Pyrolysis Of Digestate Research Articles

Effect of different digestate biochars as promoters via sludge anaerobic digestion on subsequent pyrolysis products: Focusing on the nitrogen, sulfur, and chlorine releasing characteristics

Anaerobic digestion generates a significant amount of byproduct known as digestate, necessitating resourceful disposal. However, the digestate biochar may exert certain impacts on the pyrolysis of sludge, especially regarding the release of N/S/Cl during the pyrolysis process. This may lead to environmental risks and impose restrictions on the recycling of digestate. There is limited existing research on this topic. In this study, the impact and mechanism of digestate biochar on pyrolysis products were analyzed through kinetic analysis, TG-MS, and Py-GC/MS, with a specific focus on the release characteristics of polluting elements during the pyrolysis process. The findings indicate that the Fe-containing digestate biochar, owing to the larger specific surface area, richer functional groups and catalytic activity associated with Fe3+, enhances the production of C/H/O gases during pyrolysis while reducing the release of NH3, NO2, and H2S. Particularly, Fe-containing digestate biochar significantly enhances the release of Cl-containing gases. The decrease in tar content indicates that Fe-containing digestate biochar promotes the catalytic removal of tar. A comprehensive comparison and analysis of the reaction mechanism were conducted in this study, providing a theoretical basis for the release of pollutants from digestate pyrolysis while enhancing the control and management of pollutants containing N/S/Cl.

Production of biofuel from AD digestate waste and their combustion characteristics in a low-speed diesel engine

Anaerobic digestion biogas plants generate large amounts of digestate that cannot always be valorised as fertilizer. This study proposes an alternative use through pyrolysis of the digestate for the production of liquid fuels for compression ignition engines. The digestate pyrolysis oil (DPO) and two types of biodiesel were produced and mixed with different alcohols. A total of five blends of DPO, biodiesel and alcohol were prepared and characterized, showing that their acidity and viscosity were higher than for pure diesel, and their heating value was lower. Blends containing 60 % biodiesel, 20 % DPO, and 20 % butanol were then tested in an engine, showing that the maximum in-cylinder pressure and heat release rate were 4.6 % and 3 % lower, respectively, compared to diesel, and the engine thermal efficiency at full load was 6–8% lower. The nitric oxide and smoke emissions were 7 % and 40 % lower, respectively, but the carbon dioxide emissions were 7–10 % higher than with diesel. The blends showed retarded start of combustion by 1.5° crank angle, which delays the ignition by about 6.4 %. This study concludes that blends can be used as a fuel for agriculture and marine diesel engines, although their viscosity should be reduced by improving the pyrolysis conditions.

Biochar from Digestate Pyrolysis as a Filler for Biopolymer Blends: Effect of Blend Composition

Abstract This study investigates the effect of biochar (BC) as a filler for biopolymer blends, with a focus on the effect of the biopolymer weight ratio on the final BC-added blends. The blends studied in this work were obtained by varying the weight ratio of poly-butylene adipate-co-terephthalate (PBAT) and polylactic acid (PLA) due to their great importance in packaging and agricultural fields. BC has been produced in our laboratories by the slow pyrolysis of the digestate obtained from the anaerobic digestion of the organic fraction of municipal solid waste (OFMSW). After pyrolysis, digestate-derived biochar has been milled and sieved to produce a powdery form with diameter of less than 45 μm. In order to better investigate the filler/polymer interactions, biochar particles were dimensionally, morphologically and chemically characterised. The inhomogeneity of the feedstock is responsible for content and high diversity of inorganics in biochar surface. The effect of BC on PBAT and PLA biopolymer matrices is different, and for the blend compositions the relative weight ratio between PBAT and PLA plays an important role. Furthermore, the biocomposite blend has been fully characterised: rheological, morphological, mechanical and dynamic-mechanical characterisations have been carried out, highlighting how the properties results strongly influenced by the presence of BC in the blend. In addition, a study of the viscous molar mass of the two polymer matrices when processed in the presence or absence of BC particless highlighting that a strong chemical interaction occurs between PLA and BC particles, unlike PBAT and BC. Graphical Abstract

Economic and environmental viability of biofuel production from organic wastes: A pathway towards competitive carbon neutrality

