Factors Affecting Dustcake Drag in a Hot-gas Filter System Collecting Coal Gasification Ash

This paper discusses the use of laboratory drag measurements and filter operating data to analyze factors affecting dustcake flow resistance in a hot-gas filter at the Power Systems Development Facility (PSDF). The hot-gas filter is a Siemens-Westinghouse two-tier candle filter system that is collecting coal gasification ash from a KBR Transport Gasifier. Operating experience with this system has shown that the flow resistance of the dustcake is responsible for most of the pressure drop across the hot-gas filter, and the pressure drop varies substantially with the type of coal being gasified and the operating conditions of the gasifier and filter systems. To analyze factors affecting dustcake drag, samples of gasification ash from various coals and various operating conditions were resuspended in a laboratory test apparatus, and the drag was measured as the dust was collected on a sintered metal filter. The lab-measured drag values were compared to actual values of transient drag determined from the increase in pressure drop, the inlet dust loading, and the face velocity in the hot-gas filter. After correcting the lab drag data to hot-gas filter conditions, good agreement was achieved between the lab measurements and the hot-gas filter transient drag values. Both types of measurements showed that drag was strongly influenced by coal type and carbon content. INTRODUCTION Coal currently accounts for over half of the electric power generation in the US, and coal will continue to be a major source of power well into the 21 century [1]. The sustained, long-term use of coal for power generation will require the implementation of advanced coal-based power systems that are more efficient and more environmentally acceptable than current plants. Integrated gasification combined cycle (IGCC) is one advanced coal-based power system that is attractive in terms of economics and environmental concerns. With recent increases in the cost of natural gas and with increasing pressures to reduce CO2 emissions, many US utility companies are now expressing interest in coal-based IGCC to meet future electrical demands. To maximize the potential of coal-based IGCC systems, the syngas generated in the coal gasifier must be cleaned before the gas is burned in the combustion turbine. Any particulate matter that enters the turbine could cause serious damage of the turbine blades by eroding the thermal barrier coating on the blades and by depositing alkali metals that could chemically attack the barrier coating. To date, the most promising technology for high-temperature particulate removal in IGCC systems appears to be the barrier filter using either porous ceramic or sintered metal filter elements. At the Power Systems Development Facility (PSDF), a wide variety of different ceramic and metal filter elements have been tested in a hot-gas filter collecting gasification ash from a KBR Transport Gasifier. The hot-gas filter is a Siemens-Westinghouse, two-tier candle filter system that contains up to 91 candle-type filter elements. The Transport Gasifier is an advanced circulating unit that operates at considerably higher solids recycle rates, higher velocities, and higher riser densities than a conventional circulating fluidized bed. As of May 2005, the gasifier and hot-gas filter systems have accumulated over 6,400 hours of gasification operation with several different coals, including PRB (Powder River Basin) coal and various bituminous coals and lignites. Operating experience with the Transport Gasifier has been discussed in detail in previous papers by Leonard et al [2], Smith et al [3], and Vimalchand et al [4]. Experience with the hot-gas filter system at the PSDF has been reviewed in previous papers by Davidson et al [5], Gardner et al [6], and Martin et al [7]. Recent experience with the PSDF hot-gas filter is discussed by Guan et al [8] in a separate paper presented at this symposium. Experience with the hot-gas filter system at the PSDF has shown that it can operate reliably and achieve outlet particle loadings as low as 0.1 ppmw with iron aluminide filter elements [8]. One of the remaining issues with the hot-gas filter at the PSDF is the understanding of factors that affect the flow resistance of the dustcake. Dustcake drag is an important issue in this application, because most of the pressure drop across the hot-gas filter is associated with the cake. In previous test runs, we have observed considerable variation in transient dustcake drag with changes in coal type and gasifier configuration [9]. These variations in drag need to be understood so that the pressure drop of the hot-gas filter can be predicted and so that new hot-gas filters can be sized based on the expected dustcake drag. APPROACH In this study, factors affecting dustcake drag were examined using a laboratory system for resuspending samples of gasification ash, collecting the material on a sintered metal filter, and measuring the drag of the resulting dustcake. The system was described in detail in our previous paper presented at the 5 International Symposium on Gas Cleaning at High Temperature [9]. A simplified schematic of the system is shown in Figure 1.