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Development of a Heating Process of a Slag-Tapping Hole by Syngas Burning in a 150 t/d Entrained-Bed Coal Gasifier Takuya Ishiga,*,† Takeshi Kumagai,‡ Masakazu Utano,§ Hiroshi Yamashita,§ Yasuaki Ueki,∥ Ryo Yoshiie,⊥ and Ichiro Naruse∥ †
Center for Technology Innovation, Research & Development Group, Hitachi, Ltd., Hitachi 319-1292, Japan Engineering Headquarters, Mitsubishi Hitachi Power Systems, Ltd., Kure 737-8508, Japan § Wakamatsu Research Institute, Research and Development Department, Electric Power Development Co., Ltd., Kitakyushu 808-0111, Japan ∥ Institute of Materials and Systems for Sustainability and ⊥Department of Mechanical Systems and Engineering, Nagoya University, Nagoya 464-8603, Japan
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‡
ABSTRACT: Integrated coal gasification combined cycle systems are expected to raise the power generation efficiency of coalfired power plants. In Japan, coal gasifiers are required to treat various kinds of coal imported from many countries because almost all of the coal is imported from abroad. In an entrained-bed coal gasifier, the slag-tapping hole must be heated adequately to discharge the molten slag. Thus, a heating technique has been developed to heat the hole from below, without using any auxiliary fuels. The technique is to burn the syngas, which descends through the hole at high temperature, by supplying oxygen from a pair of nozzles facing each other installed just below the hole. In this study, a design procedure to heat the hole by those nozzles was developed through thermal and flow simulations and actual gasification tests in a 150 t/d entrained-bed coal gasifier of pilot scale. established through thermal and flow simulations and gasification tests conducted in a 150 t/d entrained-bed coal gasifier in a pilot plant called EAGLE (coal Energy Application for Gas, Liquid and Electricity).
1. INTRODUCTION Coal is one of Japan’s major energy resources since it accounts for 25.9% of the domestic primary energy supply and 46.8% of it was utilized as a fuel in the electric power industry in 2015.1 To reduce CO2 emission from the thermal power plants, increased power generation efficiency is required. Integrated coal gasification combined cycle (IGCC) systems are expected to increase the efficiency.2−4 Furthermore, almost all of the coal consumed in Japan is imported, mainly from Australia and Indonesia.1 Coal gasifiers for IGCC systems are required to handle many coal types, including those with high ash fluid temperature that are used in existing pulverized coal-fired boilers.5 In entrained-bed coal gasifiers, ash present in coal is fused into molten slag. The slag flows down through the slag-tapping hole. The hole must be heated enough to maintain a smooth slag flow. There are several techniques to control the slag flow, which include adjusting the ash fluid temperature or viscosity of the molten slag by adding flux6−8 and heating the superior surface of the hole by supplying extra oxygen.6,9 A technique to monitor the aspect of the molten slag flowing down from the hole by analyzing images and sounds has also been developed.10 On the other hand, we have developed a technique to heat the undersurface of the hole without using flux or auxiliary fuel in an entrained-bed coal gasifier.11−15 The key concept of this technique is to burn the syngas that descends through the hole by supplying oxygen from a pair of nozzles, installed just below the hole and facing each other. In our previous study,12 we optimized the heating conditions of these nozzles to supply equal amounts of a mixture of oxygen and nitrogen. In this study, a design procedure to heat the hole using these nozzles was © XXXX American Chemical Society
2. METHOD TO HEAT A SLAG-TAPPING HOLE IN AN ENTRAINED-BED COAL GASIFIER 2.1. Entrained-Bed Coal Gasifier. Figure 1 shows schematic diagrams of the 150 t/d entrained-bed coal gasifier in EAGLE.12 EAGLE, which has been in use since 2002,16−18 is a pilot plant to develop a highly efficient and reliable coal gasification system through gasification, syngas clean-up, power generation with a gas turbine, and CO2 capture from the syngas. The gasification tests described in this paper, in which the gasifier was operated at 2.5 MPa, were conducted as a series of tests to demonstrate these elemental technologies. The gasifier consists of a gasification section and a slag falling section. Between these two sections, there is a slag-tapping hole located at the center of the bottom surface in the gasification section. The gasification section has a vertical cylindrical chamber surrounded by a refractory lined membrane water wall. The inner diameter of this section is defined as D. There are four burners in both the upper and lower stages. Coal fed by nitrogen and oxygen are introduced tangentially from each burner to generate a spiral downflow of syngas, which mainly contains CO and H2.19−2119−21 The O2/coal weight ratio of the lower coal burners is designed to fuse ash contained in coal into Received: December 17, 2018 Revised: March 12, 2019
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DOI: 10.1021/acs.energyfuels.8b04341 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Schematic diagram of the 150 t/d entrained-bed coal gasifier.
