Experimental Investigation On The Formation Mechanisms Of Ash Deposition During Biomass Combustion

The feasibility of biomass boiler mainly depends on its impact on ash deposition. Deposition reduces the heat transfer efficiency, and is considered as an important problem challenging the development of biomass-combustion technology. In the present work, all experiments were performed in a 25 kW downflow one-dimensional combustor under well-set and reproducible conditions. A temperature-controlled deposition probe was self-developed and employed to collect deposit samples for further analyses like composition, morphology, size, etc.Firstly, the effect of probe surface temperature on ash deposition during biomass combustion was investigated, in which 550, 600 and 650℃were carefully selected here in order to meet the increasing steam temperature of biomass boiler in practice. It was found that the low surface temperature enhances the condensation and deposition of vapor-phase species as well as the thermophoresis-driven deposition of nucleated ultrafine particles, which is termed as condensation-nucleation mechanism. Meanwhile, the high surface temperature enhances the adhesion or sintering ability of the deposited particles because the occurrence of melting and is termed as impact-melting mechanism. The lowest ash deposition at 600℃of biomass fuels can be explained by the competing between these two mechanisms. In addition, the 600℃well meets steam parameter of current biomass boilers, and thus are used as probe temperature of main experimental runs.Then, the time-dependent deposition behavior of both woody and herbaceous biomass fuels was thoroughly studied by collecting ash deposit with various time-consumption. The deposition rate variation with respect to the time exhibits as follows: at an initial beginning the rate is quite high; then it becomes decreasing and later increases again to the initial level; finally, the rate increment slows down. For a point of micro-scale view, the competition between condensation-nucleation mechanism and impact-melting mechaniam was the reason for the recurrent deposition rates along the time. Based on the composition analysis of samples, the reactive potassium (K) was found as the key ash-forming element that subsequently affects the deposit formation.Thirdly, the effect of the potassium on ash deposition was extensively discussed by introducing the atomized solution of potassium acetate into the flame zone of one-dimensional combustor to control potassium concentration in flue gas. The results indicated that the deposit tendency is linearly dependent to the potassium concentration in flue gas. Morphological and chemical analyses of ash deposits further verified the role of potassium in deposition process. The significant increase of deposition tendency can be attributed to two routes: (1) the combination of aluminosilicate with the potassium that forms new eutectics with a lower melting temperature; (2) the condensation of potassium gas as well as the adhesion of potassium submicron particle on the surfaces of big particles or probe that even increases the work of adhesion between big particles.Finally, two kinds of Chinese coals, containing greatly different content of kaolinite, were used to co-firing with various biomass streams. The results indicated that, the high-kaolinite containing coal is effective to capture the potassium from flue gases. The change of ash deposition tendency during co-combustion can be attributed to the combination of the kaolinite from coal and the potassium from biomass, which formed the new eutectics with a lower melting temperature. In this work, it was found that the deposition tendency is decreased when co-firing straw and high kaolinite coal, but is increased when co-firing straw and low-kaolinite coal. It can be explained that the reaction of kaolinite and potassium innibites the condensation-nucleation mechanism, but enhances the impact-melting mechanism.