The technical viability of a high-temperature gas clean-up system is essential for many of the new concepts in power plant design. A barrier-type filter now appears as the most appropriate solution to the problem. The most common type of barrier filter is the candle filter. The present problems with candle filters involve (1) the survivability of the filter in a harsh environment and (2) the removal of the ash deposits from the filter surface. This paper is concerned only with the latter problem. Ideally, the ash from the high-temperature dusty gas builds up on the outer surface of the candle filter. At either a prescribed time interval, or a prescribed pressure drop across the filter system, high-pressure gas is used for back-flushing the ash off the filter surface. This process is referred to as surface regeneration. At present, the surface regeneration process is not as effective as desired. Only the outer portion of the ash layer, referred to as the soft ash, is removed during surface regeneration. An inner layer of ash, the residual ash, survives the cleaning process. The residual ash layer appears to grow with time and may eventually contribute to the bridging between the candle filters. The growth and persistence of the residual ash may be noted on the performance history curves in [1], as illustrated in Figure 1. The basic goal of the filter system is to reduce the concentration of solids in the gas stream to an acceptable level with a minimum pressure drop. Many of the newer designs involving barrier filters are described in [2]. The cause(s) for the strong, stubborn, residual ash deposit has not yet been established. The two mechanisms that may account for the residual ash formation are: 1) the cohesive force(s) between the particles within the ash layer and 2) sintering [2]. Residual ash may form at low temperatures as well as high temperatures; consequently, sintering may only be significant at high temperatures. However, candle filters are intended to operate at high temperatures and the potential for sintering to occur must be considered. This study considers the problem of unburned carbon entering the ash layer on a candle filter. This unburned carbon may be due to incomplete combustion or system upsets. During surface regeneration, if air is used, a sufficient amount of oxygen will be present for combustion to occur in the ash layer. The heat generated during surface regeneration may then lead to sintering in the ash layer. Reference [2] describes occasional combustion on filters in a solid, smokeless fuel plant in England. These filtering systems were operating in the range of 200-300°C. Several of the filter fires lead to filter failures. The surface regeneration process was not performing properly and this was attributed to the pulse jet valves malfunctioning. Thermal stresses generated by the combustion of unburned carbon in the ash deposit on filters has been investigated [3]. Their research described the carbon concentration in four ash samples taken from an operating PFBC unit. This experimental study showed that when the carbon content was 16% (by weight) the change in filter surface temperature may reach 5000C. The resulting temperature rise was sufficient to predict filter failure due to thermal stresses.
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