Renewable Energy Technology Assessments - Kauai Island Utility ...

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Kaua’i Island Utility Cooperative Renewable Energy Technology Assessments Year Energy, GWh 3.0 Renewable Energy Technology Options Table 3-3. Developable Potential from Direct Fired Biomass. Capacity, MW Notes 3 97.4 13.9 Constrained to near term available fuels (tree farm trimmings and resources in Table 3-2, except MSW). Project would need to be repowering of Lihue power station to be available in 3 years. 5 714 102 Includes near-term fuels and energy crops on 20 percent of agricultural lands, excludes MSW 10 714 102 20 714 102 3.1.2 Biomass Cofiring An economical way to burn biomass is to cofire it with coal in existing plants. Cofired projects are usually implemented by retrofitting a biomass fuel feed system to an existing coal plant, although greenfield facilities can also be readily designed to accept a variety of fuels. A major challenge to biomass power is that the dispersed nature of the feedstock and high transportation costs generally preclude plants larger than 50 MW. By comparison, coal power plants rely on the same basic power conversion technology but have much higher unit capacities, exceeding 1,000 MW. Due to their scale, modern coal plants are able to obtain higher efficiency at lower cost. Through cofiring, biomass can take advantage of this high efficiency at a more competitive cost than a stand-alone direct fired biomass plant. Applications There are several methods of biomass cofiring that could be employed for a project. The most appropriate system is a function of the biomass fuel properties and the coal boiler technology. Provided they were initially designed with some fuel flexibility, stoker and fluidized bed boilers generally require minimal modifications to accept biomass. Simply mixing the fuel into the coal pile may be sufficient. Cyclone boilers and pulverized coal (PC) boilers (the most common in the utility industry) require smaller fuel size than stokers and fluidized beds and may necessitate additional processing of the biomass prior to combustion. There are two basic approaches to cofiring in this case. The first is to blend the fuels and feed them together 21 March 2005 3-10 Black & Veatch
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
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