Spark-ignition engines can be stoichiometric or lean-burn engines,
the latter common for larger sizes and having a higher efficiency.
Fuel cells are considered to become the small-scale power plant
of the future, having the potential to reach very high efficiencies
(460%) and low emissions. Special interest for biogas is focussed
on hot fuel cells (4800 1C) where CO2 does not inhibit the
electrochemical process, but rather serves as a heat carrier. Either
the solid oxide fuel cell, for small applications of a few kW, or the
molten carbonate fuel cells (up to 250kW and more) can be
envisaged.
Gas vehicles can use biogas as fuel [177], provided it is upgraded
to natural gas quality, and application in the same vehicles that use
natural gas (NGVs) becomes possible. At the end of 2005 there
were more than 5 million NGVs in the world. The number of public
transport vehicles driven on gas such as buses and waste trucks is
increasing considerably. Most of the gas driven personal cars are
converted vehicles that have been retro-fitted with a gas tank in
the luggage compartment and a gas supply system in addition to
the normal petrol fuel system. Dedicated gas vehicles run at a
better efficiency and also allow for more convenient placement of
the gas cylinders without losing luggage space. Gas is stored at
200–250 bar in pressure vessels made from steel or aluminium
composite materials. Today more than 50 manufacturers worldwide
offer a range of 250 models of commuter, light and heavy
duty vehicles. Gas vehicles have substantial advantages over
vehicles equipped with diesel or petrol engines, since CO2
emissions are reduced by more than 95%. Emissions of particles
and soot are also drastically reduced. Heavy duty vehicles are
normally converted to run on methane gas only, but in some cases
dual fuel engines can also be used. The dual fuel engine still has the
original diesel injection system and gas is ignited by injection of a
small amount of diesel oil. Dual fuel engines normally require less
engine development and maintain the same driveability as a diesel
vehicle. However, emission values are not as good as for the
corresponding dedicated gas vehicle and the engine technology
remains a compromise between spark ignition and diesel engine.
Beside the close to 100% CO2 reduction, pure gas engines with
catalytic converters demonstrate far better emission values than
the mostmodern diesel engines (EURO 4 and 5) tested according to
the European Transient Cycle (ETC) or the Enhanced Environmental
friendly Vehicle (EEV) standard at the EMPA, Switzerland. Stoichiometric
gas engines with an air-to-fuel ratio of 1 demonstrate a
better emission pattern than lean engines. However, both are far
better than dual fuel engines although at a reduced efficiency.
The number of biogas and natural gas filling stations is still
insufficient in Europe and elsewhere in the world, although the
situation is improving enormously with the number of pumping
stations multiplied over the last few years: at the end of 2005
there were 1600 pumping stations in Europe. By the end of 2006
Germany had 1000 stations in operation, Switzerland 100 and
Austria more than 50.
Biogas injection in the gas grid is possible, and various countries
of the EU have proposed standards for injecting upgraded biogas
into the grid to avoid contamination of the grid. These standards
of, e.g. Sweden, Switzerland, Germany and France, fix limits for,
e.g. sulphur, oxygen, particles and dew point. Upgrading methods
must allow treated biogas to meet these stringent quality
standards. This upgrading and associated cost outweigh the rising
costs of fossil fuels.
6.3. Biogas upgrading technologies
The major reasons for gas upgrading include the need to fulfil
the requirements of gas appliances (engines, boilers, fuel cells,
vehicles, etc.); to increase the heating value of the biogas; and/or
to standardise the biogas quality. The required quality depends
strongly on the application, as shown in Table 15.
6.3.1. Carbon dioxide removal
Removing CO2 increases the heating value and leads to a
consistent gas quality, similar to natural gas. When using removal
techniques, it is important to keep methane losses low for
economical and environmental reasons since CH4 is a greenhouse
gas 21 times stronger than CO2 [178].
There are different methods of removal, most commonly
performed as absorption or adsorption. Cryogenic separation
would also be possible, albeit expensive. Membrane separation
gains interest [179].
In absorption processes, CO2
and H2S are simultaneously
removed due to the difference in binding forces of the polar CO2
and H2S and the non-polar CH4. Water is the most common
solvent for counter-current scrubbing of pre-compressed biogas
(4–7 bar). The design of a water scrubbing system depends on the
solubility of CO2, as solubility is governed by pressure, temperature
and pH as given in Table 16: as the pressure increases, the
solubility of CO2 in water increases; but decreases as the
temperature increases.
After pressure scrubbing, CO2 and H2S are released in a flash
tank, where the pressure is reduced and the temperature possibly
increased. H2S, which is released to the air can create an emission
problem. Some of the sulphur accumulates in the water and can
cause problems of fouling or corrosion of piping. It is hence
recommended to separate H2S beforehand. Air or vacuum
stripping are seldom used since introducing O2 in the system.
Results show that 5–10% of CO2 remains in the biogas.
Of course, absorption can be nearly complete if Ca(OH)2
solutions are used to remove both CO2 and H2S, resulting in the
formation of insoluble CaCO3 and CaS.
Organic solvents such as polyethyleneglycol (Selexols, Genosorb
s) and alkanol amines (mono-ethanol-amine, or di-ethanolamine)
can be used to dissolve CO2 and H2S, which are more
soluble than CH4 in these liquids, and low-pressure operation is
possible. The chemical needs to be regenerated with steam. Only
small amounts of CH4 are removed. Reductions of CO2 to
0.5–1 vol% in biogas are possible. The organic solvent removal
units are, however, more expensive than those using water as a
solvent, and suffer from the need to periodically partly discharge,
dispose and replace its solvents.
