[Gasification] Waste wood, gasification and CHP

[Ken Boak]

Rolf & List,

I've changed the topic - because I felt we were drifting away from Doug 
Williams original No 2 Gasifier posting.

I agree that pellet stoves are pioneering current developments with "wood" 
burning stoves.  This is partly because of the uniform nature of the pellets 
that they present far fewer mechanical handling problems compared to 
woodchips or conventional firewood in the form of logs.  Gasifiers need to 
follow this same road of development if they are to find a market in 
inividual housholds.  The market is not yet supplying small woodchip stoves 
50kW to 100kW and above seems to be the introductory level - aimed at large 
houses and small institutions such as schools, businesses and hospitals etc.

Smoke is usually produced at start-up, until the steady state high 
temperatures have been achived within the stove to consume the smoke.  This 
is much reduced when you have the luxury of a fuel of uniform quality, size 
and moisture content such as pellets.

European legislation should restrict what products can be sold in Europe, 
but I am sure that there has been a considerable increase in the number of 
low cost stoves being imported from China, some of which are little more 
than a fire brick lined, steel box, with a door on the front and a flue 
opening at the back.  It is unlikely that these will score many points for 
efficient burning of wood, but their attraction is that they are cheap 
compared to European built stoves.

If there is a sudden rush to burn wood waste and other biomass, to offset 
our consumption of natural gas, then we need to address the overally 
efficiency of the stove - otherwise we will soon run into problems with the 
supply of firewood.  A 2 stage gasifying approach is likely to give the 
better performance.

It is clear that there is a lot of development possible with existing 
stoves, especially with combustion control and the mechanical handling of 
the fuel.  It would be interesting to have a comparison of the energy inputs 
required to produce pellets commercially and gasifier quality woodchips.

With Europe quickly running out of landfill sites, there are also large 
commercial opportunities for any business to convert wood waste and other 
residues (possibly MSW) into uniform pellets that can be burned cleanly in a 
gasifying stove, thus reducing the amount of combustible waste that ends up 
in landfill.

With the average UK household requiring something like 20,000kWh of heating 
fuel per annum, this equates to around 5 tonnes of wood based fuel (eg 
pellets at 4.7kWh/kg) , assuming that it can be burned at a reasonable high 
efficiency, commensurate with a natural gas boiler, say 85%.

Waste wood in the UK comes from the following sources

Construction/demolition   750,000 tonnes
Packaging/Pallets            670,000 tonnes
Household woodwaste    420,000 tonnes

Approximately 1.85 million tonnes per year available to the pellet makers, 
or enough for perhaps  200,000 households if the waste wood recovery ran at 
around 50%.  However I believe that the better solution will be to 
concentrate the utilisation of the wood waste into a number of distributed, 
small scale (<100MW) CHP plants:

Slough Heat and Power is one example of a generating station run on wood and 
fibre residues,  running  35MW and  12MW steam turbines

http://www.sloughheatandpower.co.uk/energy%20centre%20intro.htm

It consumes up to 5,500 tonnes of wood per week, of which 40% is waste wood.



Ken