Figure 2. Concept of the heating slag-tapping hole in entrained-bed coal gasifier.
molten slag and maintain flow of molten slag in the syngas at high temperatures. The molten slag is separated from the syngas since it adheres onto the surface of the refractory wall. The adhered slag flows down into the slag-tapping hole and drops into the water pool in the slag falling section.
The slag falling section also has a cylindrical shape surrounded by a membrane water wall. The inner diameter of this section is 1.2D. In this section, there are two pairs of nozzles facing each other. The heating nozzles are installed at a higher place than the existing nozzles to heat the hole by supplying a mixture of oxygen and nitrogen. The existing nozzles are the start-up B
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coal type is considered to be suitable for demonstrating the design procedure in the actual tests.
burners and are located 0.75D lower from the undersurface of the slag-tapping hole. In this study, only nitrogen is supplied from the existing nozzles to prevent them from burnout. 2.2. Heating Technique of the Hole. Figure 2 illustrates the concept of heating the slag-tapping hole in an entrained-bed coal gasifier. At the bottom of the gasification section, most of the syngas reverses its course and flows upward around the central axis. However, part of the syngas continues to descend into the slag falling section through the hole. In the lower region of the gasification section, oxygen from the lower burners is enriched so that the syngas temperature will become higher than the coal ash fluid temperature. Around the hole, the molten slag continues to be heated not only by the sensible heat of the descended syngas itself but also by the combustion heat between the syngas and oxygen supplied from the heating nozzles. In our previous study, it was effective to supply an equal quantity of the oxygen and nitrogen mixture from a pair of heating nozzles facing each other because the descended syngas immediately burned with oxygen below the slag-tapping hole.12 2.3. Monitoring Method of Heating Effect. To monitor the heating effect of the slag-tapping hole, two thermocouples are installed to measure the gas temperature close to the hole, shown as TC-1 and TC-2 in Figure 2a. The front tip of them is placed 8 mm below the refractory wall surface. The reason the temperatures were measured at these spots was that it made it possible to not only figure out the syngas temperature directly at places closer to the hole but also prevent these thermocouples from being affected by slag adherence. The heating effect of the hole was evaluated by the average temperature measured from TC-1 and TC-2. 2.4. Design Procedure to Select the Heating Conditions. With the aim of installing the heating nozzles for the 150 t/d gasifier in EAGLE, we designed both the height of the nozzles and the optimal quantity of oxygen by the threedimensional (3D) thermal and flow simulations. This enabled us to determine the target value of the average temperature of TC-1 and TC-2 by simulation. Finally, the design procedure efficacy was verified and demonstrated by actual gasification tests conducted in EAGLE.
