Journal of Hazardous Materials 310 (2016) 40–47

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Temporal measurements and kinetics of selenium release during coal combustion and gasification in a fluidized bed Fenghua Shen a , Jing Liu a,b,∗ , Zhen Zhang a , Yingju Yang a a b

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China Shenzhen Institute of Huazhong University of Science and Technology, Shenzhen 518000, China

h i g h l i g h t s • • • •

The temporal release of Se from coal combustion and gasification was measured. Kinetic laws for Se release from coal combustion and gasification were determined. The influences of temperature and chemical composition of flue gas were clarified. The interactions of Se species with mineral affect the release kinetics of Se.

a r t i c l e

i n f o

Article history: Received 16 October 2015 Received in revised form 21 January 2016 Accepted 11 February 2016 Available online 15 February 2016 Keywords: Selenium Release kinetics On-line analysis Coal combustion Gasification

a b s t r a c t The temporal release of selenium from coal during combustion and gasification in a fluidized bed was measured in situ by an on-line analysis system of trace elements in flue gas. The on-line analysis system is based on an inductively coupled plasma optical emission spectroscopy (ICP-OES), and can measure concentrations of trace elements in flue gas quantitatively and continuously. The results of on-line analysis suggest that the concentration of selenium in flue gas during coal gasification is higher than that during coal combustion. Based on the results of on-line analysis, a second-order kinetic law r(x) = 0.94e−26.58/RT (−0.56 x2 −0.51 x + 1.05) was determined for selenium release during coal combustion, and r(x) = 11.96e−45.03/RT (−0.53 x2 −0.56 x + 1.09) for selenium release during coal gasification. These two kinetic laws can predict respectively the temporal release of selenium during coal combustion and gasification with an acceptable accuracy. Thermodynamic calculations were conducted to predict selenium species during coal combustion and gasification. The speciation of selenium in flue gas during coal combustion differs from that during coal gasification, indicating that selenium volatilization is different. The gaseous selenium species can react with CaO during coal combustion, but it is not likely to interact with mineral during coal gasification. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Selenium (Se) is an essential trace element for the proper function of the immune system, but prolonged intake of a high dose of selenium is harmful for mammals [1]. Moreover, some chemical forms of selenium, such as selenates and selenites, are very toxic. Although selenium presents in only small amount in coal, the large use of coal in electricity generation leads to significant accumulative emissions of selenium [2]. In China, the coal combustion-based power plant is widely employed, and the development of coal

∗ Corresponding author at: State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.jhazmat.2016.02.031 0304-3894/© 2016 Elsevier B.V. All rights reserved.

gasification-based power plant has been established in order to produce energy in an efficient and environmentally friendly way. It is of great importance to understand the release characteristics of selenium during coal combustion and gasification for the control of its emission. Various experimental studies have been conducted to investigate the release behaviors of selenium during coal combustion and gasification. Tang et al. [3] studied the distribution of selenium in a coal-fired power plant by analyzing the solid residues. They found that the concentration of selenium in fly ash was higher than that in bottom ash and feed coal, and thus suggested that selenium was volatile during coal combustion and was likely to condense on the smaller particles. Seames et al. [4] investigated the partitioning behavior of selenium between the vapor and solid phases during coal combustion. They suggested that the reactions of sele-

