Catalytic pyrolysis of biomass and subsequent bio-oil co-processing in fluid catalytic cracking (FCC) for biofuel production
Publication Date
December 13, 2022
Creator
Abstract
Biomass from waste materials is an attractive alternative to the approach of the production of sustainable biofuels and waste management practices. However, current lignocellulosic conversion processes centered around pyro-oil hydrotreating using costly hydrogen from fossil source to produce biofuels, hence may not favorably compete with existing transportation fuels. However, if green hydrogen from, for example, water splitting via an electrolyser could be used in hydrotreating, then it will be more attractive than other upgrading processes. This is because more than 90% of oxygen can be removed in the form of water to produce pure hydrocarbons with high carbon efficiency. For now, a viable solution is to catalytically crack the pyro-oil in situ or in one thermal cycle using fluid catalytic cracking (FCC) zeolite catalyst.
This research seeks to address some of the key issues associated with bio-oil quality, presently limiting its use as a drop-in biofuel and for large-scale production. Bio-oil is a complex mixture of oxygenated compounds with low heating value, high acidity, and high polarity. Therefore, slow pyrolysis experiments of pinewood, beechwood, poplar and spent coffee ground (SCG) were investigated in a fixed bed laboratory reactor. This was to evaluate the effect of biomass compositional differences on bio-oil oxygenates content and the relative yields of gas, liquid, and solid to enable a comparison between products characteristics. Interestingly, bio-oil obtained from SCG recorded the lowest oxygen content, highest calorific value and is richer in hydrocarbons but with a high concentration of nitrogen compounds than bio-oils from other biomass samples studied. Furthermore, the effect of pyrolysis temperature has been studied to assess the optimum temperature for bio-oil yield and quality in terms of lower phenolic compounds, especially phenolic ethers which are the main precursors of coke that deactivate catalysts in FCC.
Upgrading bio-oil is a necessity for it to be utilised in processes that produce drop-in fuel. Most of the research into catalytic pyrolysis is centered around the expensive noble metal catalyst as well as fresh commercial or synthesised catalyst. In this research, catalytic pyrolysis of biomass using FCC equilibrium catalyst was carriedout and produced bio-oil with less oxygen content and better fuel qualities. However, the FCC equilibrium catalyst was not very effective in deoxygenating phenolic and acidic compounds. Therefore, thermal, and hydrothermal pre-treatment of fresh FCC catalyst using steam was conducted before the catalytic pyrolysis micro activity test (MAT). This was necessary because the catalytic activity of the fresh catalyst is an insufficient measure of its real commercial performance. It has been demonstrated that hydrothermal treatment of FCC zeolite catalyst using steam at 786 °C suppressed coke yield and with an insignificant change in oxygen content compared to the non-hydrthermal FCC catalyst. The steamed FCC catalyst formed higher bio-oil and gas yield, higher calorific value, and lower coke yield compared to thermal-only pre-treated catalyst.
Finally, despite the improvement in catalytic bio-oil fuel properties, it is still not comparable with conventional fossil fuels which contain few or no oxygenated species. To overcome this challenge, catalytic pyrolysis oil (CPO) and SCG bio-oil were further upgraded by blending with conventional vacuum gas oil (VGO) and co-processed in MAT reactor using FCC equilibrium catalyst to produce biofuels including gasoline, diesel, LPG, and other light gases. The level of bio-oil that could be incorporated into the fluid catalytic cracking (FCC) unit was investigated using bottoms fuel oil (BFO) residue from a transesterification process. It was found that an increase in the blending ratio of the bio-oil increased coke formation and lower gasoline yields. Before co-processing experiments were conducted, three different VGOs were investigated to know whether the quality of the VGO feedstock could affect the co-processing ability with the bio-oil and the best candidate VGO and FCC equilibrium catalyst were selected that gave optimum gasoline yield and less coke selectivity on the catalyst after the MAT reactions.
In conclusion, the above findings demonstrated the feasibility of co-processing bio-oil in the FCC reactor. The drawback of this process is that only a small fraction of bio-oil can be blended to maintain the closest yield characteristics compared to pure VGO cracking in an FCC reactor. Therefore, future work should focus on designing catalyst that is very effective in removing oxygen and at the same time increasing the blend ratio of bio-oil with VGO in FCC.
Item Type
ethesis
Thesis Type
PhD
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Subjects (LC)
Associated Schools / Departments
Faculty of Engineering
Department of Chemical and Environmental Engineering (UK)
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eprints ID
69613
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