4. OPTIMAL HEATING NOZZLE HEIGHT AND OXYGEN QUANTITY NEEDED TO HEAT THE HOLE 4.1. Simulation Method. The 3D simulator22,23 established in Fluent version 4.4 was applied to predict the distributions of gas temperature and syngas compositions in the 150 t/d entrained-bed coal gasifier. The region from top of the gasification section to bottom of the slag falling section is divided into approximately 300 000 hexahedral meshes. This simulator can calculate thermal flow and reactions of coal particles and gas in the steady state using standard k−ε as a turbulence model and P1̅ as a radiation model. Particle motion is calculated by the Lagrangian particle tracking method with a random walk model. In the high temperature zone, a particle adhering to the surface of a refractory wall or rebounding can be determined by a multiplicative model that takes into account gas temperature adjacent to the wall surface and the ash content in the particle. However, the flow of molten slag is not considered in the simulator. As for the boundary condition of the inner wall, convective heat transfer is considered using the overall heat transfer coefficient and external temperature. The coefficient is given for each wall surface, respectively. On the other hand, the external temperature of the wall is set to the same value as the saturated water temperature flowing inside the water wall. Table 2 shows the particle and gas reactions incorporated in the simulator.22 The reaction model consists of nine reactions using seven species, which are volatile, char consisting of fixed carbon and ash, CO, CO2, H2, H2O, and O2. Regarding the volatile as a hypothetical mixture of C, H, and O, we estimated the volatile yields from the content of volatile matter measured in the proximate analysis. During the devolatilization, we assumed that all of the hydrogen and oxygen present in coal particles were released as H2 and O2 into the gas phase and that the remaining C in the char was treated as fixed carbon. The C in the char was gasified by char combustion and gasification reactions. 4.2. Optimal Height of the Heating Nozzles. 4.2.1. Flow Conditions. Table 3 shows the simulated flow rates used to determine the height of the heating nozzles. The total coal feed rate is 6250 kg/h, which corresponds to 150 t/d. The total O2/ coal weight ratio was set at 0.74 in the gasification section, whereas the oxygen flow rate from the heating nozzles was determined from short-term gasification tests conducted previously.12 The inlet temperature of O2 and N2 was settled at 298 K throughout this paper. 4.2.2. Heights of heating nozzles for simulation. Table 4 shows the heights of the heating nozzles used in the simulation. The height is the vertical interval between the undersurface of the slag-tapping hole and the center of the nozzles. The values in the table are normalized by the inner diameter (D) of the gasification section. Since the heating nozzles, whose diameters are smaller than those of the existing nozzles, are designed to be installed at a higher position than in the existing nozzles, four heights between 0.1D and 0.6D are selected. 4.2.3. Simulation Results. Figure 3 illustrates the effects of the heating nozzle height on the temperature distribution. The results show that the nozzles located close to the hole effectively heat the hole. This is because the syngas descended in the slag falling section burns with the oxygen supplied from the heating nozzles below the hole. However, at the vertical interval 0.1D in (a) in the figure, the gas temperature near the thermocouples
3. COAL PROPERTIES Table 1 shows the properties of the coal used. The ash fluid temperature of 1693 K is the highest level applicable for existing gasifiers without using flux and auxiliary fuel.10 Therefore, this Table 1. Coal Properties item higher heating value proximate moisture analysis volatile matter fixed carbon ash ultimate analysis C H O N S ash fusion softening temperatures hemispherical fluid
unit
value
as received
base
MJ/kg wt %
dry
wt %
reducing atmosphere
K
25.5 2.8 36.4 43.4 17.4 63.6 4.7 12.5 1.1 0.3 1593 1643 1693 C
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reaction
particle phase
coal particle → volatile (CxHyOz) + char (fixed carbon + ash) C + 0.5O2 → CO C + CO2 → 2CO C + H2O → CO + H2 volatile + aO2 → bCO + cH2 volatile + dCO2 → eCO + f H2 volatile + gH2O → hCO + iH2 0.5O2 + CO → CO2 0.5O2 + H2 → H2O
devolatilization char combustion char gasification
gas phase
a a−h, stoichiometric coefficient determined by the composition of the volatile; x−z, composition of C, H, and O of hypothetical volatile gas, respectively.
