<figure><img src="image_1.png" /></figure>

HAL
open science

# Mass, carbon and energy balances of thermochemical processes for digestate valorization

Rémi Demol, Mohamed Aissaoui, Paola Gauthier-Maradei, Cecilia Sambusiti, Jasmine Hertzog, Vincent Carré, Anthony Dufour

► To cite this version:

Rémi Demol, Mohamed Aissaoui, Paola Gauthier-Maradei, Cecilia Sambusiti, Jasmine Hertzog, et al.  
Mass, carbon and energy balances of thermochemical processes for digestate valorization. Energy Con-  
version and Management, 2025, 332, pp.119759. 10.1016/j.enconman.2025.119759. hal-05041880

HAL Id: hal-05041880

https://hal.univ-lorraine.fr/hal-05041880v1

Submitted on 22 Apr 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

Check for
updates

# Mass, carbon and energy balances of thermochemical processes for digestate valorization

Rémi Demol<sup>a, ∗</sup>, Mohamed H. Aissaoui<sup>a, †</sup>, Paola Gauthier-Maradei<sup>c</sup>, Cecilia Sambusiti<sup>d, ‡</sup>,
Jasmine Hertzog<sup>b, §</sup>, Vincent Carré<sup>b, ‡</sup>, Anthony Dufour<sup>a, ∗</sup>

ARTICLE INFO

Keywords:

Digestate

Steam gasification

Combustion

Fast and slow pyrolysis

Hydrothermal liquefaction

Aspen Plus

A B S T R A C T

The aim of this study was to evaluate the valorization of digestate collected from a collective biogas plant. Raw digestate was converted by a hydrothermal process by using a fast-heating induction autoclave. Raw digestate was alternatively dewatered by a screw press (from 96.3 to 84 wt%<sub>raw</sub> basis of water content), dried and then converted by fast pyrolysis, steam gasification, and combustion (in a fluidized bed reactor at 500–900 °C) and slow pyrolysis (at 400–800 °C). The products (gas, liquid and char) were thoroughly analyzed. Surrogate molecules were defined based on high resolution mass spectrometry analysis in order to better describe the complex liquid oils. The experimental results were modeled under Aspen Plus to assess the mass, carbon and energy balances of all these processes. Most of carbon (79 wt%) and organic matter present in raw digestate remained in the liquid phase after screw press separation. Therefore, the hydrothermal conversion converts a much higher yield of organic matter because it does not require a mechanical dewatering of digestate. The energy efficiencies for all the thermochemical processes were quantified as a function of the dewatering step and the thermochemical conditions (temperature, residence time, etc.). The heat required to dry the solid fraction of the digestate (prior to its dry thermochemical conversion) can be produced by the biogas generated by the anaerobic digestion.

1. Introduction

The circular economy has recently seen increased interest in indus-
trial synergies that combine technologies to enhance energy production
and waste management [1]. Several waste processing technologies, such
as anaerobic digestion (AD), combustion, pyrolysis, gasification, and
hydrothermal liquefaction processes, have been developed to treat
wastes and produce energy simultaneously.

Anaerobic digestion allows waste management as well as bioenergy
production throw a biological process to convert organic matters from
different feedstocks into biogas. One benefit of this approach is that it
makes it possible to recover energy from wet waste biomasses by valo-
rizing the biogas produced by bacteria. On the other hand, it generates
digestate, a valuable fertilizer [2]. Due to this dual interest, the number
of AD reactors has been quickly rising in recent years. However, it
should be noted that the direct soil disposal of digestates is not always

possible depending on the chemical composition of feedstocks fed to
digester and, in some cases, without a proper pre-treatment.

Therefore, the thermochemical conversion of digestates (i.e., pyrol-
ysis, combustion, gasification, and hydrothermal liquefaction) could be
an interesting valorization route to produce renewable carbons (char)
and energy [3]. Several reviews have focused on coupling biological and
thermochemical processes, notably on char valorization [2,4]. Monlau
et al. [5] extensively reviewed the valorization of solid digestates for
energy production by thermochemical processes, mainly converting
digestates by slow pyrolysis to produce chars. However, references
dealing with the fast pyrolysis of digestates are lacking.

Wang et al. [6] reviewed the main valorization routes of chars pro-
duced from different digestates under various pyrolysis conditions.
Digestate chars are mainly envisioned as adsorbents to remove heavy
metals (or antibiotics) in wastewater or manure [7,8], to increase mi-
crobial activity during anaerobic digestion [9,10], or for soil

<footnote><sup>a</sup> Université de Lorraine, CNRS, LRGP, F-54000 Nancy, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, LCP-A2MC, F-57000 Metz, France</footnote>

<footnote><sup>c</sup> Total Energies, CRES, F-69360 Solaize, France</footnote>

<footnote><sup>d</sup> Total Energies, CSTJF, F-64000 Pau, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: anthony.dufour@univ-lorraine.fr (A. Dufour).</footnote>
amendment [11]. The conversion of digestates by hydrothermal carbonization (HTC) was also extensively studied [12–15], often conducted in lab-scale autoclaves, with some limited attempts for up-scaling in continuous processes.

Lucian et al. [16] showed an interesting synergy between AD and HTC of digestates: HTC liquids and slurries can enhance methane production by AD. Cao et al. [17] confirmed this interest in combining HTC with AD for maize silage, as the aqueous phase of HTC exhibits a high bio-methane yield (by AD). Aragon-Briceno et al. [18] also demonstrated the high interest in combining HTC and AD, as HTC increases the net energy production by using hydrochar as a fuel source.

Dieguez-Alonso et al. [19] compared slow pyrolysis and HTC of pine wood and digestate derived from corn methanization, focusing on char characterizations. They showed that chars produced from pyrolysis exhibit higher specific surface area than HTC. Funke et al. [15] presented an interesting study on the effect of water during the slow pyrolysis of digestates, showing that steam thermal carbonization allows for a higher solid content (solid mass on dry basis per total mass of feedstock) because the digestate is subjected to saturated steam instead of liquid water. The carbon losses in the liquid phase were decreased and less water was heated up during carbonization. A mechanical dewatering step of wet digestate increases the process efficiency of carbonization and it should be considered in practice.

Concerning gasification, Chen et al. [20] studied the air gasification of digestates in a batch fixed bed reactor. It was feasible despite the high ash content of digestate. An optimal equivalence ratio (ER) of 0.28 was found. Antoniou et al. [21] conducted experiments on gasification of dried digestates in a downdraft fixed bed reactor to produce a medium lower heating value syngas. Balas et al. [22] reported the air gasification of digestates in a pilot fluidized bed. No study dealing with the steam gasification of digestates was found.

Despite numerous experimental works, there is still a lack of available data on the mass and energy balances of the complete process for thermochemical conversions of digestate. For this purpose, developing detailed process modeling, such as in the software Aspen Plus [23–25], is key. Our group has previously developed advanced process models for various pyrolysis, gasification, or combustion routes (but not yet for digestates) [25–29]. To the best of our knowledge, there is still no report (neither experimental nor modeling) comparing the conversion of digestates by hydrothermal liquefaction, pyrolysis, gasification, and combustion.

For this reason, we address this lack of comprehensive data on mass and energy balances for the complete processes. The novelty of this present study relies on these 3 main points:

1) We conducted comprehensive experiments using the same digestate sample for hydrothermal liquefaction, “slow” and “fast” pyrolysis, steam gasification, and combustion processes. Detailed analyses of all products (gas, char, liquids) have been carried out, including high-resolution mass spectrometry analysis of liquids.

2) The experimental data were modeled using Aspen Plus software to assess the mass and energy balances for all thermochemical conversion processes over a wide range of operating conditions.

3) We propose a novel strategy to model complex liquids in Aspen Plus by defining surrogate molecules based on high-resolution mass spectrometry analysis.

This study provides a global comparison of the thermochemical conversion processes for carbon and energy recovery in digestates.

