Table of Contents:
In order to analyze the environmental impact in different scenarios of electric power generation plants with the coupling of a CO, capture process in post-combustion, two cases were studied: Constant fuel flow and constant energy demand. For each of these cases, four different fuels were used: Biogas, coal, non-associated gas and associated gas. The simulation of the power plant and the CO, capture process was earned out in the Aspen Plus V8.8® process simulator.
According to the information reported by (Hasan et al., 2012), the design of the power generation plant was earned out using the Peng-Robinson method to estimate the thermodynamic properties. The simulation of the combustion chamber was earned out using the RGibbs reactor module, considering a molar ratio of air to fuel of 30:1 and a fuel flow of 1000 kmol/h for all the analyzed cases. The compositions in the mass percentage of the fuels used are shown in Table 3. Please note, associated gas refers to the natural gas found in association with oil within the reservoir. There are also reservoirs that contain only natural gas and no oil, this gas is termed non-associated gas.
The CO, capture process was designed by chemical absoiption using an aqueous solution of monoethanolamine (MEA) at 30% weight of the solvent. RadFrac balance stage block was used for the simulation of the absorber and the regenerator (see Figure 5). An equilibrium stage model of a tower packed with Sulzer Mellapak 250 Y™ type packaging was used in the absorber and in the regenerator a non-equilibrium stage model of a tower packed with Sulzer Mellapak 150 Y™ packaging.
Table 3. Fuel composition m mass percent.
Figure 5. Flow diagram of a thermoelectric power plant with CO, capture system in post-combustion usmg chemical
absorption with monoethanolamme.
Table 4. Kinetics of reactions (Zhang et al., 2018).
The chemical reactions involved in the reactive absoiption desorption process are presented in equations (5)—(11). Table 4 shows the equilibrium constants.
The power plant and capture process were simulated separately considering the combustion gases of the first process as a feed of the absoiption tower. In order to cany out the LCA of the different scenarios, it was necessary to homogenize the processes so that they could be comparable to each other. In this case, the variables were adjusted in order to guarantee a 95% molar recovery in the CO, stream in the absorber. Because the components present in the absoiption process dissociate, it is necessary to achieve a recovery of CO, in the gas output stream of the same equipment. In the case of the regenerator, the distillate flow and the reflux ratio were adjusted in order to capture the greatest amount of CO, from the combustion gas stream coming from the thermoelectric power plant and, thus, reduce the CO, emissions to the atmosphere and the environmental impact that they generate. For this reason, in all the analyzed cases, they were normalized to a purity of 99 mol% of CO,.
To develop the LCA it is necessary to define the functional units. In the case of power plants, the functional unit is 1 MWh; for capture process, the functional unit is 1 kg of CO, captured. Impact Assessment was performed with SimaPro 8® software using ReciPe EndPoint (H) method. Scenarios evaluated for carbon capture at constant fuel flow and constant energy demand are presented in Figures 6 and 7.
Figure 6. Carbon capture scenarios, at constant fuel flow, analyzed with LCA.
Figure 7. Carbon capture scenanos, at constant energy demand, analyzed with LCA.
As discussed earlier, the characteristics of the flue gases depend on the type of combustion and fuel used within the power plant, particularly the volumetric flowrate and CO, content, which may affect the capture effectiveness duiing the CO, capture. In this work, the analysis of two different configurations, plant simulation with or without CO, capture system by chemical absorption, was conducted. The type and flowrate of flue gas have a significant effect on the capture effectiveness, because of the high variation in CO, content. Energy generation in power plants may vary depending on peaks of power demands and/ or variations in electricity prices, or even due to changes in the fuel characteristics. Therefore, capture plants should be able to capture different CO, loads, depending on those changes. Two different operating scenarios for the power plant were considered, the first one for a specified flowrate of fuel to be burned and the second one for specified energy production in the turbine of the power plant. This allows us to evaluate variations in the type of fuel and energy production. Four different types of fuels were selected. In order to evaluate the effect of the type of fuel and the capture plant implementation on the environmental impact of the process, an LCA was developed by means of the commercial software SimaPro.
