[Stoves] Charcoal from waste - home cooking or other markets? (Re: Crispin, Anand Karve)

Fri Oct 14 10:44:35 CDT 2016

Our experience with sawdust derived biochar has been from industrial wood
boilers. Large quantities of boiler biochar, or high carbon flyash (>60% C),
are now used as an agricultural soil amendment and, when processed, as a
component of stormwater filtration and soil remediation.

Sawdust is typically less than 6 mm (1/4 in.) and often contains 45%-60% MC
wet basis. The width, or kerf, of a saw often determines the size, typically
3 mm or 1/8 inch. Most trees contain about 45% MC wet basis in the standing
tree. An open pile of sawdust can absorb up to about 63% MC. At 63% MC there
is not enough energy in the wet wood to support combustion so it must be
heated externally. 50% MC is usually used as a design basis for burning
"wet" sawdust. 

At typical furnace temperatures of 815-980C (1500-1800F) it takes about
three seconds to completely burn a 1 mm wood particle at 50% MC: one second
to dry; one second to devolatilize; and one second to burn out the char. A
wood particle in a boiler will typically burn down to about 1 mm. At that
size and density a char particle will usually float out of the furnace with
combustion gases unless it reacts directly with combustion air. 

Industrial and utility wood boilers are large vertical shafts  that are
several meters (50-100 ft) high. Wood fuel, including bark, milled (or
"hogged") wood, and sawdust are pneumatically injected across the width of
the shaft at a height of about 2 meters (6.5 ft) above a grate. Injectors
are spaced about 1.5 m apart and "paint" a layer of fuel about 2 meters wide
and 4-5 meters long along the length of a grate The grate may be fixed or
moving. It may have air holes or can be a bubbling bed of sand. It is
normally expected that about 60-80% of the fuel will land and burn on the
grate and about 20-30% will burn in suspension above the grate. 60-80% of
the combustion air is injected through the grate where It burns fixed carbon
(charcoal) and volatile gases as they evolve from the heated fuel.  About
20% of the combustion air is injected above the fuel to create a fireball
than fills the lower 20-30% of the furnace. Total oxygen is controlled to
about 4-8% of the combustion gases. Mixing of the air (oxygen) and gases is
never perfect so there are significant volumes which are sub-stoichiometric,
where there is  not enough air to burn the combustible wood gas. Hence a wet
wood particle passes through a mixture of combustible and combusted sweep
gases for several seconds depending on its size and density.

As noted it takes time to dry the solid, convert the volatiles to gas and
burn out the carbon. If  a boiler is too short and the gas flow is high the
particles dry and devolatilize and a lot of carbon carries out of the
furnace/boiler. We find that wet sawdust injected into a large boiler
creates large quantities of high carbon flyash which is separated in the
first mechanical separators (multi-cyclones). Wet sawdust also creates high
unreacted carbon monoxide. We suspect that this CO is formed in the upper
furnace when gases from partially devolatilized sawdust/wood particles are
cooled and quenched as they enter the heat transfer (convection). CO will
not ignite in the absence of direct flame below about 760 C (1400F). The
easiest way to bring a boiler with CO and particulate emissions problems
into compliance is to remove wet sawdust from the fuel.  

The char that is formed makes a very good biochar. It has been thermally
processed in an environment of superheated steam, pyrolysis gases, N2 and
CO2. Char yield is about 2% of the fuel input. Thousands of tons of high
carbon flyash biochar are produced every day. Hundreds of tons are being
used as soil amendments. The process is similar to entrained flow
pyrolyzers, like the Ensyn process. In entrained flow pyrolysis char yield
is low (~15%) and the energy from the char is normally used to heat the
process.

Forest Concepts, in Auburn, Washington, has developed a process to make
sawdust-like particles from chips called crumbles. The particles  have a
maximum dimension of 2-3 mm. They dry and devolatilize rapidly and make a
very uniform char that makes an excellent biochar ether in a moving grate or
entrained flow pyrolyzer. www.forestconcepts.com     

The high carbon flyash, which is black, is separated from combustion gases
in a high efficiency cyclone. The gases then enter a baghouse or an
electrostatic precipitator to separate the fine particulate. This ash is
usually gray. It is mostly composed of calcium and potassium salts. Calcium
and potassium chlorides, sulfates. As the fuel is heated these elements
volatilize and condense on larger particles as fine droplets or the fly in
the flue gas stream s sub-micron particles. The particles are too small to
be captured in a cyclone so they carry over to the baghouse or ESP. They
contain little carbon, hence the gray color.