# 2. Materials and methods

## 2.1. Digestate samples

Raw, liquid and solid digestates were collected from a collective biogas plant fed with 30 wt% agro-industrial residues, 50 wt% of silage

maize residues and 20 wt% of manure.

## 2.2. Thermo-chemical processes

Digestate fractions were converted through five thermochemical processes, represented in Fig. 1. Except for hydrothermal liquefaction (HTL), all processes required a digestate dewatering step by screw press separation followed by a drying (at 40 °C for 24 h) and pelletization step. On the contrary, hydrothermal liquefaction could directly process the raw digestate (at 96.3 wt% of moisture content).

The different conditions studied for each process are presented in Table 1. The fluidized bed requires a fluidization agent corresponding to a recycled pyrolysis gas, steam or air depending on the process considered. Nitrogen was used as fluidizing agent for the pyrolysis laboratory experiments, but under industrial conditions, recycled pyrolysis gas would be used [25].

## 2.3. Experimental methods

Fast pyrolysis, gasification and combustion were conducted in a micro-fluidized bed. The experimental device and procedure are presented in Fig. S1 and in our previous work [30]. Sand was fluidized with a velocity of 10 cm/s, corresponding to 2.5 times the minimal fluidization velocity. This velocity was kept constant for all thermochemical processes (carrier gas or temperatures). All experiments were conducted with the same gas-phase residence time in the freeboard of 1.2 s. Pellets of dried digestate were injected in the preheated fluidized sand. More details about the experimental procedures are presented in supplementary material section 1. The liquid products were condensed, weighted and diluted into methanol before being analyzed by GC/MS-FID and high-resolution mass spectrometry (methods described in the next section and in supplementary material section 2). Char was collected (if not combusted) after cooling down the reactor, weighted and analyzed by elemental and proximate analysis.

Slow pyrolysis experiments were conducted in a tubular reactor with the same sampling and analysis methods as described above. Briefly, 4 g of dried digestate was positioned in a sample boat and then heated at 10 °C/min to reach the final temperature (Table 1) and at different solid residence times of 0, 15 and 30 min. The gas-phase residence time was kept constant at 2.5 s. The temperature homogeneity during the slow pyrolysis of the digestate, as positioned in the sample boat, is presented in the Fig. S2.

Hydrothermal liquefaction of raw digestate was conducted in a 300 mL autoclave (Parr, Moline, USA) stirred with pitched blade impellers. The autoclave was filled with 150 mL of raw digestate. The reactor was purged 3 times by N₂, then pressurized at an initial nitrogen pressure of 10 bar. The wall of the autoclave was heated quickly by an induction furnace to 250 °C or 300 °C in 5 min, then maintained during 15 min at this final temperature (measured inside the stirred slurry). The final pressure was 40 bar and 85 bar at 250 °C and 300 °C, respectively. The evolution of the temperature and pressure in the reactor is presented in Fig. S3. At the end of the reaction, the solid fraction (called “hydrochar”) was filtered and the aqueous phase collected.

## 2.4. Analytical characterization

### 2.4.1. Analysis of digestates and chars

Dried digestate and char were analyzed by an elemental analyzer (Vario Cube, Elementar, Germany). They were previously crushed by a ceramic mortar. The elemental analyzer was calibrated before each series of analysis. The oxygen was determined by difference with Equation (1).

$$O[wt\%] = 100 - C - N - H - S - ash[wt\%] \quad (1)$$

Dry matter (DM) and moisture content (MC) were determined by
<figure><img src="image_5.png" /><figcaption>Fig. 1. Simplified block scheme of different processes to transform a same raw digestate sample.</figcaption></figure>

<table><caption>**Table 1**

Operating conditions of the experimental trials of different processes analyzed.</caption><thead><tr><th>Type</th><th>Media</th><th>Type of reactor tested experimentally</th><th>Temperature of the reactor (°C)</th></tr></thead><tbody><tr><td>Fast pyrolysis</td><td>Nitrogen, fluidized sand</td><td>Fluidized bed</td><td>500, 700, 900</td></tr><tr><td>Steam gasification</td><td>Steam, nitrogen, fluidized sand</td><td>Fluidized bed</td><td>500, 700, 900</td></tr><tr><td>Partial combustion</td><td>Air, fluidized sand</td><td>Fluidized bed</td><td>500, 700, 900</td></tr><tr><td>Slow pyrolysis</td><td>Nitrogen</td><td>Tubular</td><td>400, 600, 800</td></tr><tr><td>Hydrothermal liquefaction</td><td>Liquid phase of the raw wet digestate</td><td>Stirred autoclave</td><td>250, 300</td></tr></tbody></table>

evaporation during 24 h at 105 °C, with Equation (2) and (3), in which $m_1$, $m_2$, and $m_3$ are the mass of empty crucible, the crucible with the solid to be analyzed and the crucible with the dried solid residue, respectively.

$$MC[wt\%] = 100 \frac{m_2 - m_3}{m_2 - m_1} \qquad (2)$$

$$DM[wt\%] = 100 - MC \qquad (3)$$

The solid residue obtained after the drying step was combusted at 525 °C (2 h heating from 20 °C to 525 °C) during 2 h in a crucible positioned in a muffle furnace under air. The organic matter was determined with Equation (4) in which $m_1$, $m_2$, and $m_3$ are the mass of the empty crucible, the crucible with dried solid and the crucible after combustion with ashes, respectively.

$$OM[wt\%] = 100 \frac{m_2 - m_3}{m_2 - m_1} \qquad (4)$$

Thermogravimetric analysis was used to analyze fixed carbon of chars. The sample (~6 mg) was placed in an alumina crucible. Measurements were conducted under an initial nitrogen flow rate of 60 mL/min. The sample was first stabilized for 10 min at 25 °C, then heated to 125 °C at a

rate of 25 °C/min. At 125 °C, the temperature was kept constant for 15 min to determine the moisture content. Subsequently, the temperature was increased to 900 °C at 25 °C/min, followed by maintaining a constant temperature during 10 min at 900 °C to determine the volatile matter content. Finally, the fixed carbon content was measured after 10 min at 900 °C under an air flow rate of 60 mL/min. The remaining material constituting the ash content.

### 2.4.2. Analysis of liquids

The liquid produced were quantified by GC/MS-FID according to Olcese et al. [31]. More details about the procedure are given in the supplementary material section 2. GC/MS analysis of the liquids was completed with high-resolution mass spectroscopy (FT-ICR MS). All the instrumental and acquisition information, as well as the data and graphical treatments, are provided in supplementary material section 2. FT-ICR mass spectrometry is the most powerful technique in terms of mass resolution ($R > 10^6$) and mass accuracy (< 1 ppm) [32]. It allows to detect hundreds or thousands of peaks and to unambiguously assign a unique molecular formula per m/z signal, respectively [33]. From the thousands of $CH_xO_yN_z$ formulae achieved in this study, it was possible to obtain some molecular parameters such as the double bound equivalent (DBE) [32]. This latter parameter describes the aromaticity and unsaturation degree of a molecule and is calculated as given by Equation (5), with $n_C$, $n_H$ and $n_N$ number of carbons, hydrogen, and nitrogen atoms, respectively.

$$DBE = 1 + n_C - \left(\frac{n_H}{2}\right) + \left(\frac{n_N}{2}\right) \qquad (5)$$

The hydrogen-to-carbon and oxygen-to-carbon ratios are also used to plot the thousands of molecular assignments on one graph, known as van Krevelen diagram [34,35], on which it is possible to differentiate biochemical classes such as lipids, carbohydrates, lignin derivatives, and peptides. Lipids are characterized by O/C ratio < 0.4 and H/C ratio close to 2; carbohydrate derivatives by O/C ratio close to 1 and H/C ratio close to 2; and peptides and amino acids by H/C ratio comprised between 1 and 2, and O/C ratio between 0.2 and 0.8.

### 2.4.3. Analysis of gas

During the experimental tests (fast and slow pyrolysis, gasification
and combustion), the gases were sampled after the cold traps in a Tedlar bag and then analyzed by micro-gas chromatography (SRA Instruments, MicroGC 3000) and Fourier Transform InfraRed gas analyzer (FTIR) within less than 30 min after the end of the thermochemical experiments.

