CEC. 2014. Estimated cost of new renewable and fossil generation in California. California Energy Commission. CEC-200-2014-003-SD. 254 pages.
CHAPTER 8: Biomass Technology
Biomass technologies are plants that use biological resources, such as forestry waste or farming by-products, to produce electricity through thermal and chemical processes. Biomass technologies are in limited production here in California. While these technologies are designed to harness biological by-products sustainably, they suffer from:
- The limitation of requiring large, reliable fuel sources to produce energy economically.
- The high cost of transporting the fuel from the origination site to the generation site. This limitation exposes the producer to the volatile market for diesel or other petroleum fuels, which can unexpectedly add significant costs.
Biomass is plant- based material, agricultural vegetation, or agricultural wastes used as fuel and has three primary technology pathways:
- Pyrolysis- transformation of biomass feedstock materials into fuel (often liquid “biofuel”) through the application of heat in the presence of a catalyst.
- Combustion- transformation of biomass feedstock materials into useful energy through the direct burning of those feedstocks using a variety of burner/boiler technologies also used for burning materials such as coal, oil, and natural gas.
- Gasification- transformation of biomass feedstock materials into synthetic gas through the partial oxidation and decomposition of those feedstocks in a reactor vessel and oxidation.
Of these technology pathways, only direct combustion of biomass is commercially available for utility-scale plants.
Gasification methods are used in some small-scale applications but are not yet viable for utility-scale applications. Active research into pyrolysis for biofuel production is ongoing but is not used for electricity production.
Combustion technologies are widespread and include the following general approaches:
Stoker boiler combustion uses similar technology for coal-fired stoker boilers to combust biomass materials, using either a traveling grate or a vibrating bed.
- Fluidized bed combustion uses a special form of combustion where the biomass fuel is suspended in a mix of silica and limestone through the application of air through the silica/limestone bed. This is similar to technology used in newer coal-fired boilers. Fluidized bed combustion boilers are classified as either bubbling fluidized bed (BFB) or circulating fluidized bed (CFB) units.
- Biomass- cofiring uses biomass fuel burned in conjunction with coal products in current pulverized- coal boiler technology used in utility-scale electricity production.
Recent sources of data and analysis have focused on the fluidized bed technology. It is also the most likely biomass technology to be installed in California. The remainder of this chapter will focus on fluidized bed technology
The inherent fuel versatility of fluidized bed systems provides a plant operator the ability to burn many biomass resource types, including those feedstocks with significant moisture variations.
Biomass fuel type and uniformity-The type and uniformity of delivered biomass fuel supply are a primary cost driver for any biomass technology. Given the variation of the delivered moisture content and heating value of biomass fuel feedstocks, along with fuel processing issues, the handling and processing costs of biomass fuels can vary greatly. As a result, the type and characteristics of the different biomass fuels can have a material impact on the capital cost of the boiler design, as well as the overall fuel handling and operations cost.
Fuel transport and handling costs- The availability of sufficient biomass fuel resources near the plant location is a critical driver for operating cost. Most biomass fuel is transported by truck to a plant site. To maintain commercially reasonable prices, the effective economic radius from the plant location to the aggregate fuel supply is limited to about 100 miles. The varied nature of biomass fuel feedstocks also necessitates special handling equipment and larger numbers of dedicated staff than are needed for coal- fired combustion power plants of equivalent size. As a result, the typical maximum size of biomass plants is limited to about 50 MW in California (McCann, et al., 1994).
Small biomass facilities lose a great deal of power over transmission lines.
Boiler island cost-Capital cost of the boiler island is a critical cost driver that can account for roughly 40 to 60 percent of the overall plant cost, depending on the type of biomass combusted and the need for postcombustion pollution controls. The choice of source and type of fuels to be combusted is an important cost driver. In addition, the escalation trends for raw materials used in manufacturing the boiler island, primarily steel cost, are factors that can influence delivered boiler island cost.
Long-term fuel supply contracts-Most current biomass fuel supply contracts are of short-term duration and can entail varying fuel qualities. A key cost barrier to promoting biomass circulating bed combustion in California is the ability to develop and achieve performance on long-term (for example, 5 years duration and longer) fuel supply contracts for available fuel sources.
Plant scale- While current CFB technology has been proven to utility-scale applications of up to 300 MW, supply availability limits potential plant scale. Steam-generator scale economies are substantial, with a 50 MW biomass plant likely to cost substantially more per kW than a 500 MW coal-fired plant of the same technology (McCann, et al., 1994).
Emissions control costs-Costs of emission control needed to satisfy air quality and permitting requirements can increase the cost of biomass plants. Post-combustion emissions control technologies, such as selective catalytic reduction/selective noncatalytic reduction technologies for NOx control, and additional particulate matter controls, are important cost drivers that can significantly increase the capital and operating costs of biomass plants.