F. Shen et al. / Journal of Hazardous Materials 310 (2016) 40–47

nium vapor on the surface or within the pores of an ash particle were essential for the transformation of selenium vapor to particles. Moreover, selenium present in the fly ash might react with calcium. Liu et al. [5] reported that selenium was highly volatile during coal gasification, and the volatility of selenium was influenced by the mode of occurrence of selenium and the coal reactivity. The analyzing residue methods can give insight into the partitioning characteristics of selenium between the vapor and solid phases during coal combustion and gasification. Nevertheless, such methods need the taking of discrete samples and only give an average value for trace element emission over a long period. Consequently, they are hard to obtain the information on trace element temporal emissions. Clarifying the temporal evolution processes of trace elements is necessary for further understanding the transformation behaviors of trace elements during coal combustion and gasification. This evolution can be investigated only by kinetic studies. When dealing with mineral matrices, such as Al2 O3 particles, the release behaviors of trace elements can be clarified by aspirating and analyzing the solid samples directly at given times since the mineral matrices will not break during the process of thermal treatment. However, it is difficult to obtain the release kinetics of trace elements during coal combustion and gasification due to the difficulty of the sampling of burning coal particles. Yan et al. [6,7] investigated the release behaviors of selenium during coal and coke combustion by applying thermogravimetric studies. The coal and coke samples used in their studies were impregnated by different selenium solutions. The influences of various factors on selenium release behaviors during coal and coke combustion were clarified. However, as other species may be simultaneously volatilized, the release rates of selenium species cannot be evaluated directly through the results of DTG curves. The release kinetics of selenium during coal and coke combustion is thus difficult to be determined accurately through their method. The application of on-line analysis system gives a meaningful approach to solve such questions. The temporal concentrations of trace elements in flue gas reveal the temporal evolution processes of trace elements from coal particles during combustion and gasification. Based on the results of instantaneous trace elements concentrations in flue gas, the release rates of trace elements during coal combustion and gasification can be evaluated by applying modeling methods, and then the kinetic laws can be obtained. Considerable efforts have been made previously to develop reliable technologies for measuring the temporal concentrations of trace elements in flue gas directly [8,9]. Among various technologies, the on-line analysis system based on inductively coupled plasma optical emission spectroscopy (ICP-OES) serves as an effective approach for measuring trace elements in flue gas quantitatively and continuously [10]. The development of on-line analysis system of trace elements in flue gas is meaningful in both the understanding of industrial processes and the measurement of emissions for legislative purposes [11,12]. By applying on-line analysis system, Falcoz et al. [13] studied the release kinetics of Cd, Pb, and Zn during artificial waste incineration. Two different kinetic laws were determined for Cd, Pb, and Zn release at different temperature range. Soria et al. [14] investigated the kinetic behavior of Cd release during waste incineration, and proposed a kinetic law for Cd release by CFD thermal analysis. In our previous study [15], the kinetic laws of Cd release during thermal treatment of mineral matrices and realistic artificial waste at 850 ◦ C were determined, respectively. A first-order kinetic law was determined for mineral matrices and a second-order kinetic law was determined for realistic artificial waste. In addition, a second-order kinetic law was determined for Cd release during coal combustion at 850 ◦ C [16]. By far, there is still lack of knowledge about the kinetic characteristics of selenium release during coal

41

Table 1 Proximate and ultimate analyses of the Shenfu coal (wt.%, air dry basis). Proximate analysis

Ultimate analysis

M

A

V

FC

C

H

N

S

O

10.19

6.5

31.16

62.34

74.03

4.8

0.89

0.37

13.41

Table 2 Ash composition of Shenfu coal (wt.%). SiO2

MgO

Al2 O3

TiO2

SO3

K2 O

CaO

Fe2 O3

35.3

1.1

20.6

0.3

8.7

1.15

15.2

14

combustion and gasification, and the influence of atmosphere on selenium release kinetics is unclear. Therefore, the release kinetics of selenium is needed to be investigated for further clarifying its transformation behaviors during coal combustion and gasification. In this work, an on-line analysis system based on ICP-OES was developed to measure the concentration of selenium in flue gas quantitatively and continuously. The release rates of selenium during coal combustion and gasification were determined based on the results of on-line analysis and inverse method. The kinetic laws governing the release of selenium during coal combustion and gasification were identified based on the experimental rates of release.

