1. Introduction
Climate change mitigation is a worldwide effort that involves all countries around the world. Among all problems, greenhouse gas (GHG) emissions have a significant impact on the environment. Regarding this matter, the Intergovernmental Panel on Climate Change (IPCC) and the United Nations Climate Change Conference have established the needs of reducing CO2 emissions (recognized as the gas mainly responsible for climate change) and mitigating the global average temperature increase. Targets were set up, proposals to reach them were provided, and some technologies were identified as a solution in counteracting this issue.
The extensive concentration of carbon dioxide in the atmosphere is a threat to environmental safety, contributing to the greenhouse effect, but CO
2 is a source of carbon for plants and can also be used as a reactant in chemical reactions [
1,
2,
3]. This concept has led to the development of Carbon Capture and Utilization (CCU) technologies, which are perceived as a more justified and socially acceptable technology for CO
2 management than Carbon Capture and Storage (CCS). However, even if they can be considered a feasible solution, their cost is still an issue. Hasan et al. proposed a national Carbon Capture Utilization and Storage (CCUS) supply chain network for a U.S. case study in their multiscale framework analysis [
4]. With the proposed solution focused on profit rather than maximizing CO
2 utilization, average profits between
$0.3 and
$17.6 per ton of CO
2 were achieved (depending on the weighted average total costs of capturing and utilizing a ton of CO
2).
There are plenty of ways to apply CCU technology wherever there is a CO2-emitting source, e.g., energy-intensive industry branches, such as energy, petrochemical and cement or iron and steel production. Additionally, CCU reactions can be supported with green technologies, such as renewable energy sources (RES).
The first step in a CCU process is the capturing of CO
2 through well-known technologies, such as oxyfuel combustion, pre-combustion or post-combustion, or as a direct-air capture process [
5,
6]. Post-combustion carbon capture can be achieved by physical or chemical separation methods, such as membranes, adsorption, absorption and cryogenic processes. Many of these technologies are already applied in industry [
7]. Pre-combustion processes capture CO
2 prior to the combustion reaction and it can be achieved with the coal gasification process or with oil or gas fuel-reforming processes [
8].
After having captured and concentrated the CO2, it can be fed to a chemical reactor for its conversion into products, such as syngas, urea, methane, ethanol, formic acid, etc. This paper is focused on the analysis of configurations to synthesize methanol and dimethyl ether (DME) from CO2.
Dimethyl ether (DME) or methoxymethane is the simplest aliphatic ether with the molecular formula CH
3OCH
3. It is a colourless, near-odourless gas under ambient conditions. It is neither a toxic nor carcinogenic compound, with properties similar to liquid petroleum gas (LPG); thus, it can be easily blended with it and used as a fuel [
9,
10,
11]. In the chemical industry, it is mainly used to produce diethyl sulphate, methyl acetate, light olefins and gasoline [
12]. Nowadays, it is considered an alternative fuel with low emissions of NOx, hydrocarbons and carbon monoxide [
2,
13]. DME can be obtained in two ways: direct synthesis and through the methanol dehydration process. The first method’s reactions are:
The process of direct DME synthesis is exothermic (c.a. 246.2 kJ/mol DME); therefore, the heat produced during the reactions has to be removed. The inlet reactant mixture is composed of CO and H2.
Methanol dehydration to produce DME is also an exothermic process. CO
2 can be used to produce methanol and then dehydrate it to DME. This has been proven to be a very economical way of utilizing carbon dioxide [
2,
14,
15]. The chemical reactions occurring during the process are presented below:
Reverse water–gas shift (rWGS)
In Ref. [
16], the authors explored the profitability of DME production from biogas; in Ref. [
17], a techno-economic assessment of bio-DME and bio-methanol production from oil palm residue was proposed. Methanol is a colourless, flammable liquid under ambient conditions with a characteristic odour. As one of the most important raw materials, it is a substrate in many syntheses of chemical compounds, including formaldehyde, acetic acid and chloromethane. It is also a very good solvent and it is easily miscible with water, alcohols and organic solvents. Due to its wide application in many industries (fuel, chemical and other industries), the demand for methanol is constantly growing, i.e., from 47 Mt/a in 2011 [
18] to 100 Mt/a in 2021 [
19]. According to the report made by the International Renewable Energy Agency [
19], in 2021, only 0.2% of global production of methanol came from renewable sources—more than 60% was converted by natural gas reformation and the rest was produced by natural coal gasification.
