Polymeric wastes include, waste plastics, tires and biomass and each represent large amounts of valuable resources that are currently wasted. The sustainable management of such wastes is a major environmental and resource management issue in the UK, Europe and throughout the world. The concern around climate change has also created great interest in producing non-fossil fuels from alternative sources.
Supercritical water is a unique reaction environment where high reaction rates are produced in the single dense phase found under supercritical conditions. Supercritical water is water above its critical point, (374.8 C temperature and 22.1 MPa pressure) where water exits as a single, dense fluid phase thus minimising mass transfer resistances and providing an environment for rapid reaction rates. Under such conditions, hydrocarbon wastes are easily solubilized and by manipulating the process conditions and with the use of suitable catalysts, higher value liquid and gaseous fuels can be obtained.
The aims of this work are to investigate the use of supercritical water liquefaction of waste plastics, tyres and waste biomass to produce product oils for use as premium grade liquid fuels. Also to investigate the supercritical water gasification of the wastes to produce a hydrogen-rich gas.
The experimental work will use a bench-scale fully instrumented sub-critical/supercritical water reactor to process the waste plastics, tyres and waste biomass over a range of process conditions. A range of sub-critical and supercritical water process conditions of temperature, pressure, feedstock:water ratio, and the use of different types of homogeneous and heterogeneous catalysts will be investigated to maximise the targeted product yield of either premium grade liquid fuels or hydrogen. Gas chromatography will be used to analyse the product gas composition in detail using a suite of gas chromatographs instruments. The product oils will be analysed by oil fractionation using HPLC and analysis of the fractions using combined gas chromatography/mass spectrometry. Such detailed analysis will aid the development of the catalytic mechanism and understand the effect on the liquefaction and gasification characteristics in relation to the supercritical water processing of the different types of waste. Catalysts will be designed and developed to improve the process and end-product properties. Detailed analysis of the catalysts will us scanning and transmission electron microscopy with energy dispersive X-ray spectrometry and X-ray diffraction.
The UK is committed to transforming its economy to ensure a low carbon sustainable future and a reduction in the waste of resources. There is therefore an urgent need to develop waste management processes for polymeric wastes such as waste plastics, tires and waste biomass that are sustainable and can ensure the development of secure low carbon future and secure energy supply. This research thereby benefits the UK by solving a waste management problem for these wastes, that are generated in high tonnages in the UK, to produce higher value premium grade liquid fuels or hydrogen-rich gas.