2. Experimental 2.1. Fluidized bed reactor The experimental setup is shown in Fig. 1. The experiments were conducted using a bubbling fluidized bed [17]. This reactor is made of Cr25 Ni20 stainless steel with an inner diameter of 40 mm. The upper section (1100 mm) of the reactor is designed for reaction, in which a given amount of coal (2 g) is injected when the desired temperature reached and the operating conditions maintained. The lower section (400 mm) of the reactor is designed for preheating, in which the fluidization gases are supplied and pre-heated. Air was employed as fluidization gas for combustion, and CO2 was used for gasification. The reactor was electrically heated by a three-section electric furnace and the heating conditions for each section could be controlled independently. Thermocouples were immersed in each section to measure the temperature of the reactor, as well as the inlet and the outlet. Batch feeding is used in the experiments. The bed material is composed of quartz sand with particle diameter of 0.55–0.83 mm (initial bed height: 35 mm). The experiments were conducted at different temperatures: 600 ◦ C, 650 ◦ C, 700 ◦ C, 750 ◦ C, 800 ◦ C and 850 ◦ C. The coal (density: 1250 kg m−3 ) used in this study is mined from the Shenfu Coalfield, which is classified as bituminous coal. The ultimate and proximate analysis for the coal sample and the chemical compositions of its ash are provided in Tables 1 and 2, respectively. 2.2. On-line selenium analysis system An on-line analysis system was developed to measure selenium concentration in flue gas continuously and quantitatively, as shown in Fig. 1. This system is based on a modified inductively coupled plasma optical emission spectroscopy (ICP-OES, Spectro Arcos Sop, Germany). It can measure multi-elements in flue gas simultaneously at various emission lines (wavelength coverage 125–770 nm), with one measurement being made per seven seconds. A demountable quartz torch was employed for measurement with an injector i.d. of 1.8 mm. This torch can realize a higher coolant gas flow, which is needed for a higher power to maintain the stability of plasma

42

F. Shen et al. / Journal of Hazardous Materials 310 (2016) 40–47

Fig. 1. Schematic diagram of experimental setup.

under atmospheric conditions [18]. The emission signal intensity of selenium was measured at 196.090 nm in this study. A small cyclone was used to remove larger particulates from the outlet gas of the fluidized bed. The first isokinetic sampling stage aspirated the flue gas from the outlet of the fluidized bed. The secondary sampling stage controlled the high flow rate of the first isokinetic sampling stage to the relatively lower flow rate to the plasma by applying a double head peristaltic pump. The sample line was maintained at 200 ◦ C in order to avoid water condensation. The sample gas flow rates were optimized based on a balance of signal intensity and background noise. The optimum sample gas flow rate is 0.14 l min−1 for measuring selenium concentration in flue gas during coal combustion, and 0.10 l min−1 for that during coal gasification. The main difficulty for calibration is the generation of standard aerosol. Standard aerosol is not available commercially and it must be made from a matrix as similar as possible to that of the sample aerosol. In addition, water can introduce changes in the transport property of the plasma. Special attention must be devoted to the water content of the standard aerosol and the sample aerosol. In this study, the calibration method was developed to permit quantitative measurements. The analysis system was first calibrated by measuring the emission intensity of standards. The standard aerosols were prepared in-situ by nebulizing calibration solutions of selenium with concentrations range from 0.1 ␮g l−1 to 2 ␮g l−1 . An ultrasonic nebulizer (USN, Cetac U5000AT+ ) was used to provide dry standard aerosols for calibrations, and to normalize the water content of the sample aerosols. Part of the fluidization gas flow was used as aerosol carrier gas, in order to keep the gas composition of standard aerosol constant with the sample aerosol. The relative standard deviation (RSD) of the calibration curve of selenium is lower than 1%. The detection limit obtained for selenium in flue gas is 0.007 mg m−3 for coal combustion, and 0.012 mg m−3 for coal gasification. The on-line analysis system is able to measure selenium in typical concentrations in coal-fired flue gas. After the calibration completed, the unknown sample aerosol was analyzed by comparing its emission signal intensity with the calibration curve, and then the concentration of selenium in flue gas was obtained. The operating parameters of the on-line analysis system are shown in Table 3.

