[Stoves] Pressed biofuel - and Fire balls. Testing

[Crispin Pemberton-Pigott]

Dear Frank

>affected by temperature.... are we talking the difference between 20 deg 
>and 100 degs? or 400 deg and 1000 deg?

Almost any amount.

>Does a slight increase (20 to 100) in the air temperature going into the 
>fuel make a big difference in fuel combustion?

Indirectly, yes. What it affects is the rate at which volatiles are driven
off, or later, (mostly) CO. If the rate of fuel burn is increased, the power
of the stove goes up. If there is not enough air or turbulence or
temperature to light the CO, emissions suffer.

This can be seen most easily by taking a controlled air stove and putting
regular wood into it, and watching how it burns. Then put in some very dry
wood of the same type. It will in all likelihood make a lot of smoke because
the pyrolisation rate is so much higher due to the missing moderating
influence of the moisture. If you optimise a stove for dry wood, on the
other hand, it will not burn moist wood well because the wood 'particles' do
not light each other well. When the stove 'expects' wood to be very dry, it
fails to burn properly when the wood is dampened. Too much energy is lost
getting rid of the water to properly keep the fire going. The emissions
profile and energy efficiency are dramatically affected by the preheating
when the fuel is air dried.

If you preheat 100 degrees and burn light biomass briquettes, it is likely
you will generate more gas than can be burned. Gasifiers might exploit this
trick to get a high rate of gas production from a hard, dense biomass.

With coal it is a little different because there are oily ingredients that
come off the fuel at a very much lower temperature than the Carbon. Raising
the primary air 50 degrees would give you a very different burn rate, and
the lighting of the adjacent particle would be easier. Once the volatiles
are gone (evaporated=cooling) the surfaces heats much faster.

One question on my mind is the practical effect of lower the air temperature
to say -10 C on the particle lighting effect and the loss of volatiles. The
longer the volatiles can be kept in the flame, the easier it is to reduce
the CO and particulate emission. So the situation to avoid is driving out
the volatiles from a charge of fuel immediately and then be left with the
challenge of trying to cleanly burn a pile of coke or charcoal in the same
device. I am referring to the challenge of burning for 6-8 hours without
refuelling.

Regards
Crispin