| It is imperative to deal with animal manure rapidly and effectivly with the development of large and medium-sized livestock and poultry breeding industry. Dry anaerobic fermentation technology which is also called as solid anaerobic fermentation has been studied widely with the advantages including water conservation, easy management, high rate of gas production, solving pollution, providing renewable sources of energy and low cost. Due to its high solid content with less free flow water of the raw materials and large concentration gradient, it resulted in difficulty of material access, non-uniform of heat and mass transmission, and causing acidosis, which restricted the extend of dry anaerobic fermentation technologies. To solve the problem, a multi-layer bed anaerobic dry fermentation reactor was designed by adding stratified beds on the internal of existed anaerobic reactor. It can divide the fermentation raw materials into several Myers and recycle the leachate to improve the effect of the reactor on heat and mass transfer of gas-liquid-solid three phases. According to experiments with pig manure as fermentation substrate and amendment of straw, the effect of stratification and bed thickness on the gas production characteristics of dry fermentation was studied. The results show that:(1)It has significant effect on the gas production rate in the earlier stage to use different kinds of thinner bed layers supporting materials in anaerobic fermentation. With the thickness of bed layer reducing from250mm to150mm, the maximum volume of biogas production per day increases by approximate14.9%rom97.4L to111.9L. Furthermore, the time to reach the maximum value of biogas production per day can be decreased greatly by5days with the decrease of bed layer thickness. In thinner bed layers reactors, reducing bed layers’thickness would promote the mass transmission efficiency of the raw materials fermentation, enhance the maximum volume of biogas production and shorten the time that needed. But it has no significant effect on the gas production rate on the later period of fermentation. When the thickness of bed layer reduces from900mm to300mm, the maximum volume of biogas production per day increases by approximate28.0%rom130.2L to166.6L in deeper bed layers reactors. Furthermore, with the increase of bed layer thickness, the daily gas production can increase on the earlier time and the time needed to reach the maximum value of biogas production per day can decrease greatly by7days. On the later stage the daily gas production can increase with the decrease of bed layer thickness.(2) Relatively thinner layering has a significant impact on accumulated gas production and reducing the bed thickness can improve the raw cumulative gas production and TS gas production rate. When the bed thickness of raw material reduces from250mm to150mm, the cumulative gas production rate increases from135.7L/kg to172.1L/kg, and the cumulative gas production increased from1896L to2519.0L, which both increase by26.8%. Relatively thicker layering has not a significant impact on accumulated gas production but has a significant impact on the beginning and end of accumulated gas production and TS gas production rate. In the early stage of the experiment, increasing bed thickness can improve the cumulative gas production and reducing the bed thickness can improve the late cumulative gas production.(3) Relatively thinner layering has some influence on the methane content and reducing the bed thickness can improve the peak of methane which increases by9%and which time is ahead of4days but has not significant impact on the late stage of material methane content. Relatively thicker layering can improve methane content at certain time and when increasing the bed depth, the time which methane content is more than45%in the first time is less than5days of before and it improves the content of the early methane fermentation gas. Reducing the bed thickness can increase the content of methane in the late stage, but generally speaking, the influence on the methane content is not obvious.(4) As the reaction proceeds, the pH value increases and COD value decreases gradually, relatively thinner layering has significant impact on the pH value of raw leachate and the concentration of COD.250mm、200mm and150mm,the three respectively different groups,of which pH values increase by8.6%,6.5%and4.8%from the initial value and COD decrease by21%,15%and14%. With the decrease of layer thickness, the relative change rate of pH value and permeability liquid are low and the depth of bed layer has significant impact on eachate infiltration and reflux effect. The relatively thinner and thicker layering shows the same characteristics. When the bed thickness of material is changed from900mm to300mm, the final pH value were increased by9.1%,6.7%,4%, and COD decreased by24%,19.2%,16.1%, but for pH value and COD the overall impact are not significant, which have respectively significant effects on pH value and COD of reactor in the first and last stage of leachate.(5) Bed stratification improved the cumulative gas production of the fermentation raw material by improving the flow characteristics of leachate.(6) When the pore sizes at the bottom of the layer bed was small, the leachate will mainly be gathered in the lower-middle bed of the stratification reactor.(7) For the fermentation reactor which depth is relatively thinner, there was a positive correlation between the internal and external temperature of raw materials to great extent in the absence of external heat source condition, and the correlation coefficient is about0.7, so it should increase the heating measures of thin bed reactor in order to ensure the temperature that anaerobic fermentation requires. The relatively thinner layer can improve the efficiency of anaerobic fermentation under reflux conditions, but this kind of layer is not conducive to improve the temperature in the aerobic stacking stage, thus it should increase the bed thickness of anaerobic fermentation reactor. |
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