Article pubs.acs.org/EF
Comparison of Combustion and Emission Characteristics of an Indonesian Lignite Washery Tailing Slurry with a Bituminous Coal in a Bench-Scale Bubbling/Circulating Fluidized Bed Combustor Dandan Chen,† Qing Yang,† Xuguang Jiang,*,† Guojun Lv,† Zengyi Ma,† Jianhua Yan,† Kefa Cen,† Xuehai Yu,‡ Haiyan Liao,‡ and Hua Zhao‡ †
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China Electric Power Research Institute of Shenhua Guohua, Beijing 100025, China
‡
ABSTRACT: In this study, the combustion and emission characteristics of slurry from an Indonesian lignite washery tailing (LWT) and a bituminous coal were studied and compared in both a bubbling fluidized bed (BFB) and a circulating fluidized bed (CFB). The effects of excess air ratio, feeding rate, secondary air ratio, and secondary air location on the flue gas emissions were analyzed to investigate the optimal operating conditions. The results show that it is feasible to directly fire LWT slurry in a CFB. The temperature of the dense-phase bed firing the LWT slurry can reach 800 °C but is lower than that of a normal BFB/CFB firing coal. Both SO2 and NO emissions of bituminous coal are higher than those of LWT slurry, but their emission characteristics are still below the limits for both EU and Chinese regulations. Improved emission characteristics are observed when firing LWT slurry and coal in CFB compared to BFB. Two main differences were found between LWT slurry and coal: (1) Low primary air (PA) flow rates facilitate the defluidization of the FB combustor when firing LWT slurry. (2) High feeding rates of LWT slurry may reduce the temperature along FB instead. These factors should be paid special attention when designing and operating an FB system. Optimal secondary air (SA) ratios were determined for both LWT slurry and coal. The SA should not be located near the distributor. All of the analyses in this study provide useful information for the design and operation of a commercial FB system.
1. INTRODUCTION Lignite has become a very important part of the coal resources in the whole world, e.g., China possessed more than 130 billion tons of lignite reserves, accounting for 13% of the coal storage. More and more attention has been paid on the utilization of lignite. A drying process is necessary for the utilization of lignite, due to its high moisture content (25−40% moisture content). Steam tube rotary dryers have proved to be a promising technology for drying lignite in power plants. However, this technology generates large quantities of pulverized lignite fines, which are easily self-ignited. Thus, wet scrubbing is usually used to collect these fines, generating wastewater with lignite fines in it. Lignite washery tailing (LWT) are obtained by precipitation of these fines from the wastewater. LWT is hard to store or utilize due to its high water content and low calorific value. It has become a threat to the environment, and there is a great need for means of disposing it. The LWT used in this study was obtained from an Indonesian power plant. Its output is approximately 10−15 t h−1,1 and it is badly in need of disposal. Fluidized-bed (FB) combustion technology has been developed as a clean combustion technology for decades. It has been proven to be suitable for most types of fuels.2 Additionally, it is a lowpollution combustion technology, which makes it promising for fuels such as LWT. In our previous work, rheological characteristics of LWT pipeline transportation were studied to determine if LWTs are suitable for use as a slurry.1 Experimental and kinetic analyses of the combustion character© XXXX American Chemical Society
istics of LWT were also performed, using TG-FTIR to obtain information regarding basic combustion kinetics of LWT.3,4 All of these works demonstrate the possibility of firing LWT in FB systems. In the last 10 years, there have been several studies of FB combustion of wet fuels, e.g., sewage sludge,5−9 coal water slurry,10−12 and wastewater.13−15 All of these studies were used as references for the present study. Yu et al.8 studied the combustion characteristics of sewage sludge (79.3% moisture content) in a pilot-scale FB. The effects of the initial temperature and feeding rate on the temperature and gas emissions were analyzed. Han et al.16 studied the combustion characteristics of sewage sludge in a small lab-scale FB. The results show that sewage sludge with a moisture content less than 40% can burn stably in FB without auxiliary fuel. Li et al.7 proposed an integrated incineration system for the disposal of sewage sludge, which combines a bubbling FB (BFB) dryer and a circulating FB (CFB) boiler. Hagman et al.6,15 studied the cofiring of animal waste, sludge, residue wood, peat, and forest fuels in a 50 MWth circulating FB (CFB) boiler. Liu et al.10 and Lee et al.5 chose overbed feeding and top feeding for the incineration of coal water slurry (CWS) and sewage sludge in FB, respectively. Based on the results of these studies, LWT was designed to be pumped into the furnace as a slurry, using a screw pump to Received: July 19, 2016 Revised: October 26, 2016 Published: October 28, 2016 A
DOI: 10.1021/acs.energyfuels.6b01772 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Proximate and Ultimate Analysis (wt %) of LWT and Lignite Coal ultimate analysis (daf)a
proximate analysis b
sample
Mad
LWT Coal
8.18 8.15
Adc
d
Vdaf
31.75 34.21
54.99 37.78
C
H
N
O
St,d
Qb,ad (J/g)e
62.92 74.32
4.55 5.30
1.15 1.42
>31.21 >18.15
0.12 0.53
16113 18227
a
daf: dry and ash-free base. bMad: moisture (air-dried base). cAd: ash (dry base, i.e., moisture-free base). dVdaf: volatile matter (dry and ash-free base). Qb,ad is the bomb calorific value (air-dried base).
