The Effect of Ash deposition on Tube Heat Transfer

Due to the abundant coal reserves in China, coal is expected to be the most important energy resource in the future. The ash deposition/slagging behavior can obviously affect the heat transfer efficiency of heat transfer tubes in boiler, and cause increasing coal consumption. In order to clarify the effect of ash deposition behavior on heat transfer characteristics of the tube in boiler, a pulverized coal combustion facility was used to test the ash deposition behavior as well as their effect on heat transfer characteristics of probe in furnace. The results shown that the vaporization and condensation behavior of alkali metals as well as the inter reaction of minerals in ash is the dominant factors for initial deposition behavior, which significantly decrease the heat transfer efficiency rapidly in the first 1.5 hrs. The reduction rate of heat transfer at the initial stage is 4~9 times higher than that at the growing stage. INTRODUCTION Approximately 290 billion tons of coal has been explored recently in Xinjiang, China, which is estimated to last more than a century at the current consumption rate[1]. Compared to bituminous coal, the newly found Xinjiang coal is notorious for its serious ash fouling/slagging problems due to the large amount of alkali metals in ash. The rapid ash deposition and dense structure cause serious problems for boilers when using this kind of coal. In light of this, most units have to be operated under low generating capacity (70~80% BMCR) or blend with other kind of coals or additive which incurs the operating cost. The factors that influence ash deposition/slaging behavior are complicated and interactive with one another, including chemical/mineral composition of original ash-forming metals in coal, aerodynamics of flue gas in boiler, temperature profile, flue gas compositions and tube surface temperature [2-5]. Extensivecharacterization on ash deposits have been conducted to clarify their properties, including particle size, chemical/mineral composition, fusion temperature, viscosity, contact angle of liquid phase on tube surface in laboratory scale tests[6-9]. Previous studies have shown that, the inner layer formed at the initial stage of ash deposition consists of fine particles (<10μm) with high content of alkali vapors, including sodium and potassium[10]. Along with the development of the initial layer on the tube surface, the initial deposition layer will capture particles entrained in flue gas and react with