Designing Ecological Habitats - Gaia Education

appropriate tecHnology: searcHing for tHe rigHt fit 147
Gasification Basics
A black-box approach to gasification is essentially the process of turning
organic material into usable energy. This of course is nothing particularly
new as gasification was used extensively during the Second World War.
What gasification does is very different from conventional energy generation
mechanisms. Most energy generating involves transferring energy from a
specific fuel source into a fluid which carries the energy through a turbine
which, in turn, powers a generator. This is true for fossil fuels, natural gas,
nuclear, and geothermal energy. Gasification, in contrast, works on the
‘operating system’ of fire itself.
First, picture a lit matchstick. What you see is a glowing red flame at the
end of a stick of wood the size of a toothpick. But there are actually several
processes that make up this flame. The visible flame isn’t the wood burning
but something else. There are four fundamental processes that make up this
flame. These are: drying, pyrolysis, combustion, and reduction.
Drying is a rather straightforward process; it is the dehydration of the
fuel in question. The material is heated to a particular temperature until
water begins to evaporate. In our matchstick system, this is the first process
to occur, right there at the tip of the match.
Pyrolysis is an underrated process that is responsible for the visible
flame of on your matchstick. Pyrolysis is accelerated decomposition under
high temperature. What happens here is that the wood begins to break up
into tar and charcoal. Other substances that were in the wood are also
released in this process. In the matchstick system, it occurs right below the
visible flame. It is the combustion (oxidation) of the released gas, tar, and
other volatile substances that is responsible for the visible flame on the
matchstick. The combustion of these materials yields carbon dioxide and
water vapor.
Reduction is the key process in gasification. During pyrolysis, gas, tar
and charcoal are formed, and these are then oxidized. Reduction occurs on
the surface of the charcoal that was formed during pyrolysis. The charcoal,
at very high temperatures, consists of large amounts of carbon. When the
carbon dioxide and water vapor are allowed to react with the charcoal, the
carbon in the charcoal takes oxygen atoms from carbon dioxide and water.
This leaves carbon monoxide and, our primary concern, hydrogen gas. This
latter substance is known as the ‘synthesis gas’ and is used to power an
engine which in turn drives a generator.
There are several types of gasifiers. A gasifier, in general, looks like a
cylindrical rocket with openings for fuel and air. One of the simplest forms
is an ‘updraft gasifier’. Fuel is loaded at the top, air enters from the bottom
and synthesis gas is piped out from fittings located at the top. This type of
gasifier can be likened to a cigarette. The tip is where combustion occurs
followed by the reduction zone, the pyrolysis zone, and then the drying
zone. This ‘smoker’, as a result, breathes in synthesis gas and tar.