Case study 1: carbon capture scenarios, at constant fuel flow
It has been stated that the type of fuel has not only an important role in energy production of the power plant but in the composition of the flue gases. As reported in several works (Nagy and Mizsey, 2013), the composition of combustion gases varies depending on the fuel (Table 5). The flue gas obtained when mineral coal is burned presented the lower CO, content, while the larger concentrations of CO, were observed for flue gases coming from burning natural gases. For all studied cases, the air flowrate was specified in a value of 33000 krnol/h, such that the oxygen to fuel ratio was around 3:1, i.e., enough to guarantee complete combustion. Due to the difference in the fuel compositions, there is a slight variation in the oxygen excess, as can be noticed in the compositions of N, and O, compositions in the flue gases (Table 5). Furthermore, CO, concentration also depends on water generation during combustion, which is larger duiing burning gases.
As expected, these variations on CO, concentration clearly influence the capture plant effectiveness, energy, and solvent requirements, as well as the CO, recovery. Table 6 summarizes the energy production- consumption among each stage of the power plant and the energy consumption due to the implementation of the capture process. When implementing the capture plant, there is a significant reduction in the efficiency of energy production, due to the energy consumption in the column for the amine regeneration. Such reductions range from 19.47% to 65.27%, wherein the larger efficiency reductions are observed for the process with mineral coal. This result can be explained in terms of lower energy production duiing electricity generation, in this case, 30% lower than that obtained by burning gases. Additionally, energy demand in the desorber column is also larger during the CO, capture; there is a direct relationship between CO, content and capture process efficiency.
Regarding impact assessment for the power plant working with the different fuels (Figure 8a), the use of mineral coal and noil-associated gas present the major impact in most categories, followed by associated gas and biogas. This could be explained because, in these two cases, the fuels are directly obtained from as raw materials, while the associated gas is obtained as sub-product from oil wells
Table 5. Flue gas composition, reported in molar fraction, from the combustion of the different fuels, Case 1.
Figure 8. Life cycle impact assessment results, Case 1. (a) potential unpact of power plant (CP), (b) potential unpact of power plant + capture process (CP+CC), (c) Power plant single score (CP), (d) Power plant + Capture process single score
and biogas is produced from residual biomass, such that all the impacts related with the ecosystems exploitation are reduced. After normalization and weighting to obtain a single score (Figure 8c), it is observed that the use of coal generates the greatest potential impact. This is due to the fact that climate change has a high weight within the single score calculation and coal has the biggest impact in this categoiy.
Figure 8b shows the potential impact when the power plant is coupled to the capture process. It is noticed that, in this case, the impact of coal is reduced in several categories, while for the gases the impact is redistributed. The efficiency and solvent flowrate have an important role in these results (Table 6, Case 1). Figure 8d shows the single score results, where it is observed that implementing the CO , capture significantly reduces the environmental impact of the energy production for the biogas and coal systems, while for the natural gases this process does not represent a friendly environmental technology. As shown in Table 6, when equivalent flows of all fuels are considered, CO, produced during energy generation with biogas and mineral carbon are ahnost double that of natural gas, such that the capture process represents a larger benefit for those systems.
It is important to highlight that natural gases present a lower environmental impact during energy production, as these systems generate a lower amount of CO, to produce a kW in the power plant than the mineral coal or biogas. For the implementation of the capture process, however, those systems present a similar requirement of solvent and energy to recover a kg of CO,, compared to the coal system, and, therefore, seem to be the less effective. Furthermore, for all gases, the impact associated with the resources is increased for the CO, capture implementation, due to the solvent requirement and energy demand.
Study case 2: carbon capture and scenarios, at specified energy production
The second scenario evaluated here considers a specified energy production within the turbine of the power plant. In this case, the feed flowrate of each fuel was adjusted such that the energy production goal is reached. As in the first case, the composition of combustion gases varies depending on the fuel. From these results, we can see that flue gases with larger CO, content are obtained from biogas and coal combustion (Table 7). Although the CO, concentration increases in this second scenario, the CO, generation per MW produced in the power plant for both these fuels is reduced in comparison to the first scenario (case 1), because of an increase in the energy production (Table 8).
Table 8 presents a comparison between the net energy of the power plant and the net energy of the same process when the capmre process is coupled. In this scenario, the study cases with the lowest reduction in the energy efficiency were the associated and the non-associated gas, with 19.50% and 19.60%, respectively, and the biogas was the one with the highest reduction, since it is the system with the larger energy requirement during the capture process.
