82
Water for Energy and Fuel Production
The product gases are cooled down to 200°C–300°C before leaving the gasifier. The
overall energy efficiency of the updraft gasifier is high.
In a “down draft” reactor, both raw materials and gasification agent flow cocurrently downward. Heat needs to be added in the upper part of the bed either
by combusting small portion of fuel or by some external sources. The produced
gas leaves the reactor at high temperature, and most of the heat is transferred to
the entering gasification agent. This creates an energy efficiency similar to that for
“updraft” reactors. Since tar passes through the hot zones within the reactor, its level
in the product gas is much lower than that in the “updraft” reactor [56,191].
The downdraft gasifier has four distinct zones [56,191]: (1) upper drying zone,
(2) upper medium pyrolysis section, (3) lower medium oxidation zone, and (4) lower
reduction zone. The temperature in the oxidation zone is 1000°C–1400°C and the
tar produced is exclusively tertiary tar. The downdraft gasifier produces clean gas
but has low thermal efficiency, and it is not suitable for handling biomass with high
moisture and ash content.
Besides updraft and downdraft gasifiers, sometimes cross-flow gasifiers are
used where raw materials (coal, biomass, etc.) flow downward and air or steam is
introduced from the side. The product gases at about 800°C–900°C are withdrawn
from the top of the gasifier. A hot combustion/gasification zone forms around the
air entrance, with both pyrolysis and drying zones being formed higher up in the
vessel. Ash is removed from the bottom of the reactor. The gasifier gives low-energy
efficiency and produces high tar content.
The fixed-bed reactors are easy to design; however, they produce gas with low
heating value and high tar content. The use of oxygen along with steam improves
the product gas heating value. The heating value also significantly depends on the
nature of the feedstock.
A novel HTAG unit was used to study biomass waste such as bark, charcoal, and
wood pellets with diameters ranging from 6 to 12 mm as well as densified and not
densified plastic wastes [56,75–84,178–181]. The facility consisted of a batch-type,
countercurrent (updraft), fixed-bed vertical column gasifier. The reactor had three
sections: a wind box, the feedstock section, and gas reaction section. The afterburning combustion chamber was coupled to a gasifier to burn completely produced fuel
gas. After burner had an inlet for fuel gas, an outlet for flue gas, and a set of air
nozzles to ensure complete combustion. Flue gas outlet channel was connected with
the afterburning combustion chamber that was equipped with the cooling system.
The air was preheated to 600°C with a capability to go up to 1300°C. The unit has
worked very successfully.
4.6.1.2 suspended Bed reactor
There are three types of suspended bed for steam gasification used in commercial
practice: fluidized bed, CFB, and entrained bed.
4.6.1.2.1 Fluidized Bed Reactor
In this type of reactor, fuel is fluidized by air (or oxygen) and steam. The ash is removed
dry or as heavy agglomerates that defluidized. In dry ash gasifier, the temperature is
relatively low, thus generating high methane content gases. The agglomerating gasifiers
Water for Energy and Fuel Production
The product gases are cooled down to 200°C–300°C before leaving the gasifier. The
overall energy efficiency of the updraft gasifier is high.
In a “down draft” reactor, both raw materials and gasification agent flow cocurrently downward. Heat needs to be added in the upper part of the bed either
by combusting small portion of fuel or by some external sources. The produced
gas leaves the reactor at high temperature, and most of the heat is transferred to
the entering gasification agent. This creates an energy efficiency similar to that for
“updraft” reactors. Since tar passes through the hot zones within the reactor, its level
in the product gas is much lower than that in the “updraft” reactor [56,191].
The downdraft gasifier has four distinct zones [56,191]: (1) upper drying zone,
(2) upper medium pyrolysis section, (3) lower medium oxidation zone, and (4) lower
reduction zone. The temperature in the oxidation zone is 1000°C–1400°C and the
tar produced is exclusively tertiary tar. The downdraft gasifier produces clean gas
but has low thermal efficiency, and it is not suitable for handling biomass with high
moisture and ash content.
Besides updraft and downdraft gasifiers, sometimes cross-flow gasifiers are
used where raw materials (coal, biomass, etc.) flow downward and air or steam is
introduced from the side. The product gases at about 800°C–900°C are withdrawn
from the top of the gasifier. A hot combustion/gasification zone forms around the
air entrance, with both pyrolysis and drying zones being formed higher up in the
vessel. Ash is removed from the bottom of the reactor. The gasifier gives low-energy
efficiency and produces high tar content.
The fixed-bed reactors are easy to design; however, they produce gas with low
heating value and high tar content. The use of oxygen along with steam improves
the product gas heating value. The heating value also significantly depends on the
nature of the feedstock.
A novel HTAG unit was used to study biomass waste such as bark, charcoal, and
wood pellets with diameters ranging from 6 to 12 mm as well as densified and not
densified plastic wastes [56,75–84,178–181]. The facility consisted of a batch-type,
countercurrent (updraft), fixed-bed vertical column gasifier. The reactor had three
sections: a wind box, the feedstock section, and gas reaction section. The afterburning combustion chamber was coupled to a gasifier to burn completely produced fuel
gas. After burner had an inlet for fuel gas, an outlet for flue gas, and a set of air
nozzles to ensure complete combustion. Flue gas outlet channel was connected with
the afterburning combustion chamber that was equipped with the cooling system.
The air was preheated to 600°C with a capability to go up to 1300°C. The unit has
worked very successfully.
4.6.1.2 suspended Bed reactor
There are three types of suspended bed for steam gasification used in commercial
practice: fluidized bed, CFB, and entrained bed.
4.6.1.2.1 Fluidized Bed Reactor
In this type of reactor, fuel is fluidized by air (or oxygen) and steam. The ash is removed
dry or as heavy agglomerates that defluidized. In dry ash gasifier, the temperature is
relatively low, thus generating high methane content gases. The agglomerating gasifiers