Removal of Fine Particulate Matter by Spraying Attapulgite

Apr 6, 2016 - scanning electronic microscopy coupled with energy dispersive spectroscopy (SEM/EDX). In order to optimize the removal process, the ...
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Removal of Fine Particulate Matter by Spraying Attapulgite Suspending Liquid Huichao Chen,*,†,‡ Wei Wu,† Cai Liang,† and Xin Wu† †

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, P. R. China ‡ School of Civil and Environmental Engineering, University of New South Wales, NSW 2052 Sydney, Australia ABSTRACT: Removing fine particles generated from coal combustion is important for air pollution abatement because of the impact such particles have on the environment. Removal of fine particles was investigated by spraying attapulgite suspending liquid. Element change in fine particles before and after spraying attapulgite suspending liquid was examined by analysis of scanning electronic microscopy coupled with energy dispersive spectroscopy (SEM/EDX). In order to optimize the removal process, the influences of variables such as attapulgite suspending liquid flow rate (FASL), air to attapulgite suspending liquid ratio (RA/L), attapulgite mass fraction (MAtt), agglomeration room temperature (TAR), pH of attapulgite suspending liquid, and molar ratio of calcium to sulfur (MCa/S) on the removal efficiency of fine particle were investigated. Results indicate that increasing FASL, MCa/S, MAtt, and RA/L is all of benefit to particle agglomeration and enhancement of fine particle removal efficiency. The improved physical adsorption properties and increased particle surface potential are favorable to particle agglomeration at low pH value. The decreased size of droplet and retention time of the suspending liquid in agglomeration room result in low PM2.5 removal efficiency attributed to increasing TAR. Many fine particles cohered at the surface of attapulgite indicates an enhancement in particles agglomeration after spraying attapuligte suspending liquid. Elements such as sodium, potassium, and sulfur are much easier to concentrate on submicron particles, while silicate and iron do not concentrate. The results highlight the potential of attapulgite suspending liquid for fine particles removal. It would be a promising application for removing fine particle if the process can be further optimized and cost is reduced. cyclones, wet scrubbers, etc., ESPs and fabric filters can capture up to 96−99%5 of total particulate emission, however, the removal efficiency for PM2.5 is greatly reduced. There is still a high concentration of PM2.5 that escapes the particle capture devices. Under the circumstance to solve the problem of frequent occurrence of extreme haze events, improve the air quality, and meet the requirement of stringent air quality standard, it is urgent to investigate and develop high efficiency low cost approaches to efficiently reduce fine particle emission. The highly effective technique for removal of fine particles from flue gas is to enlarge the size of some microns by a preconditioning technique and capture by conventional devices or reduce the formation of fine particles during coal burning. One example is using solid absorbent to capture and enlarge submicron particles, based on the mechanism of physical adsorption and chemical reaction. However, to date few reports have been presented on fine particles removal by chemical agglomeration. Only several were reported focusing on the effect of sorbents like kaolinite and bauxite on particle agglomeration. By adding limestone during coal combustion, PM2.5 in flue gas was sharply decreased.6,7 The dust removal efficiency was depended on the swirl and performance of gas− solid and gas−liquid flow in the bed by raising humidity in the circulating fluidized bed.8 Kaolinite could effectively capture

1. INTRODUCTION Coal, as one of the most abundant energy sources in China, is still believed to remain the major kind of energy for the next several decades. Coal combustion is considered one of the major sources of particulate matter emission to the atmosphere. In 2014, particulate matter (PM) emission from industries totaled 14.561 million tons in China, which occupied 83.6% of the total PM of 17.408 million tons.1 Though most of the large particles in flue gas are effectively captured by air pollution control devices, fine particles (with aerodynamic diameters not larger than 2.5 μm, PM2.5) typically have low collection efficiency and comprise a large fraction of aerosols emitted into the atmosphere.2 These particles are reported to be harmful to the human health because the hazardous matter, including trace metal, acids, organic compounds etc., is enriched on PM2.5 because of their abundant surfaces3 and is also a main contributor for other pollution phenomena, such as visibility degradation and global climate change. Once emitted into the atmosphere their long residence time will increase the potential of exposure to humans. Recently, the frequent occurrence of extreme haze and smog events in China has not only attracted global concern due to health effects4 but also triggered the Chinese government to tackle the serious air quality problem on PM2.5 pollution. It is the first time for Chinese Ministry of Environmental Protection to include PM2.5 in the National Ambient Air Quality Standards (NAAQS; GB3095-2012) published in 2012. Among the traditional particle capture technologies such as electrostatic precipitators (ESPs), fabric filters (baghouse), © XXXX American Chemical Society

