Chemical looping reforming of the micromolecular component from biomass pyrolysis via Fe_(2)O_(3)@SBA-16

To solve the problems of low gasification efficiency and high tar content caused by solid–solid contact between biomass and oxygen carrier in traditional biomass chemical looping gasification process.The decoupling strategy was adopted to decouple the biomass gasification process,and the composite oxygen carrier was prepared by embedding Fe_(2)O_(3) in molecular sieve SBA-16 for the chemical looping reforming process of pyrolysis micromolecular model compound methane,which was expected to realize the directional reforming of pyrolysis volatiles to prepare hydrogen-rich syngas.Thermodynamic analysis of the reaction system was carried out based on the Gibbs free energy minimization method,and the reforming performance was evaluated by a fixed bed reactor,and the kinetic parameters were solved based on the gas–solid reaction model.Thermodynamic analysis verified the feasibility of the reaction and provided theoretical guidance for experimental design.The experimental results showed that the reaction performance of Fe_(2)O_(3)@SBA-16 was compared with that of pure Fe_(2)O_(3) and Fe_(2)O_(3)@SBA-15,and the syngas yield was increased by 55.3%and 20.7%respectively,and it had good cycle stability.Kinetic analysis showed that the kinetic model changed from three-dimensional diffusion to first-order reaction with the increase of temperature.The activation energy was 192.79 kJ/mol by fitting.This paper provides basic data for the directional preparation of hydrogen-rich syngas from biomass and the design of oxygen carriers for pyrolysis of all-component chemical looping reforming.

化学链重整反应中含氧空位α-Fe_(2)O_(3)(001)表面CO反应行为的密度泛函理论研究

化学链甲烷重整是利用甲烷制备合成气(CO和H_(2))具有广阔前景的方法之一,其中CO在氧载体表面上的进一步反应对CO选择性起主要决定作用.本文采用密度泛函理论系统地研究了α-Fe_(2)O_(3)(001)氧空位对CO脱附、CO氧化和CO解离的影响.计算结果表明,增加氧空位浓度会导致Fe原子中的电子局域化增强,从而削弱Fe位点上CO的脱附能力.此外,氧空位浓度从1/12 ML增加到1/6 ML会导致CO氧化能垒从0.64 eV增加到1.10 eV.增加氧空位浓度虽有利于CO解离,但由于解离反应的高能垒(大于3.00 eV),CO的解离仍然很难发生.综合考虑三条反应途径,CO脱附可以在高于900K的反应温度下自发进行.增加氧空位浓度可以提高合成气选择性,因为在高氧空位浓度(1/6 ML)下,相比于CO脱附,CO氧化的竞争性减弱.这项研究揭示了在Fe_(2)O_(3)的还原程度增加时,化学链甲烷重整中CO选择性增强的微观机制.

Chemical looping gasification of maceral from low-rank coal: Products distribution and kinetic analysis on vitrinite

The product distribution and kinetic analysis of low-rank coal vitrinite were investigated during the chemical looping gasification(CLG)process.The acid washing method was used to treat low-rank coal,and the density gradient centrifugation method was adopted to obtain the coal macerals.By combining thermogravimetric analysis and online mass spectrometry,the influence of the heating rate and oxygen carrier(Fe2O3)blending ratio on product distribution was discussed.The macroscopic kinetic parameters were solved by the Kissinger-Akahira-Sunose(KAS)method,and the main gaseous product formation kinetic parameters were solved by the iso-conversion method.The results of vitrinite during slow heating chemical looping gasification showed that the main weight loss interval was 400–600℃,and the solid yield of sample vitrinite-Fe-10 at different heating rates was 64.30%–69.67%.When b=20℃·min^(-1),the maximum decomposition rate of vitrinite-Fe-10 was 0.312%min1.The addition of Fe2O_(3)reduced the maximum decomposition rate,but by comparing the chemical looping conversion characteristic index,it could be inferred that the chemical looping gasification of vitrinite might produce volatile substances higher than the pyrolysis process of vitrinite alone.The average activation energy of the reaction was significantly reduced during chemical looping gasification of vitrinite,which was lower than the average activation energy of 448.69 kJ·mol^(-1) during the pyrolysis process of vitrinite alone.The gaseous products were mainly CO and CO_(2).When the heating rate was 10℃·min^(-1),the highest activation energy for CH4 formation was 21.353 kJ·mol^(-1),and the lowest activation energy for CO formation was 9.7333 kJ·mol^(-1).This study provides basic data for exploring coal chemical looping gasification mechanism and reactor design by studying the chemical looping gasification process of coal macerals。

