A fundamental study of circulating bed absorption for flue gas desulfurization

Circulating bed absorption (CBA) is a semi-dry, lime-based, flue gas desulfurization (FGD) process that utilizes a circulating fluidized bed arrangement for contacting the sorbent with SO{dollar}sb2{dollar}-laden flue gas under "coolside" conditions. A hydrated lime slurry is injected into the bottom of a reaction chamber co-currently with the gas. The flue gas suspends, dries, and transports the sorbent through the reaction vessel. Most of the spent sorbent particles are separated from the flue gas in a cyclone and injected back into the reactor. In comparison to conventional spray drying absorption (SDA) systems, where recycle solids are slurried and atomized with the fresh sorbent, very high recycle ratios are possible in a CBA process where only dry solids are recycled. The reaction chemistry for a CBA process is thought to be similar to that of SDA. Differences between the two systems are primarily the increased solids concentration within the CBA reactor vessel and the method for re-injecting the recycle solids.; A pilot-scale fluid bed reactor was modified to simulate a generic CBA process. The objectives of the research were to: investigate the influence of basic operating parameters such as calcium-to-sulfur (Ca/S) ratio, flue gas temperature, fuel chloride, gas residence time, flyash loading, and approach-to-saturation temperature on the total sulfur capture in the CBA system; elucidate the reaction mechanisms through the analysis of spent reactor particles; and develop a simple mathematical model to describe the three-phase reaction process of the CBA system.; The overall sulfur capture varied between 45 and 99.5% depending on the specific test conditions. An increased fuel chloride level enhanced the sulfur capture in the system. Flyash was also found to enhance sulfur capture and was postulated to assist in CBA reactor particle agglomeration. While operating in the CBA mode, the reactor sulfur removal was 30 to 50% greater than that while operating in a spray dryer mode. Sulfur capture was found to be independent of the particle concentration. The liquid phase mass transfer coefficient was successfully modeled as a function of the sorbent particle spacing on the wetted surfaces.