Combustion Characteristics of CoalâWater Slurry in a Slag-Tap

Article pubs.acs.org/EF

Combustion Characteristics of Coal−Water Slurry in a Slag-Tap Vertical Cyclone Furnace through Digital Imaging Yu Bo, Zhenyu Huang, Qunxing Huang,* Yanwei Zhang, Junhu Zhou, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China ABSTRACT: This study presented a novel diagnosis for the combustion of coal−water slurry (CWS) in a pilot-scale slag-tap vertical cyclone furnace using flame images and three-dimensional temperature distributions reconstructed from these flame images through a two-step inverse radiation analysis. The accuracy of the reconstructed temperature was evaluated by infrared thermometer measurements, with a discrepancy of less than 50 K. The effects of the air/fuel equivalence ratio, the air distribution pattern, and the furnace load on combustion performance were analyzed based on the reconstructed temperature profiles. Experimental and analytical results showed that flame stability was clearly represented by the variance of the image pixel values. The three-dimensional temperature profiles also effectively illustrated the combustion characteristics in the cyclone furnace under different air/fuel settings. By appropriately supplying air, the combustion efficiency of CWS can be maintained at >99% in the proposed cyclone furnace. methods in pilot-scale boilers.9−13 The combustion characteristics of CWS from coals of different rank have also been comprehensively reported.14,15 In recent years, CWS has been commonly used for gasification and fluidized beds. Yu16 and Gong17 investigated the temperature distribution in an opposed multiburner gasifier. Atesok18 determined the combustion parameters of a laboratory-scale fluidized bed combustor by a coal−water mixture (CWM) technology. These studies have demonstrated that the combustion of CWMs is affected by parameters such as the coal/water ratio, the stoichiometric combustion ratio, and the air distribution pattern. Similar to coal-fired boilers, the distribution of flame temperature in cyclone furnaces plays a key role in evaluating ignition and burning of fuel, as well as combustion stability.19 This parameter is also important in combustion optimization to improve energy efficiency and reduce pollutant emission. Inappropriate temperature distribution may cause severe operational problems in the operation of slag-tap cyclone furnaces. However, continuously measuring the actual temperature distribution is difficult because of the high temperature and positive pressure in cyclone furnaces.20 The widely used thermocouples can only provide temperature data at a single point, and the extremely high temperature in a furnace also damages the thermocouple material. Other methods such as gas suction pyrometers have the same problems. Thus, nonintrusive optical measurement methods should be used for combustion diagnostics. In the past few years, laser-based flame temperature measurement methods have been developed to study flames in laboratories for different purposes.21−25 However, most of these techniques can only provide singlepoint data. The associated costs and resources also limit the implementation of this technique in practical systems.26−28 Therefore, the temperature reconstruction technique using

1. INTRODUCTION As a consequence of the energy crisis, the use of the “forgotten fuel”, coal, is increasing worldwide, especially in China. For decades, coal−water slurry (CWS) has been developed as a substitute for oil in many industries and power plant boilers, gasifiers, and fluidized bed combustors. CWS has been widely used in China by over 20 power plant boilers, 300 industrial furnaces, and hundreds of various kilns since 2007.1 Although the combustion of CWS generally emits less fly ash, SO2, and NOx than traditional coal-fired boilers, the quantity of fly ash in flue gas is still a major problem in using CWS as a substitute for oil and gas. To provide clean, high-temperature product gas for numerous industrial fields, the emission of fly ash can be further reduced by utilizing slag-tap cyclone furnaces. In a cyclone furnace, most of the ash melts and is discharged in the form of a flowing slag layer along the furnace wall. The proportion of fly ash in the flue gas is also significantly reduced from 80% to 15%2 of the total ash. Moreover, cyclone furnaces create an intense swirling motion with a high degree of turbulence, which combined with the high temperature level is the basis of the enhanced heat release rate.3 However, numerical modeling of the cyclone barrel carried out by the Electric Power Research Institute in the U.S. has indicated that the flow field within the cyclone barrel is relatively poorly mixed due to the staged firing configuration and large particle size.4,5 This type of furnace is mainly used in coal-fired utility boilers. Numerous studies6,7 on coal combustion using a cyclone furnace have been conducted, but only a few of these studies used CWS as fuel. Carson et al.8 reviewed the critical issues involved in CWS burning using cyclone furnaces, as well as the impact of CWS on fuel property, flame temperature, and trace metal emission. Although CWS is similar to coal, these fuel sources significantly differ from each other. CWS is combusted after it is atomized, similar to liquid fuels. Various studies on the combustion mechanism of CWS have been performed using free-falling or suspended droplets in laboratories and traditional © XXXX American Chemical Society

