sizes" and they do it
hundreds of thousands of times a day. Emissions are down more than 90%
against the baseline. If they would prepare the fuel into small round
briquettes, PM would drop another order of magnitude. Is there any other
project that has a) as many TLUD's promoted? As much reduction in the local
environmental PM? As much an advance over the baseline? As little attention?


 

>The TLUD works superbly on both rice hulls and coffee husk because these
materials are uniform and generally do not require preparation. When hulled,
coffee cherries and rice hulls are at a 12% moisture, and air flows up
through them within the reactor in a uniform manner. 

So what you are saying is that when the fuel is perfectly prepared, it
happens to work well with a particular type or types of TLUD. How is this
going to assist us all? We cannot prepare all the fuels available so they
will suit a certain burning technology. The technology is much easier to
change than 99% of the fuels.

>I venture to say the TLUD will work well on most types of irregular biomass
provided they are properly prepared. This measn that the one stove could
handle almost any type of properly prepared biomass.

An approach specifically and heartily rejected by our target audience. They
are drowning in biomass and they are not interested in a) paying for fuel
and b) preparing it to fit what someone happens to be making as a combustor.
Far easier to change the combustor. Develop once. Buy once, problem solved. 

That is my point.

Regards
Crispin

 

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