Elevating biogas upgrading performance on renewable aqueous ammonia solution via a novel “membrane method”

Biogas is usually produced by anaerobic digestion of organic waste such as animal manures and straw wastes, which is a typical green renewable energy and can be used as a fuel for power generation and heat production. China has owned large scale of biogas production, with an annual output of about 15 billion m³ biogas, and the biogas development and utilization provide a new choice for coping with the energy crisis. Factually, biogas contains about 60% CH₄ and about 40% CO₂, and other trace impurities such as H₂S and H₂O. The presence of CO₂ in biogas greatly reduces the combustion performance, and H₂S in biogas also causes a corrosion for pipelines and equipment. Hence, these impurities need to be removed to increase the share of methane in the product, in which this gas removal process is also known as biogas upgrading. The emergence of biogas upgrading technology can not only increase the economic income on biogas plants, but also reduce greenhouse gas emissions. In theory, since biogas has long been considered a typical carbon-neutral fuel, capturing, utilizing and storing CO₂ from biogas is expected to achieve negative carbon emissions.

At present, many mature biogas upgrading technologies include pressurized water scrubbing, pressure-swing adsorption, chemical absorption, membrane separation and cryogenic technology. Among them, water scrubbing and pressure-swing adsorption can simultaneously remove CO₂ and H₂S from biogas, while they have the fatal disadvantages of high CH₄ loss (between 2% and 20%). It is well known that the greenhouse effect of CH₄ is about 21 times that of CO₂. Therefore, many researchers focused on the chemical absorption method, where it presents the negligible loss of CH₄ and the high reaction rate. Coincidentally, because the chemical reaction between the absorbent and the acid gas can form a stable salt, the chemical absorption process has existed the problems of high system energy consumption and CO₂ absorbent loss.

In order to solve the problems of the above chemical absorption method, Prof. Shuiping Yan and his team proposed to a renewable ammonia aqueous absorbent from biogas slurry (a byproduct of anaerobic digestion) combining with the principle of green energy engineering. CO₂-rich renewable ammonia aqueous absorbent can be used as an ammonium nitrogen fertilizer to directly apply into the farmland, thus avoiding the problems on traditional chemical absorbents. Additionally, the team members achieved efficiently simultaneous removal of CO₂ and H₂S from biogas by using renewable ammonia aqueous solvent in a gas-liquid membrane contactor. Compared with the typical physical absorption (H₂S removal efficiency: 48%), the 0.1 mol·L⁻¹ NH₃ renewable ammonia aqueous solvent could remove 97% H₂S from biogas, and the removal efficiency of H₂S is less affected by impurities in this absorbent. When the renewable ammonia aqueous solution was adjusted to 0.5 mol·L⁻¹ NH₃, the biogas can be purified to the pipeline biomethane. Moreover, the optimum operating conditions for the simultaneous removal of CO₂ and H₂S in the hollow fiber membrane contactor were determined through exploring the influence of various operating parameters such as temperature and gas-liquid flow rate during the membrane absorption process, which provided a certain theoretical basis and technical support for the green development of biogas upgrading process. This study has been published in the Frontiers of Agricultural Science and Engineering in 2023, 10(3). DOI: 10.15302/J-FASE-2022473.