1. Introduction
As a greenhouse gas, CO
2 is directly related to global warming. Coal-fired power plants are one of the main sources of CO
2 emissions. Therefore, the capture, storage, and utilization of CO
2 in coal-fired power plants have important practical significance for mitigating global warming [
1,
2,
3]. The current CO
2 emission reduction technology routes of coal-fired power plants mainly are: integrated gas gasification combined cycle (IGCC) technology [
4], oxygen-rich combustion system [
5,
6], chemical chain combustion technology [
7,
8], physical adsorption [
9,
10], and chemical absorption method [
11]. For existing traditional coal-fired power plants, physical adsorption and chemical absorption methods are more practical methods because they do not require much change in the layout of the power plants. The alkali metal adsorbent combines the process characteristics of physical adsorption method and chemical absorption method, which not only overcomes the shortcomings of low adsorption capacity and poor selectivity to CO
2 of physical adsorption method, but also eliminates the equipment corrosion problem of chemical absorption method. At the same time, it can react with CO
2 at low temperature (60–80 °C) and can achieve complete regeneration, and it has small secondary pollution and good cycle performance, so it has received widespread attention [
12,
13,
14,
15].
However, this method also has serious disadvantages, such as slow carbonation reaction rate, low conversion rate, and excessive energy consumption for regeneration of the adsorbent. In order to improve the adsorption efficiency, some scholars have supported K
2CO
3 on the surface of different supports such as Al
2O
3 [
16], activated carbon (AC) [
17], and 5A molecular sieve [
18] to prepare supported potassium-based adsorbents. The experimental research by thermogravimetry found that under the condition of 1% CO
2 concentration, when the load of K
2CO
3 adsorbent supported on Al
2O
3 support was increased from 12.8% to 36.8%, the adsorption rate of the adsorbent was increased by 62%. However, the above research used a thermogravimetric analyzer, and the mass of the adsorbent used in the carbonation reaction is relatively small, which was quite different from the actual situation of the power plant.
Studies have shown that the CO
2 adsorption performance of potassium-based adsorbents mainly depends on the microstructure of the support. Aerogel has a large specific surface area and a high porosity with a regular pore structure, which has a high load capacity and good load characteristics for K
2CO
3. Low-cost silica aerogel materials with high porosity and high surface area had been widely used as supports for the synthesis of solid carbon dioxide adsorbents [
18,
19,
20,
21], but their active components diethylene triamine (DETA), 3-Aminopropyltrimethoxysilane (APS), and triethylenetetramine (TETA) had poor stability and toxicity. They were easily lost in the flue gas flow and during the carbonation reaction, causing secondary pollution.
Recently, some scholars [
22,
23] used K
2CO
3 solution to impregnate wet gel to prepare silica aerogel-supported K
2CO
3 adsorbent. When the design load was 20%, CO
2 adsorption amount was 1.32 mmol/g, and carbonation conversion rate was 88.62%. However, previous studies had focused on low-concentration CO
2 capture at room temperature, and the conclusions of the study were not suitable for emission reduction of coal-fired power plants. Based on the previous research by scholars, this article studies the CO
2 capture characteristics of the adsorbent under the reaction conditions which concentration of CO
2 from 5% to 15%, water vapor concentration from 10% to 20%, and the reaction temperature from 50 °C to 80 °C.
Adsorption kinetics is an important factor in adsorption characteristics, which directly affects the adsorption efficiency of adsorbents for CO
2. At present, the adsorption kinetics of amine-based CO
2 adsorbents has been thoroughly studied [
24]. However, there are few studies on the adsorption kinetics of potassium-based adsorbents. At the same time, the adsorption models used in the research are relatively few, and the mechanism of adsorption is not fully explained.
Based on these, in this paper, a two-step sol–gel method was used to prepare silica aerogel support then K2CO3/silica aerogel modified potassium-based adsorbent was obtained by wet loading. Combined with the microscopic characteristics of modified potassium-based adsorbents, a self-designed fixed-bed reactor was used to study the adsorption reaction characteristics of the adsorbents. Based on the experiments, different adsorption kinetic models were used to fit the experimental data, the model suitable for describing the adsorption process of CO2 on the modified potassium-based adsorbent was selected, and the adsorption kinetics was studied.
