The behavior of inorganic material in biomass-fired power boilers: field and laboratory experiences

Abstract

This paper highlights some of the major findings of the Alkali Deposits Investigation, a collaborative effort to understand the causes of unmanageable ash deposits in biomass-fired electric power boilers. A group of interested industrial institutions and the US DOE Energy Efficiency and Renewable Energy Office's Biomass Power Program through the National Renewable Energy Laboratory jointly sponsored the project. The industries contributed both funding and, in most cases, use of facilities to the project and included Mendota Biomass Power and Woodland Biomass Power (both associated with Thermo Electron Energy Systems), CMS Generation Operating (formerly Hydra-Co Operations), Wheelabrator, Shasta and Hudson Energy, Sithe Energy, Delano Energy, the Electric Power Research Institute, Foster Wheeler Development, and Elkraft Power of Denmark. Research contracts with Thomas R. Miles Consulting Design Engineers, Sandia National Laboratories, and The National Renewable Energy Laboratories provided the government portion of the funding. In addition, the University of California at Davis and the Bureau of Mines performed significant work in close collaboration with the other researchers. This summary highlights the major findings of the project more thoroughly discussed in a recent report [2]. We highlight fuel properties, bench-scale combustion tests, a framework for considering ash deposition processes, pilot-scale tests of biomass fuels, and field tests in commercially operating biomass power generation stations. Detailed chemical analyses of 11 biomass fuels representing a broad cross-section of commercially available fuels reveal their properties that relate to ash deposition tendencies. The fuels fall into three broad categories: (1) straws and grasses (herbaceous materials); (2) pits, shells, hulls and other agricultural by-products of a generally ligneous nature; and (3) woods and recycle fuels of commercial interest. Woods and wood-derived products represent the most commonly used biomass fuels. Herbaceous fuels contain silicon and potassium as their principal ash-forming constituents. They are also commonly high in chlorine relative to other biomass fuels. These properties portend potentially severe ash deposition problems at high or moderate combustion temperatures. The primary sources of these problems are shown to be: (1) the reaction of alkali with silica to form alkali silicates that melt or soften at low temperatures (can be lower than 700°C, depending on composition), and (2) the reaction of alkali with sulfur to form alkali sulfates on combustor heat transfer surfaces. Alkali material plays a central role in both processes. The mobility of alkali material, defined as its ability to come in physical contact with other materials, is measured using chemical extractive techniques. Potassium is the dominant source of alkali in most biomass fuels. The analyses below indicate that essentially, all of the biologically occurring alkali, in particular potassium, has high mobility. The non-biologically occurring alkali is present as soil contaminants and additives to the fuels, such as clay fillers used in paper production. This non-biologically occurring alkali exhibits far lower mobility than the biological fraction. The relative amounts of biologically vs. non-biologically occurring material depend on fuel type and fuel handling. In the fuels investigated here, the dominant form of alkali was biologically occurring potassium. Some traditional indicators of deposit behavior, most notably ash fusion temperatures, poorly predict ash behavior compared with a more mechanistic interpretation of the data. Many of the agricultural by-products also contain high potassium concentrations with equally high potassium mobility. Some woods, on the other hand, contain far less ash overall, differing by as much as a factor of 40 from high-ash straws, for example. In addition, the ash-forming constituents contain greater amounts of calcium with