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CFB Refractory Improvements for Biomass Co-Firing
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The increasing demand for circulating fluidized bed (CFB) boilers to co-fire biomass and waste fuel products creates increased stress on refractory systems resulting in more frequent replacement cycles and increased maintenance costs.  The problems with refractory systems in this application are accentuated due to improper application and/or inappropriate material selection.
The primary failure is due to corrosion/erosion associated with varied biomass materials. This failure mechanism is the result of alkali-related chemical attack which creates compounds that be easily eroded. A secondary concern is the inappropriate use and application of the refractory material. This is less of a concern in CFBs that are not used to co-fire biomass.
Chemical attack, or refractory corrosion, is the reaction between the material matrix and the interface of alkalis introduced into the unit (usually from the fuel source, mainly sodium, potassium and calcium in the forms of carbonates, sulfates and chlorides).  These reactions occur within the microscopic pores of the refractory lining causing weakened bonds, volumetric expansion and, ultimately, failure by erosion.
Refractory technology has changed dramatically over the past several years. Although various material products are promoted to be more resistant to alkali attack, many of these products rely on high surface temperatures for the formation of silica glass to seal porosity and reduce the likelihood of alkalis penetrating the surface. Unlike other industrial applications using refractory linings, CFB operating temperatures are lower than the 1,850 F required for the formation of the silica glass seal. Because of this, refractory material choices become more limited, relying more on the types of raw constituents, bonding mechanism, ultimate porosity, alumina type and installation method to achieve higher resistance to chemical attack and extended refractory life.
There have been worldwide trends over the past several years steering away from standard coal fuels such as bituminous, anthracite and lignite toward the combustion of alternate solid fuels such as petroleum coke, wood waste, peat, agricultural waste, tire and refuse derived fuels. Depending upon regional availability, these alternative fuels can result in lower plant operating costs and reduced emissions while meeting government guidelines for replacing fossil fuels with carbon-neutral, renewable energy sources. Given the CFB boiler’s ability to burn alternative fuels with little or no modifications, many existing traditional coal-fired facilities have selected or are in the process of selecting alternative fuel sources for co-firing.
Refractory linings and coal firing have coexisted for over a century. However, the refractory lining systems in existing CFB plants are often not considered when alternative fuels are selected for combustion. Many existing CFB facilities that began operating in the late 1980’s and early 1990's were lined with older refractory products. Recently, they have begun co-firing with biomass fuels. Although the existing refractory linings have been successful with minimal repair or replacement over the operating life of the plant, annual maintenance costs of these current lining systems have increased due to premature failures and increased unplanned outages. The initial mode of failure appears to be primarily due to erosion and excessive cracking that has not been previously experienced. As a result, larger replacement areas of the combustor and cyclone linings are required, often resulting in longer planned outage cyclones or more frequent emergency outages to maintain the thermal and tube protection required for the CFB environment.
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