[Stoves] Burning wet wood

[Paul Olivier]

Crispin,

My point of departure is a top-lit, updraft gasifier that makes biochar.
Everything that I have written previously has to be understood in this
light. This could be contrasted with a direct combustion unit, but this was
not my point at all. But direct combustion units do not have a very great
appeal among the Vietnamese.

I do not think that I could interest very many households in Vietnam to
switch from bottled gas to a biomass fuel if I were to ask them to directly
combust biomass in their kitchens. Most Vietnamese might not make a lot of
money per month, but they are sharp, intelligent and enterprising. They see
direct combustion units associated with poverty. They are not attracted to
handling a messy biomass of a low bulk density, and there is also a strong
social resistance even against handling loose rice hulls. This is
especially true in an urban setting.

Rice is a major export item for Vietnam, and it is grown on over 80% of the
agricultural land in Vietnam. Therefore, Vietnam produces a lot of rice
hulls. In the highland areas where rice is not grown, we find coffee husks.
So for the Vietnamese, there are these two readily available fuels. We also
see rice hulls or coffee husks in many other Asian countries. Now let me
pose the following question to you.

Do you think that you could build a small direct combustion unit that would
handle loose rice hulls and loose coffee husks as neatly and cleanly as a
TLUD? I have seen many people who try to burn these two waste products, but
there is always a lot of smoke. The coffee husk is especially nasty to
combust. When burned, it emits a strong, black, pungent smoke. I would
insist that this direct combustion unit for rice hulls and coffee husks not
emit a whiff of visible smoke at any time during its entire operation.

Finally I must caution that the direct combustion of rice hulls is not
ideal, since the ash might easily contain cristobalite. How would
cristobalite formation be avoided in a direct combustion unit processing a
biomass that has an appreciable content of amorphous silica?

Thanks.
Paul


On Thu, Jun 13, 2013 at 8:29 PM, Crispin Pemberton-Pigott <
crispinpigott at gmail.com> wrote:

> 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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-- 
Paul A. Olivier PhD
26/5 Phu Dong Thien Vuong
Dalat
Vietnam

Louisiana telephone: 1-337-447-4124 (rings Vietnam)
Mobile: 090-694-1573 (in Vietnam)
Skype address: Xpolivier
http://www.esrla.com/
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