Received: October 22, 2015 Revised: March 26, 2016

A

DOI: 10.1021/acs.energyfuels.5b02491 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analysis of U.S. Bituminous Coal (%) proximate analysis

ultimate analysis

coal type

Cad

Had

Oad

Nad

Sad

FCad

Vad

Aad

Mad

U.S. bituminous coal

67.42

4.14

7.98

1.04

3.05

48.29

35.34

9.85

6.52

Table 2. Composition of Limestone (Mass Fraction, %) sample

CaO

SiO2

Al2O3

MgO

Fe2O3

K2O

Na2O

TiO2

Others

LOI

limestone

55.99

0.392

0.1

0.538

0.050

0.01

0.041

0.004

0.175

42.7

Table 3. Composition of Attapulgite (Mass Fraction, %) sample

C

O

Mg

Al

Si

Cl

K

Fe

Ca

attapulgite

33.3

31.14

4.85

5.13

20.01

0.13

0.875

3.765

0.8

Figure 1. Six kW CFB experimental apparatus. 1-CFB furnace; 2-feeding system; 3-cooling water; 4-windbox; 5-degreaser and dehumidifier; 6-buffer tank; 7-flowmeter; 8-back pass; 9-nozzle; 10-agglomeration room; 11- peristaltic pump; 12-suspending liquid tank; 13- ELPI; 14-flue gas analyzer; 15-sample collector.;16-vacuum pump.

emission during coal combustion in a 6 kW circulating fluidized bed to find an effective and low cost way to remove fine particles in flue gas.

sodium vapor, but the efficiency depended on coal type during coal combustion.9 These research works are mainly focused on the effect of some conventional sorbents like kaolinite, limestone and TiO2 on heavy metals removal. The promotion of fine particle agglomeration are limited, i.e. removal efficiency of fine particles by adding kaolinite as low as 60%,10 and the cost is high for TiO2 with low recycling rate. Thus, to find the effective and low cost sorbents to promote fine particle agglomeration is essential for the chemical agglomeration technology. Attapulgite is used because it is a kind of crystalloid hydrous magnesium−aluminum silicate mineral, having a special laminated chain structure in which a crystalline lattice displacement exists and high surface area up to 350 m2/g and strong absorbability of 150% and stable property.11 The high surface area and the absorption center in the surface of attapulgite may make it suitable for promoting fine agglomeration. Therefore, experimental work is carried out to investigate the effect of attapulgite suspending liquid on PM2.5

2. EXPERIMENTAL SECTION 2.1. Materials. U.S. bituminous coal, of the particle size of 0.6−1.5 mm, is burned in a 6 kW circulating fluidized bed. The proximate and ultimate analysis of coal is displayed in Table 1. The limestone with the particle size of 0.105−0.9 mm is from Huaibei, China. The attapulgite is from Mingguan, China, with the particle size of less than 74 μm. The surface area and pore volume of the attapulgite have been tested to be 143.63 m2 g−1 and 0.35 cm3 g−1, respectively, with an average pore size of 9.72 nm. Compositions of the limestone and attapulgite are presented in Tables 2 and 3, respectively. Sodium dodecylbenzenesulfonate of 0.48 g/L is used as a surfactant.12 The attapulgite suspending liquid, with the pH of 7, is made by water and attapulgite. A blender stirring at 300 r/min is used to prevent depositions of attapulgite in the suspending liquid. 2.2. Methods. The fine particulate matter in flue gas is from coal combustion in a 6 kW circulating fluidized bed system. Figure 1 illustrates the experimental apparatus. It is composed of a circulating B

