The large carbon dioxide emission (CO2) from anthropogenic sources is mainly associated with energy production processes, such as the combustion of non-renewable fuels. Since this gas is one of the main responsible for the global warming, its capture - sustainably made - from such processes, for further use as a resource in another industrial application, is an important contribution to mitigate its effects on climate change and to make those processes more environmentally friendly.
The current study was performed to analyze the energetic and solvent needs to capture the CO2 in a flue gas stream originated from natural gas (NG) combustion in boilers using ethanolamines chemical absorption. The selection of the NG was made due the fact that this fuel represents 21.3% of the global energetic supply and 36.7% of it is used by the industrial sector, besides it has a low amount of impurities, ensuring a better quality of the produced CO2. The chemical absorption using ethanolamines was chosen because it has a high CO2 selectivity, since the CO2 is an acidic gas while the ethanolamines have basic characteristics, besides there is the possibility of CO2 recovery with consequent solvent regeneration. In order to execute this study, both the NG combustion stage and the CO2 capture by chemical absorption were carried out using the Aspen Plus® simulator.
The NG combustion stage was simulated in a thermodynamic equilibrium reactor with an excess of oxygen to ensure the complete combustion of the fuel, whose composition was considered as an approximation of the average NG composition that is available in the Midwest, Southeast and South regions of Brazil. Three aqueous ethanolamine solutions and an aqueous ethanolamine solution plus an additive were considered, whose composition were: monoethanolamine (MEA) at 22% (w/w), diethanolamine (DEA) at 28% (w/w), metyl-diethanolamine (MDEA) at 50% (w/w) and MDEA with Piperazine (PZ) as an addictive at 40% (w/w) and 10% (w/w), respectively. The studied cases were defined as Cases 1, 2, 3 and 4 for the processes using the MEA, DEA, MDEA and MDEA plus PZ, respectively. The energetic and solvent needs analysis were calculated in relation to the amount of CO2 from the flue gas.
The ranging CO2 recovery from 15% to 98%, the solvent needs, expressed in kmol of solvent / kmol of CO2, fed in the absorber varied from 4.27 to 28.3 in Case 1; 5.76 to 42.3 in Case 2; 7.23 to 73.6 in Case 3; and 3.51 to 25.4 in Case 4. And the energetic needs in GJ / kmol of CO2 fed in the absorber, in the same range of recovery percentage, varied from 0.28 to 1.89 in Case 1; 0.046 to 0.31 in Case 2; 0.044 to 0.28 in Case 3 and 0.078 to 0.52 in Case 4. These figures in energy percentage, related to the energy obtained by NG combustion, ranged from 36.6% to 244% in Case 1; 5.98% to 39.5% in Case 2; 5.73% to 36.1% in Case 3 and 10.1% to 67.5% in Case 4.
Furthermore, an analysis of the specific cost was performed based on the utility cost of the process (cooling water and steam), once the solvents were recovered and recycled, and the CO2 revenue. This analysis was based on the average costs from the literature and the local market. The specific costs in US$ per kg of CO2 were defined from the difference between the expenses and the revenue, resulting in -14.07 for the Case 1; 7.93 for the Case 2; 8.25 for the Case 3; and 4.94 for the Case 4; considering 98% of CO2recovery.
The results indicate that Cases 2 and 3 are more attractive due to their lower energy consumption, even though they required larger quantities of solvent, consequently requesting larger equipment. The best net income, based on the specific costs presented, were obtained from the Cases 2 and 3 as well, according to this initial analysis, this must be enhanced with an estimative of the overall project implementation cost. Therefore, the results ensure that the use of DEA and MDEA solutions would be more appropriated alternatives for the CO2 recovery from the NG flue gas, for future use as a value-added product.