[Gasification] Making charcoal with engine exhaust.

[AJH]

On Mon, 03 Jul 2006 19:40:37 +0100, AJH wrote:

>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.

I sent that off too fast

3kW is 3000J/sec which is 0.003MJ/sec, wee need 0.3MJ/kg so we can
carbonise 0.003/0.3kg/sec = 10 grammes/sec

Someone else had better check the rest of my rushed maths!

AJH