Research On The Mechanism Of Demand Oriented Biogas Supply By Codigestion Of Sewage Sludge And Food Waste

With the rapid development of urbanization in China,the generation amount of excess sludge and food waste is increasing accordingly.How to realize the recycling and energization of these two types of waste more effectively,economically and safely has become a major challenge for our nation.Co-digestion of sewage sludge with food waste is an effective way to overcome the relatively low biogas production rate when the sludge is solely digested,and to enhance the renewable energy utilization rate of the sewage plant,as well as to realize the stabilization and recycling of these two municipal wastes simultaneously.But the biogas output is not able to be regulated according to the demand is one of the main reasons which limit the large-scale application of the recycling means.Demand-oriented biogas supply mechanism can provide a pathway for solving this dilemma,but the problem is whether the co-digestion of sewage sludge and food waste is suitable for realizing the demand-orienetd biogas supply has not been scientifically verified.In this context,this paper systematically explores the technical feasibility of co-digestion of sewage sludge and food waste to achieve demand-oriented biogas supply based on laboratory-scale experiments and computer simulation,then designs and optimizes the realization scenario of demand-oriented biogas supply,with the aims of improving the recycling level of food waste and sewage sludge,enhancing the efficiency of waste-to-energy process,so as to promote"zero waste city"strategy by providing new research ideas and relevant theoretical basis.Firstly,this research focuses on the flexible biogas production,which is the main method to realize the demand-oriented biogas supply mechanism,laboratory-scale experiments were carried out to analyze the technical feasibility of flexible biogas production by co-digestion of sewage sludge and food waste.The experiments simulated the pulse feeding under the flexible biogas production mode by setting different feeding control paramertes which mainly include the organic loadings,feeding frequencies and so on;the effects of pulse feeding on the biogas production and methane content were analyzed,and the anaerobic digestion process parameters were monitored to evaluate the stability of the co-digestion system under the pulse feeding process.In addition,a prediction model of biogas production in the co-digestion system under the flexible biogas production mode is established.The experimental results show that the biogas production rate can reach the peak value within 2 hours under each pulse feeding parameters,which proves that the sewage sludge and food waste,as the adjustable substrate of flexible biogas production,owns the high instant feeding response;the parameters of p H,NH₃-N and FOS/TAC fluctuated within the acceptable ranges,which further illustrates that the sudden increase of the instantaneous load from pulse feeding will not affect the stability of the co-digestion system.At the same time,according to the response parameters of biogas production to pulse feeding,it is suggested that 1.9 g COD·L^-1 d^-1 should be selected as the feeding OLR under the flexible biogas production;the paper also discussed how to identify the optimal feeding control parameters according to the actual power demand.Then,the operational effects of flexible biogas production were evaluated by comparing the capacity of biogas storage volume under the continuous biogas production mode and flexible biogas production mode.Finally,based on the experimental data,the accuracy and validity of the biogas production prediction model are verified.Then,based on the matching degree of biogas supplied for power generation and total electricity demand,the paper designs the biogas supply for power generation scenarios,which consist of fully flexible and semi-flexible demand-oriented scenario,and the mixed integer linear programming model was used to obtain the optimal biogas dynamic demand curve based on maximizing the economic benefits of the two scenarios.On this basis,the paper designs and optimizes the realization scenario of demand-oriented biogas supply to adjust the biogas production from co-digestion system,so that the biogas produced can meet the dynamic biogas demand curve.In this paper,a conceptual model of demand-oriented biogas supply mechanism of co-digestion system is constructed,and a combined demand-oriented biogas supply scenario of continuous biogas production and flexible biogas production which is suitable for co-digestion system is proposed.Furthermore,system dynamics model is used to simulate the operation and net present value of the co-digestion system under the combined demand-oriented biogas supply scenario.In addition,in order to reduce the external cost of the demand-oriented biogas supply mechanism and give full play to its advantages,the paper analyzes the auxiliary effect of the government's incentive policy on the demand-oriented biogas supply mechanism,so as to stimulate the operation of the system under the renwable energy quota trading system.The result of NPV calculation shows that the RPS policy can effectively stimulate the demand-oriented biogas supply mechanism,79%of the total electricity demand can be covered by power generated from biogas under this mechanism;at the same time,it is proved that the hybrid demand-oriented biogas supply mechanism is able to integrate the advantages of continuous biogas production and flexible biogas production,and produce higher economic benefits;the hybrid mode should adopt continuous biogas production to supply most of the biogas demand,and the remaining demand should be supplemented by flexible biogas production.In addition,according to the results of NPV accounting and sensitivity analysis,the optimal operation parameters and incentive policies of demand-oriented biogas supply mechanism are determined.Finally,in order to evaluate the sustainability of demand-oriented biogas supply mechanism of co-digestion system,the paper calculates the input of natural environment resources of system from the perspective of"Emergy"pespective,and quantitatively evaluates the safety risk of the system.The results of Emergy analysis show that the sustainable development index(ESI)of the system increases by 55.2%under demand-oriented biogas supply.The results of Emergy flow analysis and security risk quantification were processed by the CCWH model,during which the comprehensive benefit ranking of each system is obtained.The results show that the increase of comprehensive benefit is only4%after the introduction of demand-oriented biogas supply,it is indicated that demand-oriented biogas supply will bring some safety risks due to the expansion of biogas storage volume,but its lower Emergy input and higher economic output will still lead to the highest comprehensive benefits,which proves that the mechanism of demand-oriented biogas supply is an effective mean to improve the sustainability of the system.The results show that it is technically feasible to achieve on-demand biogas supply by the co-digestion system,and the win-win of economic and environmental benefits can be achieved through suitable scenario design,operation parameter optimization and incentive policy assistance.