Stabilization and upgrading of pyrolysis oil, mixed model compounds, and catalyst deactivation
Doctoral thesis, 2024
Pyrolysis oil derived from fast pyrolysis of lignocellulosic biomass, as well as hydrogenated vegetable oils (HVO) from waste sources, are promising alternative energy sources used to produce renewable biofuels. However, the high reactivity of unsaturated oxygenated compounds in pyrolysis oil results in aging during storage as well as charring and deactivation of the catalyst under the hydrotreatment upgrading conditions. Catalytic mild hydrotreatment is suggested as a means to stabilize the oil before proceeding to hydrodeoxygenation (HDO) for upgrading. This thesis focuses on reactions during stabilization and upgrading of such oils, specifically, the undesirable carbon loss to solid char as well as aspects of catalyst deactivation during biofuel production. The selection of reaction parameters, such as residence time, temperature, oil composition and type of catalyst are crucial for the process.
In this context, a set of catalysts; Pt/Al2O3, Rh/Al2O3, Pd/Al2O3, Re/Al2O3, and NiMo-S/Al2O3 was used for the mild catalytic hydrotreatment of a simulated pyrolysis oil, containing a comprehensive mixture of various oxygenated groups in a batch reactor. The solid products were extracted into soluble and insoluble solid fractions to determine the degree of polymerization. Firstly, it was found that at elevated temperatures (180-300°C) using NiMo-S/Al2O3 the soluble solids transformed into the bio-liquid product and solid insoluble yield was suppressed and its composition changed to fully developed char. It was also accompanied by a higher degree of HDO causing the stabilization of light oxygenates, and a significant decrease in the formation of heavy oligomers in the liquid phase. Further, furan and sugar compounds were identified to significantly influence the yield of liquid products containing stabilized compounds and contributed to solid formation through oligomerization and polymerization reactions. Then, the catalyst screening results showed that Pd/Al2O3 at 180°C was significantly better in terms of producing a high yield (66 wt%) of stabilized oil with the highest selectivity to low molecular weight products and a high char-suppressing potential (3.3 wt% of solids) in which soluble polymers were more pronounced. The results also indicated that NiMo-S/Al2O3 showed a good performance in catalyzing the conversion of reactive compounds, except furans, into stable light oxygenates with the formed solids rich in heavy insoluble solids (13.6 wt%). The Rh/Al2O3 was comparable to NiMo-S/Al2O3, however, Pt/Al2O3 and particularly Re/Al2O3 rendered poor performances with the lowest yields and qualities of the liquid products, consisting mainly of heavy soluble oligomers, which resulted in a high degree of polymerization. Also, the results indicate that accelerated aging converted 79% of the simulated pyrolysis oil into large oligomers, increasing solid formation to almost double after catalytic stabilization. This suggests that aging before stabilization is highly detrimental to the industrial process. Additionally, dewatered pyrolysis oil can be derived by azeotropic distillation, and the treated bio-oil was compared to pyrolysis oil. The dewatering resulted in the elimination of certain volatile compounds or those highly soluble in mesityl oxide/water, particularly acids. Our findings suggest that dewatering the pyrolysis oil before an upgrading process can open new reaction pathways which lead to improved hydrodeoxygenation efficiency.
Furthermore, sulfided metal catalysts, NiMo/Al2O3, NiMo/SiO2-Al2O3, and NiW/Al2O3 along with bare supports (Al2O3, SiO2-Al2O3, and zeolite Y) were investigated after being used in a refinery green hydrotreating unit for several months. Phosphorus, alkali metals like potassium and sodium, common in renewable feedstocks, were identified as major poisons affecting these catalysts. It revealed that metal catalysts specially the NiW catalyst exhibited an increased affinity for poisons compared to the bare supports, leading to a significant decrease in HDO activity for fatty acid HDO in the lab-scale experiments. Attempts to recover NiMo/Al2O3 catalyst activity through solvent washing with DMSO were not successful as this led to further reduced surface area, although the poisons declined.
In this context, a set of catalysts; Pt/Al2O3, Rh/Al2O3, Pd/Al2O3, Re/Al2O3, and NiMo-S/Al2O3 was used for the mild catalytic hydrotreatment of a simulated pyrolysis oil, containing a comprehensive mixture of various oxygenated groups in a batch reactor. The solid products were extracted into soluble and insoluble solid fractions to determine the degree of polymerization. Firstly, it was found that at elevated temperatures (180-300°C) using NiMo-S/Al2O3 the soluble solids transformed into the bio-liquid product and solid insoluble yield was suppressed and its composition changed to fully developed char. It was also accompanied by a higher degree of HDO causing the stabilization of light oxygenates, and a significant decrease in the formation of heavy oligomers in the liquid phase. Further, furan and sugar compounds were identified to significantly influence the yield of liquid products containing stabilized compounds and contributed to solid formation through oligomerization and polymerization reactions. Then, the catalyst screening results showed that Pd/Al2O3 at 180°C was significantly better in terms of producing a high yield (66 wt%) of stabilized oil with the highest selectivity to low molecular weight products and a high char-suppressing potential (3.3 wt% of solids) in which soluble polymers were more pronounced. The results also indicated that NiMo-S/Al2O3 showed a good performance in catalyzing the conversion of reactive compounds, except furans, into stable light oxygenates with the formed solids rich in heavy insoluble solids (13.6 wt%). The Rh/Al2O3 was comparable to NiMo-S/Al2O3, however, Pt/Al2O3 and particularly Re/Al2O3 rendered poor performances with the lowest yields and qualities of the liquid products, consisting mainly of heavy soluble oligomers, which resulted in a high degree of polymerization. Also, the results indicate that accelerated aging converted 79% of the simulated pyrolysis oil into large oligomers, increasing solid formation to almost double after catalytic stabilization. This suggests that aging before stabilization is highly detrimental to the industrial process. Additionally, dewatered pyrolysis oil can be derived by azeotropic distillation, and the treated bio-oil was compared to pyrolysis oil. The dewatering resulted in the elimination of certain volatile compounds or those highly soluble in mesityl oxide/water, particularly acids. Our findings suggest that dewatering the pyrolysis oil before an upgrading process can open new reaction pathways which lead to improved hydrodeoxygenation efficiency.
