Assessment of alternative waste technologies for energy recovery from solid waste in Australia

Solid waste can be considered either as a burden or as a valuable resource for energy generation. Therefore, identifying an environmentally sound and technoeconomically feasible solid waste treatment is a global and local challenge. This study focuses on identifying an Alternative Waste Technology (AWT) for meeting this global and local demand. AWT recovers more resources from the waste flow and reduces the impact on the environment. There are three main pathways for converting solid waste into energy: thermo-chemical, biochemical and physicochemical. This study deals with thermochemical conversion processes. Mainly four thermo-chemical conversion processes of AWTs are commonly used in Australia: anaerobic digestion, pyrolysis, gasification and incineration. The main aim of this study is to identify and test the most suitable AWT for use in Australia. A decision-making tool, Multi-Criteria Analysis (MCA), was used to identify the most suitable AWT. MCA of the available AWTs was performed using five criteria, that is, capital cost, complexity, public acceptability, diversion from landfill and energy produced, from which Gasification technology has been identified as the most suitable AWT for energy recovery from solid waste. This study then mainly focused on assessing the performance of gasification technology for converting solid waste into energy both experimentally and numerically. Experimental investigation of solid waste gasification was performed using a pilotscale gasification plant of Corky’s Carbon and Combustion P/L plant in Mayfield, Australia. In this experiment, wood chips were used as feedstock (solid waste) under specified gasifier operating conditions. Syngas composition was measured at different stages of gasification, such as raw, scrubbed and dewatered syngas. Mass and energy balance was analysed using the experimental measured data. It was found that 65 per cent of the original energy of solid waste was converted to syngas, 23 per cent converted to char and 6 per cent converted to hot oil. The remaining 6 per cent was lost to the atmosphere. Firstly, a numerical investigation was performed by developing a computational process model using Advanced System for Process ENgineering (ASPEN) Plus software. Computational models were developed for both fixed bed gasification and fluidised bed gasification processes. A simplified, small scale fixed bed gasification model was initially developed in order to observe the performance of the solid waste gasification process. The model is validated with experimental data of Municipal Solid Waste (MSW) and food waste from the literature. Using this validated model, the effects of gasifier operating conditions, such as gasifier temperature, air-fuel ratio and steam-fuel ratio were examined and performance analyses were conducted for four different feedstocks, namely wood, coffee bean husks, green wastes and MSWs. Secondly, a computational model was developed for the fluidised bed gasification process. The model was validated with experimental data for wood chips (solid waste) measured at Corky’s Carbon and Combustion plant. A very good agreement was found between simulation and experimental results, with a maximum variation of 3 per cent. The validated model was used to analyse the effects of gasifier operating conditions. Using the fluidised bed gasification model, a detailed analysis was done for both energy and exergy in order to achieve a complete picture of the system outcome. Energy efficiency of 78 per cent and exergetic efficiency of 23 per cent were achieved for the system. The developed fixed bed and fluidised bed gasification models were useful to predict the various operating parameters of a solid waste gasification plant, such as temperature, pressure, air-fuel ratio and steam-fuel ratio. This research outcome contributes to a better understanding by stakeholders and policy makers at national and international levels who are responsible for developing different waste management technologies. In future, this research can be extended for other feedstocks, such as green waste, sugarcane bagasse and mixed MSW.