[Gasification] Fwd: RE: Whole log pyrolysis for char production was Re: W...

[Jason]

Ken,

I find these sorts of exchanges useful to sharpen my own thinking on these
issues.

*"I suppose that firebricks are a simple capacitive thermal mass, to
prevent the outer casing of the traditional cast iron stove from seeing the
worst effects of thermal cycling, and to prevent excessive surface
temperature."*  -

Steel and cast iron also have capacitance, what is different is the heat
transfer coefficient. Ceramics are much much slower to absorb heat so your
fire gets hotter quicker. The most polluting part of the burn cycle in a
(traditional) stove is after the introduction of new fuel so using ceramics
to help ramp up the temperature quickly will help a great deal. My system
naturally ramps up the temperature quickly so ceramics are not required. In
comparison to the absorption rate the emissivity rates of ceramic and iron
are much closer.

*"What is the problem with pyrolysis occurring too early?  Is it simply
because fuel is pyrolysing in the wrong place, and there is no means to
transfer the pyrolysis gases to the combustion chamber, or is the problem
tar generation in the fuel magazine?"*

The problem is driving off all the volatiles too early so the gas mix is
largely carbon monoxide, which is much harder to combust, so while a
glowing bed of coals might seen great there will be a lot of wasted, but
unseen, energy going up the flue. Catalytic combustors work by bringing
down the combustion temperature of carbon monoxide. I did work this out
this until I had some instrumentation on it at the Decathlon.

*"My motivation for design is a more efficient woodstove, which radiates
more heat into the room in which it's located - say the living room, plus
provides adequate hot water via a heat exchanger to provide heating for
some additional rooms and hot water."* .....*."Traditional stoves generally
lose a lot of heat straight up the chimney. Whilst this generates draft, it
is a major cause of inefficiency."*

Taking heat from the top of the flue is a much better solution as at that
point you are capturing waste heat rather than heat needed for combustion.
Not easy to do though.

*"The nominal 8kW stove I have at the moment fails to produce much radiant
heat, and I am sure that the simple heat-exchanger tank at the back of the
combustion chamber seriously effects the combustion temperatures resulting
in more emissions and poor, inefficient combustion. For this reason I
believe that the only way to control emissions and combustion temperatures,
is to first gasify the wood fuel and then burn the wood gas at high
temperature with preheated secondary air."*

The simplest solution I can see for you is to put ceramics between the heat
exchanger tank and the combustion chamber. Bring your combustion efficiency
up and it will improve the whole system. Pre-heated secondary air is
overated, good design can make it unnecessary. I have a prototype stove
built and tested in October that I believe would be very close to the
capacity and dimensions of your stove. I did four sequential loads of 45%
moisture content wood without any visible smoke from the stack. The test
was duplicating NZS 4012/4013 compliance testing as best I could (stove on
scales + testing and weighing the fuel and its input times) and this test
was witnessed by a Justice of the Peace using a ringlemann card and the
subsequent report endorsed by him.

*"Some heat could be recuperated for secondary air pre-heating, using a
simple concentric heat exchanger made from twin-wall fluepipe."*

Check out the Mulciber from the Wood Stove Decathlon. They still had issues
with carbon monoxide though, too much heat too early.

*"A good stove should be easy to light, be easy to load, easy to clean out
ash. Additionally it should have a convenient batch burn time, and the
ability to control the heat (turn down), without too much loss of
efficiency.  The stove should be capable of handling the predominant fuel
type (say split logs) without additional fuel preparation."  *......*"These
are the features that I consider necessary to meet customer expectations."*

You can have any two of those. Lol.

Bringing it back to ceramics, my retrofitted stove in the Wood Stove
Decathlon beat all the EPA certified stoves for efficiency, so in the
83-84% region but I don't have the official report on that yet from
Brookhaven National Laboratory. The curve of diminishing return comes to
mind. I did not use any ceramics. I also achieved a zero emission
(particulates and carbon monoxide) test run that was supposed to take a 15
minute sample but had to be terminated at the 12 minute mark because the
gas analyser overheated.

Regards

Jason




On Mon, Jan 6, 2014 at 12:11 AM, Ken Boak <ken.boak at gmail.com> wrote:

> Jason,
>
> Thanks for the interesting comments. I suppose that firebricks are a
> simple capacitive thermal mass, to prevent the outer casing of the
> traditional cast iron stove from seeing the worst effects of thermal
> cycling, and to prevent excessive surface temperature.
>
> Taking this capacitive idea to the max, I guess is the masonry stove,
> which is all thermal mass intended to absorb and slowly release the heat
> from a brief but intense fire.
>
> I have magnetite bricks left over from an electric storage heater (common
> in the UK). My intention was to experiment with these for heat retention.
>
> What is the problem with pyrolysis occurring too early?  Is it simply
> because fuel is pyrolysing in the wrong place, and there is no means to
> transfer the pyrolysis gases to the combustion chamber, or is the problem
> tar generation in the fuel magazine?
>
> My motivation for design is a more efficient woodstove, which radiates
> more heat into the room in which it's located - say the living room, plus
> provides adequate hot water via a heat exchanger to provide heating for
> some additional rooms and hot water.
>
> The nominal 8kW stove I have at the moment fails to produce much radiant
> heat, and I am sure that the simple heat-exchanger tank at the back of the
> combustion chamber seriously effects the combustion temperatures resulting
> in more emissions and poor, inefficient combustion. For this reason I
> believe that the only way to control emissions and combustion temperatures,
> is to first gasify the wood fuel and then burn the wood gas at high
> temperature with preheated secondary air.
>
> Traditional stoves generally lose a lot of heat straight up the chimney.
> Whilst this generates draft, it is a major cause of inefficiency. Some heat
> could be recuperated for secondary air pre-heating, using a simple
> concentric heat exchanger made from twin-wall fluepipe.
>
> A good stove should be easy to light, be easy to load, easy to clean out
> ash. Additionally it should have a convenient batch burn time, and the
> ability to control the heat (turn down), without too much loss of
> efficiency.  The stove should be capable of handling the predominant fuel
> type (say split logs) without additional fuel preparation.
>
> There may be good reason to have the stove non-reliant on electrical
> power, relying on natural draft and thermosyphoning for it's normal
> operation.
>
> These are the features that I consider necessary to meet customer
> expectations.
>
> Having intensively run my existing stove for around 14 hours per day for
> the last 16 days, as the primary source of heat over the festive holiday
> period, I am tolerating its less than ideal performance, but am now certain
> that there must be a better design.
>
>
> regards
>
>
> Ken
>
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