e
bucket, and connected to a screw pump with a rated flow from 1 to 33.6 L h−1 and a rated pressure of 1.8 MPa. The feeding rate of the LWT slurry was controlled by a variable-frequency motor. 2.2. Fluidized Bed Combustor Test Rig. The schematic diagram and photographs of a 35 kWth FBC test rig for LWT slurry are shown in Figure 1a,b, respectively. The system consists of a fluidized bed, an ignition system, a feeding system, and a cleaning system for air and flue gas. The main components of the FB system include a furnace (dense and dilute phase zone), a cyclone separator, an economizer, and a bag filter. The inner diameter of the dense phase zone is Φ 100 mm, and the inner diameter of the dilute phase zone is Φ 140 mm. In the system, the furnace is made of 110 mm thick refractory castable. Outside the refractory castable is 65 mm thick aluminum silicate fiber refractory cotton. Outside the aluminum silicate fiber is 150 mm thick insulation cotton. The whole height of the furnace is 4900 mm. The LWT slurry feeding entrance is at the top of the FB furnace, and the coal feeding entrance is at 470 mm above the air distributor. A button-type air distributor was chosen in the system, with four air distribution buttons (24 mm) evenly distributed and with six holes (5 mm) in each air button. Three secondary air inlets were installed above the air distributor at 880, 1080, and 1380 mm. A high-efficiency cyclone separator was placed in the furnace outlet. The inner diameters of its exhaust pipe and ash discharge pipe were 50 and 60 mm, respectively. The whole height of the cyclone separator was 680 mm. Two gate valves were installed at the FB furnace outlet. The FB system can be operated as a BFB or CFB by closing one of the two valves. The fluidized bed was wrapped in aluminum silicate fibers for thermal insulation. Quartz sand particles within a size range of 1−1.2 mm were used as bed materials, with a static height of 400 mm. To measure the temperature along the fluidized bed, seven thermocouples were installed above the air distributor at 225, 475, 725, 1525, 2225, 3625, and 4325 mm. Gas concentrations (CO2, CO, O2, NO, and SO2) were measured in this study using a GASMET FTIR Dx4000 gas analyzer (Temet Instrument Oy, Finland).
account for its high water content. Top feeding was also chosen in this study. In our previous work, 3 trial tests were conducted on a 0.5 m × 0.5 m pilot-scale CFB (0.5 MWth) to study the basic combustion and emission characteristics of LWT. However, this pilot-scale CFB consumes too much LWT. Three trial tests are not sufficient for studying the effects of different operation conditions on the combustion and emission characteristics. Thus, a new self-heating bench-scale fluidized bed (35 kWth) was designed for firing LWT. Most current FBCs of a similar scale use electrical heaters to maintain the furnace temperature.17,30,31 However, for the closest possible simulation of real combustion characteristics of fuels, no electrical heaters were employed to maintain the furnace temperature in the present system. Without knowledge of the optimal operating conditions for LWT, there will be limits to the operation of firing LWT in a CFB. Varol et al.17,18 conducted an in-depth study on the effects of excess air ratio, secondary air, and secondary air location on the emission characteristics of cocombustion of lignite and biomass. The detailed analysis in these studies gives useful information for designing the operating conditions for FB incineration. Shimizu et al.19,20 analyzed the emissions of NOx and N2O during the combustion of sewage sludge in both BFB and CFB combustors. Okasha et al.21 presented a stagedair technique of secondary air feeding in a new FB system, which has been proven to be more effective in controlling NOx. Zhou et al.22combined a CFB and a postcombustion chamber in the new system, which provided improved control of NOx compared to that of traditional methods. To obtain the optimal operating conditions, the effects of operating conditions (PA flow rate, excess air ratio, and secondary air) were studied systematically and comprehensively in the new self-heating, bench-scale fluidized bed (35 kWth) in this paper. Twenty-two tests conducted while firing LWT in the fluidized bed were compared and analyzed. Moreover, 10 tests firing traditional bituminous coal were also conducted in this paper, to study the difference between LWT and traditional bituminous coal. All the tests and analyses in this paper are expected to be very useful for commercial applications of LWT in CFB and will also provide useful information for the design and simulation of a BFB/CFB system.