DOI: 10.1021/acs.energyfuels.5b02491 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels fluidized bed with an inner diameter of 66 mm in dense bed and 79 mm in dilute bed and the height of 2400 mm, a cyclone, and a return feeder. The unburned carbon and coarse particles can be collected by a cyclone and returned to the bed through the return feeder to ensure their complete combustion. Coal is continuously supplied by a feed screw, and air is preheated before it is supplied to the furnace for coal combustion. An online flue gas analyzer is used to monitor the complete combustion of coal. Bed temperature (TB) is controlled at around 900 °C during coal combustion. Fine particles in flue gas are collected by an electronic low pressure impact (ELPI) made by Dekati company, Finland. There are 13 stages (12 channels), and the measurement size range is 0.023−9.314 μm aerodynamic diameter. An agglomeration room made with 316SS is 25 × 2.5 mm in diameter and 1600 mm in height to be used for fine particle agglomeration. Temperature of the agglomeration room (TAR) can be set and controlled during the experiment. Water and attapulgite suspending liquid is sprayed in the flue gas by a pump and atomizer. The retention time of flue gas in the agglomeration room is about 2 s. The samples gas is routed through an isokinetical sampling gun to a cyclone where particles larger than 9.314 μm is separated. The samples gas is then diluted with particle free, hot dry air (150 °C) prior to entering the ELPI measurement system. In this way, condensation of water vapor on the wall of sampling pipelines and the impact plate of the ELPI could be avoided. 2.3. Analysis. Particle size distribution in number concentration and mass concentration of the collected particles is measured by ELPI. The total removal efficiency of fine particulate matter after spraying attapulgite suspending liquid is determined by measuring inlet and outlet number concentration of the particles. The grade efficiency of fine particulate matter is calculated by inlet and outlet measurement of each particle size range. eq 1 and 2 are used for the calculation of removal efficiency and grade efficiency, respectively. Ninlet − Noutlet × 100% Ninlet

RE =

REi =

Ninlet, i − Noutlet, i Ninlet, i

Figure 2. Effect of attapulgite suspending liquid flow rate FASL on size distribution of PM2.5. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass fraction MAtt: 1%; air to suspending liquid ratio RA/L: 0.5; original: without spraying any water or suspending liquid; water: spraying water at 38 mL/min).

particles and saturated water vapor which is formed at the agglomeration room after spraying water. It helps to grow into large droplets containing fine particles and remove fine particles to some extent.13 Both the number concentration (denotes as NPM2.5) and mass concentration of fine particles are further decreased with attapulgite suspending liquid sprayed. At a flow rate of 38 mL/ min attapulgite suspending liquid, the NPM1 and NPM1−2.5 are reduced by about 82% and 65%, respectively, and the MPM2.5 is reduced by about 72%. With flow rate increased, further decrease in NPM2.5 is observed but the reduction becomes small. Spraying attapulgite suspending liquid, fine particles in flue gas can be easily wetted and cohered to each other due to the liquid bridging force among the wet particles when they passed through the agglomeration room. The scattered particles are connected to be clumps or chains because of the bridging form of attapulgite atom chains, which is mainly due to the adsorptive flocculation of attapulgite suspending liquid.14 When the liquid in particle surface vaporizes, the liquid bridging force is converted into solid bridging force and the agglomerating force between particles is strengthened. Moreover, because of the low hydration free energy of the solved ions like Cl− and SO42− in fine particles, which can be the first to be dissolved into water and the cation will locate at the particle surface and enhance the particle surface potentials, which is in favor of fine particles agglomeration.15 The grade efficiency presented in Figure 3a illustrates that spraying water in flue gas removes some fine particles of all size ranges. The grade efficiency is further increased with the attapulgite suspending liquid flow rate increased, and a high removal efficiency of particles less than 0.1 μm is achieved. This is mainly attributed to the strong effect of particle collision and diffusion, especially for particles less than 0.1 μm, in which the diffusion effect is predominant. Equations 4 and 516 can be used to calculate the removal efficiency of particles (ηBD) less than 0.1 μm because of particle diffusion:

