Assessment of Conversion Technologies for Bioalcohol Fuel ...

Syngas Production
The thermochemical conversion of biomass to synthesis gas (syngas) encompasses
processes that are carried out in closed systems under reducing (oxygen depleted) or
oxidizing (partial oxygen) conditions at high temperatures (typically 1500-2000 o F). The
primary chemical processes that occur include pyrolysis, oxidation, steam reforming
and gasification.
Carbon-containing compounds in the biomass feedstock are converted to synthesis
gas (syngas), which is composed primarily of hydrogen (H 2 ), carbon monoxide (CO),
methane (CH 4 ) and carbon dioxide (CO 2 ). Syngas may be utilized as a substitute for
natural gas in cogeneration engines, gas turbines or boilers to produce power and/or
heat. In addition, syngas can be an excellent feedstock for fuel production via catalytic
synthesis.
In air-blown systems, significant amounts of nitrogen (N 2 ) will also be present due to
the air supplied for partial oxidation. Syngas can also contain minor constituents
including higher hydrocarbons and tar compounds, and other trace constituents. As
discussed in the following section, syngas cleanup and conditioning is important for
making a useful fuel product.
The types of syngas production systems include air-blown gasification, oxygen
gasification, thermal pyrolysis (no oxygen) and steam reforming. Systems that are
supplied with air or oxygen are autothermal with heat from the partial oxidation of the
biomass. Thermal pyrolysis and steam reforming of biomass are endothermic and
typically require a secondary fuel to supply heat to the reaction chamber. This is often
supplied with clean syngas recycled back to externally heat the reactor.
When syngas production takes place in a carefully controlled, closed system, there
should be no direct emissions of criteria and toxic air pollutants. Externally heated
systems may have some emissions from the secondary burners, but these can be
minimized with low-emission nozzles and controls typical for boiler systems. In
addition, oxygen gasification systems typically require an oxygen generation plant that
consumes energy, with associated emissions. These systems produce a raw syngas
that may require cleanup and conditioning to insure the proper function of downstream
processing of the syngas.
Chevron Texaco, Conoco Phillips (Global Energy) and Shell (Lurgi) have developed
economically viable biomass-to-syngas production systems for the production of
electricity in the 100-1,000 MW output range (NREL, 2002). However, these
technologies have not proven to be economical for small scale power generation
applications (1-25 MW).
During the past several years approximately 110 organizations have focused their
efforts on the development of small (1-25 MW), economical systems for generation of
electricity from waste materials. However, very few of these companies have
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