Projected Costs of Generating Electricity - OECD Nuclear Energy ...

may be obtained by the process of fast or flash pyrolysis at moderate reaction temperatures, whereas slow
pyrolysis produces more charcoal (35% to 40%) than bio-oil. A main advantage (with respect to energy
density, transport, emissions, etc.) of fast pyrolysis is that fuel production is separated from power
generation.
Gasification is a form of pyrolysis carried out with more air and at higher temperatures in order to
optimise the gas production. The resulting gas is more versatile than the original solid biomass. The
gas can be burnt to produce process heat and steam or used in internal combustion engines or gas turbines
to produce electricity. It can even be used as a vehicle fuel. Biomass gasification is the latest generation
of biomass energy conversion processes and offers advantages over direct burning. In techno-economic
terms, the gas can be used in more efficient combined-cycle power generation systems, which combine
gas turbines and steam turbines to produce electricity. The conversion process – heat to power – takes
place at a higher temperature than in the steam cycle, making advanced conversion processes thermodynamically
more efficient. In environmental terms, the biogas can be cleaned and filtered to remove
problematic chemical components.
Anaerobic digestion (AD) is a biological process by which organic wastes are converted to biogas –
usually a mixture of methane (40% to 75%) and carbon dioxide. The process is based on the breakdown
of the organic macromolecules of biomass by naturally-occurring bacteria. This bioconversion takes place
in the absence of air, thus anaerobic, in digesters, i.e. sealed containers, offering ideal conditions for
the bacteria to ferment (“digest”) the organic feedstock to produce biogas. The result is biogas and
co-products consisting of an undigested residue (sludge) and various water soluble substances. Anaerobic
digestion is a well-established technology for waste treatment. Biogas can be used to generate heat and
electricity through gas, diesel or “dual fuel” engines at capacities of up to 10 MW. About 80% of
industrialised global biogas production stems from commercially exploited landfills. The methane gas
produced at landfills (“landfill gas”) can be extracted from existing landfills by inserting perforated pipes
through which the gas travels under natural pressure. If not captured, this methane would eventually
escape into the atmosphere as a greenhouse gas. Another common way of producing biogas by AD is by
using animal manure. Manure and water are stirred and warmed inside an air-tight container (“digester”).
Digesters range in size from around 1 m 3 for a small household unit to as large as 2 000 m 3 for a large
commercial installation.
Geothermal
Geothermal technology depends on the type and location of the natural resource. Since it is not
practical to transport high-temperature steam over long distances by pipeline due to heat losses, most
geothermal plants are built close to the resource. A geothermal system consists of three main elements:
a heat source, a reservoir and a fluid – the last being the carrier for transferring heat from the source to
the power plant. The heat source can be either a very-high-temperature (> 600°C) magmatic intrusion that
has reached relatively shallow depths (5 to 10 km) or, as in certain low temperature systems, the Earth’s
normal temperature, which increases with depth. The heat source is natural, whereas the fluid and the
reservoir can be introduced to the subterranean media by the project.
Geothermal power plants tend to be in the 20 MW to 60 MW range and the capacity of a single
geothermal well usually ranges from 4 MW to 10 MW. Typical minimum well spacing of 200 m to 300 m
is established to avoid interference. Three power plant technologies are being used to convert hydrothermal
fluids to electricity. The type of conversion depends on the state of the fluid (steam or water) and
on its temperature:
● Dry steam power plants use hydrothermal fluids primarily in the form of steam. The steam goes
directly to a turbine, which drives a generator that produces electricity. This is the oldest type of
geothermal power plant and was originally used at Larderello in 1904. This steam technology is still
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