Received: January 21, 2013 Revised: May 28, 2013

A

dx.doi.org/10.1021/ef400112a | Energy Fuels XXXX, XXX, XXX−XXX

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flame images is receiving considerable interest because of its ability to retrieve the desired temperature distribution under restricted conditions29 by inverse radiation analysis. Liu et al.,30,31 Park and Yoo,32 as well as Namjoo et al.33 theoretically studied the inverse radiative transfer problems concerning temperature determination. Wang and Liu34 constructed a model for simultaneously measuring the two-dimensional temperature and particle concentration distribution for pulverized-coal flame by neglecting scattering. Huang35 developed a two-step inverse radiation analysis method for rapid three-dimensional temperature reconstruction. In this paper, a pilot-scale cyclone furnace is proposed for a CWS combustion test. The image of the CWS flames under different combustion conditions were captured using chargecoupled device (CCD) cameras. Then, the three-dimensional flame temperature distribution was retrieved from these images through inverse radiation analysis by the two-step reconstruction method. The flame image pixel values and reconstructed flame temperature profiles were used to assess the effects of air/ fuel equivalence ratio, air distribution pattern, and furnace load on combustion characteristics. We also discussed the influence of primary air ratio on CWS combustion. Based on the combustion tests, the combustion efficiency of different test cases was calculated and analyzed.

Is,̂ K = Is,̂ K − 1 e−τs,̂ K −1 + Ss*,̂ K − 1(1 − e−τs,̂ K −1) = Is,̂ K − 2 e−τs,̂ K −1+ τs,̂ K −2 + Ss*,̂ K − 1(1 − e−τs,̂ K −1) + Ss*,̂ K − 2(1 − e−τs,̂ K −2)e−τs,̂ K −1 ⋮

∑ Ss*,̂ k(1 − e−τ

s,̂ k

K−1

)e− ∑m=k +1 τs,̂ m

k=0

(2)

or K−1

Is,̂ K =

∑ αs,̂ kSs*,̂ k

(3)

k=0

In eq 3, the contribution coefficient αŝ,k represents the fraction of the intensity contributed by the cell k and received by the camera in direction ŝ. This term can be expressed as K−1 ⎧ ⎪(1 − e−τs,̂ k )e− ∑m = k + 1 τs,̂ m for volume element αs,̂ k = ⎨ K−1 ⎪ ⎩ e− ∑m = 0 τs,̂ m for wall element

(4)

The radiative source term of volume and wall elements can be expressed as

2. THREE-DIMENSIONAL TEMPERATURE RECONSTRUCTION METHOD The computation strategy of flame temperature reconstruction is similar to the traditional tomographic method while using flame self-emission instead of the attenuation of extra intruding laser or X-ray beam. The foundation of this technique is inverse radiation analysis, that is, solving the radiative transfer equation with a known boundary condition.36,37 According to Planck’s radiation law, flame temperatures can be obtained from local radiative intensities emitted by the participating medium in the flame. The local radiative intensities can also be retrieved from the flame emission measured at the boundary through inverse radiation analysis. In this study, the two-step inverse radiation analysis method developed by Huang35 was used to retrieve local radiative intensities for temperature calculation. The two-step method combines the discrete transfer method and discrete ordinate method (DOM) to solve the radiative transfer equation. Before calculation, the volumetric space of the medium is divided into small volume elements with uniform size and radiative properties. CCD cameras that detect visible light are installed at the furnace boundary to capture flame emission images, which are used to infer flame emission intensities through camera calibration. Based on the two-step method, the transfer of radiative intensity is formulated as Is,̂ k + 1 = Is,̂ k e−τs,̂ k + Ss*,̂ k(1 − e−τs,̂ k )

K−1

K−1

= S*s,̂ w e− ∑k =0 τs,̂ k +

ω ⎧ (1 − ωk )Ib , k + k Ik(s′̂ )Φ(s′̂ ,s)d ̂ Ω ⎪ 4 π 4π ⎪ for volume element Ss*,̂ k = ⎨ ⎪ ρ ⎪ εIb , w + Iw(s)̂ |n̂ ·s|̂ dΩ for wall element ⎩ π n̂·s ̂′ < 0