## 2.5. Methodology to close the atom balances

The closure of the mass and atom balances, required for process modeling, was done based on analytical characterizations as follows:

(1) The mass yields of the three phases produced (gas, char, liquid) are based on experimental yields (Table S2).

(2) Gases and char were comprehensively analyzed; therefore, their mass yields and elemental compositions are experimentally known (Tables S3 and S5). For the partial combustion, it was not possible to separate char and ashes from sand (low mass of the solid and high dilution in the silica bed). In this case, the char yield was deduced by difference and its elemental composition was estimated assuming all ashes are in char and the same elemental composition as for fast pyrolysis char produced at the same temperature.

(3) Some molecular species in the liquid phase were quantified by GC/MS-FID. Six representative molecules (called hereafter “surrogates”) of families (acid, methoxyphenols, phenol, alkyl phenols, furans, aromatic hydrocarbons) were defined based on GC/MS-FID. They are presented in Table S4.

The FT-ICR MS analysis was used to define three surrogate molecules representing the other heavy fractions of the liquids not analyzable by GC/MS. FT-ICR MS analysis gives H/C, O/C, number of C atoms in molecules (#C), #H, #O, #N, and DBE. These surrogates and their yields are chosen to be representative of all conditions investigated. They match the FT-ICR MS analyses as presented in 4 “maps”: H/C vs O/C, DBE vs. #C, #O vs. #C and #N vs. #C.

(4) The overall atom balances were closed with these 3 surrogates and H₂O using a solver with constraints. The stoichiometry of the digestate consumption was defined based on experimental yields for all species quantified. In order to close the balance, the solver was used to determine the FT-ICR MS surrogate yields with the objective of minimizing the mass of atoms not attributed. The target values have to follow some constraints: yields of surrogate must be inferior to the produced liquid yield, balance of each atom must be positive or equal to zero and O/C and H/C ratios must be close to FT-ICR MS analyses.

## 2.6. Process modeling

Aspen Plus V8.8 was used to simulate the different processes and operating conditions. Fonseca [36] compared with experimental data the VLE estimated by several thermodynamic models among Peng-Robinson (PR), Soave-Redlich-Kwong (SRK) and modified PR and SRK models. In the context of pyrolysis oil including oxygenates, SRK model modified with the Kabadi-Danner mixing rules leads to the lowest average deviations. SRK Kabadi-Danner was chosen as thermodynamic model because it is well adapted for the liquids from biomass pyrolysis [36]. SRK model was also previously used for modeling the hydrothermal conversion of digestates [37]. In the model, digestate and chars were defined as non-conventional solid with their ultimate and proximate analysis. An additional non-conventional specie (trash) was added to close all atom balances. The model of Mott and Spooner was selected for the determination of the heat of combustion for non-conventional solids (digestate and chars). This method is more adapted to carbonaceous solids with high oxygen content [38].

Two surrogates (n-acetylgalactosamine and benzofuran) were missing in the classical Aspen Plus databanks. They were defined based on their molecular structure in Aspen Plus. The missing properties were evaluated with TDE NIST (Table 2).

The main operation unit models are described in Table 3. The detailed flowsheets are provided in Fig. S6–S8. The energy flows were

<caption>**Table 2**</caption>

Method selected on the Aspen Plus simulations for each digestate fraction, including the surrogate compounds describing the liquid fraction.

<table><caption>**Table 3**</caption><thead><tr><th>Sample</th><th>Specie</th><th>Methods</th><th>Definition</th></tr></thead><tbody><tr><td>Digestate</td><td></td><td>Non-conventional solid</td><td>From TGA, CHNS-O Ultimate &amp; proximate analysis</td></tr><tr><td>Char</td><td></td><td>Non-conventional solid</td><td>From TGA, CHNS-O Ultimate &amp; proximate analysis</td></tr><tr><td rowspan="7">Liquid</td><td>Acid (acetic acid)</td><td>Databank specie</td><td></td></tr><tr><td>Methoxyphenols (4-methyl-2-methoxyphenol)</td><td>Databank specie</td><td></td></tr><tr><td>Phenol</td><td>Databank specie</td><td></td></tr><tr><td>Alkyl phenols (cresol)</td><td>Databank specie</td><td></td></tr><tr><td>Furans (benzofuran)</td><td>User-defined conventional specie</td><td>Structure, TDE NIST</td></tr><tr><td>Aromatic hydrocarbons (naphthalene)</td><td>Databank specie</td><td></td></tr><tr><td>SURROG-1 (n-acetylgalactosamine)</td><td>User-defined conventional specie</td><td>Structure, TDE NIST</td></tr><tr><td></td><td>SURROG-2 (n-eicosanoic acid)</td><td>Databank specie</td><td></td></tr><tr><td></td><td>SURROG-3 (tryptophan)</td><td>Databank specie</td><td></td></tr></tbody></table>

Model assumptions for the estimation of the process model.

<table><thead><tr><th>Unit</th><th>Assumptions</th></tr></thead><tbody><tr><td>Screw press</td><td>Electrical consumption: 11 kWh/t<sub>solid wet</sub> [39]<br/>Solid digestate/raw digestate: 4 wt%.</td></tr><tr><td>Drier</td><td>Adapted from François et al. [40]: liquid water separated from raw digestate, liquid evaporation up to the 6 wt %-moisture target to estimate the enthalpy needs and remix of the remaining water and solid.</td></tr><tr><td>Pump</td><td>Discharge pressure 2 (for water in steam gasification) or 100 bar (for HTL). Efficiency 80 %.</td></tr><tr><td>Compressor</td><td>Discharge pressure 1.5 bar. Polytropic efficiency 80 %, mechanical efficiency 98 %.</td></tr><tr><td>Thermochemical reactor</td><td>Stoichiometric reactor (RSTOIC) with reaction defined by user from experimental balance.</td></tr><tr><td>Cooler</td><td>Heat exchanger. Outlet temperature 80 °C.</td></tr></tbody></table>

defined as enthalpy flows given by Aspen Plus at 20 °C as reference temperature.

All models were conducted by considering 1 t/h of raw digestate at 96.3 wt% moisture content. The screw press produced 40 kg/h of solid digestate (at 84 wt% moisture content) and 960 kg/h of liquid digestate (at 97 wt% of moisture content), according to industrial data (more details presented in supplementary material, section 4).

The energy efficiency η is defined by Equation (6). HHV [kJ/kg] is the mass higher heating value, ṁ [kg/s] is the mass flowrate, ḇ [kW] is the net heat produced and ḋ [kW] is the electrical power consumption.

$$ \eta = \frac{\frac{\text{HHV}_{\text{gas}} \cdot \dot{m}_{\text{gas}} + \text{HHV}_{\text{char}} \cdot \dot{m}_{\text{char}} + \text{HHV}_{\text{liquid}} \cdot \dot{m}_{\text{liquid}} + \dot{Q}_{\text{net}} - \dot{W}}{\text{HHV}_{\text{digestate}} \cdot \dot{m}_{\text{digestate}}} \quad (6) $$

Considering the high water content in the liquids produced by gasification or HTL, their HHV was not calculated and, thereby, they were not integrated in the calculation of the energy efficiency.
# 3. Results

## 3.1. Characterization of digestate, char, liquid and gas

### 3.1.1. Digestate characterization

Ultimate and proximate analyses are presented in Table 4. These analyses are consistent with the literature [41,42].

A high moisture content was obtained for all digestate fractions, including the solid fraction recovered after a digestate dewatering step by screw press. As reported in Table 4, “raw digestate” corresponds to the digestate with 96.3 wt% of moisture content, “solid digestate” to the solid phase after screw press separation at 84 wt% of moisture content and “liquid digestate” is the second output of the screw press (at 96.6 wt % wet basis of moisture).

Carbon content was similar in raw and liquid digestate (around 40 wt % TS) and slightly higher in solid digestate after the dewatering step (49.5 wt% TS).