Production of renewable C1 transport biofuels (such as biomethane and biomethanol) through the integration of anaerobic digestion (AD) with carbon capture, utilization and sequestration technologies may offer a solution to reduce greenhouse gas (GHG) emissions. This paper presented a detailed techno-economic and environmental assessment of four cases for biomethane or biomethanol production by incorporating AD, CO2 utilization via biomethanation (CU), solid digestate pyrolysis (Py) and methanol synthesis (MeOH). The results reflected the current state of technologies and potential future scenarios within a mature market. Under optimistic scenarios (scaled-up systems and reduced hydrogen price of 1 €/kg), the minimum potential GHG abatement cost for the AD-Py-CU case was −111.1 €/t CO2-eq when biomethane was sold at 1.03 €/Nm3 (a contract gas price in 2022), while the abatement cost rose to −58.2 €/t CO2-eq when H2 was purchased at 3.40 €/kg. When methanol was sold at 425 €/t (global weighted average value), the marginal abatement cost for the AD-Py-CU-MeOH (with H2 at 1.0 €/kg) case was 136.5 €/t CO2-eq, which is higher than current carbon credits at 33.5 €/t CO2. This study suggests that biomethane produced by incorporating AD, CO2 biomethanation and pyrolysis technologies may be economically and environmentally competitive over natural gas.

Synergistic interaction of tween 20 and magnesium@ functionalized graphene oxide nano-composite for dual productivity of biohydrogen and biochar from onion peel waste

This study aimed at developing a novel nano-composite enhancing fermentation/pyrolysis strategy for dual productivity of biohydrogen and biochar from onion peel waste (OPW). Biohydrogen productivity was maximized by adding tween 20 (T20) and magnesium@ functionalized graphene oxide (Mg@FGO) onto the fermentation of OPW. Three semi-continuous stirred tank reactors (CSTR1, CSTR2 and CSTR3) were respectively supplemented with OPW (control), OPW + T20, and OPW + T20+Mg@FGO and operated at constant retention time of 4.0 d. The hydrogen production rate (HPR) of 343.2 ± 36.2 mL/L/d and hydrogen yield (HY) of 30.30 ± 4.6 mL/g CODadded was obtained by CSTR2 which were greater than those achieved in the control (CSTR1) by 2-folds. Further improvement of H2 productivity >200% was achieved in CSTR3 supplemented with T20+Mg@FGO compared with CSTR1. The addition of T20+Mg@FGO in CSTR3 enhanced acetate fermentation pathway, the enzymatic activities and the biodegradation of phenolic compounds. Moreover, microbial diversity analysis of CSTR3 showed the dominance of the H2-producing bacteria related to Firmicutes, Planctomycetes, and Proteobacteria. Further, digestates collected from the CSTR3 were pyrolyzed at temperature of 550 °C, delivering biochar having multiple surface functional groups. Results confirm that adding T20+Mg@FGO onto the fermentation of OPW followed by digestate pyrolysis could be a sustainable strategy for fulfilling the renewable energy requirements.

Electrofuels in a circular economy: A systems approach towards net zero

Decarbonising the hard-to-abate sectors will be necessary in realising a future net-zero economy. Electrofuels store electricity as low carbon energy vectors such as hydrogen or methane which can be used in areas where electrification is not ideal, and as such can facilitate decarbonisation of sectors such as transport, agriculture, and wastewater treatment. In this study, the production of electrofuels was analysed at an industrial site by storing renewable electricity as green hydrogen produced using electrolysis. The analysis highlighted the need for scale in hydrogen production. The cost of hydrogen was calculated at €8.92/kg when a 122 kW electrolyser operated solely on curtailed electricity generated from the industry site was situated at a 65,000 person equivalent municipal wastewater treatment plant. A subsequent integrated and circular approach to electrofuels production was investigated. The oxygen by-product from electrolysis could be utilised for wastewater aeration and reduce the annual electricity usage at the wastewater treatment plant by 3.6%. Furthermore, the carbon dioxide in biogas generated from sewage sludge could be converted to methane through a Sabatier reaction (4H2 + CO2 → CH4 + 2H2O) as a means of carbon capture and utilisation. The hydrogen produced from the 122 kW electrolyser could convert only 40% of the total carbon dioxide (within the biogas) in a biomethanation process, again supporting the argument for larger electrolyser systems with increased hydrogen production. Pyrolysis of digestate to produce biochar was investigated as a negative emissions technology. If pyrolysis is coupled with anaerobic digestion of feedstocks within 10 km of the industry site savings of 42.7 kt CO2/a could be achieved. In essence, a circular economy approach to electrofuel production could integrate existing electrical, gas and water infrastructure, whilst treating waste, improving the environment, decarbonising agriculture, and storing energy in the form of new low carbon energy vectors for use in heavy transport. Such an approach is vital to progressing future net-zero strategies, however future emissions accountancy processes must adapt to facilitate the benefits of a circular economy.