Spark-ignition engines can be stoichiometric or lean-burn engines,
the latter common for larger sizes and having a higher efficiency.
Fuel cells are considered to become the small-scale power plant
of the future, having the potential to reach very high efficiencies
(460%) and low emissions. Special interest for biogas is focussed
on hot fuel cells (4800 1C) where CO2 does not inhibit the
electrochemical process, but rather serves as a heat carrier. Either
the solid oxide fuel cell, for small applications of a few kW, or the
molten carbonate fuel cells (up to 250kW and more) can be
envisaged.
Gas vehicles can use biogas as fuel [177], provided it is upgraded
to natural gas quality, and application in the same vehicles that use
natural gas (NGVs) becomes possible. At the end of 2005 there
were more than 5 million NGVs in the world. The number of public
transport vehicles driven on gas such as buses and waste trucks is
increasing considerably. Most of the gas driven personal cars are
converted vehicles that have been retro-fitted with a gas tank in
the luggage compartment and a gas supply system in addition to
the normal petrol fuel system. Dedicated gas vehicles run at a
better efficiency and also allow for more convenient placement of
the gas cylinders without losing luggage space. Gas is stored at
200–250 bar in pressure vessels made from steel or aluminium
composite materials. Today more than 50 manufacturers worldwide
offer a range of 250 models of commuter, light and heavy
duty vehicles. Gas vehicles have substantial advantages over
vehicles equipped with diesel or petrol engines, since CO2
emissions are reduced by more than 95%. Emissions of particles
and soot are also drastically reduced. Heavy duty vehicles are
normally converted to run on methane gas only, but in some cases
dual fuel engines can also be used. The dual fuel engine still has the
original diesel injection system and gas is ignited by injection of a
small amount of diesel oil. Dual fuel engines normally require less
engine development and maintain the same driveability as a diesel
vehicle. However, emission values are not as good as for the
corresponding dedicated gas vehicle and the engine technology
remains a compromise between spark ignition and diesel engine.
Beside the close to 100% CO2 reduction, pure gas engines with
catalytic converters demonstrate far better emission values than
the mostmodern diesel engines (EURO 4 and 5) tested according to
the European Transient Cycle (ETC) or the Enhanced Environmental
friendly Vehicle (EEV) standard at the EMPA, Switzerland. Stoichiometric
gas engines with an air-to-fuel ratio of 1 demonstrate a
better emission pattern than lean engines. However, both are far
better than dual fuel engines although at a reduced efficiency.
The number of biogas and natural gas filling stations is still
insufficient in Europe and elsewhere in the world, although the
situation is improving enormously with the number of pumping
stations multiplied over the last few years: at the end of 2005
there were 1600 pumping stations in Europe. By the end of 2006
Germany had 1000 stations in operation, Switzerland 100 and
Austria more than 50.
Biogas injection in the gas grid is possible, and various countries
of the EU have proposed standards for injecting upgraded biogas
into the grid to avoid contamination of the grid. These standards
of, e.g. Sweden, Switzerland, Germany and France, fix limits for,
e.g. sulphur, oxygen, particles and dew point. Upgrading methods
must allow treated biogas to meet these stringent quality
standards. This upgrading and associated cost outweigh the rising
costs of fossil fuels.
6.3. Biogas upgrading technologies
The major reasons for gas upgrading include the need to fulfil
the requirements of gas appliances (engines, boilers, fuel cells,
vehicles, etc.); to increase the heating value of the biogas; and/or
to standardise the biogas quality. The required quality depends
strongly on the application, as shown in Table 15.
6.3.1. Carbon dioxide removal
Removing CO2 increases the heating value and leads to a
consistent gas quality, similar to natural gas. When using removal
techniques, it is important to keep methane losses low for
economical and environmental reasons since CH4 is a greenhouse
gas 21 times stronger than CO2 [178].
There are different methods of removal, most commonly
performed as absorption or adsorption. Cryogenic separation
would also be possible, albeit expensive. Membrane separation
gains interest [179].
In absorption processes, CO2
and H2S are simultaneously
removed due to the difference in binding forces of the polar CO2
and H2S and the non-polar CH4. Water is the most common
solvent for counter-current scrubbing of pre-compressed biogas
(4–7 bar). The design of a water scrubbing system depends on the
solubility of CO2, as solubility is governed by pressure, temperature
and pH as given in Table 16: as the pressure increases, the
solubility of CO2 in water increases; but decreases as the
temperature increases.
After pressure scrubbing, CO2 and H2S are released in a flash
tank, where the pressure is reduced and the temperature possibly
increased. H2S, which is released to the air can create an emission
problem. Some of the sulphur accumulates in the water and can
cause problems of fouling or corrosion of piping. It is hence
recommended to separate H2S beforehand. Air or vacuum
stripping are seldom used since introducing O2 in the system.
Results show that 5–10% of CO2 remains in the biogas.
Of course, absorption can be nearly complete if Ca(OH)2
solutions are used to remove both CO2 and H2S, resulting in the
formation of insoluble CaCO3 and CaS.
Organic solvents such as polyethyleneglycol (Selexols, Genosorb
s) and alkanol amines (mono-ethanol-amine, or di-ethanolamine)
can be used to dissolve CO2 and H2S, which are more
soluble than CH4 in these liquids, and low-pressure operation is
possible. The chemical needs to be regenerated with steam. Only
small amounts of CH4 are removed. Reductions of CO2 to
0.5–1 vol% in biogas are possible. The organic solvent removal
units are, however, more expensive than those using water as a
solvent, and suffer from the need to periodically partly discharge,
dispose and replace its solvents.
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