(1)

× 100% (2)

where RE is the removal efficiency, Ninlet is the number concentration of particles from inlet, Noutlet is the number concentration of particles from outlet, REi is the grade efficiency, Ninlet,i is the number concentration of particle size range of i from inlet, and Noutlet,i is the number concentration of particle size range of i from outlet. The morphologies and the elements analysis of the collected particles are observed by field emission scanning electron microscopy/ energy dispersive spectrometry (FESEM-EDS, Model FEI SIRION 200). The mass percentage of the element (wt) detected in collected fine particles is calculated by eq 3 as follows:

wt =

mi × 100% m

(3)

where mi is the mass of the element in the particles and m is the mass of the collected fine particles.

3. RESULTS AND DISCUSSION 3.1. Effect of Attapulgite Suspending Liquid Flow Rate (FASL) on PM2.5 Emission. Figure 2 presents the effect of attapulgite suspending liquid flow rate on PM2.5 emission. Dp is the particle diameter of the collected particles. It is observed that spraying 38 mL/min water (without attapulgite) in flue gas reduces fine particle emissions. The number concentration for particles of less than 1 μm (denotes as NPM1) and 1−2.5 μm (denotes as NPM1−2.5) in flue gas is reduced by about 44% and 32%, respectively, and the mass concentration of PM2.5 (denotes as MPM2.5) is reduced by 39%. The reduction in number and mass concentration of fine particles is mainly attributed to the heterogeneous condensation between fine

ηBD =

Pe = C

1.71Pe−2/3 (2 − ln ReD)1/3

u0Dc D

(4) (5) DOI: 10.1021/acs.energyfuels.5b02491 Energy Fuels XXXX, XXX, XXX−XXX

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increase in attapulgite suspending liquid flow rate. It suggests the great effectiveness of attapulgite suspending liquid on particle agglomeration. 3.2. Effect of Air to Attapulgite Suspending Liquid Ratio (RA/L) on PM2.5 Emission. The effect of RA/L on PM2.5 emission is great as observed in Figure 4 that both NPM2.5 and

Figure 4. Effect of air to liquid ratio RA/L on size distribution of PM2.5. (Coal combustion temperature TB : 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass faction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/min; original: without spraying any water or suspending liquid).

MPM2.5 are decreased with increasing RA/L. Compared with the case at RA/L of 0.125, the NPM1 and NPM1−2.5 are decreased by about 38% and 18%, respectively, when at RA/L is 0.25. The NPM2.5 is further decreased with increasing RA/L, which indicates the benefit of particle agglomeration by increasing RA/L. This is attributed to the important effect of RA/L on droplet size and the higher RA/L is applied, the smaller droplet is obtained. Generally, the suitable RA/L is between 0.1 and 10. However, no appropriate droplets can be obtained with a RA/L less than 0.1; in addition, a huge amount of energy will be consumed for atomization at this ratio. The droplet size may not always be decreased at a RA/L higher than 10. Whereas, good contact would be achieved for fine particles with droplets of about 25 μm, which can be obtained at RA/L of about 0.5, based on the experience of airflow nozzle technology, and relatively few energy is consumed as well.17 Better contact between fine particles and attapulgite suspending liquid is achieved by increasing RA/L from 0.125 to 0.5, which is favorable for fine particle agglomeration. The grade efficiency of all size ranges increased with increasing RA/L and removal efficiency reaches 51% at RA/L of 0.125 and rises up to 69% at RA/L of 0.25, and it continues to grow with further increase in RA/L as shown in Figure 5. This indicates an enhanced particle agglomeration achieved at a suitable RA/L. 3.3. Effect of Attapulgite Mass Fraction in Suspending Liquid (MAtt) on PM2.5 Emission. The effect of MAtt on PM2.5 emission is critical for the determination of appropriate amount of attapulgite in the liquid from the economic aspect. The results in Figure 6 illustrate that both NPM2.5 and MPM2.5 are decreased with an increase in MAtt, with a bimodal distribution in MPM2.5 whose peaks located at about 0.1 and 0.6 μm, respectively. The NPM1 is decreased while NPM1−2.5 a little increased with MAtt increased from 0.1% to 1%, indicating a possible shift in particle size of ultrafine to submicron or micron. A reduction of about 84% in NPM1, 63% in NPM1−2.5, and 65% in MPM2.5 is achieved at a MAtt of 1%, compared with