Table 3. Simulated Flow Rates Used to Determine the Height of Heating Nozzles gasification section
slag falling section
total coal feed rate from upper and lower burners
total O2 flow rate from upper and lower burners
total O2/coal
kg/h 6250
mN3/h 3238
kg/kg 0.74
vertical interval between the hole and heating nozzles (-)
(a)
(b)
(c)
(d)
0.1D
0.2D
0.4D
0.6D
total flow rate from existing nozzles N2 O2 mN3/h mN3/h 60.0 0.0
and the refractory wall is approximately 2000 K. When the gas at high temperature reaches the thermocouples and the refractory wall, erosion damage occurs, which is a cause for concern. Therefore, it was determined that the 0.1D in (a) was not a
Table 4. Heights of Heating Nozzles Used in Simulations pattern
total flow rate from heating nozzles N2 O2 mN3/h mN3/h 105.0 70.0
Figure 3. Effects of the heating nozzle height on temperature distribution. D
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4.2.4. Selecting Optimal Height of the Nozzles. Figure 4 shows how the heating nozzle height affects the average gas temperature immediately below the slag-tapping hole. It is clear that a smaller interval between the hole and the nozzles heats the hole effectively. Although the average temperature at 0.1D (a) was the highest, the interval of 0.1D (a) was evaluated as unsuitable due to concerns about the reliability described in Section 4.2.3. With both heating effect and reliability taken into account, 0.2D (b) was determined to be the optimal height for effectively heating the hole. However, 0.4D (c) was selected as the height at which the nozzles should be added to the gasifier in EAGLE because it was impossible to install the nozzles around 0.2D (b) due to space restrictions. The average gas temperature at 0.4D (c) is 1294 K, which is approximately 300 K lower than the ash softening temperature shown in Table 1. In accordance with the gas temperature distribution shown in Figure 3c, temperatures near the refractory wall surface inside the hole are maintained equivalent to 1600 K or higher. This temperature distribution can be estimated to maintain smooth flow of the molten slag. When the temperature level falls short, the total O2 flow rate from the heating nozzles enlarges. How enlarging the O2 flow rate from the nozzles affects the heating performance will be discussed in Section 4.3. 4.3. Optimal Quantity of Oxygen Supplied from the Heating Nozzles. 4.3.1. Flow Conditions in the Slag Falling Section. This section shows an evaluation of how the total O2 flow rate from the nozzles affects the heating performance. Determining the flow rate in simulations made it possible to
Figure 4. Effects of the heating nozzle height on heating.
Table 5. Simulated Flow Rates in the Slag Falling Section
case (1) (2) (3) (4) (5)
total flow rate from heating nozzles
total flow rate from existing nozzles
N2 (mN3/h)
N2 (mN3/h)
105.0
O2 (mN3/h) 0.0 35.0 70.0 105.0 157.5
60.0
O2 (mN3/h) 0.0
suitable height for ensuring reliability of the thermocouples and the refractory wall.
Figure 5. Effects of the total O2 flow rate from heating nozzles on temperature distribution. E
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from the nozzles and the gas temperatures, whereas Figure 6b shows the relationship between the total O2 flow rate and the CO mole fraction. In Figure 6a, there are two kinds of gas temperatures, the first being the average gas temperature of the cross-sectional surface at the bottom of the hole and the second being the average gas temperature of TC-1 and TC-2. The obtained results indicate that both kinds of temperatures increase in accordance with an increase of the O2 flow rate from the heating nozzles. To heat the average gas temperature of the cross-sectional surface at the bottom of the hole to higher than 1593 K, i.e., the ash softening temperature shown in Table 1, the total O2 flow rate from the nozzles is required to be higher than 75 mN3/h. The reason that the ash softening temperature is defined as a criterion is to ensure fluidity of the molten slag flowing down in the slagtapping hole. In Figure 6b, the averaged CO mole fraction of the crosssectional surface at the bottom of the hole decreases in accordance with an increase of the O2 flow rate. This tendency indicates that CO contained in syngas burns with O2 in the slag falling section. To heat the hole efficiently, the minimum quantity of the total O2 flow rate from the nozzles is required. Therefore, the best condition of the total O2 flow rate is determined to be 75 mN3/h.