2. Materials and Methods
2.1. Experimental Materials and Preparation
2.1.1. Ingredients
For the preparation of highly efficient solid CO2 adsorbents, the drugs used were as follows:
Potassium carbonate (K2CO3, Bodi Chemical Company Limited, Tian Jin, China), tetraethyl orthosilicate (TEOS, Tianli Chemical Reagent Company Limited, Tian Jin, China), absolute ethanol (EtOH, Beichen Founder Reagent Factory, Tian Jin, China), 36.5% hydrochloric acid (Sinopharm Chemical Reagent Company Limited, Shanghai, China), ammonium hydroxide (Kaitong Chemical Reagent Company Limited, Tian Jin, China).
2.1.2. Preparation of Aerogel Support
K2CO3 was used as the active component, tetraethyl orthosilicate (TEOS) was chosen as the precursor of the silica gel support. The two-step sol–gel wet loading method was used to prepare the adsorbent.
TEOS reacts with water and it is hydrolyzed. The hydrolysis reaction is
The condensation reaction is
The polymerization reaction is
The total reaction of hydrolysis, condensation and polycondensation is
Completely hydrolyzing 1 mol of TEOS requires 2 mols of H2O. Properly increasing the ratio of H2O can promote the reaction. Ethanol is solvent for TEOS to react with H2O. HCl is used to adjust pH value. Under acidic condition, TEOS is prone to hydrolysis.
The operation steps were as follows: TEOS, water, absolute ethanol, and 36.5% hydrochloric acid were mixed in a molar ratio of 1:3:15:0.001. The mixture was stirred for 1 h using a magnetic stirrer (Guang Ming, Bei Jing, China) under heating in a 50 °C water bath. After cooling to room temperature, ammonia water was added dropwise according to the ratio of TEOS: ammonia water = 1:0.002. Alkaline condition helps the polycondensation reaction to occur, so ammonia water is added to change the pH value. Then stirred the mixture at room temperature for 1 min using a magnetic stirrer to form a sol. After stewing for 2 h, the beaker was tilted at 45° without liquid flowing out meant a wet gel was formed. The wet gel was aged for 48 h, then placed in an oven (Gang Yuan, Tian Jin, China) dried at 120 °C. The dried gel was placed in a muffle furnace (Ke Jing, Zheng Zhou, China) and calcined at 300 °C for 2 h to obtain a silica gel support.
2.1.3. K2CO3 Loading into Aerogel Support
K2CO3 was dissolved in an aqueous solution of ethanol at a 25% theoretical load, and a certain amount of carrier was weighed and added. It was stirred at room temperature for 12 h using a magnetic stirrer and then placed in an oven for drying. In order to avoid changing the active components, no calcination was carried out after drying. Finally, it was ground and sieved to obtain the adsorbent.
2.2. Experimental Methods
The small fixed bed experimental system used in CO
2 adsorption experiment is shown in
Figure 1, N
2 and CO
2 are supplied through a gas cylinder. The water vapor is generated by electric heating and metering controlled by the Series III metering pump (SSI, Cincinnati, OH, USA). N
2, CO
2, and water vapor are mixed in the gas mixing chamber. After passing into the dehydration device, the exhaust gas of the adsorption reaction is dried and dehydrated, then connected to an S2000 CO
2 gas analyzer (Xin Ze, Shan Dong, China) to perform on-line monitoring of the concentration.
In the adsorption experiment, the mass of the modified potassium-based adsorbent used was 2 g. In order to simulate the actual flue gas environment of the power plant, the basic experimental reaction atmosphere was: 10% CO2 concentration, 10% water vapor concentration, and the rest was N2. The parameters, reaction temperature (50 °C, 60 °C, 70 °C, 80 °C), water vapor concentration (10%, 15%, 20%), CO2 concentration (5%, 10%, 12.5%, 15%), and total gas volume (400 mL/min, 500 mL/min, 600 mL/min) were changed to obtain the influence of operating conditions on the adsorption reaction characteristics.
