Performance and techno-economic evaluations of co-processing residual heavy fraction in bio-oil hydrotreating
Graphical abstract
Introduction
Crude oil is still the major source of transportation fuels. Concern for the sustainability and impact of crude oil on global climate change has increased the importance for renewable and carbon-neutral energy sources for transportation fuel production. Lignocellulosic biomass (e.g., wood, grass, energy crops, and agricultural waste) is a renewable, abundant, and economical source for liquid fuels and carbon-based chemicals production [1,2]. Fast pyrolysis of lignocellulosic biomass offers a fast and relatively simple step to produce bio-oil or pyrolysis oil; however, the bio-oil product has encountered difficulties when directly used as transportation fuel or been blended with petroleum-based materials. This is because raw bio-oil has high oxygen (20−50 wt.%) and water contents (depending on feedstock moisture content) which lead to high acidity, high corrosiveness, high viscosity, low heating value, and poor thermal and chemical stability [[2], [3], [4], [5], [6], [7]]. Consequently, improving the bio-oil’s physical and chemical properties are necessary prior to using raw bio-oil in either existing petrochemical infrastructure or traditional gasoline or diesel engines.
Extensive oxygen and water removal are required to upgrade bio-oils to fuel-range hydrocarbons, while hydrotreating is one of the most common and cost-effective processes already available in existing oil refineries [[8], [9], [10], [11], [12], [13]]. Catalytic hydrotreating involves removal of heteroatoms, oxygen, sulfur, and nitrogen by hydrodeoxygenation, hydrodesulfurization, and hydrodenitrogenation, separately, along with hydrogenation of olefins and aromatics [[14], [15], [16]]. Whereas long-term operation of the hydrotreater system with raw pyrolysis oil is not feasible because of catalyst coking, deactivation, and severe reactor bed fouling/plugging [[17], [18], [19]]. This is mainly owing to the active carbonyl compounds (such as aldehydes and ketones) and phenolics in bio-oil that can react further at higher upgrading temperature through condensation or polymerization reactions to form high molecular weight carbonaceous species [[20], [21], [22]]. Therefore, a critical step for reliable long-term processing is to stabilize the reactive species in bio-oil. This could be implemented by low-temperature hydrogenation that converts carbonyls to more stable alcohols [18,20,21]. These approaches have been integrated together to obtain deeper heteroatom removal and higher fuel-range hydrocarbon yields.
Hydrocarbon products yielded from a combination of stabilizer and hydrotreater can be further fractionated into gasoline, diesel, and jet fuel range products with heavy residues [23,24]. Using the conventional catalytic hydrocracking process, the heavier-than-diesel fraction (residue with boiling point >338 °C) can be broken down to light hydrocarbons in a separate hydrocracker under high temperatures [24]; however, adding a standalone hydrocracking step increases the capital and operating costs for bio-oil upgrading. Techno-economic analysis (TEA) has been used to assess process economics and identify cost drivers of the technology. TEA studies of this fast pyrolysis and upgrading pathway have been performed and published in research literatures [25,26]. The previous study [25], based on projected yields and process model simulation, reported that having a separate hydrocracking section to process residual materials is responsible for about 10 % of the fuel selling price; thus, this work aims to investigate whether the price could be reduced by eliminating the hydrocracker section. The proposed process configuration includes a stabilizer and a high-temperature hydrotreater. Residue materials from the upgrading section will be recycled to the hydrotreating reactor where the residual oil and stabilized bio-oil are co-processed.
We recently briefly reported our success in co-processing of recycled residue and stabilized bio-oil for catalytic hydrotreating and the research and development undertaken at Pacific Northwest National Laboratory on fast pyrolysis and bio-oil upgrading in the last decade including catalyst and process development [10]. Here we provides detailed results on the co-processing performance of recycled residue and stabilized bio-oil for catalytic hydrotreating and their economic impact. First, a stabilized oak bio-oil was tested in a lab-scale, fixed-bed, continuous-flow reactor with commercial NiMo sulfide catalyst to establish the hydrotreating performance baseline. Then, a residue from fractionation of a hydrotreated pine bio-oil was co-fed to the hydrotreater with other conditions (such as temperature, bio-oil liquid hourly space velocity (LHSV), and H2/bio-oil) unchanged to evaluate the co-processing performance. The hydrotreated products were collected at steady-state and analyzed to determine their properties. The overall conversion of co-processed residue was then determined. Based on these experimental results, the TEA of two different process flowsheets—a base case process flowsheet with a separate hydrocracking section and an improved process flowsheet without a separate hydrocracking section—was performed to present the detailed TEA impact of recycling and co-processing residue in the hydrotreating reactor.
Section snippets
Hydrotreating experiment
Two wood-derived pyrolysis oils were used in our work, including oak and pine bio-oils, which were provided by the National Renewable Energy Laboratory and derived from hardwood forest residues and pine sawdust, respectively. Subsequent bio-oil stabilization and hydrotreating used these two bio-oils and detailed properties of the bio-oil samples are discussed in the later section. For bio-oil stabilization, a Ru/TiO2 catalyst (3.0 wt.%) was prepared by the wet impregnation method and employed
Bio-oil stabilization and direct hydrotreating
Pyrolysis bio-oils from oak and pine feedstock were used in this study. Oak bio-oil was initially used to determine its performance for stabilization, hydrotreating, and co-processing with residues. Pine bio-oil was then used to further verify the co-processing performance at a higher residue content for co-processing.
Stabilization of the oak bio-oils was conducted by a standalone hydrotreater using a single-bed Ru/TiO2 catalyst at a bio-oil LHSV of 0.23 h−1 and a temperature of 140 °C (Table 1
Conclusion
Although intermediate bio-oil production from fast pyrolysis and hydroprocessing of bio-oil to fuel range hydrocarbon as fuel blendstock have been demonstrated in laboratory-scale continuous-flow reactors, technical advancements on process development are still needed, such as elimination of a residual oil hydrocracking step, to reduce capital cost. In this work, we recycled the residual heavy fraction from hydrotreating of stabilized pine bio-oil and co-fed with stabilized bio-oils in the
Disclaimer
The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately
CRediT authorship contribution statement
Huamin Wang: Conceptualization, Methodology, Writing - review & editing, Supervision. Pimphan A. Meyer: Methodology, Formal analysis, Writing - review & editing. Daniel M. Santosa: Investigation, Resources. Cheng Zhu: Writing - original draft. Mariefel V. Olarte: Validation, Investigation. Susanne B. Jones: Conceptualization, Methodology, Formal analysis, Supervision. Alan H. Zacher: Conceptualization, Project administration, Supervision.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
The authors gratefully acknowledge the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office for supporting this work. We also thank Marie Swita, Theresa Lemmon, Igor Kutnyakove, and Richard Lucke for their research contributions. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle under Contract DE-AC06-76RL0183.
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