Study on CO2 capture using nanostructure carbon adsorbents

Abstract

The increasing trend of CO2 concentration in the atmosphere is the main cause for the enhancement in global warming which is known as the biggest environmental problem. The combustion of fossil fuels by humans raises the concentration of CO2 in the atmosphere day by day. Increase in the fossil fuels burning leads to enhance the CO2 emissions in the atmosphere which increases the greenhouse effect. Among the various mitigation pathways, Carbon capture and storage (CCS) is the most significant technology to reduce the CO2 concentration in the environment by capturing it from the large point sources. The CCS has three techniques i.e. post-combustion, pre-combustion and oxy-combustion capture, among which post-combustion capture is known to be effective in the reduction of greenhouse gas (GHG) emissions. It includes several methods like absorption, cryogenics, adsorption, membrane, and other techniques. Among them, adsorption via solid materials is one of the effective techniques in CO2 capture practical applications due to its cost-effectiveness, high CO2 uptake, and low energy necessities. Porous carbons are known to be most efficient adsorbents because of their high surface area and large pore volume. These adsorbents can be developed from various low cost sources using different methods like carbonization of low-cost precursors, sol-gel method, nanocasting technique, chemical activation method, etc. Nanocasting and chemical activation methods are known to be effective methods for the development of high surface area carbon adsorbents. In the nanocasting technique, generation of porous structure and tuning of their surface area can be easily done in the preparation of carbon adsorbents. This method consists of three major steps: synthesis of the template, precursor impregnation and then template removal. On the other side, chemical activation method consists of the impregnation of carbon precursor with the chemical activating agents followed by the carbonization process (inert atmosphere) and then acid washing. This results in the formation of micropores in the structure via dehydration process and incorporates various heteroatoms like oxygen, sulfur, boron, nitrogen in the carbon framework. Such heteroatoms help to increase the Lewis basic character of the adsorbent via addition of the various basic functionalities on the adsorbent surface and beneficial to capture CO2. xxi In the present work, two different types of nanostructured carbon adsorbents have been developed i.e. nanocasted carbon monoliths and polyacrylonitrile (PAN) activated carbons. Carbon monoliths were prepared by using nanocasting technique and activated PAN adsorbents were synthesized by using chemical activation process. The CO2 adsorption experiments were performed using the fixed bed adsorption system under various conditions (5-12.5 % CO2 and 30-100 °C temperature). Regeneration, kinetics, adsorption isotherm and thermodynamic studies have been also performed in details. Finally, energy duty for desorption of adsorbed CO2 was also estimated. Porous carbon monoliths were obtained through nanocasting technique from silica monoliths (hard template) and furfuryl alcohol (precursor). These carbon adsorbents were evaluated as sorbents for CO2 capture by using fixed-bed adsorption set up under dynamic conditions. Carbonization at different temperatures (550 to 950°C) was carried out and resulted in the generation of different carbon adsorbents containing oxygen functional groups. The textural characterization results reveal the effect of nanocasting technique, which is confirmed from the generation of mesopores (0.41 cm 3 g -1 ), micropores (0.85 cm3 g -1 ) and high surface area (1225.1 m 2 g -1 ) of adsorbent carbonized at 950 °C. It shows highest CO2 uptake of 1.0 mmol g-1 at 30 °C and 12.5 % CO2 concentration. The increase in the adsorption capacity with increasing CO2 concentration and decrease with the increasing adsorption temperature confirms the physisorption process. Five adsorption-desorption cycles show established materials with excellent regeneration stability as an adsorbent. Furthermore, three kinetic models along with three isotherms were used in the present study to analyze the adsorption data and found that fractional order kinetic model and Temkin isotherm fitted best. Thermodynamic studies suggest the exothermic, spontaneous as well as the feasibile nature of the adsorption process. PAN-based activated carbon adsorbents have been synthesized by using the simple and cost-effective route of carbonization followed by chemical activation process. The effect of different activating agents like NaNH2, NaOH, K2CO3, and KOH on the textural properties of PAN and its adsorption potential for CO2 under dynamic conditions was investigated. The KOH activated carbon adsorbent exhibited the surface area of 1890 m 2 g -1 whereas, NaNH2, NaOH, and K2CO3 activated carbons showed the surface area of 833 m2 g -1 , 1020 m2 g -1 and 1250 m 2 g -1 respectively. The porosity of the adsorbents was affirmed by SEM and HRTEM analysis. Whereas, XPS analysis have revealed the various types of basic functional groups which contain oxygen and nitrogen elements on the carbon surface. The adsorbent, PANKOH shows the best CO2 uptake of 1.2 mmol g-1 which is about four times the adsorption xxii capacity of the carbonized PAN (0.32 mmol g-1 ) with the flowing 12.5 % CO2 concentration. Moreover, the adsorbents showed a stable adsorption capacity over multiple sorption cycles. The best information of the adsorption at all adsorption temperatures was given by fractional order kinetic model whereas, the best fit of Freundlich isotherm model with the adsorption data and high Qst values confirms the adsorbents’ surface heterogeneity. Thus, the present study provides a two-step synthesis process to produce nitrogen and oxygen-containing activated carbons from low-cost and commercially available PAN for its use in the CO2 capture practical applications.