Table 3 Operating parameters of the on-line analysis system. Parameter

Value

Generator frequency (MHz) Power RF (kW) Plasma gas flow rate (l min−1 ) Auxiliary gas flow rate (l min−1 ) Aerosol carrier gas flow rate (l min−1 ) Sample gas flow rate, combustion (l min−1 ) Sample gas flow rate, gasification (l min−1 ) Injector tube diameter (mm) Integration time (s) Nebulizer desolvator temperature (◦ C) Nebulizer condenser temperature (◦ C)

27.15 1.42 13 0.8 0.8 0.14 0.1 1.8 3 140 3

3. Results and discussion 3.1. Temporal concentrations of selenium in flue gas The mass balance experiment was performed by calculating the percentage of the area under selenium concentration curve and the concentration of selenium in the final residue to the initial selenium concentration in the coal. The mass balance rates (85.4% for coal combustion and 86.2% for coal gasification) were attained, indicating that the results obtained by on-line analysis system are accurate. The quantitative results of selenium concentration in flue gas during coal combustion and gasification are shown in Fig. 2. The selenium concentration in flue gas begins to increase rapidly after the injection of coal sample, and reaches a peak almost instantaneously, and then it declines slowly. The duration time of the process of selenium concentration decreasing is much longer than that of its increasing. This indicates that the start of selenium release represents a less part of the total Se vaporized. The peak value of selenium concentration in flue gas for each temperature is quite distinct, which indicates that the temperature has a strong influence on selenium release behavior. In all the temperature range, the concentration of selenium in flue gas as well as the area under the concentration curve increase with increasing temperature. This is because a higher reaction temperature improves the decomposition of coal particles [19], and thus facilitates the release of selenium during coal combustion and gasification. It seems that the atmosphere has a significant influence on selenium

F. Shen et al. / Journal of Hazardous Materials 310 (2016) 40–47

(a) Coal combustion

(a) Coal combustion 0.06

1.6

850oC 750oC 650oC

1.4 1.2

800oC 700oC 600oC

1.0 0.8 0.6 0.4 0.2

Se release rate, r (mg⋅kg-1⋅s-1)

Se concentration in flue gas (mg⋅Nm-3)

43

850oC 750oC 650oC

0.05 0.04 0.03 0.02 0.01 0.00

0.0 0

30

60

90

120

150

180

210

0

240

30

60

90

120

150

180

210

240

Time (s)

Time (s) (b) Coal gasification

(b) Coal gasification

0.12

2.8 850oC 750oC 650oC

2.4 2.0

800oC 700oC 600oC

1.6 1.2 0.8 0.4

Se release rate, r (mg⋅kg-1⋅s-1)

Se concentration in flue gas (mg.Nm-3)

800oC 700oC 600oC

850oC 750oC 650oC

0.10 0.08

800oC 700oC 600oC

0.06 0.04 0.02 0.00

0.0 0

30

60

90

120

150

180

0

210

30

60

Time (s) Fig. 2. Temporal concentrations of selenium in flue gas: (a) coal combustion, (b) coal gasification.

