Sorbent performance in fluidized bed coal combustion

The sulfation behavior of natural limestones and dolomites has been assessed using a bench scale fluidized bed reactor. The investigations were performed under the experimental conditions which are applicable to bubbling atmospheric fluidized bed condition.; There was a significant advantage of using precalcined sorbent when operating at a low temperature or under a high CO{dollar}sb2{dollar} concentration. By increasing the temperature or reducing the CO{dollar}sb2{dollar} partial pressure in the calcining gas, both precalcined and uncalcined sorbents exhibited relatively similar behavior on sulfation. Under these conditions calcination was relatively fast and therefore did not influence the overall sulfation process. When preparing precalcined sorbents, their reactivities towards SO{dollar}sb2{dollar} depended on the calcining conditions, in particular on the CO{dollar}sb2{dollar} concentration in the calcining gas and the calcining temperature. The apparent effects of calcining conditions on the subsequent sulfation behavior were directly related to the physical structure of the solid, which changes as a consequence of sintering. In general, pore size is an important property in determining the ultimate sorptive capacity of large sorbent particles. For a particular sorbent there is an optimum pore size beyond which the sulfur uptake begins to decrease.; Besides optimum calcination conditions, there was also an optimum sulfation temperature which varied depending on the physical structure of the sorbent. The low sulfur uptake at high temperatures was due to early pore blockage which restricted the transport of SO{dollar}sb2{dollar} into the particle. For uncalcined sorbents the optimum sulfation temperature depended on the calcination rate. Contrary to a number of earlier conclusions reported in the literature, higher calcium utilization was achieved using impure limestones. When distributed uniformly within the particles, impurities dilute the CaO concentration and help to delay pore blockage upon reaction with SO{dollar}sb2{dollar}. In the sulfation of uncalcined sorbents, the presence of impurities will increase the overall rate of calcination. In addition, some impurities will affect the sintering process and ultimately produce a favorable solid structure for sulfation. It can be concluded that the chemical composition of a sorbent should be considered in addition to its physical properties in selecting suitable candidates for reducing SO{dollar}sb2{dollar} emissions from fluidized bed combustion.