3. RESULTS AND DISCUSSION 3.1. Preparation of the FB Test. 3.1.1. Cold Test of FB. There are three types of fluidization states, i.e., bubbling, fluidization turbulent fluidization, and fast fluidization. The fluidization state evolves with the increase in gas velocity. When the gas velocity exceeds the terminal velocity (Ut) of the bed particles, the bed particles transform from the fluidized state into a transport reactor. Moreover, the fluidization state only occurs when the gas velocity exceeds the minimum fluidizing velocity (Umf). Thus, it is important to know the value of Umf before tests. Therefore, the cold test of FB must be conducted first. The material layer resistance versus primary air (PA) flow rate during the cold test is given in Figure 2a. The cold test was conducted under static bed layers of three heights, i.e. 300, 400, and 500 mm. The “bed material layer resistance” represents the pressure loss of air flow through the material layer. It is obtained from the following equation: bed material layer resistance (ΔPm) = static pressure of wind box (ΔPw) − air distribution plate resistance (ΔPp). ΔPw and ΔPp were calculated based on the pressure measurement points along the fluidized bed system. Figure 2a shows that the material layer resistance of the bed increases with increasing PA flow rate in
2. EXPERIMENTAL SECTION 2.1. Materials. The Indonesian LWT used in this study was a solid waste obtained from an Indonesian power plant, SHENHUA PT.GH EMM, Indonesia, in which a lignite from South Sumatra was used as the fuel. The proximate and ultimate analyses of LWT and lignite coal are presented in Table 1. In the FB test, LWT was pumped up into the furnace as a slurry, by using a screw pump. Coal was fed by a screw feeder. The original LWT was first mixed with water in a pot to prepare a LWT slurry. A common dispersant for coal water slurry, sodium methylene bis-naphthalenesulfonate (NNO), was used to improve the rheological behavior of the LWT slurry. Its dosage was 1.0 wt % of the dried LWT. The moisture content of the LWT slurry was 54.69%. The LWT slurry was sieved (4 mm), poured into a slurry B
DOI: 10.1021/acs.energyfuels.6b01772 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. (a) Schematic diagram and (b) photographs of the FB system.
