Environmental Engineering Reference
In-Depth Information
Silo
(storage)
Mechanical size
reduction
Gasification
Truck unload wood chips
Synthetic natural gas
Alcohol
CO
hydroge-
nation
Water
gas
shift
Gas
clean
up
Jet fuel
Gasoline
Synthesis gas
Diesel
Heating oil
H
2
Power, heat
FIGure 8.4
Process flow diagram for biomass gasification-based processing to energy products.
interfere with rotating power generation equipment, (2) ammonia and other nitrogen-containing
gases will be converted to oxides of nitrogen (NO
x
) in engines or turbines, and (3) H
2
S poisons
catalysts and ammonia blocks catalyst active sites and therefore both must be removed.
Conversion to fuels and chemicals (CO hydrogenation in Figure 8.4) occurs in the presence
of specific metal catalysts and requires unique ratios of CO to H
2
to form the different fuel and
chemical products. The CO/H
2
ratio can be adjusted through a catalytic water gas shift reaction in
which CO is combined with H
2
O to form additional H
2
and CO
2
. Other approaches are possible to
adjust the final CO/H
2
ratio, including the use of methane in a steam-reforming reaction (CH
4
+ H
2
O
→ CO + 3 H
2
) or a H
2
-selective membrane separation step. Depending on application, separation of
final products and co-products is accomplished through distillation, membranes, and other devices.
8.3.1.1 Gasification of Biomass
Gasification of biomass can be carried out in several reactor configurations. Biomass chips can be
introduced at the top of the reactor to be contacted with a co-current or countercurrent stream of
the gasification medium (air, oxygen, and water vapor). Another configuration utilizes a bubbling or
circulating fluidized bed reactor in which heat is transferred by a circulating stream of solid media
such as sand. The gasification product composition is dependent on process conditions, gasification
medium, and feedstock type, but with CO/H
2
ratio usually less than 1 and with a heating value
between 5 and 15 MJ/Nm
3
(Nm
3
is normal cubic meter). Thus, biomass-derived synthesis gas is
considered a low to medium fuel gas compared with natural gas (35 MJ/Nm
3
).
The gasification reaction itself is endothermic and requires a source of external heat. Volatile
matter from heating of biomass is initially oxidized forming CO
2
and H
2
O and releasing heat, which
leads to further volatilization of biomass and H
2
O. Partial oxidation and thermal decomposition
reactions also take place leading to the product mixture discussed in Section 8.3.1.
Ash from the biomass is mostly bound up with particulate char and can be separated easily from
the gas product using filtration or other means. The char can be oxidized yielding additional process
heat or partially oxidized with O
2
or reformed with H
2
O to increase carbon conversion in synthesis
gas. Tars are high-molecular-weight aromatic hydrocarbons and must be removed to protect
