Impurities in biogas: Analytical strategies, occurrence, effects and removal technologies

The growing concerns on the sustainability of our planet have been leading the way to the development of alternative and renewable forms of energy. One example is the use of waste (e.g. sewage sludge, forest residues, agroindustry and food processing) to produce biogas. Comprised mostly of methane and carbon dioxide, it can be used as fuel in cogeneration systems, automotive mobility and domestic heating, among other applications. The increasing interest in the use of biogas has raised some questions regarding its quality for energy valorization. To start, the carbon dioxide content must be adjusted to upgrade the calorific power and economic value and then common potentially hazardous and even toxic impurities such as carbon monoxide, hydrogen sulfur, halogens, or ammonia should be removed to ensure a correct operation. Recently, a close attention was drawn to the presence of siloxanes. These widespread emerging pollutants compromise the use of biogas for energy production due to the formation of abrasive silicon oxides after combustion that deposit on and damage several engine parts. In addition, some of them are potentially toxic and persistent, involving a threat to humans and the environment, which increased drastically the literature devoted to them in the last years. This review provides a comprehensive assessment of the literature available on biogas impurities, presenting their main effects, sampling and analytical strategies, and concentrations. A summary of removal methodologies is also presented to address the technological challenges to cope with these unwanted compounds.