Abstract: The present invention relates to the manufacture of high calorific fuel gas through a process of anaerobic digestion involving retention of a slurry of organic material, maintained at a predetermined volume-to-interface (V/I) ratio and for a desired liquid retention time, pH and temperature to produce substantial amounts of methane gas while minimizing the production of carbon dioxide gas. When operating in the preferred range of volume-to-interface (V/I) relationships, the faster molecular diffusion rate of methane from the slurry as compared with carbon dioxide gas allows virtually all of the methane to be collected from the slurry while a considerably smaller amount of carbon dioxide gas is collected. Instead, the bulk of the carbon dioxide remains in the spent slurry which is transferred as sludge to a loop for carbon dioxide stripping. Liquid retention is controlled by a recirculation-stripping loop.

Abstract: Conventional anaerobic digestion, as practiced in municipal waste treatment plants, is modified to yield high-quality methane gas. Upwards of 98 percent methane is produced rather than the normal 60 to 70 percent methane (diluted with carbon dioxide).The process, wherein digestion is conducted under several atmospheres of pressure, involves the application of Henry's Law. Digesting sludge is used as a scrubbing agent. According to one system a recirculation loop features pressure release and degassing of carbon dioxide. Degassed sludge is then pumped, under pressure, back into a digestion tank. The recirculation rate is designed to maintain sludge in the digester in an unsaturated state with regard to carbon dioxide solubility. This keeps the carbon dioxide from precipitating out of the sludge within the digestion tank and results in high purity methane production. Another system accomplishes similar results by periodically depressurizing the digestion tanks to allow carbon dioxide to escape.