Spatially explicit economic and environmental optimisation of bio-SNG production systems integration with the Irish gas network

Abstract

Biomass-derived synthetic natural gas (bio-SNG), produced from biomass through gasification and cleaning and conditioning of syngas, is an energy carrier compatible with the existing gas infrastructure and appliances, is a promising solution for the decarbonisation of transport, heating and electricity generation. However, low energy density and scattered distribution of the resources, technological complexity and high cost of the conversion process, undermine its sustainability. This thesis aimed to deliver a techno-economic and environmental assessment of bio-SNG production, and to build a model that allows identification of the sites and the sizes of the plants for bio-SNG production that minimises the cost and the environmental impact of the bio-SNG produced in the case of the Republic of Ireland. Published life cycle assessments (LCAs) of bio-SNG and biomethane have been reviewed. None of the studies integrated their LCA with both process and geospatial modelling. Therefore a framework including process modelling, to characterise the conversion, and geospatial modelling, to investigate the supply chain configuration, has been proposed in order to assess the environmental and economic sustainability of a spatially-explicit bio-SNG production system and applied in this work. An initial resource assessment identified the distribution and the quantity of all the available resources that can be converted into bio-SNG, showing that 2.5 PJ a-1 of bio-SNG can be provided by forestry residues. A thermodynamic model has been built to simulate the GoBiGas process, a large-scale bio-SNG production plant. This real-case process has been also used as a term of comparison to satisfactorily validate the mass and energy balances and efficiencies resulting from the model. The economies of scale of the process are calculated and finally a geospatial model has been built in order to find the size and the sites of the plant that minimise the unitary cost of bio-SNG produced. The resulting configuration showed that 66.7% of the available forestry residues is processed into a single 71-MW plant, at a cost of 86.3 €/MWh. The environmental impact is then included in the optimisation, as an aggregated single-score of 10 impact categories (climate change, ionising radiation, ozone depletion and resource depletion, eutrophication and acidification potentials, photochemical ozone formation and particulate matter). The single-plant configuration is the one that presents Pareto optimal solutions when compared to multiple plants and considering the minimisation of LCOE and environmental impact as objectives. A 67.6-MW plant minimises environmental impact per Euro spent, presenting benefits for 4 out of 10 of the impact categories considered (climate change, ionising radiation, ozone depletion and resource depletion). The economic and energy potential of bio-SNG from digestate produced from anaerobic digestion (AD) of the organic fraction of municipal solid waste is also assessed as a waste management alternative, resulting in a cost between 89.5 and 109.6 €/MWh, showing results comparable to AD and landspreading of the wet digestate or combustion of the digestate. The methodology presented in this thesis can be applied to any other region where geospatial data are made available and extended to other resources and/or products, providing gas transmission system operators with a tool to integrate renewable gas production technologies into existing infrastructure, minimising the costs and the environmental impact, and evaluating the competitiveness of the renewable gas produced when compared to natural gas.