Biomass Feasibility Project Final Report - Xcel Energy

Thermal treatment of SSO also involves a mixing drum. Pressurizing the drum for ten to thirty
minutes with steam breaks down the cell walls of the biomass to reduce its size. Thermal
treatment typically increases the moisture content of the feedstock by 10% and adds roughly
$3.00 to $7.00 per ton to processing costs.
Chemical Composition and Physical Characteristics
Chemical composition and physical characteristics influence fuel handling and processing
decisions. Some straws, like flax and hemp, are abrasive. Flax chive, a by-product produced by
pounding, not only is abrasive; in handling it compresses into small, hard balls that bridge in
storage bins and conveying equipment to cause blockages. Many straws contain silica. Others
contain corrosive chemicals.
Some feedstocks corrode handling equipment, some attack boilers, and some pollute the air.
Some do several of these things at once. Within a few months of operation, municipal solid
waste can corrode augers and steel conveyors to the point of uselessness. Chlorides destroy
boilers, which is why oat hulls, which contain chlorine, usually are mixed with other biomass, like
wood, to make boiler fuel. Wood off the stump is benign, but wood waste from factories that
make secondary wood products, like millwork, furniture and cabinets, often contains resins and
plastics that off-gas chlorides that corrode boilers and pollute the air. Some biomasses contain
high levels of nitrogen that contribute to NOx emissions. This is a manageable problem that
includes such strategies as mixing materials with low-nitrogen wood to mitigate the problem.
Another problem in burning some biomass feedstocks is slagging caused by certain salts and
high pH. Potassium can contribute to slagging in boilers, but silica and silicon dioxide can be
more problemmatic culprits. Silica has a melting point of 1710 o C, but in high pH environments,
melting temperatures might be considerably lower than that. When silica melts in a boiler or
gasifier, it forms glass that hardens on heat exchangers, pipes, and exhaust systems. It also forms
a toxic compound called cristobalite when it cools. Removing slag is a tough job that adds to
downtime and operating costs.
The following table shows the silica content of ash from various biomass feedstocks for
comparison. Silica itself may not be a problem unless it is introduced into a high pH
environment. As a previous chapter suggests, a thorough chemical analysis of feedstocks and
their potential effects on all stages of power production is an important preliminary to planning a
biomass facility.
Identifying Effective Biomass Strategies: Page 43
Quantifying Minnesota’s Resources and Evaluating Future Opportunities