| As a carbon-neutral energy utilization technology,chemical looping combustion（CLC）has the potential to capture CO2 in the combustion process by using the circulating oxygen carrier（OC）to transfer lattice oxygen.The CLC process spatially divides conventional combustion into several sub-steps,providing an N2-free atmosphere for fuel conversion and CO2 purification.This novel combustion technique has been considered as one of the most promising energy-effective and cost-effective CO2 capture schemes.However,the slow char gasification rate and the complex reaction mechanisms complicate the structure of CLC reactor as coupling carbon stripper and extra circulation paths.It is inevitable to raise challenges for stable fluidization operation and efficient fuel conversion.Therefore,it is necessary to enhance the CLC performance by optimizing operational strategies in a CLC reactor with a simplified circulation mechanism design criterion.In the present work,the characteristics of hematite OC were investigated firstly in a fluidized bed thermogravimetric analysis（FB-TGA）with respect to kinetic parameters and reaction mechanisms.Secondly,the influence of internal on fluid dynamics and bubble behaviors were explored in a micro interconnected fluidized bed（MIFB）,contributing to the design for the 3 k Wth CLC plant.Then,a simulation model of fuel reactor（FR）with coupling internals was developed to analyze the intensified gas-solid reaction mechanism of internal and offer the optimal FR structure with coupling internals.Finally,a 3 k Wth multi-staged interconnected fluidized bed was designed to evaluate the potential of operational strategies on improving the CLC performance based on the gas evolution law and carbon migration path.FB-TGA can realize the measurement of sample weight variation under a fluidizing state,which eliminates mass and heat transfer limitations in the conventional TGA with providing a novel method to characterize the gas-solid process.The FB-TGA results showed that the lattice oxygen of hematite had high reactivity during the reduction stage of Fe2O3-Fe3O4.Over 6 Umf fluidization velocity was required to exclude the influence of external diffusion as reducing 1 g hematite by CH4and CO.However,only the co-existence of kinetic control and diffusion control was effectuated as reducing hematite by hydrogen within the applicable operation conditions of FB-TGA.The shrinking nuclear model was suitable for describing the reaction process of hematite OC with CO,CH4,and O2.The reaction order and activation energy of CO,CH4,and O2 were 0.9 and 58.89k J/mol,0.5 and 122.61 k J/mol,and 1 and 25.75 k J/mol,respectively.The reduction process of hematite with hydrogen was consistent with the mechanism of the zero-order reaction model.The reaction order and activation energy of H2 were 1 and 31.97 k J/mol,respectively.Only 350 g bed inventory was required in MIFB with hematite serving as OC,which could significantly reduce the time and cost of synthetic OC development.Flexible and stable circulation fluidization was realized in MIFB through the coordinated controlling of bed material distribution and OC circulation rate.A slight delay was found in the self-stabilizing process of MIFB,resulted from the intermittent particles transportation of the loop-seal.The stability of particle transportation was enhanced by increasing the downcomer stacking height and the aeration gas flow.The fluidization in the upper chamber was reconstructed under the effect of baffle internals,which also increased the uniformity of gas-solid distribution,and strengthened the gas diffusion conditions in the dense phase.Besides,the internal could inhibit the slugging duration and evolve the slugging in the lower chamber into the seepage flow.Although the internal hindered the particle up-flow rate,it considerably prolonged the particle residence time and reduced the OC demand of the CLC reactor.For the depth analysis of the intensified process of gas-solid reaction,a 1.5-dimensional FR simulation model was established by combining the K-L bubbling bed model with EMMS theory.The bubble behavior played a dominant role in the gas conversion efficiency.Large bubbles weakened the reaction intensity in the emulsion phase and reduced the mass transfer rate between the bubble and emulsion phases.As a result,the gas-solid reaction rate in the upper zone of the dense phase was dozens of times lower than that in the bottom zone.Installing internals could break bubbles and transform the inefficient conversion zone with large-scale bubbles into the high-efficiency reaction zone.However,the decrease of bed inventory and OC circulation rate were inevitable after installing internals.The gas conversion efficiency reached the highest as coupling 3internals,91%higher than that obtained without internal.In addition,the gas conversion was improved to a certain extent by increasing the height of the dilute phase.The optimal height ratio between dense phase and dilute phase was 2.93.The installation height of internals could affect the OC circulation rate and axial distribution of bubble size.The highest conversion efficiency was profited as installing the internal at 4/5 of the FR height.A 3 kWth multi-staged CLC plant with a simplified circulation mechanism was designed by combining the MIFB structure and the numerical analysis.The operational strategy was optimized in this multi-staged CLC plant to improve CLC performance.The efficient gas conversion was obtained with the result of 3.6%oxygen demand,attributing to the enhanced gas-solid contact by coupling internals in FR.Based on the analysis of particles and furnace gas sampled from FR chambers,more than 80%of combustible gases were entirely converted in the lower three FR chambers.The further reduction of oxygen demand should focus on intensifying CH4 conversion and increasing OC circulation rate.However,the particle residence time in FR was insufficient due to the lack of an internal circulation path and carbon stripper.The residence time was less than 100s,resulting in 65.6%CO2 capture efficiency in the preliminary test.By analyzing the carbon migration path,coal pyrolysis was completed in the lower two FR chambers,and char gasification mainly occurred in the upper three chambers.Only 8%char was converted into a gaseous form in one chamber with 250mm height.Although blending Ni-based OC showed no apparent effect on increasing CO2 capture efficiency,but significantly promoted CH4 conversion by improving lattice oxygen activity.Increasing steam concentration could promote char gasification and enhance steam reforming reaction within a specific range.After introducing secondary fluidization gas and optimizing the staged gas flow,the particle residence time had a sharp increase to 280 s.As a result,the excellent CLC performance was available under 870°C with 80%CO2 capture efficiency,96%carbon conversion efficiency,and 3.7%oxygen demand.Around 300-342 h hematite OC lifetime was obtained in this 3 k Wth plant,in which medium-sized particles with 0.1-0.25 mm size were the central part of the abrasion particles.The fluidization behaviors in FR had a negligible effect on OC lifetime,but the gas-solid separation in the cyclone could significantly exacerbate OC attrition.There was a conflicting demand for the cyclone operation.Increasing the inlet gas velocity of the cyclone could enhance carbon capture but also seriously increase the OC abrasion. |