Figure 3. Effect of attapulgite suspending liquid flow rate FASL on PM2.5 removal efficiency. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass fraction MAtt: 1%; air to suspending liquid ratio RA/L: 0.5; water: spraying water at 38 mL/min).

where Pe is the Piclet number, ReD is the Reynolds number, u0 is the gas velocity, m/s, Dc is the particle diameter, m, and D is diffusion coefficient, m2/s. It indicates that removal efficiency is inversely proportional to the piclet number, that is, the removal efficiency decreases with increasing Piclet number. Based on eq 5, the Piclet number is inversely proportional to particle diameter and proportional to the diffusion coefficient. At the same gas velocity, the diffusion effect is enhanced and the piclet number decreases with particle diameter decreased and thus the removal efficiency is increased. This explains why the grade efficiency of particles less than 0.1 μm is relatively higher when spraying attatpulgite suspending liquid into flue gas. Grade efficiency of particles larger than 1 μm is also high, mainly attributed to the strong effect of particle collision. Whereas, both the diffusion and collision is relatively small for particles with diameter of 0.3−1 μm, and this leads to low removal efficiency. Meanwhile, a great proportion of particles of less than 0.1 μm are agglomerated into larger particles of these sizes which also decreases the removal efficiency of particles of 0.3−1 μm. Nevertheless, the removal efficiency is still much higher than the case with water sprayed in flue gas. The removal efficiency of PM2.5 reaches 44% with water sprayed in flue gas and quickly ramps up to 75% at a flow rate of 9.5 mL/min attapulgite suspending liquid as shown in Figure 3b. Slow increase in removal efficiency is observed with further D

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Figure 7. Effect of pH on PM2.5 emission. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass fraction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/min).

Figure 5. Effect of air to liquid ratio RA/L on PM2.5 removal efficiency. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass faction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/min).

conditions and the surface charge of attapulgite is changed leading to a charge change of attapulgite suspending liquid. The aggregated bundles of attapulgite are scattered and the cationic of the octahedron are gradually dissoluted. Microporous channels are expanded, and the attapulgite is purified. The ion exchange capacity as well as the surface area of attapulgite is enhanced, resulting in an improvement in physical adsorption properties.20 Another possible reason is that low free energy of dissolved ions like Cl− and SO42− in fine particles helps particles easily wetted in water and the particle surface potential is increased due to the location of cations at the surface of fine particles, which is favorable for fine particle agglomeration and this process can be strengthened under acidic conditions. Though the surface area of attapulgite can also be increased under alkaline conditions to some extent, more antions will stay at particle surfaces and thus decrease particle surface potential, which does not benefit particle agglomeration. That is why more fine particles are emitted under basic conditions than that under acidic conditions.21 The grade efficiency is clearly presented in Figure 8 demonstrating a decrease with pH of attapulgite suspending liquid increase. Compared with the original flue gas, the removal efficiency of PM2.5 reaches 85% at a pH of 5, is decreased to 82% at a pH value of 7, and continues to decline with pH further rising. Accordingly, keeping an acidic condition

Figure 6. Effect of attapulgite mass fraction MAtt on size distribution of PM2.5. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite suspending liquid flow rate FASL: 38 mL/min; original: without spraying any water or suspending liquid).

