DOI: 10.3303/CET24109033 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 4 December 2023; Revised: 15 March 2024; Accepted: 22 March 2024 
Please cite this article as: Ha D.T., Tong H.D., Tran K.-Q., Trinh T.T., 2024, Molecular Simulation of Hydrothermal Gasification of Digestate 
Sludge with Copper Catalyst Addition, Chemical Engineering Transactions, 109, 193-198  DOI:10.3303/CET24109033 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 109, 2024 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Leonardo Tognotti, Rubens Maciel Filho, Viatcheslav Kafarov 

Copyright © 2024, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-09-0; ISSN 2283-9216 

Molecular Simulation of Hydrothermal Gasification of 

Digestate Sludge with Copper Catalyst Addition  

Do Tuong Haa, Hien Duy Tongb, Khanh-Quang Tranc,*, Thuat T. Trinhd,* 

aFaculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh city, Vietnam  
bFaculty of Engineering, Vietnamese-German University (VGU), Thu Dau Mot City, Binh Duong Province, Vietnam  
cDepartment of Energy and Process Engineering, Norwegian University of Science and Technology - NTNU, Norway  
dPorelab, Department of Chemistry, Norwegian University of Science and Technology - NTNU, Norway  

khanh-quang.tran@ntnu.no; thuat.trinh@ntnu.no  

Hydrothermal gasification (HTG) has emerged as a promising technology for converting wet and low cost 

biomass and organic waste resources to valuable fuel gases including hydrogen and methane. In this study, 

hydrothermal gasification of digestate sludge, a promising approach for waste management and energy 

recovery from biogas plants, is investigated through molecular simulations. Copper catalyst addition is explored 

to enhance the process conversion efficiency. The results reveal significant improvements in gas yield and 

conversion rates upon copper nanoparticle catalyst addition. This work also sheds light on the catalyst aging 

mechanisms and offers valuable insights into optimizing hydrothermal gasification conditions for digestate 

sludge treatment, contributing to sustainable waste management practices and renewable energy generation.  

1. Introduction 

Digestate sludge originates from anaerobic digestion (AD) processes in wastewater treatment plants, where 

microorganisms break down organic materials without oxygen to produce combustible gas mixtures containing 

primarily methane and carbon dioxide (Li et al., 2011). The resulting digestate contains non-digestible matter 

and valuable nutrients such as nitrogen and phosphorus, making it suitable for agricultural use (Mata-Alvarez 

et al., 2014). However, challenges associated with digestate sludge include its variability in composition due to 

factors like feedstock type and pre-treatment methods used during anaerobic digestion (Nkoa, 2013), as well 

as potential risks linked to contaminants such as heavy metals and pathogens (Verstraete et al. 2011). 

Consequently, researchers are exploring alternative ways to effectively utilize digestate sludge, including HTG. 

Optimization of biomass conversion, like HTG of digestate sludge, is crucial for sustainable waste-to-energy 

technologies (Verstraete et al., 2011; Zhang et al., 2010; Tran et al., 2020; Mahmoodinia et al., 2017). 

HTG is a process that occurs in supercritical water, typically at temperatures ranging between 500-700°C and 

corresponding pressures. The use of water as the reaction medium renders HTG highly compatible with the 

valorization of wet feedstock such as digestate sludge (Promdej & Matsumura, 2011). Due to its potential for 

generating hydrogen-rich syngas with reduced environmental impact compared to conventional gasification 

methods, HTG has significant potential. Using catalysts plays a crucial role in optimizing HTG reaction 

conditions, lowering the gasification temperature and pressure, as well as modulating product selectivity (Arregi 

et al., 2018; Guo et al., 2010 ). Copper-based catalysts have been widely studied for their effectiveness in 

improving the conversion of biomass to syngas during HTG (Guo et al., 2010). Another important aspect of 

using catalysts in HTG is their potential impact on the overall economics of the process. It is because, by 

lowering the temperature and pressure required for gasification, catalysts can contribute to cost savings and 

reduced energy consumption. This is particularly relevant when considering large-scale implementation of HTG 

technologies for waste management or renewable energy production (Nkoa, 2013). 

In this paper, Reactive force field (ReaxFF) is utilized for investigating the HTG of digestate sludge. By 

employing ReaxFF simulations, researchers can analyze the molecular-level interactions between the digestate 

sludge components and the copper nanoparticle catalyst, as well as explore the impact of catalyst on gas yield 

and composition products.  

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mailto:thuat.trinh@ntnu.no


This computational approach allows for a deeper understanding of the underlying reaction mechanisms and 

provides valuable guidance for optimizing HTG conditions, ultimately contributing to more efficient waste 

management practices and renewable energy generation. 

2. Models and methods 

In this study, we investigated the HTG of digestate sludge using molecular simulations. Three simple model 

compounds were chosen for investigation: aspirin (acetylsalicylic acid), xylylenediamine, and sacrylamide. 

