Fuels can be made by natural products by fermentation. But all oxygen must be excluded for anaerobic respiration to occur. Biogas is mainly methane can be mad by using a range of oragnic waste materials conatining carbohydates.
Many different microorganisms in digestion for waste materials. Waste from sugar factories or sewage works can provide bigas on a large scale. Biogas generators can supply the energy of individual families or farms on a large scale.