Framework for multistage pre-treatment of anaerobic digestion for maximizing electrical energy production

Abstract

Anaerobic digestion (AD) is a complex process involving several dependent variables. Among critical factors are pH value, temperature and type of pre-treatment of raw material. The change in these parameters affects the overall performance of the system in terms of biogas and methane yield, resulting into varying power output. Different pre-treatments of biomass have different impact on the kinetics of AD. Therefore, the overall electrical output power varies with varying the type of pretreatment and to which extent it is used. In this regard, most of the existing approaches focused only on the multistage reactor design and economic evaluations with single pre-treatment technique. They did not consider the effect of multistage pre-treatment techniques on electrical power output. This research proposes a novel methodology of multistage pre-treatment of organic matters which has the potential to increase the power output from AD to its maximum. The modelling of most common pre-treatment techniques (chemical, mechanical and thermal pre-treatments) of organic matters is presented to calculate the effect of these treatments on the electrical energy production. A framework is developed to evaluate the whole process from pre-treatment to the power output. Multistage pre-treatment is proposed in this research to enhance the electrical energy production from AD. The first order kinetic model of AD is used to calculate the biogas and methane yields and electrical energy as existing literature illustrates that this model is a good choice acceptably for the solution of chemical reactions involved in AD. Three different pre-treatment scenarios, AD with single pretreatment (Case 1), AD with two stage pre-treatment (Case 2) and AD with three stage pre-treatment (Case 3) are considered for the application of the proposed methods. The proposed scenarios are simulated to use different possible number of combinations in all three pre-treatment cases. The highest production of electrical energy achieved was 0.62 kWh, 0.75 kWh and 0.87 kWh for 1 kg of animal wastes for Case 1, Case 2 and Case 3 respectively. The results are compared with the experimental results of pilot scale plant and Anaerobic Digestion Model No. 1 (ADM1). This shows that biogas, methane yield and electrical energy output can be enhanced to approximately two fold by using multistage pre-treatment. The proposed technique is useful for the prediction of bioenergy yield for different organic matters as well as for other bioenergy conversion routes.