# Application of Nanofluids in CO\({}_{2}\) Absorption: A Review
## 1 Introduction  
The European Union (EU) has long term strategies to reduce carbon-based energy systems till 2030 and 2050. This energy strategy is set to reduce Greenhouse Gas (GHG) emissions, increase the efficiency of energy systems, and gradually substitute fossil fuels with renewable energy sources (RES). One of the targets is to reduce carbon emissions by about 40% by 2030 [1]. CO\({}_{2}\) is the main greenhouse gas and constitutes about 77% of the emissions all around the world. The emission of an excessive amount of CO\({}_{2}\) intensifies global warming. Fossil fuel burning and the production of cement are the two main CO\({}_{2}\) emitting sources [2]. One way of decreasing the emission rate of CO\({}_{2}\) to the atmosphere is its separation and collection, utilization, or storage for further application. There are several separation methods to capture CO\({}_{2}\) from various gas streams. Adsorption, absorption, cryogenic, and membrane separation are the main technologies used in CO\({}_{2}\) capture [3, 4]. Much progress has been made in CO\({}_{2}\) absorption by gas-liquid, some of them are proposed to reduce the energy demand. The gas-liquid absorption technique can also be applied to other renewable energies for the absorption of carbon dioxide. Typically, the novel absorption methods that use solvents benefit from physical and/or chemical phenomena to improve the absorption process. This technique has higher efficiency in low concentrations of CO\({}_{2}\), therefore it has a better performance in separating CO\({}_{2}\) from the flue gas mixtures of post-combustion processes that contain low concentrations of CO\({}_{2}\)[5]. The probable chemical absorption mechanism of CO\({}_{2}\) is that one NH\({}_{2}\) group attacks CO\({}_{2}\) with the free electron pairs of its N atom and as a result, a hydrogen bond is formed with another NH\({}_{2}\) group. The chemical absorption of CO\({}_{2}\) in flue gas by a solvent is a selective and reversible chemical reaction. One of the major challenges is to minimize the energy demand of this process [6]. Many different solvents have been used for CO\({}_{2}\) chemical absorption so far. The selection criteria of a suitable solvent are: fast kinetics of the reaction; high capacity ofabsorbent; low regeneration energy demand; and low rate of solvent degradation [7]. Today, amine solvents are the most widely utilized chemical solvents for CO\({}_{2}\) capture. The reasons are their high capacity, high thermal stability, and the high reaction rate of amine solvents with carbon dioxide molecules [8]. The sterically hindered amines and simple alkanolamines are the two major types of amine absorbents. Monoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine (MDEA) are primary, secondary, and tertiary amines that are classified as simple alkanolamines. All amine absorbents are different in some respects, such as stability, absorption capacity for CO\({}_{2}\), the kinetics of reaction with CO\({}_{2}\), and corrosion products. MEA is considered the standard amine solvent for carbon dioxide absorption; the reason being the high efficiency (90%) of CO\({}_{2}\) capture, fast kinetics, high mass transfer rate, and capability of CO\({}_{2}\) absorption even at atmospheric and low pressures of blue gases [9]. Nevertheless, since absorption of CO\({}_{2}\) with chemical solvents containing amines is a mature and economically feasible method, that can be used on large scale to treat blue gas streams with high volumes, it has been at the center of attention in recent years [10; 11]. This method also has some drawbacks including high energy demand for solvent recovery, expensive materials, weak performance in selective absorption from acidic gases, corrosivity, the occurrence of side reactions, and resultant water-saturated gas, and being detrimental to the environment [12; 13]. CO\({}_{2}\) absorption processes with the physical solvents Selexol and Rectisol need high-pressure conditions and are governed by Henry's law.