Study Of Chemical-Looping Combustion Based On Cu-based Oxygen Carrier

Technologies of carbon capture&storage (CCS) are crucial in solving the problem of CO2emission in fossil energy fields. The main obstacle to the application of CCS is the substantial energy consumption involved in the capture process of CO2. Chemical-looping combustion (CLC) of fossil fuels has not only an inherent advantage of nearly zero energy consumption in capturing CO2, but also a high thermal efficiency and nearly zero emission of pollutants. All of these advantages show that CLC has significant potential. However, current CLC systems, which mainly focus on the applications of thermal power generation, use interconnected fluidised beds working at above900℃. Thus, several problems such as carbon deposition, sintering and attrition of oxygen carriers (OC) have resulted in short OC cycle life and poor system sealing. In this paper, a new CLC process based on Cu-based OC has been proposed, and the performance and cycle life of Cu-based OC were investigated.A key point of this paper is based on the application of Cu-based OC, whose reduction reaction and oxidation reaction are both exothermic. With Cu-based OC, the application of a fixed bed reactor and continuous heat supply through reduction and oxidation become possible. The main disadvantage of Cu-based OC is sintering because of the low melting point of metallic Cu. The reduction characteristics of Cu-based OC with H2, CO and CH4were studied using a fixed bed reactor, TPR and TGA. Results showed that temperatures for the complete reduction of Cu-based OC with H2and CO are300℃and225℃, respectively, while the corresponding temperature with CH4is650℃. The carbon deposition from CH4occurred at over550℃. CO chemisorption experiments were also conducted on the OC, and it was indicated that Cu-based OC sinter seriously at700℃. In order to lower the required reduction temperature of OC, a new CLC process with adding a CH4steam or CO2reforming step has been presented. It has the potential to replace the conventional gas-fired middle-and low-pressure steam and hot water boilers. Along with the realization of clean and high effiecient heating, CO2can be captured with little energy consumption, and liquid CO2with high purity can be by-produced.Based on the new CLC process, a variety of OCs, which were prepared with Cu as active component and Al2O3or SiO2as inert support by impregnation and mechanical mixing, were evaluated. Results showed that OC prepared with impregnation exhibits a good reactivity. However, its oxygen content ratio (Ro) was low due to the limitation of saturation loading of active component. In contrast, OC prepared with mechanical mixing, using Cu particle with a size of25～38μm and a suitable amount of inert support of Al2O3or SiO2, whose size is significantly smaller than that of OC, had both higher Ro and good reactivity. Ro of Cu-Ni/Al2O3-M and Cu/SiO2-M were0.177and0.167, respectively. Meanwhile, Cu-Ni/Al2O3-M and Cu/SiO2-M reacted with H2and O2completely at a GHSV (H2) of200h-1in reduction and a GHSV (O2) of95h-1in oxidation at500℃. The reduction utilization ratio and oxidation utilization ratio of OC achieved above95%and91%when H2concentration and O2concentration reached1%.The cycle performance of Cu-Ni/Al2O3-M and Cu/SiO2-M were also investigated. The results showed that in the oxidation step, the oxidation utilization ratio of Cu-Ni/Al2O3-M decreased obviously from91.0%in the first cycle to27.7%in120cycles. In contrast, the oxidation utilization ratio of Cu/SiO2-M maintained above50%in360cycles and decreased to30.6%in420cycles. In the reduction step, both of reduction utilization ratios of Cu-Ni/Al2O3-M and Cu/SiO2-M decreased mildly, and it maintained86.8%in120cycles for Cu-Ni/Al2O3-M and80%in420cycles for Cu/SiO2-M. According to the characterization of the degraded OC, the main reason for the performance degradation of OC was speculated to be the powdering of OC particles and local aggregation of the powdered fine particles, which worsened the distribution and diffusion of the reactive gases in the packed bed. A regeneration method of the degraded OC based on re-granulation has been proposed and applied, and its mechanism was illustrated. Actually, through re-granulating the degraded OC, the performance of OC was recovered to a level close to the initial performance.This paper has also presented a new thermochemical method for CO2pressurization, which can be applied in the filed of central heating with natural gas. The method is based on the chemical absorption process at a cost of lowering the quality of CLC reaction heat, and its purpose is to reduce the power consumption for CO2compression without paying heat consumption essentially. The mechanism of CO2thermochemical pressurization is that the reduction product gas, which is almost CO2, is absorbed in absorbing tower at atmospheric pressure and a temperature higher than60℃, which is a common temperature of heating return water, and CO2-enriched absorbing solution is regenerated in the pressurized stripping tower at higher temperature with a part of CLC reaction heat introduced to the reboiler of stripping tower. As a result, the pressurized CO2is obtained at the outlet of condenser of stripping tower. As the both heat of absorbing reaction and vapor condensation can be recovered with the heating return water, there is little heat comsuption in the CO2pressurization.Combined with the thermochemical method of CO2pressurization, a10kW class scale-up test stand was devoleped. Here, Ni-based catalyst was used as reforming catalyst, and Cu-Ni/Al2O3-M or Cu/SiO2-M was used as OC. The effect of temperature and GHSV(CH4) on methane reforming with CO2, and reduction and oxidation in CLC were investigased. Results showed that the conversion of CH4maintained above85%and H2and CO converted from the methane reforming reacted with OC completely. Meanwhile, the reduction utilization ratio and oxidation utilization ratio of OC achieved80.7%and74.1%when H2concentration and O2concentration reached1%. By stripping the absorbing solution, which absorbed CO2at70℃from the reduction flue gas in the reduction step, with a stripping temperature of137℃in the oxidation step, the pressure of stripping chamber reached2.5MPa, indicating that the thermochemical method of CO2compression was effective.