Liquid chemical looping gasification of biomass:Thermodynamic analysis on cellulose

Liquid chemical looping technology is an innovation of chemical looping conversion technology.Using liquid metal oxide as the oxygen carrier during gasification process could prolong the service life of oxygen carrier and improve the process efficiency.In this paper,based on Gibbs minimum free energy method,the thermodynamic characteristics of biomass liquid chemical looping gasification were studied.Cellulose and lignin,the main components of biomass,were taken as the research objects.Bismuth oxide and antimony oxide were selected as liquid oxygen carriers.The results showed that when the temperature increased from 600℃to 900℃,the output of H_(2)and CO in the products of cellulose gasification increased from 0.5 and 0.3 kmol to 1.3 and 2.6 kmol respectively.Different ratios of oxygen carriers to gasification raw materials had the best molar ratio.The addition of steam in the system was beneficial to the increase of H_(2)content and the increase of H_(2)/CO molar ratio.Bi_(2)O_(3)and Sb_(2)O_(3)with different mass ratios were used as mixed oxygen carriers.The simulation results showed that the gasification temperature of biomass with different mixed oxygen carriers had the same equilibrium trend products.It could be seen from the results of product distribution that the influence of the mixing ratio of Bi_(2)O_(3)and Sb_(2)O_(3)on gas product distribution could be neglected.These results could provide simulation reference and data basis for subsequent research on liquid chemical looping gasification.

Role of surface frustrated Lewis pairs on reduced CeO2(110)in direct conversion of syngas

Direct syngas conversion to light olefins on bifunctional oxide-zeolite(OX-ZEO)catalysts is of great interest to both academia and industry,but the role of oxygen vacancy(Vo)in metal oxides and whether the key intermediate in the reaction mechanism is ketene or methanol are still not well-understood.To address these two issues,we carry out a theoretical study of the syngas conversion on the typical reducible metal oxide,CeO2,using density functional theory calculations.Our results demonstrate that by forming frustrated Lewis pairs(FLPs),the VOs in CeO2 play a key role in the activation of H2 and CO.The activation of H2 on FLPs undergoes a heterolytic dissociative pathway with a tiny barrier of 0.01 eV,while CO is activated on FLPs by combining with the basic site(O atom)of FLPs to form CO2^2-.Four pathways for the conversion of syngas were explored on FLPs,two of which are prone to form ketene and the other two are inclined to produce methanol suggesting a compromise to resolve the debate about the key intermediates(ketene or methanol)in the experiments.Rate constant calculations showed that the route initiating with the coupling of two CO*into OCCO*and ending with the formation of ketene is the dominant pathway,with the neighboring FLPs playing an important role in this pathway.Overall,our study reveals the function of the surface FLPs in the activation of H2 and CO and the reaction mechanism for the production of ketene and methanol for the first time,providing novel insights into syngas conversion over OX-ZEO catalysts.

Particle-scale and sub-grid drag models coupled CFD for simulating the CO methanation in a CFB riser

The diffusion and chemical reactions inside the catalyst particles and the heterogeneous flow structure in the computational cells are key factors to affect the accuracy of the coarse-grid simulation in circulating fluidized bed(CFB)methanation reactors.In this work,a particle-scale model is developed to calculate the effective reaction rate considering the transient diffusion and chemical reactions in the particle scale,i.e.,the scale of the single catalyst particle.A modified sub-grid drag model is proposed to consider the effects of the meso-scale and chemical reactions on the heterogeneous gas-solid interaction,where the meso-scale is between the single particle and the whole reactor and featured with the particle cluster.Subsequently,a coupled model is developed by integrating the particle-scale and modified sub-grid drag models into CFD.Moreover,the coupled model is validated to achieve accurate predictions on the CO methanation process in a CFB riser.Notably,the coupled model can be performed with a coarse grid(∼58 times particle diameter)and a large time step(0.005 s)to accelerate the simulation.By simply changing the reaction kinetics,different gas-solid catalytic reaction systems can be simulated by using the coupled model.

