Cold plasma initiated valorisation of triglyceride containing feedstocks

Abstract

This thesis reviewed current methodologies for producing liquid fuels from triglycerides. With ever increasing demands for sustainable liquid fuels compatible with modern engines, waste cooking oils (WCO) are an attractive source of glycerides. However, transesterification and thermochemical conversion of impure glycerides face significant challenges, including lower quality fuels; unwanted waste glycerol; poor selectivity and catalyst fouling via saponification. To overcome those challenges, a two phase dielectric barrier discharge plasma reactor was developed for the valorisation of oil rich biomass and waste oils by cold plasma with and without catalysts. This thesis reports the valorisation of WCO and transesterification waste products via direct decomposition by cold plasma catalysis within a dielectric barrier discharge reactor. Non-catalytic cold plasma achieved 25% conversion of methanol to hydrogen, carbon oxides and hydrocarbons using a nitrogen carrier gas, and the use of a helium carrier gas notably generated propanal and methyl methanoate at trace yields (0.96 and 0.73wt.%). The synergistic effect of plasma catalysis has been demonstrated by the combination of cold plasma and various catalysts (Ni/Al2O3, HZSM-5, Y-Zeolite and zirconia). For example, 99% conversion of waste glycerol and 50wt.% yields of acetol can be obtained using a Ni/Al2O3 catalyst compared to 91% and 34wt.% with no catalyst. Triglyceride decomposition primarily produces gaseous and liquid hydrocarbons, fatty acid esters and hydrogen, with other products not observed thermochemically (e.g. acetol and formic acid). The two phase plasma system permits formation of thermally unfeasible liquid products as the liquid phase prevents electron induced reactions, stabilising high energy species and inhibiting decomposition of liquid phase products. Full reaction pathways were derived for all feedstocks. Unlike conventional methods, the process is highly tolerant of poor feedstocks and selectivities are tunable to any product. For example, WCO decomposition produced over 50wt.% yields of fatty acid esters in under 11 seconds without requiring methanol, heating or high pressures. Use of hydrogen carrier gas permits high selectivities to low energy reactions, e.g. inducing esterification of free fatty acids with glycerides without a catalyst. Experimental decomposition of glucose and cellulose to valuable furan and pyran products suggests that cold plasma treatment of oil rich biomass for one-step generation of liquid fuels is feasible, offering cost and energy savings, expanding the range of potential sources and valorising waste. Similarly, two phase effects and the tunablity of iv cold plasma presents significant research opportunities to utilise novel synthesis pathways to create value added products. The current study demonstrated that two phase cold plasma could potentially utilise the by-products of commercial transesterification and waste oils to generate sustainable liquid fuels and negate the environmental impact of waste disposal. The findings could also be extended to other liquid feedstocks for applications such as pyrolysis oil upgrading.