1. Introduction
To meet the Paris 1.5 °C target, 3.75 gigatons per year of concentrated CO
2 will be generated from carbon capture facilities [
1]. As CO
2 is a stable chemical, the utilization of this CO
2 source as a raw material for the chemical industry is a challenging task [
2]. There are many articles on the reaction pathways or techno-economic assessment of large-scale technologies for the conversion of CO
2 into chemicals and fuels [
2,
3,
4,
5]. Some identified chemicals which can be produced from CO
2 with existing mature or emerging technologies including urea, methanol, salicylic acid, formaldehyde, formic acid, cyclic carbonates, ethylene carbonates, di-methyl carbonate [
5]. Among these chemicals, methanol is the feedstock for the production of formaldehyde, methyl tertiary-butyl ether and acetic acid. Methanol can also be used as a fuel or fuel blend. [
6,
7]. In addition, the methanol-to-olefins and methanol-to-propylene processes allow for the production of feedstock for consumer plastics. Methanol is a building-block commodity chemical with an annual production of over 100 million tons but the annul utilization of CO
2 as its feedstock is only 2 million tons [
8]. It is worthwhile to investigate the possibility to increase the use of CO
2 for methanol production.
The process technology of methanol synthesis is very mature and can be classified into high-pressure, low-pressure, and liquid-phase technology [
9]. Industrial production of methanol mainly uses the steam reforming of methane. A new development of methanol production technology is by direct hydrogenation of CO
2 using green hydrogen obtained from renewable energy [
10,
11] and has been demonstrated in a capacity of 1 ton/day [
12]. The technology still needs to overcome the hurdles of high cost of obtaining green hydrogen.
Two fundamental types of methane reforming processes are steam reforming (SR) and autothermal reforming (ATR) [
9]. The reactions involved are:
The SR reaction is endothermic and external heat supply is necessary. Thermal balance can be obtained in the ATR reactor via partial oxidation followed by the steam reformation of methane. The former reaction of partial combustion generates exothermic heat, which can be utilized by the endothermic reforming reaction; as such, an energy balance is achieved for the reactor. Two variations of integrating SR and ATR are the two-step reforming (TSR) and combined reforming (CR) [
13]. TSR uses a steam reformer followed by an autothermal reformer, i.e., the two reformers are connected in series. On the other hand, in CR configuration, the two reformers are connected in a series-parallel way. Part of the methane feed is added to the steam reformer and the other part is sent directly to the autothermal reformer. The output from the steam reformer is sent to the autothermal reformer for further reaction. On methane reforming, much literature has reported the industrial operation conditions of various reforming processes [
6,
7,
9,
14].
The utilization of CO
2 in natural gas reforming, by the dry reforming reaction, has been extensively studied.
However, the carbonaceous deactivation of the catalyst due to high CO
2 concentration is a critical problem [
15,
16]. The concept of combining steam reforming, dry reforming, and partial oxidation of methane, i.e., tri-reforming, was proposed for the production of syngas with desired H
2/CO ratios [
17]. The selection of appropriate reforming technologies has been conducted by an optimization study for maximum economic profit with the product H
2/CO ratio constraint [
18].
The reactions involved in methanol synthesis are [
9]:
In industrial practice, the syngas feedstock for the methanol process is specified to have a CO
2/CO ratio of about 0.5 or lower and an M module value of 2.04–2.06, in order to limit H
2O formation and carbon deposition [
7]. The M module is defined as
.
