Thermo-chemical conversion of biomass for sustainable aviation fuel/fuel additives

Research in renewable energy is essential because of the limited supply of fossil fuel, particularly liquids, and the problem resulting from emissions of greenhouse gases, NOx and H₂S. For aviation/jet fuel, organic liquids cannot yet be replaced by electricity, solar cells, or gas. Currently, CO₂ emission from aviation fuels appears to be a small problem compared to that from road transport, but CO₂ emissions per passenger from fossil derived aviation fuel is actually higher than that from any other mode of transport. Therefore, it is important to look for a renewable substitute which will lead to less net CO₂ emission. The aim of this study has been to investigate and to develop thermochemical routes for the production of sustainable aviation fuels from various biomass sources. The work was focused on liquefaction under CO/H₂O with experiments under N₂ and H₂ for comparison and studied, mainly, the liquid fraction of the product. Widely different types of biomass were investigated to explore which biomass types gave high yield, high quality products as potential precursors to jet fuels. The feedstocks studied were poly-strain microalgae, single-strain microalgae (Chlorococcum sp.), grape marc, soft-wood (radiata pine), hard-wood (blue gum), and fossil wood (or woody coal) for comparison. The biomass types were analyzed by ultimate and proximate analyses, infrared spectroscopy, X-ray fluorescence spectroscopy, pyrolysis gas chromatography-mass spectrometry (Py-GC/MS), and by solid state 13C NMR and, in the case of the algae, by hexane extraction. The biomass types were reacted in 27 mL, 70 mL and 100 mL autoclaves under N₂, H₂, N₂/H₂O, H₂/H₂O and CO/H₂O under a range of pressures (1, 3, 5 MPa (cold)) and temperatures (285-425 °C) and , in the case of reactions under CO, with a range of water to biomass ratios (1:1, 3:1). Catalysts (Na₂CO₃, NaAlO₂, Fe) impregnated into the biomass were added in the case of CO/H₂O reactions. Most experiments were carried out in a batch autoclave system but for comparison some were conducted in a semi-continuous or flow-through configuration. High yields of material soluble in dichloromethane (DCM) and in hexane were obtained under suitable conditions. Thermo-chemical reactions of algae produced significantly higher dichloromethane solubles and generally higher yields of oil and asphaltene than other feedstocks under the reaction conditions used. CO/H₂O tended to give higher yields than N₂, or H₂. CO/H₂O was only effective in improving product yields for biomass or feedstock containing a reasonable amount of carbonyl (-C=O) or free –OH groups. Catalysts added generally did not greatly increase the yield of DCM solubles but did increase the amount of oil (hexane solubles) and thus improved the quality of the products. Different types of biomass behaved differently in different reaction systems. In batch autoclaves, algae gave higher yields of DCM solubles than grape marc or radiata pine, but in the flow-through system, radiata pine gave the highest yields. Thus, the results from the batch autoclave system have to be treated with caution to predict what would happen in more realistic conditions. The DCM solubles and in the case of flow-through products, the condensate, were analysed by elemental analysis, ¹H and 13C NMR, GC-MS and Py-GC/MS. The high heating value (HHV) of the DCM solubles and, in the case of flow-through reactions, of the organic phase of the condensate was always much higher than that of the original biomass. The DCM solubles always contained much less oxygen than the original biomass, but, in the case of algae and grape marc, which have a high nitrogen content, the DCM solubles and the organic phase of the condensate from flow-through reactions were always high in nitrogen, which would have to be removed in an upgrading step to obtain a useful fuel. The alkaline catalysts increased the atomic H/C ratio of the algal products and, at low temperatures, the proportion of product boiling in the jet fuel range. For grape marc, blue gum, radiata pine, and fossil wood catalysts reduced the oxygen content, at least partly by promoting decomposition of the fatty acids to hydrocarbons. The algal DCM solubles obtained at higher temperatures contained a large concentration of aliphatic hydrocarbons and also contained some indoles, phenols, aromatics, nitriles and amides. The grape marc products were intermediate between those from algae and radiata pine or blue gum. The radiata pine, blue gum and fossil wood products were more aromatic and contained few nitrogen compounds. Analysis of the DCM solubles using Py-GC/MS gave more representative results than ordinary GC-MS because some high boiling point, highly polar materials were not detectable by GC-MS. It also gave insight into the chemical transformations of the nitrogen compounds and some of the oxygen compounds in the original biomass during CO/H₂O reactions. The high product yields from algae liquefaction and the large concentration of aliphatic carbons in algal bio-oil indicated algae as the most promising sources of jet fuel, so that the oils derived from algae were subjected to hydrotreating to improve the quality of the products. Upgrading of the bio-oil derived from algae was attempted by hydrogenation in the presence of NiMo/Al-SBA-15 catalysts. To obtain a uniform sample for upgrading, a large quantity of DCM solubles was prepared by multiple reactions in 100 mL autoclaves under N₂ at 355 ⁰C. Attempted upgrading by catalytic hydrogenation resulted in severe coking, particularly with less acidic supports (higher Si/Al ratio). Incorporation of NiMo reduced coking and increased the product yields to up to 65 wt.%. Non-catalytic hydrogenation reduced the nitrogen and oxygen content of the bio-oil significantly and adding catalysts had little effect on either. However, adding supports gave an increased fraction boiling in the jet fuel range (C₁₂ -C₁₇) consisting mainly of hydrocarbons. NiMo catalysts enhanced cracking and eliminated nitriles. The fraction boiling in the jet fuel range of the upgraded product was increased much more by adding NiMo to the supports, particularly when NiMo was deposited on more acidic Al-SBA-15 and when the hydrogen pressure was increased. Under suitable conditions, up to 45% of the upgraded product boiled in the jet fuel range. The main findings are that, of the biomass types reacted, algae are the most suitable starting point for jet fuel and under suitable conditions a high yield of oil can be obtained which can then be upgraded to a product with a reasonable proportion boiling in the jet fuel range. The NiMo catalysts greatly increased the proportion of product in the lower boiling point jet fuel range, by stabilizing products against repolymerization and by promoting cracking and decarboxylation of fatty acids. Thus the catalyst synthesized as described is useful. However, the extent of coking and the nitrogen content of the upgraded oil remained too high, so that improvement in experimental method, by e.g. altering the reactor design and/or by carrying out upgrading in several stages, would be required to obtain a practically useful fuel.