174
Alternative Fuels for Transportation
catalyst to produce the syngas. The heat of the rich combustion converts the
natural gas to syngas, with the added oxygen combining with the carbon to
give the CO needed for the syngas. Systems are also known where the steam
reformer and autothermal reactor are combined. This is called the combined
reforming process. Typically, the steam reformer comes first, followed by the
autothermal reactor. Steam can also be added to an autothermal reactor to
adjust the product composition.
In this case, the overall reaction for formation of syngas from natural gas
can be written
2CH + O + H O → 2CO + 4H + H O ∆H =− 36.6 kJ/kMol CH
−
. . (6.3)
4
2
2
2
2
R
4
The composition is well suited to methanol formation.
In the direct process where a ratio of H 2 /CO = 1 is desired, this can be
obtained by recirculating the CO 2 from the products of the DME reactor back
to the reformer. Then the reforming reaction can be written
2CH + O + CO → 3CO + 3H + H O ∆H =− 16.5 kJ/kMol CH
−
. . (6.4)
4
2
2
2
2
R
4
This process gives a ratio of H 2 /CO = 1, which is the stoichiometric mixture
for DME formation from syngas.
For coal gasification, the process is more complicated since the fuel is in
the solid state and contains many impurities, and various types of gasifiers
are used to convert the solid coal to syngas. One type is a fixed bed system,
where solid coal particles are found in a bed in the reactor, and hot gasses are
fed over them. The volatile portions of the coal are driven off by heating, and
may be partially burned by adding an oxidizer to give higher temperature
gasses that gasify the remainder of coal, which is predominantly carbon.
As the gasification proceeds, slag and ash drop to the bottom of the reactor and are removed. Particles in the gas leaving the gasifier often contain
unconverted carbon, and are typically removed from the exit gas in a cyclone
or filter, and recycled to the gasifier, where the carbon can be converted to
syngas. Fluidized bed gasifiers and entrained flow gasifiers with pulverized
coal are also used. Because of slag, ash, and other impurities in coal, catalysts
are not used in coal gasification.
Gasification of biomass and waste products is possible but is complicated
by variable composition, and a variety of impurities that must be removed.
Research is being conducted in Japan relative to the gasification of syngas
from waste plastic (JDF 2007).
There is an energy loss associated with the production of syngas. The term
used to define this is the cold gas efficiency, which is the ratio of the heating
value of the products to that of the reactants. For natural gas as a feedstock,
the efficiency is on the order of 80–90%. For coal, it is about 10% lower. For
biomass and waste about 15–20% lower (JDF 2007). On an efficiency basis
Alternative Fuels for Transportation
catalyst to produce the syngas. The heat of the rich combustion converts the
natural gas to syngas, with the added oxygen combining with the carbon to
give the CO needed for the syngas. Systems are also known where the steam
reformer and autothermal reactor are combined. This is called the combined
reforming process. Typically, the steam reformer comes first, followed by the
autothermal reactor. Steam can also be added to an autothermal reactor to
adjust the product composition.
In this case, the overall reaction for formation of syngas from natural gas
can be written
2CH + O + H O → 2CO + 4H + H O ∆H =− 36.6 kJ/kMol CH
−
. . (6.3)
4
2
2
2
2
R
4
The composition is well suited to methanol formation.
In the direct process where a ratio of H 2 /CO = 1 is desired, this can be
obtained by recirculating the CO 2 from the products of the DME reactor back
to the reformer. Then the reforming reaction can be written
2CH + O + CO → 3CO + 3H + H O ∆H =− 16.5 kJ/kMol CH
−
. . (6.4)
4
2
2
2
2
R
4
This process gives a ratio of H 2 /CO = 1, which is the stoichiometric mixture
for DME formation from syngas.
For coal gasification, the process is more complicated since the fuel is in
the solid state and contains many impurities, and various types of gasifiers
are used to convert the solid coal to syngas. One type is a fixed bed system,
where solid coal particles are found in a bed in the reactor, and hot gasses are
fed over them. The volatile portions of the coal are driven off by heating, and
may be partially burned by adding an oxidizer to give higher temperature
gasses that gasify the remainder of coal, which is predominantly carbon.
As the gasification proceeds, slag and ash drop to the bottom of the reactor and are removed. Particles in the gas leaving the gasifier often contain
unconverted carbon, and are typically removed from the exit gas in a cyclone
or filter, and recycled to the gasifier, where the carbon can be converted to
syngas. Fluidized bed gasifiers and entrained flow gasifiers with pulverized
coal are also used. Because of slag, ash, and other impurities in coal, catalysts
are not used in coal gasification.
Gasification of biomass and waste products is possible but is complicated
by variable composition, and a variety of impurities that must be removed.
Research is being conducted in Japan relative to the gasification of syngas
from waste plastic (JDF 2007).
There is an energy loss associated with the production of syngas. The term
used to define this is the cold gas efficiency, which is the ratio of the heating
value of the products to that of the reactants. For natural gas as a feedstock,
the efficiency is on the order of 80–90%. For coal, it is about 10% lower. For
biomass and waste about 15–20% lower (JDF 2007). On an efficiency basis