These representative model compounds were selected as they are present in digestate sludge solutions (He et 

al., 2022). Figure 1 shows the initial system containing Cu nanoparticle (Cu-NP) of a size of 4 nm and the three 

model components. 

 

Figure 1: A model of digestate sludge with copper nanoparticle (Cu-NP) catalyst used in ReaxFF MD simulation 

of HTG process. The color yellow, green, blue, red and white represent Cu, C, N, O, and H, respectively.  

ReaxFF (Reactive Force Field) is a bond-order based force field method used for molecular dynamics 

simulations of chemical reactions processes. It was developed to overcome the limitations of traditional force 

fields, which often fail to accurately describe the dynamic behavior of reactive systems due to their fixed bonding 

topology and limited parameter sets (van Duin et al., 2001). ReaxFF uses a reactive potential energy function 

that allows for the formation and breaking of chemical bonds in real-time during the simulation. The method is 

based on the bond order concept, where the strength of a chemical bond is determined by its overlap with 

adjacent atoms. This approach enables ReaxFF to accurately model complex reactions involving multiple bond-

forming and breaking events, as well as changes in molecular structure and composition. For a more detailed 

description of the ReaxFF force field and its underlying methodology, interested readers should be referred to 

the original paper introducing ReaxFF (van Duin et al., 2001). 

In this study, we employed the ReaxFF parameters proposed by Monti et al. (2013) to investigate the HTG 

process. To accelerate the reaction kinetics and overcome the time and length scale limitations inherent in 

molecular dynamics (MD) simulations, we conducted the simulations at elevated temperatures beyond those 

reported in the literature for experimental conditions (Reddy et al., 2014; Ha et al., 2024). The temperature range 

used for the HTG simulations in this work was between 3,000 K and 4,000 K, which is consistent with the 

temperature range employed in a previous study by Liu et al. (2020). It was common practice to speed up the 

chemical reaction in nanoscale simulations by simulating the HTG process at a temperature higher than the 

experimental settings (Liu et al. 2020). The total simulation time was set to 250 ps with a time step of 0.25 fs 

allowing for an accurate and efficient exploration of the HTG process. 

 

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3. Results and discussions 

3.1 Formation of product phase  

In this section, we present the gas product composition for both cases: with and without the catalyst addition. 

By comparing these compositions, we can observe the impact of the addition of Cu-NP catalyst on enhancing 

HTG efficiency and improving overall process performance. 

 

(a)                                       (b) 

Figure 2: Mass fraction (%) of different phases during HTG simulation time at temperature T=3200K in the case 

without catalyst (a) and with Cu-NP catalyst (b).  

Figure 2 presents the mass fraction (%) of different phases during HTG simulation time at a temperature of 

3,200 K, highlighting the significant impact of the Cu-NP catalyst on the conversion process. In Figure 2(a), 

which illustrates the case without a catalyst, it is evident that the gas yield remains relatively low throughout the 

simulation, remaining around 60%. In contrast, Figure 2(b) displays the mass fraction distribution for the case 

with the Cu-NP catalyst. As demonstrated in this figure, the introduction of the catalyst leads to a substantial 

improvement in the gas yield, which increases to approximately 95% during the simulation period. This marked 

increase in gas yield underlines the crucial role that the Cu-NP catalyst plays in facilitating the conversion of 

digestate sludge components into valuable gaseous products.  

It is interesting to observe that in the case without a catalyst, there is a very small amount of solid phase (less 

than 3%) present throughout the HTG process. However, when employing the Cu-NP catalyst, no solid phase 

is observed in the entire HTG simulation, highlighting the superior performance of the catalyst in promoting the 

conversion of digestate sludge components into gaseous products. This observation emphasizes the 

importance of employing an appropriate catalyst to enhance the efficiency and effectiveness of the HTG 

process. 

3.2 Composition of gaseous product 

This section focuses on the composition of gaseous products generated during the HTG process, both with and 

without a Cu-NP catalyst. Figure 3 illustrates the evolution of the number of different gas molecules against 

simulation time at a temperature of 3200 K, highlighting the distinct gaseous product compositions in the 

absence and presence of the Cu-NP catalyst. 

 

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       (a)       (b) 

Figure 3: Evolution of number of different gas molecules against the simulation time of HTG process at 

temperature T=3200K in the case without catalyst (a) and with Cu-NP catalyst (b).  

As demonstrated by Figure 3(a), which shows the case without a catalyst, H2 was found to be the most abundant 

gaseous product, followed by a lower concentration of carbon dioxide (CO) and other less significant gas 

molecules. However, no methane (CH4) was observed in this context. In Figure 3(b), which depicts the case 

with the Cu-NP catalyst, hydrogen was the most abundant gaseous product. Nonetheless, a higher 

concentration of hydrogen product was observed for the case with in the presence of the Cu-NP catalyst 

compared to the case without it.  