The decarburization performance of the adsorbent was studied by the CO
2 adsorption breakthrough rate. The calculation method is shown in Equation (6).
In the equation, η is CO2 adsorption breakthrough rate, %; is CO2 concentration in the mixed gas before the adsorption reaction, %; is CO2concentration in the mixed gas after the adsorption reaction, %.
The adsorption capacity of the adsorbent was calculated by the amount of CO
2 (mmol/g) adsorbed by the potassium-based adsorbent per unit mass, and the calculation method is shown in Equation (7).
In the equation, q is CO2 cumulative adsorption amount, mmol/g; Q is total gas flow rate, mL/min; m is the mass of the adsorbent before the reaction, g; T is the temperature of the adsorption reaction, K; T0 is 273 K.
In this paper, JEOL JSM-7800F (Japan Electronics Corporation, Tokyo, Japan) ultra-high-resolution field emission scanning electron microscope was used to observed the microscopic morphology of the adsorbent surface. ASAP 2460 nitrogen analyzer (Micromeritics, Norcross, GA, USA) was used for N2 adsorption and desorption experiments, and the specific surface area was calculated using BET method. BJH method was used to obtain the pore structure parameters. DX-2700 X-ray diffractometer (Fang Yuan Instrument Company Limited, Dandong, China) was used to obtain the crystal structure characteristics of the adsorbent before and after the reaction. Epsilon1 scientific research X-ray fluorescence spectrometer (PANalytical B.V., Almelo, the Netherlands) was used to detect the experimental loading of each component in the prepared adsorbent.
2.3. Kinetic Models
The intrinsic reaction of potassium-based adsorbent for CO
2 adsorption is the chemical reaction of K
2CO
3 with CO
2 and H
2O, which is a non-catalytic heterogeneous gas–solid reaction. It mainly includes the following basic processes: CO
2 and H
2O diffuse into the surface and pores of the modified potassium-based adsorbent; CO
2 and H
2O react with the active sites of modified potassium-based adsorbents; a dense product layer is formed. As the reaction proceeds, the product layer becomes thicker, and intra-particle diffusion becomes difficult. There are many kinetic models used to describe the adsorption kinetic properties on solid adsorbents currently. Among these models, pseudo-first order and pseudo-second order kinetic models have been widely used to represent the gas–solid adsorption process [
25,
26,
27]. Among them, the pseudo-first order kinetic model, as shown in Equation (8), mainly studies the adsorption process controlled by surface diffusion. Pseudo-second order kinetic model, as shown in Equation (9), based on the Langmuir adsorption isotherm equation, a hypothesis is established that the chemical reaction is the rate-controlling step of the adsorption process on the gas–solid interface. It mainly describes the chemical adsorption process and the formation of chemical bonds. Both the pseudo-first order kinetic model and the pseudo-second order kinetic model are applicable to gas–solid adsorption reactions on porous adsorbents. Although they can be used to obtain kinetic parameters of the adsorption process, the gas–solid adsorption process of modified potassium-based adsorbents may involve multiple processes and is more complicated. These two models may have some limitations. The Weber–Morris kinetic model [
28] mainly describes the diffusion process of substances in the internal pores of the particles during the solid adsorption process. It is not suitable for surface diffusion controlled reactions, as shown in Equation (10). The Elovich kinetic model [
29] is based on the Temkin adsorption isotherm equation and describes a series of reaction mechanism processes, including surface diffusion, internal diffusion, activation, and deactivation, etc. It is suitable for processes with large changes in activation energy, such as the Equation (11) shows. The Avrami fractional kinetic model is a crystallization kinetic model, which is suitable for describing the process of random nucleation and subsequent growth. Rodrigo Serna-Guerrero [
30] used the Avrami fractional kinetic model to describe the kinetics of CO
2 adsorption by amine-functionalized mesoporous silica gel, as shown in Equation (12). Aliakbar Heydari-Gorji [
31] proposed a modified Avrami fractional kinetic model. The model already includes multiple adsorption pathways, including surface diffusion, intra-particle diffusion, and interaction with active sites (physical and chemical). It was used to describe the CO
2 adsorption kinetics of an adsorbent with an amine active site, as shown in Equation (13).
