[Gasification] Charcoal Gasifiers

[Tom Miles]

Thanks for the comments.

 

Everything counts because outside of India there are probably only a few
hundred gasifiers generating power. How many of those work 4000-6000 hours
per year? Very few I would think. 

 

The context is Subsaharan Africa (SSA) where charcoal is the primary cooking
fuel, and Southern Africa where abundant biomass exists that is
underutilized. One project in The DRC promotes fuel wood plantations for
charcoal. The villages where charcoal is produced are not electrified ad
will not likely be electrified in the future. Where there is off-grid power
from gensets it is expensive. So the challenge is how to recover power from
waste charcoal kiln gases.  

 

In most charcoal production the kilns are vented as they go through drying,
torrefaction, pyrolysis and cooling. During drying and torrefaction the
offgas is mostly CO2 with some CO. The calorific value of the gas is quite
low (<5 MJ/m3) for the first thirty to forty hours until kiln temperatures
reach about 280 C. Combustible gases do evolve which should be incinerated
in an afterburner. From 35-80 hours, a period of about 45 hours, gases
evolve with a calorific value of about 16 MJ/m3, depending on the peak
temperature (at 325-380C, 19 MJ/m3 at 500C). During that time the chemical
energy in the gas is considered to be recoverable. The gas of course
contains tars and acids. The challenge is how to recover this energy for
heat or local power generation. 

 

In the US charcoal kilns are required to use afterburners to burn out the
gas and pollutants. Solid fuel (e.g. sawdust) afterburners have been largely
unsuccessful. Most kilns use propane piloted burners that consist of an
eductor which draws the gases from several (usually four) kilns. The
standing propane flame ignites the mixture to insure combustion at 1600F as
required by the EPA. (The most recent batch kiln was installed with a
catalytic afterburner followed by a lime scrubber and a baghouse.) In
Brazil, where some large charcoal producers are supplied entirely by
plantations, companies have studied the evolution of gases and the potential
for energy recovery. They have tested arrangements of six or more kilns to
obtain a steady gas flow in overlapping kiln cycles. Some kilns have
afterburners or combustors. As yet there are no commercial systems
recovering heat and power from kilns. 

 

I am inspired by a local biochar producer who uses a downdraft wood fired
gasifier to heat his batch pyrolyzer. He starts the heating process with
wood gas. Once combustible gas appears from pyrolysis he valves it into the
combustor where the evolving gas combines with the flame from the producer
gas to heat the carbonizer. The wood gasifier provides a stable source of
heat and ignition. The combustor is the afterburner for the kiln gas. He
uses a chipper to prepare the wood for the gasifier and pyrolyzer. Chips,
nuts and shells make great fuels for downdraft gasifiers. If you don't have
a chipper (making gasifier fuel by hand is tedious) but you do make
charcoal, why not use charcoal as the pilot fuel? 

 

So my thought is to use charcoal from the kiln production to run a small
gasifier. No need to chip the wood. The gas could be used two ways: A) the
gas could be used as a pilot for an afterburner which could provide heat to
dry the fuel to be charred; and B)  cold clean gas could be used to power a
genset in dual fuel or 100% producer gas mode. A charcoal gasifier requires
steam which could be supplied from the kiln gas during the early cycle.
Introduce the kiln gas with the air to the charcoal gasifier. Thus the kiln
gas could fuel the charcoal gasifier reducing the fuel required to fired the
engine-genset. You could generate 50 kWe from about six small (12 m3) kilns.

 

While a downdraft gasifier might work, an updraft or crossdraft gasifier
might be more appropriate to this application because of the amount of fines
generated in the kiln.  A Missouri kiln makes about 23 tons of charcoal and
7 tons of fines from 100 tons of wood. In Brazil the fines are spread on the
plantations (what else in the land of terra preta)? They could also be used
to fuel an updraft gasifier if fired together with coarser charcoal.  

 

Putting a gasifier(s) together with a charcoal kiln might be a useful
"marriage of convenience."

 

Tom           

 

  

 

From: gasification-bounces at lists.bioenergylists.org
[mailto:gasification-bounces at lists.bioenergylists.org] On Behalf Of Robert
Kana
Sent: Sunday, May 08, 2011 10:08 AM
To: gasification at lists.bioenergylists.org
Subject: Re: [Gasification] Charcoal Gasifiers

 

Dear Tom,
We have tried using charcoal for engine quality gas in 2 types of gasifiers.
One was our own design, downdraft with tuyers and throat, the other design
was also downdraft grateless design. At first we had some clingers problems,
later fixed with adjusting air volume. Gas comes out pretty clean. We washed
for cooling gas, in a venturi type washer, used wood chips/sawdust/charcoal
for filter medium. Charcoal used was kemiri (some type of nuts grow in
Indonesia) shells, coconut shells, regular wooden charcoal pieces and
briquette charcoal pieces. 
The result was OK. Because charcoal cost here is more, we stopped with the
charcoal gasification, now concentrating on rice husk and wood chips.
Any other details you need to know, we will try to answer. 
Actually I have almost 30 tons of charcoal pieces in stock, we were
originally thinking to built gasifier for use in our own generator. But the
charcoal pieces we can sell for $ 300.00 for ton. Rice husk here cost $
39.00 a ton and we get almost as clean gas from rice husk as charcoal.
Robert

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