Page 227 - Synthetic Fuels Handbook

FUELS FROM SYNTHESIS GAS 213

certain conditions, heat from the reactor could be used for district heating, industrial steam
production, or water desalination plants.
A wide range of materials can be handled by gasification technologies and specific pro-
cesses have been optimized to handle particular feedstock (e.g., tire pyrolysis and sewage
sludge gasification), while others have been designed to process mixed wastes. For exam-
ple, recovering energy from agricultural and forestry residues, household and commercial
waste, and materials recycling (autoshredder residue, electrical and electronic scrap, tires,
mixed plastic waste, and packaging residues) are feasible processes.
Biomass gasification could play a significant role in a renewable energy economy, because
biomass production removes CO from the atmosphere. While other biofuel technologies such
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as biogas and biodiesel are also carbon neutral, gasification runs on a wider variety of input
materials, can be used to produce a wider variety of output fuels, and is an extremely efficient
method of extracting energy from biomass. Biomass gasification is therefore one of the most
technically and economically convincing energy possibilities for a carbon neutral economy.
However, a disadvantage cited against the use of using biomass as a feedstock in gas-
ification reactors is that biomass generally contains high levels of corrosive ash and cause
damage to the gasifier. The difference between the biomass minerals and coal minerals is
not entirely clear as both can, obviously, lead to corrosion.

7.5 FISCHER-TROPSCH

Before synthesis gas can be used in the Fisher-Tropsch reaction there are several cleaning
steps required lest the impure gas deactivate the catalyst.
The general gas cleaning protocol (after as cooling) involves: (a) a primary water
scrubber, (b) a carbonyl sulfide, (c) a secondary water scrubber, (d) an olamine absorber,
and (e) a combustor to convert sulfur compounds to sulfur dioxide (Speight, 2007a).
Other aspects of synthesis gas cleaning may involve other steps which include: (a) tar
removal, (b) dust separation in a cyclone separator, (c) a bag filter, (d) carbonyl sulfide
(COS) hydrolysis in the scrubber (not if amine washing is used) at 100 to 250°C, (e) tar
condensation, (f ) ammonia (NH ) and hydrogen cyanide (HCN) scrubbing with sulfuric
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acid, and hydrogen sulfide scrubbing. The methods are readily available (Speight, 2007a).
Particulate removal systems from synthesis gas must remove ashes, char particles, and
fluidized bed solids from the gas, because of potential erosion and emission problems.
Techniques used are filtration (hot or cold) and scrubbing. Bag filters use plastic fibers for
filtration up to 230°C; ceramic and metallic dust filters can be used at maximal running
temperatures of about 800°C, or even more. Particles removed can be as small as 0.1 μm
and cleaned gas contains less than 1 to 5 ppm. Alkali metal oxides condense at 550°C and
stick to the surface of particles present and are removed together with them in particulate
filter. Hydrogen sulfide present in the synthesis gas is removed by scrubbing with lime or
caustic soda. Sulfur can be removed from gas by addition of ferrous sulfate and subsequent
filtration and disposal of the dust. When the fluidized bed contains calcite and/or dolomite,
sand reacts with sulfates and forms salts.
Although the focus of this section is the production of hydrocarbons from synthesis gas,
it is worthy of note that all or part of the clean syngas can also be used as: (a) chemical
building blocks to produce a broad range of chemicals (using processes well established in
the chemical and petrochemical industry), (b) a fuel producer for highly efficient fuel cells
(which run off the hydrogen made in a gasifier) or perhaps in the future, hydrogen turbines
and fuel cell-turbine hybrid systems, and (c) a source of hydrogen that can be separated
from the gas stream and used as a fuel or as a feedstock for refineries (which use the hydro-
gen to upgrade petroleum products).