Regarding the LCA, the fuel with the greatest impact in most of the categories is mineral coal, to mention some categories, a noticeable impact is observed in climate change and human health, human toxicity, terrestrial acidification, terrestrial toxicity, and eutrophication, among others. After normalization and weighting to obtain a single score, results shown in Figure 9b seems to have a different trend than that observed for case 1. However, it is important to point out that, in both cases, natural gases present a single score close to 3, similar than that obtained in case 1. Hie real difference is observed for biogas and mineral coal, wherein the respective Eco points are significantly reduced as a result of the increase in energy production.
Table 7. Flue gas composition, reported in molar fraction, from the combustion of the different fuels, Case 2.
Table 8. Case 2. Simulation of the Power plant (CP) + Capture process (CC), with energy production m turbine equal to
Figure 9. Life cycle impact assessment results, Case 2. (a) potential impact of power plant (CP), (b) the potential impact of power plant + capture process (CP+CC), (c) Power plant single score (CP), (d) Power plant + Capture process single score
In this case, the single score (Figures 9c and 9d) indicates that implementing the capture process to the power plant working with biogas greatly reduces the environmental impact from 1.8494 kPt to 0.5684 kPt. This result can be attributed to: (i) the energy production was increased in the power plant so that the CO, generated per MW produced was diminished, (ii) A larger concentration of CO, in the flue gas that reduces the solvent requirement in the capture process.
On the other hand, the global impact of implementing this CO, capture technique to the process with the other fuels seems to be only redistributing among the different categories, and does not show a significant difference among them when the single scores are compared.
It is important to highlight that, even if the CO, capture implementation may reduce the environmental impact associated with the human health and/or ecosystems, the impact associated with the exploitation of the resources is always increased due to the energy and solvent requirements during the capture process. As discussed, the efficiency reduction observed during the capture implementation has an important effect on the environmental impact, so there is a clear incentive for optimizing the operating conditions of this post-combustion alternative in order to enhance its effectiveness.
The implementation of the CO, capture process in power plants has thus far been considered as the most mature technology to reduce the environmental impact associated with electricity production. Most research efforts in this field have been focused on performing techno-economic analysis and optimizing the energy efficiency of the capture process. However, it is essential to analyze the process from a holistic point of view, considering not only the CO, capture as a strategy to reduce the negative effects of the power plant but also by identifying new environmental effects due to the implementation of such capture process.
In this work, a Life Cycle Analysis was conducted in order to evaluate the environmental impact of different scenarios duiing the generation of electricity, as well as the energy and environmental implications of coupling a CO, capture process. For a specified feed flowrate of fuel to a power plant, the fuel with the lowest environmental impact is associated gas (natural gas found in association with oil within the reservoir), with a single score of 3.52 kEcopoints. When the CO, capture process is coupled to the power plant, the process with the greatest reductions in overall impact is coal, with 2.14 kEcopoints. For the scenario of specified energy production, the fuel with the lowest environmental impact is biogas, with a single score of 1.85 kEcopoints, and such impact is further reduced to as low' as 0.57 kEcopoints, when the CO, capture plant is implemented.
Regarding energy efficiency, power plants suffer important energy penalties when the capture process is implemented, mainly due to the energy required for solvent regeneration. For the systems considered here, those processes working with associated gas and noil-associated gas remain the most efficient in terms of net energy produced.
On the other hand, it is clear that the global demand for electricity is continuously growing and several efforts are centered on switching to less carbon-intensive systems by means of increasing the use of renewable energy sources. In this work, we evaluated the hypothetical case of using a biogas during electricity generation, bearing in mind that the environmental impact of this system should be minimal in comparison to the fossil fuels and that, for renewables sources, the implementation of a capture process should not be necessary. However, our findings indicate that even for this green fuel the environmental impact depend on the process parameters.
The environmental impact of the transport and storage of the captured CO, to depleted wrells, mines or depth saline aquifers should be examined in a next stage, in order to cousiderer the long-term effect of these processes. However, in the light of the obtained results, we consider that, before developing a more comprehensive study, it is necessary to enhance the efficiency of coupling the power plant and CO, capture process. Ош research group is undertaking this challenge by developing a model to solve a multiobjective optimization problem for each process variable and design parameter in both processes.
Further discussions should focus on exploring the benefits and weaknesses of CO, capture during real operation, considering combined technologies that are able to use different fuels, as well as variable energy demands.
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