Computational fluid dynamics simulation of hydrodynamics in the riser of an external loop airlift reactor

Local hydrodynamics in the riser of an external loop airlift reactor （EL-ALR） are identified and the performances of three drag models are evaluated in computational fluid dynamics simulation. The simulation results show that the Schiller-Naumann drag model underestimated the local gas holdup at lower superficial gas velocity whereas the Tomiyama drag model overestimated that at higher superficial gas velocity. By contrast, the dual-bubble-size （DBS）-local drag model gave more reasonable radial and axial distri-butions of gas holdup in all cases. The reason is that the DBS-local drag model gave correct values of the lumped parameter, i,e., the ratio of the drag coefficient to bubble diameter, for varying operating conditions and radial positions. This ratio is reasonably expected to decrease with increasing superficial gas velocity and be smaller in the center and larger near the wall. Only the DBS-local drag model correctly reproduced these trends. The radial profiles of the axial velocity of the liquid and gas predicted by the DBS-local model also agreed well with experimental data.

Chemical looping gasification characteristics and kinetic analysis of Chlorella and its organic components

Chemical looping gasification(CLG)characteristics and kinetic analysis of Chlorella(CHL),simulated Chlorella(V-CHL)and medium-chain triglycerides(MCT)were investigated using a thermogravimetric analyzer coupled with an online mass spectrometer.The apparent activation energy was obtained via Kissinger-Akahira-Sunose(KAS)method.In the result of the weightless behavior,the addition of oxygen carrier inhibited the decomposition of V-CHL at lower temperatures but promoted its decomposition at high temperatures.The values of chemical looping process characteristic parameters showed that a 10 wt%oxygen carrier would maximize the release of volatile products in the CLG of MCT,with 5.12×10^(-6)%⋅min^(-1)⋅℃^(-3).Oxygen carriers also affected gaseous products.The LHV of gaseous products of CHL reached the largest when the oxygen carrier was 10 wt%,which was 8.13 MJ/m^(3).And the gaseous product of MCT had the largest LHV with 30 wt%oxygen carrier,which was 8.83 MJ/m^(3).According to the kinetic analysis,the minimum value of apparent activation energy of MCT chemical looping gasification was 89.54 kJ⋅mol^(-1) with the oxygen carrier of 30 wt%,which was 50%less than that of MCT pyrolysis.And the minimum value for V-CHL was obtained when the mass fraction of Fe2O3 was 50 wt%.This paper could provide a reference for the choice of algae,the design of reactors,and the targeted regulation of the gaseous product for the algae CLG process.

Understanding the Initial Decomposition Pathways of the n-Alkane/Nitroalkane Binary Mixture

Addition of nitroalkanes into n-alkanes can lower the activation barriers of free-radical production and acceler- ate the decomposition of n-alkanes at relatively low temperatures. Four initial decomposition mechanisms of the n-butane/nitroethane binary mixture were proposed for the promoting effect and considered theoretically at the B3LYP, BB1K, BMK, MPW1K, and M06-2X levels with MG3S basis set. Energetics above was compared to high-level CBS-QB3 and G4 calculations. Calculated results confirm the feasibility of the four initial decomposition pathways： （I） the C--NO2 bond rupture of nitroethane to produce ethyl and .NO2, （II） HONO elimination from nitroethane followed by decomposition to .OH and .NO, （1II） rearrangement of nitroethane to ethyl nitrite which further dissociates into CH3CH20. and .NO, and （IV） direct hydrogen-abstraction ofnitroethane with n-butane.