release behavior. The concentration of selenium in flue gas during coal gasification is higher than that during coal combustion. For example, the maximum of selenium concentration in flue gas during coal gasification at 850 ◦ C is found to be 2.37 mg m−3 , whereas it is 1.48 mg m−3 for that during coal combustion. The duration time of selenium release during coal gasification is shorter than that during coal combustion. The results of on-line analysis indicate that a reducing atmosphere of coal gasification can facilitate the release of selenium. The dynamic characteristics of selenium during coal combustion and gasification cannot be described by a same kinetic law. It can be concluded that the on-line analysis system developed in this study obtained successfully the quantitative data on temporal concentration of selenium in flue gas. This system detected accurately the start of selenium release, the maximum selenium concentration in flue gas, and the end of selenium release during coal combustion and gasification. 3.2. Kinetic laws of selenium release during coal combustion and gasification Based on the temporal concentration of selenium in flue gas, the release rate (r) of selenium during coal combustion and gasification were determined respectively by applying the inverse method [15]. The modified Kunii and Levenspiel model was adapted in the inverse method. The bed was assumed to be isothermal. Thus, the internal phenomenon within a solid particle, including heat and mass transfer as well as chemical reactions, would have no effect on the results. This macroscopic approach uses only the global flux of generation of species at the external surface of the particles. Global release flux is related to the release rate in the inverse method. Therefore, the inverse method could predict the release rates of trace elements from coal particles based on the results of on-line analysis. The detailed description of the inverse model can be found in our previous paper [15].

90 120 Time (s)

150

180

210

Fig. 3. Temporal release rates (r) of selenium: (a) coal combustion, (b) coal gasification.

Before being injected into the inverse model, the concentration profiles of selenium in flue gas were fitted by the following sigmoid equation. 1

y = y0 + a

w1 ⁄2 (1 −

x-xc + − w2

1+e

1 1+e

w1 ⁄2 )

(1)

x-xc − − w3

The temporal release rates of selenium during coal combustion and gasification are shown in Fig. 3. In each case of different temperature and atmosphere, the release rate of selenium increases rapidly immediately after the injection of coal sample, reaches a peak in a short time, there after decreases slowly and, after a certain period, becomes substantially constant. The release rate of selenium from coal increases with a rise of temperature, indicating that temperature has a strong effect on selenium release dynamics. It appears that the release rate of selenium during coal gasification is faster than that during coal combustion. This suggests that the release kinetics of selenium during coal combustion differs from that during coal gasification. A general mathematical expression for the kinetic law of trace elements during coal combustion and gasification is described expressed as follows: r(x) = k(T )f (x)

(2)

where k(T) is the release rate constant. f(x) is the mechanism function, which describing the relationship of release rate and release degree (x). The degree of released selenium during coal combustion and gasification is defined as: x = (q0 − q)/(q0 − qf )

(3)

where qf is the final concentration of selenium in the coal particle, and q is the instantaneous concentration of selenium in the coal particle. The initial selenium concentration (q0 : 4.8 mg kg−1 ) in the coal used was measured by ICP-OES coupling with ultrasonic nebulizer after acid digestion of particles.

44

F. Shen et al. / Journal of Hazardous Materials 310 (2016) 40–47

-2.2

(a) Coal combustion 1.2

-2.4

(b) Coal gasification y = -5427.651x-2.482 R2 = 0.99

-2.6

-3.0

rad = r/rmax

ln (rmax)

-2.8

-3.2 -3.4 -3.6 -3.8 -4.0

850oC 750oC 650oC f(x)

1.0

(a) Coal combustion y = -3204.595x-0.063 R2 = 0.99

0.8

800oC 700oC 600oC

0.6 0.4 0.2

0.00090 0.00095 0.00100 0.00105 0.00110 0.00115 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

1/T

x Fig. 4. Determination of parameters of Arrhenius from the results of release rate of selenium: (a) coal combustion, (b) coal gasification.

(b) Coal gasification 1.2

Ea k(T ) = Aexp(− ) RT

1.0

rad = r/rmax

As indicated in Fig. 3, the influence of temperature on the release rate of selenium from coal is of great importance. This influence is included in the release rate constant k(T), which is expressed using the Arrhenius equation:

