Evolution Of Fuel-N And Mechanism Of NO_x Formation During Oxy-fuel Fluidized Bed Combustion

Coal is the main production source of SO2, NOx and CO2 in China, and the excessive consumption of coal in power plant results in serious environmental problem, including greenhouse effect. CFB oxy-fuel combustion combines the advantages of CFB techonology and oxy-fuel combustion techonology, which can capture CO2 and control the emission of NOx very efficiently. Plenty of studies have been developed on the generation of NOX in CFB oxy-fuel combustion, but most of them were focused on the influences of operation parameters, and the evolution of fuel-N and the mechanism of NOx generation under this condition are still unclear. Thus, based on the transformation routes of fuel-N in combustion, the influence of CO2 on fuel-N evolution and the mechanism of low NOx emission in oxy-fuel combustion will be researched systematically through the following four aspects:the transformation of fuel-N during pyrolysis, the oxidations of volatile-N and char-N during combustion and the reduction of NO.Firstly, in order to study the producing characteristics of NOx precursors and the influence of CO2 on this, rapid pyrolysis experiments were carried out in a suspension bed reactor under N2 or CO2 atmosphere. Combined with the deduction of the evolution of hydrogen groups and char structure during pyrolysis/gasification which were measured by FTIR spectrometry, Raman spectrometry and X-ray diffractometry, the formation mechanisms of HCN and NH3 were revealed. The results indicate that nitrile (-CN) groups and quaternary-N are the important intermediates of HCN and NH3, respectively. With the increase of temperature, the amounts of HCN and NH3 both increase. The effects of CO2 on HCN and NH3 are complicated, which can be divided into two periods. In the pyrolysis stage, the release of H and the cracking of N-containing groups are promoted by the gasification of char with CO2, enhancing the production of HCN. However, when pyrolysis temperature is too high or too low, the generation of HCN will be inhibited by CO2. At the same time, CO2 can suppress the polymerization of aliphatic chains, and more H is retained as-CH2 or-CH3 in chars during coal pyrolysis, which is helpful to produce abundant H radicals for the formation of NH3 in the stage of gasification. CO2 can promote the cracking of N-containing groups, which is also beneficial to the formation of NH3. Eventhough, if the reactivity of nascent char is poor, CO2 can block access of the H radicals to the N-sites and less NH3 will be produced.According to the kinetic analysis of NO catalytic reduction by CO over CaO, it shows that as temperature increases, both the activation energy of the reaction and the reaction order of CO decrease. With the increase of both CO concentration and catalyst amount, the reaction order of CO decreases. The catalytic effect of CaO on NO reduction by CO is seriously inhibited by CO2.Simultaneously, the experiments of NO reduction by char were also developed in the suspension bed reactor to study the reaction characteritics and the internal mechanism. The influences of CO, O2 and CO2 have been studied as well. It has been proven that the surface oxygen complexes are essential for the reduction of NO by char through the reaction C(O)+NO+Cf?CO2+C(N)+*, particularly at low temperatures, and the main carbon containing product is CO2. As temperature increases, because of the acceleration of C(O) desorption, the promotion effect of C(O) on NO reduction will be weakened and then CO becomes as the main carbon containing product. At the lower temperature, both CO and O2 can enhance the NO reduction by char, which may be attributed to the formation of C(O) on char surface. But with the temperature increasing, the promotion effect will decrease gradually. CO2 appear to suppress the NO reduction by char and the inhibition effect will be promoted with the increase of gasification intensity.Through the experiments of char-N oxidization, the effects of the important related factors, such as temperature, char concentration and coal rank, were investigated and the formation mechanisms of N-containing products (NOx, N2O etc.) from char-N have been revealed. With the increase of specific surface area, char reactivity, ash content and char concentration, the ratio of NO produced increases. At the same time, the influences of temperature, coal rank, the amount of feeding coal and coal particle size on the generation of NOx from coal combustion were also been studied.Then, NH3 was chosen as the model compound of volatile-N, and it was oxidized in O2/CO2 atomosphere. The results show that the transformation of NH3 to NO in oxidizing atmosphere is suppressed by CO2. According to the experiments of char combustion in O2/CO2 atomosphere, CO2 has two quite different effects on NO generation:(1) CO2 suppresses the reduction of NO; (2) CO2 can react with char directly, leading to the high selectivity of N2 from char-N. If the supply of oxygen is enough, the suppression effect of CO2 on NO reduction will play a leading role in char combustion, which is helpful to the formation of NO. Otherwise, the gasification of char with CO2 may play a more important role, and the generation of NO will be inhibited.Finally, the O2/CO2 combustion experiments were carried out on a fluidized bed multi-functional test bench to study the influences of oxygen concentration, air-staged proportion and staged oxygen injection on NOx formation. Based on the previous research conclusions in the paper, the reasons for low NOx emission in oxy-fuel combustion were discussed in detail. It may not be caused by the rise of CO concentration in the furnace, which is benefical to the reduction of NO, but by the inhibition effect of CO2 on the transformation of volatile-N and char-N to NO.