∫

∫

(5)

For the participating medium in the CWS flame, soot, which contributes the most to flame radiation in visible range, is assumed to be the only emission source. The particle size of soot is very fine, and the scattering is considered as isotropic (i.e., Φ(ŝ′,ŝ) = 1). The reflection of the wall is also considered as diffusive, so the local source term is nondirectional (i.e., S*ŝ,k = Sk*). Thus, eq 3 can be transformed into the following matrix form: I = A·S*

A = [AV , AW ] ∈ RN × M

S* = [S*V , S*W ] ∈ R M

I ∈ RN

(6)

Based on the two-step method, eq 6 can be solved by optimizing the regularized least-squares equation as follows: f (S*) = min{|| A·S* − I ||22 + λ 2||D(S* − S*′)||22 }

(7)

where λ is the regularization parameter and D is the vector deviation operator. When the local source term S* is obtained, the local radiative intensity in eq 5 can be deduced by replacing the scattering term with a numerical quadrature using DOM as follows:

(1)

ωk 4π

(1 − ωk)Ib , k = Sk* −

where Iŝ,k and Iŝ,k+1 denote the radiative intensity entering and leaving the kth discrete volume element in direction ŝ. The optical thickness is τŝ,k, and S*ŝ,k is the local radiative source term that consists of emission from local media and scattering from the surrounding media. To determine Iŝ,k+1 from Iŝ,k for all cells (i.e., k = 1, 2, ...) that the ray has passed, eq 1 can be recursively used from the starting point at the opposite boundary wall of the camera (k = 0) to the camera (k = K). Thus, we obtain

K ′l

L

∑ wl[ ∑ S*k′[1 + (δ(k′) − 1)e−τ

]

sl̂ , k ′

l=1

k ′= 0 K l −1

′

e− ∑m=k ′+1 τsl̂ ,m]

(8)

and B


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ρ = Sw* − π e−τsl̂ ,k ′]e

K ′l − 1

L

∑

wl[ ∑ Sk*′[1 + (δ(k′) − 1)

l = 1, n·̂ sl̂ < 0

K l −1 − ∑m′= k + 1 τsl̂ , m

′

k ′= 0

]

(9)

where wl is the quadrature direction weight associated with direction ŝl, δ(k′) = 0 if k′ ≠ 0, and K′l is the number of discrete volume elements that the ray has passed in direction ŝl. Before solving eq 7 to obtain the radiative source term, the radiative properties of the participating medium for each discrete volume element must first be determined to calculate the contribution coefficient of each element αŝ,k. In this study, the particle concentration for the CWS combustion flame is assumed to be uniform because of the strong turbulence. The optical thickness can also be calculated from the Mie scattering model.38 When the local radiative source term is determined, we can retrieve Ib,k and Ib,w by solving eqs 8 and 9 to deduce flame temperature by Wien’s equation or two-color method. The temperature reconstruction approach can be summarized in the following steps. Step one: calculate the contribution coefficient matrix A according to the particle concentration in the flame and the optical projection parameters of the camera.35,38 Step two: retrieve the flame emission I from captured images through calibration.39 Step three: solve eq 7 through the fast algorithm introduced elsewhere35 to obtain the local radiative source term S*. Step four: solve eqs 8 and 9 to obtain the local radiative intensity, and calculate the temperature from Ib,k and Ib,w.

Figure 1. Combustion test rig of the slag-tap vertical cyclone furnace.

Table 1. Properties of the CWS Used proximate analysis (weight %, as received basis) moisture

ash

37.48

7.31

fixed carbon

volatile matter

20.50 34.71 net calorific value (kJ/kg)

concn 65.67

17238 ultimate analysis (weight %, as received basis)

