OCR Text |
Show porization of the metal is expected to correspond to the feed rate of the metal. Particle Entrainment If a metal has not completely vaporized at combustion conditions (saturated metal), additional partitioning of the metal to the gas phase can occur due to ash entrainment. Metals partitioning to the gas phase due to ash entrainment can be accounted for through an estimate of the percent of ash entrained in the combustor. The percent of ash entrained is calculated using the particulate loading in the gas and the coal ash content, or by using entrainment models which simulate the conditions of the combustion system[ 17]. The amount of ash entrained depends on the size, shape, and density of the ash, as well as on the coal type, combustor design, and operating characteristics of the coal combustion system. In many systems, the fraction of particulate entrainment is known (Table 1 [18]) for a given coal and operating conditions. The total uncontrolled emissions of metals were calculated by adding the vaporized and entrained portions of the metals. Table 1. Coal Ash distribution by boiler type [18]. Furnace Type % Fly AsfVO/o Bottom Ash (Bituminous Coal)t Pulverized Dry Bottom 80/20 Pulverized Wet Bottom 65/35 Cyclone 13.5/86.5 Stoker 60/40 tBased on several studies of coal ash from large and intermedi-ated size coal-fired boilers. Aerosol Dynamics By knowing the vapor pressure of metals at the combustor exit and control equipment inlet, the entrained particle size distribution at the coal combustor exit, and the temperature profile of the . gas upstream of the APCD, aerosol dynamics (nucleation, condensation, and coagulation) modeling can be performed and the fraction of metals condensed onto or into each particle size range can be calculated at the APCD inlet. Nucleation and condensation are key processes because they account for the enrichment of metals on small particles. A computer model simulating both nucleation and condensation has been developed. Details of the condensation and nucleation submodels are presented elsewhere.[5] Particle coagulation is simulated using the computer model developed by Gelbard and Seinfeld[19]. 7 |