Catalytic hydrothermal liquefaction of industrial lignin to bio-oil

Abstract

The negative implications of the world’s current fossil fuel systems are driving researchers to search for alternative energy resources. One of the alternative energy resources that has received attention is renewable fuels produced from oils other than crude oil. However at the moment, renewable fuels cannot compete with traditional fossil fuels due to complications from the production process, lower energy contents and high costs associated with the production. Therefore, if it is possible to improve on the process or even the quality of the feed (bio-oil), an end product with the potential to compete with gasoline or diesel may be produced. Bio-oil produced through hydrothermal liquefaction of non-edible lignocellulosic waste could be the solution. However, the utilisation of bio-oil produced by means of hydrothermal liquefaction as feedstock for biofuel production poses challenges in terms of varying quality of the bio-oil. This can be attributed to the utilisation of different types of biomass and the application of different reaction conditions during hydrothermal liquefaction. This study was aimed at improving the quality of the bio-oil in order to be more compatible with current processes and to improve the efficiency of the renewable fuel production process. In this study, bio-oil was produced from sodium lignosulphonate using ethanol as the solvent and sodium hydroxide as a catalyst at varying process conditions such as reaction temperature (240–320˚C), residence time (20–80 min), and catalyst loading (2–6 mass%). The feedstock, solvent and catalyst were mixed together and placed under anaerobic conditions, using nitrogen, inside a hydrothermal liquefaction reactor for the selected time and temperature to produce the bio-oil and other products. After which the products were separated by venting the gas and washing the liquid product from the biochar with acetone and a Büchner funnel. Rotary evaporation was used to obtain the bio-oil. The bio-oils’ properties and composition were determined using elemental analysis for the elemental composition, Gel Permeation Chromatography (GPC) to determine the average molecular weight and polydisperse index, Fourier Transform- Infrared spectroscopy (FTIR) to identify functional groups present in the bio-oil, calorific values (CV) to determine the energy content, Nuclear Magnetic Resonance spectroscopy (NMR) to assist in clarifying which functional groups are present and Gas Chromatography coupled with mass spectrometry (GC-MS) to determine which chemical compounds were present after hydrothermal liquefaction. The findings showed that the reaction variables affected the bio-oil’s yield and quality. A combined effect was also observed and measured with a severity factor. Residence time

had a larger effect on bio-oil yield at low temperatures, this could be seen in the variation of the severity factor from 5.42 to 6.03 at 240°C. As the reaction temperature increased however, residence time was less influential as seen when the severity factor variation decreases. To achieve higher yields of bio-oil, a higher catalyst loading (5 mass%) was advantageously combined with low temperatures (240°C) and longer residence times (40 min) as it increased the bio-oil yield (20.83 mass%) at the expense of gas production (11.04 mass%). For improved bio-oil quality, higher temperatures were required combined with lower catalyst loadings and shorter residence times. Longer residence times might have provided deeper depolymerisation but promoted repolymerisation; thus, increasing the number of heavy compounds that are present in the bio-oil as seen when the molecular weight and PDI increased from 198 g/mol and 1.8 at 60 min and 320°C to 342 g/mol and 2, respectively when the residence time was increased by 20 min. Shorter residence times produced a bio-oil with a larger diesel fraction (±77 mass%), smaller molecular weights and less heavy compounds, with a high carbon recovery. This study found that a change in reaction temperature had a larger effect than residence time on the quality and quantity of the bio-oil. It seemed as though temperature mostly encouraged decomposition and depolymerization, while residence time controlled the rate of repolymerization. However, it is important to consider the combined effect of the two variables. The loading of an alkali catalyst effected the process by enhancing the effects of reaction temperature and residence time while also decreasing the rate at which cracking reactions are taking place. However the rate of repolymerization was accelerated by the increase in catalyst loading. This study showed that due to the combined effect of the variables, the severity factors can be used as a design tool to predict the outcome of a process with different parameter requirements. It is recommended that more hydrogen-donating solvents and their effect on the bio-oil and Lignex® fractionation be investigated. This study also found that the produced bio-oil contained a large amount of compounds that may have pharmaceutical applications.