the first stage and then tends to stabilize in the second stage. The cutoff point of the two stages corresponds to the minimum fluidizing velocity (Umf). In this study, the Umf for the bed particles within a size range of 1−1.2 mm was approximately 0.53 m/s. The operating superficial gas velocity (Us) was
approximately 1.6 m/s for the BFB and 2.1 m/s for the CFB, and the ratio Us/Umf was 3 for the BFB and 4 for the CFB. 3.1.2. Ignition Test of FB. An ignition burner was installed beneath the FB for boiler ignition, as shown in Figure 1. Both lignite and bituminous coal were used for ignition to shorten the time of the ignition process. The temperatures at each point C
DOI: 10.1021/acs.energyfuels.6b01772 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
ment above the air distributor (225, 475, 725, 1525, 2225, 3625, and 4325 mm). As described in Figure 2b, it took approximately 45 min to heat the dense-phase bed up to 330 °C. This temperature (330 °C) was chosen to be the point at which the ignition burner shuts down and the lignite feeding starts. It took approximately 35 min to heat the dense-phase bed up to 900 °C, at which point the LWT slurry feeding was started. It took approximately 2 h for the whole ignition process. 3.1.3. Stability Test of the FB System. A stable test is the basis for an accurate measurement of emission characteristics. Thus, the stability of tests was examined in this study, as shown in Figure 2c. All temperatures were found to be stable throughout the whole process. The temperature of the densephase bed was approximately 690 °C. However, the dilutephase zone had a lower temperature because the system was arranged in a BFB pattern and the combustion heat could not reach above the FB. The LWT-slurry-fired FB system was very stable, and the temperatures and gas concentrations used in the following text were all obtained based on the average values for a stable test. All the tests conducted in this study are listed in Table 2. 3.2. Emissions Characteristics of LWT in Bubbling Fluidized-Bed Combustor. 3.2.1. Effect of PA Flow Rate. The temperature of the dense-phase bed and the emission characteristics at different PA flow rates for the BFB firing LWT slurry are presented in Figure 3. The concentration of pollutants in this study is expressed with respect to 6 vol % O 2 [mg/Nm 3 ] at ambient temperature (273 K) and atmospheric pressure (1 atm). According to Figure 3a, the highest temperature of the dense phase bed is 730 °C, and the temperature at the furnace outlet is 241 °C. It is obvious that the temperatures of the FB in these tests (tests 1−4) cannot meet the requirements of commercial applications. More tests were conducted to study the optimal operating conditions. As the PA flow rate increases, the temperatures of both dense and dilute phase zones decrease in most tests, as shown in Table 2 (tests 1−4). The PA flow rate was first decreased to 67 L/h to decrease the excess air, but this caused the excess air ratio to increase to 2.80 instead. This means that less fuel was fired in this test, which indicates that defluidization may have occurred in this test. In addition, the temperature of the densephase bed in this test was also observed to decrease obviously, which proves the defluidization phenomenon. Therefore, there is a minimum PA flow rate required to avoid defluidization in the FB directly firing LWT slurry. In this study, the minimum PA flow rate for this system was approximately 74.56 L/h. The low temperatures of the BFBC in Figure 3a occur because the system is on a bench scale (35 kWth) and no electrical heaters were used to maintain the furnace temperature. Until now, most FBCs17,30,31 of a similar scale have used electrical heaters to maintain the furnace temperature.
Figure 2. Tests for preparation of FB: (a) cold test, (b) ignition test, and (c) stability test.
of measurement during this period are given in Figure 2b, in which T0 denotes the temperature of the wind chamber and T1−T7 represent the temperatures of each point of measureTable 2. Tests for LWT test
fuel
FB
PA flow (L/h)
SA flow (L/h)
SA ratio (%)
1−4 5−9 9−12 13−16 17−19 19−22
LWT LWT LWT LWT LWT LWT
BFB BFB BFB BFB CFB CFB
67.01−86.62 75.82 75.82 79.58 91.39 91.26
0.00 0.00 0.00−24.00 13.21 16.94−27.05 19.98
0.00 0.00 0.00−24.04 14.23 15.68−22.93 17.96 D
SA location
feeding rate (L/h)
excess air ratio
all 1, 2, 3, all all all
13.20 9.90−16.50 11.42 13.20 19.80 9.90−19.80
2.08−2.88 1.08−3.56 2.98−3.37 1.57−2.68 1.33−1.57 1.33−2.83
DOI: 10.1021/acs.energyfuels.6b01772 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Madhiyanon et al. gave two reasons to explain this phenomenon: (1) The FB combustor was cooled due to the increasing PA flow rate, thus causing higher CO emissions. (2) As the PA flow rate increased, the fluidizing air velocity increased, thus shortening the resident time. The shorter resident time was not enough for the conversion of CO into CO2. The amount of CO emissions in these tests varied from 2900 to 4600 mg/Nm3. According to the studies by Varol et al.17 and Armesto et al.,24 CO emissions during combustion in the FB ranged from 200 to 2000 mg/Nm3. According to the study by Okasha et al.,21 CO emissions during combustion in the FB ranged from 2300 to 4700 mg/Nm3. These differences are a consequence of the varying operating conditions. Armesto et al. proved that temperature and fluidization velocity have a great influence on CO emissions. In these tests, the temperature of the FB was low and the fluidization air velocity was high, resulting in high CO emissions. Figure 3b shows that the amount of NO emissions in these tests varied from 590 to 840 mg/Nm3. NO emissions were also observed to increase with increasing PA flow rate. In this study, at FB temperatures
