Renewable Energy Technology Assessments - Kauai Island Utility ...

Kaua’i Island Utility Cooperative
Renewable Energy Technology Assessments
3.0 Renewable Energy Technology
Options
to the coal processing equipment (crushers, pulverizers, etc.). In a cyclone boiler,
generally up to 10 percent of the coal heat input could be replaced with biomass using
this method. The smaller fuel particle size of a PC plant limits the fuel replacement to
perhaps 3 percent. Higher cofiring percentages (around 10 percent) in a PC unit can be
accomplished by developing a separate biomass processing system at somewhat higher
cost.
Even at these limited cofiring rates, plant owners have raised numerous concerns
about negative impacts of cofiring on plant operations. These include:
• Negative impact on plant capacity
• Negative impact on boiler performance
• Ash contamination impacting ability to sell coal ash
• Increased operation and maintenance costs
• Limited potential to replace coal (generally accepted to be 10 percent on an
energy basis)
• Minimal nitrogen oxide reduction potential
• Boiler fouling/slagging due to high alkali in biomass ash
• Negative impacts on selective catalytic reduction air pollution control
equipment (catalyst poisoning)
These concerns have been a major obstacle to more widespread biomass cofiring
adoption. Most of these concerns can be addressed by using an external biomass gasifier
to convert the energy of the solid biomass into a low energy gas ("syngas") to be fired in
the boiler. Using gasification technology, it is expected that 25 percent or more of the
coal heat input could be displaced without significant operational problems.
Additionally, the syngas can be used as a reburn fuel to significantly reduce NOx
emissions. The gasification system has a higher cost than the other cofiring approaches,
but still a fraction of the cost of a new direct-fired plant.
Coal and biomass cofiring may also be considered for new power plants.
Designing the plant from the outset to accept a diverse fuel mix would allow the
specifications for the boiler to incorporate the biomass fuel into the design, ensuring high
efficiency with low operational and maintenance impacts. Fluidized bed technology is
often the preferred boiler technology as it has inherent fuel flexibility. There are many
fluidized bed units around the world that burn a wide variety of fuels, including biomass.
An example is the 240 MW CFB owned by Alholmens Kraft Oy in Finland which burns
a mix of wood, peat and lignite. This unit was supplied by Kvaerner Pulping and was
commissioned in 2001. The plant is shown in Figure 3-3.
21 March 2005 3-11 Black & Veatch