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< prev - next > Energy Fuels and engines producer_gas (Printable PDF)
Producer gas is formed by the partial oxidation at elevated temperature of a carbonaceous
feedstock such as biomass or coal in a suitable furnace, generator, or producer unit; hence
the name producer gas. The process is known as gasification and is a relatively old
technology that was used extensively throughout Western Europe to produce ‘Town’ gas from
coal before natural gas became widely available. During the Second World War many
vehicles towed wood gasifiers as their fuel supply. More recently there has been a resurgence
of interest in biomass gasification for the production of fuel for power generation. Partial
oxidation, or gasification, takes place when there isn’t enough oxygen present for full
oxidation to occur i.e. when less than the stoichiometric amounts of oxygen needed for
complete combustion are present. As a result of this, partially oxidised products are formed.
If the temperature is sufficient, the primary products from the gasification of biomass are
carbon monoxide (CO)
carbon dioxide (CO2)
trace amounts of higher hydrocarbons such as ethane and ethene
nitrogen (if air is used as the oxidising agent)
Various contaminants such as small char particles, ash, tars and oils are also present with the
gases. If the temperature or contact time is insufficient, there will be an undesirably high
level of carbon dioxide (CO2). If the fuel is damp there will be hydrogen present and a little
water or steam may be injected to raise the calorific value of the gas and reduce the
proportion of nitrogen dilutant. Excessive amounts of hydrogen or moisture are a
disadvantage, and as metering is difficult, this provision may be omitted.
Typically producer gas has a low calorific (energy) value of 4-12 MJ/m3. This is due to the
dilution of the product gases with nitrogen from air during the gasification process. In
addition to its low calorific value producer gas is difficult to liquefy or compress and small-
scale gasifiers, in particular, can be difficult to control to match variations in load. Therefore
producer gas is best suited for use in stationary (i.e. power generation) rather than
automotive (i.e. mobile) applications. For power generation applications it is usual to use
producer gas to fuel internal combustion (IC) engines to drive a generator although on large-
scale plants (>5MW) gas turbines can be used.
At high temperatures, charcoal and liquids are either minor products or not present in the
product mixture. The partial oxidation can be carried out using air, oxygen, steam or a
mixture of these. There are three basic gasification techniques: air gasification, oxygen
gasification and pyrolitic gasification.
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