CO-PRODUCTION OF HYDROGEN, METHANE AND SILICA FROM RICE STRAW USING ANAEROBIC DIGESTION AND THERMAL TREATMENT PROCESSES IN AN INTEGRATED ENERGY SYSTEM

Abstract

The response to electricity inadequacies and waste management issues in Nigeria is projected to be achieved by designing and employing an integrated energy system in predominantly farm communities to complement the traditional fossil energy. Although the design of the energy unit encompasses detailed aims, the primary objective of the research study is "A solution to Nigeria's energy challenges with hydrogen and methane co-produced from pretreated rice straw in a three-stage anaerobic digestion process and to calculate the energy value of the produced biogas via the combined cooling, heat and power strategy". These research objectives were achieved using acidogenic and methanogenic processes in batch, semi-continuous and continuous systems after pretreating rice straw with mechanical and chemicals/agro-industrial wastes followed by a biological agent. At the same time, silica precursors were produced from the various RS digestate using a thermal procedure, while the energy assessments were done using data from the laboratory study and the literature. The research findings indicated that
 At acidogenesis stage, hydrogen yield was insignificant when chemical agents and agro-industrial wastes were employed alone. However, the daily H2 production increased when the pre-treated (PT) RS residues were biologically hydrolysed with NaOH-PT samples (114 NmL H2 g-1 TS d-1) and PE-PT RS residues (103 NmL H2 g-1 TS d-1) having the highest values at steady states and raw RS producing the least (30 NmL H2 g-1 TS d-1).
 Mechanical and chemical/agro-industrial wastes pre-treatments followed by enzymatic hydrolysis of RS improved the daily specific methane production by 18%, 31.7% and 41.5% for HCL, PE and NaOH-PT RS residues than the raw RS (control) sample.
 In the methanogenesis phase, the methane production efficiency was 80% for NaOH, 75% for PE, 68% for HCl RS PT samples, while the control (raw RS) was 48%.
 Pre-treated and digested methanogenic RS ash samples contained amorphous silica materials confirmed by the scanning electron microscope, Energy-dispersive X-ray fluorescence spectrometer and X-ray diffraction results.
 Using the energy-integrated unit's PEIO data, the net energy (bio-methane, electricity, thermal and cooling) from Nigeria's RS annual yield was estimated. It was found that the net bio-methane at 1.6mpa ranged from 100 to 170 hm3 if small scale processes were applied and 122 to 200 hm3 when large scale operations were used. In the same vein, the predicted electricity produced in TWh available for use was within the range of 8.4 to 9.3. In addition, the net thermal energy available for use was between 12.7 to 13.4 TWh which gave 8.9 to 9.4 TWh of cooling fluid.
 Finally, whereas the Firmicutes, especially the Clostridium, Ruminococcus and Thermoanaerobacterium were the dominant microbial community in acidogenic digestates, the Euryarchaeota typified by Methanobacterium, Methanosarcina and Methanosaeta were the principal phyla for most methanogenic reactors