Coal-fueled Chemical Looping Combustion For CO₂ Capture Coupling With O₂ Gasification

CO₂ is a kind of greenhouse gas with the largest human emissions.Its amount produced in fossil fuel combustion accounts for about three-quarters of the total emissions from human activities.In order to mitigate the greenhouse effect and its ecological problems,it is an urgent problem to develop efficient carbon capture technologies for fossil fuels'utilization.As an inherent CO₂ separation technology during combustion process,chemical looping combustion?CLC?has the advantages of zero-energy CO₂ separation cost,energy cascade utilization and so on,which has widely drawn attentions.Comparing with gaseous fuel,solid-fueled CLC is more complex and of low reaction rate.Thereof,investigation on the CLC of solid fuel is of significance and necessity.A kind of low-cost and medium-reactivity iron ore was chosen as the oxygen carrier?OC?in this work.The auto-thermal operation of the coal fueled CLC was achieved both in a two-stage fixed bed reactor and a circulating fluidized bed?CFB?system for separated gasification CLC?SGCLC?.The three-phase gas-solid flow characteristics in the CFB SGCLC system were further investigated based on computational fluid dynamic?CFD?simulations.For operation safety of the future large-scale CLC system,the thermal states of the system were monitored and diagnosed in real time.First,the feasibility and reactivity of a low-cost iron ore as oxygen carrier for CLC were investigated in thermogravimetric analyzer.The reaction performances and mechanism of this oxygen carrier with CH₄ were studied at different temperatures.Based on the experimental data,the kinetic parameters of the reduction of OC were calculated.Results demonstrated the reduction of OC obeys a parallel reaction mechanism,which consists of two reactions with the apparent activation energies of 157.5 kJ/mol and 126.9 kJ/mol respectively.Comparing with other kinds of iron ores,this iron ore has medium and proper reactivity for CLC application.Meanwhile,multi-cycle redox reactions were conducted in thermogravimetric analyzer to test the cyclic stability of this OC.Further,the auto-thermal condition and performances of coal-fueled SGCLC were explored in a two-stage fixed bed reactor system.Adopting the coal and iron ore as solid fuel and oxygen carrier respectively,the coal gasification and combustion of production gases were separated and proceeded in different reactors.When the oxygen concentration in gasification agent varied,the bed temperature was recorded and analyzed to obtain the auto-thermal operation condition.Under the auto-thermal condition,the temperatures of the gasifier?GR?and reduction reactor?RR?were changed to investigate the effects of the reaction temperature on the CLC performances.Under auto-thermal condition,the gas combustion efficiency increased from 97.91%to 99.96%when the temperature of RR was elevated from 850°C to 1050°C.Meanwhile,the rise of GR temperature from 850°C to1050°C could bring the benefit of a higher coal conversion of 96.64%from 61.02%.In 15cyclic SGCLC experiment at 950°C,the reactivity of the oxygen carrier could maintain in a high level,demonstrating a satisfying cyclic stability.A bubbling bed,a fast fluidizing bed and a countercurrent moving bed were employed as GR,RR and air reactor?AR?separately in this work.Then,a CFB system with two gas-solid separators and OC refeed valves for solid-fuelled SGCLC was established.In this reactor system,the coarse sand particles and medium OC were used as bed materials to investigate the operation performances and pressure distribution under cold condition when the fluidizing numbers in RR and AR?N_r and N_a?were changed.The SGCLC system exhibited a good operation stability in a relatively long test.In the adjustment ranges of the experiment,the AR and fuel side showed relatively slight dependence.And the pressure distributions were obtained.In hot SGCLC test,the Shenhua bituminous coal was used as solid fuel and the steam was used as gasification agent.When the temperatures of AR and RR were set as900°C and 850°C,the CO₂ yield at the outlet of RR could reach to more than 92.7%while the contents of CH₄ and CO were low.Furthermore,the gasification agent coupled with some O₂ was employed to realize the auto-thermal operation of this SGCLC CFB system.Under different operation conditions with variable reactor temperatures,coal feeding rates,steam mass flows,oxygen flows and loosen gas kinds,the auto-thermal condition,reaction performance and carbon capture property of this SGCLC system were deeply investigated.When the ratio of oxygen to coal R_oc was no lower than 27.6%,the CFB system could run auto-thermally with high gas combustion efficiency and carbon capture efficiency.While the auto-thermal operation temperature increased from 830°C to 930°C,the gas combustion efficiency,coal conversion and carbon capture efficiency increased from 95.30%,87.36%and 97.05%to 97.67%,91.87%and 97.77separately.The accelerating of the coal feeding rate could improve the operation load,but it led to the fuel conversions in system.On the contrary,the raise of the steam amount into the reactor could enhance the coal conversion in GR,however,the gas combustion efficiency and carbon capture efficiency showed slight decreasing trend.In order to offer reliable information of the gas-solid characteristics for system design and operation optimization,the CFD method was used to simulate the hydrodynamics and the distribution properties of the gas,sand,and OC phases before the construction of CFB system.The gas-solid hydrodynamic model in SGCLC CFB system was established on the Eulerian-Eulerian model.Based on simulation results,the CFB system was constructed.Then,the feasibility and reliability of this model were validated by the experimental data.When the fluidizing number N_r increased 19.40 to 26.80,the bed expansion in GR became larger and more obvious and the sand volume fraction at the bottom zone dropped from 0.31 to 0.21,and there was no sand flowing into RR.Meanwhile,the gas velocity decreased along the axial direction of GR.Similarly,the OC fraction in RR decreased with the increase of N_r,forming dense region in lower part and dilute region in upper part.With the development of the gas-solid flow,the gas-solid slip velocity in RR became stable in upper region.Using the simulation results in three tests,the local gas-solid slip velocity was fitted as a function of the N_r and height Z.The results in other two conditions were employed to validate the fitting accuracy.Results demonstrated that the fitted function had high accuracy and it was suitable for the prediction of the gas-solid flow under variable conditions.Finally,for the operation safety,the thermal states of large-scale CLC system were monitored during operation using simulation method.Based on the inverse analysis method coupling the fuzzy inference and Kalman filter,the measured temperatures on the outer surface of the CLC reactor were used to monitor the internal thermal condition inversely.Comparing the monitoring results using constructed methods of FI-KFW and FI-AKF,it was found that the FI-KFW was feasible for the accurate and real-time monitoring.Under an abnormal condition,the effects of the number of measurement points and measurement noises on the monitoring results of internal heat flux and reactor temperature were investigated.When the standard deviations of measurement noise were 0.10,0.50,1.00 and 1.50,the relative root mean square errors of monitored heat flux were 1.36%,1.89%,3.42%and 7.55%and the temperature monitoring errors were in the range of?-7.5°C,7.5°C?,which demonstrated that the FI-KFW has strong robustness and high performance for the safe operation of the CLC system.