[Stoves] Stove comparison coming - Is it really coming?

[Paul S. Anderson]

Readers,

Did I miss the posting of the stove comparison?  Please send it.

Meanwhile, the discussion under this subject line is certainly not  
about the comparison.

It is possible that you are missing readers because the subject line  
does not inform people of the actual contact?

Anyway, let me know when there actually is a Stove comparison as  
indicated, but please use a different subject line because the old one  
was not accurate.

Paul
-- 
Paul S. Anderson, PhD
Known to some as:  Dr. TLUD    Doc    Professor
Phone (USA): 309-452-7072   SKYPE: paultlud   Email: psanders at ilstu.edu



Quoting Crispin Pemberton-Pigott <crispinpigott at gmail.com>:

> Dear Andrew
>
> On Sunday 20 March 2011 19:49:03 Crispin Pemberton-Pigott wrote:
>
>>> Lots to think about. I am looking for common elements in the clean
>>> combustion and it make take a while to decide much. At the moment it
>>> is clear that if the environment is hot enough, and the excess air low
>>> enough, and the flame path long enough,
>
>> OK this is re stating 2 of the 3 Tees
>
> Yup.
>
>> Excess air is also related to temperature as it increases mass flow which
> has to be heated by the fire and thus lowers temperature
>
> That is what I  have been trying to get Rocket stove builders to understand.
> Heat transfer efficiency is greatly lower by excess air.
>
>>> From your description I conclude that it is the high temperature
>>> coke-surfaced burns that are providing the best result.
>
>> I'm not sure I follow that but one thing is apparent and that is coke (and
>> coal) has a higher cv/kg than even dry wood so less mass to distribute the
> heat over.
>
> Well...not sure that has any bearing on it. It seems not to be mass related.
> Is it about PM not being easy to make when burning high carbon fuel, at
> least with domestic natural draft stoves.
>
> You know, there is a lot of noise being made about fans greatly reducing PM.
> Wonders and alla that. Well, maybe what is happening is that the draft of
> the stoves I am working with these days are producing the proper mixing that
> a fan would produce. It means all one needs to do is attaché a short chimney
> and the whole fan thing can be dispensed with. I mean, the results are there
> so it is not as if I am speculating about what could happen, it does happen
> and we are left to look for the reasons.
>
>> ....Yes that's what I thought, the question still remains which bits of the
> original feedstock is it? With cellulose it looks like a CH2OH group would
> be the first to split off but after that...
>
> From what I see the perfect combustion conditions for the volatiles portion
> of the burn are more demanding that the later carbon-rich burn. I think the
> gasifiers (meaning the people, not the products) should look into this
> because if the whole fuel can easily be burned without changing any
> settings, it means they can systematically use a greater % of the fuel
> collected. Yes some stoves burn the char when the secondary air is opened
> near the end of the burn, but why bother? Just work out how to burn while
> fuel continuously. Possible?
>
>>> What I observe is that particles are formed far more when there is
>>> badly burning whole fuel than when there is badly burning carbon. Why
>>> is that?
>
>> There's simply no scope for these secondary compounds to exist if char is
>> burned, they have to be formed during the combustion of the pyrolysis
>> offgas.
>
> That is my emerging conclusion.
>
>> Actually I suggest that the temperatures would be similar if you could
>> keep the layers stratified. It's the limits of what heat is released in
>> the various zones and the massflow that makes clean gasification of wood
>> far more difficult than gasifying coal, coke or char.
>
> Agreed, and hence the efforts that have to be made to homogenise the fuel to
> get a reasonably gasifier running without hiccups.
>
>>> It is becoming clear to me that seeing as coal, and pretty crummy coal
>>> at that, can be burned with such low particulate matter in the exhaust
>>> stream, why can't wood or any biomass be burned just as cleanly?
>
>> For one thing because even poor coal has a higher cv than dry biomass and
>> hence adiabatic combustion temperatures are higher. This is why wood was
>> able to make bronze but charcoal was necessary to produce iron,
>
> I have a very different interpretation of that. It relates largely to gas
> volume and not so much the heating value of the components (CV). H2 burned
> to H2O creates 2 times as many molecules of products as C burned to CO2.
> That means more dilution (total volume) which means lower overall enthalpy
> per cubic meter of rushing-past gases. In other words it is not the CV it is
> the number of molecules generated. H2 has a higher CV but doesn't make a
> small enough final volume to compensate for it.
>
>>> So maybe the problem is that biomass doesn't burn with low PM if it is
>>> damp, or cold, or both.
>
>> Actually these relate to the stove rather than the fuel.
>
> You get a gold star for that answer!
>
>> Depending on one's definition of clean, and I think you posted some figures
> from EPA for
> what is acceptable from a US wood heater, it's pretty certain that
> there's a temperature and retention time which pretty much guarantees
> clean combustion if air and fuel are right. I'd suggest a couple of
> seconds above 850C but others may prove different. This sort of
> temperature can be reaches with as harvested wood in a large well
> insulated combustor with getting on for 50% excess air.
>
> I am beginning to like hot reflective surfaces late in the burn. This flies
> in the face of insulative brickwork, but I was always encouraged by Peter
> Scott's ETHOS paper showing the truth of my long-standing claim of better
> performance from solid bricks than insulating ones on large stoves. 'Large'
> is of course arbitrary but there is something in the statement that is
> generally true. We are getting very good results from TLUD's and crossdraft
> products that rely absolutely not at all on anything insulating in the
> combustion chamber.
>
> I am hoping as always for a new era in stove combustor development to
> appear. Although not much is said about refuellable gasifiers, I had a lot
> of fun with two of Roger Samson's interns improving the Mayon Turbo Stove -
> a continuously refuellable rice hull gasifier with extremely low emissions.
> And I got more of the char to burn as well showing that it is not just a
> pretty face.....
>
> Regards
> Crispin back in the land of rabbits, squirrels and spring
>
>
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