Challenges in biogas valorization, effect of impurities on dry reforming process during H2 generation: a review

Abstract

Growing concerns about our planet's sustainability have prompted the need to find alternative and renewable sources of energy. One example is the transformation of bio-waste (e.g., sewage sludge, forest leftovers, agriculture, and food processing) to generate biogas by methanisation process. Biogas is a very promising renewable energy source and is mostly composed of CH₄ and CO₂, and by far is mostly used as fuel in cogeneration systems, vehicle mobility, and household heating, among additional uses. The combustion of biogas releases greenhouse gases, so it is important to convert biogas into cleaner energy carriers such as hydrogen. Dry reforming is one of the possible ways to use the biogas as in this process two of the primary greenhouse gases found in biogas react to form syngas. The efficacy of the dry reforming process is impacted by the inability to completely remove some impurity gases from raw biogas during pretreatment. The main components of biogas are CH₄ (40–70%) and CO₂ (10–50%), with other impurities including water vapor (H₂O), nitrogen (N₂), oxygen (O₂), ammonia (NH₃), hydrogen sulphide (H₂S), carbon monoxide (CO), volatile organic compounds (VOCs; hydrocarbons, organic alcohols, aromatic hydrocarbons, halogenated compounds, and sulphur compounds like carbon disulphide and mercaptans) and volatile methyl siloxanes (VMSs) being the most common components among them. Moreover, the natural process of methanisation of organic waste produces biogas which vary in nature and impurity depending on several factors, such as the initial waste input, the microbial community involved in anaerobic digestion and the process conditions. The present study examines the effects of several contaminants on the dry reforming process, such as NH₃, H₂S, siloxanes, O₂, etc. Reported studies have focused on the development of sulfur-resistant catalysts, as H₂S has the potential to poison catalysts. In contrast, little research has been carried out into the impact of other contaminants present in biogas. This article provides a comprehensive summary of research into biogas impurities, covering their main effects, analytical and sampling techniques, and concentrations for dry reforming reactions for hydrogen production.