Evaluation of digestate-derived biochar to alleviate ammonia inhibition during long-term anaerobic digestion of food waste

The feasibility of using food waste anaerobic digestate-derived biochar (FWDB) to mitigate ammonia toxicity in an anaerobic digester was evaluated. The optimal conditions for preparing and adding the activated FWDB were explored using response surface experiments, and the long-term effects of adding activated FWDB on digester performance under optimum conditions were verified in semi-continuous experiments. The results showed that the optimal preparation and addition conditions for activated FWDB were pyrolysis temperature of 565 °C, particle size of 0–0.30 mm, and dosage of 15.52 g·L−1. During the long-term operation of the digesters, when the total ammonia nitrogen (TAN) concentration was higher than 2000 mg·L−1, the control and experimental digesters showed deteriorated reactor performance. Volatile fatty acids in the control digester accumulated to 20,306 mg·L−1 after the TAN concentration increased to 3391 mg·L−1, the methane yield decreased to 31 mL·g VS−1, and the digester experienced process failure. In contrast, the experimental digester with added activated FWDB only suffered a slight short-term accumulation of acetate and a slight decline in methane yield. This may be attributed to the adsorption of NH4+/NH3 by activated FWDB, which reduced the TAN concentration in the anaerobic digestion (AD) system and mitigated ammonia toxicity. Microbial analysis and metagenome predictions demonstrated that the community richness, diversity, and evenness, as well as the abundance of acetogens and related key genes (ACSM1, paaF, and acdA) were higher in the experimental digester than in the control digester. This study provides a closed-loop AD enhancement strategy by pyrolysis of digestate and in-situ supplementation into the digester.

Pyrolysis characteristics of anaerobic digestate from kitchen waste and availability of Phosphorus in pyrochar

Pyrolysis of digestate from anaerobic digestion of kitchen waste yielded combustible gas and liquid with biochar as solid product. In this study, the pyrolysis characterization of the digestate and the P availability of the obtained pyrochar were discussed. Thermogravimetry (TG) analysis showed that 15.56 % of the weight loss occurred from 600 °C to 750 °C due to CaCO3 decomposition. When the final temperature increased from 500 °C to 700 °C, the HHV of pyrogas was slightly affected elevated from 17.20 MJ/Nm3 to 18.12 MJ/Nm3, while the relative content of polycyclic aromatic hydrocarbons (PAHs) in the liquid raised from 1.36 % to 6.28 %. SMT analysis showed that phosphorus in the pyrochar was fully converted to apatite phosphorus (AP) due to an excessive Ca content. Neutral ammonia citrate (NAC) extraction indicated that the availability of P in pyrochar was significantly decreased from 7.97 mg/g at 500 °C to 2.29 mg/g at 700 °C. Overall, the results showed that pyrolysis at 500 °C was suitable to obtain high-quality fuel and pyrochar with high P availability.

Preparation of aromatic hydrocarbons from catalytic pyrolysis of digestate

Catalytic pyrolysis of digestate to produce aromatic hydrocarbons can be combined with anaerobic fermentation to effectively transform and utilize all biomass components, which can achieve the meaningful purpose of transforming waste into high-value products. This study explored whether catalytic pyrolysis of digestate is feasible to prepare aromatic hydrocarbons by analyzing the thermogravimetric characteristics, pyrolysis characteristics, and catalytic pyrolysis characteristics of digestate. For digestate pyrolysis, an increase in temperature was found to elevate the CO, CH4, and monocyclic aromatic hydrocarbon (benzene, toluene, and xylene; BTX) content, whereas it decreased the contents of phenols, acids, aldehydes, and other oxygenates. Furthermore, the catalytic pyrolysis process effectively inhibited the acids, phenols, and furans in the liquid, whereas the yield of BTX increased from 25.45% to 45.99%, and the selectivity of xylene was also increased from 10.32% to 28.72% after adding ZSM-5. ZSM-5 also inhibited the production of nitrogenous compounds.

