Reaction Characteristics Of Coffee Grounds & Phosphogypsum Solid Waste Processing By Chemical Looping Technology

In the backgrounds of energy shortage and environmental pollution, various countries strive to develop clean energy utilization technology. Chemical looping combustion（CLC） is a kind of novel and efficient technology by using oxygen carriers transport lattice oxygen to fuels which air and fuel have no direct contact, not only can realize the capture of CO₂ but can realize the gradient utilization of energy. Chemical looping gasification（CLG） is a kind of technology which controls the ratio of fuel, oxygen carriers and gasification agent to obtain the gasification products. Coffee grounds is one of biomass wastes generated in the procession of coffee drinks production, phosphogypsum is a by-product of wet phosphoric acid production process. This paper illustrates the resourceful treatment of coffee grounds and phosphogypsum by chemical looping technology, achieving treatment of these two kinds of solid waste in the way of coffee grounds chemical looping gasification, using phosphogypsum as Ca-based oxygen carriers in coal chemical looping combustion, respectively. The main findings of this thesis are summarized as follows:（1） Using sol-gel method, the Fe4ATP6K1 oxygen carriers modified by KNO₃ were prepared with Fe₂O₃ as an active component, natural attapugite（ATP） as an inert support. The effects of reaction temperature, steam flow as well as O/C molar ratio on coffee grounds chemical looping gasification were investigated in a fluidized bed using steam as gasification agent. The results indicated that the Fe4ATP6K1 oxygen carrier could facilitate the conversion of coffee grounds. Compared with SiO₂ as bed material, the carbon conversion in CLG increased from 71.38% to 86.25%. The optimized conditions were presented as follows: the reaction temperature was 900°C, the water flow was 0.23 g·min-1, the O/C molar ratio was 1. Under these conditions, it was found that the average concentration of H₂ reached a maximum value 52.75%, with the syngas production of 1.30 m3·kg-1 and H₂ production of 83.79 g·kg-1, respectively. 20 redox tests demonstrated although there exists a small loss of K in the process of reaction, Fe4ATP6K1 oxygen carrier has a good cyclic stability, the carbon conversion and cold gas efficiency both remained above 75%, while the average gas concentration of gases were nearly stable.（2） Through mechanical-mixing method, PG/Ben oxygen carrier was prepared with phosphogypsum（PG） as an active component and bentonite（Ben） as an inert support. In order to improve its reactivity, PG/Ben oxygen carrier decorated with NiO、CuO、Fe₂O₃ were also prepared by mechanical mixing-impregnation method. The physical and chemical properties of these oxygen carriers were characterized by XRD, SEM and N₂ absorption-desorption measurements. The reactivity of these OCs in coal chemical looping combustion was performed in a fluidized bed reactor. The results indicated that the reactivity of the decorated OCs was improved as its surface area increased and Ni-PG/Ben oxygen carrier exhibits better performance than other oxygen carriers. Moreover, the reactivity of Ni-PG/Ben oxygen carrier was enhanced by the increasing of impregnation increment. Redox tests show that the concentration of CO₂ was above 80% in 12 cycles and the carbon conversion rate decreased from 0.114 min-1 in the first redox test to 0.077 min-1 in the fifteenth redox tests.（3） The reaction characteristic of phosphogypsum with qujing lignite was investigated by TG-FTIR. The Effect of different atmosphere and Ca/C molar ratio to gas/solid phase products were also analyzed. The results indicated that the addition of lignite could obviously lower the decomposition temperature of PG under CO₂ atmosphere than N₂ atmosphere. Moreover, the decomposition process of phosphogypsum could be altered by adding different quantities of lignite. According to the infrared spectrum under the DTG peak time, the intensity of infrared characteristic peak of SO₂ decreased with the increasing of Ca/C molar ratio. The activation energy of reduction of phosphogypsum to Ca S by lignite was calculated using Kissinger equation and Flynn-Wall-Ozawa equation. Also the dynamic equation of phosphogypsum reduction to Ca S was achieved by the Coats-Redfem equation.