Experimental and theoretical study of submicrometer particle formation and capture mechanisms in coal combustion environments.

摘要

A systematic study on submicrometer particles generation, evolution and collection in combustion processes was conducted. In the first section, experiments were conducted to examine the effects of temperature and residence time on particle size distributions and the transformations of trace metal species from pulverized coal combustion processes. The submicrometer ash particles were formed by means of a vaporization-nucleation-condensation mechanism. The modes of occurrence of the elements in raw coal affected the formation of the submicrometer particles. A decreasing enrichment of iron and silicon with increasing temperature was observed, whereas the trace species associated with the volatile fraction of the coal matrix were significantly enriched in the submicrometer sizes.; Experiments and theoretical calculations on transportation of submicrometer particles in an electrostatic precipitator were also carried out. Collection efficiencies were measured under various operating conditions and it was found that lower electrical resistivity of particles resulted in the decrease of collection efficiency because of reduced attractive force at collector surfaces after deposition. A decrease in collection efficiency was observed (both in a one-stage and two-stage ESP for different kinds of particles, NaCl, Al₂O₃, SiO₂, and submicrometer ash) with the reduction in particle size below 0.1 μm. A statistical approach for particle charging process, which assigned discrete charge levels to particles with the same size, in conjunction with the transport equation was used to support the experimental observations. This approach of fractional charging was more accurate than an assumption of uniform particle charging.; To further improve capture efficiency of submicrometer particles in electric field, UV charging experiments are designed to investigate the effectiveness of ultraviolet light on charging process for submicrometer particles as well as the electrical precipitation process. The total particle number concentration at the outlet of the electrostatic precipitator was reduced in the presence of UV irradiation. Also, the UV irradiation could enhance the corona generation. The corona current increased in the presence of UV light due to the added photon energy in the corona zone.; Another novel approach used to control the emission of submicrometer particles was the vapor phase sorbent injection technology. The vapor-phase sorbent injection methodology suppressed the nucleation mode of the ash particles by a reduction of particle concentrations at smaller sizes. The calcium coated sorbent demonstrated a very reasonable capture efficiency of heavy metals from Guizhou (China) coal where a physical condensation mechanism seemed to be dominant.; Simultaneously, a kinetic study was performed to investigate the interactions of elemental mercury vapor with entrained fly ash particles from coal combustion in a flow reactor. The rate of transformation of elemental mercury on fly ash particles was approximately first order (0.91 for total fly ash, and 0.98 for submicrometer fly ash), and the reaction rate increased with increasing temperature, indicating a chemisorption process. Single component particle experiments (titania, silica, and iron oxide) were performed to establish their contributions to the transformation of elemental mercury. Iron oxide caused the conversion of elemental mercury more efficiently than that of fly ash particles, while titania and silica did not result in oxidation or adsorption of the elemental mercury.