3. EXPERIMENTAL SECTION 3.1. Slag-Tap Furnace. Combustion experiments were performed using a vertical type slag-tap cyclone furnace composed of a combustion chamber and reburning chamber. The full furnace load was 250 kg/h, with a heat capacity of 1.5 MW, and the boiler was an atmospheric pressure boiler. The diameter and height of the combustion chamber were 450 and 3000 mm, respectively. A highchromium refractory was used for lining the furnace with an air cooling jacket to reduce the heat loss through the furnace wall. The preheated air was then injected into the furnace at 348 K. A slag tapping port was placed at the bottom of the combustion chamber to collect the molten slag drops in a quench tank. The high-temperature product gas from combustion entered the reburning chamber and exited into the heat recovery unit. The combustion test rig is shown in Figure 1. The properties of the spent CWS are shown in Table 1. The atomizing nozzle used in this experiment can produce the slurry droplets with the Sauter mean diameter (SMD) of 90 μm. SMD is defined as the diameter of a sphere that has the same volume/surface area ratio as a particle of interest. To guarantee the quick evaporation of water in the CWS, a burner with a preheating chamber (size, Φ350 mm ×150 mm) was used for the ignition and combustion of CWS. Therefore, some of the water in the CWS had evaporated once the CWS was injected into the main combustion chamber, and the preheating chamber also can compensate for the delay of the ignition to a certain extent. The fuel was then quickly ignited, and compressed air was pneumatically injected into the burner. The compressed air mixed with a swirling flow of preheated primary air and secondary air, which were injected into the furnace at velocities of 15 and 40 m/s, respectively. The tangential secondary air was injected into the furnace through a series of specially designed ports at a velocity of 50 m/s to form an intense swirling motion with a circulating diameter of 360 mm. 3.2. Experimental Imaging System and Reconstruction Zone. To capture flame images from different angles simultaneously, the imaging system that we developed consisted of three CCD cameras installed at two existing viewing ports in different directions

C

H

N

S

O

44.26

3.11

0.66 ash fusion point (K)

0.43

6.73

deformation temp.

softening temp.

hemisphere temp.

flow temp.

1482

1599

1626

1668

beside the burner. The other CCD camera was placed in an open port that was located 250 mm below the burner, as shown in Figure 2. The angle between the CCD cameras beside the burner along the vertical

Figure 2. Experimental imaging system layout and reconstruction zone. C


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axis of the furnace was 15°. The industrial camera used in this study was provided by Micro Vision Company. The resolution of the camera was 752 × 480 pixels with 24 bits depth, and the individual square pixel size of the sensor was 6 × 6 μm. The dynamic range of the camera was 62 dB. The recording rate was 60 frames per second, and the camera was equipped with a Computar 12 mm/1.4f lens with a viewing angle of approximate 85°. Three cameras capturing images simultaneously were triggered by a synchronizer and Giga Ethernet cable was used for image transfer. The camera’s spectral domains were, respectively, from 400 to 550 nm for blue color, from 450 to 620 nm for green color, and from 570 to 700 nm for red color. The shutter speed of the camera can be adjusted from 1/30 s to 1/31000 s for continuous image capture mode. For the coal−water slurry flame discussed in this paper, to obtain enough flame radiation and to avoid pixel saturation at the same time, the shuttle speed was set to 1/1000 s during experiments, and no neutral density filter was used. To protect the CCD camera and its accessories from the excessive heat in the combustion chamber, the optical probes/CCD was shielded with an air-cooled jacket. During the experiment, a jet of air was spouted to prevent dust and slag deposition on the optical guide. Before experiments, a simple CFD calculation was carried out, and the volume fraction of soot particles is approximately 0.5 to 1.5 ppm within the flame region at thermal equilibrium condition. So, the concentration was set to be 1.0 ppm, and the scattering of particles was assumed to be isotropic. To evaluate the impact of uniform assumption for actual nonuniform distribution of solid particles, numerical assessments were used. First, the temperature and volume fraction of particles inside discrete reconstruction element was assumed to be nonuniform, and then, the monochromatic radiative intensities measured by CCD cameras were calculated according to the optical projection parameters of the camera and optical beam tracking algorithm. Finally, the monochromatic radiative intensities were used to reconstruct the temperature distribution using the coefficients obtained with uniform assumption and the reconstructed temperatures were compared with assumed values. Results indicated that the error can be controlled below 5% because of the relatively low volume fraction. For system calibration, a high temperature resolution silicon carbide cavity furnace was designed as the camera calibrator. The calibration method was introduced in detail in our previously published paper.39 A similar method has also been used by Zhou40 for power plant boiler temperature reconstruction. The reconstruction zone of the imaging system was divided into 11 × 11 × 11 subcells. Theoretically, the size of the subcell is expected to be as small as possible to increase the spatial resolution of reconstructed temperature distribution. However, the number is limited by the camera number and computational cost. If the size of the cell is too small or the number of the cell is too large, the reconstruction problem will become extremely ill-conditioned and the reconstruct may fail. The reconstruction zone was located 50 mm below the outlet of the burner and had the same height and diameter (450 mm). The flame temperature and particle volume fraction were treated as uniform. The wall was considered diffusive, with an absorption coefficient of 0.85. The temperature of the wall was measured using a thermocouple. During temperature reconstruction, the temperature was set at a constant value. As shown in Figure 2, the X- and Y-axes corresponded to the radial direction, and the Z-axis corresponded to the axial direction. The coordinates of the center cross sections were X = 5, Y = 5, and Z = 5. The cross-section of #1 tangential secondary air was located at Z = 3. The root region of the flame, which was the primary reaction zone of combustion in terms of energy conversion and emission formation, was fully observed. 3.3. Test Cases. Three-dimensional temperature distributions under eight different experiment air/fuel setting cases were reconstructed to evaluate the effect of operation loads, air feeding patterns, and equivalence ratios on combustion performance. Table 2 shows the details of the test cases. Four air feeding patterns were used. For the concentrated air distribution pattern, primary air (15%), secondary air (28%), and #1 and #2 tangential secondary air (57%) were used. For the staged air distribution pattern, the same air flow