Furthermore, sulfided metal catalysts, NiMo/Al2O3, NiMo/SiO2-Al2O3, and NiW/Al2O3 along with bare supports (Al2O3, SiO2-Al2O3, and zeolite Y) were investigated after being used in a refinery green hydrotreating unit for several months. Phosphorus, alkali metals like potassium and sodium, common in renewable feedstocks, were identified as major poisons affecting these catalysts. It revealed that metal catalysts specially the NiW catalyst exhibited an increased affinity for poisons compared to the bare supports, leading to a significant decrease in HDO activity for fatty acid HDO in the lab-scale experiments. Attempts to recover NiMo/Al2O3 catalyst activity through solvent washing with DMSO were not successful as this led to further reduced surface area, although the poisons declined.
Catalyst deactivation
Insoluble solid/char
Pyrolysis oil
NiMo/Al2O3 catalyst
Stabilization
Biofuels
Hydrodeoxygenation
Aging
Author
Elham Nejadmoghadam
Chalmers, Chemistry and Chemical Engineering, Chemical Technology
Stabilization of fresh and aged simulated pyrolysis oil through mild hydrotreatment using noble metal catalysts
Energy Conversion and Management,;Vol. 313(2024)
Journal article
Stabilization of bio-oil from simulated pyrolysis oil using sulfided NiMo/Al<inf>2</inf>O<inf>3</inf> catalyst
Fuel,;Vol. 353(2023)
Journal article
Nejadmoghadam, E., Achour, A., Öhrman, O., Creaser, D., Olsson, L., Understanding catalyst deactivation in an industrial green hydrotreater and its correlation with catalyst composition
Nejadmoghadam, E., Öhrman, O., Creaser, D., Olsson, L., Impact of dewatering on pyrolysis oil upgrading: A comparative study of properties and hydrodeoxygenation
Industries are progressively aiming to reduce their dependence on petrochemicals and petroleum-derived materials/fuels by adopting biobased, sustainable alternatives. This transition is primarily motivated by the pressing necessity to combat global warming and decrease greenhouse gas (GHG) emissions. Sweden has set an ambitious goal of achieving net-zero greenhouse gas emissions by 2045, with aspirations for negative emissions beyond that. To achieve this, the demand for renewable, cost-effective, and environmentally friendly substitutes has become crucial, with biofuel production emerging as a prominent substitute for fossil fuels. Today, biomass resources sourced from waste streams—such as animal fat, food waste, agricultural residues, wood and forest residues, and industrial byproducts like tall oil—are gaining significant attention as the large-scale organic carbon source to produce fuels and chemicals. The chemical composition of biomass, which includes carbon, hydrogen, oxygen, and trace amounts of inorganic materials, differs markedly from that of fossil fuels, which primarily consist of carbon and hydrogen. Consequently, refining biomass is not compatible with the existing oil refinery infrastructure. The high inherent oxygen content in biomass reduces its energy density, necessitating the selective removal of oxygen molecules through using robust catalysts via hydrodeoxygenation (HDO) and the elimination of impurities when the target products are fuels.
This thesis investigates the utilization of bio-oil mixed model compounds, and pyrolysis oil as renewable feedstocks for producing biofuel through catalytic hydrotreatment. A major challenge in catalytic hydrotreating pyrolysis oils is the formation of undesired char residues; therefore, they are first hydrotreated into a more stabilized bio-intermediate before undergoing HDO to produce renewable hydrocarbons. We have gained insights into the causes of undesired solid production from pyrolysis oil and its mixed model compounds by examining various reaction conditions and exploring strategies to mitigate char formation.
Additionally, catalysts used in hydroprocessing undergo deactivation, leading to a decline in their efficiency over time. Our research delves into the factors contributing to catalyst deactivation, focusing on the mechanisms arising from impurities in renewable feedstocks. This knowledge can aid in the development of processes that prevent or mitigate catalyst deactivation by designing effective guard beds, similar to those used in fossil fuel refineries, to selectively remove unwanted impurities prior to hydrotreatment, thereby improving the efficiency of biofuel production.
This thesis investigates the utilization of bio-oil mixed model compounds, and pyrolysis oil as renewable feedstocks for producing biofuel through catalytic hydrotreatment. A major challenge in catalytic hydrotreating pyrolysis oils is the formation of undesired char residues; therefore, they are first hydrotreated into a more stabilized bio-intermediate before undergoing HDO to produce renewable hydrocarbons. We have gained insights into the causes of undesired solid production from pyrolysis oil and its mixed model compounds by examining various reaction conditions and exploring strategies to mitigate char formation.
Additionally, catalysts used in hydroprocessing undergo deactivation, leading to a decline in their efficiency over time. Our research delves into the factors contributing to catalyst deactivation, focusing on the mechanisms arising from impurities in renewable feedstocks. This knowledge can aid in the development of processes that prevent or mitigate catalyst deactivation by designing effective guard beds, similar to those used in fossil fuel refineries, to selectively remove unwanted impurities prior to hydrotreatment, thereby improving the efficiency of biofuel production.
Driving Forces
Sustainable development
Areas of Advance
Energy
Subject Categories
Chemical Process Engineering
ISBN
978-91-8103-115-7
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5573
Publisher
Chalmers