Enhanced Removal of Methylene Blue Dye by Sustainable Biochar Derived from Rice Straw Digestate

The bioconversion of biomass-based feedstocks to carbonaceous adsorbents is considered a cost-effective and environmentally friendly route for pollutant removal from wastewater. This study focused on preparing biochar from the pyrolysis of digestate of anaerobically processed rice straw, namely digested rice straw biochar (DRSB). The DRSB material was employed as an adsorbent to eliminate methylene blue (MB) from aqueous solutions. The adsorption mechanism was illustrated regarding DRSB characterization via Scanning Electron Microscopy (SEM) related surface morphology and Fourier Transform Infrared Spectroscopy (FTIR) associated surface functional groups. The effect of adsorption factors (solution pH, initial dye concentration, contact time, and adsorbent dosage) on the MB removal efficiency was investigated. The optimum adsorption pH value was 7, achieving MB removal efficiency of 92.98% using adsorbent dosage = 6 g/L and initial MB concentration = 20 mg/L, within 60 min. The experimental data fitted well onto the Langmuir (R2= 0.94) and Freundlich (R2= 0.99) adsorption isotherm models. The Langmuir maximum adsorption capacity (Qm) was estimated as 18.90 mg/g and the Freundlich intensity parameter (1/n) was derived as 0.58, indicating the favorability of MB adsorption onto DRSB. Adsorption kinetics were also tested and explained using pseudo-first-order and pseudo-second-order models, suggesting the contribution of both chemisorption and physisorption mechanisms for MB uptake. The synthesis of DRSB revealed a feasible economic adsorbent with a total cost of 0.3022 US$/kg. This study depicted that the utilization of digested residues resulting from the anaerobic digestion of agricultural wastes for preparing biochar adsorbent would be considered for the real application of dye-laden textile wastewater treatment.

The effects of digestate pyrolysis liquid on the thermophilic anaerobic digestion of sewage sludge — Perspective for a centralized biogas plant using thermal hydrolysis pretreatment

The use of pyrolysis process to valorize digestate from anaerobic digestion (AD) of municipal sewage sludge for biochar production was piloted in a central biogas plant. The pyrolysis also generates pyrolysis liquid with high organics and nutrient contents that currently has no value and requires treatment, which could potentially be done in AD. As the pyrolysis liquid may contain inhibitory compounds, we investigated the effects of adding the pyrolysis liquid on AD of sewage sludge and thermal hydrolysis pretreated sewage sludge (THSS) simulating the full-scale centralized biogas plant conditions. In batch assays, the pyrolysis liquid as such did not produce any methane, and the 1% and 5% (v/w) shares suppressed the methane production from THSS by 14–19%, while a smaller decrease in methane production was observed with sewage sludge. However, in the semi-continuous reactor experiments, pyrolysis liquid at a 1% (v/w) share was added in sewage sludge or THSS feed without affecting the methane yields or digestate characteristics. The laboratory results indicated that pyrolysis liquid can be treated in AD, while extrapolating the results to the centralized biogas plant indicated minor increase in the overall methane production and an increased potential for ammonium recovery.

Preparation of Aromatic Hydrocarbons from Catalytic Pyrolysis of Digestate

Preparation of Aromatic Hydrocarbons from Catalytic Pyrolysis of Digestate

Industrial symbiosis of anaerobic digestion and pyrolysis: Performances and agricultural interest of coupling biochar and liquid digestate

The sustainability of the anaerobic digestion industry is closely related to proper digestate disposal. In this study, an innovative cascading biorefinery concept coupling anaerobic digestion and subsequent pyrolysis of the digestate was investigated with the aim of enhancing the energy recovery and improving the fertilizers from organic wastes. Continuous anaerobic co-digestion of quinoa residues with wastewater sludge (45/55% VS) exhibited good stability and a methane production of 219 NL CH4/kg VS. Subsequent pyrolysis of the solid digestate was carried out (at 500 °C, 1 h, and 10 °C/min), resulting in a products distribution of 40 wt% biochar, 36 wt% bio-oil, and 24 wt% syngas. The organic phase (OP) of bio-oil and syngas exhibited higher and lower heating values of 34 MJ/kg and 11.8 MJ/Nm3, respectively. The potential synergy of coupling biochar with liquid digestate (LD) for agronomic purposes was investigated. Interestingly, coupling LD (at 170 kg N/ha) with biochar (at 25 tons/ha) improved the growth of tomato plants up to 25% compared to LD application alone. In parallel, co-application of biochar with LD significantly increased the ammonia volatilization (by 64%) compared to LD application alone, although their simultaneous use did not impact the C and N mineralization rates.

