[Stoves] Burning wet wood

Thu Jun 13 19:57:27 CDT 2013

Dear Paul

To comment on your important question:
"How would cristobalite formation be avoided in a direct combustion unit processing a biomass that has an appreciable content of amorphous silica?",
I would suggest for your consideration the possibility of adding alkaline earths or metals to the rice husks, with a view to forming harmless silicate compounds.

1: The first one I would try is Ca(OH)2, or "Hydrated lime". It might have "binding characteristics" that would enable you to produce a pellet or "Rice Hull Aggregate" if some sort, but on combustion, the CaO would certainly react with the SiO2, to produce "Calcium Silicate" of some sort, having the generalized formula "XCaO.YSiO2" This would tend to produce a pH neutral ash. Or, it might also yield an ash with cementatious properties.

2: Another approach you might try is to add a Sodium or Calcium Bentonite Clay to the RH fuel, firstly to act as a "binder/aggregator", and then to act as a "Silica Getter" during combustion.

3: You might even try sodium carbonate, as a 'silica reactant. "

4: Iron Oxide has a strong affinity for silica also. If you have a 'high iron clay", it might also be worth a try.

Note that these are not necessarily "simple suggestions." Too much of the alkalis may cause clinkering. Significant experimentation may be required.

Best wishes,

Kevin

  ----- Original Message ----- 
  From: Paul Olivier 
  To: Discussion of biomass cooking stoves 
  Sent: Thursday, June 13, 2013 9:21 PM
  Subject: Re: [Stoves] Burning wet wood

  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
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  Dalat
  Vietnam

  Louisiana telephone: 1-337-447-4124 (rings Vietnam)
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