On the use of CO
2 as a raw material for methanol synthesis, Wiseberg et al. [
19] compared the direct methanol production process via the hydrogenation of CO
2 and the bi-reforming (steam reforming plus dry reforming) of natural gas process. The study concluded that the direct hydrogenation process is economically viable if the hydrogen price is lower than 1000 USD/t but the bi-reforming process is not feasible. Direct hydrogenation can reduce 87% of emissions from the CO
2 source but bi-reforming results in the increase of emissions. For the direct hydrogenation of CO
2 process, compared to the conventional methanol process, Pérez-Fortes et al. [
20] concluded that there is a net but small potential for CO
2 emissions reduction. Luu et al. [
21] compared different syngas production configurations, including steam reforming, dry reforming, bi-reforming, and tri-reforming, and concluded the dry reforming scheme with H
2 addition configuration significantly outperformed others in CO
2 emission intensity and methane reliance. A study reported that the methane uptake and the combined CO
2 emissions of the power plant and methanol plant can be reduced by adding high-purity CO
2 to the syngas feedstock for the methanol plant [
22]. The exergy analysis of a similar integrated process, i.e., steam reforming with CO
2 addition to the syngas for gas conditioning, with a steam cycle for energy recovery revealed that the major exergy losses come from the reformer, steam cycle, and methanol synthesis reactor [
23]. Zhang et al. [
24] focused on the methanol plant using tri-reforming syngas production, optimal reforming conditions in terms of reaction temperature, methane/flue gas ratio and pressure were determined by a simulation study. They concluded the plant is economic.
For the natural gas-based methanol processes, alternative process schemes can be evolved via the employment of (1) various methane reforming, i.e., SR, ATR, CR or TSR, which are applied in industry, and (2) different types of CO
2 addition. The alternative processes are depicted in
Figure 1. In addition to the major feedstock of methane and the CO
2 input, the steam and oxygen inputs are shown. Three types of CO
2 addition to the process can be identified. Type A processes use only methane as the carbonaceous feedstock of the plant, in other words, there is no utilization of CO
2. Type B processes use CO
2 as part of the feedstock of the reforming step. Type C processes add CO
2 to condition the syngas fed to the methanol synthesis step. Note that the addition of CO
2 to the SR reactor makes it involve not only steam reforming but also dry reforming. However, for easy identification, the process is referred to as B-SR in this paper.
The paper presents the comparison of various industrial natural gas-based methanol processes, each operated with different arrangements of CO
2 utilization as feedstock, as shown in
Figure 1, in terms of the KPIs (key performance indicators) of energy consumption, exergy loss, CO
2 utilization, and economic profit. Adopting industrial common operation conditions and process/product constraints, heat integrated design of each process was determined. Rigorous process simulation results were used for the analysis of KPIs.
4. Conclusions
As methanol is a key building-block commodity chemical and energy feedstock, the utilization of CO2 in the production of methanol is an important research topic. For the industrial low-pressure methanol production process with different synthesis gas generation reactors, i.e., SR, ATR, CR, and TSR, employing two CO2 utilization approaches, i.e., to serve as feedstock to reforming or methanol synthesis, this paper presents a comprehensive and in-depth study. The study accomplishes the process design with energy integration and the performance index analysis, including the energy consumption and exergy loss, net CO2 utilization, and cost and profit.
The results of this study show that:
With the utilization of CO2 in the methanol process, the major energy effects are on the SR and ATR processes. Less energy input to the process is needed (SR) or more energy output from the process (ATR) can be obtained.
With the utilization of CO2 in the methanol process, the total exergy loss is increased significantly for the ATR processes, in particular from the reactors.
The use of CO2 as part of the reforming feedstock (Type B processes) can utilize more CO2 than the direct use of CO2 as the methanol synthesis feedstock (Type C processes). The process uses the highest CO2 feed rate is B-ATR.
The utilization of CO2 in both the reforming step and methanol synthesis step of the methanol process is beneficial to the carbon dioxide reduction. The process with the highest NCR value is the B-ATR process with a value of 0.23 kg CO2/kg methanol.
The utilization of CO2 in methanol production does not necessarily lead to the increase of capital cost or manufacturing cost.
The utilization of CO2 in methanol production does not necessarily lead to a reduction of the profit. The B-ATR process has the highest IRR with a value of 41%. The process with the lowest IRR is B-TSR with a value of 12.5%.
The use of CO2 as the feedstock to the autothermal reforming in the methanol production process gives the best performance index, including the highest amount of CO2 usage, the highest net carbon dioxide reduction (0.23 kg CO2/kg methanol) as well as the highest internal rate of return (41%). This study concludes that the utilization of CO2 in the industrial methanol process can realize substantial carbon reduction and is beneficial to process economics.