3.2.2. σ-Potential

The σ-potential describes the a ffinity of the DESs to biogas impurities (CO2, H2S, L2, L3, D4) (Figure 6). The σ-potential diagram can also be divided into the same three fragments as in the σ-profile. The higher negative value of μ(σ) [kcal/molÅ] indicates stronger interaction between compounds. On the other hand, the higher positive values of μ(σ) sugges<sup>t</sup> stronger repulsive interactions.

**Figure 6.** σ-potential of ChCl:U (1:2), ChCl:OA (1:2), CO2, H2S, L2, L3, and D4.

The graphic results show that all contaminants of the model biogas have parabolic curves of σ-potential. The negative values of μ(σ) in the non-polar segmen<sup>t</sup> indicate the non-polar nature of CO2, H2S, and siloxanes. The σ-potential of ChCl:U (1:2) and ChCl:OA (1:2) show negative values in the HBD, HBA, and non-polar region. This indicates that both DESs will tend to interact with hydrogen bond acceptor and donor surfaces and nonpolar molecules. The positive values of σ-potential in the HBA and HBD region of all impurities sugges<sup>t</sup> that electrostatic interactions are probably the main driving force of the absorption process. In addition, the high negative value in the non-polar region of the DESs suggests a strong affinity to all biogas impurities. In addition, the similar σ-potential shape of both DESs suggests similar dissolution capabilities for all of the impurities.

## *3.3. Economic Evaluation*

The main factor that determines the success of an investment is the economic cost [71]. The capital and running costs of biogas upgrading technology depend primarily on the size of the installation, type of technology, type of installed devices (their number and power), degree of technological advancement (degree of modernity and automation), system configuration, etc. Therefore, these costs are a function of many factors. The described technology for biogas upgrading assumes that the resulting bio-methane product will meet the quality standards of natural gas [72]. This enables the bio-methane to be introduced into natural gas installations. This is very important from an economic point of view because bio-methane does not require a specially dedicated infrastructure, which increases investment costs.

In order to better compare the cost of applying DESs, the size of installations and process streams were adopted from previous studies [55,56]. Based on an assumed biogas flow rate (813 m<sup>3</sup>/h), estimated annual DESs consumption, and assumed biogas composition (CH4 (64.9%; 31.0% CO2; 3.0% H2O; 1.04% H2S, and 0,02% of L2, L3, and D4 [8,54]) the amount of raw biogas (7.13 Mm3) supplied for installation per year was calculated. In addition, methane losses of 5% during the biogas upgrading process were assumed based on COSMO-RS theoretical calculations. The annual bio-methane production was calculated as 4.27 Mm<sup>3</sup> per year. Based on the solubility of individual biogas components in DESs, the saturation time of absorbents was calculated using the COSMO-RS model (Table 5). In order to obtain reliable information about the cost of 1 m<sup>3</sup> of pure bio-methane, the complete cost analysis including the total investment, operating, and maintenance costs was calculated.


**Table 5.** List of individual impurities and their solubility in DESs.

## 3.3.1. Investment Cost

The literature review indicated that the process scale of the biogas upgrading technology is the most important factor in the total capital investment cost (*TCIC*) calculations [73,74].

In this study, an absorption capacity of 427 m<sup>3</sup>/<sup>h</sup> was obtained for the assumed flow rate of raw biogas, absorption and desorption column dimensions, and 8600 operating hours per year (Table 6). The assumed process parameters enabled the estimation of the individual equipment cost (*EC*) according to Equation (6). The *EC* costs (Table 6) include *EC* for the upgrading biogas section but do not include the biogas production sections. The values presented in Table 6 are average amounts from previous works [55,56]. Nevertheless, to minimize the risk of overly optimistic calculations, fluctuations in the market price of individual materials, i.e., steel and electronic components in the years from 2015–2020 were included [75].



SD—standard deviation.

A total *EC* cost estimate was necessary to calculate the total capital investment cost (*TCIC*). *TCIC* was estimated mainly on the basis of the value of equipment cost (*EC*) [55]. In addition, statistical data for absorption technologies and laboratory processes scaling data were used for the estimation of the *TCIC* [55]. The general *TCIC* analysis for ChCl:U (1:2) and ChCl:OA (1:2) is presented in Table 7. The calculated *TCIC* for absorption using DESs was in the range of 3,152,088–3,164,929 EUR. The obtained *TCIC* is comparable to the *TCIC* of amine scrubber (3,166,000 EUR), pressure swing adsorption (3,140,000 EUR), and membrane separation (3,033,000 EUR) calculated for installations with a capacity of 500 m<sup>3</sup>/<sup>h</sup> bio-methane. A much lower *TCIC* was obtained for the water scrubber (2,794,000 EUR) [76].


