1.5.4 Gas Cleaning
Gas cleaning is the biggest challenge to the development of a successful BTL plant.
The impurities in syngas need to be reduced to the level demanded by the catalytic
fuel synthesis processes.
For CFB gasification, a cyclone can be used for particulates separation, and then
syngas should pass through a tar cracker, where tars are destroyed at 875
C by
addition of oxygen and steam. The tar-free syngas is then cooled down to 280
C
using a heat exchanger. The cooled syngas passes through a bag filter (Hofbauer
et al. 2009) and then is fed to the Rectisol unit, where CO 2 and sulfur compounds are
removed (Hofbauer et al. 2009).
For the EF gasification concept, if the H 2 /CO molar ratio of the dust-free syngas
produced by the entrained flow gasifier is lower than the required ratio (H2/CO ¼ 2)
for FT and methanol synthesis, a water-gas-shift (WGS) reactor should be added
before the Rectisol process. In that way the carbon dioxide produced in the WGS
unit can be removed soon after in the Rectisol unit. So the dust-free syngas is fed to a
direct water quench where it is cooled to the operating temperature of the WGS
reactor (200
C) (Swanson et al. 2010). The cooled syngas then passes through a bag
filter to remove particulates and then enters the Rectisol unit.
1.5.5 Fuel Synthesis
After the Rectisol unit, liquid fuels can be produced from syngas using:
– FT synthesis
– Methanol synthesis followed by the MTG process
– The TIGAS process
These three processes are currently the most reliable syngas conversion
technologies for transport fuel production available on the market. FT synthesis
has already been used in large-scale coal-to-liquid (CTL) and gas-to-liquid (GTL)
plants worldwide (Mangena 2012; Fleisch et al. 2002). Both the MTG and the
TIGAS technologies have been successfully proven at demonstration scale
(Fürnsinn 2007; Topp-Jorgensen 1988).
1.5.5.1 Fischer-Tropsch Synthesis
Fischer-Tropsch synthesis is a process for catalytically converting syngas to mainly
hydrocarbon products of different chain lengths (typically from C1 to C100). Among
the most widely known fuel synthesis plants in the world are:
– The CTL Fischer-Tropsch plants operated by Sasol in South Africa, which is the
world’s largest CTL production facility producing 27% of South Africa’s total
liquid fuel production (Mangena 2012).
1 Biofuels: Types and Process Overview
21
Gas cleaning is the biggest challenge to the development of a successful BTL plant.
The impurities in syngas need to be reduced to the level demanded by the catalytic
fuel synthesis processes.
For CFB gasification, a cyclone can be used for particulates separation, and then
syngas should pass through a tar cracker, where tars are destroyed at 875
C by
addition of oxygen and steam. The tar-free syngas is then cooled down to 280
C
using a heat exchanger. The cooled syngas passes through a bag filter (Hofbauer
et al. 2009) and then is fed to the Rectisol unit, where CO 2 and sulfur compounds are
removed (Hofbauer et al. 2009).
For the EF gasification concept, if the H 2 /CO molar ratio of the dust-free syngas
produced by the entrained flow gasifier is lower than the required ratio (H2/CO ¼ 2)
for FT and methanol synthesis, a water-gas-shift (WGS) reactor should be added
before the Rectisol process. In that way the carbon dioxide produced in the WGS
unit can be removed soon after in the Rectisol unit. So the dust-free syngas is fed to a
direct water quench where it is cooled to the operating temperature of the WGS
reactor (200
C) (Swanson et al. 2010). The cooled syngas then passes through a bag
filter to remove particulates and then enters the Rectisol unit.
1.5.5 Fuel Synthesis
After the Rectisol unit, liquid fuels can be produced from syngas using:
– FT synthesis
– Methanol synthesis followed by the MTG process
– The TIGAS process
These three processes are currently the most reliable syngas conversion
technologies for transport fuel production available on the market. FT synthesis
has already been used in large-scale coal-to-liquid (CTL) and gas-to-liquid (GTL)
plants worldwide (Mangena 2012; Fleisch et al. 2002). Both the MTG and the
TIGAS technologies have been successfully proven at demonstration scale
(Fürnsinn 2007; Topp-Jorgensen 1988).
1.5.5.1 Fischer-Tropsch Synthesis
Fischer-Tropsch synthesis is a process for catalytically converting syngas to mainly
hydrocarbon products of different chain lengths (typically from C1 to C100). Among
the most widely known fuel synthesis plants in the world are:
– The CTL Fischer-Tropsch plants operated by Sasol in South Africa, which is the
world’s largest CTL production facility producing 27% of South Africa’s total
liquid fuel production (Mangena 2012).
1 Biofuels: Types and Process Overview
21