that of original flue gas. The absorption bridging action is the main mechanism of fine particle agglomeration. More volatile element will be diffused and collected on the attapulgite surface because of the Brown proliferation and the role of van der Waals force based on absorption and chemical reaction when the MAtt increases, and more places can be provided for solid particles formed due to the homogeneous condensation by attapulgite in the suspending liquid, resulting in PM1 reduction and particle size increase.18,19 3.4. Effect of pH of Attapulgite Suspending Liquid on PM2.5 Emission. The attapulgite suspending liquid is neutral and has a pH of 7, based on the preliminary experiment. The change of pH may affect the characteristic of attapulgite liquid which in turn influences its effect on particle agglomeration. Thus, pH of the suspending liquid is adjusted by adding phosphoric acid and sodium hydroxide to determine how pH of the suspending liquid influences PM2.5 emission. It is observed in Figure 7 that both NPM2.5 and MPM2.5 are increased with the pH rise. At a pH of 5, the NPM1 and NPM1−2.5 is decreased by about 86% and 75%, respectively, compared with the case without attapulgite suspending liquid. While, at a pH of 9, more fine particles are produced. This implies that fine particle agglomeration is promoted under acidic condition, resulting in lower PM2.5 emission. One possible reason for this is that the impurities in attapulgite are removed under the acidic

Figure 8. Effect of pH on PM2.5 removal efficiency. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass fraction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/min). E

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Energy & Fuels of the attapulgite suspending liquid is more favorable for particle agglomeration and will result in low PM2.5 emission. 3.5. Effect of Agglomeration Room Temperature (TAR) on PM2.5 Emission. To determine the appropriate agglomeration room temperature for particle agglomeration, PM2.5 emission is investigated at different temperature. As can be seen in Figure 9 that both the NPM1 and NPM1−2.5 are decreased,

Figure 10. Effect of temperature of agglomeration room TAR on PM2.5 removal efficiency. (Coal combustion temperature TB: 900 °C; attapulgite mass fraction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/min).

However, how the limestone influences PM2.5 emission when spraying attapulgite suspending liquid is a question. Figure 11

Figure 9. Effect of temperature of agglomeration room TAR on PM2.5 emission. (Coal combustion temperature TB: 900 °C; attapulgite mass fraction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/ min).

by about 82% and 65%, respectively, and MPM2.5 is decreased by about 72% when the agglomeration room temperature is controlled at 150 °C, while a decrease of about 50% for NPM1 and 55% for NPM1−2.5 is achieved with the agglomeration room temperature increased to 250 °C. The increase in NPM2.5 and MPM2.5 suggests the negative effect on particle agglomeration by increasing agglomeration room temperature even though spraying the same amount of attapulgite suspending liquid. According to the Kelvin equation, the supersaturation is decreased with flue gas temperature increase and the steam condensed around microscopic dust particles is reduced,22 thus reducing the droplet size which is unfavorable for steam condensations at fine particle surfaces and particle growth. Meanwhile, flue gas is rapidly heated at high agglomeration room temperature. The temperature difference as well as the heat transfer driving force between flue gas and suspending liquid is boosted, and the time for suspending liquid evaporation and particle coherence is shortened, which results in low removal efficiency. As is clearly shown in Figure 10, the grade efficiency of PM2.5 is decreased with increasing agglomeration room temperature. A removal efficiency of 82% is achieved at the agglomeration room temperature of 150 °C and decreased to 44% with the temperature increased to 250 °C, and a further decrease is observed with agglomeration room temperature rising up to 450 °C. It implies that the negative effect of agglomeration room temperature increase is far stronger than the positive effect of spraying suspending liquid on particle agglomeration; that is, higher agglomeration room temperature is not good for particle agglomeration. While keeping an agglomeration room temperature at around 150 °C for particle agglomeration would achieve high removal efficiency and result in low PM2.5 emission. 3.6. Effect of Molar Ratio of Calcium to Sulfur (MCa/S) on PM2.5 Emission. During coal combustion, limestone is normally added in the furnace for SO2 capture. Results showed that PM2.5 emission can be decreased by adding limestone.6,7

Figure 11. Effect of molar ratio of calcium to sulfur MCa/S on PM2.5 removal efficiency. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass fraction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/ min).

