Development of the Next Generation CaO-Sorbent for Sulfur- and Nitrogen-Compound Removal

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Limestone and lime are very important materials as sorbents for S-and N-compound removal. However their porosities are usually low, and their capacity are low. To solute this problem, the clarifications of the reaction mechanisms and the clarification of the effect of pore size distribution on the reactivity are very important in addition to the development of a preparation method of a porous lime which porosityis suitable for the removal. We present new preparation methods of macro-porous CaO using acetic acid, water and a mixture of acetic acid and water. By using these methods, we could prepare the samples, which pore size distributions were different each other, from a natural lime. The effects of pore size distribution of the CaO- and CaCO_3- sorbent on the removal of SO_x using the serial samples were studied. It was concluded that if we can prepared a lime that has an adequate number of macropore, mesopore and micropore, and good serial connection of these different size pore to … More ensure the permeability of reactant(SO_2 and O_2) and prevent pore plugging during sulfation, the reactivity and utility of the lime will be drastically improved. From the results of study on the CaCO_3-sorbent, it was also suggested that if the macropores in the CaCO_3 sample are developed enough to SO_2 removal, the reactivity of the CaCO_3 sample is same as that of CaOWhen CaO is used for H_2S removal, CaS formed from the CaO-absorbent has to be converted to CaSO_4 before disposal because H_2S is released from the reaction between CaS and water. To find out a method for the perfect conversion of CaS to CaSO_4 and CaO, the basic characters of the oxidative decomposition of CaS were examined using reagent grade CaS in the presence of H_2O because hot combustion gas contained moisture is usually used for the decomposition of CaS. It was found that the presence of H_2O affected drastically to the conversion of CaS to CaO and SO_2 : H_2O directly reacted with CaS and formed H_2 reacted immediately with O_2.The mechanism of the simultaneous removal of SOx and NOx by slaked lime was clarified : A reaction scheme of this reaction was proposed : SO_2 reacted with H_2O and formed SO_3^<2->, then NO and NO_2 might participate the reaction in one step and the other hand, accelerated the conversion of SO_3^<2-> to SO_4^<2->. The main part of NO_2 was converted to nitrate salts in the presence of H_2O. SO_3^<2-> formed from the adsorption is supposed to be significant for the removal of NO, and the enhancement of NO to the SO_2 was ascribed to its catalytic effect in the oxidation of SO_3^<2-> to SO_4^<2->. Less