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玉米秸秆和猪粪好氧-厌氧一体化产甲烷特性

Integrated aerobic-anaerobic characteristics of maize stovers and pig manure for methane production

  • 摘要: 高固体负荷下的好氧预处理是一个好氧微生物的产热过程,能有效降低后续厌氧发酵过程原料的增温能耗。而目前好氧预处理过程通常采用强制连续通风或间歇通风,当环境温度较低时,冷空气易对微生物产生冲击,造成热量过度损失,降低好氧预处理过程的温升及预处理效果。该研究采用好氧-厌氧一体化发酵工艺,并在好氧预处理过程增设基于反应体系氧浓度变化的动态曝气,结合接种、渗滤液回流等手段,系统研究不同环境温度下,好氧预处理的温升特性、干式厌氧发酵的产甲烷(CH4)特性及一体化运行的能耗与产能。结果表明,好氧-厌氧一体化工艺能实现好氧预处理、干式厌氧发酵过程的稳定运行及好氧阶段到厌氧阶段的快速转换与无缝衔接;基于动态曝气的好氧预处理能显著提高原料的温度,在11.7~17.3 ℃环境温度范围内,预处理前72 h期间均可保持升温,曝气不加热时物料最高温度可达57.9 ℃,同时可产生一定量的挥发性脂肪酸,有效地促进了干式厌氧发酵的快速启动,并显著提高了CH4产量;好氧预处理过程的动态曝气能耗较少,仅占一体化运行过程总能耗的7.64%~12.69%,而厌氧发酵过程的增温能耗占比达87.31%~92.36%;在3~5 mm秸秆粒径、曝气不加热的好氧预处理和20%总固体负荷(TS)、40%接种物添加量、渗滤液日回流4次的干式厌氧发酵的最优组合条件下,CH4产率、单位电耗的CH4产量和CH4产能分别达到218.99 L/kg(以volatile solids VS计)、6.352 L/kWh和7.840 MJ/kg;各因素对CH4产率的影响强度由强到弱依次为秸秆粒径、渗滤液日回流次数、TS、接种物添加量、曝气温度;固、液两相微生物群落在水解酸化和产CH4 2个阶段差异较大,液相中微生物群落的相对丰度随时间变化显著,而固相微生物群落的相对丰度较稳定。该研究可为干式厌氧发酵技术的推广应用,尤其北方寒区厌氧发酵成本的降低提供新的思路和基础依据。

     

    Abstract: Aerobic pretreatment is one of the important heat-producing steps by aerobic microorganisms at high solids loading. The energy consumption can be effectively reduced to warm the feedstocks for the subsequent anaerobic digestion. Among them, aerobic pretreatment can usually employ forced continuous ventilation or intermittent ventilation. The cold air can often tend to impact the microorganisms and the pretreatment at the low ambient temperature. Excessive heat loss can also occur, leading to serious challenges during methane (CH4) production. In this study, an integrated aerobic-anaerobic fermentation was adopted to regulate the dynamic aeration in the aerobic pretreatment, according to the oxygen concentration during the reaction. A systematic investigation was made on the temperature rise during aerobic pretreatment, the methanogenic characteristics during dry anaerobic fermentation, as well as the energy consumption and production capacity under different ambient temperatures. The inoculation and leachate reflux were also combined for the integrated operation. The results showed that the aerobic-anaerobic process was integrated to realize the stable aerobic pretreatment. Dry anaerobic digestion was used to realize the rapid conversion and seamless connection of the aerobic to the anaerobic stage. Furthermore, the aerobic pretreatment with dynamic aeration significantly increased the temperature of the feedstocks. The fluctuations impacted the warming of the aerobic pretreatment under the ambient temperature. The temperature also elevated in the first 72 h under the ambient temperature of 11.7-17.3 ℃. The highest temperature reached 57.9 ℃ under the conditions where the aeration was not preheated. Meanwhile, a large amount of the volatile fatty acids (VFAs) was produced during aerobic pretreatment. The rapid startup of the dry anaerobic digestion promoted significantly the CH4 production. There was less energy consumption for the dynamic aeration during aerobic pretreatment. There was only accounted for 7.64% to 12.69% of the total energy consumption of the integrated operation. While the warming energy consumption during the anaerobic fermentation accounted for 87.31% to 92.36%. The particle size of the corn straw dominated the CH4 production. The cumulative CH4 production under 3-5 mm particle size increased by 113.93% and 81.95%, respectively, compared with that under ≤1 mm and 7-8 mm particle sizes. There was no significant difference in the effects of the different aeration temperatures on the CH4 production and the VFA concentrations. The maximum threshold of the total solids (TS) was 20% for dry anaerobic digestion. The cumulative CH4 production increased by 24.24% and 78.98%, respectively, under 20% TS, compared with the 15% and 25% TS. The cumulative CH4 production decreased from 183.92 to 156.76 L and 158.04 L, as the inoculum concentration increased from 40% to 60%. There was no significant difference in the CH4 production when the leachate was refluxed 3 to 4 times per day. The optimal combination of the aerobic pretreatment was obtained with a straw particle size of 3-5 mm, dynamic aeration without heating, and dry anaerobic digestion with a total solids load of 20%, an inoculum concentration of 40%, and leachate refluxing four times per day. The CH4 yield, CH4 production per unit of electricity consumption, and CH4 production capacity reached 218.99 L/kg, 6.352 L/kWh, and 7.840 MJ/kg, respectively. The bacterial communities in the liquid and solid phases also differed outstandingly from the two phases of hydrolytic acidification and CH4 production. There was a great variation in the relative abundance of the bacterial community in the liquid phase during anaerobic digestion. The relative abundance of the Bacteroidota decreased from 62% to 34%. Both the relative abundances of the Firmicutes and Proteobacteria increased from 22% and 9% to 25%. There was a more stable relative abundance of the bacterial community in the solid phase. The relative abundances of the Firmicutes and Bacteroidota increased from 52.0% and 12.3% to 52.7% and 15.3%, respectively. While the relative abundance of the Proteobacteria decreased from 15.3% to 8%. This finding can also provide a strong reference to reduce the consumption loss of dry anaerobic digestion in the northern cold area.

     

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