Nitrogen And Sulfur Conversion Mechanism During Biomass Combustion

Among various biomass utilization technologies, combustion for electricity is the most developed and widely applied because of its low costs and high reliability. However, although the nitrogen and sulfur contents in biomass fuels are relatively lower compared to coal and other fossil fuels, the nitrogen oxides and sulfur oxides during thermo-chemical conversion processes still can not be ignored with increasingly stringent environmental regulations. Funded by National Key Technology R&D Program of China (2012BAA09B01), the nitrogen and sulfur conversion mechanism during biomass combustion were systematic investigated in the present study. The main contents and conclusions in current dissertation are as follows:(1) Five domestic representative agricultural wastes (rice straw, wheat straw, corn stalk, sugarcane leave, eucalyptus bark) were selected for this study, to investigate the nitrogen transformation characteristics during biomass pyrolysis process. The results showed that, most part of nitrogen was released in the lower temperature range (<400℃), and there was no more than30%of fuel-N retained in the char when the the pyrolysis temperature of900℃. NH3and HCN are the main N-containing species from slow pyrolysis of rice straw. Howerver, HCN yield is much higher than that of NH3during fast pyrolysis.(2) Biomass model char samples was prepared by using amino acid as N precusors. It was found that strong coupling reactions occurred during co-pyrolysis of amino acids and cellulose. Morover, through XPS analysis, it was found that the model char samples have similar characteristics compared to low-grade coal char, concerning the type of nitrogen-containing functional group.(3) The NO and N2O formation characteristics during biomass combustion were investigated in a well-controlled fixed bed system. It showed that there was still considerable degree of N2O conversion for biomass fuels, although NO was formed in much larger amount. Most part of NO and N2O yields were formed during devolatilization stage for biomass fuels, therefore, it can be expected to achieve ultra-low emissions of nitrogen oxides by optimizing the supply of air and fuel during biomass combustion in actual boilers. In the temperature range of700℃-900℃, the fuel-N conversion to NO increased first and then reduced slightly, while the conversion to N2O showed a continuous decreasing trend. The fuel-N conversion to NO and N2O shows a general increase trend with the increase of inlet oxygen concentration, and this phenomenon seems more obvious at higher temperatures.(4) Compared to coal char, biomass char has excellent capability for NO reduction, which is largely influenced by char properties and reaction conditions. Chars made at lower pyrolysis temperature showed higher reactivity with NO. Moreover, the NO reduction was proportionally higher with increasing reaction temperature.(5) The sulfur transfonnation characteristics during rice straw combustion was deeply studied. It was found that most part of fuel-S is released in the lower temperature range (<400℃), which mainly caused by decompsition of organic sulfur. SO2can be largely captured by char in the temperature range of700-900℃. And most part of the captured sulfur was incorporated with organic char matrix rather than directly retained by inherent alkali and alkaline-earth matters in the form of inorganic salts. During char combustion, substantial amounts of the captured sulfur could be retained in the ash, which was mainly limited by the alkali and alkali-earth matters available.(6) Within the CFB combustor, the excellent gas-solid contact would greatly enhance the self-desulfurization effect during biomass combustion. And there was nearly no SO2emission when the combustion temperature was controlled below800℃. Increase the excess air ratio would greatly reduce CO emission, but lead to higher NO concentration, while there is no clear effect on N2O emission. Raise the bed temperature would increase NO emission, but at the same time reduce N2O concentration. In high temperature condition, air-staging combustion is conducive for NO reduction, but there exists an optimized secondary air ratio. However, in the low temperature condition, air-staging combustion increase NO emission on the contrary.