81
Steam Gasification and Reforming Technologies
The overall stoichiometry gives a maximum yield of 11.2% based on wood. The
overall reaction is
CH 1.9 O 0.7 + 1.26 H 2 O → CO 2 + 2.21 H 2
(4.74)
For this process, bio-oil from regional facility is generally transported to central
reforming facility. The process is compatible with other organic waste streams such
as aqueous steam fractionation processes used for ethanol production and trap grease.
Methanol and ethanol can also be produced from biomass by a variety of technologies and used for on-board reforming for transportation. Methane from anaerobic
digestion could be reformed along with natural gas. A system analysis has shown
that biomass gasification/shift conversion is economically unfavorable compared to
natural gas reforming except for very low-cost biomass and potential environmental
incentives.
4.6 steam GasiFiCatiOn and reFOrminG reaCtOrs
4.6.1 STeAm gASiFiCATion reACTorS
Fundamentally, three types of gasifiers are used in the commercial processes: fixed
bed, fluidized bed and/or CFB, and entrained bed [5,8–10,33,75–83,125,127,156,
175 –191]. In some specific applications, plasma and free radical gasifiers as well as
molten salt gasification reactors are also used. Although in most conventional applications the first two types are most commonly used, all types of gasification reactors
are briefly described below.
4.6.1.1 Fixed-Bed Gasifiers
There are two major types of fixed-bed gasifiers: countercurrent or “updraft” and cocurrent or “downdraft” gasifiers. In countercurrent gasifier, the carbonaceous materials (coal, biomass, waste, etc.) flow downward, whereas steam, oxygen, and/or air
flow upward in the reactor. The ash is removed either dry or as slag. The slagging
gasifiers have a lower ratio of steam to carbon achieving a temperature higher than the
ash fusion temperature. The fuel must be permeable and noncaking. The throughput
for this type of gasifier is relatively low. In this type of reactor, while thermal efficiency is high, both tar and methane productions are high and the product gases need
to be extensively cleaned. The tar can be recycled. In gasification of rice hulls, the gas
gets very hot (up to 1000°C) and has to be forced (by fan) into the reactor.
In both updraft and downdraft gasifiers, drying and devolatilization occur at the
top of the reactor [56,191]. In the updraft reactor, this is followed by reduction and
combustion. But in the downdraft gasifier, combustion precedes reduction. In an
updraft reactor, in the devolatilization zone volatile species are released and considerable quantities of tars are formed. In the reduction zone, permanent gases are
formed and finally char and remaining solids are combusted in the final combustion
zone. The updraft reactor produces low tar content and the temperature in the gasification zone can also be controlled by co-feeding steam and air or humidifying air.
Steam Gasification and Reforming Technologies
The overall stoichiometry gives a maximum yield of 11.2% based on wood. The
overall reaction is
CH 1.9 O 0.7 + 1.26 H 2 O → CO 2 + 2.21 H 2
(4.74)
For this process, bio-oil from regional facility is generally transported to central
reforming facility. The process is compatible with other organic waste streams such
as aqueous steam fractionation processes used for ethanol production and trap grease.
Methanol and ethanol can also be produced from biomass by a variety of technologies and used for on-board reforming for transportation. Methane from anaerobic
digestion could be reformed along with natural gas. A system analysis has shown
that biomass gasification/shift conversion is economically unfavorable compared to
natural gas reforming except for very low-cost biomass and potential environmental
incentives.
4.6 steam GasiFiCatiOn and reFOrminG reaCtOrs
4.6.1 STeAm gASiFiCATion reACTorS
Fundamentally, three types of gasifiers are used in the commercial processes: fixed
bed, fluidized bed and/or CFB, and entrained bed [5,8–10,33,75–83,125,127,156,
175 –191]. In some specific applications, plasma and free radical gasifiers as well as
molten salt gasification reactors are also used. Although in most conventional applications the first two types are most commonly used, all types of gasification reactors
are briefly described below.
4.6.1.1 Fixed-Bed Gasifiers
There are two major types of fixed-bed gasifiers: countercurrent or “updraft” and cocurrent or “downdraft” gasifiers. In countercurrent gasifier, the carbonaceous materials (coal, biomass, waste, etc.) flow downward, whereas steam, oxygen, and/or air
flow upward in the reactor. The ash is removed either dry or as slag. The slagging
gasifiers have a lower ratio of steam to carbon achieving a temperature higher than the
ash fusion temperature. The fuel must be permeable and noncaking. The throughput
for this type of gasifier is relatively low. In this type of reactor, while thermal efficiency is high, both tar and methane productions are high and the product gases need
to be extensively cleaned. The tar can be recycled. In gasification of rice hulls, the gas
gets very hot (up to 1000°C) and has to be forced (by fan) into the reactor.
In both updraft and downdraft gasifiers, drying and devolatilization occur at the
top of the reactor [56,191]. In the updraft reactor, this is followed by reduction and
combustion. But in the downdraft gasifier, combustion precedes reduction. In an
updraft reactor, in the devolatilization zone volatile species are released and considerable quantities of tars are formed. In the reduction zone, permanent gases are
formed and finally char and remaining solids are combusted in the final combustion
zone. The updraft reactor produces low tar content and the temperature in the gasification zone can also be controlled by co-feeding steam and air or humidifying air.