Sequential SO₂/CO₂Capture Behavior Of Ca-Based Sorbents At High Temperture

The greenhouse effect caused by the emission of CO2from fossil fuel combustion have an important impact on the global climate change. The cyclic calcination/carbonation reaction with Ca-based sorbents to capture CO2from flue gases has been one of the most feasible technology, due to its economics by using inexpensive and widely distributing limestone or other Ca-based minerals as CO2absorbent. Studies have shown that during the process of Ca-based absorbent loop capturing CO2, SO2in the flue gas can react with CaO, generate a layer of CaSO4in the surface of CaO, which hinder the absorption of CO2, causing sharp decline of the CO2capturing capacity. Sequential SO2/CO2capture with Ca-based absorbents is an effective way to solve this problem. The technology using spent sorbents after cyclic carbonation/calcination to remove SO2can reduce the use of Ca-based sorbents, and also can achieve waste reuse, especially suitable for the SO2/CO2capture in coal-fired circulating fluidized bed boiler.The effects of sulfation of CaO on average carbonation conversion, CO2capture efficiency and energy requirement in a calcinier for the regeneration of CaO at existence of SO2in flue gas which enters a carbonator are investigated in this paper. Compared with unsulfation, sulfation induces a decrease in average carbonation conversion. When the limestone supplement flowing ratio is0.14, SO2/CO2volume ratio from0.02increased to0.04, the average carbonation conversion reduced from0.18to0.03. When the volume ratios of SO2to CO2is1%and CO2capture efficiency is90%, the energy requirement in the calciner with the sulfation of CaO is290kJ/molCO2, which shows a14%rise compared with that without the sulfation.The behavior of calcium-based sorbents sequently removal SO2/CO2are studied in a fixed bed reactor and a thermalgravimetric analyzer. Researching the influence of temperature, reaction time, particle size, number of cycles and other factors on the carbonation and sulfation efficiency of calcium-based sorbents. Combined with scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray spectroscopy analyzer (EDS), N2adsorption and other means to analying the micro-structural changeing characteristic during the reaction process, in order to obtain the reaction mechanism.Study the impact of reaction conditions on the sulfation characteristics of limestone and dolomite after different numbers of carbonation/calcination cycle. Cyclic carbonation/calcination numbers have great effect on the sulfation characteristics of the calcium-based sorbents. With the number of cycles, limestone sulfation conversion first decreases rapidly, after20cycles sulfation conversion stabilized at around21%, a slight increase in conversion after150cycles about24%, after200cycles the sulfation conversion is16%. These changes have a closely relation with limestone microstructural changes in the cyclic process. Dolomite sulfation conversion decreases with the number of cycles, compared with limestone, dolomite shows a higher sulfuation conversion under the same reaction conditions.The crude pyroligneous acid (CPA) was proposed to modify limestone for enhancing the CO2capture capacity In the same number of cycle, the carbonation rate of the modified limestone is higher than that of unmodified limestone. The carbonation conversion of the modified limestone undergone103cycles can achieve0.33,4.2times than that of unmodified limestone under the same conditions. The modification of limestone increases the surface area and pore volume of its calcines during cycles. The price of crude pyroligneous acid (CPA) is low, so using the modified limestone as CO2capturing sorbent at high temperature has application prospect. Study the sulfution behavior of the modified limestone after cyclic carbonation/calcination. In the same reaction conditions, the sulfution conversion of modified limestone is significantly higher than that of unmodified limestone. After20cycles carbonation/calcination, the sulfation conversion of the modified limestone is2times than that of unmodified limestone.Analyse the sulfation kinetics of limestone modified with crude pyroligneous acid after different carbonation/calcination cyclic numbers using equivalent particle model, and obtaine the changing rules of kinetic parameters with the number of cycles. With the number of cycles, time for the chemical reaction-controlled phase of modified limestone is shorter, and the reaction into the product layer solid diffusion-controlled phase is ahead of time. Before20cycles, the activation energy for the chemical reaction zone of the modified limestone is between33.0～58.0kJ-mol-1, the pre-exponential factor is between2500.0～11000.Ocm-min-1, while the activation energy for the product layer diffusion zone is between159.0～179.OkJ·mol-1, the pre-exponential factor is between400-12000cm2·min-1.