Current knowledge on the air quality impacts and greenhouse gas emissions of methane valorization or production facilities - Study report. Study synthesis

The high potential of biogas activities development raises the question of the real impacts of the biogas sector. This study establishes the state of knowledge of impacts of biogas production and recovery plants on air pollutants and greenhouse gases emissions. This state of art is a statement of direct impacts (for any biogas plant, throughout his life cycle) and indirect impacts (for the particular case of agricultural biogas plants), aiming to propose technical recommendations to control air emissions and research subjects to further knowledge. To date, four priority thematic areas to deepen have been identified: ammonia and nitrous oxide emissions for the digestate recovery step, the uncontrolled emissions of methane in the biogas plant, odorous compounds emissions during feedstock storage and ammonia and methane emissions during digestate storage and treatment. Knowledge about indirect impacts is limited and does not allow to identify and quantify them into details. A mapping of the changes caused by the establishment of anaerobic digestion plant on a farm is proposed in the study. This is a methodological basis for reflection for further developments. The quantitative study of two cases of agricultural biogas plants is a first attempt to quantify the impacts, based on the lessons learned from the state of knowledge. Recommendations by step of anaerobic digestion process are proposed and analyzed according to their technical feasibility, maturity, efficiency and the level of investment needed. Finally, research subjects are presented: they aim at achieving measurement campaigns in installations which are functioning, at producing reference values and at developing methodologies of assessment of the impacts. (authors)