Table 2. Test Cases case

load (%)

air distribution patterns

excess air ratio α

1 2 3 4 5 6 7 8

75 100 100 100 100 100 100 100

staged concentrated concentrated concentrated staged staged all tangential secondary air no primary air

1.4 1.2 1.4 1.6 1.2 1.4 1.4 1.4

rate of primary air and secondary air as that in the concentrated air distribution, as well as #1 and #3 tangential secondary air (57%), was used. For the all tangential secondary air distribution pattern, all secondary air was injected through the tangential ports (#1 to #3). For the no primary air distribution pattern, the primary air supply was shut and the air was added to the secondary air. During each test, the CWS flame was stabilized for approximately half an hour.

4. RESULTS AND DISCUSSION 4.1. Typical Test Case. The second test case with 100% load, concentrated air distribution, and an excess air ratio of α = 1.2 was selected as the representative test case. In the beginning of the experiment, an oil burner was used as an additional heat source for preheating the combustion chamber. After the temperature reached 1550 K, the CWS was injected into the furnace for heating and ignition. Given that the flame was unsteady because of the turbulent flow, 500 measurements were conducted to obtain time-averaged information. Figure 3 shows the effective zone of the original red, green, and blue channel flame images obtained from the CCD camera. Figure 4 illustrates the local radiative intensities emitted from the participating medium in the flame and its variance recorded by camera 0. The variance refers to the standard deviation of pixel value in image, and for a single flame image, the value of the variance can be used to describe the uniformity of flame radiation in camera viewing angle. If the change of the variance from one time to next was regular, the flame fluctuation can be considered to be stable. The intensities recorded by all three cameras from different angles show a similar distribution rule. The intensity at the red channel wavelength was the highest and had a tendency to be saturated. Although the intensity at the blue channel wavelength was relatively low, its variance level was the lowest. This finding indicated that the intensity at the blue channel wavelength was the closest to its mean value and most reliable. During the tests, some undesirable effects were observed in several captured videos because of the deposition of small particles on the camera lens, which cannot be prevented in spite of the purge system. However, these effects did not influence the conclusions. These flame images were then used to reconstruct the flame temperature profile by inverse radiation analysis using the twostep reconstruction method. The inferred temperature profiles at different cross sections are shown in Figure 5. All the three axes go from 0 to 10, and the coordinate of the centerline in every direction is 5. The units are the numerical order of the subcells. Despite the delay of the ignition and larger required ignition heat of the CWS fuel, the temperature range of the reconstruction zone indicates that the CWS had already ignited because of the strong central backflow of the high temperature product gas in the cyclone furnace. In the cyclone furnace, the strong swirling motion produced the sufficient backflow of the high temperature product gas, which is an important heat D


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Figure 3. Effective zone of the original flame images: (a) camera 0, (b) camera 2, and (c) camera 3.