Methanol can be synthesized from a gas containing either carbon monoxide or carbon dioxide when it reacts with hydrogen:
The main properties of DME and methanol in comparison with LNG and diesel oil are shown in
Table 1. Moreover, several potential major applications of DME and methanol are shown below in
Figure 1 and
Figure 2.
State-of-the-Art Production of DME/Methanol from Green Hydrogen and CO2
Converting CO
2 into DME requires an energy input in the form of hydrogen, as shown in Equations (1)–(9). In order to meet the goal of reducing carbon emissions, green hydrogen is needed [
1,
5,
26,
27].
Hydrogen can be synthesized in different ways: chemical, biological, catalytic, electrochemical and thermal. To reduce the carbon footprint of the CCU process, a low-emission hydrogen production technology has to be applied. Among several possible approaches, the production of “green hydrogen” using an electrolyser powered by a renewable energy source is the most interesting [
28,
29].
Barbato et al. [
24] presented a process for carbon dioxide conversion to green methanol where CO
2 captured from a power plant was combined with hydrogen from an electrolyser powered by hydropower energy. In the calculations and conclusions, the price (with cost calculations at the prevailing rate) of methanol production resulted to be 294 €/ton and it was successfully reduced by 20% compared to the price of methanol produced traditionally.
While there are a number of pilot or demonstration-scale facilities, there are a few commercial units producing either DME or methanol on an industrial scale. Regarding existing facilities with a power plant as a carbon dioxide source, two projects developed in Germany can be taken into consideration.
The first was funded by Europe’s Horizon 2020 program (MefCO
2, Project No. 637016). Nine partners established in 2014 an international cooperation to research the feasibility of CCU technology along with the production of green methanol. The main aims of the project were to demonstrate the economic feasibility of utilizing captured CO
2 by converting it into a usable fuel, such as methanol, and further providing green hydrogen produced from excess energy from renewable sources. The source of carbon dioxide was a lignite-fired power plant located in Niederaussem, Germany. The project ended in 2019 with the development of one of the largest facilities in the European Union to synthesize methanol from CO
2 from flue gases, capable of producing 1 ton of methanol per day while capturing more than 1.5 tons of carbon dioxide per day [
30,
31,
32,
33,
34,
35,
36,
37].
Carbon Recycling International Ltd. (CRI, Reykjavik, Iceland), known for producing renewable methanol since 2007, participated in this project. CRI’s pilot unit is located near Iceland’s capital—Reykjavik. Industrial-scale green methanol production began in 2012 in the first pilot plant with an annual capacity of about 4000 tonnes of methanol (c.a. 12 t/d). The process is based on the conversion of CO
2 from geothermal sources with the hydrogen produced by water electrolysis using geothermal energy [
1,
36,
38]. The methanol produced in this facility is used in a number of applications, including blending with gasoline, biodiesel production, and wastewater denitrification. Additionally, the CRI methanol production process reduces the environmental impact by 90% compared to conventional methods [
19]. Furthermore, CRI, in cooperation with China Henan Shuncheng Group, developed in 2021 the world’s first green methanol plant with a capacity of 110,000 tons per year [
39].
