以沼液为纽带的沼气中CO2吸收强化与农业生态利用

CO2 capture and storage from raw biogas generated from the anaerobic digestion of biomass (biogas-CCS) is a typical carbon-negative technology because it can capture CO2 from the atmosphere to achieve the net carbon negative emissions during the life cycle of the conversion and utilization of biomass. In addition, the bio-natural gas can also be achieved to contribute to alleviating the shortage of natural gas. And the keystone of this technology is determining how to reduce the cost of biogas-CCS. A novel idea to reduce biogas-CCS was put forward in this study, in which as the byproducts of the biogas anaerobic digestion of biomass, biogas slurry containing considerable amounts of plant nutrients was adopted to capture CO2 from biogas, and then CO2-rich biogas slurry was successively applied into the agroecosystem to accelerate the growth of plants, and consequently CO2 captured would be successfully fixed into the plant and soil. Firstly, in terms of the reaction characteristics between CO2 and biogas slurry, the reaction sequence between CO2 and biogas slurry will be resolved, and then the active components chemically absorbing CO2 will be determined precisely. Based on it, in terms of higher CO2 absorption performance, lower phytotoxicity to reproduction of plants and lower energy requirement, the mechanism of biogas slurry concentrating by using membrane technology with high concentrating efficiency and low cost will be explored to increase the concentration of the active compenents in the biogas slurry. Additionally, enhancement mechanism of reaction between CO2 and biogas slurry will also be discussed to screen these optimal absorbents. And the appropriate mechanism of mass transfer enhancement of CO2 absorption will also be developed. Secondly, the feasibility of CO2 rich biogas slurry applied into the agroecosystem will be delved in order to confirm the positive influence of CO2 rich biogas slurry on plant growth promotion. And after the determination of the transport law of carbon coming from biogas into soil and plant, the net negative emission amount of CO2 will be estimated. Finally, this new method will be assessed to erect an expected novel idea having the potential to reduce the cost of CCS.

对生物质厌氧发酵所产生的沼气中CO2进行捕集和储存（沼气-CCS），可获得生物天然气，有助于缓解天然气供应的不足，亦可从大气中固定CO2，实现CO2的净负排放，是典型的碳负性技术。沼气-CCS关键在于如何降低成本。本项目提出了一种低成本沼气-CCS新思路：以生物质厌氧发酵后的副产物沼液为吸收剂进行沼气CO2吸收，并将产物进行农业生态利用，促进植物生长，使CO2固定于植物机体和土壤中。通过研究沼液与CO2的反应特性，解析反应历程，精确标定CO2反应组分，依此研究沼液CO2吸收反应强化机理和吸收过程强化机制，并以CO2吸收性能、富CO2沼液的植物生理毒性和能耗等为指标，掌握高效低耗的沼液浓缩提升CO2反应组分浓度的机制、筛选合适的添加剂和优化传质强化过程；通过开展富CO2沼液的农业生态利用及碳在土壤中迁移机理研究，估算系统CO2净负排放量，并进行系统评估，期望为低成本CCS提供一种新思路。

本项目提出以生物质厌氧发酵后产生的可再生沼液为吸收剂，在吸收剂不循环的CO2吸收工艺中分离沼气中CO2，并将富CO2沼液（富碳沼液）进行农业利用，将CO2固定于植物机体中，可同时实现CO2分离成本降低与CO2净负排放。项目对沼液CO2吸收机理与强化机制、富碳沼液农业应用与富碳沼液中CO2的迁移转化机理等进行了深入研究，结果表明：（1）沼液吸收CO2主要依靠其自由氨与CO2之间的化学反应。（2）沼液氨氮含量决定了富碳沼液的植物生理毒性，但通过CO2吸收引入到沼液中的碳酸氢根离子可提高植物的氨氮耐受性。（3）77℃和40kPa下对沼液自带CO2进行减压再生，可提高沼液净CO2吸收能力至0.125mol/L，为原沼液的3.18倍，且富碳沼液植物生理毒性更低。同时，20kPa和35-75ºC条件下CO2再生反应活化能为57.78 kJ/mol。（4）提出了“沼液浓缩-吸收剂添加-CO2吸收-沼液稀释”的外源吸收剂浓度增量添加方案，MEA和DEA在CO2吸收性能强化、富碳沼液植物生理毒性及生物降解等指标上表现较优。MEA适合于4倍浓缩沼液情形（添加量0.4 mol/L），而DEA适合于5倍浓缩沼液（添加量0.5mol/L）。（5）为大幅强化沼液CO2吸收性能，提出从沼液中回收可再生氨水并用于沼气CO2吸收的新思路。可再生氨水的CO2吸收潜力为0.987mol/mol，二级反应动力学常数为3933L/(mol•s)。可再生氨水中丁酸含量对其CO2吸收速率的负影响最大，其次为乙酸、乙醇和CO2。可再生氨水回收时平均热耗为3.26MJ/kg，真空泵电耗0.45-1.24MJ/kg。当充分利用余热和清洁能源时，沼液CO2吸收工艺可真正实现CO2净负排放。而当系统消耗的电能来源于煤时，只有当可再生氨水回收能耗较低时，才能实现CO2的近零排放。（6）沼液膜CO2吸收性能排序为：可再生氨水>KOH强化沼液 >CaO沼液>减压再生沼液。（7）富碳沼液可用于水培生菜，且优于化学营养液。（8）富碳沼液中以碳酸氢根离子形式存在的外源C被生菜根系吸收后，一部分直接运输到叶片中，被分解后参与光合作用，另一部分则直接在根系中被分解，生成的水用于保持根系渗透压，而CO2则自由扩散到叶片中参与光合作用。