### 3.1.2. Analysis of chars

Mass yields, proximate and ultimate analyses of the chars are presented in Tables S2 and S3.

### 3.1.3. Determination of surrogate molecules to represent the liquid phase

The chosen molecules are presented in Fig. 2 for the liquids produced by fast pyrolysis at 500 °C in this example. The results of all operating conditions are provided in supplementary material section 6. To the best of our knowledge, we present here the first use of high-resolution mass spectrometry (FT-ICR MS) to define surrogates of complex liquids for a process modeling (like Aspen Plus). Fonts et al. [43] have proposed surrogate molecules for a bio-oil produced by lignocellulosic biomasses but not from digestates. The proposed molecules did not contain N-moieties of high importance for protein-based conversion products.

Three surrogate molecules were chosen for their representability, in terms of sample occurrence and intensity, in the high-resolution MS analysis and also based on the availability of the thermodynamic data in Aspen Plus. Molecule a, n-acetylgalactosamine (C<sub>8</sub>H<sub>15</sub>NO<sub>6</sub>), is a representative surrogate of carbohydrates pyrolysis in the presence of N-moieties [44,45]. Molecule b, eicosanoic acid (C<sub>20</sub>H<sub>40</sub>O<sub>2</sub>), is a surrogate of lipid pyrolysis [46]. Molecule c, L-tryptophan (C<sub>11</sub>H<sub>12</sub>N<sub>2</sub>O<sub>2</sub>), is a significant protein-based pyrolysis product [44]. These molecules are consistent with previous work on digestate pyrolysis of Wang et al. [46] and Cheng et al. [44]. Furthermore, these molecules allowed us to depict most of the species analyzed over the wide range of liquids composition for the studied conditions (see Supplementary material section 6).

## 3.2. Mass, carbon and enthalpy flowrate during the pre-treatment of wet digestate

Sankey diagrams in Fig. 3 report the overall mass, carbon and enthalpy balances of the pretreatment step, consisting in a dewatering step through screw press, followed by a drying step. The pelletizing step was not represented.

<table><caption>**Table 4**
Ultimate and proximate analyses of raw, liquid and solid digestates.</caption><thead><tr><th></th><th>Raw digestate</th><th>Liquid digestate</th><th>Solid digestate</th></tr></thead><tbody><tr><td>Moisture (wt% wet basis)</td><td>96.3 ± 0.6</td><td>96.6 ± 0.7</td><td>84.0 ± 0.3</td></tr><tr><td>VS (wt% TS)</td><td>59.1 ± 0.5</td><td>71.5 ± 0.6</td><td>76.6 ± 0.7</td></tr><tr><td>Ash (wt% TS)</td><td>14.8 ± 0.5</td><td>17.5 ± 0.5</td><td>7.7 ± 0.5</td></tr><tr><td>Fixed carbon (wt% TS)</td><td>26.2 ± 0.4</td><td>11.0 ± 0.4</td><td>15.7 ± 0.4</td></tr><tr><td>C (wt% TS)</td><td>40.1 ± 0.10</td><td>38.8 ± 0.09</td><td>49.5 ± 0.12</td></tr><tr><td>H (wt% TS)</td><td>4.6 ± 0.10</td><td>4.6 ± 0.10</td><td>5.7 ± 0.13</td></tr><tr><td>N (wt% TS)</td><td>4.2 ± 0.62</td><td>4.3 ± 0.64</td><td>2.3 ± 0.34</td></tr><tr><td>S (wt% TS)</td><td>2.4 ± 0.33</td><td>2.7 ± 0.37</td><td>1.1 ± 0.15</td></tr><tr><td>O (wt% TS) by difference</td><td>33.9</td><td>32.1</td><td>33.8</td></tr></tbody></table>

The screw press can handle large flow rates of digestate at low energy input, but the recovery of solid matter is not optimal, as reported in literature [39]. Indeed, as shown in Table 4, after screw press separation, the liquid fraction represented 96.6 wt% wet basis of the liquid digestate, a value close to that of the raw digestate. Consequently, 83 % of the solid phase in the raw digestate was transferred into the liquid digestate (Table 4). These results are in agreement with Guilayn et al. [47] (more details are presented in supplementary material section 4). In terms of carbon, 79 % was transferred into the liquid digestate, which means again that the screw press does not allow to recover optimally the carbon fraction in the solid phase. Moreover, an additional drying step is necessary for the thermochemical processes to reduce the moisture content of the solid digestate from 84 wt% to 6 wt% for further conversion of the dried digestate. This step implies an important heat input: 24 kJ/s to dry 40 kg/h of wet solid digestate obtained from the screw press (Fig. 3).

## 3.3. Mass and carbon balances of the thermochemical conversion processes

The overall mass balances along the thermochemical processes are presented with Sankey diagrams in Fig. 4 for five operating conditions which were previously selected. The detailed mass balances are presented in supplementary material section 8.

The water flows are highlighted in blue in Fig. 4 for HTL and steam gasification processes. As explained before, except for Hydrothermal Liquefaction (HTL), in this study all experimental tests required a digestate dewatering step followed by a drying and pelletization step. Then, HTL experimental tests were performed at a moisture content of 96.3 wt%. On the other hand, in the gasification, steam is used in excess and in mixture with N<sub>2</sub>, to keep a constant fluidization velocity of 10 cm/s.

Nitrogen, air and steam used in the processes were accounted for in the Aspen plus model (including heat recovery from the produced hot gas to pre-heat this carrier gas). However, the flow rate of nitrogen (carrier-gas for pyrolysis) required for the fluidized bed is not represented in Fig. 4.

As expected, the gas was the main product obtained for steam gasification (see Fig. 4, 5.8 kg of syngas/t<sub>raw digestate</sub>, 80 wt% based on anhydrous digestate mass) and combustion (41.5 kg/t<sub>raw digestate</sub> of flue gas diluted in air) at the same temperature (900 °C).

The main product obtained by fast or slow pyrolysis is bio-oil with a mass yield close to 0.4 kg/kg<sub>dried digestate</sub> (Table S2) (2.5 kg/t<sub>raw digestate</sub>, Fig. 4). This value is lower compared to lignocellulosic feedstocks (50 – 70 wt% bio-oil by fast pyrolysis). A broad range of pyrolysis oil mass yields (between 10 to 65 wt%) was reported in the literature [48] depending on reactor configuration (such as gas-phase residence time). For instance, 36 wt% oil was quantified for food wastes digestates and a pyrolysis reactor of 500 °C [49].

The mass yield of chars is similar for slow (600 °C) and fast pyrolysis (500 °C) (2.2 kg/t<sub>raw digestate</sub> see Fig. 4). Our char yields (35 wt% based on anhydrous digestate mass) are in agreement with Opatokun et al. [50] and Zhao et al. [49] on food waste digestates under similar pyrolysis conditions as ours.

The char mass yield is lower for HTL (14 wt% on dry digestate basis, Table S2) than for pyrolysis. Our HTL char yield is lower than in the literature [48] probably due to a faster heating in our HTL device. The HTL process globally produces more char (5.2 kg/t<sub>raw digestate</sub> see Fig. 4) than the pyrolysis routes because an important loss in solid material is induced by the screw press (for the pyrolysis routes).

Fig. 5 presents the carbon balance of the five processes. The carbon present in the dried digestate is mainly transferred to gas phase for combustion and steam gasification. The last one produces an energy-rich gas (Table S5) which can be used for combined heat and power production or fuel production (methanol or Fischer-Tropsch liquids). Combustion may be of interest when heat production is exclusively
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Example of HRMS data and definition of the 3 model surrogates for Aspen Plus. ⭐ n-acetylgalactosamine (C<sub>8</sub>H<sub>15</sub>NO<sub>6</sub>) ◐ eicosanoic acid (C<sub>20</sub>H<sub>40</sub>O<sub>2</sub>), △ L-tryptophan (C<sub>11</sub>H<sub>12</sub>N<sub>2</sub>O<sub>2</sub>). These 3 surrogates are positioned on the different graphical representations of the HRMS analysis of the complex liquids. They enable to model the broad range of liquids composition in terms of atomic composition (C, H, N, O) and double bond equivalent (DBE).</figcaption></figure>

considered as the energy valorization pathway. Carbon present in digestate is equally transferred to solid char and liquid (in bio-oils) for both slow and fast pyrolysis. 78 % of the carbon from the digestate is converted to the liquid phase for the HTL process. The flow rate of carbon in the liquid phase of the HTL process (11.6 kgC/h) is about eight times the ones produced by fast pyrolysis (1.5 kgC/h) due to the important loss in carbon during raw digestate dewatering by the screw press. Nevertheless, the organic molecules present in the HTL liquid are highly diluted in water.

