Mechanism Of Sulfur Transformation And Interaction With Calcium During Oxy-coal Combustion

Coal plays a dominant role in the energy mix and is responsible for the majority (over 70%) of electricity generation in China. The use of coal is one of the biggest contributors to global emission of SOx, NOx, PM and CO2. Coal combustion under oxy-fuel conditions is recognised to be a key step-change technology for the clean and efficient utilization of coal and the mitigation of CO2 emissions. Presently, oxy-fuel combustion has been recognized as one of the most promising technologies to implement CO2 capture and simultaneously realize low NOx emission and high desulfurization efficiency as well as other pollutants control. Therefore, it has great scientific and practical significances to investigate the formation mechanisms, evolvement process and emission characteristic of pollutants during oxy-fuel combustion.This thesis provided an overview of the present researches on oxy-fuel combustion fundamentals, including convective, radiative heat transfer, devolatilization and ignition, volatile and char burnout, boiler tube corrosion and gaseous pollutants emissions, ash deposition chemistry, especially focusing on SO2 formation mechanisms, evolvement process and emission characteristic and significant control during oxy-fuel combustion. This thesis investigated on sulfur transformation during CO2-pyrolysis, and then focused on effects of coal characteristics and combustion parameters on SO2 emission, desulfurization efficiency and simultaneous inhalable particles control during O2/CO2 coal combustion with in-boiler desulphurization by injection of limestone. The key findings of dissertation are as follows:Firstly, effect of organic and inorganic sulfur on the transformation during CO2-pyrolysis has been analyzed. The experimental results indicated that SH* evolved by organic sulfur and active S decomposed by pyrite were the two key intermediate radicals during CO2-pyrolysis. SH* radicals prefered to react with the char to form H2S, COS, and elemental sulfur, or new sulfur structures retained in the char. Active S radicals prefered to undergo secondary reactions with the char to form nascent organic sulfur structures, as well as SO2, COS. Meanwhile, inherent minerals had significant influences on the SH* radical-char and active S radical-char reactions, thus promoted the sulfur retention.Secondly, effect of combustion parameters on the calcination and sintering behaviors of limestone in O2/CO2 atmosphere has been discussed. There were some obviously different calcination and sintering characteristics in O2/N2 and O2/CO2 atmosphere. The initial decomposition temperature and the activation energy in O2/CO2 atmosphere needed much higher than that in conventional coal combustion. In O2/CO2 coal combustion with in-boiler desulphurization by injection of limestone, the higher decomposition temperature not only increased the desulfurization temperature range and extended the time of limestone decomposition, but also shortened the sintering time, lightened the nascent CaO sintering and improved structure characteristic of CaO, making SO2 and O2 easier entry into product lager and enabling higher degrees of sulfation. Calcination temperature and time simultaneity affected calcination and sintering, increasing temperature leaded to much increase of calcination rate and sintering rate. It was found that an optimum temperature for the highest specific surface area and porosity of CaO was between 900-1000℃. SEM energy spectrum analysis and XRD phase quantitative analysis proved that in the end of limestone decomposition, calcination time made the crystallite size of CaO increase and the sintering more seriously.Thirdly, effect of different mechanisms on calcination, sintering and sulfation of limestone has been discussed. The mechanism of direct sulfation under high CO2 concentration in O2/CO2 atmosphere, different from that of CaO-SO2 sulfation in conventional coal combustion, is an important reason for the high sulfation efficiency. The experimental results showed that direct sulfation enabled higher sulfation degrees and desulfurization efficiency than those observed from CaO-SO2 sulfation, because the counter-diffusion of CO2 generated from limestone decomposition formed a porous product layer. What's more, the calcination and sulfation characteristics of limestone were investigated in a laboratory drop tube furnace (DTF). The experimental results indicated that the desulfurization efficiency in O2/CO2 coal combustion was much higher than that of conventional coal combustion. More importantly, the desulfurization efficiency in O2/CO2 coal combustion maintained a high value in a wider and higher range of temperatures.Finally, experiments of limestone addition have been carried out in a laboratory drop-tube furnace to simultaneously study high desulfurization efficiency and reduction of inhalable particles. The experimental results indicated that the element S was an important constituent of submicron particles (PM1). In O2/CO2 coal combustion with limestone injection into furnace, limestone reacted with S to produce supermicron particles and the ratio between fine and coarse particles shifted significantly toward larger supermicron particles. Meanwhile, the influence of combustion temperature and oxygen content during O2/CO2 coal combustion with limestone injection into furnace has been analyzed. With the increase of temperature, PM1 and PM10 emission first decreased and then increased. Otherwise, with the increase of oxygen content, PM1 emission first decreased and then increased, while PM10 emission always increased due to large production of supermicron particles of calcium sulfate. The results showed that in-boiler desulphurization by injection of limestone could simultaneously realize high desulfurization efficiency as well as inhalable particles control. Furthermore, compared with in-furnace desulfurization in conventional coal combustion, O2/CO2 atmosphere provided a better control of PM emission.