| The Calcium Looping (CaL), i.e.,carbonation/calcination cycles of the Ca-based sorbent, which is based on the reversible reaction between CaO and CO2 has attracted extensive attention due to the low cost of the sorbent precursor, high CO2 adsorption capacity, low energy penalty, and opportunity for integration with cement manufacture. This technology appears promising not only for hydrogen production by fuel gasification and reforming(e.g., Zero Emission Coal, ZEC), but also for post-combustion CO2 capture from flue gas in power plant. Currently, millions of the carbide slag are produced in China. It is a difficult task how to carry on the large-scale treatment and disposal of this kind of Ca-based industrial waste. Early studies have shown that carbide slag can be used as CO2 sorbent in CaL process, which avoids limestone mining’s destruction on the surrounding ecological environment. Sulfur in coal presents in the form of H2S and SO2 during the process of gasification and combustion of coal, respectively, which endangers CO2 capture during CaL process, resulting in a significant drop in CO2 capture capacity. More seriously, H2S present in H2 leads to catalyst inactivation and SOFC anodic poisoning. Therefore, it is necessary to develop efficient removal technologies of H2S present in the fuel-gasification hydrogen production process and SO2 present in the process of post-combustion CO2 capture based on CaL to reduce the adverse effects of sulfur.To solve the above-mentioned problem, a novel CO2/H2S capture technology based on the coupled system of CaL and hydrogen production and a high-efficiency CO2/SO2 capture technology based on the coupled system of CaL and coal combustion were proposed, which contributes to the novel technology combining resource utilization of carbide slag in large scale and low-cost, effective and simultaneous capture of carbon/sulfur. Main work carried out as follows:In this work, the cycled Ca-based sorbent discharged from the CaL/reforming process in ZEC was proposed to remove H2S in the raw gas from coal gasification, which contributes to the novel, low-cost and step-by-step removal technology of CO2/H2S during hydrogen production by coal.The cycled carbide slag and limestone which had experienced multiple carbonation/calcination cycles for CO2 capture in a dual fixed-bed reactor (DFBR) at atmospheric pressure was sent to a fixed-bed sulfidation reactor (FBSR) at atmospheric pressure for H2S adsorption. The effects of sulfidation temperature, H2S concentration, particle size and cycle number on the sulfidation performances of the cycled carbide slag and limestone were investigated. The results show that the cycled carbide slag and limestone experienced long-term carbonation/calcination cycles for CO2 capture still have high H2S adsorption capacity. After sulfidation for 120 min, the sulfidation conversions of the carbide slag and the limestone after 100 cycles are 0.73 and 0.74 mol/mol, respectively. The optimum sulfidation temperature is 900℃. In the previous 50 cycles, the H2S adsorption capacity of the carbide slag is lower than that of the limestone. Nevertheless, the situation is reverse after 50 cycles. The H2S adsorption capacity of the carbide slag is higher than that of the limestone as the cycle number increase sequentially. The difference of H2S adsorption capacity between the cycled carbide slag and limestone is closely related with the change in the pore structure of carbide slag during multiple carbonation/calcination cycles.In this study, aluminate-modified (the mass ratio of CaO/Al2O3 is 90:10) and magnesia-modified (the mass ratio of CaO/MgO is 80:20) carbide slags with high CO2 capture reactivity were synthesized by combustion synthesis method using carbide slag as raw material. Using modified carbide slag as composite CO2 sorbent in ZEC was proposed. The cycled modified carbide slag from CaL/hydrogen production process was used to remove H2S. The highly effective CO2/H2S capture was achieved and the efficience of hydrogen production was improved. The long-term CO2 capture capacities of the 2 kinds of modified carbide slags were discussed on the basis of the previous research. And the H2S adsorption performance of the cycled modified carbide slags from CaL for CO2 capture was investigated in the FBSR. The impact mechanism about the inert carrier in the modified carbide slag was revealed synchronously. The results show that the 2 kinds of modified carbide slags remain relatively high CO2 capture capacities during the multiple carbonation/calcination cycles and the cyclic stabilities are better than that of limestone. The CO2 capture capacities of aluminate-modified and magnesia-modified carbide slag are 0.35 and 0.30 g/g, which are greatly higher than that of carbide slag (0.19 g/g). It is found that both of the cycled aluminate-modified and magnesia-modified carbide slags from long-term carbonation/calcination cycles can achieve high HbS removal capacity, and their optimum sulfidation temperatures are 900-950℃ and 850-900℃, respectively. The sulfidation reaction rates of the cycled aluminate-modified and magnesia-modified carbide slags are higher obviously than that of the cycled carbide slag, especially