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Show 1. INTRODUCTION AND SCOPE OF STUDY It is common practice to fire large water tube furnaces with pulverized coals. Coals contain a number of mineral impurities, which are distributed in separate coal particles. As combustion proceeds, these impurities come under the influence of several complex processes, principally the pyrolysis of the coal particles followed by their rapid hetrogeneous combustion. After complete combustion of coal particles, the mineral impurities contained within are converted into new crystallographic phases, which are generally referred to as coal ash. The temperature in the furnace is sufficient for some of this ash to melt and the molten ash suspended in the combustion gases forms small droplet of glass. Also, some fraction of the ash normally agglomerates and falls to the bottom of the furnace or builds up on the walls near the burners. Molten ash is known as slag. In operation, slag may accumulate on the water tube walls, reducing the output and causing derated output or sometimes complete shutdowns. In addition, some fraction of the molten ash droplets may adhere in the upper part of the furnace and develop a thin layer of solidified droplets on tube surfaces. Since this layer is exposed to a hot radiant flame, it continuously receives heat by radiation and becomes sufficiently hot to fuse, sinter or melt even though gas temperature may be below the melting point of the ash. The continuous impingement of the other suspended particles against this layer can create a thick sintered layer which has a high thermal gradient between the outer surface and the surface next to the water tubes. This type of slag deposit creates two major problems. Firstly, the thick deposit can crack off as the temperatures of the walls change when load is changed on the plant, and the deposit falls from the top to the bottom (which is typically 100 feet in height) and damages the bottom tubes, or blocks the ash removal hole. Secondly, it 2-3 |