太阳能热催化二氧化碳转化机理研究进展

太阳能热催化还原技术是二氧化碳资源化的重要技术路线,不同催化剂可实现CO_(2)向CO、CH_(4)、HCOOH、MeOH和合成气的转化。转化过程中光激发热电子及处于一定热环境中的热力系统的状态同时受到光热转化效率、材料选择性以及系统结构和工作参数的影响,直接决定了系统的转化效率。本文分析了热驱动二氧化碳催化转化相关的科学技术问题、挑战和需求,系统总结了二氧化碳热化学转化过程的反应热力学和动力学机制,以及新型反应器研发方面的重大进展。指出了考虑入射能量光谱特性与光热催化剂匹配能够实现低能量输入条件下的高转化率和高选择性,通过二氧化碳加氢生产C_(1+)和C_(2+)燃料能够有效扩大太阳能光热耦合利用规模并使之与化学工业接轨,具有重要的研究价值和广阔的应用前景。

CH_(4)/H_(2)O/H_(2)辅助热催化CO_(2)转化机制

随着热化学技术及相关反应机制认知的不断进步,二氧化碳回收转化为高附加值燃料或其他化工产品的清洁能源技术正逐步走向成熟。同时,因社会工业进步及人类生产活动急剧增加了大气中的二氧化碳浓度,且已远超地球正常发展的浓度阈值,这使得基于二氧化碳转化的零碳高效可再生清洁能源技术成为世界各国为应对全球能源与环境问题亟待实现的关键技术。鉴于二氧化碳热催化转化的基本原理、关键材料和反应系统的相关研究对于推动该技术的工业化进程有着至关重要的作用,本文报道了二氧化碳热催化转化机制相关的最新进展,重点阐述了对热催化过程具有辅助作用的活性气体(包括H_(2)O、CH_(4)和H_(2))在实现高水平二氧化碳回收转换与高效合成气生产方面的催化机制,其核心体现在优化的反应动力学与温和条件下的热力学优势。

Numerical study on waste polyethylene pyrolysis driven by self-sustaining smoldering

Polyethylene is the type of waste plastic that accounts for the most significant proportion of municipal solid waste.Waste polyethylene can be valorized via pyrolysis and produce value-added oil,gas,and char.On the other hand,self-sustaining smoldering is an emerging technical means to deal with sand/soil contaminated by organic matter.The high-temperature heat generated by smoldering can be used as a heat source for pyrolyzing waste polyethylene.Therefore,this study investigates numerically the pyrolysis of waste polyethylene driven by self-sustaining smoldering.A novel 4-step lumped kinetic model is proposed for simulating the pyrolysis of waste polyethylene.The results indicate that the operating parameters can determine the pyrolysis product yields by regulating the pyrolysis temperature and the volatile residence time.Note that higher temperatures and longer residence times favor the generation of shorter-chain pyrolysis products because of the intensified volatiles’secondary cracking.It can be concluded that a high interface-wall heat transfer coefficient(400 W m^(-2)K^(-1)),a low PE content(0.20),a high char concentration(2.4%),and a moderate air velocity(0.040 m s^(-1))are beneficial to oil yield.To some extent,this study may broaden the boundaries for the application of self-sustained smoldering-driven pyrolysis.

Application of CoFe_(2)O_(4)-NiO nanoparticle-coated foam-structured material in a high-flux solar thermochemical reactor

The splitting of carbon dioxide through the two-step solar thermochemical cycle presents enormous potential,for it holds the dual functionalities of solar fuel production and carbon-based energy recovery.However,the industrialization of this technology is impeded by two critical factors:The absence of fully developed oxygen carriers and advanced reaction devices that deliver exceptional performance.In order to identify a potentially effective oxygen carrier,50 wt%NiO-modified Co Fe_(2)O_(4)is selected as the active component and characterized by means of thermogravimetry,scanning electron microscopy,and energy dispersive spectroscopy,so as to clarify its oxygen exchange capacity,micromorphology and elemental composition in high-temperature thermochemical cycles.Further,nanoparticle-coated foam-structured materials are prepared in combination with Si C ceramic foam for experimental testing in a high-flux solar reactor.The results indicate that a peak CO yield of 1.96 m L min^(-1)g^(-1)can be gained in a 1500–1250 K preliminary test,demonstrating the application potential of the material.In contrast to conventional redox materials,the CO_(2)activity of the materials synthesized in this study exhibits an enhancement with rising oxidation temperature.It means that isothermal cycles can potentially achieve higher conversion and fuel yield than non-isothermal cycles,while simultaneously reducing the amount of irreversible heat loss during high-temperature cycling.Although the estimated steadystate thermal efficiency of the solar reactor can reach up to 42%,further optimization of the reactor design is necessary to enhance energy conversion efficiency,as it is partially limited by the dimensions of the reaction chamber.

Radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina RPC structures as integrated solar thermochemical reactor

This paper investigated radiative heat transfer and thermal characteristics of Fe-based oxides coated SiC and Alumina reticulated porous ceramic structures as integrated solar thermochemical reactor.High-flux solar radiation absorption and axial temperature distribution in the ceramic foams reactor were analyzed by adopting surface-to-surface radiation model coupled to the P1approximation for radiation heat transfer.The radiative heat transfer and thermal characteristics of different foam-type RPC structures,including SiC,CeO_2,FeAl_2O_4,NiFeAlO_4,Fe_3O_4/SiC,and NiFe_2O_4/SiC were evaluated.The mass flow rate and foam structural parameters,including the permeability,pore mean cell size,and extinction coefficients have significantly affected the axial temperature distribution,pressure drop,heat transfer,and fluid flow.Integrated porous structure to the solar receiver could maximize the incorporation of redox powder in the reacting medium,lower the pressure drop,and enhance the thermal performance of the thermochemical reacting system.SiC structure was the candidate materials in the case where more heat flux and high axial temperature distribution is needed.However,Fe-based oxide coated Al_2O_3structure could be considered regarding the heat transfer enhancement along with the catalyst activity of oxygen carriers for solar thermochemical reacting system performance.

Analysis of heat flow diagram of small-scale power generation system:Innovative approaches for improving techno-economic and ecological indices

In this paper,the heat flow diagram of steam turbine model K-6-35 has been analyzed for innovative approaches towards improving the techno-economic and ecological indices of the small-scale power generation system.The numerical analysis is performed using IPSEpro process simulation software based on heat balance method under four different cases.It was found that the study of Solar Assisted Power Generation(SAPG)system has important practical significance in power generation with minimum pollutants and maximum efficiency.Both fuel-saving(FS)and power-boosting(PB)operation modes of the SAPG system are considered.Various types of stand-alone solar thermal power plants exhibited very low overall efficiency with many ecological advantages compared to the conventional thermal power plant based on fossil fuels.Besides,SAPG system with FS mode presented higher techno-economic indices and operation performance.An important reduction in fuel consumption and pollutant emissions could be obtained with SAPG system.Considering the hourly,daily,monthly,and yearly amount of saved fuel and reduced pollutants in the whole power plant,the SAPG system with FS mode can largely contribute to high ecological indices power generation.A thermal efficiency increased by 1.31%with specific equivalent fuel consumption decreased by 22.54 g/kWh was obtained with SAPG system.The coal consumption was reduced by 4.75%when SAPG system operates in FS mode.

Thermal characteristics and thermal stress analysis of solar thermochemical reactor under high-flux concentrated solar irradiation

Nowadays, using a solar-driven thermochemical reaction system to convert greenhouse gases into high-quality liquid fuels has been proven to be an effective way to address the growing depletion of traditional fossil fuels. However, the utilization of highlyconcentrated solar irradiation runs the high risk of reactor damage issues resulting from thermal stress concentration, which seriously threatens the security and reliability of the total reactor system. In this study, the thermal radiation distribution and thermo-mechanical process in a volumetric reactor were numerically investigated by combining Monte Carlo ray-tracing method with computational fluid dynamics method. Based on the experimental results and thermal characteristic analysis, the formation mechanism of thermal stress concentration and the strategies of improving thermal stress distribution were discussed in detail.The simulation results indicate a great possibility of reactor damage at about 1000℃ operating temperature and 9.0 k W lamp power, which is well-matched with related experimental results. The ceramic damage typically occurs at the inner edges of the through-holes, including the aperture, the gas inlet, and the thermocouple hole, then extends along the lines connecting these holes and finally causes brittle fracture. By reasonable control of the opening direction and the distance between the throughholes, the maximum compressive stress can be reduced by 21.78%.