along the refractory wall. Therefore, the temperature decreased toward the furnace wall in the external region because of the heat flux through the refractory wall. In the Z direction, the temperature along the central axis of the furnace continually increased, whereas the temperature at the tangential air entrance had peaks and troughs. The tangential secondary air was only preheated to 348 K before it was injected into the furnace. Thus, the temperature near the ports at the crosssection of X = 5 quickly decreased. Conversely, after the fresh tangential air mixed with the fuel, the CWS combustion increased until the temperature peaked at 1720 K at the crosssection of Y = 5. Due to the high temperature and pressure in the cyclone furnace, only a rapid test using the infrared thermometer was employed to verify the temperature at 230 and 275 mm below the burner, which was also shown in Figure 6c as comparison. As the pressure inside the cyclone furnace is higher than atmosphere, the flame would blow out from the opening port for the infrared thermometer measurement under normal condition. So for safety reasons, when the infrared thermometer was measuring, we decreased the pressure in the furnace by increasing the load of the induced fan. Considering the uncertainty caused by the pressure variation of the infrared thermometer measurement, the discrepancy was acceptable and reasonable. 4.2. Effect of Different Excess Air Ratios. The air/fuel equivalence ratio affects the combustion economical efficiency of a cyclone furnace. This ratio is also directly related to the ability of a furnace to facilitate normal combustion, and guarantee the normal operation and safety of a boiler. To determine the effects of different excess air ratios, the other operational parameters were maintained as constant as possible during the tests. However, some difficulties related to variability, control, and repeatability cannot be avoided using the pilot-scale facility. Figure 7 shows the temperature distribution along the centerline of the furnace in the Z direction. The reconstructed temperature profiles from different heights at cross-section Y = 5 are depicted in Figure 8. In the early stage of the combustion process, the primary air and secondary air only provided 43% of the air required for CWS combustion. Therefore, a high air/fuel equivalence ratio was used to supply more air for ignition and primary combustion. Thus, the temperature increased by approximately 100 K with increased excess air ratio by 0.2. However, when the tangential secondary air was injected into the furnace, the high excess air ratio introduced a large quantity of cold air into the furnace. The temperature then rapidly decreased, especially at α = 1.6. In the test case with α = 1.4, the general temperature was higher than that in the other two

Figure 4. Intensities and variances at the red, green, and blue channel wavelengths recorded by camera 0.

source for the rapid ignition of the CWS atomization torch. The flame in this area showed significant axial symmetry at the X and Y cross sections. The swirling flow of the primary and secondary air resulted in the formation of an annular hightemperature region in the initial stage of combustion. With increased distance from the burner, the swirling motion of the air was weakened because the swirling jet continually entrained surrounding gases. While the tangential secondary air was not supplied at this height, the annular high-temperature region faded away at Z = 5. However, the tangential secondary air with high velocity and momentum again formed a strong swirl motion at the lower part of the reconstruction zone. Thus, the intense turbulence and mixing at this area promoted CWS combustion, resulting in an annular high-temperature region with a larger radius. The reconstructed temperature profiles at typical cross sections are depicted in detail in Figure 6. The temperature profiles from different heights at the cross sections of X = 5 and Y = 5 had similar distribution characteristics. The temperature profile can be divided into three typical regions from the furnace center to the wall. In the central region, the temperature profile had a trough because of the low fuel concentration caused by the atomization angle of 45° and swirling motion of the air. The temperature in this region mainly reflected the central backflow region. In the middle region, the intense combustion of most of the fuel and strong swirling motion quickly increased the temperature to peak values, resulting in an annular high-temperature region, as shown in Figure 5. The temperature in the reconstruction zone was not adequately high to form a molten slag layer flowing E


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Figure 5. Temperature profiles at different cross sections: (a) X = 5, (b) Y = 5, (c) Z = 10, (d) Z = 5, and (e) Z = 0.

with the temperature distribution at the lower part of the reconstruction zone, where the temperature was the highest when α = 1.4. The test case with α = 1.6 had the lowest intensity at the blue channel wavelength. The variance indicated that the intensity at the blue channel wavelength with α = 1.4 was the closest to the mean value. The intensity with α = 1.2 was relatively diffused under higher noise levels because of the slag deposition during the measurement.

cases, which reflected the strongest swirling motion and the most intense mixing. However, no significant difference was observed between α = 1.4 and 1.2 at the lower part of the reconstruction zone. Furthermore, the air distribution pattern had no influence on the temperature distribution at different air/fuel equivalence ratios, which confirmed this result. Figure 9 illustrates the intensity at the blue channel wavelength recorded by camera 0 and its variance. For both air distribution patterns, the intensities at the blue channel wavelength was coincident F


