Study On Methane Production By Anaerobic Digestion Of Banana Pseudo-stem

Faced with the energy crisis and environmental pollution caused by the increasing consumption of traditional fossil fuels,scientists urgently need to seek sustainable and more ecologically alternative energy.Among them,methane produced by anaerobic digestion（AD）of biomass is a renewable energy source and has important benefits in organic waste disposal,agro-ecology and environmental protection.Banana pseudo-stem（BPS）is a low-lignin-lignocellulosic biomass and suitable for methane production.However,BPS has the characteristics of the high-water content,resistance structure,high C/N ratio,and easy to form scum during AD.These drawbacks are likely to cause an unstable AD process and lower methane productivity.However,an elaborate information for AD treated BPS and its efficient methanation strategy is lacking in the existing literatures,leading to many uncertainties during AD of BPS in practice.In order to improve methane production efficiency from BPS,a systematic study have been conducted as follows:（1）Optimizing the basic conditions of methane production form BPS during AD process.The results showed that the optimal process of AD treated BPS were 10 mm of particle size,10 g VS/L of inoculum,10 g VS/L substrate,7.3 of initial p H,and 26 of initial C/N.Based on the optimal conditions,the effects of four laboratory-scale anaerobic reactor volumes（250,500,1000,and2000 m L）on methane production were studied,and it was found that the larger the volume,the higher the methane yield.Specifically,a methane yield with222.3±10.5 m L/g VS in 2000 m L anaerobic system was closer to the actual yield.However,the trend of daily methane yield curves was basically consistent in different reactor volume.（2）Comparing the effects of different pretreatment methods on methane production form AD of BPS.It was found that the squeezing pretreatment can remove excess water about 50%of BPS mass,and the methane production from squeezed BPS reached 211.5±14.8 m L/g VS,which was 41.2%higher than that of untreated sample.The methane increase was attributed to the improvement of physical properties,such as increasing specific gravity and porosity,as well the destruction of resistant structure of BPS.On the basis of squeezing BPS,further pretreatment using Na OH increased the methane yield by 15.5%.However,a process（biogas plant treated 1 million metric tons of fresh BPS）simulation using Super Pro Designer found that,since adding Na OH treatment for BPS during AD,the return on investment was reduced by 1.12%,and the payback period was increased by 0.86 years.Thus,the additional Na OH-treated process is not economically attractive during AD of BPS.Specially,the transportation of BPS feedstock account for 29.23%of operating cost,which is primary reason why the factory is not profitable.Therefore,the biogas plant should be built near the banana plantation.（3）Evaluating the effects of anaerobic co-digestion（co-AD）of BPS,sugarcane bagasse（SCB）,and chicken manure（CM）on methane production.It was found that a higher proportion of BPS in co-substrate was beneficial to methane production and a significant synergy effect（37%）between three feedstocks above mentioned on methane production.The best combination of co-substrate based on VS was 45.7%of BPS,26.2%of SCB,and 28.1%of CM,correspondingly methane yield was 186.5 m L/g VS,increasing by 16.2%than the mono-AD of BPS.Furthermore,in the semi-continuous test with a low organic loading rate（OLR）（1 and 2 g VS/（L.d））,the methane yield was 0.313–0.469 L/（L.2 d）during co-AD,increasing by 11.4–24.1%than mono-AD of BPS.Due to the stronger buffer capacity at high OLR（3 or 4 g VS/（L.d））,co-AD can still maintain a high methane yield（0.587–0.668 L/（L.2 d））,which was 35.9–101.8%higher than mono-AD of BPS.Compared with the mono-AD based on economic analysis,the return on investment of co-AD increased by 1.6%,while the payback period shortened by 0.98 years.The environmental impact assessment using Open LCA showed that the co-AD can reduce greenhouse gases by 291.2 kg CO₂ eq/t TS organic waste,with an increase by 28.3%than mono-AD of BPS.Therefore,mastering the complementary fermentation characteristics between BPS,SCB and CM can improve the methane production and economic environmental benefits of AD.（4）Developing a conical hydraulic biogas digester with capacity of controlling scum formation.After 30 d of batch and 120 d of semi-continuous tests using BPS,the scum volumes of in cylindrical digesters（simulating a traditional household biogas digester in China）were 4.12 and 2.12 times that in the conical digesters,respectively.Finally,the highest methane yield and its biodegradability of the conical digester can reach 240.6±7 m L/g VS and 80±0.7%,respectively,increasing correspondingly 14.6%and 7.2%than those of cylindrical digester.Therefore,the cone-shaped biogas digester has good performance in controlling scum formation and is beneficial to methane production.More importantly,conical digester is simple to constructed,economical,and attractive in practice.