illustrates the effect of MCa/S on PM2.5 removal efficiency when burning U.S. bituminous coal at 900 °C with attapulgite suspending liquid sprayed. It is observed that by adding limestone at MCa/S of 1.5 the grade efficiency for particle diameter less than 1.0 μm is increased. For particles of diameter less than 0.5 μm, the grade efficiency is further increased with increasing MCa/S, while that for particles of diameter larger than 0.5 μm is decreased. Based on the size distribution of fine particles (not shown here), the NPM1 and NPM1−2.5 are decreased by about 82% and 65%, respectively, when spraying attapulgite suspending liquid without limestone, compared with the original flue gas. When spraying attapulgite suspending liquid and adding limestone at a MCa/S of 1.5, the NPM1, NPM1−2.5 and MPM2.5 are further decreased, by about 86%, 53%, and 62%, respectively. A further decline in NPM2.5 is observed by about 8% at a MCa/S of 3.5, compared with the case at a MCa/S of 1.5. However, both the number concentration and mass concentration of the micron particles are increased, and the submicron particles are declined with increasing MCa/S. It implies a shift in particle size from fine particles to larger ones. F

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particles can be observed in Figure 12e after spraying attapulgite suspending liquid, mainly due to their formation mechanism. Minerals in coal vaporize at high temperature and condense in the form of homogeneous or heterogeneous nucleation at low temperature zone to form spherical particles mainly because of the dominated surface tension of fine particles. Submicron particles are formed mainly based on the mechanism of nucleation-condensation-coagulation after its evaporation from the solid matrix at high temperature.23,24 The supermicron particles have irregular shapes as shown in Figure 12b−d, as the large agglomerates consisting of primary particles, largely attributed to the fragmented larger ash particles after experienced a molten phase at a relatively high temperature. To know more about the elements change in PM2.5 before and after spraying attapulgite suspending liquid, EDX is used to detect the elements distribution in particles. The distribution of elements such as sodium (Na), potassium (K), sulfur (S), iron (Fe), and silica (Si) in particles versus attapulgite mass fraction in suspending liquid is displayed in Figure 13. It is observed that contents of Na, K, S, Fe, and Si in particles of the typical size are all decreased when spraying water and attapulgite suspending liquid. Contents of Na, K, and S in particles decrease with particle size increasing from 0.1 to 2.5 μm at a certain attapulgite mass fraction in suspending liquid indicating

The removal efficiency of PM2.5 is 82% when spraying attapulgite suspending liquid without limestone and reaches 86% at a MCa/S of 1.5. A further growth is noted with increasing MCa/S when spraying the same amount of liquid. This demonstrates the beneficial effect of limestone on particle agglomeration and the contribution cannot be missed. 3.7. Particle Morphologies and Element Distribution of Fine Particles. In a more straightforward way to find out what happens for the fine particles, SEM images of attapulgite and fine particles before and after spraying attapulgite suspending liquid are observed and presented in Figure 12.

Figure 12. SEM images of particles before and after spraying attapulgite suspending liquid. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite mass fraction MAtt: 1%; attapulgite suspending liquid flow rate FASL: 38 mL/ min).

As can be seen in Figure 12a, the attapulgite is rod-shaped crystal aggregates, while PM2.5 particles look spherical and have a smooth surface without or with quite few fine particles connected to each other as shown in Figure 12b. The ball-like particles become irregular and some fine particles are connected at the surface of larger ones after spraying attapulgite suspending liquid as shown in Figure 12c. It indicates that spraying attapulgite suspending liquid helps promote fine particle agglomeration to be larger particles and reduce fine particle emissions. Some fine particles are connected to each other and some adhered at the rod-shaped particles like attapulgite, implying the beneficial effect of attapulgite suspending liquid on particle agglomeration as observed in Figure 12d. Morphologies of PM1 and PM0.1 are presented in Figure 12e and f, respectively. Smooth and spherical submicron

