Global warming and environmental problems have attracted much attention in the 20th century, and many researchers throughout the world attempt to find a solution for one of the main challenges of mankind. Beside the nature of global warming, the rapid increase of population and high consumption of energy have intensified this problem. According to the report of the Energy Information Administration (EIA) the energy consumption will raise by 57 % until 2030 (EIA, 2010). Although, there are several types of greenhouse gases (GHGs), and methane (CH4) and chlorofluorocarbons (CFCs) have much greater negative effects on the environment than CO2; but carbon dioxide is the reason of 70% of global warming (Olivares-Marín et al., 2011). Tangang et al. declared that the climate changes cause the rise of surface temperature of earth to around 3-5 0C at the end of this century and it can contribute to the ice and glacier melting, which increases sea level up to 95 cm and also the rainfall patterns (Tangang et al., 2012, Hamdan et al., 2013). These new environmental conditions can influence the food security and harm humans, settlements and infrastructures.
Carbon dioxide emission has four major sources including industrial processes, fossil fueled power plants, de-carbonization (production of hydrogen from carbon rich feed stock), and transportation (Rackley, 2010). Among the various sources, fossil fueled power plants are the first ranked to produce carbon dioxide. Fossil fuels supply 81 percent of the required commercial? energy of the ?world (Rackley, 2010), while consumption ?of? fossil? fuels ?produces ?nearly?30? Pg (petagram) of carbon dioxide annually, and it contributes to three-fourths of the enhancement of carbon dioxide in the atmosphere (McCarthy et al., 2001) (see Table 1-1). Due to these industrial activities, the CO2 concentration has had 70 ppm enhancement in the atmosphere from pre-industrial period until now (from 280 ppm to 400 ppm) while its maximum value should not exceed 350 ppm (Wennersten et al., 2014).
At present, the amount of carbon dioxide is increasing by 2 ppm per year in the atmosphere which suggests that more than a third of the carbon dioxide emitted remains in the atmosphere (Rackley, 2010). According to the Intergovernmental Panel on Climate Change (IPCC) (McCarthy et al., 2001), the atmosphere may contain up to 570 ppm of carbon dioxide in 2100 causing a rise of approximately 1.9 °C in the mean global temperature, and an increase of 3.8 m in the mean sea level (Stewart & Hessami, 2005).
There are several theoretical strategies to decrease the amount of carbon dioxide emission into the atmosphere including improving the efficiency of using the energy, replacing the fossil fuels by other energy sources (ex., renewable energy, hydrogen, nuclear energy, solar energy and so on) and carbon dioxide capturing and sequestrating by developing new carbon capture technologies (Yang et al., 2008). While every one of these techniques have several benefits and some problems, the general approach for CO2 capture has been designed by CCS strategy and it has the ability to reduce, control and optimize the overall mitigation costs by enhancing a great reduction in the greenhouse gas (GHG) emissions (Metz et al., 2005, Li & Fan 2008). CCS is a group of technologies that can reduce the emission from ?xed industrial sources to the atmosphere; which based on the BLUE Map Scenario of the International Energy Agency (IEA), this route can contribute to a 19% reduction of CO2 emissions (as most costly component of the CCS process), by 2050 (Figueroa., 2008). The simple schematic of CCS has been presented in Fig. 1.
2. CARBON DIOXIDE CAPTURE TECHNOLOGIES
The approaches to reduction of carbon dioxide from power generation plants are classified in three main categories, including: post-combustion, pre-combustion and oxy-fuel combustion. In the following, a summary of each process with its advantages and disadvantages for carbon capturing is discussed.
Post-combustion capture is a downstream process that removes the carbon dioxide from the combustion reaction product streams before emission to the atmosphere, and it can be considered as an extension to the flue gas treatment process for other gases removal such as NOx and SOx. But carbon dioxide removal is the main challengeable step in this process because of its high quantities in the gas stream (typically 5-15% v/v) (Rackley, 2010), low partial pressure of carbon dioxide in the flue gas, and relatively high temperature of flue gases (Olajire, 2010). Thus, this process requires high energy consumption and powerful adsorbents with high uptake capacity for CO2 capture. Despite these challenges, post-combustion process is a promising strategy because of its ability to capture carbon at existing units (Figueroa, 2008). In the next sections, various technologies for carbon dioxide capture at post-combustion processes including physical adsorbents and aqueous solutions have been discussed, extensively. In addition, a summary of this process has been depicted at Fig.2.
The next approach for carbon dioxide adsorption is pre-combustion capture which includes de-carbonation by gasification of the primary fuel, coal or biomass. In this way, the fuel produces mainly carbon monoxide and hydrogen due to its reaction with oxygen or air and the higher carbon dioxide concentration and partial pressure of carbon dioxide. Thus, this process requires smaller equipment and technologies than post-combustion processes with lower energy for regeneration, but the total capital cost of the generating facilities is very high (Olajire, 2010). Simple schematic of this process has been shown in Fig. 3.
As can be observed in Fig. 4, oxy-fuel combustion is a process in which the combustion of the fuel gases occurs in oxygen rather than in air or other gases, thus it contributes to the mostly pure carbon dioxide. This process is similar to the post combustion one, in which the combustion step has been modified and it contributes to the higher concentration of carbon dioxide in the flue gas. Also, higher required amounts of oxygen have increased the capital costs and energy consumption. The concentration of carbon dioxide in the flue gas of oxy-fuel combustion is higher than 80%, thus only separation and purification of this component is required, and it is one of the main benefits of this process (Figueroa, 2008). Thus, predominant strategy in oxy-fuel combustion for separation is physical adsorption, which contributes to the lower operating costs and less environmental problems (Sabouni, 2013).
In the following subsections different adsorbents in the carbon dioxide capture technologies such as amine-based absorption technology as conventional one, and new emerging technologies such as zeolites, metal organic frameworks (MOFs) and carbon-based materials are discussed in summary.