Environmental assessment of a multifunctional process coupling anaerobic digestion and pyrolysis

Anaerobic digestion (AD) is a waste treatment technology based on organic matter degradation by microorganisms in the absence of oxygen. This process generates two valuables products, biogas and digestate. While biogas is recognized and exploited for its energy potential, digestate could be an interesting fertilizer thanks to its rich-nutrient composition. However, implementation of AD still raises concerns about energy transportation, consequences of digestate spreading and poor conversion to carbon during digestion. To overcome the issues linked to AD, to integrate anaerobic digestion with the pyrolysis of digestate is an innovate path. This study aims to analyze sustainability of a multifunctional process coupling anaerobic digestion and pyrolysis. The process is applied to the treatment of sewage sludge and quinoa residue in co-digestion. To evaluate environmental impacts, life cycle assessment (LCA) is realized from a cradle-to-gate perspective, using SimaPro software (V8.5.2) and Environmental Footprint EF 3.0 method. Results underline that the main contributor to environmental impacts is the treatment of bio-oil, a pyrolysis product, while other impacts could be counterbalance thanks to energy production.

Char derived from food waste based solid digestate for phosphate adsorption

Recovery of phosphorus from waste streams is desirable as natural reserve of phosphorus is being depleted. This study explores the potential of phosphate adsorption using chars produced from anaerobically digested food waste. Two types of feedstocks, dried solid digestate (DSD) and pelletized solid digestate (PSD) are used to produce chars via pyrolysis between 300 and 900 °C. The maximum P sorption capacity of DSD char (100.7 mg-P/g-char) was more superior than that of PSD char (66.0 mg-P/g-char). The X-ray powder diffraction (XRD) result suggests that P was mainly captured by Ca in the chars via the formation of hydroxyapatite. The P speciation and exterior surfaces in chars before and after P exposure (P-laden chars) are quantitatively characterized using 31P solid-state nuclear magnetic resonance spectroscopy and transmission electron microscope equipped with energy dispersive X-ray spectroscopy. Orthophosphate was found to be dominant in both digestate chars and P-laden chars, and the P capture role of nano-sized CaO formed from digestate pyrolysis at 700 °C was observed. The results suggest that chars produced from food waste digestate can reclaim exogenous phosphate. In addition, the P-laden chars rich in hydroxyapatite can be applied as a slow-release P fertilizer or soil remediation agents.

Pyrolysis of digestate from anaerobic digestion on tungsten oxide catalyst

This study presents the catalytic pyrolysis of conditioned digestate from anaerobic digestion co-digestion plants, in the presence of WO3/γ-Al2O3 catalyst under nitrogen atmosphere. The experimental program was performed at atmospheric pressure, temperature in the isothermal reaction zone 425 °C and feeding flow rate of 0.33 mL/min. The analysis of bio-oil composition suggested the presence of a large number of linear and branched aliphatic hydrocarbon components, alcohols, phenols and alkyl aromatic hydrocarbons with two or three methyl-type substituents. The major components present in the cracking gas are methane and carbon oxide. The simulation data obtained for compression at equilibrium of cracking gas indicates an olefin content of LPG higher than the values imposed in the technical specifications of similar products on the market.

Thermochemical processing of digestate from biogas plant for recycling dairy manure and biomass

This paper describes the experimental results of the thermal decomposition of the digestate obtained as a result of anaerobic digestion of dairy manure and dry biomass of Amaranthus retroflexus L. The high yield of Amaranth green mass in the main soil and climate zones of Russia is its advantage over other crops. The ratio of amaranth stems, amaranth leaves, and dairy manure for volatile solids was equal to 1:5:16. Amaranthus retroflexus L. is a weed; therefore, a thermochemical digestate processing is proposed to avoid the seed integrity. The higher heating value (HHV) of digestate was 18.6 MJ/kg. The HHV of the char residue was 19 MJ/kg. In the inorganic part of the digestate sample, the oxides CaO, SiO2, and K2O are prevailed. Thermal decomposition of digestate was studied in the temperature range 25–1000 °C in an inert atmosphere using thermogravimetric and differential scanning calorimetry (TG-DSC) at the heating rates of 5, 10, and 20 °C/min. Char residue mass was ranged from 39.8 to 41.08%. Consequently, the rest of the organic matter components pass into the pyrolysis gas and liquid phase. Additionally, studies of the digestate pyrolysis process were carried out in a laboratory setup (in an inert atmosphere) at the heating rate of 10 °C/min and the temperature of 550 °C. As a result, 31% of pyrolysis liquid, 28% of gas, and 41% of char residue were obtained. The HHV of the char residue increased on 4.8% compared with the initial value of digestate. The main components of the pyrolysis liquid are acetic acid (71.44%) and propionic acid (6.12%).

