Study Straw Dry Anaerobic Fermentation Characteristics

Preparing biogas by crop straw anaerobic fermentation is a new trend of crop straw energy and resource utilization in China. However, this preparation method was initiated later and less experience is owned through large-scale project operation compared to developed countries. This thesis demonstrates effective approaches of process selection, process parameter design and economic efficiency calculation for large-scale biogas project of crop straw fermentation, combining theoretical analysis with experimental validation. The major research results and conclusions of this thesis are summarized as follows:(1) According to the biochemical reaction kinetic theory of fermentation, two dynamic models of biomass anaerobic fermentation are built. The process of biomass anaerobic fermentation can be simulated based on these two models to predict changes of biogas composition and rules of biogas production. The prediction can provide guides to design and operation of biomass anaerobic fermentation system.(2) The major methods of crushing straw are introduced. Energy consumption analysis of crop straw crushers produced by different manufacturers is performed. It shows that the larger particle size is, the more power is required under the same crush force. Crush energy consumption can be reduced based on particle requirements in practical applications.(3) Based on fractal theory, a mass fractal model of crop straw is built: y=(R/Rmax)3-D. Energy consumption of crushing different sizes of particles can be calculated with this model.(4) Using corn stalk as the raw material, the influence of different factors on biogas production characteristics is studied through batch anaerobic fermentation. Experimental results show that culture addition can dramatically improve the gas production rate and shorten the fermentation period; the gas production rate increases when the temperature rises, however, the gas production rate under50℃does not rise much compared to that under38℃. Considering energy consumption, the fermentation temperature of38℃as well as the fermentation concentration of25%is selected. In addition, the gas production rate does not rise much as well by reducing the particle size of corn stalk from20to10mm to less than2mm. Hence, it is recommended to use the corn stalk with a particle size of20to10mm. (5) The calculation method of straw cost is introduced. The curve showing the relationship between straw cost and profit is plotted. Based on statistical data of grain-straw ratio, straw amount estimation method is developed. Studied economy calculation method about crop straw biogas project. Moreover, the method of calculating economic efficiency of biogas preparation by crop straw anaerobic fermentation is proposed, which can be used to calculate the economic efficiency of correspondingly biogas preparation project.