Fuel derived from plants could have a big impact in both cutting
carbon dioxide emissions and supporting the agricultural economy.
Now, researchers have developed a reactor to produce "bio-fuel" from
widely available renewable resources such as wood and cereal crops.
The fuel can be burned in gas turbines, diesel engines, and Stirling
engines, as well as in conventional, oil-fired boilers. The objective
is to develop a small-scale, bio-fuel electricity generator for farms
and small communities. Above:
of the CFB (circulating fluidised bed) pyrolyser.
As governments seek ways of reducing their
dependence on fossil fuels, researchers are investigating new sources
of renewable energy. One of these is "bio-fuel", a term used to describe
any kind of fuel derived from cultivated crops. Not only are bio-fuels
infinitely renewable, but they can be burned without increasing the
amount of carbon dioxide in the atmosphere - the CO2 released
in burning is balanced by an equal amount removed from the air via
the photosynthesis process when the crop is growing. This energy resource
thus reduces the problem of global warming.
Pasquali Macchine Agricole, an Italian manufacturer of farm machinery,
is leading a JOULE project to develop a small electricity generator
which runs on fuel made from various kinds of crops. The project was
helped by Italian state subsidies for electricity generation from
renewables, explains Henk de Lange, the project's technical co-ordinator.
"There was a lot of tension in the agricultural community because
the European Commission obliged farmers to set aside a significant
part of the land they used to use for food production - often as much
as 15 or 20%. But if the land could be used to grow energy crops,
you would have a new important source of green fuel while maintaining
farm employment. Currently, this problem is reduced because the Commission
is thinking of abolishing the set-aside policy. Still, growing energy
crops on unused land will contribute to employment in the agricultural
One way to use cultivated material - known as biomass - is to simply
to burn it like coal, using the heat to generate electricity in
conventional power stations. But this is not very efficient, and
a better way is to turn the biomass into gas first, and then use
it to drive a gas turbine generator. "But these schemes are only
useful for electricity production for base load," Mr de Lange says.
"You have a continuous cycle and you just go on producing electricity."
They are not suitable for small-scale generation schemes.
From biomass to bio-
fuel The process chosen in this JOULE project is called pyrolysis,
in which the biomass is heated to around 500°C. A limited amount
of oxygen is allowed to enter the reactor to provide the heat to
sustain the pyrolysis process. The biomass does not burn; instead,
it produces bio-fuel vapours, which condense to a dark-brown, mobile
liquid that can substitute for fuel oil. The gasification process
takes place at a higher temperature and longer residence times,
and involves the partial combustion of the fuel to obtain a fuel-gas
whose combustible compounds are carbon monoxide, hydrogen, and a
small fraction of very light hydrocarbons. Pyrolysis has the advantage
over gasification that bio-fuel has a much higher energy density.
Moreover, it can be stored for long periods and easily pumped and
transported. "This is one of the first attempts to go from biomass
to electricity production using this route," says Mr de Lange. "No
other systems are widely available on the market yet. The objective
of this project was to develop a commercial application for this
The design and construction of the pyrolysis reactor (the pyrolyser)
and power plant were the responsibility of project leader Pasquali.
The Umbrian Region (Italy) offered its support for investigating
the development, harvesting, transport, and storage of the fuel
crops. Stadtwerke Saarbrücken (Germany) carried out a market survey
in their area to find out how this technology could be implemented
there. They also investigated the use of this bio-fuel in Stirling
The prototype power plant consists of a pyrolyser coupled to a
100-150 kWe generator. Early tests showed that 55-65% of the biomass
was being converted into fuel.
Sorghum performs best
In principle any kind of biomass can be used to make bio-fuel by
this process, but the quality depends on crop used. "So far the
best results have been obtained with wood, but from a strategic
point of view wood is not the best energy crop," explains Mr de
Lange. "Wood typically gives 15 dry tonnes per hectare per year,
whereas sorghum could give you as much as 40. So sorghum and certain
other types of cereal grasses would be more interesting, but it
is more difficult to pyrolyse them."
Sorghum is especially useful because the juice extracted from it
can be fermented to make ethanol, while the solid residue is processed
by pyrolysis. The mixture of these two fuels performs even better
than bio-fuel alone, especially in existing engines. One part of
the project was the design of a harvesting tool to harvest, cut,
and dry the sorghum directly in the field.
Mr de Lange is cautiously optimistic about the future for bio-fuel.
"It was a very useful project. We have developed a pyrolyser which
works. We have looked into using it in gas turbines, diesel engines,
and also conventional boilers. It's not really commercial yet, because
a number of problems remain to be solved and a number of issues
have to be looked at further."
Although Pasquali is concentrating on small-scale applications
of up to a few hundred kilowatts - specifically for farms and agricultural
co-operatives - there is also a lot of interest in Europe in generating
electricity from bio-fuel at the megawatt scale. Similar projects
are underway elsewhere, especially in Britain, Sweden and Finland.
"In Sweden they don't have their own oil resources, but they have
large forests and also stringent objectives in reducing carbon dioxide
emissions. On a European level there is quite some interest in this