| In China, the environmental pollution is serious, and the emission quantity of major pollutant is now still very high. The flue gas from coal combustion is the main origin of air pollution, and the major pollutants are soot and sulfur dioxide (SO<sub>2). About 87% SO2 in air conies from coal combustion, especially from the coal-fired power plants. The fluidized bed combustion and desulphurization is the effective and economical clean coal technique, and the absorbents such as limestone, dolomite or lime are very ubiquitous and cheap, so they are widely used in the desulphurization of fluidized bed all over the world. The seashells are like limestone in the contents, their major content is calcium carbonate (CaCO3). The aim of this paper is go deep into studying the desulphurization of calcium-based absorbent like limestone and shell at various conditions, and the difference of various absorbent at microstructure as well as its influence on the desulphurization through experiments and theoretical analysis.In this paper, 27 kinds of sea shells and 18 kinds of limestone were selected as sulfur absorbent; the calcinations and desulphurization of every absorbent were studied using thermal-gravimetric analysis (TGA) method at atmospheric or simulate flue gas respectively. The effects of absorbent type, particle diameter, furnace temperature and concentration of SO2 as well as O2 on the calcium sulphation ratio were investigated. As results, the desulphurization of most shells are excellent than that of limestone as their content of alkali metals such as sodium oxide (Na2O) kalium oxide (K2O) et al, and the optimal temperature of shells are 50 to 150℃ higher than that of limestone. The order of sulphation reaction for SO2 is between 1 and 1.6, and the calcium sulphation ratio increases with the SO2 and O2 concentration when the O2 concentration was less than 4.85%.A bench scale fluidized bed reactor was established for the experiments and a serious of desulphurization experiments of 5 kinds of sea shells were carried out on this reactor, as contrast, 1 kind of limestone was chosen also. As results, the desulphurization efficiency of shells are higher than that of limestone, and increase with Ca/S mole ratio and temperature in some range; the optimal temperature are little lower than that in TGA.Scan electronic microscope (SEM) and mercury porosimeter were used to measure the microstructure of some absorbents. Contrast to the limestone, the original shells has big pore diameter, big specific volume and big specific surface area; when they are calcined, the major range of pore diameter is between 0.2 and 5jim. While in the limestone, in despite of larger specific surface area, but the specific volume is very low, the pore diameter is only between 0.005 and 0.1 μm. Farther, within the shells, the high specific surface area corresponding to the high calcium sulphation ratio, so we can conclude that the reasonable pore size in absorbent for desulphurization may larger than 0.2 um and as small as possible on this base. With the temperature rise, the shell, which have big original pore size, have a decrease in pore size, while the limestone whose original pore size are small have an increase in pore size and a decrease in specific surface area, these lead to the decrease of reaction rate of sulphation. On the basis of the pore structure, the fractal dimension of the pore of absorbent was investigated. The results show that the shells have big fractal dimension but the limestone have small one respectively, and the absorbent, which have big fractal dimension, corresponding higher optimal sulphation temperature.A particle-pore size distribution model was made to describe the sulphation of the absorbent, which was on the basis of an un-reacted core model and a distributed pore sizemodel and the researching of the chemical reaction and the diffusion of reacted gases of the desulphurization. This model was coherent with the experimental results and being considered to used...
|
PDF Full Text Request