Article pubs.acs.org/EF
Effects of H2O and HCl on Particulate Matter Reduction by Kaolin under Oxy-coal Combustion Dong Chen, Xiaowei Liu,* Chao Wang, Yishu Xu, Wei Sun, Jiang Cui, Yu Zhang, and Minghou Xu* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China ABSTRACT: Little work has been performed on the effects of H2O and HCl to particulate matter (PM) reduction by kaolin under oxy-coal combustion. To determine the effect mechanism, a combustion experiment of pulverized coal mixed with kaolin was performed in a drop tube furnace. First, a low-pressure impactor was used as a sampling device to investigate the particle size distribution of PM when H2O and HCl were added during the first stage, and then a special sampling tube was used to collect the combustion products at 950 °C. Besides, the HSC Chemistry 6.0 thermodynamic software was also used for equilibrium calculations of sodium species in the drop tube furnace. The results indicate that H2O can enhance the absolute amount of PM0.2 captured by kaolin through promotion of the generation and diffusion of sodium hydroxide during oxy-coal combustion. However, the increase in PM0.2 generated from coal in the presence of H2O overrides the increase in the PM0.2 captured by kaolin; thus, the ratio of PM0.2 captured by kaolin to the total PM0.2 produced decreases during oxy-coal combustion. HCl can suppress the capture of sodium by kaolin and inhibit the efficiency of PM0.2 reduction by kaolin during oxy-coal combustion. and SO2 in flue gas during oxy-coal combustion are several times higher than those under air combustion.20−22 These high concentrations of CO2, H2O, HCl, and SO2 may have some effects on PM formation during oxyfuel combustion. Wang et al.18 studied the role of exhaust gas recycling on submicrometer-sized particle formation during oxy-coal combustion. The results show that H2O can increase the growth rate of submicrometer-sized particles, which was attributed to enhancement of particle nucleation. According to the previous work of our team, H2O significantly enhances the vaporization of silica and trace elements in coal and appreciably increases the formation of ultrafine particles.23,24 Our previous work24 also found that the formation of PM increases as the H2O content increases during oxy-coal combustion and revealed that H2O can enhance char conversion, promote the fragmentation of char, enhance the diffusion of O2, and increase the generation of submicrometer-sized particulates. In addition, high concentrations of these gases may also influence the reduction of PM by kaolin. Chen et al.25 revealed that a higher concentration of CO2 can enhance the adsorption efficiency of submicrometersized particles by kaolin. According to our previous work and Glarbog’s results,24,26 a higher concentration of SO2 has almost no effect on particle generation and silicon aluminate formation and thus has a limited effect on the control of particle emissions by kaolin. Nevertheless, few studies on the reduction of PM by kaolin in the presence of high concentrations of H2O and HCl have been reported. It is not known whether H2O and HCl enhance or inhibit the process of PM capture by kaolin during oxy-coal combustion or if they have no influence. The role of H2O and HCl and the related mechanisms remain unclear. To clarify the effects of H2O and HCl on PM reduction by kaolin during oxy-coal combustion, several experiments were
1. INTRODUCTION Currently in China, a large amount of particulate matter (PM) is produced by coal-fired power plants. The widely used electrostatic precipitator (ESP) can remove ∼99.9% of the total PM,1 but it has particularly low efficiency in the reduction of submicrometer-sized particles.2 Submicrometer-sized particles contain large amounts of harmful substances, such as sulfur, chlorine, and trace amounts of toxic elements.3,4 Therefore, it is necessary to reduce the emission of these particles. From previous studies,5−7 it is known that adding sorbents during coal combustion can effectively decrease the production of ultrafine particles and make up for the defects of an ESP. Many studies have demonstrated that kaolin is one of the most promising sorbents for reducing PM formed during coal combustion.6−10 The mechanism of kaolin reduction of PM has been shown to be the reaction of kaolin with volatile metals such as alkali and trace elements.6−9,11,12 Gale and Wendt11 conducted further studies of the reaction process of kaolin and alkali, elaborating the adsorption mechanism of alkali by kaolin. Previous studies have shown that the adsorption of alkali and trace elements by kaolin and the PM removal efficiency are affected by many factors, such as coal type, reaction temperature, adding mode, adding proportion, and flue gas composition.8,10,12−15 Takuwa and Naruse8 found significantly different