### 3.4. Energy balances

The enthalpy balances of each scenario are presented on Fig. 6, including the heat required and the potential heat recovery. The

energetic performances of the scenarios modeled are gathered in Table 5. It is important to notice that the liquid products are considered in the energy efficiency calculation only for pyrolysis processes (fast and slow). For gasification, combustion and HTL, the liquid was highly diluted in water (e.g. scrubber to clean the tar from the syngas in gasification), consequently the organic molecules could not be easily separated from water. Therefore, their heating value was not considered in the calculation.

The gases produced by steam gasification (≥700 °C) and fast pyrolysis (900 °C) presented the highest HHV (Table 5). The flue gas of combustion still contains some CO that is why its HHV is not strictly zero. In a fluidized bed, it is known that as combustion temperature increases, CO/CO₂ ratio increases too [51–53]. The target was not to optimize the combustion in this study but to conduct experiments with
<figure><img src="image_7.png" /><figcaption>Fig. 3. Mass flowrate (kg/h wet basis) (a), carbon flowrate (kg/h) (b) and enthalpy flowrate (kJ/s) (c) for 1000 kg/h of raw digestate for the pretreatment (screw press separation and drying).</figcaption></figure>

the same constant gas-phase residence time to better compare com-
bustion, gasification and fast pyrolysis.

The highest HHV of char was obtained from pyrolysis experiments (fast or slow) notably at higher final temperature or longer residence time of the solid in the reactor (30 min). The energy efficiencies (Equation (6) referred to the raw digestate of thermochemical processes were evaluated between –20 % and 6 %. This negative or very low ef- ficiency is due to the high moisture content in the digestate (both raw and solid). This results in low recovery of dry matter in the screw press and requires a high amount of energy to heat the raw digestate to 250 – 300 °C for the HTL process or to dry the solid digestate before pyrolysis, steam gasification and combustion processes. To be note that the heat recovered along the processes by heat exchanger, is not sufficient to fulfill the energy and heat needs. Moreover, the global energy effi- ciencies were determined by assuming that the heat contained in the steam that came out of the drier was not recovered. Thus, the energy efficiency could be increased, up to 11 – 24 % depending on the sce- narios considered.

The HTL process exhibits the lower energy efficiency (around –13 to –20 % on raw digestate basis), this can be explained by the fact that the entire flowrate of the raw digestate, with a high moisture content, needed to be heated and processed (without dewatering step) and thus the energy demand was drastically higher. Nevertheless, the HTL reactor

is approximately athermal (under isothermal condition) producing 4 kW for 1 t/h of raw digestate (or 376 kJ/kg of anhydrous digestate). This value is lower but in the same range of magnitude than that reported by Ghavami et al. [37]: 1.46 MJ/kg of dry solid, due to different operating conditions (200 °C, 1 h, 30 wt%TS). These same authors have defined an interesting criterion to assess the energy efficiency of HTL conversion: the ratio between the energy content of the hydrochar per unit mass of reactor inlet (including the water in the digestate) [37]. This ratio lies between 0.7 to 5 MJ/kg depending on the processed wet biomass and operating conditions. In this study, this ratio was calculated to 0.11 MJ/kg reactor inlet (32 kW of hydrochar for 1 t/h of raw wet digestate) because of the very low solid content of the digestate (3.7 wt%TS).

The energy efficiencies calculated for the other processes could seem
low in comparison with literature. The dewatering step appeared as a
clear bottleneck. Indeed, as explained before, the liquid fraction after
screw press separation contains 78 wt%TS of the total carbon flow rate.
This carbon not recovered is thus not valorized. The enthalpy recovered
in the solid digestate in the screw press separation was estimated to
around 17 % (Fig. 3). This is confirmed by looking closer the results in
terms of TS basis. In this case, the energy efficiencies are in good
agreement with classical values [54–56], between 73 and 93 % for py-
rolysis (fast and slow), between 49 and 62 % for steam gasification and
60 – 82 % for combustion.
<figure><img src="image_8.png" /><figcaption>Fig. 4. Mass flowrates (kg/h) for 1000 kg/h of raw digestate (corresponding to 6.8 kg/h of dried digestate) for different scenarios: combustion at 900 °C, fast pyrolysis at 500 °C, steam gasification at 900 °C, slow pyrolysis at 600 °C (30 min solid residence time), hydrothermal liquefaction at 300 °C (15 min of residence time).</figcaption></figure>

### 3.5. Discussion

From an energy perspective, the impact of the moisture content of the solid digestate on the drying energy needs is discussed in the previous section 3.4. It should be noted that the global energy efficiencies were determined without considering the heat contained in the steam produced during the drying process. Besides, it is necessary to consider the digestate valorization considering the global biogas chain. Indeed,

the digestate is produced from anaerobic digestion in which biogas is generated. For instance, Monlau et al. evaluated the biogas production at 92 $Nm^3/t_{fresh matter}$ [57].

In a global context of environmental circularity of the biogas plants, part of biogas could be burned in order to generate the energy required for the valorization of digestate. In our case, the biogas produced could represent around 2300 $kWh_{LHV}$ per ton TS of raw digestate (see Table S7 in supplementary material). From our previous results, the drying step
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Fate of carbon (kgC/h) for 1000 kg/h of raw digestate for different scenarios: combustion at 900 °C, fast pyrolysis at 500 °C, steam gasification at 900 °C, slow pyrolysis at 600 °C (30 min of solid residence time), hydrothermal liquefaction at 300 °C (15 min of residence time).</figcaption></figure>

required 643 kWhth per 1 ton TS of raw digestate. This means that part of biogas can be used to the plant energy needs whereas the additional remaining biomethane can be injected into the gas network, after upgrading and complying with the gas network standards (pressure, quality).

Alternatively, the biogas can feed a combined heat and power plant (CHP). The electricity generated can fulfill the power requirement of the site and/or be injected on the grid for an estimated yield of 1.9 kWh/e/Nm³biogas [57]. The heat produced (2.6 kWhth/Nm³biogas [57]) usually cannot be valorized in heat networks because anaerobic digestion units are usually located in rural areas. This heat could fulfill the heat requirement of the digestate valorization process (Fig. 7). The heat available according to Monlau et al. is around 975 kWhth/t [57], which is higher than our heat requirement for drying (643 kWhth/t).

From an economical point of view, the thermochemical conversion of digestates does not require to be economically viable by itself but it should be less expensive than traditional treatment processes for digestates disposal. The choice of the technology would be driven by the most economical and environmentally friendly options, which are out of the scope of this present work because they depend on the final targeted energy carrier and local socio-economic factors (like market price, incentives, etc.).

According to this study, if power production is targeted, valorization of the syngas from gasification in an internal combustion engine may be targeted. If heat is targeted and associated to a local heat demand (e.g. heat for agro-industries), combustion of digestates could be considered. If the production of liquid fuels is targeted, the fast pyrolysis and HTL scenarios would be more adapted, but the downstream up-grading steps of bio-oils remains to be developed. If the target is to maximize carbon

recovery from raw digestate to char, hydrothermal conversion is the best option.

