| Calcium looping,as one of the most feasible large-scale CO2 capture technologies,realizes CO2 capture from the coal-fired power plants via the calcination/carbonation of CaO.There are many kinds of pollutants in coal-fired power plants,which are removed in different reactors.In this paper,a novel strategy aiming at combining de-NOx with the carbonation of calcium looping for CO2 capture is proposed.The simultaneous NO/CO2 removal performance of biochar/CaO in the carbonation step of the calcium looping was investigated.The effects of CaO and its carbonation on NO reduction were elucidated.To improve simultaneous NO/CO2 removal performance of biochar/CaO,biochar was modified by Cu and CaO was doped by Mn.The effect mechanisms of Cu and Mn were revealed by density functional theory calculations and microstructure characteristic analyses.A series of studies were carried out using coconut shell char as a typical representative of biochar in this paper.The effects of reaction conditions,mass ratio of char to CaO and number of cycles on simultaneous NO/CO2 removal performance of biochar/CaO were investigated.It is found that CaO absorbs CO2 and catalyzes NO reduction by CO formed by biochar oxidation,leading to that NO removal efficiency of biochar/CaO is 24%higher than that of biochar alone.High reaction temperature,O2 concentration and biochar addition amount are beneficial to NO reduction by biochar and the generation of CO,however,they also increase CO2 release generated by biochar oxidation,which reluts in the decrease in CO2 capture efficiency.Therefore,the appropriate temperature,O2 concentration and mass ratio of biochar to CaO are 650?,4%and 3:16,respectively.At the first cycle,NO removal efficiency and CO2 capture efficiency of biochar/CaO are up to 97%and 80%,respectively.As the number of cycles increases to 9,CO2 capture efficiency remains at 80%,but NO removal efficiency decreases to 84%due to the sintering of CaO.To obtain a high NO removal efficiency of biochar with a low addition amount in the carbonation step of the calcium looping,Cu-modified biochar was prepared by the wet impregnation method.The effects of preparation process,mass ratio of Cu-modified biochar to CaO,O2 and calcium looping cycles on simultaneous NO/CO2 removal by Cu-modified biochar/CaO were studied.The mechanisms of Cu-modified biochar on NO reduction and CO oxidation were analyzed by microstructure characteristic analyses and density functional theory calculations.It is found that Cu significantly promotes NO reduction by CO and char.Under the catalysis of CaO and Cu on NO reduction by CO,the addition amount of biochar decreases by 2/3 while NO removal efficiency increases by 15%.Cu catalyzes CO oxidation,and the generated CO2 leads to the decrease in CO2 capture efficiency.Therefore,the suitable molar ratio of Cu/C is 0.77:100,obtained by preparing Cumodified biochar under 3wt%CuSO4 solution and pretreation duration of 60 min.Density functional theory calculations show that the energy barrier of NO-C,NO-CO and CO-O2 reactions decrease by 37%;60%and 110%,respectively,which indicates Cu promotes the reactions above.When the mass ratio of Cu-modified biochar to CaO,temperature and O2 concentration are 1:16,650? and 3%,respectively,in the first 11 cycles,CO2 capture efficiency of Cu-modified biochar/CaO remains at 80%and NO removal efficiency decreases from 94%to 83%due to the sintering of CaO.NO removal efficiency of biochar or Cu-modified biochar/CaO gradually decreased with the number of calcium looping cycles.The catalysis of CaO on NO reduction by CO in the carbonation step was investigated to explore the reasons.It is found that CaO has a strong catalytic effect on NO reduction by CO as CO concentration is high(3%).At the initial stage of the first cycle,NO removal efficiency of CO is up to 95%,However,NO removal efficiency gradually decreases to 66%during the chemical reaction-controlled stage of CaO carbonation.The catalytic effect of CaO on NO reduction by CO is weakened due to the CaCO3 product layer.Carbonation of CaO inhibits its catalysis on NO reduction by CO.At the 20th cycle,the initial NO removal efficiency decreases to 78%.The aggravated sintering of CaO with the number of cycles leads to the decrease in porosity of sorbent,hindering the diffusion of CO and NO in CaO sorbent.The catalysis rate significantly decreases but the carbonation rate in the the chemical reaction-controlled stage is barely affected by the sintering,which causes further inhibition of catalysis of CaO on NO reduction by CO.This is the reason why the NO removal efficiency of simultaneous NO/CO2 removal process decreases with the number of cycles.To reach high NO removal efficiency by low concentration CO,Mn was doped into CaO to mitigate the adverse effects of carbonation and sintering on the catalysis on NO reduction by CO.Mn-doped CaO prepared by combustion method and wet impregnation method have higher CO2 capture capacities than the original CaO.Under the severe calcination conditions,as the steam concentration,molar ratio of Mn/Ca and carbonation duration are 15%,0.75:100 and 5 min,the carbonation conversion at the 10th cycle of Mn-doped CaO is 2.4 times as high as that of CaO.Mn enhances the carbonation rate via enhancing electron transportation,and it also enhances the sintering resistance by limiting the growth of CaO grains.In order to be suitable for the utilization in bubbling fluidized bed,Mn modified limestone(denoted by Mn-CaO)was prepared by impregnation method.The catalytic NO removal performance of low concentration CO under the catalysis of Mn-CaO in the carbonation step of calcium looping was studied in the bubbling fluidized reactor.Under mild calcination conditions,when the molar ratio of Mn/Ca,temperature and CO concentration are 3.5:100,650? and 0.4%,respectively,compared with the original CaO,CO2 capture efficiency of Mn-CaO increases by 21%and NO removal efficiency significantly increases from 0%to 99%.NO removal efficiency maintains stable(99%)during the carbonation chemical reaction-controlled stage in the 10 cycles.Under the severe calcination conditions,CO2 capture efficiency of 85%and NO removal efficiency of 99%can still be achieved in 10 cycles.The density functional theory calculation shows that Mn enhances the adsorptions of CO,NO and CO2 by 39%,61%and 96%,respectively.Mn supplies active sites for catalyzing NO reduction by CO,breaking the competitive stalemate between the catalysis and the carbonation of CaO due to the limited O-tops.Thus,the catalysis on NO reduction by CO of Mn-CaO remains stable and efficient as the carbonation of CaO proceeds.Meanwhile,Mn is a bi-function additive for catalyzing NO reduction by CO and improving CO2 capture by CaO. |
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