Lignocellulosic Biomass Processing - Food and Industrial Bioproducts and Bioprocessing - page 297

12.3.2.3 Gasification

There are several types of gasifiers (such as downdraft, updraft and crossdraft) widely used
in syngas production. Syngas is a chemical building block to produce biopolymers,
biomaterials, or biofuels. Generally, an updraft gasifier has air passing through feedstocks
from the bottom and the combustible gases come out the top of the gasifier, while the
downdraft gasifier lets the air pass through in the opposite direction. In a crossdraft gasifier,
the air enters from one side and syngas is taken out from the other side. All existing gasifiers
fall into the above categories with slight modification.
In the research on gasification of biomass at Mississippi State University, a downdraft
gasifier has been used for feedstocks such as hard and soft wood chips and switchgrass
pellets (Wei et al ., 2008). The study found that the downdraft gasifier produced syngas with
similar quality regardless of the three feedstocks used. Additional feedstocks, such as
forestry wastes, chicken litter, gin trash, and crop residues, are scheduled to be gasified in
the near future.
Syngas from a downdraft gasifier system generally contains high contents of nitrogen,
hydrogen, carbon monoxide and carbon dioxide as well as a small amount of impurities.
One by-product of the gasification process is tar. Some tar compounds may be toxic to the
catalysis process. Tar elimination reactions are known to be kinetically limited, so reaction
rates can be increased through the use of catalysts. Many different catalysts have been
explored for reducing tars. The most widely used catalysts include dolomite, alkali metals
and nickel. Dolomite is a cheap and disposable catalyst that has resulted in significant
improvement in tar conversion. Nickel catalysts are widely used in hydrocarbon reforming
and are known to be useful in control and adjustment of gas composition. Dual systems have
also been proven effective, where dolomite may be used in the reactor bed, followed by
further catalytic cleaning of the hot gas downstream with a nickel-based reforming catalyst
(Boerrigter et al ., 2003). Physical removal of tars and particulates in the syngas is also well
studied using different wet and dry scrubbing processes (Devi et al ., 2003 ). Acetone is one
of the extensively tested solvents for scrubbing out condensables from biomass syngas,
particularly in tar and particulate measurement (Patil et al ., 2006 ; Cateni, 2007 , 2008).
Researchers at MSU have also demonstrated the proof of concept that syngas can be
converted to chemicals, such as ethylene, alcohols, and hydrocarbons using a combination
of commercial and proprietary catalysts and reactor design (Liu et al ., 2009 ). They showed
that the enhanced conversions of higher alcohols (e.g. ethanol, propanol, and butanol) to
hydrocarbon (30-40% by mass) at pressures to 70 bar compared to that observed for
methanol conversion at (1-2% by mass). The analyses of hydrocarbon products showed an
octane number of 80-90 with API gravities of 50-55.
Syngas from biomass gasification can also be utilized to produce ethanol or acetic acid
via fermentation (Ahmed and Lewis, 2006; Cateni, 2007). Virtually all fermentation
bioreactors of technical importance deal with heterogeneous systems involving at least two
phases. Many challenges exist to optimize synthesis gas fermentation, including overcoming
mass transfer limitations, increasing cell densities, reducing the production of unwanted
by-products, and minimizing the effects of inhibitory synthesis gas constituents generated
via biomass gasification (Worden et al ., 1997). Work has shown the effects of gas constituents
on cell growth, product distribution, and hydrogenase activity (Ahmed and Lewis 2006). In
particular, it was shown that nitric oxide can inhibit hydrogenase activity (Ahmed and
Lewis, 2007). It has also been observed that the carbon monoxide partial pressure plays a
critical role in the cell concentration and the product distribution (Hurst and Lewis, 2010).