Figure 13. Effect of attapulgite mass fraction MAtt on element distribution of PM2.5. (Coal combustion temperature TB: 900 °C; temperature of agglomeration room TAR: 150 °C; attapulgite suspending liquid flow rate FASL: 38 mL/min; original: without spraying any water or suspending liquid; Water: sprayed water at 38 mL/min). G

DOI: 10.1021/acs.energyfuels.5b02491 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels an enrichment of Na, K, and S in submicron particles. While, contents of Si and Fe increase with particle size increasing from 0.1 to 2.5 μm at a certain attapulgite mass fraction in suspending liquid indicating no enrichment of Si and Fe in submicron particles. This is largely due to the different forming mechanism of submicron and supermicron particles. The submicron particles are formed mainly based on the vaporization−condensation during volatiles combustion. Many inorganic in atomic states and volatile constitutes of minerals are vaporized and thus many fine particles are formed due to homogeneous condensation, while very little nonvolatile compounds are vaporized which is not concentrated on submicron particles.25 Volatile elements like Na, K, and S are largely reduced, while reduction of nonvolatile elements like Fe and Si is relatively small by spraying attapulgite suspending liquid. Elements such as Na, K, S, Fe, and Si are all reduced in fine particles of a certain diameter with increasing attapulgite mass fraction in the suspending liquid. It indicates that submicron and supermicron particles are more likely to collide with attapulgite and the absorption bridging force is enhanced which is beneficial to particle absorption at the surface of attapulgite and particle agglomeration, especially for those volatile elements like Na, K, and S formed based on vaporization-condensation mechanism, resulting in low content of Na, K, and S in PM2.5.

optimization are needed to be considered and conducted to make it applicable and cost-effective for practical application.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 25 83790147; +61 2 9385 5037. Fax: +86 25 83790147; +61 2 9385 6139. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the National Natural Science Foundation Project of China (51206024) and National Basic Research Program (2013CB228505) are sincerely acknowledged. The author (H.C.C.) also acknowledges the award of a UNSW Vice-Chancellor’s Post-Doctoral Research Fellowship with Supplementary Research Support Grant (No. RG142405).

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REFERENCES

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4. CONCLUSIONS The research work presents a promising technique to remove fine particles in flue gas by spraying attapulgite suspending liquid. Spraying attapulgite suspending liquid can effectively reduce PM2.5 emissions generated by burning coals in a 6 kW circulating fluidized bed. Both NPM2.5 and MPM2.5 in flue gas are decreased attributed to fine particle agglomeration mainly based on absorption bridging action. The scattered fine particles are cohered to each other in the form of bridge of attapulgite molecule chains to form balls or chains. When water in the suspending liquid vaporizes, the liquid bridge forces between particles are converted to the solid bridge forces and the particle agglomeration is enhanced. The removal efficiency of PM2.5 increases with increasing attapulgite suspending liquid flow rate and attapulgite mass fraction in the suspending liquid. Increasing ratio of air to attapulgite suspending liquid helps to enhance particle agglomeration and thus increases the removal efficiency of PM2.5. Increased pH of the suspending liquid decreases the removal efficiency of PM2.5 as more anions located at the surface of attapulgite lowering the surface potentials which is unfavorable to particle agglomeration. Increasing agglomeration room temperature increases PM2.5 emissions whose reason needs to be further detailed. The removal efficiency of PM2.5 can be further increased by adding limestone when spraying attapulgite suspending liquid and it is increased with increasing MCa/S. SEM images present that many fine particles cohered at the surface of attapulgite which indicates an enhancement in particles agglomeration by spraying attapuligte suspending liquid. Elements such as Na, K, S, Fe, and Si are all reduced in fine particles with spraying attapulgite suspending liquid, and their contents are decreased with increasing attapulgite mass fraction in the suspending liquid. These interesting results achieved suggest the highly potential and perspective to industries, even though the work was based on only a specific type of coal and attapulgite, and further detailed research and H

DOI: 10.1021/acs.energyfuels.5b02491 Energy Fuels XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.energyfuels.5b02491 Energy Fuels XXXX, XXX, XXX−XXX