[Terrapreta] Economics of biochar

[andrew]

On Tuesday 08 January 2008 18:57, Sean K. Barry wrote:

>
> Is the total energy, 15.4 MJ/kg, estimated from the BTU of the dry
> arisings?

Yes but there will be rounding errors, I used 18.6MJ/kg for dry 
biomass but less the 17% of inert material that was ash and soil 
contamination.
>
> 2.97 MJ is the estimated energy that is left remaining in the ~11%
> charcoal yield (dw% = 100% x (0.323 - 0.210)/1.22)) (with >80%
> fixed Carbon (fC) in the charcoal byproduct)?

Yes, that's the calculation for the yield of char on an ash free 
basis expressed as a fraction of the oven dried sample. I was 
guessing a bit on the calorific value of the char but 27MJ/kg should 
be in the ball park.
>
> I see, (15.4 MJ/kg) * (1.22 kg) = ~18.8 MJ total energy content of
> the sample and ~18.8 MJ - 2.97 MJ = ~15.8 MJ released ?

Well I had moved the calculation from the actual sample to one on a 
per kg basis.
>
> How do you arrive at the amount of energy released?  You state
> some dry matter was lost in the carbonization because of
> combustion. Was that, 0.22 kg, so that ...

No I ignored the loss, if it were significant it would make the 
calculation a pessimistic one.
>
> (15.4 MJ/kg) * (1.22 kg - 0.22 kg)  - 2.97 MJ = ~12.5 MJ ?

I think your .22kg is due to the change from the sample weight to a 
per kg basis.
>
> You also stated that the other released heat energy of the process
> was enough to dry 3-5 times the biomass, plus bring that to the
> exothermic temperature.  What is the energy conversion efficiency
> of the pyrolysis process then?

I think it is a bit difficult to calculate because we are dealing 
with a mixture of chemicals: cellulose, hemicellulose and lignin, 
which all break down at different tempratures but perceives wisdom 
seems to be that the process is largely self sustaining between 330C 
and 440C. As both mass and energy are conserved and we know how much 
energy is left in the char then we know that the remainder is given 
off in the process, we just have to harness it.

> What portion of that released energy (12.5 to 15.8 MJ) is in the
> product gas BTUs as combustible fuel gases and what portion is
> heat?

We can calculate the energy in the sensible heat of the offgas from 
its specific heat times the process temperature and the remainder 
shout be the calorific value of the fuel gas less the heat losses 
through the equipment but it probably isn't worthwile unless we have 
a need for the heat elsewhere because there is easily sufficient to 
do the job, i.e. process waste organic matter on site and leave it 
on site.
>
> Would it be possible to tune down this reaction by moving the
> biomass feedstock through the reaction faster, producing less
> gases, less heat loss, and more charcoal? 

Yes, I'm sure changing the residence time will have a large effect.


> Assuming that we wish 
> to pyrolyze completely (react until the exothermic stops), then
> how long does the biomass need to stay above the exothermic
> temperature?

This depends on the size, shape and moisture content, getting the 
moisture out of the middle of a particle is the limiting step as it 
is so endothermic.

> Once it reaches that temperature, then how long 
> before we move it out of the reaction zone, where it is cooler, or
> has less oxygen?

As I said this would be trial and error but we actually don't want 
any oxygen to survive into the reaction zone.
>
> I guess what I am grappling with trying to ask here, is if there
> is a way to throttle this reaction, to control the relative
> amounts, composition, and qualities of the byproduct outputs;
> charcoal, gases, and heat, by adjusting feedstock, gas, and/or
> charcoal flow controls, insulating the reaction, and while using
> temperature as the feedback?

Yes to all those things
>
> It has been discussed here, as well as in some referenced papers,
> that control over maximum internal particle temperature can best
> help manage VM% and porosity of the resulting charcoal byproduct. 
> Is seems that control over these properties is desirable for
> potential users of agrichar.  Shouldn't we then maintain the
> quality of the charcoal product, primarily by controlling the
> process temperature?

Probably but most of the time I've worked with charcoal the 
temperatures have sort of self limited themselves around the 500C 
mark, probably mostly because of heat losses through the uninsulated 
equipment.
>
> The 62 to 84% energy "released" as heat and gases is another
> concern.  Should more of that energy go into the charcoal or
> should some of it be harvested and converted to usable work,
> heating, or electrical power?

My colleague, briefly, successfully fuelled a gas turbine with it but 
by then the capital cost had escalated so far as to make it 
prohibitively expensive on the farm scale. There is obviously scope 
to make use of heat but that means moving the raw material to point 
of use, at which point you may be better off burning it out 
completely.
>
> It seems to me, that the ratio of the two byproducts; charcoal
> and/or usable process energy could depend on the economic values? 

Yes and at present it's not worth harvesting this waste for energy, 
so it is discarded to rot. I was just seeing if there could be 
potential for it as a carbon sink.

AJH