the cycled aluminate-modified carbide slag. In the quick stage, the cycled aluminate-modified carbide slag exhibits the best H2S adsorption capacity among the 3 kinds of sorbents. The cycled magnesia-modified carbide slag can get a more pronounced better H2S adsorption capacity than the cycled carbide slag when calcined under high concentration of CO2. It has been proved that Ca3Al2O6 and MgO as inert carrier in the aluminate-modified and magnesia-modified carbide slags can markedly enhance H2S adsorption. The results show that aluminate-modified and magnesia-modified carbide slags experienced long-term carbonation/calcination cycles remain high H2S adsorption capacities The inert carrier, Ca3Al2O6, and a multilayer mesh-shaped pore structure were formed during combustion synthesis process of the aluminate-modified carbide slag. A large amount of clusters of CaO-MgO grains exist in magnesia-modified carbide slag, and the pore structure is well-developed. Inert materials, Ca3Al2O6 and MgO, work as mechanism support. Pores in the range of 20-150 nm are present in the 2 kinds of modified carbide slags, which is beneficial for the diffusion of H2S. Meanwhile, the inert carrier increases sintering resistance of CaO during cycles, which is related to the favourable cyclic stability and H2S adsorption performance of the modified carbide slag.Simultaneous CO2/SO2 capture performance of CaO derived from carbide slag in multiple adsorption/desorption cycles was investigated in a dual fixed-bed reactor. The results show that SO2 presence appreciably impedes the cyclic CO2 capture capacity of the carbide slag. A longer adsorption duration and high SO2 concentration cause a sharper drop in the carbonation conversion due to the thicker product layer of CaSO4. The compact CaSO4 product layer covers the surface of CaO derived from the carbide slag during the multiple cycles, which results in a sharp drop in the surface area and pore volume with the cycle number and impedes CO2 capture. When the concentration of SO2 present in the reaction gas is 0.2%, carbide slag shows similar carbonation conversion with limestone, but better sulfidation conversion, higher total Ca utilization. Thus, carbide slag is a better CO2/SO2 sorbent under high temperature. SO2 removal by cycled carbide slag from the multiple carbonation/calcination cycles was proposed. It is found that the cycled carbide slag experienced cycles for CO2 capture still has high H2S removal capacity. The sulfation conversion of carbide slag decreases with cycle number and the optimum sulfation temperature is 950℃.SO2 present in the carbonation atmosphere appreciably impedes the cyclic CO2 adsorption capacities of aluminate-modified and magnesia-modified carbide slags. A high-efficiency CO2/SO2 capture technology based on the coupled system of CaL and coal combustion was proposed, which to the greatest degree possible can avoid the adverse effect of SO2 from flue gas on CO2 capture capacity for modified carbide slag. The SO2 removal capacity using modified carbide slag from multiple carbonation/calcination cycles as SO2 sorbent was investigated on TGA, and the effects and mechanisms of calcination under high concentration of CO2 and steam on CO2 capture capacity of modified carbide slag and sulfidation performance of the cycled modified carbide slag were illustrated. The results show that the sulfation reaction retes of the aluminate-modified and magnesia-modified carbide slag are both higher than that of the carbide slag. The modified carbide slags after multiple carbonation/calcination cycles achieve better SO2 removal capacity than carbide slag and also have a wider optimum range of sulfidation temperature. After sulfidation for 7200 s at 900℃, the sulfidation conversions of alnuminate-modified and magnesia-modified carbide slags after 100 cycles are 0.68 and 0.64 g/g, respectively, which is 1.8 and 1.7 times of carbide slag, respectively. The existence of Ca3Al2O6 and MgO as inert supports enhances the sintering resistance, which decreases the adverse effects of calcination under high concentration of stream and CO2 on cyclic carbonation and sulfidation performance of modified carbide slag. In the condition of calcination under high concentration of steam and CO2, modified carbide slag shows better carbonation and sulfidation performance than carbide slag. High concentration of steam is a relatively moderate atmosphere than high concentration of CO2, and the carbonation performance and sulfidation performance after carbonation/calcination cycles of modified carbide slag are much better than those under high concentration of CO2. The utilization of calcination under O2/steam is a promising way to provide necessary heat for calciantion process.In summary, the use of carbide slag as Ca-based industrial waste and the corresponding modified products to capture carbon/sulfur in the process of hydrogen production by fuel and post-combustion CO2 capture is expected to form sustainable resource development integrated with control of atmospheric pollutants and the development of clean and efficient energy technology route, which has a good application prospect. |
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