Fuel and Combustion

Fuel and Combustion

Thermodynamic Equilibrium Investigation of Sustainable Dimethyl Ether Green Fuel Production from Syngas by Gibbs Free Energy Minimization

Document Type : Original Article

Authors
Reza Khoshbin 1
Mohammad Haghighi 2
1 2- Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST),
2 3- Faculty of Chemical Engineering, Sahand University of Technology, P.O.Box 51335-1996, Sahand New Town, Tabriz, Iran
10.22034/jfnc.2026.545105.1454
Abstract
Dimethyl ether (DME) is a clean fuel with a high cetane number and soot-free combustion, making it a suitable substitute for fossil fuels. However, optimizing the direct DME synthesis process from syngas (STD) requires a deep understanding of equilibrium limitations and the interaction of operating conditions. In this study, with the aim of determining the optimal thermodynamic conditions as well as evaluating the final product quality from a combustion perspective, the equilibrium of the STD process was analyzed using the Gibbs free energy minimization method. Calculations were performed using Aspen Plus software and the Peng Robinson equation of state over a temperature range of 100 to 500 °C, a pressure range of 1 to 50 bar, H₂/CO molar ratios from 0.5 to 4, and CO₂/CO ratios from 0 to 2. The effects of adding steam and inert gas (N₂) on conversion and selectivity were also evaluated. All calculations were based on the assumption of single-phase gas equilibrium. At 250 °C and 40 bar, CO conversion reached about 95% and DME selectivity exceeded 92%. Increasing the temperature to 500 °C reduced DME selectivity to below 70% and increased CO₂ selectivity to above 30%. Adding 20% steam to the feed increased hydrogen production by up to 20% but decreased DME selectivity by about 12%. In contrast, adding excess CO₂ reduced DME selectivity without significantly changing hydrogen conversion. Comparison of the direct and indirect routes showed that CO conversion in the direct route exceeds 95%, while in the indirect route it remains below 50%.

Highlights

Thermodynamic equilibrium analysis based on Gibbs free energy minimization demonstrated that high pressure (40–50 bar) and moderate temperature (approximately 250 °C) provide the optimum conditions for direct DME synthesis from syngas. Under these conditions, high CO conversion (>90%), DME selectivity (≈92%), and DME yield (≈50%) were achieved. Steam addition promoted hydrogen production but reduced DME selectivity, whereas the presence of an inert gas had only a minor influence on the process. Furthermore, the direct synthesis route exhibited significantly higher CO conversion than the indirect route, confirming its thermodynamic superiority.

 

Keywords
Dimethyl ether
Syngas
Thermodynamic equilibrium
Gibbs energy minimization
Subjects

در این پژوهش، تعادل ترمودینامیکی تولید DME به کمک روش حداقل سازی انرژی آزاد گیبس مورد بررسی قرار گرفت. نتایج نشان داد که افزایش فشار تا 50-40 بار باعث شد که تبدیل CO از حدود 60 درصد (در فشار پایین) به بیش از 90 درصد برسد. همچنین، در شرایط بهینه°C 250 و 40 بار، انتخاب‌پذیری DME به حدود 92 درصد و بازده آن به حدود  50 درصد بر پایه خوراک CO و CO₂ رسید. افزایش دما از °C 250 به °C 500 سبب افت انتخاب‌پذیری DME به کمتر از  70 درصد شد، در حالی‌که انتخاب‌پذیری CO₂ به بیش از30 درصد افزایش یافت. افزودن بخار آب به خوراک، تولید هیدروژن را تا 20 درصد افزایش داد ولی انتخاب‌پذیری DME را حدود 10 تا 15 درصد کاهش داد. افزودن گاز بی‌اثر (N₂) تنها اثر اندکی بر کاهش تبدیل CO و هیدروژن داشت. مقایسه دو روش نشان داد که در مسیر مستقیم (STD)، تبدیل CO به بیش از 95 درصد می‌رسد، در حالی‌که در روش غیرمستقیم این مقدار کمتر از 50 درصد باقی می‌ماند. در مجموع، نتایج نشان می‌دهد که بهترین شرایط برای دستیابی به بازده بالا و انتخاب‌پذیری مناسب DME، فشارهای بالا (50-40 بار) و دماهای متوسط ‌(حدود°C 250) است. با این حال، کیفیت DME تولیدی به عنوان سوخت سبز، به شدت به شرایط عملیاتی فرایند وابسته است. در دمای °C 500، محصول نهایی حاوی بیش از ۳۰ درصد CO₂ است که به مراتب فراتر از حداکثر مجاز تعیین‌شده در استانداردهای بین‌المللی سوخت DME (معمولاً کمتر از ۸ تا ۱۵ درصد حجمی) است‌. از دیدگاه احتراق، حضور چنین غلظت بالایی از CO₂، افزون بر کاهش مستقیم ارزش حرارتی حجمی سوخت (به نسبت کسر مولی(CO₂، از طریق اثرات شیمیایی ‌)واکنش (CO+OH⇌CO₂+H و حرارتی (ظرفیت گرمایی بالا) باعث کاهش شدید سرعت شعله آرام، افزایش زمان تأخیر اشتعال و احتراق ناپایدار می‌شود. اگرچه CO₂ می‌تواند به دلیل کاهش دمای شعله، انتشار NOx را کاهش دهد، این مزیت به هیچ وجه جبران‌کننده افت شدید کیفیت احتراقی و عدم انطباق با استانداردهای سوخت تجاری نیست. بنابراین، برای دستیابی به DME با قابلیت کاربرد عملی به عنوان سوخت سبز، اجتناب از دماهای بالا (به ویژه بیش از°C 450) و رعایت محدودیت غلظت CO₂ در خروجی فرایند (زیر آستانه استاندارد) ضروری است.

