Study On The Mechanisms For Reduction Of SO₂ And Nox By Calcium-enriched Bio-oil

Coal-fired power plants are the primary sources of SO2 and NOx emissions. With the increasingly stringent environmental requirements, the simultaneous removal of SO2 and NOx from coal-fired power plant has been imperatived.Spraying limestone inside furnace is the most commonly method to reduce SO2 emissions in china numerous small and medium-sized industrial coal-fired boilers. But desulfurization by limestone has the disadvantage of low desulfurization efficiency and the problem that can not denitration simultaneously. Researches abroad indicated that organic calcium salts (such as calcium acetate) had high desulfurization efficiency and certain denitration efficiency simultaneously, which were very suitable for coal-fired boilers absorbents. But a large number of organic acids will be need for preparing organic acids calcium salts, and the cost of producing organic acids is too high, which causes the fact that this method is still in academic research stage.The bio-oil can be obtained easily by rapid pyrolysis of any kind of biomass in the absence of oxygen. The bio-oil mainly contains acetic acid, formic acid, propionic acid and a small amount of benzoic acid, which can react with Ca(OH)2 or CaCO3 and the product contains numerous organic acids calcium salts, which called" calcium-enriched bio-oil(CEB)". It can be act as a succedaneum for organic acids calcium salts.This paper focuses on the novel organic calcium "calcium-enriched bio-oil(CEB)", which is capable of desulfurization and denitrification simultaneously. The effect principles of factors on preparating of CEB were investigated firstly. Limestone particle size and pH value of bio-oil and reaction temperature were investigated during the process of CEB preparation. Bio-oil pH value and reaction temperature play a key role in the process, the smaller pH value of bio-oil and high temperature are conducive to increase calcium concentration. The results of CEB components analysis indicated that CEB mainly contain Ca, H, O, C. The test results of CEB molecular structure showed that CEB mainly contain organic acids calcium salts, but also containing alcohol, benzene, phenol and other substances. The calcination process of CEB was investigated by thermogravimetric analyzer, the results showed that CEB calcination process could be divided into four stages, which were dehydration of CEB, CO2 and H2O precipitation from part component of bio-oil, decomposition of organic carboxylic acid calcium salt, decomposition of calcium carbonate respectively. Heating rate and particle size had little effect on CEB decomposition processes, but CO2 had significantly inhibitory effect on CEB decomposition processes. The kinetics of the second and third stage was studied, and the kinetic parameters were calculated. The mechanism functions were also determined by both the universal integral method and the differential equation method. The results indicate that the shrinking cylinder model with surface reaction rate controlling mechanism was the model fitting the latter two stages during CEB calcination process. The calcium oxide particles obtained from decomposition and calcination of CEB were analyzed by different methods to determine their physicochemical characteristics. Pore structure parameters indicated that decomposition of organic carboxylic acid calcium salt had important influences on product pore structure. The cavitations effected by gas precipitation and carbonization deposition phenomenon of macromolecular compound coexist in the range of 450～600℃, but the latter dominated. The carbonization deposition phenomenon of macromolecular compound mainly occurred in the range of 500～600℃. After 600℃, with the decomposition of calcium carbonate and carbon dioxide release, new micropores had been formed in calcined product. Under the same calcination temperature the pore characteristics of CEB calcined product obviously superior to calcium carbonate.CEB desulfurization process was investigated on thermogravimetric analyzer and a small fixed bed reactor. The results showed that CEB desulfurization temperature should be the best around at 900℃, and CEB desulfurization process could be divided into three stages:surface desulfurization, sulfur dioxide slow diffusion stage and the reaction suspension stage. Slow diffusion phase of sulfur dioxide plays a decisive role in CEB desulfurization process. Grain model had been used to simulate CEB desulfurization process, and simulant results in good agreement with the experimental results. Then evaluation method established by principal component analysis was used to assess the reaction activity of CEB desulfurization, the results showed that CEB desulfurization performance mainly decided by the CaO content, the ratio of specific surface area and average pore size, porosity.CEB desulfurization and denitrification simultaneously was was investigated on a small fixed bed reactor. The reuslts showed that during CEB desulfurization process SO2 precipitation decline with temperature increasing in initial combustion stage, SO2 precipitation increase with temperature increasing in late combustion stage. SO2 precipitation peak gradually moves up with the increase of excess air ratio. The active group CHi generated by the decomposition of organic gases during CEB calcination process had obvious reduction effect to NOx first precipitation peak, while the CaO formed by CEB calcination process had a dual role on NOx second precipitation peak.1150℃was the appropriate temperature for CEB denitrification; poor oxygen atmosphere was conducive to CEB denitrification efficiency.CEB reburning process was simulated, the results showed that the temperature and atmosphere were major factors on NOx reduction; poor oxygen condition was conducive to CEB denitrification efficiency. While the effect of the temperature related with excess air ratio, excess air ratio of 0.6 to 0.8 and the temperature in the range of 1100～1400℃were idea condition for CEB denitrification. CEB reburning process could be divided into two stages, in the rapid response stage, CH2CO generated during CEB calcination process continue to decompose as H, HO2, HCCO, which reduce the NOx concentration.