Kinetic Study And Process Simulation Of The Chemical Absorption-biological Reduction Integrated Process For NO_x Removal

The chemical absorption-biological reduction(CABR)integrated process is deemed as an appropriate technology for advanced NOx removal from flue gas in middle-and small-scale coal-fired boilers.There were kinds of lab-scale CABR denitrification devices had been developed and studied recently.Nowadays,the CABR integrated process is facing the challenge of industrial application.In this dissertation,the kinetic characterizations of the absorption and bioreduction were studied based on the understanding of the process mechanisms in different CABR systems.Kinetic models of one-stage,two-stage,and bioelectrochemical CABR systems were developed to characterize both the physicochemical and biological processes.At last,the scale-up designs of the two-stage and bioelectrochemical CABR systems were accomplished,and the different design results could provide us informations on the application and improving direction of the CABR process.Based on the mass balance principle and two-film theory,a kinetic model considering the mass transfer and reactions in the gas,liquid,and biofilm phases of the one-stage CABR system was developed.The biophysicochemical model was validated by the experimental results and subsequently was used to predict the system performance under different operating conditions.NO distribution along the biofilter was calculated,and the results showed that the sublayer contributed mostly for NO removal.The liquid flow rate and total iron concentration were optimized to achieve a>90%NO removal efficiency.Furthermore,sensitivity analysis of the model revealed that the performance the CABR process was controlled by the bioreduction activity of Fe(?)EDTA.A steady-state kinetic model was developed for the two-stage CABR system and then used for the estimation of the footprints for treating a 5×104 m3 h-1 flue gas from a 20t h-1 coal-fired steam boiler.For a baseline case,the designed absorption column size was 3.60×8.75 m(d×h),while the bioreduction column was set at 3.60×8.50 m(d×h).Sensitive analysis including the influence of gas concentrations,packing properties,and microorganism activities was investigated to optimize the design and operation of two-stage CABR process.The footprint of the bioreactor can be downsized by 27.2%through the enhancement of the bioreduction activity of Fe(?)EDTA,and the corresponding size of the absorption column was 3.60×9.05 m(dxh)?The electron transfer mechanism of the biocathodes with the function of Fe(?)EDTA and Fe(?)EDTA-NO reduction in the bioelectrochemical CABR system was investigated.As the mediated electron transfer supported by H2 and self-excreted compounds was excluded,direct electron transfer was the main mechanism of Fe(?)EDTA reduction,while Fe(?)EDTA-NO was mainly mediately reduced via Fe(II)-assisted autotrophic denitrification.The Nernst-Monod equation suited for electron transfer kinetics in bioanode was improved to characterize the electron transfer in biocathodes in the bioelectrochemical CABR system.The model parameters estimation showed that the half saturation concentration of the electron acceptor was 4.46 mol m-3 under the experimental conditions,while the half saturation potential was-223±26.9 mV vs.SHE.The kinetics of Fe(III)EDTA reduction of the biocathodes in the bioelectrochemical CABR system was obtained on the basis of the improved Nernst-Monod equation and faraday efficiency.The kinetic data changed with the cathodic potential,and the maximum reduction rate was 4.35 mmol h-1 at-300 mV vs.SHE,while the half saturation concentration of Fe(?)EDTA was 4.41 mol m-3.The kinetic analysis of the Fe(II)EDTA-NO reduction in the biocathodes was conducted based on the double-Monod model.The maximum reduction rate of Fe(?)EDTA-NO was 14.66 mmol h-1,and the half saturation coefficient of Fe(?)EDTA-NO was 8.50 mol m-3 while that of Fe(II)EDTA was 0.16 mol m-3.To treat the flue gas of a 20t h-1 coal-fired boiler,the designed size of the absorption column of the bioelectrochemical CABR system without added organic carbon source was 3.60×8.12 m(d×h),while the biocathodic volume was 1113.5 m3.While there was organic carbon source addition,the absorption column size and biocathodic volume were 3.60×8.54 m(d×h)and 454.4 m3,respectively.The design results showed that the Fe(III)EDTA reduction ability of the MEC reactor in the bioelectrochemical CABR system should be further enhanced.