68
Water for Energy and Fuel Production
Yamada et al. [31] also produced useful syngas from a mixture of coal and biomass
briquettes. Kumabe et al. [27] examined steam gasification (with air) of a mixture
of Japanese cedar and mulia coal and obtained useful syngas for the production of
DME. Their gasification results were very similar to those described earlier and they
obtained cold gas efficiency of 65%–85% during the gasification process. Pan et al.
[96] examined steam gasification of residual biomass and poor coal blends. Pine
chips from Spain were used as biomass, and the two types of coal—black coal (low
grade) from Escatron, Spain, and Sabero coal from Sabero, Spain—represented
poor-grade coals. Once again, reasonable quality of syngas was produced with an
overall thermal efficiency of about 50%. Satrio et al. [97] examined steam gasification of coal–biomass mixture with the specially designed catalyst pellets with outside
shell consisting of nickel on alumina and core consisting of calcium and magnesium
oxides that can adsorb carbon dioxide. This catalyst design gave higher production
of hydrogen. Finally, Ji et al. [32] studied steam gasification of a mixture of low-rank
fuel mixture of biomass, coal, and sludge in a fluidized bed reactor at 900°C temperature. Just like other studies, higher temperature gave more gas and hydrogen but
not high heating value of gas. The calorific value of syngas produced from sludge
mixture, sludge, wood chips, and lignite was 13, 10, 6.9, and 5.7 MJ/m 3 , respectively.
An excellent review of problems associated with co-firing of coal and biomass
fuel blends was given by Sami et al. [29]. This review critically assesses the effectiveness of this mixed feedstock for combustion and pyrolysis—two extreme cases of
gasification. While they specifically do not discuss steam gasification and reforming,
significant parts of their analysis are applicable to the process of steam gasification
and reforming. Indrawati et al. [98] examined partial replacement of fossil energy
by renewable sources such as rice husk, palm kernel shell, sawdust, and municipal
waste in the cement production. While this study also does not specifically address
steam gasification and reforming of mixed feedstock, the study points to another
application of the mixed feedstock.
4.5.1.4 tar
As indicated earlier, formation of tar is a major issue with steam gasification. Tar
is a complex mixture of condensable hydrocarbons and it can contain one-ring to
five-ring aromatic compounds with other oxygen containing hydrocarbon species
[99–103]. Generally, tar is defined as C 6
+ aromatic organics produced under gasification conditions. Tar is a problem during gasification because (1) it can deposit on
the outlet pipes of the gasifier and also on the particulate filters; (2) it can clog fuel
lines and injectors in the internal combustion engine; and (3) it reduces the gasifier’s
efficiency to produce additional useful fuel products such as hydrogen, carbon monoxide, carbon dioxide, and methane.
Baker et al. [99] illustrated the conceptual relationship between tar disappearance
and the temperature during thermal steam gasification of carbonaceous materials.
They also divided tar components in four different categories (Equation 4.55) [99].
The first category is easiest to crack and the fourth category (which mostly contains polycyclic aromatic hydrocarbons) is the most difficult to crack. Their analysis
showed that at low temperatures (400°C), a significant amount of tar is produced,
and at temperatures higher than around 1000°C, very little tar is produced. The
Water for Energy and Fuel Production
Yamada et al. [31] also produced useful syngas from a mixture of coal and biomass
briquettes. Kumabe et al. [27] examined steam gasification (with air) of a mixture
of Japanese cedar and mulia coal and obtained useful syngas for the production of
DME. Their gasification results were very similar to those described earlier and they
obtained cold gas efficiency of 65%–85% during the gasification process. Pan et al.
[96] examined steam gasification of residual biomass and poor coal blends. Pine
chips from Spain were used as biomass, and the two types of coal—black coal (low
grade) from Escatron, Spain, and Sabero coal from Sabero, Spain—represented
poor-grade coals. Once again, reasonable quality of syngas was produced with an
overall thermal efficiency of about 50%. Satrio et al. [97] examined steam gasification of coal–biomass mixture with the specially designed catalyst pellets with outside
shell consisting of nickel on alumina and core consisting of calcium and magnesium
oxides that can adsorb carbon dioxide. This catalyst design gave higher production
of hydrogen. Finally, Ji et al. [32] studied steam gasification of a mixture of low-rank
fuel mixture of biomass, coal, and sludge in a fluidized bed reactor at 900°C temperature. Just like other studies, higher temperature gave more gas and hydrogen but
not high heating value of gas. The calorific value of syngas produced from sludge
mixture, sludge, wood chips, and lignite was 13, 10, 6.9, and 5.7 MJ/m 3 , respectively.
An excellent review of problems associated with co-firing of coal and biomass
fuel blends was given by Sami et al. [29]. This review critically assesses the effectiveness of this mixed feedstock for combustion and pyrolysis—two extreme cases of
gasification. While they specifically do not discuss steam gasification and reforming,
significant parts of their analysis are applicable to the process of steam gasification
and reforming. Indrawati et al. [98] examined partial replacement of fossil energy
by renewable sources such as rice husk, palm kernel shell, sawdust, and municipal
waste in the cement production. While this study also does not specifically address
steam gasification and reforming of mixed feedstock, the study points to another
application of the mixed feedstock.
4.5.1.4 tar
As indicated earlier, formation of tar is a major issue with steam gasification. Tar
is a complex mixture of condensable hydrocarbons and it can contain one-ring to
five-ring aromatic compounds with other oxygen containing hydrocarbon species
[99–103]. Generally, tar is defined as C 6
+ aromatic organics produced under gasification conditions. Tar is a problem during gasification because (1) it can deposit on
the outlet pipes of the gasifier and also on the particulate filters; (2) it can clog fuel
lines and injectors in the internal combustion engine; and (3) it reduces the gasifier’s
efficiency to produce additional useful fuel products such as hydrogen, carbon monoxide, carbon dioxide, and methane.
Baker et al. [99] illustrated the conceptual relationship between tar disappearance
and the temperature during thermal steam gasification of carbonaceous materials.
They also divided tar components in four different categories (Equation 4.55) [99].
The first category is easiest to crack and the fourth category (which mostly contains polycyclic aromatic hydrocarbons) is the most difficult to crack. Their analysis
showed that at low temperatures (400°C), a significant amount of tar is produced,
and at temperatures higher than around 1000°C, very little tar is produced. The