A dry-desulfurization process using Ca(OH)2/fly ash sorbent and a circulating fluidized bed (CFB) was developed. Its aim was to achieve high SO2 removal efficiency without humidification and production of CaSO4 as the main byproduct. The CaSO4 produced could be used to treat alkalized soil. An 83% SO2 removal rate was demonstrated, and a byproduct with a high CaSO4 content was produced through baghouse ash. These results indicated that this process could remove SO2 in flue gas with a high efficiency under dry conditions and simultaneously produce soil amendment. It was shown that NO and NO2 enhanced the SO2 removal rate markedly and that NO2 increased the amount of CaSO4 in the final product more than NO. These results confirmed that the significant effects of NO and NO2 on the SO2 removal rate were due to chain reactions that occurred under favorable conditions. The amount of baghouse ash produced increased as the reaction progressed, indicating that discharge of unreacted Ca(OH)2 from the reactor was suppressed. Hence, unreacted Ca(OH)2 had a long residence time in the CFB, resulting in a high SO2 removal rate. It was also found that 350 degrees C is the optimum reaction temperature for dry desulfurization in the range tested (320-380 degrees C).
The effects of NO x , CO 2 , and reaction temperature on the SO 2 removal process have been investigated in a fluidized bed reactor by using CaO/fly ash sorbent in order to demonstrate any available method to promote the calcium utilization rate and form valuable gypsum as a byproduct for the dry desulfurization process. It was found that NO x increased the selectivity of CaSO 4 as the final product in the sorbent during the desulfurization process. The effect of NO x on SO 2 removal rate was also investigated and results showed that the presence of NO x enhanced the SO 2 removal rate, even when mole ratio of NO/SO 2 was less than 1.0. It was deduced that the reaction mechanism involved NO x -related chain reactions. On the basis of FTIR analysis of reactions product, the chain reaction mechanism could be outlined that Ca(NO 3 ) 2 was formed from the reaction of Ca(OH) 2 with NO 2 , and then Ca(NO 3 ) 2 reacts with SO 2 to form CaSO 4 + NO. On the other hand, it was found that the CO 2 negative effect on SO 2 removal was reduced with NO x presence.
Ca(OH)2/fly ash sorbent has been studied as an effective method for SO2 removal. The effect of iron and
other species for enhancing the ability of Ca(OH)2/fly ash sorbent was investigated in this study. At first,
Fe(NO3)3 was added in the preparation of the sorbent, and TG analysis was carried out. The Ca utilization rate
over a period of 90 min was about 10% greater than that for Ca(OH)2/fly ash sorbent. However, it was found
that iron is not effective for enhancing the ability of Ca(OH)2/fly ash sorbent but that NO3
- was the most
effective factor to enhance it. The mechanism of enhancing the Ca utilization rate was also investigated, and
it was found that Ca(NO3)2 was produced in the sorbent and reacted with SO2, so that the reaction Ca(NO3)2
+ SO2 → CaSO4 + 2NO + O2 proceeded.
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