**Table 7.** The general estimate the total capital investment cost (*TCIC*) for DES.

> SD—standard deviation.

#### 3.3.2. Operation and Maintenance Cost

The annual fixed operating costs (FC) included the operation and maintenance cost (OC and MC) of biogas upgrading plants. The OC and MC included the costs of maintenance, operating, labor, and taxation, which are presented in Table 8. The cost of DESs was calculated for the scrubber volume (2.35 m3), which was doubled in order to maintain the continuity of the process.



SD—standard deviation.

Due to the di fferent absorption capacity of DES and regeneration cycles, the energy consumption in the absorption processes was di fferent. Based on previous studies, it was assumed that ChCl:U (1:2) and ChCl:OA (1:2) can be regenerated 73 and 60 times, respectively, without loss of absorption capacity. From an economic and industrial point of view, recycling and reuse of DES after the absorption process is highly desirable because it reduces annual operating costs and the amount of waste. Numerous regeneration cycles can be achieved due to highly reversible absorption, which mainly depends on the structure and thermal stability of DESs. HBDs play the main role in the thermal stability of DESs, which depends mainly on the weak intermolecular interaction. The decomposition temperature of urea in ChCl:U is about 172.40 ◦C [77], while the decomposition temperature of oxalic acid in ChCl:OA is about 134.84 ◦C [78]. Both temperatures are higher than the temperature required for regeneration, which is enough to ensure long absorption–desorption cycles. However, the ChCl:OA structure and its lower decomposition temperature result in a slightly lower number of regeneration cycles. After a number of regeneration cycles, DESs must be replaced to further ensure the high quality of bio-methane. The other costs of OC and MC was estimated based on the literature [74,79] and using percentage factors of *TCIC*. The costs in Table 8 (FC, DPC, GE) are averaged values for selected European Union countries, i.e., Sweden, Germany, France, Norway, and Poland for which standard deviations have been determined. The one-time cost of replacing the absorbent is 66,816 EUR and 74,277 EUR for ChCl: U and ChCl: OA, respectively. Due to the 5-fold (ChCl: U) and 6-fold (ChCl: OA) exchange of absorbents to ensure the high quality of bio-methane, the total cost of replacement is 334,080 and 445,662 EUR for ChCl:U and ChCl:OA, respectively.

The total OC and MC cost for ChCl:OA (703,047 EUR) is comparable with amine scrubber (688,000 EUR) and membrane separation (662,000 EUR), while the total OC and MC cost for ChCl:U (591,465 EUR) is more comparable with water scrubber (513,000 EUR) and pressure swing adsorption (557,000 EUR) [76].

#### 3.3.3. Economic Comparison of the Overall Biogas Upgrading Process

It is di fficult to clearly estimate the costs of individual technologies due to the di fferences in the cost of components, materials and utilities, and local conditions. Therefore, is important to consider the total annual cost (*TAC*) of the biogas upgrading process, which was 982,510 ± 78,601 EUR (ChCl:U) and 1,095,685 ± 87,654 EUR (ChCl:OA) in the economic analysis. The *TAC* cost for ChCl:U was very similar to the *TAC* for pressure swing adsorption (970,000 EUR), while the *TAC* obtained for ChCl:OA was very similar to the *TAC* for amine scrubber (1,104,000 EUR) and membrane separation (1,061,000 EUR). The lowest *TAC* is for the water scrubber (880,000 EUR). Based on the above calculations, the unit cost of 1 m<sup>3</sup> of pure bio-methane was determined by means of Equation (4). The obtained unit cost of 1 m<sup>3</sup> of bio-methane was 0.35 ± 0.03 EUR/m<sup>3</sup> and 0.37 ± 0.03 EUR/m<sup>3</sup> for the physical absorption process using ChCl:U (1:2) and ChCl:OA (1:2), respectively. The unit cost for various biogas treatment technologies can be ordered as follows: amine scrubber > membrane separation > ChCl:OA (1:2) > ChCl:U (1:2) > PSA > water scrubbing [76] (Table 9). The values include the average standard deviation (8%), which was adopted based on the above calculations. Table 9 contains only the total *TAC* and UC values without standard deviations due to the lack of data from other studies. The main advantage of the innovative method based on DES is the cost of biogas upgrading compared to the most commonly used absorbents.


**Table 9.** Comparison of economic analysis.

The application of traditional absorbents (water, amine), requires further biogas refinement operations, which involves additional costs, while the use of DES ensures that high-quality bio-methane is obtained in a one-step process. The obtained results indicate that biogas upgrading technology by means of DESs is a competitive technology for all currently used methods in the industry.
