Development of nondestructive evaluation methods for hot gas filters

Rigid ceramic hot gas candle filters are currently under development for hi gh-temperature hot gas particulate cleanup in advanced coal-based power sys tems. The ceramic materials for these filters include monolithics (usually non-oxides), oxide and non-oxide fiber-reinforced composites, and recrystal lized silicon carbide. A concern of end users in using these types of filte rs, where over 3000 may be used in a single installation, is the lack of a data base on which to base decisions for reusing, replacing or predicting r emaining life during plant shutdowns. One method to improve confidence of u sage is to develop nondestructive evaluation (NDE) technology to provide su rveillance methods for determination of the extent of damage or of life-lim iting characteristics such as thermal fatigue, oxidation, damage from ash b ridging such as localized cracking, damage from local burning, and elongati on at elevated temperatures. Although in situ NDE methods would be desirabl e in order to avoid disassembly of the candle filter vessels, the possible presence of filter cakes and/or ash bridging, and the state of current NDE technology prevent this. Thus, off-line NDE methods, if demonstrated to be reliable, fast and cost effective, could be a significant step forward in d eveloping confidence in utilization of rigid ceramic hot gas filters. Recen tly, NDE methods have been developed which show promise of providing inform ation to build this confidence. Acousto-ultrasound, a totally nondestructiv e method, together with advanced digital signal processing, has been demons trated to provide excellent correlation with remaining strength on new, as- produced filters, and for detecting damage in some monolithic filters when removed from service. Thermal imaging, with digital signal processing for d etermining through-wall thermal diffusivity, has also been demonstrated to correlate with remaining strength in both new (as-received) and in-service filters. impact acoustic resonance using a scanning laser vibrometer has be en demonstrated to allow detection of changes in frequency which may be cor related to remaining strength. These methods have been shown to be applicab le to clay-bonded SiC filters, recrystallized SIC filters, CVI-SiC composit e filters and oxide composite filters. Other NDE methods under development include: (a) fast, high spatial-resolution X-ray imaging for detecting dens ity variations and dimensional changes; (b) air-coupled ultrasonic methods for determining through-thickness compositional variations; and (c) acousti c emission technology with mechanical loading for detecting localized bulk damage.