The ALIGN-CCUS (Project No 271501) demonstration plant is also located at Niederaussem, Germany. The project was funded through the ERA-NET ACT program and it was co-funded by the European Commission under the Horizon 2020 program ACT [
40]. The source of the captured CO
2 is a 1000 MW power plant unit, and the hydrogen comes from a 140 kWel alkaline electrolyser providing 22 kg of hydrogen per day. The daily production of DME is about 50 kg. For DME synthesis, a Mitsubishi Power bifunctional catalyst was used, which was responsible for both methanol synthesis and the dehydration process [
41].
In this paper, we evaluate the energetic performance of a DME and methanol synthesis process fuelled by the CO
2 captured from a natural gas combined cycle (NGCC) power plant and by the green hydrogen produced using an electrolyser (alkaline). The process configuration performance is assessed by means of an exergetic analysis and compared to a post-combustion CCS. Through exergy, these two different processes can be evaluated [
42,
43,
44], assessing the quality degradation of energy and individuating sources of irreversibility.
The post-combustion CCS is a common basis to compare new strategies to avoid carbon emissions. Olaleye et al. deeply discussed post-combustion carbon capture from a coal-fired power plant [
45,
46].
Blumbert et al. presented an NG-based low-pressure synthesis process for the production of methanol with CO
2 utilization [
47]. In their paper, the authors conducted an extended exergy analysis dividing the methanol production plant into various subsystems and evaluated the performance of each of them. Nakyai et al. [
48] conducted an energy and exergy analysis of the DME production from CO and CO
2 in a single-stage process. Farooqui et al. [
49] evaluated from an energetic and an exergetic point of view a polygeneration plant with oxyfuel carbon capture for combined power and DME production. In our paper, we proposed a new configuration for DME and methanol production in a single-stage process, in which a 90% CO
2 conversion rate was achieved. We compared this process with post-combustion CCS and evaluated all of the chain from the NGCC power plant to the DME and methanol production. Through the exergetic analysis, we spotted the irreversibilities, calculated the conventional performance indicators [
45,
46,
47] and defined new performance indicators to assess the goodness of our configuration with respect to energy transformation and exergy destruction.
The proposed configuration is described in
Section 2.
Section 3 introduces the adopted methodology and the process assumptions that were made.
Section 4 presents the results of the exergy analysis and
Section 5 gathers the main conclusions of the work.
3. Assumptions and Methodology
All processes were simulated with the software Aspen Plus V10. The Peng–Robinson model was applied to the Brayton–Joule cycle of the power plant, while the Steam–Table model was chosen for the Rankine cycle of the plant. To control the process, eight design specifications were introduced to adjust the temperature at the outlet of the GT, the pinch temperature differences in the HRSG, the vent purge in the deaerator, the water make-up and the steam to be sent at the reboiler of the stripper in the PCCC unit. A calculator was used to evaluate the power generated by the plant. The natural gas properties were taken from [
51]. The combustion chamber was simulated with an RGibbs reactor, which minimized the Gibbs free energy, and the deaerator with a Flash2 separator.
With respect to the PCCC unit, for all of the blocks within this unit, the ENRT-RK model was used, which consisted of an unsymmetric electrolyte NRTL model with the Redlich–Kwong equation of state and Henry’s law for electrolyte systems. The electrolyte species were due to the presence of the amine MDEA as solvent.
To control the PCCC process, four design specifications were introduced to adjust the CO2 content in the gas stream leaving the absorber from the top, the molar composition of the residue of the stripper, the water make-up and the solvent make-up. The absorber was simulated with a RadFrac column with no condenser nor reboiler of 10 theoretical stages working at 3.8 bar. The stripper was simulated with a RadFrac column too, without the condenser, but with a Kettle reboiler fed with the LP steam from the power plant. It had five theoretical stages and a working pressure of 1.1 bar.