Catalytic Co-Pyrolysis of Food Waste Digestate and Corn Husk with CaO Catalyst For Upgrading Bio-Oil

The catalytic co-pyrolysis behavior of food waste digestate (FD) and corn husk (CH) with CaO catalyst for upgrading bio-oil was studied. Two kinds of CaO catalysts derived from calcining calcium hydroxide (CH-CaO) and calcium acetate monohydrate (CA-CaO) were used in this work. The char yield decreased while the bio-oil and non-condensable gas yields increased with the CH/FD mass ratio increased. Gas chromatography/mass spectrometry results showed that bio-oil from food waste digestate pyrolysis was rich in nitrogen-containing compounds (NCCs), resulting in the pH of bio-oil exhibited weak alkaline. The acid compounds in bio-oil could adjust the pH to be neutral or weakly acidic with the addition of CH. For catalytic co-pyrolysis, the relative contents of acids and esters in bio-oil decreased while the phenols increased. The contents of NCCs especially the nitrogen-containing heterocyclic compounds (pyridines, pyrroles and indoles) showed an increased trend. Ulteriorly, a reaction pathway of main NCCs during co-pyrolysis process of FD and CH was proposed in this study. Additionally, the use of CaO catalysts reduced the content of CO2 and increased the contents of H2, CO and CH4 in non-condensable gas. Compared to CH-CaO, CA-CaO exhibited better catalytic performance for upgrading bio-oil.

Comparative evaluation on municipal sewage sludge utilization processes for sustainable management in Tibet

In recent years, a sharp increase in the amount of municipal sewage sludge (MSS) in Tibet has posed serious threats to the fragile ecological environment. Tibetan sludge, with a high content of volatile and low heavy metals, has re-utilization advantages, and thus, the selection of appropriate utilization processes for Tibetan MSS is of great importance. In this study, not only the processes themselves, but also other factors including legislations and environmental pollution were investigated. This study introduced the current waste management legislation situation (especially for MSS) in China and Tibet, China. Moreover, a series of SWOT (strength & weakness and opportunity & threats) analyses were conducted to compare anaerobic digestion (AD), incineration, pyrolysis (PY), gasification, and anaerobic digestion coupled with pyrolysis. The results showed that anaerobic digestion coupled with pyrolysis was the optimal treatment option, because anaerobic digestion was suitable for the low oxygen content in Tibet. Although only 50–60% of the organic matter in MSS could be degraded by anaerobic digestion, the residual organic matter (energy) could be further decomposed by pyrolysis, converting it into pyrolytic gas, bio-oil, and biochar, as valuable products. Sludge digestate pyrolysis could reduce environmental risks, save energy, recover materials, and produce high value-added materials. Moreover, it provides a “zero waste” solution for sludge disposal and promotes a “Circular Economy.” The challenges and obstacles of MSS anaerobic digestion coupled with pyrolysis in Tibet were also investigated. This study provides an important technical reference for the comprehensive utilization of Tibetan MSS.

Molten carbonate pyrolysis of digestate with metal-modified HZSM-5 for bio-based monophenols: Kinetics and mechanism study

Molten carbonate pyrolysis is proposed to convert digestate (DE) into biobased monophenols coupled with a metal-modified HZSM-5 catalyst. The results indicated that Fe/HZSM-5 was the most suitable catalyst for this process. The optimum reaction conditions for the process were WHSV = 15 h−1 at a pyrolysis temperature of 430 °C and a catalytic cracking temperature of 475 °C, and 79.22 % of monophenol components were achieved in the bio-oil. A kinetic study on the digestate with carbonates indicated that the alkali metal ion in the molten carbonates attached to the β-O-4 bonds and branch methoxy group of lignin, which promoted the removal of these functional groups, resulting in a reduction in the activation energy during monophenol generation. The Fe/HZSM-5 catalyst reinforced the generation of monophenol compounds in both concentration and yield. Compared with molten carbonate pyrolysis, the Fe on the catalyst not only benefited the breaking of β-O-4 bonds and methoxy groups of lignin but also lowered the temperature during molten carbonate pyrolysis to minimize the secondary cracking of phenolic compounds.