kaolin efficiencies for capturing sodium in two different coal types. The adsorption efficiency increases with rising temperature in a specific range and then declines when the temperature reaches a certain value.13,14 The above research has offered informative details about kaolin removal of PM during air combustion. Oxy-fuel combustion has been a popular research topic, because of its potential for reducing CO2 emissions. The formation characteristics of PM during oxy-coal combustion have been studied in recent years.16−19 Our previous work16 revealed that oxy-coal combustion could substantially affect the size distribution of submicrometer-sized particles. Because of the recycling of flue gas, the concentrations of CO2, H2O, HCl, © XXXX American Chemical Society
Received: January 10, 2017 Revised: May 9, 2017 Published: May 9, 2017 A
DOI: 10.1021/acs.energyfuels.7b00077 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Properties of the Coal Sample and Sorbent Proximate Analysis (ad, wt %)
a
moisture
ash
volatile matter
3.14
16.57
11.95
Ultimate Analysis (daf, wt %) fixed carbon
C
Oa
N
S
22.62
1.01
0.97
K2O
CaO
Fe2O3
TiO2
0.98
3.69
3.44
0.88
H
68.34 72.63 2.78 Low-Temperature Ash Composition (wt %)
Na2O
MgO
Al2O3
SiO2
1.64
1.43
34.25
49.02
P2O5
SO3
0.45 4.22 Sorbent Composition (wt %)
sample
Na2O
MgO
Al2O3
SiO2
K2O
CaO
Fe2O3
kaolin
0.71
0.15
47.07
51.42
0.14
0.12
0.39
The oxygen content is calculated by subtraction.
Figure 1. Schematic diagram of experimental setup.
performed in an electrically heated drop tube furnace (DTF). A low-pressure impactor (LPI) system was used as the sampling device to investigate the effect of kaolin on the particle size distribution (PSD) of PM, and a high-temperature sampler was designed to study the adsorption mechanism of alkali vapor by kaolin under different experimental atmospheres.
Table 2. Experimental Conditions sample
sampling method
atmospherea
coal coal + kaolin sodium acetate + kaolin
I I II
A, B, C A, B, C A, B, C
a
Atmosphere A: O2/CO2 = 29/71; atmosphere B: O2/CO2/H2O = 29/51/20; and atmosphere C: O2/CO2 = 29/71 doped with 150 ppm of HCl.
2. EXPERIMENTAL SECTION 2.1. Properties of Materials. To simulate the actual conditions of coal combustion in coal-fired power plants, experiments were conducted in a DTF with a temperature of 1500 °C. A ShanXi coal sample was chosen, which has an ash fusion point of 1554 °C, minimizing the effect of slagging in the furnace. The burnout rate of the coal under different experimental conditions was >99.6%. A pure kaolin standard was used as the sorbent to investigate the effects of H2O and HCl on PM reduction by kaolin. Properties of the prepared coal and sorbent are listed in Table 1. In addition, the particle size of the coal sample was 45−90 μm, and the Sauter mean diameter of sorbent particles was 5.78 μm. 2.2. Experimental Methods. As observed in Figure 1, the experiment was conducted in a high-temperature DTF system, and the details of the furnace have been described in our previous studies.22,23 An LPI sampling system (I) was used to obtain the size distribution of the PM, and a high-temperature sampling probe (II) was used to collect the sorbents that reacted with alkali vapor. The detailed experimental conditions are presented in Table 2. In all of the experiments, the furnace temperature was set to 1500 °C, the feeding rate was maintained at 0.2 g/min with a SANKI piezo bowl vibratory feeder, and the total gas flow rate was 5 L/min. In addition, the residence time of the feeding particles in the furnace was ∼1.2 s. Three types of samples were tested in the DTF under atmospheres A, B, and C (described below). Based on the coal properties in Table 1 and the total flow rate of the gas, the calculated H2O content in the flue gas contributed by coal combustion was ∼1.5%. Atmosphere A represented typical oxy-coal combustion conditions without H2O
recycling. In atmosphere B, an extra 20% of H2O was added to simulate the recycling of H2O. Similarly, under atmosphere C, 150 ppm of HCl was added to simulate increased HCl content that was due to the recycling of the flue gas. By comparing the PM captured by kaolin under the three atmospheres, the dominant effects of the recycled H2O and HCl on PM captured by kaolin could be revealed, although the coal itself could release a small amount of H2O and HCl. 2.2.1. Experiments with Kaolin and Coal. To determine the effects of H2O and HCl on PM reduction by adding kaolin during oxy-coal combustion, kaolin was evenly blended at a kaolin-to-ash (in coal) ratio of 20 wt % with pulverized coal in a physical way. The combustion of the coal sample with and without kaolin was conducted under an atmosphere of (A) O2/CO2 = 29/71, (B) O2/CO2/H2O = 29/51/20, and (C) O2/CO2 = 29/71 doped with 150 ppm of HCl. The PM was collected by the LPI sampling system, which primarily consisted of a cyclone, an LPI, and a vacuum pump. During the experiment, the combustion product got into the cyclone and LPI, in turn, after being quenched with pure N2 (5 min/L). The entire LPI sampling system was heated up to 130 °C during the experiment to ensure that the sulfates and H2O would not condense on the particulate surface. In the cyclone, coarse particles with aerodynamic diameters of >10 μm were removed. The rest of the PM was collected and segregated into 13 stages from 10 μm down to 0.0281 μm by LPI. The aluminum foils and polycarbonate membranes were used to B
