Environmental Engineering Reference
In-Depth Information
water, which may then cause corrosion. Particles and siloxanes can also
cause problems since they can clog small parts (e.g. tubes) in the boiler.
However, small boilers are often not designed for the utilisation of raw
biogas, but only for natural gas; this opens up the possibility of using
upgraded biogas.
Industrial boilers are sometimes certified for using raw biogas. As with
small boilers, hydrogen sulphide together with condensed water may cause
corrosion. Since the components in large boilers are not so small, there are
less problems with particles and siloxanes than in small boilers. After the
boiler is adjusted to a certain biogas composition it should not vary too
much. However, if the boiler is equipped with an oxygen or carbon
monoxide sensor in the flue gas, more variations in gas composition can be
tolerated.
Microturbines are generally flexible in operation and tolerant to trace
compounds. Biogas that is burned in a microturbine is first compressed,
which means that it has to be dry in order to avoid condensation of water
from the gas. Some microturbines can tolerate hydrogen sulphide
concentrations of up to 1000 ppm, but particles and siloxanes can cause
problems in microturbines.
As with microturbines, the gas used in gas turbines is compressed, which
sets quality demands on the gas when it comes to water content. Quality
demands on the biogas used in gas turbines must often be discussed and
agreed with the equipment producer. Gas turbines can tolerate different
biogas compositions, but must be more finely adjusted to perform well.
Producers of gas turbines often specify a maximum tolerance of hydrogen
sulphide and particles in the biogas fuel.
Engines used for heat and power production can tolerate different gas
compositions but, as with other applications, hydrogen sulphide can cause
problems. Siloxanes can also cause problems.
Fuel cells can also be used for power production from biogas. Different
types of fuel cells use different fuels and have different sensitivities to
impurities in the gas. High-temperature fuel cells are more fuel flexible and
are also more tolerant to impurities in the fuel. High-temperature fuel cells
(e.g. molten carbonate fuel cells (MCFCs)) can use methane from biogas as
a fuel, but for fuel cells that operate at lower temperature (e.g. proton
exchange membrane (PEM) fuel cells), the biogas has to be catalytically
reformed to hydrogen which can then be used as a fuel. Compounds that
can be toxic to fuel cells and thus should be removed from the biogas are
hydrogen sulphide, halogenated hydrocarbons, ammonia and siloxanes.
These compounds should also be avoided in the catalytic reforming process
prior to reformed biogas being used in PEM fuel cells operating at low
temperature.
Search WWH ::
Custom Search