Methane production performance comparison of different agricultural residues

Agricultural organic residues are considered as excellent substrates for anaerobic fermentation, producing clean energy and reducing environmental problems.Many researches have proved that agriculture residues can yield substantial biogas.Methane production performance of 7 typical agriculture residues including corn stover (CS), wheat straw (WS), rice straw (RS), banana stem (BS), chicken manure (ChM), cow manure (CM) and swine manure(SM) were investigated under the same fermentation conditions.The volatile solid(VS) of lignocellulosic materials was 91.47%~93.53%, higher than manure residues (42.98%~57.86%), which meant more biodegradable mass could be fermented to biogas in lignocellulosic materials.The unbalanced nutrient would limit biogas production when the substrates were anaerobically digested directly due to the inappropriate C/N.Adjusting C/N to 25~30 by co-digestion and adding urea would be necessary.Contents of sulphur were low in all 7 substrates, which avoided substrate competition between methanogens and sulfate-reducing bacteria, and was beneficial for methanogenesis procedure.Theoretical methane potential(TMP) based on the elemental composition, and biochemical methane potential(BMP) based on the fermentation results were evaluated.The TMP values of the different substrates mentioned above were 500.99, 521.36, 523.21, 449.25, 676.09, 771.63 and 805.98 mL/g, respectively.Data showed that the methane production potential from the substrates of manure was higher than that of lignocellulose.Structural carbohydrates and lignin of CS, WS, RS and BS were investigated in this paper.Results showed that CS, WS and RS, belonging to grass family, had little difference in cellulose, hemicellulose and lignin content.BS, belonging to herbage, contained more cellulose(48.67%) and less lignin(17.40%) than grass family.Biodegradable component in BS was the highest(76.13%), which indicated better methane production potential of BS.EBC(easily biodegradable component) including protein, lipid and NSCD(non-structural carbohydrate) in manure was analyzed.Protein content in ChM was the highest and reached 38.32%.Lipid content in SM was higher than ChM and CM, and the lipid contents for the 3 materials were 11.18%, 1.51% and 4.14%, separately.EBC content in ChM, SM and CM was 59.73%, 47.08% and 39.49%, respectively.The high content of EBC in ChM was evidence of better methane production potential, and adding protease or microorganism that could produce protease would improve methane production from ChM.Anaerobic digestion of lignocellulosic materials was investigated, with the substrate particle size under 40 mesh and the inoculum-substrate ratio of 2∶1.A total of 2~3 peaks appeared in all experiments.For the 1st peak observed, no significant difference existed among the 4 materials.The 2nd and 3rd peaks from BS were higher than other materials.Cumulative methane production of BS was 186.10 mL/g, 1.37 times of the average methane production of other 3 materials.Manure substrates were also anaerobically digested, and 2 peaks were observed during procedure.Daily and cumulative methane production from ChM were higher than SM and CM.Final methane produced from ChM was 224.85 mL/g, which was 1.85 and 2.81 times of SM and CM.Biodegradability(BDA) and digestion time were calculated for 7 substrates, and the results showed that the BDA of BS and ChM was significantly higher than other substrates, which reached 41.42% and 33.28%, separately.Generally, the BDA of lignocellulosic materials was higher than manure.The digestion time of ChM was the shortest(7 d), and that of CS and WS was the longest (15 d).Based on methane production and digestion time, monthly production of methane was calculated, and the results showed that ChM was the best anaerobic fermentation substrate(867.28 mL/g).The BMP data were used by the Gompertz model to determine the optimum parameters to fit 7 substrates, and the maximum methane production was close to the BMP.The reaction rate constant values of ChM and BS were 44.36 and 36.78 mL/(g·d), separately, which were obviously high than other substrates (10.18~18.34 mL/(g·d)).Data in this paper show that BMP can be a better standard compared to TMP, and BS and ChM are more suitable for anaerobic digestion.