[Gasification] Torrified woodchips as a gasifier fuel

[Ken Boak]

Max, Harmon,

" The idea of trapping the waste carbon and using the waste heat from a WVO 
powered diesel using biomass destined for gasification sounds excellent to 
me and is a great example of whole cycle thinking that will truely overcome 
todays challenges in the future."


For small scale distributed power generation, and domestic scale CHP we 
really only have 2 viable options for the prime mover, namely spark ignition 
or compression ignition internal combustion engines.

Unfortunately suitable Stirling engine based systems have failed to appear 
on the commercial market - sadly the recent demise of Microgen in the UK was 
yet another blow for the commercial viability of the Stirling engine.  Steam 
engines or turbines or gas turbines have yet to feature in this arena, and 
it is unlikely that they will find application, except in the minority of 
specialist applications.

With an IC engine based generator converting fuel to electricity at about 
25% at best, it is imperative that the waste heat is channelled back into 
the system to impove the overall system efficiency. Whilst the utility 
company can afford to run a traditional coal plant at just 35% efficiency - 
the home generator has to be a lot more efficient if it is to succeed.

IC engines will remain the mainstay of home power generation and backup 
supply for several years to come.  If we continue to run them on 
conventional fossil fuels, clearly we are going to be at the mercy of the 
rising price of fossil fuels.
Any means of offsetting fossil fuel consumption, by using alternative liquid 
or gaseous fuels, will be of importance in the near future.

There are two technologies that offer some promise in this field, and have 
already been shown to be practical on a small scale  - namely gasification 
of biomass and biodigestion to produce biogas.  Both of these techniques 
will become increasingly important as fossil fuel prices rise.

I believe that both these techniques will be used together, in varying 
degrees, to make use of local waste resources to produce a mix of flammable 
gases that can run the IC engine.

It will be important to adapt the IC engine to run on different mixes of 
fuels, as the exact gas composition nor calorific value cannot be 
guaranteed.  The IC engine will have to become truly omnivorous to makefull 
use of the various alternative fuels  - this is no different to 100 years 
ago when engines were regularly run on whatever was locally available - coal 
gas, producer gas,  sewage gas, shale oil, kerosene, ethanol, naptha etc, 
etc.

Here are a few non-conventional fuels being offered in my region:

In the summer months, my near neighbours cut their lawns and once a week put 
out several dozen plastic bags of fresh grass cuttings - to be collected by 
the local council, for disposal in a garden waste composter plant.  I could 
quite easily divert 100kg per week of this material from my neighbourhood to 
keep a biogas digester running.

Tree surgery generates considerable amounts of woody biomass waste. With 
very little local market for this product, woodchips are selling for about 
$80 per tonne. This is half the price of firewood and about half the price 
of natural gas - on an equivalent energy basis.

Waste vegetable oil is still available from pubs and restaurants.  Much of 
this waste is now being diverted into commercial biodiesel manufacture, but 
it is still possible to obtain about 10 gallons per week with little effort. 
After filtration through a woven polpropylene felt bag, this WVO can be used 
directly in the slow speed diesel engine.  However 10 gallons a week isnot 
going to go very far. This is why it is imperative to run the engine in a 
dual fuel mode, and  make up the rest of the fuel requirement (60 to 80%) 
from producer gas or biogas.

The oily sludge, left over from the filtration process make an ideal 
additional feedstock for the biogas reactor, and this is a good way of 
disposing of this less than pleasant product.  Alternatively it could be 
mixed with woodchips and used to fuel the gasifier.

No one technology is going to dominate - we are likely to have to draw on 
all available techniques to make biomass conversion practical at such a 
small scale.

Once you have started your diesel generator on WVO, you now have no shortage 
of mechanical power, electrical power and high grade/low grade heat for 
utilising in the bioconversion processes.  Further experimentation will show 
how these energy inputs can be combined to improve the gasification and 
biodigestion processes.


Ken