[Gasification] Making charcoal with engine exhaust.

[Ken Boak]

Andrew,

Thanks,- This is just the information I was needing.

I think the charcoal production is likely to be 3 grammes/sec not 300.

Based on a more conservative estimate of 2 grammes/sec this will yield 7.2kg
of charcoal per hour, and produce significant off-gases that can be further
reduced elsewhere, probably using the previous day's batch production of
charcoal.




Ken






----- Original Message ----- 
From: "AJH" <list at sylva.icuklive.co.uk>
To: "Ken Boak" <kenboak at stirlingservice.freeserve.co.uk>
Cc: <GASIFICATION at LISTSERV.REPP.ORG>
Sent: Monday, July 03, 2006 7:40 PM
Subject: *** SPAM *** Re: [Gasification] Making charcoal with engine
exhaust.


On Mon, 3 Jul 2006 17:13:25 +0100, Ken Boak wrote:

>My question now has to be, given an exhaust stream of nominal 300 C,
containing 77% N2 by mass and about 36kg per hour (between 2.5 and 3kW
heat ), how much wood chip could this char?

The perceived wisdom seems to be that pyrolysis remains endothermic up
till ~330C then it becomes mildly exothermic till ~440C outside of
this range you need to put energy in. My take is that this energy
fills 3 needs, 1 to boil off free water, 2 to raise the wood's
temperature to pyrolysis temperature and 3 to supply energy to break
bonds, the third is probably a small amount.

I would worry about being able to flare  the offgas done in the way
you describe because it will be diluted with the combustion products
of the diesel engine. I don't think the oxygen in the exhaust would be
significant at 300C. Bear in mind that a diesel running flat out will
have a lower oxygen content in the exhaust and we would expect to see
500C+.

I think I would choose to use the exhaust stream for drying only and
then I think you would need to dilute it to cut the risk of VOCs being
vented to atmosphere.

The carbonising could then be done in a simple furnace along the
principles Yuri described on the stoves list. I believe you have seen
this done in a burner made to my design. This has the advantage in
incinerating all the offgas cleanly.
>
>
>Re-phrased, how many joules of energy do I need to put into a kilo of 25%
MC woodchips (2.6GJ/tonne) to convert to charcoal?

>From first principles and with no heat losses:

1kg chips @ 25%mc wwb is .25kg water plus .75kg of dry wood

First you need to boil off the water that requires about .25 times
2.7MJ to get it away as steam, I make that .675MJ

Then you have to raise the wood to >270C, in fact anything less than
450C is hardly charcoal, more like torrefied wood. The specific heat
of wood is about 1700J/kg/deg C. You wish to raise it through about
250C. So we need 1700*250*0.75 Joules or about .3MJ.

It looks like about 1MJ/kg of your woodchips. Mind it will have to be
a strictly counter flow device or else the drying phase drops the
exhaust temperature below the carbonising temperature. I think it
would be highly polluting unless you can ensure a stable flare.

3kW looks like it would be 300grammes/sec.

AJH