Characteristics Of Mass Transfer And Hydrogen Production In Biofilm Photo-Bioreactor With Photosynthetic Bacteria

Economic growth in the last few decades was strongly dependant on fossil fuels as sources of energy, while these resources were not unlimited on a long view. Today absence of energy supply in the world remains in strain that lead to the always rising of international price of oil.The primary obstacle affecting the development of society and economy presently is the increasing shortage of energy resources. Meanwhile, the exploitation and application of fossil fuel brought seriously environmental problems, especially the resent worldwide'greenhouse effect'attributed to an increasing concentration of carbon dioxide in the air due to combustion of fossil fuels, while the acid rain was ascribed to the emitted NOx and SO2. These pollutants disturbed the sustainable development of earth and human being. Compared with other country, there was less energy resources in Chinese. The storage amount per capita of oil and natural gas in our country is only 7.7% and 7.1% of word average level. So our requirement for new technologies to obtain sustainable and environmentally acceptable energy was more urgently.Hydrogen has the highest gravimetric energy density among any known fuel and is compatible with electrochemical and combustion processes for energy conversion without producing emissions that add to environmental pollution. Therefore, offers tremendous potential as clean, renewable energy supplyment. And in the 11th Five-Year Plan of energy resource promulgated the exploitation of hydrogen energy will become one of the important and advanced technologies needed to study. However, hydrogen production by conventional methods needed fossil energy consumption thus destroyed our living environment also. Solar energy is the most abundant one of various renewable energy sources. Its radiation provides the biggest flow of energy on the earth. Hydrogen production via biological process by photosynthetic bacteria can operate in moderate reaction condition that coupled with the organic contamination degradation and solar energy utilization, leading to a solution for the confliction of energy requirements and environment protection perfectly.It should be pointed out that one necessary premise of practical photobiological hydrogen production is to make the process operate continuously. Cell immobilization was the feasible technique to realize the continuous hydrogen production. A few studies on immobilized-cell techniques for biological hydrogen production had been carried out revealed that the reactor using cell immobilization technology ,gel entrapment and biofilm, could effectively increase the biomass amount in the unit bioreactor space, enhance the tolerance ability of the bacteria and increase the hydrogen production rate and bio-degradation rate. For gel entrapment, the main drawbacks were low hydrogen production rate, insufficient light supply, weak mechanical strength and poor stability for long-term operation. Immobilized-cell systems with biofilm formation were considered as suitable means for continuous and efficient hydrogen production. Measures on mass transfer enhancement, biomass immobilization increasing and light conversion efficiency improvement are motivations of present study. Three novel photobiological reactors of groove-type photobioreactor, annular fiber-illuminating biofilm photobioreactor and annular bundle of optical-fiber-illuminating photobioreactor were developed for continuous and efficient hydrogen production by immobilized PSB. The different performances with respect to hydrogen production rate, substrate consumption rate, hydrogen yield and light conversion efficiency of the photobioreactor were investigated and effects of different operation conditions on the performance of photobioreators were obtained also.Results of parallel experiments on groove-type photobioreactor and flat panel photobioreactor indicated that groove structure with large specific surface area was beneficial to cell immobilization and biofilm formation of the photosynthetic bacteria on photobioreactor surface to increase the amount of biomass in the bioreactor. The fluctuant structure could also enhance the convective mass transfer of the substrate from the bulk flow to the biofilm and metabolic production transfer from the biofilm to the bulk flow to attain a better microenvironment in biofilm zone. Moreover, the groove structure with a high surface-to-volume ratio offered a larger illumination area for the photobioreactor and this, in turn, leaded to even distribution of the light and less light attenuation in the biofilm zone, hence improved the light conversion efficiency. The maximum hydrogen production rate, H2 yield and light conversion efficiency in the groove-type photobioreactor were 3.816 mmol/m2/h, 0.75 molH2/molglucose and 3.8%, respectively, which were about 75% higher than those in the flat panel photobioreactor. The groove-type structure offered a reference about the improvement of the carrier within bioreactor for photobiological hydrogen production with biofilm technology.Our newly developed annular fiber-illuminating biofilm photobioreactor firstly solved the confliction of cell immobilization and light transfer enhancement in the study field of photobiological hydrogen production. A comprehensive investigation of operational conditions on the performance of AFIBR was carried out by a series experiments. A novel increasement of light conversion efficiency of 47.9% and hydrogen production rate of 0.83 mmol/g dry cell/h were attained by monochromatic light illumination at 530 nm and even light intensity distribution character of inside light source in AFIBR at 4.15 W/m2. A mathematic model on substrate transfer and degradation of AFIBR was built based on experimental results. It described effects of different operating conditions on characters of the AFIBR, model predictions were accordant with experimental results on the whole. The newly developed annular bundle of optical-fiber-illuminating photobioreactor based on the previous studies attained sufficient utilization of inner space and evenly light intensity distribution in the bioreactor. It achieved excellent hydrogen production rate (0.6 mmol/L/h) and light conversion efficiency (3.64%) under sunlight simulative conditions of 5.1 W/m2. These results have instructional effects on the study of efficient photobiological hydrogen production in large scale.So far, the cell immobilization technology has become positive in biotechnology, but there was few study on the transfer characteristics of immobilized PSB cells in the process of photobiological hydrogen production. Our researches, which would set a primitive effort for application of immobilized cells technology in bioenergy production to solve the problem of mass transfer limitation within immobilized cells, low light conversion efficiency and low hydrogen production rate of the bioprocess. These studies offered valuable referents on the research of efficient photobioreactor for continuous hydrogen production in large-scale, they could give necessary aidances for both therotical and practial studies in the future.