## 4. Conclusion

Five thermochemical processes for the valorization of digestates were evaluated both by experiments and process simulation. All these processes were tested on the same initial feedstock and on the same basis (1 t/h of raw digestate) to give an unambiguous comparison. An advanced modelling approach under Aspen Plus was proposed which is based on a complete characterization of the products and on high resolution mass spectrometry of the liquids. Global fate of mass, carbon and energy for the thermochemical processes were presented.

Our main findings can be summarized by the following points:

1) Our methodology to define surrogate molecules based on high resolution mass spectrometry presents a valuable approach to model complex liquids. This approach can be used for all other complex organic liquids.

2) The HTL process globally produces more char (5.2 kg/t<sub>raw digestate</sub>) than the pyrolysis routes (2.2 kg/t<sub>raw digestate</sub>) on the basis of the raw digestate because of the important loss in solid material by the screw press (for the pyrolysis routes).

3) In terms of carbon, 79 % of the initial carbon present in the wet raw digestate remain in the liquid fraction after dewatering step involving the use of a screw press. This means that the screw press does not optimally recover the carbon fraction in the solid phase.

4) 78 % of the carbon from the raw digestate is converted to the liquid phase by the HTL process. The flow rate of carbon in the liquid phase
<figure><img src="image_10.png" /></figure>

b. Fast pyrolysis (500°C)

<figure><img src="image_11.png" /></figure>

c. Steam gasification (900°C)

<figure><img src="image_12.png" /></figure>

d. Slow pyrolysis (600°C – 30 min)

<figure><img src="image_13.png" /></figure>

e. Hydrothermal liquefaction (300°C)

<figure><img src="image_14.png" /><figcaption>**Fig. 6.** Enthalpy flowrate (kJ/s) for 1000 kg/h of raw digestate for different scenarios: combustion at 900 °C, fast pyrolysis at 500 °C, steam gasification at 900 °C, slow pyrolysis at 600 °C (30 min of solid residence time), hydrothermal liquefaction at 300 °C (15 min of residence time).</figcaption></figure>

of the HTL process (11.6 kgC/h) is about eight times the ones produced by fast pyrolysis (1.5 kgC/h) due to the important loss in carbon during raw digestate dewatering by the screw press.

5) All dry thermochemical processes (fast and slow pyrolysis, gasification and combustion) were characterized by a low global energy efficiency due to a high-water content in digestate (raw and solid

phase). However, in a global context of the biogas plants, the heat demand required to dry the digestate for the dry thermochemical processes could be fulfilled by converting energetically a portion of the biogas produced in a CHP system.

Concerning the perspectives of this work, the organic molecules are
<table><caption>**Table 5**

Higher Heating Value for gas and char and energy efficiencies for different thermochemical processes at different operating conditions.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Temperature</th><th colspan="2">Higher Heating Value; HHV (MJ/kg)</th><th colspan="5">Energy efficiency (%)</th></tr><tr><th>Gas</th><th>Char</th><th>Total: gas, char, liquid, heat net</th><th>Dried dig.<sup>a</sup></th><th>Raw digestate basis<sup>b</sup></th><th>gas</th><th>char</th><th>heat</th><th>Liquid</th></tr></thead><tbody><tr><td rowspan="3">Combustion</td><td>500</td><td>0.2</td><td>9.8</td><td>60.1</td><td>-1.5</td><td>2.4</td><td>1.3</td><td>-4.9</td><td></td></tr><tr><td>700</td><td>0.7</td><td>2.6</td><td>80.6</td><td>3.0</td><td>5.5</td><td>0.2</td><td>-2.4</td><td></td></tr><tr><td>900</td><td>1.0</td><td>-</td><td>81.7</td><td>3.3</td><td>7.1</td><td>-</td><td>-3.2</td><td></td></tr><tr><td rowspan="3">Fast pyrolysis</td><td>500</td><td>3.6</td><td>22.3</td><td>74.9</td><td>1.8</td><td>1.2</td><td>8.8</td><td>-14.1</td><td>6.7</td></tr><tr><td>700</td><td>8.5</td><td>23.0</td><td>73.4</td><td>1.5</td><td>4.3</td><td>8.1</td><td>-13.8</td><td>3.6</td></tr><tr><td>900</td><td>14.3</td><td>23.6</td><td>93.3</td><td>5.9</td><td>9.7</td><td>8.3</td><td>-13.4</td><td>2.0</td></tr><tr><td rowspan="3">Steam gasification</td><td>500</td><td>4.1</td><td>23.4</td><td>49.4</td><td>-3.9</td><td>0.7</td><td>8.4</td><td>-12.7</td><td></td></tr><tr><td>700</td><td>12.0</td><td>22.7</td><td>53.4</td><td>-3.0</td><td>5.4</td><td>7.2</td><td>-15.3</td><td></td></tr><tr><td>900</td><td>13.8</td><td>13.8</td><td>61.5</td><td>-1.2</td><td>13.6</td><td>2.7</td><td>-17.1</td><td></td></tr><tr><td rowspan="3">Slow pyrolysis</td><td>400-30<sup>c</sup></td><td>4.5</td><td>25.0</td><td>75.5</td><td>1.9</td><td>1.2</td><td>11.1</td><td>-14.4</td><td>4.6</td></tr><tr><td>600-30<sup>c</sup></td><td>5.3</td><td>25.2</td><td>74.7</td><td>1.7</td><td>2.0</td><td>9.5</td><td>-14.0</td><td>4.9</td></tr><tr><td>800-30<sup>c</sup></td><td>5.6</td><td>23.6</td><td>73.7</td><td>1.5</td><td>2.4</td><td>8.1</td><td>-13.7</td><td>5.4</td></tr><tr><td rowspan="2">Hydrothermal liquefaction</td><td>250</td><td>0.4</td><td>20.6</td><td>-</td><td>-19.7</td><td>0.1</td><td>15.3</td><td>-29.5</td><td></td></tr><tr><td>300</td><td>5.1</td><td>21.9</td><td>-</td><td>-13.5</td><td>2.2</td><td>19.4</td><td>-29.4</td><td></td></tr></tbody></table>

a Value calculated on dried digestate basis after dewatering and drying i.e. on a mass flowrate of 6.8 kg/h (84 wt% TS).

b Value calculated on raw digestate basis including dewatering and drying i.e. on a mass flowrate of 1000 kg/h (3.7 wt% TS).

c For slow pyrolysis experiments, the second figure is related to the solid residence time in min.

<figure><img src="image_15.png" /><figcaption>**Fig. 7.** Integration of AD CHP and digestate valorization: the heat available from biogas CHP plant is sufficient for the drying needs of solid digestates.</figcaption></figure>

highly diluted in water after the HTL process. Therefore, the Chemical Oxygen Demand (COD) could be further assessed in order to determine the overall performance in terms of energy conversion. Nevertheless, COD is not yet well handled in process modeling tools. Another more global perspective could be to further conduct techno-economical and life cycle assessments of digestate conversion routes.

### CRediT authorship contribution statement

**Rémi Demol:** Writing – original draft, Visualization, Software, Methodology, Formal analysis, Conceptualization. **Mohamed H. Ais-saoui:** Investigation, Formal analysis. **Paola Gauthier-Maradei:** Writing – review & editing, Supervision, Resources. **Cecilia Sambusiti:** Writing – review & editing, Supervision, Resources. **Jasmine Hertzog:** Writing – review & editing, Formal analysis. **Vincent Carré:** Supervision, Formal analysis. **Anthony Dufour:** Writing – original draft, Supervision, Project administration, Methodology, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial

interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgement

This work was partially supported by the MassLor research infrastructure at the University of Lorraine, for FT-ICR MS analyses.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.enconman.2025.119759.

### Data availability

All data are provided in supplementary material

### References

[1] Tayibi S, Monlau F, Bargaz A, Jimenez R, Barakat A. Synergy of anaerobic digestion and pyrolysis processes for sustainable waste management: A critical review and
[2] future perspectives. Renew Sustain Energy Rev 2021;152:111603. https://doi.org/10.1016/j.rser.2021.111603.

