Optimisation of biogas production from co-digestion of water hyacinth, municipal solid waste and cow dung

Abstract

The acceleration and integration of renewable energy technologies (RETs) is at the core of research and development in a bid to deal with climate change issues as well as to ensure sufficient energy access to all. The incorporation of RETs requires thorough investigations to ensure that energy generation via these routes is maximised to obtain optimal yields and that their assimilation into the existing energy demand and supply mix is smooth and competitive. Anaerobic digestion is one such renewable energy technology avenue, which produces bioenergy in the form of biogas, a biofuel. Anaerobic co-digestion of different substrates is reported to increase biogas output volumes owing to the optimistic interactions created in the digestion medium, microbial variations in diverse substrates as well as provision of missing nutrients by the co-substrates. This will help to deal with the issues of environmental sustainability since wastes will be converted to energy as well as help to strike a balance between energy demand and supply. In order to maximise the overall biogas yield from co-digestions, modelling and optimisation using specific substrates such as water hyacinth, cow dung and municipal solid waste is necessary. This necessitates the need for mathematical modelling and application of optimisation tools in biogas production to accurately arrive at optimal parameters such as the co-digestion substrate mixing ratios as opposed to just the experimental approaches which are more of a trial and error way of getting these optimal feed ratios. The overall optimal yields are affected by the time of the year and the environment from which the substrates are derived from since these dictate the amount and quality of the same. Biogas production and optimisation models developed to date do not account for the accuracy of co-digestion blending ratios, the improvement of the quality of biogas, geographical/environmental and seasonal variation of substrates. The integration of biogas in hybrid systems to cater for energy demand has not been dealt with in an in-depth way targeting energy cost reductions and minimisations of fossil fuel usages. This study aims to enhance biogas production from the co-digestion of varied substrates (water hyacinth, municipal solid waste and cow dung are used in this research work) by way of exploring simulation, modelling and optimisation approaches in combination with mathematical analytical tools. Firstly, a survey of previous works on the subject matter is conducted to investigate the status, current trends and future perspectives of anaerobic co-digestion, modelling and optimisation with focus on enhancing biogas yields. A model for biogas production is built based on Boyle’s modified Buswell and Mueller equation (3.2) in which carbon, hydrogen, oxygen, nitrogen and sulphur make up the elemental constuents of the biomaterial composition and methane, carbon dioxide, ammonia and hydrogen sulphide constitute the biogas product. Baseline biogas potential yields of 747.4 Nml/gVS, 790.83 Nml/gVS and 884.24 Nml/gVS were obtained from water hyacinth, municipal solid waste and cow dung respectively in a case study. The formulated model is further developed and optimised to give optimal co-digestion substrate blending ratios for varied co-digestion mixtures of which water hyacinth, municipal solid waste and cow dung are used for the purposes of this study. The optimisation problem is solved using a linear programming mathematical approach in MATLAB. Optimal co-digestion results in co-digestion percentage substrate blending ratios of 53.27 : 24.64 : 22.09 for water hyacinth, municipal solid waste and cow dung respectively in a case study. 1 kg of substrate mixture yields 124.56 m3 of biogas which translates to 124 560 Nml/gVS. Co-digestion and optimisation of substrate blend mix proportions increased the biogas output by 157.11 %. Seasonal variations in the availability of co-digestion substrates are incorporated in an advanced formulation and development of a co-digestion model in which the methane component of biogas is maximised whilst the other components (carbon dioxide, ammonia and hydrogen sulphide) are minimised so as to improve the quality of the biogas. The formulated problem was solved using the Optimisation Interface tool (OptiTool) in combination with the Solving Constraint Integer Programs (SCIP) toolbox in MATLAB. Finally the methane-optimised biogas is hybridised with liquid petroleum gas in a bid to cut down import costs as well as to lower pollutant emissions from the liquid petroleum gas fossil fuel which is conventionally used (by a community in a case study) for heating and cooking purposes. Consideration of seasonality changes in the availability of substrates in the modelling and optimisation led to an increase of 174.58 % in annual biogas output. A 6.97 % annual lowest cost savings was realised in winter and 18.24 % annual highest cost savings was realised in summer from the methane-optimised biogas-liquid petroleum gas hybrid system. Physical laboratory experimental approaches towards biogas production enhancement and optimisation are out of scope of this study. However, their integration with this particular kind of work together with multi-stage co-digestion is recommended for future studies.