The kinetic models are as follows:
1. Pseudo-first order kinetic model
In the equation, q is adsorption amount per unit mass of adsorbent, mmol/g; qe is adsorption amount per unit mass of adsorbent at equilibrium, mmol/g; t is adsorption time, min; k1 is pseudo-first order rate constant, min−1.
2. Pseudo-second order kinetic model
In the equation, k2 is pseudo-second order rate constant, mmol/(g·min).
3. Weber–Morris kinetic model
In the equation, kWB is Weber–Morris diffusion rate constant, g/(g·min1/2); C is constant related to boundary layer thickness.
In the equation, kE is initial adsorption rate, g/(g·min1/2); β is constant related to surface coverage and activation energy; t0=1/(kE·β).
In the equation, ka is Avrami rate constant, min−1; n is Avrami exponent.
6. Modified Avrami fraction kinetic model
In the equation, kma is modified Avrami rate constant, gn−1min−mmmol1−n; m,n are modified Avrami exponent.
4. Conclusions
In this paper, a silica aerogel support was prepared by two-step sol–gel method, and the active component K2CO3 was supported on the silica aerogel support according to theoretical loading of 25% to obtain a modified potassium-based adsorbent. Combined with the microscopic characteristics of the adsorbent, the self-designed fixed-bed reactor was used to study the adsorption characteristics of the modified potassium-based adsorbent with the help of nitrogen adsorption instrument, scanning electron microscope, and X-ray diffractometer. The kinetic models were fitted to the experimental data. The results show:
(1) The specific surface area and cumulative pore volume of the active component K2CO3 are small. After loading it on the prepared silica aerogel support, the microstructure of the modified potassium-based adsorbent obtained is improved. The specific surface area is increased to 110.46 m2/g, and the cumulative pore volume is 0.0857 cm3/g. The mesopores reaches more than 98%. The developed microstructure is favorable for the CO2 capture of the adsorbent.
(2) Among the six kinetic models, the Avrami fractional kinetic model and the modified Avrami fractional kinetic model show the highest correlation coefficients with R2 more than 0.97. 60 °C is the optimal reaction temperature. The influence of temperature on the adsorption characteristics is both positive and negative. Increasing the temperature can promote the increase of the adsorption rate. According to the characterization results, the reaction of the adsorbent to capture CO2 is that K2CO3 reacts with CO2 to generate KHCO3. This reaction is reversible and exothermic, and the excessively high temperature results in a decrease in the amount of adsorption.
(3) According to the kinetic studies, increasing the water vapor concentration increases the adsorption rate. When the water vapor concentration is 15%, the cumulative adsorption amount reaches a peak. The increase of the water vapor content can promote the formation of active sites. However, the excess water vapor makes the microstructure of the adsorbent poor and the mass transfer worse, which is not conducive to adsorption.
(4) There is an optimal value of CO2 concentration. Increasing the CO2 concentration within a certain range can increase the driving force for surface diffusion of the adsorption reaction, thereby increasing the amount of adsorption. After the CO2 concentration exceeds 12.5%, the amount of adsorption no longer increases.
(5) Increasing the gas flow rate can increase the adsorption rate as kinetic studies shows, while its effect on the amount of adsorption is two-fold. The lower gas velocity can make the contact time between the reactant CO2 and the active site longer, and the higher gas velocity can make the diffusion resistance less. Generally, with the gas velocity increases, the cumulative adsorption capacity increases first and then decreases. The gas flow rate of 500 mL/min is optimal.
The results of this article lay the foundation for the practical application of CO2 emission reduction in coal-fired power plants. The next work will study the law and mechanism of the failure of the adsorbent for multiple cycles, and the adsorption performance of the adsorbent in the presence of other acidic gases in the flue gas.