Experimental Study On Coal Char Reactivity Under Circulating Fluidized Bed Combustion Conditions

The combustion conditions in a Circulating Fluidized Bed (CFB) Boiler are much different from a pulverized coal firing (pf) boiler. However, this dissertation has confirmed that coal char in CFB boiler will also experience reactivity loss and ordering of turbostratic carbon structure, which have been reported by many researchers whose work were mainly restricted to pf conditions. Three low rank coals and one anthracite have been studied. Interferences to experimental results by effects of coal inhomogeneity, particle size and volatile release on char properties have been greatly reduced by special methods. Pyrolysis chars have been obtained under various well-controlled conditions close to those of CFB combustion. Residual carbons have also been collected from two industrial CFB boilers and a pilot CFB combustor, as well as from coal particles burning statically in open air. Characters of the pyrolysis chars and residual carbons such as reactivity, turbostratic carbon structure and specific surface area have been examined by thermal gravimetric analysis, powder X-ray diffraction and mercury intrusion methods respectively. Catalytic effect of minerals has been identified. With the increase of temperature, chars from low rank coals deactivate quickly and significantly. Through a strict and systematic analysis, char deactivation has been found to be due to the combined effect of the ordering of turbostratic carbon structure and the loss of catalytic effect of minerals during pyrolysis or combustion process. Specific area changes are negligible and irrelative to char deactivation. The minerals in the 4 coals studied act in very different ways, showing significant catalytic effect in one lignite, while no such effect observed in the other coals. With the increasing of temperature, the mineral catalytic effect is losing quickly, and completely lost above 1200(C, which agrees with the results by other researchers. The study of residual carbons from CFB fly ashes and its comparison with pyrolysis chars have confirmed both deactivation and ordering of turbostratic carbonstructure of coal char particles in CFB boilers. Residual carbons contain particles of a distribution of true density. The heavier portion (>1.9g/cm3), having a lower reactivity and even more difficult to burn out, makes up of over one quarter of the total amount of residual carbon. This unexpected fact shows that efforts in order to improve combustion efficiency by increasing cyclone efficiency or sending back to furnace the high carbon content ashes from electrostatic separator will suffer a consequential limitation. Experimental results support the universal relationship of kinetic parameters of coal developed by Fu and Zhang in some aspects. The universal relationship assumes irrelativeness of activation energy to coal properties. For combustion under temperatures below 900(C, dispersed minerals in coal chars may play a very strong role upon char reactivity, and different coals will develop chars of different carbon structures. These factors, which have been identified by our experimental work, are obviously not counted into the proximate analysis data. Consequently, when the universal relationship is applied in low temperature combustion, there might be a risk of considerable inaccuracy. A simple deactivation model has been constructed and used in a one-dimension CFB combustion model to evaluate effects of char deactivation upon the prediction of carbon content in fly ashes by the combustion model. Modeling efforts have been checked by experiments in the pilot CFB combustor.