6 - Anaerobic digestion of various feedstocks for second-generation biofuel production

Abstract

Methanol is a second-generation biofuel that reduces emissions and provides a clean environment. Methanol can be produced from concentrated carbon sources such as natural gas and biogas. Biogas can be produced from various feedstocks such as food waste, kitchen waste, and wastewater sludge in a floating drum digester by anaerobic digestion. Biogas conversion efficiency depends on the carbon to nitrogen ratio, temperature, and pH value. The carbon to nitrogen ratio is maintained between 20 and 30 for better biogas conversion because bacteria consume carbon at a rate 30 times faster than nitrogen. The optimum thermophilic temperature is varied from 50–60°C and a pH value of 6.7–7.5 has to be maintained. Sodium carbonate, potassium, and sodium hydroxide are added to maintain the pH level. Biogas contains a mixture of methane and carbon dioxide gases as the major content as well as traces of hydrogen, nitrogen, hydrogen sulfide, ammonia, and water vapor. The digestive waste that is rich in nutrients can be used as a soil fertilizer. The biogas produced was purified with ammonia and a silica gel scrubber to remove carbon dioxide, hydrogen sulfide, and traces of other gases.