38. Cogeneration in Anaerobic Sludge Digestion, Biogas Pretreatment, Desulfurization, and Utilization

Biogas produced as a by-product of anaerobic digestion in UWTPs contains 60–70% methane, 30–40% carbon dioxide, and 1–2% of small quantities of other impurities such as ammonia, halogenated organic compounds, hydrogen sulfide, siloxanes, and water vapor. With regard to further biogas utilization, methane is the most valuable component of biogas and can be utilized in cogeneration units. However, although the concentrations of impurities are relatively insignificant, they can have a negative effect upon the cogeneration unit and a varying level of biogas treatment is therefore required that matches the cogeneration technology used. The employment of cogeneration plant at UWTPs can provide an economic, efficient, and sustainable solution for their heat and power requirements. Nonetheless, while biogas utilization is promising from a carbon footprint perspective, challenges remain, due largely to the presence of impurities in the biogas. The most problematic contaminants are H₂S, water vapor, and siloxanes, and a variety of methods for their removal are available. With the exception of biological H₂S removal, all the other methods use physicochemical processes and in addition, there are many different cogeneration technologies for biogas utilization. This review demonstrates that internal combustion engines are by far the most frequently applied technology in the biogas utilization market, but from environmental perspective, some other techniques such as fuel cells and Stirling engines may become increasingly attractive in the future, as they have inherently low NOX, CO, and VOC emission profiles.