Gas cleaning at high temperature 659 removal of such particulate catalysts should therefore prevent emissions of these In many applications, it is conventional to carry out the preliminary coarse particle capture in one or two stages of cyclones. However, cyclones have been found to be ineffective for collection of particles much below about 7 um so that it is conventional to apply a so-called tertiary gas cleaning stage, which at near-ambient temperature is frequently a fabric or bag filter. There is still debate about necessity for tertiary gas cleaning and the newest generation of PFBC plants has only two stages of cyclones to protect the turbine Of the available tertiary cleaning methods, rigid ceramic barrier filters are the most promising and the most highly developed [4], although advances continue to be made in the development of high-temperature fabrics for bag filters, and there are likely to be specific application for metal filters [5, 6], granular bed filters [7] and electrostatic precipitators [8]. It is the intention here to review the recent advances understanding of the behavior of rigid ceramic filters. For a wider view of hot cleaning in general, the reader is referred to the Proceedings of the four International Conferences on 'Gas Cleaning at High Temperatures[9-12] 2. ADVANCES IN DESIGN AND UNDERSTANDING For those unfamiliar with the technology, Fig. I represents a typical filter arrange ment. Conventionally, the filter medium is provided in the form of long hollow tubes, or,, closed at one end and hung vertically from a tube plate such that the gas to be filtered passes from the outside inwards, depositing a dust cake on the CLEANED CAS HOT DUSTY LOCH Figure 1. Typical candle filter house(after British Coal)Gas cleaning at high temperatures 659 removal of such particulate catalysts should therefore prevent emissions of these damaging compounds. In many applications, it is conventional to carry out the preliminary coarse particle capture in one or two stages of cyclones. However, cyclones have been found to be ineffective for collection of particles much below about 7 ¹m so that it is conventional to apply a so-called tertiary gas cleaning stage, which at near-ambient temperature is frequently a fabric or ‘bag’ lter. There is still debate about the necessity for tertiary gas cleaning and the newest generation of PFBC plants has only two stages of cyclones to protect the turbine. Of the available tertiary cleaning methods, rigid ceramic barrier lters are the most promising and the most highly developed [4], although advances continue to be made in the development of high-temperature fabrics for bag lters, and there are likely to be specic application for metal lters [5, 6], granular bed lters [7] and electrostatic precipitators [8]. It is the intention here to review the recent advances in understanding of the behavior of rigid ceramic lters. For a wider view of hot gas cleaning in general, the reader is referred to the Proceedings of the four International Conferences on ‘Gas Cleaning at High Temperatures’ [9 – 12]. 2. ADVANCES IN DESIGN AND UNDERSTANDING For those unfamiliar with the technology, Fig. 1 represents a typical lter arrange￾ment. Conventionally, the lter medium is provided in the form of long hollow tubes, or ‘candles’, closed at one end and hung vertically from a tube plate such that the gas to be ltered passes from the outside inwards, depositing a dust cake on the Figure 1. Typical candle lter house (after British Coal).