Biomass Feasibility Project Final Report - Xcel Energy

Besides buying harvest waste, the utility is planning to develop a “closed loop” biomass supply
from hybrid poplar and willow plantations. A total of 1,650 acres were planted by LEA in 2004-
2005. The poplar harvest will begin 5 years later.
Laurentian Energy’s selection of boilers demonstrates that real-world constraints sometimes
matter more than efficiency. The project is using traditional direct-burn stoker-grate boilers
because a short financing timeline precluded a lengthy design process. The equipment also
had to fit within limited spaces at the Authority’s existing plants. Even with 1970’s technology,
however, the facility should be profitable. It will sell biomass power to Xcel Energy at the price of
10.2 cents/kWh (PUC, 2005); more than double the market price of power from other fuels.
BIO-SOLIDS
Two factors suggest that bio-solids will play an increased role in energy production in Minnesota.
The first is an increasing interest in using them, and the other is the development of new
technologies that convert them into useful energy without consuming too much of it in the
process. Bio-solids soon may begin to generate marketable electricity and thermal energy.
Incinerators, anaerobic digesters and gasifiers all can be used to convert bio-solids, which in
Minnesota are found mainly in wastewater treatment facilities and animal barns. Most of
Minnesota’s smaller waste water treatment plants use anaerobic digesters to process bio-solids.
But three large facilities that process 58% of the state’s municipal waste use incinerators.
Anaerobic digesters are used most often to convert animal manures, but a new facility in
Benson, Minnesota, is incinerating turkey litter (manure mixed with bedding) to generate
electricity.
Converting Bio-Solids into Energy
Considering that around 70% of dry bio-solids are volatile solids containing around 10,000 BTUs
per pound, one would expect wastewater to be a great source of renewable fuel. But the
amount of energy needed to dry wet bio-solids has discouraged their use as fuel. Waste water
entering treatment facilities typically contains only 2% solids. New dewatering technologies
involving clarifiers, polymers, filter presses and other means can increase solids content to a
range of 8% to 45%. That helps, but solids have to reach 85% for bio-solids to work as combustion
fuel. Drying to that level takes much of the energy that bio-solids can produce in combustion.
Gasification might become an alternative to combustion in converting bio-solids to energy.
Heavy metals in bio-solids would be released in gasification, but most bio-solids in Minnesota are
quite free of them. Gasifiers have lower capital and operating costs than other advanced
technologies like fluidized bed incinerators, but since they are less tolerant of moisture, their biosolids
fuel also must be dried to 85% solids.
WASTEWATER TREATMENT
The possibility of creating electric and thermal energy from wastewater treatment may become
a reality, however, with some recent innovations in treatment technologies. Upflow sludge
blanket digesters, wetland treatment systems, and the Cannon Process use up to 70% less
energy than the older systems now in use. In addition to these more energy efficient
technologies, advances have been made in maximizing the distribution of heating and cooling
in waste water facilities. New plants can distribute heat from heat pumps and chillers much
more efficiently than plants built just ten years ago. Retrofits in the commercial sector often
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Identifying Effective Biomass Strategies:
Quantifying Minnesota’s Resources and Evaluating Future Opportunities