Biomass Feasibility Project Final Report - Xcel Energy

prevent grate damage. Updraft gasifiers work well with fuels with high water and inorganic
content as long as they are large, dense, and uniformly sized.
Fixed bed-downdraft. These gasifiers are best for processing biomass with high volatile content.
They introduce both the biomass fuel and the air/oxygen mixture from the top of the combustion
chamber.
Fluidized bed. This design can handle a wide range of relatively unprocessed feedstocks.
Biomass fuel feeds continuously into the chamber and air rises through a bed of inert sand or
crushed limestone that distributes heat evenly. In a directly heated design, char burns in the
gasifier vessel to heat it. In an indirectly heated design, char burns in a separate vessel.
There is good news and bad news about fluidized bed gasifiers. The good news is their superior
operating performance. They confer temperature uniformity, tolerance of wet fuels with
moisture contents up to 55%, and short reaction times. But fluidized bed gasifiers don’t produce
gas of the highest quality. The high temperature of the gas exiting the gasifier leaves alkali
compounds in a vapor phase, and the gas is high in particulates that have to be filtered
(Williams and Larson, 1996).
Low pressure. In this process, two separate chemical reactions occur in two separate reactors.
In the first reactor, hot sand surrounds biomass, whose volatile gasses in pyrolysis arise from the
char, ash and sand to enter a cyclone that divides them into their separate constituents. In the
second reactor, the remaining char is burned to heat the first reactor. Gas from this process has
a BTU content of around 550 per square foot. Scrubbed of impurities, it can power a gas turbine.
Operational Concerns in Gasification
Alkali can cause as much trouble in biogas fueling a turbine as it does in biomass fueling a
boiler. Since alkali fouls turbine surfaces, it must be chemically scrubbed from gas going into a
turbine. Another way to control alkali is to keep gas at a cooler temperature as it exits the
gasifier. Research in the coal industry discovered that at low exit temperatures, alkali
compounds condense on particulates in the gas stream instead of on surfaces of equipment
(Williams and Larson, 1996). Certain chemical additives also might reduce alkali (Miles et al.
1995).
Research on gas exit temperatures hasn’t been incorporated into the design of equipment yet.
Until it is, alkali compounds are removed by electrostatic filters, barrier filters, or wet scrubbers.
Those devices can’t go to work until hot gas from the gasifier has cooled (Stevens, 2001).
Unfortunately, however, losing that heat reduces the overall efficiency of the system. Alkali
“getters” that can remove alkali at high temperatures may resolve that problem, but they
haven’t yet been tried in a commercial system (Stevens, 2001).
Tars are organic materials in the product stream of the gasification process, mostly aromatic
compounds. If the tar condenses, it can do a number of undesirable things:
• form coke on fuel reforming catalysts;
• deactivate sulfur removal systems in co-firing systems;
• erode compressors, heat exchangers indirect combustion systems, and ceramic filters;
• damage gas turbines and engines; and
• frustrate compliance with emissions standards.
In a well designed combustion boiler, tar clean-up is not a critical issue. But in a gasifier, it has
critical importance. Cleaning tar brings:
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Identifying Effective Biomass Strategies:
Quantifying Minnesota’s Resources and Evaluating Future Opportunities