Abstract:

As an effort to achieve carbon neutrality, utilization of biomass gasification-based syngas (mainly consisting of CO, CO₂, and H₂) has been extensively studied. Conversion of syngas to fuel ethanol has the dual effects of carbon emission reduction and energy production. Since syngas can not be completely converted in chemical catalytic process, the tail gas produced, known as low-carbon syngas, is considered to be further utilized by biological process. In this study, fermentation of Clostridium autoethanogenum for ethanol production was investigated from the aspects of gaseous substrate and fermentation technology. Results showed that Clostridium autoethanogenum preferentially consumed CO other than CO₂ and H₂ in syngas fermentation, the ratio of CO consumption to H₂ consumption was 0.99±0.12, and the molar ratio of ethanol and acetic acid achieved was 0.78±0.10; when 100% CO was used as substrate, the ratio of CO consumption to CO₂ production was 1.72±0.21, and the molar ratio of ethanol and acetic acid was 1.00±0.11. During the process of xylose-syngas co-fermentation, xylose was consumed slowly, and compared to syngas fermentation, the cell density increased, but ethanol concentration was lower. Cell activities were not affected when 4g/L of sodium acetate was added in the medium, while significant inhibition on cell growth was observed under higher concentrations (≥12g/L). Fermentation results in the 3L stirred-tank reactor showed that the ethanol yield was low once "acid crash" happened, while lowering temperature at the late logarithmic phase of cell growth could prevent "acid crash". Consequently, the maximum ethanol concentration obtained was 3.46g/L, maximum ethanol and acetic acid molar ratio was 2.01, and the highest ethanol yield was 0.35g/(L∙d). This study provided references for the process optimization of ethanol production from low-carbon syngas via Clostridium autoethanogenum fermentation.

Key words: syngas, fuel ethanol, gaseous fermentation, acetate, Clostridium autoethanogenum

摘要：

在碳中和大目标下，生物质气化合成气（主要成分为CO、CO₂和H₂）的资源化利用受到广泛关注，将合成气转化为燃料乙醇具有碳减排和能源生产的双重功效。针对化学催化转化过程存在合成气不能被完全利用的问题，采用生物法对其尾气（低碳合成气）进行进一步利用, 以提高合成气的利用率。本文从气体底物、发酵工艺等方面对乙醇梭菌（Clostridium autoethanogenum）发酵产乙醇过程进行了研究。结果表明：在合成气发酵中，菌株会优先利用CO，之后再利用CO₂和H₂，CO和H₂消耗的比值为0.99±0.12，产物中乙醇与乙酸的摩尔比为0.78±0.10；以100% CO为底物时，CO消耗与CO₂生成的比值为1.72±0.21，乙醇与乙酸的摩尔比为1.00±0.11。木糖-合成气共发酵时菌株对木糖的利用比较缓慢，与合成气发酵相比，菌体浓度较高，但乙醇浓度较低。培养基中添加乙酸钠浓度为4g/L时对菌株活性没有影响，高浓度（≥12g/L）则对菌株生长产生明显的抑制作用。3L搅拌罐式反应器的发酵实验显示，当发酵过程出现“酸崩溃”时，菌株的产乙醇代谢受到抑制，导致乙醇产量低；在菌株对数生长后期降低发酵温度可有效避免“酸崩溃”，发酵得到的最高乙醇浓度为3.46g/L，最高乙醇与乙酸的摩尔比达到2.01，最大乙醇产率达到0.35g/(L∙d)。上述研究结果可为Clostridium autoethanogenum发酵产乙醇过程工艺优化提供参考。

关键词: 合成气, 燃料乙醇, 气体发酵, 乙酸, 乙醇梭菌