| Energy exhaustion and environmental pollution are the most two important issues in 21st centrury. Developping clean fuel energy and constructing new energy system have strategic importance to realize the target of the human sustainable development. Biohydrogen production has been paid more and more attention because it could be produced at ambient temperature and pressure, as well as its environmental friendly and low energy costing due to the source of renewable biological waste.This research is aimed to study biohydrogen production technology using CO as the carbon source and pure culture of Carboxydothermus hydrogenformans as the fermentative bacteria. Anaerobic fermentation for biohydrogen production experiments are carried out in batch and continuous feeding conditions, respetively. The biohydrogen producing mechanism, biomass growing charateristics, substrate consumption and CO inhibition has been studied. Operating parameters of anerobic fermentation are determined in a hollow fiber membrane bioreactor (HFMBR) for continuous biohydrogen prodcution. The main content of the study and the results are as follows:1 C.hydrogenformans is known to use CO as the sole source of carbon and energy to produce biohydrogen under anaerobic fermentation, in which CO is oxidized to CO2, while H+ is reduced to H2. High specific hydrogen production rate (SHPR) is obtained, as well as the high hydrogen yield (Yield) of 96%. Also C.hydrogenformans can use pyruvate as the carbon source to grow and produce hydrogen, however, low Yield of 17% is observed, most of the fementative productions are volatile fatty acids (VFA) and ethanol. The characteristic that C.hydrogenforans can gather Ca and P from the medium to form the crystal of hydroxyapatite is found though the SEM and EDS analysis.2 Orthogonal experimental design are performed to optimize the medium composition, the influence order and optimal concentration of PO43, HCO3-, Ca2+ and Mg2+are observed as 1 mM,5 mM,0.1 mM and 0.5 mM, respectively. The final optimized medium culture could not only avoid the formation of inorganic crystal in the biofilm but also keep the best SHPR and Yield.3 The reaction kinetics of anaerobic fermentation for biohydrogen production has been assessed, the biomass decay efficiency and maximal growth rate are calculated as 0.022 h-1 and 0.017 h-1, respectively. The best Yield (97%) and SHPR (3.0 mol/g-VSS.d) are obtained at initial biomass densities (Xo) of 5 mg-VSS/L and 8 mg-VSS/L. The mass transfer is studied and the optimal feed/microorganism (F/M) is observed at 6.3 mol-CO/g-VSS, that is to say the CO substrate in the head space should be over 176 g-CO(gas)/g-VSS(1 atm,70℃,100 r/min). Also the kinectic courve is plotted by the CO conversion rate as function of different CO concentration in the medium, CO inhibited concentration 0.55 mmol/L is observed. Also a nonlinear regression has been applied to fit Monod extended equation to estimate the maximal specific rates of substrate depletion, critical concentration of the inhibitory substrate and half-saturation constant.4 In this thesis, biohydroen fermentation technique and membrane bioreactor are united; a new biohydrogen production method is proposed—biohydrogen production from membrane bioreactors. The feasibility of continuous anaerobic fermentation for hydrogen production using CO in a HFMBR is improved. The objective of the reactor is to improve hydrogen producing rate (HPR) and CO conversion efficient (η) by evaluating the effects of paramenters of CO partrial pressure PCO, CO loading Qg, liquid recirculation rate Q1 and temperature T on the hydrogen producing ability in the reactor. The results indicates that Qg is function of PCO; the bestηof 97.6% and HPR of 0.46 mol/d are observed at Qg=0.22 mol/L and 1.15 mol/L, respectively. While increasing Q1 can increase CO-H2O mass transfer which improves the HPR in the reactor. The highest mass transfer coefficient of 1.72 h-1 is obtained at Q1=1500 ml/min. However, the latter augmentation of liquid recirculation coincides a drop of the immobilized C. hydrogenoformans, which is probably due to a severe alteration of the biofilm. Also, T is decreased to improve the mass transfer; however the biological activity of biomass is inhibited at lower T which limited the hydrogen producing ability in the reactor. The HFMBR has a long term working stability of 4 months, no membrane fouling is observed; 84.5±1.6% of the microorganism is cultivated and fixed on the hollow fiber as the biofilm, also high organic active ingredients is observed in the biofilm with a VSS/SS of 86±5.9%. EDS analysis is conducted in the biofilm, no crystals that containing Ca and P are observed any more, which indicates the validity of the optimized medium by orthogonal experimental design. The best SHPR 0.85 mol/g-VSS.d in the HFMR is 0.8 times higher than the best SHPR traditional bioreactors for hydrogen production of 0.57.The theoretical research, experimental analysis and model calculation are proceeded in this study, also experiments are carried out based on improving mass transfer and increasing biohydrogen producing ability by evaluating operating parameters in the reactor, which establishs theory study and practical application for the method of biohydrogen production in the membrane bioreactors. |
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