 
[1] M. A. Adnan and N. I. F. Mukti, "Dimethyl ether as a future energy vector: Economic feasibility and environmental sustainability," Renewable Energy, p. 124043, 2025.
[2] V. Pirouzfar, A. Zarringhalam Moghaddam, and B. Mirza, "Physicochemical properties and combustion performance of gas oil–fuel additives," J. Energy Resour. Technol., vol. 134, no. 4, p. 041101, 2012.
[3] N. K. Abiyazani, V. Pirouzfar, and C.-H. Su, "Enhancing engine power and torque and reducing exhaust emissions of blended fuels derived from gasoline–propanol–nanoparticles," Energy, vol. 241, p. 122924, 2022.
[4] G. Thomas, B. Feng, A. Veeraragavan, M. Cleary, and N. Drinnan, "Emissions from DME combustion in diesel engines and their implications on meeting future emission norms: A review," Fuel Process. Technol., vol. 119, pp. 286-304, 2014.
[5] C. Arcoumanis, C. Bae, R. Crookes, and E. Kinoshita, "The potential of di-methyl ether (DME) as an alternative fuel for compression-ignition engines: A review," Fuel, vol. 87, no. 7, pp. 1014-1030, 2008.
 
[6] D. Junfa, S. Yongsheng, Y. Zhenzhong, and S. Zhiqiang, "Combustion and emissions characteristics of diesel engine operating on composite combustion mode of DME and diesel," in Proc. 1st Int. Conf. Mechanical Engineering and Material Science (MEMS), 2012, pp. 290-293.
[7] E. Kraleva, U. Armbruster, M. L. Saladino, F. Giacalone, T. Mizugaki, and I. S. Pieta, "From CO₂ to DME: Catalytic advances, challenges, and alternatives to conventional gas-phase routes," Catal. Sci. Technol., 2025.
[8] I. M. Sivebaek and J. Jakobsen, "The effect of gasses on the viscosity of dimethyl ether," Tribol. Int., vol. 41, no. 9-10, pp. 839-843, Oct. 2008.
[9] D. A. Rodriguez-Pastor, V. Soltero, and R. Chacartegui, "Methanol to dimethyl ether (DME) assessment toward thermochemical energy storage," Chem. Eng. J., vol. 509, p. 161286, 2025.
[10] P. Styring, G. R. Dowson, and I. O. Tozer, "Synthetic fuels based on dimethyl ether as a future non-fossil fuel for road transport from sustainable feedstocks," Front. Energy Res., vol. 9, p. 663331, 2021.
[11] M. Hashemzehi, V. Pirouzfar, H. Nayebzadeh, and A. Alihosseini, "Application of response surface methodology to optimize high active Cu-Zn-Al mixed metal oxide fabricated via microwave-assisted solution combustion method," Adv. Powder Technol., vol. 31, no. 4, pp. 1470-1479, 2020.
[12] M. Hashemzehi, V. Pirouzfar, H. Nayebzadeh, and C.-H. Su, "Modelling and optimization of main independent parameters for biodiesel production over a Cu0.4Zn0.6Al2O4 catalyst using an RSM method," J. Chem. Technol. Biotechnol., vol. 97, no. 1, pp. 111-119, 2022.
[13] R. Khoshbin, M. Haghighi, and S. Oruji, "Nanostructural evolution by applying various fuels in combustion design of CuZnAl mixed-oxides over HZSM-5 used in one-step production of CH3-O-CH3," Mater. Chem. Phys., vol. 329, p. 130063, 2025.
