Township-based bioenergy systems for distributed energy supply and efficient household waste re-utilisation: Techno-economic and environmental feasibility
Introduction
Over the past two decades the global population has increased by over 1.5 billion, leading to ever greater energy demand and waste volume [1]. A Municipal Solid Waste (MSW) generation rate of 2.2 billion tonnes per annum is expected by 2025 worldwide [2]. In 2015, 31% of all MSW was still landfilled in the EU and about 25% in the UK. This represents a significant percentage. Furthermore, the UK is the largest exporter of waste in Europe, mostly shipping their waste to other European countries, India, Turkey, and China [3]. In Glasgow, UK, the council disposes of around 30 million bin collections every year. To improve the waste treatment and collection process, the first Cleansing Waste Strategy and Action Plan was implemented by the local government in 2010 [4]. Some of the government goals include that no more than 5% of all waste can be landfilled by 2025 and that 70% of all waste will be recycled, composted or prepared for re-use by 2025 [4]. All this clearly indicates that suitable solutions to these issues need to be found.
Significant effort has been put to design sustainable waste treatment systems based on various waste-to-energy (WTE) technologies, such as anaerobic digestion (AD) and gasification [5]. Anaerobic digestion is an attractive way for recovering energy from organic waste, whilst potentially generating a valuable by-product in the form of digestate. Digestate can be utilised as fertilizer for agricultural land application to displace mineral fertilisers [6]. Gasification can recover energy from organic and non-organic waste, making it a more versatile technology. The biochar generated from the gasification process has various potential uses, e.g. soil amendment being one of the most common ones.
The economic and environmental feasibility of AD- and gasification-based WTE systems have been extensively explored by existing studies using cost-benefit analysis (CBA) and life cycle assessment (LCA). For example, Ahamed et al. [7] compared three different food waste (FW) management technologies (incineration, AD, and food waste-to-energy biodiesel) for Singapore and found incineration was the least favoured option for FW treatment on environmental and economic basis. Whiting and Azapagic [8] evaluated the life cycle environmental impacts of AD plants treating agricultural wastes for combined heat and power (CHP). They found that using energy crops, such as maize, as an alternative feedstock reduced the overall global warming potential (GWP) at the cost of increasing 8 of the 11 impact categories considered. Luz et al. [9] evaluated the techno-economic feasibility of municipal solid waste (MSW) gasification and found that the net present value (NPV) was positive for municipalities with more than 35,000 inhabitants based on an annual rate of interest of 7.5%.
In recent years, much research has been conducted on designing decentralised WTE systems due to their advantages over centralised systems in terms of transportation reduction and pathogen transmission alleviation. You et al. [10] evaluated the economic feasibility and environmental impact of a decentralised palm biomass gasification system in Indonesia and found that the electrical efficiency and capital cost both had a significant impact on the economic feasibility of the proposed systems. Patterson et al. [11] compared centralised and distributed biogas infrastructures. CHP with 80% heat utilisation was found to be the most environmentally friendly alternative.
This study explores the techno-economic and environmental feasibility of decentralised waste treatment utilising AD and gasification to tackle the issues of waste pile-up and a need for renewable energy in Glasgow. This is in line with the local government goal of increasing landfill diversion as defined in the Cleansing Waste Strategy and Action Plan [4]. Monte Carlo simulation-based CBA is used to evaluate the economic feasibility. Environmental feasibility is explored using LCA with various impact categories, such as global warming potential. The novelty of this work is twofold. Firstly, the focus of this work lies on decentralised waste treatment systems utilising a combination of AD and gasification. Secondly, the feasibility of such a system is studied in terms of different sub-areas (townships) in a European city (Glasgow). This allows for a comparison of different degrees of decentralisation, as well as a comparison of different WTE technologies. Thus, the focus does not lie on comparing AD and gasification to more commonly employed waste treatment alternatives such as landfilling and incineration; but rather on finding the most suitable waste treatment system, based on AD and gasification.
Section snippets
Gasification
Gasification is a thermochemical conversion technology capable of converting solid waste to syngas (also called synthesis gas) in an oxygen-deficient environment at a temperature range of generally around 550 °C to 1000 °C where the oxidation is too low for stoichiometric combustion [12,13]. The syngas generally comprises of CO, H2, CH4, CO2 and potentially N2 if air is used as a gasifying agent. Moharir et al. suggests a typical H2 content of 33.7% for syngas produced from MSW [12].
This study
Results and discussion
The total electricity generated throughout all six townships of Scenario 6A&G was found to be 204,562 MWh/year. In comparison, scenarios 6G and 3A&G yielded 198,672 MWh/year and 239,301 MWh/year respectively. Households in Glasgow consumed on average 3332 kWh of electricity per year [65]. Based on this, it was calculated that each waste treatment system in Scenario 6A&G covers the annual electricity demand of on average 10,232 households located in its township. In comparison, each system in
Conclusions
This study examined the economic feasibility and environmental impacts of township-based bioenergy systems based on AD and gasification using Monte Carlo simulation-based CBA and LCA. It was found that all scenarios resulted in an avoidance of over 300 kg CO2-eq. per tonne of waste treated (excluding biogenic CO2). Digestate and biochar can have a significant impact on the economic feasibility of a distributed bioenergy system. The BCR distribution lies under 1 for the baseline Scenarios,
Acknowledgments
This research is funded by the Engineering and Physical Sciences Research Council under its EPSRC Vacation Scholars scheme.
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