0.8

0.4

where A, Ea and T are the pre-exponential factor, activation energy and temperature, respectively. The initial particle size may have a significant influence on the experimental kinetics. This phenomenon can be integrated into the theoretical laws by defining the release rate with respect to the external surface (unit: mg kg−1 s−1 m−2 ) [13]. The activation energy and pre-exponential factor were obtained by plotting ln(rmax ) versus 1/T at the experimental temperature range, as shown in Fig. 4. A high value of the coefficient R2 has been obtained. The calculated activation energy and the pre-exponential factor for selenium release during coal combustion are 26.58 kJ mol−1 and 0.94 mg kg−1 s−1 m−2 , respectively. For selenium release during coal gasification, the activation energy is 45.03 kJ mol−1 and the pre-exponential factor is 11.96 mg kg−1 s−1 m−2 . The activation energy and pre-exponential factor for selenium release during coal gasification are higher than those during coal combustion. The mechanism function f(x) was furtherly determined by plotting the normalized experimental curves r/rmax versus x for each temperature, as shown in Fig. 5. These rate curves were fitted into several n-order polynomials, and the mean values of both increasing and decreasing points of these polynomials were recognized as the coefficients of f(x). The start of selenium release (x < 0.1) has insignificant physical meaning only, and the concentration of selenium in flue gas is ignorable in this period. In fact, selenium concentration in coal decreases gradually once selenium release started during combustion and gasification. Therefore, it is reasonable to neglect the initial stage of the selenium release processes during coal combustion and gasification for each temperature. Such approximation is beneficial for simplifying the final mathematical equation of the kinetic law. The mathematical expression of f(x) of selenium release during coal combustion is given by:

0.2

(5)

The mathematical expression of f(x) of selenium release during coal gasification is given by: f (x) = −0.53x2 −0.56x + 1.09 These two mathematical expressions are valid for 0.1 ≤ x ≤ 1.

(6)

800oC 700oC 600oC

0.6

(4)

f (x) = −0.56x2 −0.51x + 1.05

850oC 750oC 650oC f(x)

0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

x Fig. 5. Normalized mechanism functions fitted by f(x) for all temperatures: (a) coal combustion, (b) coal gasification.

Therefore, the kinetic law governing selenium release during coal combustion can be expressed as: r(x) = 0.94e−26.58/RT (−0.56x2 −0.51x + 1.05)

(7)

The kinetic law governing selenium release during coal gasification can be expressed as: r(x) = 11.96e−45.03/RT (−0.53x2 −0.56x + 1.09)

(8)

The mathematical equations of the proposed kinetic law (7) and (8) are very simple, and take into account the influence of temperature. This is beneficial for them to be used in complex combustion and gasification models. Fig. 6 shows the comparison between the experimental kinetics and the kinetic laws mathematically determined. The red lines were calculated by kinetic laws. A good agreement between the kinetic laws and experimental data can be seen for all the temperatures. This proves that the kinetic law (7) and (8) successfully predict the kinetic characteristics of selenium release during coal combustion and gasification, respectively. The dynamic characteristic of selenium release during coal combustion differs from that during coal gasification. This reveals the influence of operating condition on selenium release kinetics. The release rate of selenium changed more obviously during coal gasification than that during coal combustion with the increasing of temperature. This suggests that selenium release kinetics during coal gasification is more temperature-sensitive than that during coal combustion. When the release rate of selenium reaches a peak, the remained selenium concentration in coal particles during coal gasification is similar to that during coal combustion. This indicates that the influence of residue selenium concentration in coal particles on selenium release kinetics is very small, and this is most probably related with the speciation of selenium in coal.

F. Shen et al. / Journal of Hazardous Materials 310 (2016) 40–47

(a) Coal combustion

Se release rate, r (mg⋅kg-1⋅s-1)

Se concentration in coal, q (mg⋅kg-1)

(a) Coal combustion 0.07

850oC 800oC 750oC 700oC 650oC 600oC Kinetic Law

0.06 0.05 0.04 0.03 0.02 0.01 0.00 1.6

2.0

2.4

2.8

45

3.2

3.6

4.0

4.4

4.8

5.0

850oC 800oC 750oC 700oC o 650 C 600oC Kinetic Law

4.5 4.0 3.5 3.0 2.5 2.0 1.5

0

30

60

90

Se concentration in coal, q (mg⋅kg-1) (b) Coal gasification

Se concentration in coal, q (mg⋅kg-1)