[2] Pecchi M, Baratieri M. Coupling anaerobic digestion with gasification, pyrolysis or hydrothermal carbonization: A review. Renew Sustain Energy Rev 2019;105: 462-75. https://doi.org/10.1016/j.rser.2019.02.003.

[3] Wen Y, Wang S, Shi Z, Nuran Zaini I, Niedźwiecki L, Aragon-Briceno C, et al. H2-rich syngas production from pyrolysis of agricultural waste digestate coupled with the hydrothermal carbonization process. Energy Conver Manage 2022;269:116101. https://doi.org/10.1016/j.enconman.2022.116101.

[4] Sikarwal VS, Pohorelý M, Meers E, Skoblia S, Mosko J, Jeremiás M. Potential of coupling anaerobic digestion with thermochemical technologies for waste valorization. Fuel 2021;294. https://doi.org/10.1016/j.fuel.2021.120533.

[5] Monlau F, Sambusiti C, Ficara E, Aboulkas A, Barakat A, Carrère H. New opportunities for agricultural digestate valorization: current situation and perspectives. Energy Environ Sci 2015;8:2600-21. https://doi.org/10.1039/C5EE01633A.

[6] Wang W, Lee DJ. Valorization of anaerobic digestion digestate: A prospect review. Bioresour Technol 2021;323:124626. https://doi.org/10.1016/j.biortech.2020.124626.

[7] Jiang B, Lin Y, Mbog JC. Biochar derived from swine manure digestate and applied on the removals of heavy metals and antibiotics. Bioresour Technol 2018;280: 603-11. https://doi.org/10.1016/j.biortech.2018.08.022.

[8] Ngigi AN, Ok YS, Thiele-Bruhn S. Biochar-mediated sorption of antibiotics in pig manure. J Hazard Mater 2019;364:663-70. https://doi.org/10.1016/j.jhazmat.2018.10.045.

[9] Cavali M, Libardi Junior N, Mohedano R de A, Belli Filho P, da Costa RHR, de Castilhos Junior AB. Biochar and hydrochar in the context of anaerobic digestion for a circular approach: An overview. Science of The Total Environment 2022;822: 153614. DOI: 10.1016/j.scitotenv.2022.153614.

[10] Basinas P, Rusín J, Chamárová K, Kaldis SP. Pyrolysis of the anaerobic digestion solid by-product: Characterization of digestate decomposition and screening of the biochar use as soil amendment and as additive in anaerobic digestion. Energy Conver Manage 2023;277:116658. https://doi.org/10.1016/j.enconman.2023.116658.

[11] Farru G, Dang CH, Schultze M, Kern J, Cappai G, Libra JA. Benefits and Limitations of Using Hydrocarbons from Organic Residues as Replacement for Peat on Growing Media. Horticuiturae 2022;8:325. https://doi.org/10.3390/horticuiturae8040325.

[12] Taufer NL, Benedetti V, Pecchi M, Matsumura Y, Baratieri M. Coupling hydrothermal carbonization of digestate and supercritical water gasification of liquid products. Renew Energy 2021;173:934-41. https://doi.org/10.1016/j.renene.2021.04.058.

[13] Okoro OV, Sun Z. The characterization of biochar and bioocrude products of the hydrothermal liquefaction of raw digestate biomass. Biomass Conv Bioref 2021;11: 2947-61. https://doi.org/10.1007/s13399-020-00672-7.

[14] Rodriguez Correa C, Bernardo M, Ribeiro RPL, Esteves IAAC, Kruse A. Evaluation of hydrothermal carbonization as a preliminary step for the production of functional materials from biogas digestate. J Anal Appl Pyrol 2017;124:461-74. https://doi.org/10.1016/j.jaap.2017.02.014.

[15] Funke A, Reefs F, Kruse A. Experimental comparison of hydrothermal and vapothermal carbonization. Fuel Process Technol 2013;115:261-9. https://doi.org/10.1016/j.fuproc.2013.04.020.

[16] Lucian M, Volpe M, Merzari F, Wüst D, Kruse A, Andreottola G, et al. Hydrothermal carbonization coupled with anaerobic digestion for the valorization of the organic fraction of municipal solid waste. Bioresour Technol 2020;314:123734. https://doi.org/10.1016/j.biortech.2020.123734.

[17] Cao Z, Hülsemann B, Wüst D, Illi L, Oechsner H, Kruse A. Valorization of maize silage digestate from two-stage anaerobic digestion by hydrothermal carbonization. Energy Conver Manage 2020;222:113218. https://doi.org/10.1016/j.enconman.2020.113218.

[18] Aragón-Briceño CI, Ross AB, Camargo-Valero MA. Mass and energy integration study of hydrothermal carbonization with anaerobic digestion of sewage sludge. Renew Energy 2021;167:473-83. https://doi.org/10.1016/j.renene.2020.11.103.

[19] Dieguez-Alonso A, Funke A, Anca-Couce A, Rombolà AG, Ojeda G, Bachmann J, et al. Towards biochar and hydrochar engineering-influence of process conditions on surface physical and chemical properties, thermal stability, nutrient availability, toxicity and wettability. Energies 2018;11. https://doi.org/10.3390/en11030496.

[20] Chen G, Guo X, Cheng Z, Yan B, Dan Z, Ma W. Air gasification of biogas-derived digestate in a downdraft fixed bed gasifier. Waste Manag 2017;69:162-9. https://doi.org/10.1016/j.wasman.2017.08.001.

[21] Antoniou N, Monlau F, Sambusiti C, Ficara E, Barakat A, Zabaniotou A. Contribution to Circular Economy options of mixed agricultural wastes management: Coupling anaerobic digestion with gasification for enhanced energy and material recovery. J Clean Prod 2019;209:505-14. https://doi.org/10.1016/j.jclepro.2018.10.055.

[22] Balás M, Milcák P, Elbl P, Lisý M, Lachman J, Kracík P. Gasification of fermentation residue in a fluidised-bed gasifier. Energy 2022;245:123211. https://doi.org/10.1016/j.energy.2022.123211.

[23] Spath P, Aden A, Eggeman T, Ringer M, Wallace B, Jechura J. Biomass to Hydrogen Production Detailed Design and Economics Utilizing the Battelle Columbus Laboratory Indirectly-Heated Gasifier 2005. https://doi.org/10.2172/15016221.

[24] Abdelouahed L, Authier O, Mauviel G, Corriou JP, Verdier G, Dufour A. Detailed Modeling of Biomass Gasification in Dual Fluidized Bed Reactors under Aspen Plus. Energy Fuels 2012;26:3840-55. https://doi.org/10.1021/ef300411k.

[25] Demol R, Dufour A, Rogaume Y, Mauviel G. Production of Purified H2, Heat, and Biochar from Wood: Comparison between Gasification and Autothermal Pyrolysis

Based on Advanced Process Modeling. Energy Fuels 2022;36:488-501. https://doi.org/10.1021/acs.energyfuels.1c03528.

[26] Olcese RN, Francois J, Bettahar MM, Petitjean D, Dufour A. Hydrodeoxygenation of Guaiacol, A Surrogate of Lignin Pyrolysis Vapors, Over Iron Based Catalysts: Kinetics and Modeling of the Lignin to Aromatics Integrated Process. Energy Fuels 2013;27:975-84. https://doi.org/10.1021/ef301971a.

[27] Pelletier C, Rogaume Y, Dieckhoff L, Bardeau G, Pons M-N, Dufour A. Effect of combustion technology and biogenic CO2 impact factor on global warming potential of wood-to-heat chains. Appl Energy 2019;235:1381-8. https://doi.org/10.1016/j.apenergy.2018.11.060.

[28] Francois J, Fortin M, Patisson F, Dufour A. Assessing the Fate of Nutrients and Carbon in the Bioenergy Chain through the Modeling of Biomass Growth and Conversion. Environ Sci Technol 2014;48:14007-15. https://doi.org/10.1021/es5032823.

[29] Francois J, Mauviel G, Feidt M, Rogaume C, Rogaume Y, Mirgaux O, et al. Modeling of a Biomass Gasification CHP Plant: Influence of Various Parameters on Energetic and Exergetic Efficiencies. Energy Fuels 2013;27:7398-412. https://doi.org/10.1021/ef4011466.