[14] W. H. Zhihua Gao, L. Yin, L. Hao, and K. Xie, "The structure properties of CuZnAl slurry catalysts prepared by a complete liquid-phase method and its catalytic performance for DME synthesis from syngas," Catal. Lett., vol. 127, no. 3-4, pp. 354-359, 2009.
[15] M. E-Moghaddam, N. Dahmen, U. Santo, and J. Sauer, "Gasoline synthesis from biomass-derived syngas comparing different methanol and dimethyl ether pathways by process simulation, based on the bioliq process," Energy Fuels, vol. 38, no. 5, pp. 4229-4243, 2024.
[16] S. Oruji, R. Khoshbin, and R. Karimzadeh, "Thermodynamic modeling of gasoline production via thermal cracking of heavy hydrocarbons," Fuel Combust., vol. 10, no. 1, pp. 75-85, 2017.
[17] R. Khoshbin and R. Karimzadeh, "Thermodynamic analysis of ethane thermal cracking for light olefin production," Fuel Combust., vol. 9, no. 2, pp. 59-69, 2017, (In Persian).
[18] H.-J. Chen, C.-W. Fan, and C.-S. Yu, "Analysis, synthesis, and design of a one-step dimethyl ether production via a thermodynamic approach," Appl. Energy, vol. 101, pp. 449-456, 2013.
[19] W.-H. Chen, C.-L. Hsu, and X.-D. Wang, "Thermodynamic approach and comparison of two-step and single-step DME syntheses with carbon dioxide utilization," Energy, vol. 109, pp. 326-340, 2016.
[20] Z. Wang, J. Wang, F. Ren, M. Han, and Y. Jin, "Thermodynamics of the single-step synthesis of dimethyl ether from syngas," Tsinghua Sci. Technol., vol. 9, no. 2, pp. 168-176, 2004.
[21] R. Khoshbin and M. Haghighi, "Direct conversion of syngas to dimethyl ether as a green fuel over ultrasound-assisted synthesized CuO-ZnO-Al2O3/HZSM-5 nanocatalyst: Effect of active phase ratio on physicochemical and catalytic properties at different process conditions," Catal. Sci. Technol., vol. 4, no. 6, pp. 1779-1792, 2014 ,(In Persian).
[22] A. Kakoee and A. Gharehghani, "Comparative study of hydrogen addition effects on the natural-gas/diesel and natural-gas/dimethyl-ether reactivity controlled compression ignition mode of operation," Energy Convers. Manage., vol. 196, pp. 92-104, 2019.
[23] J. Jeon and C. Bae, "The effects of hydrogen addition on engine power and emission in DME premixed charge compression ignition engine," Int. J. Hydrogen Energy, vol. 38, no. 1, pp. 265-273, 2013.
[24] Xi, Zhongya, Tianyou Lian, Xiaoxiang Shi, Zundi Liu, Sibo Han, Xiaoyuan Yang, Jianguo Zhang, Wei Li, Bowen Mei, and Yuyang Li., "Co-firing hydrogen and dimethyl ether in a gas turbine model combustor: Influence of hydrogen content and comparison with methane," Int. J. Hydrogen Energy, vol. 47, no. 90, pp. 38432-38444, 2022.
[25] F. Dadgar, R. Myrstad, P. Pfeifer, A. Holmen, and H. J. Venvik, "Direct dimethyl ether synthesis from synthesis gas: The influence of methanol dehydration on methanol synthesis reaction," Catal. Today, vol. 270, pp. 76-84, 2016.
[26] A. T. Aguayo, J. Ereña, I. Sierra, M. Olazar, and J. Bilbao, "Deactivation and regeneration of hybrid catalysts in the single-step synthesis of dimethyl ether from syngas and CO₂," Catal. Today, vol. 106, no. 1-4, pp. 265-270, 2005.