Formation of Ash Aerosols and Ash Deposits of Coal Blends

Abstract

Many coal-fired power plants now burn coal blends instead of a single raw coal because of required low sulfur attainment levels. Mineral matter for the coal blends is likely to be different from that for their parent coals and is unlikely to be predictable from simple averaging rules. The problem is important because deposit buildup alters the characteristics of heat transfer and pollutant emissions of the boiler. In this work, experiments were conducted on a 100-kW rated pilot-scale down-fired self-sustained combustor, firing an Illinois coal, a Powder River Basin (PRB) coal, and a 60 % Illinois/40 % PRB coal blend. Such a 60/40 % blend had been planned for the FutureGen 2.0 project. Air combustion as well as oxy-coal combustion with recycled flue gas (RFG) was investigated. The intent was not only to test how deposit was formed from coal blend but also to relate the size-segregated composition of the ash aerosol to the spatially resolved composition within the deposits. To this end, a Berner low-pressure impactor (BLPI), a scanning mobility particle sizer (SMPS), and an aerodynamic particle sizer (APS) were utilized to acquire size-segregated ash aerosol samples and to measure particle size distribution (PSD). A novel surface temperature-controlled ash deposition probe system was used for fouling deposits collection. The results from air combustion show that PSDs measured by BLPI and SMPS/APS agree well with each other. Combustion of Illinois coal will likely produce more ultrafine particles compared to PRB coal. However, combustion of Illinois-PRB blended coal could somewhat reduce the formation of these ultrafine particles. Aerosols from combustion of Illinois coal have higher Si and Al, and corresponding lower Ca, Mg, S, and Na compared to those from combustion of PRB coal. The elemental concentrations in aerosols from combustion of blended coal lie between those of the parent coals. Comparing to PRB coal, the inside deposits from combustion of Illinois coal have higher Al, K, Fe, and Si, while lower S, Ca, Na, and Mg, which is consistent with the trends of ash aerosol composition measurements. This agrees with our previous theory that vaporization mode ash aerosols are the main contributor to build up inside layer deposits, and their composition depends on coal composition. Blended coal increased S retention in ash due to higher alkaline earth metal (AAEM, especially Ca) concentration in PRB coal. Deposits of the blend do not obey simple averaging rules for the two components of the blend, which is not surprising given that in order to understand mechanisms of deposit formation, one must have access to both the size-segregated composition of the ash aerosol and the spatially resolved composition of the deposits.