DOI: 10.1021/acs.energyfuels.7b00077 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels collect ash particles in each stage of LPI for mass analysis and chemical analysis, respectively. 2.2.2. Experiments with Kaolin and Sodium Acetate. The reaction of kaolin with coal is complicated, which is affected by many factors, and some of the factors are coupled together. With the purpose of studying the single effect of H2O and HCl on the PM reduction by kaolin, the experiment of sodium acetate/kaolin was performed. Sodium is a typical alkali metal found in coals that has a strong association with submicrometer-sized ash particulates. Sodium acetate was selected as the sodium source in this experiment, because it contributes to particulate formation but does not introduce chlorine, which would impact the ability to study the effect of HCl.27 In addition, sodium acetate has been used as the alkali source in many studies.7,8 The kaolin used in the present study was uniformly dry mixed with sodium acetate particles and then fed into the DTF by the feeding device. In order to achieve the above experimental purpose, sodium acetate was combusted to promote a stable and typical sodium source. To make the effect of the H2O and HCl on alkali adsorption ability of kaolin clearer, the feeding rate of sodium in the experiments of pure sodium acetate is much higher than that in the coal experiments, to make the absorbed sodium reach a sufficient amount. Moreover, the molar ratio of kaolin to sodium acetate was 10:1. In this way, kaolin was in excess to ensure that sodium could be captured as much as possible. This makes the fact that the sodium content in sodium acetate/kaolin experiments is higher than that in coal unimportant, because all the sodium can be absorbed if the reaction is complete. At each sodium/kaolin experiment, the ratio of kaolin and sodium acetate and the feeding rate remains constant, to make the total sodium unconverted, only allowing the change of atmosphere. With these controls of experimental conditions, the effect of H2O and HCl can be explained. In addition, note that the atmosphere is the same as that in the coal combustion experiments, except that the sorbents that are reacting with the alkali vapor were collected by a high-temperature sampling probe system instead of the LPI system. The high-temperature sampling probe system mainly consisted of a sampling tube, a wire mesh, a flowmeter, and a vacuum pump. The outer diameter and inner diameter of the tube was 20 mm and 6 mm, respectively, and the length stretched into the furnace could be adjusted to ensure a more suitable sampling temperature. During the experiment, the sampling probe was immersed in the reactor, to ensure that the temperature where the stainless steel wire mesh was located was ∼950 °C. This temperature is higher than the saturated steam temperature of sodium vapor under these conditions, and metallic sodium existed in form of vapor at 950 °C. Therefore, the sodium contained in the sample collected by the stainless steel wire mesh was combined by sorbents chemically, avoiding physical condensation of the sodium vapor.7 After the adsorption reaction was complete, most alkali-metal-adsorbing kaolin particles could be held up by the wire mesh at the top of the sampling tube. Finally, the particles were scraped down for further analysis. 2.3. Analytical Methods. In the PM sampling experiment, the aluminum foils in each stage of the LPI were weighed by a Sartorius M2P microbalance before and after the sampling, to determine the mass distribution of the PM. The polycarbonate membranes were used to collect the ash particles in each stage for further chemical analysis. In addition, X-ray fluorescence (XRF) was employed to analyze the main composition of the coal samples, kaolin, and particles in each stage of LPI. The size of the kaolin particles was determined by a Malvern laser particle size analyzer. In the alkali adsorption experiment, the collected sorbent samples were digested by HNO3/ HF (3 mL/7 mL) in a microwave digestion system and the solutions were subsequently measured by inductively coupled plasma−mass spectrometry (ICP-MS).
that, at the small mixed ratio of kaolin to coal, the PM from kaolin was
