Technical assessment of bioenergy recovery from cotton stalks through anaerobic digestion process and the effects of inexpensive pre-treatments

Methane yields from cotton stalks have been reported in the range of 0.022–0.150 Nm3/kg-VS in the preceding studies. In this study, hydrothermal pre-treatment of cotton stalks was performed under atmospheric conditions prior to anaerobic digestion process through which, three agents were examined; hot water, dilute ammonia solution containing 2% NH3 (dry matter basis), and recycled liquid from anaerobic process. All three options increased methane yield but the highest results (over 0.200 Nm3/kg-VS) were obtained by utilization of recycled process liquid (PL) under an adjusted ammonia content and C:N ratio, which was achieved by dilution of PL with water as well as application of stabilized swine sludge as a nitrogen rich inoculum. The energy balance evaluation of the overall process revealed that bioenergy recovery from pre-treated cotton stalks by anaerobic digestion is feasible and viable only at feed streams with above 10% solids content, methane yield above 0.175 Nm3/kg-VS and with heat recovery from hydrothermal pre-treatment tank. A methane yield of 0.184 Nm3/kg-VS was observed under semi-continuous feeding of PL treated stalks to a hybrid plug-flow pilot reactor.