Se release rate, r (mg⋅kg-1⋅s-1)

o

850 C 800 C 750oC 700oC 650oC 600oC Kinetic Law

0.10 0.08 0.06 0.04 0.02 0.00 1.6

2.0

2.4

2.8

150

180

210

240

(b) Coal gasification

0.12 o

120

Time (s)

3.2

3.6

4.0

4.4

4.8

Se concentration in coal, q (mg⋅kg-1) Fig. 6. Validation of the kinetic laws at each temperature: (a) coal combustion, (b) coal gasification.

3.3. Temporal evolutions of selenium in coal and thermal equilibrium analysis Fig. 7 shows the total amount of selenium remained in the coal particles for each temperature. The red lines were calculated by the kinetic law. In all the cases, the residue selenium amount in coal particles decreases gradually and becomes constant after a certain period. The decreasing rate of residue selenium amount at high temperature is faster than that at low temperature. This corresponds to the increasing of selenium release rate with a rise of temperature. The process of selenium release during coal combustion continued for about 150 s, whereas selenium release process during coal gasification lasted about 120 s. The release percent of selenium during coal combustion and gasification increased obviously with a rise of temperature. The release percent of selenium increased from 36.3% to 60.5% as the temperature increased from 600 ◦ C to 850 ◦ C for coal combustion. The release percent of selenium increased from 20.3% to 64.3% as the temperature increased from 600 ◦ C to 850 ◦ C for coal gasification. It can be concluded that the kinetic law (7) and (8) predicted accurately the temporal evolution processes of selenium during coal combustion and gasification, respectively. The released selenium during coal combustion and gasification can distribute between the vapor and solid phases in different proportions. Understanding of the possible reactions of trace elements with mineral matter [20,21], and the influences of flue gas are important for clarifying their release behaviors during coal utilization [22,23]. The gaseous species of selenium during coal combustion differs from that during coal gasification due to the different composition in the combustion and gasification atmospheres. The differences of selenium species present in flue gas and the reactions of gaseous selenium species with fly ash can affect the partitioning of selenium between the vapor and solid phases

5.0

850oC 800oC 700oC 650oC Kinetic Law

4.5

750oC 600oC

4.0 3.5 3.0 2.5 2.0 1.5

0

30

60

90

120

150

180

210

Time (s) Fig. 7. Comparison between the kinetic laws predicted and the experimental concentration profiles in coal: (a) coal combustion, (b) coal gasification. Table 4 Parameters for equilibrium calculation. Element

Combustion (mol)

Gasification (mol)

C H O N S Se Ca Fe Si CO2

61.69 48.00 176.33 628.60 0.19 4.5E-5 0.18 0.11 0.38

61.69 48.00 9.40 0.64 0.19 4.8E-5 0.18 0.11 0.38 200.00

[4,24]. In order to predict the transformation behaviors of selenium during coal combustion and gasification, thermodynamic equilibrium calculations were conducted for simulated coal combustion and gasification atmosphere including selenium and mineral matter based on the minimization of the free Gibbs energy. The input parameters for equilibrium calculation are listed in Table 4. The results of equilibrium calculation reveal the speciation and distribution of selenium when the reactions reach equilibrium. Fig. 8 shows the results from thermodynamic predictions. During coal combustion (Fig. 8a), the dominant gaseous species of selenium is SeO2 (g) at temperatures higher than 420 ◦ C. SeO(g) occurs as the minor species between 1000 and 1600 ◦ C. The results are consistent with the previous theoretical studies [25,26]. Solid CaSeO4 becomes the dominant species at lower temperature (

Temporal measurements and kinetics of selenium release during coal combustion and gasification in a fluidized bed.

The temporal release of selenium from coal during combustion and gasification in a fluidized bed was measured in situ by an on-line analysis system of...
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