2. CO2 Capture
The production of polyglycolic acid can be carried out through different reaction pathways, shown in
Figure 1. The routes have in common the need for methanol synthesis, where hydrogen reacts with CO and CO
2. In this case, a CCU concept could be employed to replace syngas production, with CO
2 hydrogenation to methanol addressed by use of renewable H
2 from electrolysis. The CO
2 could be obtained through different capture technologies, with the choice being mainly a function of the characteristics of the gas carrying the CO
2 to be captured and the energy resource of the process; in a general way, the lower the CO
2 partial pressure and the carbon content of the resource gas, the more expensive the capture process. The capture strategies are generally classified into three main groups when applied to power generation processes: pre-combustion, post-combustion, and oxy-combustion
[20,21][20][21]. They are briefly described below.
In pre-combustion CO
2 capture, the fuel (e.g., biomass, coal, natural gas) is firstly converted into syngas and then subjected to shift conversion to react CO and increase H
2 content, as illustrated in
Figure 2. Syngas generation occurs between 700 and 1000 °C, and the required heat is usually supplied in situ by partial oxidation or indirectly by combustion. Then, H
2/CO
2 fractionation takes place, usually by chemical or physical absorption, and the H
2 stream experiences combustion. The advantage of the strategy relies mainly in performing separation with relatively high CO
2 fugacity, in comparison with typical flue gases of the post-combustion route. The major drawback is the high capital investment, which is a consequence of the much greater plant complexity
[22,23][22][23].
Figure 2.
Overview of main conceptual routes for CO
2
capture in power generation processes.
Post-combustion CO
2 capture concept consists of removing CO
2 from flue gas (
Figure 2). This method is particularly advantageous when the CO
2 content is relatively low
[24,25][24][25]. The concept allows easy CCS adaptation to various industrial and power settings, requiring minimum change in the original plant. In this case, chemical absorption with aqueous alkanolamines is a mature solution for this separation service, readily available for commercial implementation
[26]. The technology is well known for its use in natural gas processing, with more than 6 decades of application
[20]. The main drawback is the high operating cost linked to CO
2 removal in low fugacity from a low-pressure N
2-rich stream. Absorption processes are sensitive to the presence of NOx and SOx and require a solvent makeup to compensate for losses from volatilization, inactivation, or degradation
[23].
The process of oxy-combustion (
Figure 2) involves burning fuel with pure oxygen instead of air to minimize nitrogen introduction to the system, resulting in exhausts primarily composed of CO
2 + H
2O, which has the advantage of dismissing a separation process for CO
2 removal in exchange for a process for oxygen production. The economic competitiveness of this concept is thus highly dependent on air separation performance
[27], which usually entails high power demand and high capital investment
[28].
The various possible technologies for CO
2 separation can be categorized into five generic groups (
Figure 3): absorption, adsorption, membrane permeation, cryogenic distillation, and chemical looping combustion (CLC)
[24]. Chemical absorption with amines is the most mature technology, given the experience of decades of large-scale plant operation. These chemical solvents have high reactivity with CO
2, relatively high thermal stability, and high absorption capacity. The major drawback is the relatively high heating demand for solvent regeneration
[20]. Post-combustion capture by such amines in a thermal power station involves a countercurrent contact of the gas with the solvent in a packed column operated at nearly atmospheric pressure and a temperature of 40–70 °C
[25]. Some substances commonly used for this purpose are monoethanolamine (MEA), methyl-diethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP)
[29,30][29][30]. The mechanism involved in CO
2 chemical absorption by MEA is shown in Equations (1) and (2). Its regeneration heat, expressed by mass of captured CO
2, is nearly 4 GJ/ton
(CO2) if it is applied to mitigate emissions from a natural gas power plant. An alternative to minimize heating demand is to employ phase change solvents
[31], e.g., by addition of an alcohol to MEA, which allows reduction of the amount of solvent to be regenerated, thus decreasing the heating demand associated with CO
2 capture. The general flowsheet of a standard chemical absorption plant for CO
2 removal is shown in
Figure 4 [32]. Besides chemical absorption, physical absorption is also mature and commercially available (e.g., SELEXOL, RECTISOL, NMP PURISOL), being applicable when the stream is pressurized and when CO
2 has enough fugacity. Physical solvents are less selective—implying lower CO
2 purity—but can be regenerated at lower temperatures by stream depressurization.