[30] Aissaoui MH, Hertzog J, Sambusiti C, Gauthier-Maradei P, Pons M-N, Carré V, et al. Thermochemical conversion of solid digestates: Effects of temperature and fluidizing gas on products composition. J Anal Appl Pyrol 2025;186:106928. https://doi.org/10.1016/j.jaap.2024.106928.

[31] Olcese R, Carré V, Aubriet F, Dufour A. Selectivity of Bio-oils Catalytic Hydrotreatment Assessed by Petroleomic and GC*GC/MS-FID Analysis. Energy Fuels 2013;27:2135-45. https://doi.org/10.1021/ef302145g.

[32] Abou-Dib A, Aubriet F, Hertzog J, Vernet-Loset L, Schramm S, Carré V. Next Challenges for the Comprehensive Molecular Characterization of Complex Organic Mixtures in the Field of Sustainable Energy. Molecules 2022;27:8889. https://doi.org/10.3390/molecules27248889.

[33] Krajewski LC, Rodgers RP, Marshall AG. 126 264 Assigned Chemical Formulas from an Atmospheric Pressure Photoionization 9.4 T Fourier Transform Positive Ion Cyclotron Resonance Mass Spectrum. Anal Chem 2017;89:11318-24. https://doi.org/10.1021/acs.analchem.7b02004.

[34] Hertzog J, Carré V, Aubriet F. Chapter 22 - Contribution of Fourier transform mass spectrometry to bio-oil study. In: Kanawati B, Schmitt-Kopplin P, editors. Fundamentals and Applications of Fourier Transform Mass Spectrometry, Elsevier; 2019, p. 679-733. DOI: 10.1016/B978-0-12-814013-0-00022-3.

[35] Schmitt-Kopplin P, Gabelica Z, Gougeon RD, Fekete A, Kanawati B, Harir M, et al. High molecular diversity of extraterrestrial organic matter in Murchison meteorite revealed 40 years after its fall. Proc Natl Acad Sci 2010;107:2763-8. https://doi.org/10.1073/pnas.0912157107.

[36] Fonseca FG. Process simulation and optimization of biomass fast pyrolysis. PhD Thesis KIT 2023.

[37] Ghavami N, Özdenköç K, Chianese S, Musmarra D, De Blasio C. Process simulation of hydrothermal carbonization of digestate from energetic perspectives in Aspen Plus. Energy Conver Manage 2022;270:116215. https://doi.org/10.1016/j.enconman.2022.116215.

[38] Rönsch S, Wagner H. Calculation of heating values for the simulation of thermochemical conversion plants with Aspen Plus. DBFZ Germany 2012.

[39] Fechter M, Kraume M. Digestate treatment techniques 2016.

[40] Francois J, Abdelouahed L, Mauviel G, Patisson F, Mirgaux O, Rogaume C, et al. Detailed process modeling of a wood gasification combined heat and power plant. Biomass Bioenergy 2013;51:68-82. https://doi.org/10.1016/j.biombioe.2013.01.004.

[41] Tayibi S, Monlau F, Marías F, Thevenin N, Jimenez R, Oukarroum A, et al. Industrial symbiosis of anaerobic digestion and pyrolysis: Performances and agricultural interest of coupling biochar and liquid digestate. Sci Total Environ 2021;793:148461. https://doi.org/10.1016/j.scitotenv.2021.148461.

[42] Miliotti E, Casini D, Rosi L, Lotti G, Rizzo AM, Chiaramonti D. Lab-scale pyrolysis and hydrothermal carbonization of biomass digestate: Characterization of solid products and compliance with biochar standards. Biomass Bioenergy 2020;139: 105593. https://doi.org/10.1016/j.biombioe.2020.105593.

[43] Fonts I, Atienza-Martínez M, Carstensen H-H, Benés M, Pinheiro Pires AP, García-Perez M, et al. Thermodynamic and Physical Property Estimation of Compounds Derived from the Fast Pyrolysis of Lignocellulosic Materials. Energy Fuels 2021;35: 17114-37. https://doi.org/10.1021/acs.energyfuels.1c01709.

[44] Cheng F, Bayat H, Jena U, Brewer CE. Impact of feedstock composition on pyrolysis of low-cost, protein- and lignin-rich biomass: A review. J Anal Appl Pyrol 2020; 147:104780. https://doi.org/10.1016/j.jaap.2020.104780.

[45] Mainali K, Garcia-Perez M. Understanding organic N interactions with cellulose during pyrolysis to produce N-doped carbonaceous materials. J Anal Appl Pyrol 2023;169:105791. https://doi.org/10.1016/j.jaap.2022.105791.

[46] Wang S, Mandfloen P, Jönsson P, Yang W. Synergistic effects in the copyrolysis of municipal sewage sludge digestate and salix: Reaction mechanism, product characterization and char stability. Appl Energy 2021;289:116687. https://doi.org/10.1016/j.apenergy.2021.116687.

[47] Guilayn F, Jimenez J, Rouez M, Crest M, Patureau D. Digestate mechanical separation: Efficiency profiles based on anaerobic digestion feedstock and equipment choice. Bioresour Technol 2019;274:180-9. https://doi.org/10.1016/j.biortech.2018.11.090.

[48] Wang Z, Li J, Yan B, Zhou S, Zhu X, Cheng Z, et al. Thermochemical processing of digestate derived from anaerobic digestion of lignocellulosic biomass: A review. Renew Sustain Energy Rev 2024;199:114518. https://doi.org/10.1016/j.rser.2024.114518.
[49] Zhao J, Wang Z, Li J, Yan B, Chen G. Pyrolysis of food waste and food waste solid digestate: A comparative investigation. *Bioresour Technol* 2022;354:127191.
https://doi.org/10.1016/j.biortech.2022.127191.

[50] Opatokun SA, Kan T, Al Shoaibi A, Srinivasakannan C, Strezov V. Characterization of Food Waste and Its Digestate as Feedstock for Thermochemical Processing. *Energy Fuels* 2016;30:1589-97. https://doi.org/10.1021/acs.energyfuels.5b02183.

[51] Kwong KY, Marek EJ. Combustion of Biomass in Fluidized Beds: A Review of Key Phenomena and Future Perspectives. *Energy Fuels* 2021;35:16303-34. https://doi.org/10.1021/acs.energyfuels.1c01947.

[52] Kwong KY, Mao R, Scott SA, Dennis JS, Marek EJ. Analysis of the rate of combustion of biomass char in a fluidised bed of CLOU particles. *Chem Eng J* 2021; 417:127942. https://doi.org/10.1016/j.cej.2020.127942.

[53] Morin M, Pécate S, Hémati M. Kinetic study of biomass char combustion in a low temperature fluidized bed reactor. *Chem Eng J* 2018;331:265-77. https://doi.org/10.1016/j.cej.2017.08.063.

[54] Wong WY, Lu Y, Nasserzadeh VS, Swithenbank J, Shaw T, Madden M. Experimental investigation into the incineration of wool scouring sludges in a novel rotating fluidised bed. *J Hazard Mater* 2000;73:143-60. https://doi.org/10.1016/S0304-3894(99)00171-5.

[55] Venderbosch R, Prins W. Fast pyrolysis technology development. *Biofuels Bioprod Biorefin* 2010;4:178-208. https://doi.org/10.1002/bbb.205.

[56] Schuster G, Löffler G, Weigl K, Hofbauer H. Biomass steam gasification – an extensive parametric modeling study. *Bioresour Technol* 2001;77:71-9. https://doi.org/10.1016/S0960-8524(00)00115-2.

[57] Monlau F, Sambusiti C, Antoniou N, Barakat A, Zabaniotou A. A new concept for enhancing energy recovery from agricultural residues by coupling anaerobic digestion and pyrolysis process. *Appl Energy* 2015;148:32-8. https://doi.org/10.1016/j.apenergy.2015.03.024.