Table 1: Mass fraction of main gaseous product from HTG process obtained from ReaxFF MD  

System Simulation 

temperature 

H2 CO NH3 CH4 CO2 

Without 

catalyst 

3,200 K 36% 17% 6% 0% 0% 

       

With Cu-NP 

catalyst 

3,200 K 62% 27% 1% 0% 0% 

 

The mass factions of main gaseous products from the simulation of HTG with and without the catalyst are 

numerically tabulated in Table 1.  Clearly, the addition of Cu-NP catalyst significantly enhances the HTG 

process, leading to a considerable increase in hydrogen gas yield. In the case without a catalyst, H2 gas yield 

was found to be approximately 36%, while in the presence of the Cu-NP catalyst, this value increased to around 

62% (Table 1). This substantial improvement in H2 gas yield can be attributed to the catalytic effects of copper 

nanoparticles, which facilitate the conversion of digestate sludge components into hydrogen. 

Interestingly, the presence of the Cu-NP catalyst also led to a reduction in the formation of ammonia (NH3) from 

6% without the catalyst to just 1% when using the catalyst. This observation can be explained by the fact that 

copper catalysts are known to promote the conversion of NH3 to nitrogen (N2) and hydrogen (H2). By 

accelerating this reaction, the Cu-NP catalyst effectively reduces the formation of ammonia during the HTG 

process. 

3.3 Catalyst aging mechanism 

The long-term stability and performance of Cu-NP catalysts in practical applications is crucial, as well as for 

optimizing their design to enhance their durability and effectiveness. The aging mechanism of Cu-NP catalysts 

during HTG processes involves the incorporation of oxygen atoms into the nanoparticles, which can lead to the 

formation of copper oxide (CuxOy).  

196



This process is primarily driven by the reaction: 2Cu + 1/2O2 -> Cu2O. However, at high temperatures and under 

HTG conditions, a multitude of radicals are present, leading us to observe that the primary oxidation reaction of 

Cu involves its reaction with hydroxyl radicals (•OH). 

 

Figure 4: Number of Oxygen atom incorporated into Cu-NP during HTG process at temperature T=3,200 K.  

As illustrated in Figure 4, which presents the number of oxygen atoms incorporated into Cu-NP during the HTG 

process at a temperature of 3200 K over time, it becomes evident that the aging mechanism follows a specific 

evolution pattern. Initially, most of the oxide forms after approximately 100 ps, indicating a rapid onset of the 

aging process. As the simulation progresses, the number of oxygen atoms incorporated into Cu-NP gradually 

increases, reaching around 25 atoms after 250 ps. 

By analyzing these findings, it can be concluded that approximately 25 oxygen atoms have been incorporated 

into the Cu-NP catalyst, leading to an oxygen content of 1% of the total atomic composition of the catalyst. We 

can estimate that complete deactivation of the Cu-NP catalyst takes place after approximately 100 catalytic 

cycles. However, it is crucial to emphasize that this estimation requires further simulations to confirm its accuracy 

and to gain a more comprehensive understanding of the aging mechanism and its implications on the catalyst's 

performance over extended periods. Another intriguing question is at which level of oxygen incorporation could 

influence the catalytic properties and performance of the Cu-NP catalyst in the HTG process. 

4. Conclusions 

In conclusion, this study has demonstrated the significant impact of the Cu-NP catalyst addition on enhancing 

the hydrothermal gasification process, leading to a notable increase in hydrogen gas yield and reduced 

formation of ammonia. By simulating the aging mechanism of Cu-NP catalysts during HTG processes, we have 

gained valuable insights into the incorporation of oxygen atoms into the nanoparticles, which can lead to the 

formation of copper oxide and potentially affect the catalytic properties and performance of the Cu-NP catalyst. 

Further research is required to validate our estimation of complete deactivation of Cu-NP catalysts after 

approximately 100 catalytic cycles, as well as to explore the impact of varying temperature conditions and 

different digestate sludge feedstocks on the long-term stability and effectiveness of these materials. By gaining 

a more comprehensive understanding of the aging mechanism and optimizing the design of Cu-NP catalysts, 

we can contribute to the development of more durable and efficient catalysts for practical applications in waste 

management and renewable energy production processes. 

Acknowledgments 

The research leading to these results has been co-funded by the Norway Grants 2014-2021 via the National 

Center for Research and Development, within the frame of the project: “Negative CO2 emission gas power 

plant” - NOR/POLNORCCS/NEGATIVE -CO2-PP/0009/2019-00 which is co-financed by program “Applied 

research” under the Norwegian Financial Mechanisms 2014-2021 POLNOR CCS 2019 - Development of CO2 

capture solutions integrated in power and industry processes. TTT acknowledge funding from the Research 

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Council of Norway (RCN), the Center of Excellence Funding Scheme, Project No. 262644, PoreLab. Co-

financing and support from Ton Duc Thang University, Vietnamese-German University, and Norwegian 

University of Science and Technology are gratefully acknowledged. 

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	Molecular Simulation of Hydrothermal Gasification of Digestate Sludge with Copper Catalyst Addition