Biogas that has been upgraded for use in place of fossil natural gas. The principal constituents of the biogas are methane and carbon dioxide. Renewable natural gasrenewable natural gas Biogas that has been upgraded for use in place of fossil natural gas. The principal constituents of the biogas are methane and carbon dioxide. Renewable natural gas projects capture and recover methane produced at a landfill or anaerobic digestion facility. (U.S. Environmental Protection Agency). projects capture and recover methane produced at a landfill or anaerobic digestion facility. (U.S. Environmental Protection Agency).