Thermochemical biorefineries for the conversion of lignocellulosic biomass: improvements of fast pyrolysis and hydrothermal processes for fuels and chemicals precursors

In order to mitigate ongoing effects of the climate change due to the use of fossil sources for the production of fuels and chemicals, strong efforts are necessary for gradually switch to renewable sources, thus improving circular carbon utilization. The transportation sector is one of the major contributors to global green-house gas (GHG) emissions and the CO2 generation is expected to grow due to increasing demand for transportation driven by growth in world population as well as economies for developing countries. Additionally, the integrated production of fuels and chemicals is nowadays a standard approach in petroleum refinery where 85% of the entire oil barrel is used for fuel production and the remaining 15% to produce chemicals, that account for about 50% of the overall profits. Thermochemical biorefineries for the conversion of lignocellulosic biomass are a promising route to produce precursors for fuels and chemicals for the transportation sector and chemical industry. However, technical challenges still need to be tackled to improve the thermochemical technologies through economically viable approaches for wider industrial applications. In this work, thermochemical conversion-based biorefineries for the valorization of lignocellulosic biomass were investigated to improve the process technologies toward industrial scale-up and commercialization. More in details, studies on the optimization of the thermochemical conversion process, pathway for co-products extraction and scale-up/commercialization routes were addressed for fast pyrolysis (FP) and hydrothermal liquefaction (HTL) biorefineries. In collaboration with the U.S. National Renewable Energy Laboratory (NREL), fast pyrolysis biorefinery for the conversion of lignocellulosic feedstock (e.g. forest residues) has been investigated through advanced approaches for process optimization and co-products extraction as well as the integration of bio-intermediates into existing petroleum infrastructures. Fast pyrolysis (FP) or catalytic fast pyrolysis (CFP) is a suitable process for the conversion of dry lignocellulosic sources through a thermal and catalytic cracking of the molecular structures. The process take place in absence of oxygen where the solid matrix composing the lignocellulosic biomass (cellulose, hemicellulose and lignin) is thermally cracked to form lighter compounds in form of vapors that are quickly condensed in order to maximize the liquid bio/oil percentage. Therefore, FP/CFP are efficient processes that leads to the production of high yields of liquid bio-oil (up to 80 wt.%). Raw bio-oil from FP has high oxygen content and high acidity as well as chemical instability from these properties that make it incompatible to be directly used as conventional fuels substitute or in blends. Thus, vapor or liquid upgrading is needed to improve the overall bio-oil quality and reduce the costs of final upgrading to fuel hydrocarbons. In this work, experimental studies have been carried out on micro-scale equipment and pilot plant, showing the possibility of preconditioning the FP vapors through a catalytic hot gas filter (CHGF) in order to partially deoxygenates the organic streams forming aromatic hydrocarbons, increasing the presence of valuable phenolics compounds as well as reducing the presence of unwanted corrosive compounds (such as carboxylic acids). Furthermore, the controlled fractional condensation of the partially upgraded vapors was used to validate the upgrading efficacy and to propose a separation pathway that has the potential of roughly concentrate interesting fraction for biorefinery co-products extraction. Thus, potential co-products pathway can be achieved through compounds separation directly from the vapor phase (e.g. through fractional condensation) or from the liquid phase (e.g. distillation or liquid-liquid extraction). In this regard, bio-based co-products from CFP biorefinery have been investigated, extracting organic molecules from pyrolysis liquid streams that can be used as bio-derived active ingredients in insecticides formulations. Fractions of CFP oil were obtained by vacuum distillation and then tested to evaluate the insecticidal activity. A correlation study between the dosage and the insect mortality allowed to individuate the most active compounds. The results showed that compounds with longer alkyl chain in the substituted group of phenol have higher effect on insect mortality. After the experimental campaign, a techno-economic analysis (TEA) as well as a life cycle analysis (LCA) were carried out to validate the plant economic advantages and the environmental benefits in term of energy demand and GHG emissions. An existing CFP plant model was modified in order to include an extraction step comprising two distillation columns. Different simulations have been carried out varying the amount of CFP oil used for the extraction of co-products in order to obtain a correlation between minimum product selling price (MPSP), defined as the plant gate selling price of the product that makes the net present value of the project equal to zero, and the product yield. In addition, starting from the biorefinery model outputs, a life cycle inventory (LCI) has been extrapolated and used as input for the evaluation of the GHG emissions and energy demand of the bio-based insecticides supply chain. The results were compared to the supply chain of traditional insecticides (Pyrethroids and Organo-phosphates) showing large benefits in term of overall CO2 emissions and energy demand when active ingredients are produced from lignocellulosic biorefineries. In addition, in order to improve the growth and commercialization of the FP/CFP technology, and at the same time accelerate the insertion of renewable carbon into the transportation fuels sector, a potential approach would be the integration of biorefineries into existing petroleum refineries through co-processing. Previous studies demonstrated the possibility of introducing FP/CFP bio-intermediates into refinery units, such as fluid catalytic cracker (FCC) or hydrotreaters (HT) without modifying the existing infrastructure. However, the accurate quantification of