Regarding the DME production plant, the NRTL and the Peng–Robinson models were implemented. The former was for the distillation columns, which worked below 20 bar, and the latter was used for the other blocks, which worked at higher pressures. The single-stage reactor was modelled with an RGibbs reactor minimizing the Gibbs free energy with an implemented temperature approach of equilibrium of 20 K. The distillation columns used to separate the reaction products were all modelled with the RadFrac column. Every distillation column had 15 theoretical stages and a Kettle reboiler. Columns C-1 and C-2 were equipped with a partial condenser and their distillate was vaporous, while column C-3 had a total condenser providing a liquid distillate. DME was the primary product of the plant, while MeOH was a secondary product. The simulation included seven design specifications with the objective of setting the molar ratio between hydrogen and carbon at the inlet of the reactor and the distillates’ flow rate and purity at each distillation column.
The exergy analysis with the aim of evaluating thermodynamics inefficiencies and rigorous performances of the process was based on the conventional methodology described in the following and found in some similar papers [
45,
47].
The ambient conditions were assumed at 298.15 K and 1.013 bar. The exergy flow rate
of the
i-th stream was the sum of the physical and chemical exergies, as reported in Equation (13), and was obtainable from the Aspen Plus simulations, while the kinetic and potential exergies were neglected. For further details, please refer to [
44].
The exergy destruction rate in the
j-th component was calculated as follows:
where
,
and
are respectively the exergy of the streams fed to the
j-th component, the exergy of the streams leaving the
j-th component, and the exergy destroyed in the
j-th component.
Table 5 shows how
and
were defined for each component.
For every component, the following parameters were calculated:
where
is the sum of the exergy of the inlet streams in every process and
is the exergy of all the products of every process. For every unit, the exergy efficiency
is quantified by Equation (17).
The novel performance parameter
is introduced for the PCCC, the underground storage and the DME production plant, defined by Equation (18) as the ratio between the destroyed exergy in the unit and the non-emitted CO
2 :
The values of the mass exergy of natural gas, DME, methanol and hydrogen were miscalculated by the software Aspen Plus V10; in fact, the software underestimated those values when the components were below their ignition point. The software mass exergy values for the streams carrying the above-mentioned components were corrected by adding the mass lower heating value weighted on the components’ mass fraction in the stream. The standard lower heating value of natural gas was taken from [
50]. The standard lower heating values of DME, methanol and hydrogen were taken from [
53].
5. Conclusions
In the present study, an exergy analysis of a process configuration able to avoid carbon emissions and to produce DME and methanol is presented. The carbon-emitting source in this study was a 189 MW power plant. The Brayton–Joule cycle in the power plant was mainly responsible for exergy destruction, 75% of the produced carbon dioxide was captured to be stored underground and the remaining 25% was to be transformed into DME and methanol via reactions with hydrogen. In the plant, the total CO2 generated by the power cycle was 119,068 kg/h, the amount stored was 89,288 kg/h, the stream converted was 26,815 kg/h and the amount vented to the atmosphere was 2965 kg/h (2.5% of the total). The reaction sections converted the CO2 stream into DME (10,381 kg/h) and methanol (3214.1 kg/h). A second route was considered, i.e., the storage of 100% of the CO2 generated by the power plant.
Of those two routes to avoid carbon emissions, the underground storage offers a much lower exergy destruction per mass of non-emitted carbon. Inside the DME production plant, the main contribution to exergy destruction was from the distillation column separating the reactor outlet stream and, in particular, the top-stage condenser was found to be the component with the highest irreversibility. This DME synthesis showed a higher efficiency than the underground storage unit, but a higher exergy destruction per non-emitted carbon dioxide ratio. As a consequence, the process configuration we propose has higher exergetic efficiency and exergy destruction than full geological storage, and produces valuable compounds, such as DME and methanol. Its feasibility is strictly correlated with the development of the hydrogen industry, and a future study should evaluate the impact of hydrogen production at different carbon dioxide shares between DME production and underground storage. Future works should investigate the optimal CO2 split between Route 1 and Route 2 and extend exergy analysis to green hydrogen production.