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Figure 7. Temperature distribution along the central axis under different excess air ratios: (a) concentrated air distribution and (b) staged air distribution.

combustion pollutant control, the increased air/fuel equivalence ratio from α = 1.2 to α = 1.6 under concentrated air distribution increased the NOx emission level from 1148 mg/ m3 to 1462 mg/m3. Under the staged air distribution, the NOx emission level increased from 752 mg/m3 to 780 mg/m3 with increased α from 1.2 to 1.4. Therefore, to obtain a sufficiently high temperature in the cyclone furnace and complete combustion, the excess air ratio should be maintained at α = 1.2−1.4 or lower. 4.3. Effect of Different Air Distribution Patterns. To evaluate the effect of air distribution patterns, the furnace load and air/fuel equivalence ratio were maintained during the tests as 100% and α = 1.2, respectively. The reconstructed temperature profiles from Z = 4 at cross-section Y = 5 are shown in Figure 10. The intensities and variances at the blue channel wavelength recorded by camera 0 shown in Figure 11 reflect the same information based on the combustion performance. When the tangential secondary air was injected, the test case with staged air distribution had the highest temperature in all four air distribution patterns. However, the no primary air distribution pattern had the lowest temperature. Although the differences among all four cases were unclear, the smallest quantity of tangential air (28.5%) in the upper zone of the furnace with staged air distribution still had the least air cooling effect on temperature. The lower temperature of the all

Figure 6. Temperature distribution in different directions: (a) temperature distribution from different heights at the cross sections of X = 5, (b) temperature distribution from different heights at the cross sections of Y = 5, and (c) temperature distribution in the Z direction.

Considering the operation conditions of cyclone furnaces, a very low excess air ratio may result in low burnout rate and high heat loss of incomplete combustion. However, when the excess air ratio was too high, the exhaust gas heat loss increased and the temperature of the cyclone chamber decreased. The low temperature also caused incomplete combustion and low combustion efficiency. As shown in Section 4.5, the test case with α = 1.6 had lower combustion efficiency than that of the other two cases because of the low temperature. With regard to G


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Figure 8. Temperature distribution from different heights at the cross sections of Y = 5 under different excess air ratios: (a) concentrated air distribution and (b) staged air distribution.

Figure 9. Intensities and variances at the blue channel wavelength recorded by camera 0 under different excess air ratios: (a) concentrated air distribution and (b) staged air distribution.

tangential air distribution case and no primary air distribution case was caused by the insufficient oxygen supply in the initial stage of combustion. However, the air quantity, velocity, and entrance location of the secondary air in the cyclone chamber played a dominant role because of its large momentum. In the all tangential secondary air distribution case, the large quantity and high velocity of the tangential air generated a stronger swirling motion and more intense disturbance to enhance heat and mass transfer. This result counteracted the effect of insufficient air supply in the initial stage of combustion and increased the burnout rate, as mentioned in Section 4.5. Compared with the secondary air, the primary air of the cyclone furnace cannot be directly used to determine the aerodynamic characteristics of the cyclone chamber. Nonetheless, the combustion performance, especially in the ignition stage, was closely related to the primary air. The increased ignition heat and delayed ignition time of CWS combustion caused by the evaporation of water led to difficult CWS ignition. Without the primary air, the required ignition heat of CWS was larger than the heat supplied by the backflow of the high temperature product gas. Therefore, some of the fuel was unfired and the temperature decreased. In addition, the backflow of the high temperature product gas formed in the ignition stage was weak, and the air/fuel mixture temperature was low. Thus, the combustion performance and burnout rate decreased, as discussed in Section 4.5.

Figure 10. Temperature distribution from Z = 4 at the cross sections of X = 5 under different air distribution patterns.

In terms of combustion pollutant control, the staged air distribution pattern significantly reduced the NOx emission. The NOx concentration in the tail flue of the staged air distribution case was 780 mg/m3, with 6% oxygen concentration. The NOx concentration in the tail flue of the concentrated air, all tangential secondary air, and no primary air distribution cases were 1270, 1075, and 1190 mg/m3, respectively. H


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tangential secondary air, the temperature increased by 50 K. Accordingly, the intensity at the blue channel wavelength recorded by camera 0 (Figure 13) increased with increased

Figure 13. Intensities and variances at the blue channel wavelength recorded by camera 0 under different furnace loads.

