[Bioconversion] Veg Lister - torrified woodcips

[Ken Boak]

Geoff, & list,

I see torrefaction priarily  as a means to improve the overall efficiency of
th gasification process, resulting in a more homogenous fuel-stock and one
that will have less tar content. It also seems a useful means of exploiting
the high thermal energy in the exhaust gas stream, and it might be possible
to use a condensing heat exchanger to recuperate some of the flue losses.

Separating the volatiles from the torrefied wood and burning them somewhere
where they do not re-enter the main engine gas flow, or condense out as tars
would appear to be beneficial. As they carry about 10% of the energy content
of the woodchips, they could be burnt in a  burner zone  - ie flared off,
and used for fuel drying or heating water  - as this is a CHP system, hot
water is in great demand.  Compared to the energy retained in the exhaust
gas stream, these volatiles only represent about 1/3rd.

As yet I cannot say whether the resulting mix of engine exhaust and the
driven off volatiles can be sensibly recirculated into a char bed and do
anything useful for us. I just don't know without trying.

At some point, the exhaust gases are going to have to be dumped, as I
suspect the high nitrogen content will cause a problem and they cannot just
be recirculated through the gasifier and engine ad infinitum,  if we assume
the engine is drawing in a 50% fresh air charge on each induction stroke,
there is always a similar quantity of exhaust N2 to get rid off.

However, before the N2 is dumped, it might as well give up its heat load and
use its energy (pressure) as a transport mechanism - possibly even
transporting finely divided torrefied wood and char particles back into the
main gasifier by way of a wind sieve as porposed by Kalle.

In this respect, I see torrefaction unit and gasifier as two separate units
working in batch mode.  Part of the reason for the torrefaction unit isto
utilise the waste heat in the exhaust, absorb the pulsation energy and
utilise any waste CO, CO2 and excess air that might be present.  There might
turn out to be sufficient excess air in the exhaust gas to support an
incandescent charcoal bed at the base of the torrefaction unit, where the
exhaust gases first enter the woodchip column.

With regards to fuel utilisation, I have run the Lister at 1/3rd of rated
rpm using between 320 and 360ml of veg oil per hour. The dc output of the
generator was measured as  297W.  If we assume that a similar amount of oil
will be needed for pilot injection (approximating 20% of fuel input) then
the woodgas will need to supply the other 4/5ths.

If we are running at full speed,  650 rpm,  and the swept volume of the
engine is 1433cc,  then the estimate of air   required per hour is   650/2 x
1.433 x 60 =  27944 litres

Typical gas calorific value will be around 4600 KJ/m3 . For full power
output the Lister will need to draw in approximately 11.3kWh in gaseous
form, equating to 8.84m3 of gas per hour.

This suggests that the air/gas mix will be more likely be 2:1  than 1:1 and
reducing further with improved calorific value gas.

I hope to be having a small Chinese gasifier
http://www.wattpower.com/wpindex6.html  assessed by a colleague for its
suitability for duel fuelling the Lister.

These units are $250 FOB China,  and come with scrubber, cleaner/cooler and
80W blower fan for starting. At that price it is hardly worth considering
manufacturing your own gasifier.




Ken