biogenic carbon ending up in gasoline, diesel, and jet fuels is a key barrier that still needs addressing, as refineries use this measurement to demonstrate compliance to local, state and government regulatory mandates. In this part of the work, the individuation of the limitation of traditional biogenic carbon accounting techniques have been investigated, proposing some new advanced approaches for the implementation of analytical methods directly at refinery sites. Traditional biogenic carbon tracking methods include mass and energy balances or 14C analysis through accelerated mass spectrometry (AMS) or liquid scintillation counting (LSC). However, the overall balances are not precise due to potential small differences in yields when co-processing; on the other hand, traditional 14C techniques are precise, reliable and robust but they are expensive and time-consuming with few facilities available around the world, thus they cannot be implemented directly in-line in refineries. Potential alternatives such as 14C optical analysis and 13C/12C ratio analysis have been selected as the most promising approaches to develop advanced in-line analytical instruments to be applied on refinery units. The other biorefinery concept investigated in this work is based on hydrothermal liquefaction (HTL) for the valorization of wet lignocellulosic residual streams. The work has been carried out in collaboration with the RE-CORD consortium of the University of Florence, studying the hydrothermal liquefaction of lignin-reach stream derived from lignocellulosic ethanol biorefineries. More in details, the HTL biorefinery has been studied through experimental campaigns for process optimization, potential co-product recovery pathways, as well as scale-up perspectives (i.e. model and pilot plant design and commissioning). In contrast to FP or CFP biorefinery, where the thermochemical reactions require very low moisture content, HTL is a suitable process for organic-rich wet streams, in form of slurries. This interesting characteristic makes the technology suitable for several industrial and domestic organic waste streams, such as paper pulp, food residues, sewage sludge and lignin-rich co-products from lignocellulosic ethanol biorefineries. The advantage is that a dewatering step can be bypassed since the liquefaction happens in presence of water at near critical or critical state (i.e. 374 °C and 22,064 MPa). In these conditions, the feedstock first undergoes to an initial degradation into water soluble organics, with water and carbon dioxide as byproducts. Then, repolymerization reactions occurs by different recombination and condensation mechanisms to form water-insoluble liquid organic compounds (biocrude) and water-insoluble solid organic/inorganic compounds (biochar or hydrochar). Among suitable feedstocks, lignin-rich stream from lignocellulosic ethanol biorefineries is a potential candidate for a valorization through hydrothermal processing due to an expected high availability in the mid-long term. Hydrothermal liquefaction offers large opportunities for this wet feedstock but implies several technical challenges that still needs to be deeply tackled. The first part of this study was mainly focused on the HTL overall process characterization through an experimental campaign using batch micro reactors to observe how the operative conditions affected the biocrude yield. Among the parameters evaluated during the experimental tests, high temperature and the use of catalytic additives were found to be the most effective approaches to maximize the biocrude yield. Then, the identification of the main compounds in the biocrude light fractions and dissolved in the aqueous phase have been evaluated, observing how the process conditions influenced the detectable-monomers generation. In addition, since part of the original carbon always remains trapped in the residual aqueous phase a pathway for compounds recovery through liquid-liquid extraction (LLE) followed by adsorption over a polymeric resin was proposed. The backbone idea is to recover valuable monomers that can enter the market as bio-based commodity chemicals (e.g. phenol or acetic acid) or to be used as precursors for materials manufacturing (e.g. precursors for polymers). LLE recovery was experimentally tested processing HTL aqueous phase with four different organic solvents, evaluating the fraction of mass extracted and the selectivity towards some oxygenated aromatic species. Moreover, the raffinate aqueous phase post-LLE was processed over a polymeric resin to adsorb the remaining valuable acidic species. The potential chemicals production in EU and USA trough these approaches was estimated from the experimental data as baseline and the current and future lignin availability data. In order to evaluate the benefit of the aqueous phase valorization, a TEA model has been implemented to compare the introduction of an aqueous phase recirculation coupled with a LLE unit to reduce the costs associated with traditional wastewater treatments. First a HTL plant model have been realized designing every part of the plant for a capacity of 180.000 tonnes/year of lignin. Then, a modification has been implemented introducing the aqueous phase recycle into the process and the LLE for the recovery of commodity chemicals. The minimum biocrude selling price (MBSP) was calculated in both configurations obtaining positive economic benefits of recirculating the aqueous phase and extracting organic mixtures for the chemical industry. In addition, since the technology needs to be investigated at larger scale in perspective of industrial applications, a continuous HTL pilot plant was designed, built and tested to convert 1-2 l/h of lignin slurries at 350°C and 250 bar. To sum up, improvements of fast pyrolysis and hydrothermal liquefaction lignocellulosic biorefineries have been investigated through theoretical and experimental studies, highlighting benefits of specific processes modifications to maximize the main products quality and evaluate co-products extraction routes to improve the overall plant economy. Moreover, pathways for industrial scale-up and commercialization have been individuated or experimentally tested, highlighting technical challenges that needs to be tackled for future broader industrial implementations of the technologies.