A comparative evaluation of recarbonated CaCO₃ derived from limestone under oxy-fuel circulating fluidized bed conditions

SO₂ emissions from coal-fired boilers are air pollutants and a source of acid rain, causing extensive environmental pollution. Limestone (CaCO₃) is a Ca-based sorbent which is injected into circulating fluidized bed (CFB) boilers, where it combines with SO₂ to produce calcium sulfate (CaSO₄). As a result, SO₂ emissions from a power plant are reduced. In this study, CaCO₃ addition was proposed and the desulfurization efficiency improved. The direct desulfurization reaction is dominant in a commercial CFB boiler due to the high CO₂ partial pressure, but CaO is formed at a fast reaction rate by calcination in the high temperature or in the low CO₂ partial pressure region. When CaO remains in the loop seal, it is exposed to a high CO₂ partial pressure condition moving through the recirculation section for an extended period and re-injected into the furnace as recarbonated CaCO₃. To analyze the direct desulfurization reaction kinetics, a shrink core model in which the reaction proceeds inside the particle was adopted. Surface observations through FE-SEM of CaSO₄ produced by the 180 minute long desulfurization experiment using TGA suggest that the CaSO₄ crystal growth rate increased after the pre-treatment (recarbonation) of limestone. Recarbonation lowered the limestone crystallinity, causing a faster reaction. The CaCO₃ recarbonation increased the Ca utilization by more than 20% when the direct desulfurization reaction occurred. The TGA experiments show that recarbonation contributes to CaSO₄ conversion. Increasing the desulfurization efficiency using recarbonation can reduce the fixed investment and operating costs of oxy-fuel CFB plants because only desulfurization in the furnace is able to meet SO₂ emission regulations or lower the flue gas desulfurization (FGD) dependence. Accordingly, the desulfurization conversions of recarbonated CaCO₃ and limestone were compared in this study. Morphological changes in the limestone were also evaluated using XRD, FE-SEM, and other analysis methods.