Charcoal from community boilers or gasifiers could be used as a cooking fuel
or as a soil amendment. We see charcoal fines from earth kilns agglomerated
with clays to make cooking fuels. Alternatively we could make charcoal from
wet sawdust by injecting it into a hot gas stream, like in a boiler, but
there has to be enough heat and residence time to carbonize it. 

Tom   

-----Original Message-----
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of
ajheggie at gmail.com
Sent: Friday, October 14, 2016 3:59 AM
To: Discussion of biomass cooking stoves <stoves at lists.bioenergylists.org>
Subject: Re: [Stoves] Charcoal from waste - home cooking or other markets?
(Re: Crispin, Anand Karve)

[Default] On Thu, 13 Oct 2016 15:07:35 -0700,Tom Miles <tmiles at trmiles.com>
wrote:

>A cyclone sawdust gasifier was developed and tested by Jim (JW) cousins 
>in New Zealand in the early 1980s. Google "cyclone gasifier" 
>http://pubs.acs.org/doi/abs/10.1021/i200031a063
>
>There have been various research versions since but nothing commercial. 
>
> 
>
>Jaques Lede worked on a fast pyrolysis cyclone reactor. I suspect that the
char yields were pretty low. 
>
>https://www.researchgate.net/publication/231393861_The_Cyclone_A_Multif
>unctional_Reactor_for_the_Fast_Pyrolysis_of_Biomass
>
> 
>
>Cyclonic pyrolysis and gasification of lighter ag residues in the 1970s and
1980s required recirculation of pyrolysis gases to maintain high mas flow in
the reactor while limiting air or reactive gases. 

A few points:

pyrolysis of individual particles from sawdust will inevitably lead to a
lower yield of char than the pyrolysis of the same mass solid piece of wood,
this is because secondary reactions  which deposit solid carbon in the
matrix of char cannot occur and lighter pyrolysis products are carried away
as gases and vapours.

The processes cited by Tom don't appear to be aimed at making char and
yields as low as 3% are mentioned. But these are probably concurrent or co
current devices with through flow rather than one ended?

My experiments with a device, which Ken will remember seeing, (it was
intended for burning fairly high moisture content wood chip with no smoke
and  low CO) showed that a flame curtain developed all the time volatiles
were evolved and the nascent char could be seen below the flame, adding
small quantities of wood chip resulted in the fresh wood falling onto the
char surface and pyrolysing before being burned out once primary air made it
past the flame curtain.

By changing the air supply it is possible to change the characteristic in
the pot. The flame then becoming the feedback path for the pyrolysis.

As to the limiting reactive gases, yes there will always be some complete
combustion taking place but I speculate with  increasing air pressure and
smaller nozzles to deliver the sawdust into the burner air mass could be
minimised at the cost of more power consumption in the fans. Also more dry
sawdust could be metered in separately from the air:sawdust supply

I'd made bulk char many years earlier by burning "lop and top" in a charcoal
ring kiln and I believe I posted pictures of the flame curtain to the list
in the 90s. It was the fastest way of producing char from otherwise waste
material but only 50% made the size class required for lump wood charcoal in
this country and it was difficult to preserve the char without quenching
with water.

If the requirement is to maximise the char yield then briquetting then
pyrolysis would enable the secondary reactions to take place within the
briquette.

One thing we learned very early on is that in a kiln the yield is
drastically affected by moisture content. This is because the enthalpy of
vaporisation of the water content is so much higher than raising the
particle temperature above 100C to the pyrolysis temperature and all this
energy is supplied by the feedstock. We see this in TLUD burning but that is
self limiting to an extent as high mc material will not support the
pyrolysis front.

So I can envisage a simple continuous feed retort  for fresh sawdust which
still uses combustion of the pyrolysis offgas to drive the drying and
initial heating to pyrolysis temperature but keeps the processes separate so
that steam does not dilute the offgas.

I have an old colleague from many years back when we used to harvest timber
together who now runs a sawmill by Billingshurst which is about
30 miles from where you lived Ken, should you wish to progress anything on
the sawdust front. I also still have a collection of parts from wood chip
and pellet burners to play with.

My worry would be finding a use for the fine char and the realisation that
fine carbon dust would not be good for the environment, especially if it got
into the waterways.

Andrew

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