Figure 3.
Technologies applied for CO
2
separation process. Reprinted with permission from [30].
Figure 4.
Typical flowsheet of CO
2
chemical absorption by amine-based solvent. Adapted from [32].
Separation by adsorption can occur through physical (e.g., using zeolites, activated carbon, or metal-organic frameworks) or chemical mechanisms (e.g., metal oxides, hydrotalcites, lithium zirconate)
[33], among which physical adsorption has been more frequently used for CO
2 capture. It involves a selective interaction between the target adsorbate CO
2 and a solid material, which retains the CO
2 in its surface, to later be regenerated, usually by pressure or temperature variation. At least two vessels installed in parallel are required for continuous cyclic operation: while one tower is regenerated, another one is active in the process. The cycle duration depends on adsorbent capacity and regeneration method (it usually operates for a few hours without regeneration if it is temperature-swing, but only a few minutes if it is pressure-swing)
[34].
A relatively new concept is the use of selective membranes to separate CO
2 from a gas stream. Membranes are semipermeable barriers that can be manufactured using different materials, which can be an organic (e.g., polymer) or inorganic type (e.g., ceramic, metallic). Separation by polymeric membranes has been more relevant in the field, and it is already utilized commercially for natural gas processing, since the stream is already found at high pressure, where it offers significant advantages of operational flexibility. The following two perspectives are important in the evaluation of membrane performance: permeability (for certain pressure drop) and selectivity (permeability ratio) of desired components. These determine component recoveries and stream purities after the separation process. The main drawbacks of gas permeation are low scalability (it is manufactured in modules), low product purity, and the need to compress the feed stream to generate separation driving-force if it is received at low pressure, which generally makes the option economically unattractive when compared to other separation methods
[25]. In addition, the material may be sensitive to the presence of certain contaminants in the gas (e.g., sulfur compounds). However, membranes can be advantageous to promote process intensification in reactors and to improve reaction performance by in situ separation, as discussed in later sections
[25].
Another separation method is cryogenic distillation, which is capable of producing high-purity streams and CO
2 already pressurized and liquefied, ready to be pumped for transportation. The process involves high capital investment (due to feed gas pre-treatment, large amount of involved equipment, and requirement of resistant material for low-temperature operation) and high operating costs (linked to refrigeration), being economically competitive in large-scale applications where the feed stream has high CO
2 content (usually above 50%)
[22]. Some further advantages of this process—besides the production of pure liquid CO
2—are the absence of solvents and good scalability (economic performance is substantially improved by process scale-up). Some of the existing process designs differ in how CO
2 freeze-out is avoided or managed (CO
2 solidification may be allowed at certain conditions, depending on the process)
[22,35][22][35].
Chemical-looping combustion (CLC) uses metal oxides as oxygen carriers to convert the fuel and generate heat, in order to produce CO
2 + H
2O flue gas as in oxy-combustion (it is often classified in this category). A cyclic process of oxidation with air and reduction with fuel takes place to avoid the direct contact of air and fuel. The concept efficiently promotes CO
2 capture with low energy requirements, while avoiding the presence of N
2 in the flue gas, which not only increases the CO
2 content but also has the further advantage of avoiding the formation of NO
x. Besides high oxidation/reduction activity, the material should present long-term stability, with good mechanical resistance and minimum agglomeration. Additionally, the material should enable complete oxidation of the fuel for maximum system efficiency. Oxygen carriers meeting these requirements are under development
[36].