[Gasification] Torrified woodchips as a gasifier fuel

[Ken Boak]

Whilst most of the recent discussion has been about producing engine grade 
gas, without the tars that cause problems to the valve gear, I wonder 
whether there might be another possibility, particularly for running 
stationary generator sets for CHP.

Whilst the Scandanavian pioneers soon learnt that the charcoal gasifier was 
the best a producing engine grade gas for their vehicles,  the problem was 
that by converting wood block fuel to charcoal,  that you are losing a 
significant proportion of the energy contained in the wood.  In charcoal - 
depending on the process of manufacture, only 25 to 55% of the original wood 
energy remains.

However, with some CHP systems, particularly when heat is required for space 
heating in the ratio of roughly 10 parts heat to 1 part power,  a typical 
diesel generator set cannot provide enough thermal output.

If torrified woodchips were used for the gasifier - these would contain more 
energy than charcoal,  less tars than the original wood, and because of 
their friable nature, would be more suited to mechanical handling and 
possibly pulverisation.

This lead me to the idea of using the hot, "inert"  exhaust gases from the 
diesel engine, to heat the woodchip fuel to roughly 300 C, and to drive off 
the volatiles and tars, where they could be burned with secondary air to 
produce usable heat - more than that contained in the exhaust gases alone.

After the wood chips have been torrified in this manner,  the retort is then 
reconfigured to gasifier mode, and the resulting ugases used to dual fuel 
the engine.

By using two retorts, one which is being torrified by the exhaust gases and 
one which is gasifying the previous batch of torrified wood chips, the 
engine can be run continuously, with minimum amount of gas cleaning, and the 
unwanted tars being consumed to produce additional thermal energy.

A retort, filled with woodchips, makes a great exhaust silencer fora noisy 
dlow speed diesel engine.  The woodchips would also work as a good exhaust 
filter, capturing the unburnt carbon from the exhaust,  so that these 
particles could be consumed when the contents of the retort are next 
gasified.


Ken