4.4. Effect of Different Furnace Loads. The reconstructed temperature distribution also reflects the load characteristics of a cyclone furnace. With increased furnace load from 75% to 100%, the increased CWS feed led to increased releasing heat and flame temperature, as shown in Figure 12. In the high-temperature area near the entrance of the

furnace load. A cyclone furnace has a certain ability to adapt to changes in furnace load. Increasing the load of the combustion chamber within certain limits results in a higher temperature level in the furnace and improves combustion and slag melting. The critical load of a cyclone furnace is the lower limit of the load at which the slag tapping process is still smooth. The size of the critical load depends on coal quality, boiler capacity, structural characteristics, hot air temperature, and other factors. In this study, the normal operation of the cyclone furnace at 75% load indicated that the critical load was lower than 75%. Based on the experiment and discussion in Section 4.5, no obvious change was observed in the burnout rate of the CWS under the different loads. Therefore, load had little influence on the combustion efficiency within a large load range. However, the economical efficiency of the combustion of the cyclone furnace would significantly decrease if the load was too low. 4.5. Combustion Efficiency. To evaluate the combustion performance in the different test cases, the residual carbon content of fly ash from the tail flue and inlet of the reburning chamber, as well as the combustion efficiency, was determined (Figure 14). The residual carbon content of fly ash from the tail flue gas and inlet of the reburning chamber was less than 4% in

Figure 12. Temperature distribution from Z = 4 at the cross sections of X = 5 under different furnace loads.

Figure 14. Residual carbon content of fly ash and combustion efficiency of different test cases.

Figure 11. Intensities (a) and variances (b) at the blue channel wavelength recorded by camera 0 under different air distribution patterns.

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Notes

most test cases, except for the no primary air case. Considering the low CO concentration level and residual carbon content in the slag, the combustion efficiency of these cases was calculated to be between 99.2% and 99.6%. The high combustion efficiency level was attributed to the inherent characteristics of cyclone furnaces, such as long residence time and high heat release rate. The atomization combustion of CWS further stabilized its flow organization to achieve stable ignition and combustion. However, the high velocity of an atomization torch extended the ignition distance. In the cyclone furnace, the CWS and air underwent a strong swirling motion with a high degree of turbulence and mixing, which was caused by the high velocity tangential secondary air. Therefore, the adverse effect of the high momentum of the CWS atomization torch was mitigated. The molten slag layer attached onto the refractory wall can accumulate the heat, resulting in a stable and intensive flame, as well as complete combustion. The no primary case only had 97.67% combustion efficiency, and its residual carbon content of fly ash was the highest among all cases. As discussed in Section 4.3, the combustion performance of the no primary air case decreased because of the insufficient air supply in the ignition stage. The low temperature provided insufficient heat for CWS ignition, which decreased the burnout rate.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2012CB214906 and 2009CB219802).

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5. CONCLUSIONS Slag-tap vertical cyclone combustion technology is a promising solution for reducing the particulate matter emission of CWSfueled power plants. Temperature distribution is extremely important for CWS combustion and melting of the fly ash in a cyclone furnace. In this paper, a novel combustion diagnosis was presented using flame images and reconstructed threedimensional temperature distributions. Based on the assessment of the influence of air/fuel equivalence ratio, air distribution pattern, and furnace load on the combustion characteristics and combustion efficiency of CWS, the following conclusions were drawn. (1) The reconstructed temperature distribution can well reflect actual situations under various combustion conditions in the cyclone furnace. The consistency between the reconstructed results and measured results using an infrared thermometer demonstrated that the proposed reconstruction method had good reliability. (2) The inherent characteristics of cyclone furnaces can well overcome the problem of CWS combustion, such as ignition delay and larger required ignition heat. The combustion performance suggested that the cyclone furnace was capable of CWS combustion. (3) For the enhancement of combustion efficiency and combustion pollutant control, the CWS combustion in the cyclone furnace with rated load, excess air ratio of α = 1.2 to 1.4 or slightly lower, and staged air distribution pattern had the best combustion conditions and combustion organization. The combustion efficiencies under the four distribution patterns were between 99.2% and 99.6%, except for the no primary air case, which was 97.67%.

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REFERENCES

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