5. DEMONSTRATING THE DESIGN PROCEDURE TO HEAT THE HOLE IN EAGLE GASIFICATION TESTS 5.1. Test Conditions. Table 6 shows the flow conditions in the gasification test that was conducted in EAGLE. The values in Table 6. Flow Conditions Conducted in the Gasification Test Figure 6. Effects of the total O2 flow rate from heating nozzles on the heating performance.
gasification section total coal feed rate from upper and lower burners kg/h 6250
design the optimum O2 quantity of the gasifier in EAGLE. The vertical interval between the hole and the nozzles was fixed at 0.4D, which was selected in Section 4.2.4. Table 5 shows the simulated flow rates in the slag falling section. The oxygen flow rate from the heating nozzles was varied from 0 to 157.5 mN3/h, whereas the nitrogen flow rate from the heating nozzles and the existing nozzles was fixed. The flow conditions in the gasification section were fixed at the values shown in Table 3. 4.3.2. Simulation Results. Figure 5 illustrates how the total O2 flow rate from the heating nozzles affects the temperature distribution. In case (1), gas temperature in the slag-tapping hole is lower than 1500 K and tends to decrease in the slag falling section because the descended syngas does not burn in the section. On the other hand, in case (2), the descended syngas burns with O2 supplied from the nozzles and thus the gas temperature near the heating nozzles is higher than that in case (1). As seen in case (2) to case (5), the gas temperature between the hole and the nozzles tends to become higher as the O2 flow rate increases. This tendency indicates that the quantity of burned syngas in the region increases in accordance with an increase of the O2 flow rate. From these results, it is confirmed that supplying O2 from the nozzles is effective for heating the region. 4.3.3. Determining the Optimum Quantity of Oxygen from the Heating Nozzles. Figure 6 shows the effects of the total O2 flow rate from the heating nozzles on the heating performance. Figure 6a shows the relationship between the total O2 flow rate
total O2/coal
kg/kg 0.74
slag falling section total flow rate from heating nozzles N2
O2
total flow rate from existing nozzles N2 O2
mN3/h 90.0−120.0
mN3/h 60.0−80.0
mN3/h 60.0
mN3/h 0.0
Figure 7. Verification of the heating performance with heating nozzles.
the gasification section are the same as those shown in Table 3. The total O2 flow rate from the heating nozzles is in the range between 60 and 80 mN3/h. This range covers 75 mN3/h, evaluated as the optimum value described in Section 4.3.3. Since the O2 concentration in the nozzles was fixed at 40 vol %, the total N2 flow rate was controlled in accordance with the total O2 flow rate. The reason why the O2 concentration was controlled F
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Table 7. Flow Conditions in Simulations for Comparison with the Test gasification section total coal feed rate from upper and lower burners
total O2/coal
kg/h 6250
kg/kg 0.74
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +81-294-525111. Fax: +81-294-27-5081.
slag falling section total flow rate from heating nozzles N2 O2 mN3/h mN3/h 75.0 50.0 105.0 70.0 135.0 90.0
Article
total flow rate from existing nozzles N2 O2 mN3/h mN3/h 60.0 0.0
ORCID
Takuya Ishiga: 0000-0002-5178-9158 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
Electric Power Development Co., Ltd. conducted all of the gasification tests in EAGLE. Part of these tests was included in the cooperative study with NEDO (New Energy and Industrial Technology Development Organization). The cooperation of NEDO is gratefully acknowledged.
constant was to prevent the nozzles from burnout especially in the conditions of O2 more than 70 mN3/h. On the other hand, the total N2 flow rate from the existing nozzles was fixed at the same values as those given in Table 3. 5.2. Verification Results. The whole gasification test continued 368 h, during which stable yielding of the molten slag was confirmed. It was confirmed because more than 90 wt % of ash contained in coal was recovered as granulated slag from the water pool and no problematic deposits were observed around the slag-tapping hole. Figure 7 shows the test results obtained in verifying the heating performance with the heating nozzles. The average gas temperature immediately below the hole was measured every 1 min in a 75 h continuous test extracted from the 368 h operation. The measured values fell within the range of ±150 K, in contrast to the simulated values. Flow conditions in simulations comparing the test are shown in Table 7. The measured values were estimated to fluctuate due to fluctuation of flow of the descending syngas and molten slag around the hole. These results demonstrate the adequacy of the design procedure and the monitoring method suggested in this study.
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6. CONCLUSIONS In a 150 t/d entrained-bed coal gasifier of pilot scale, a design procedure was developed to heat a slag-tapping hole by supplying oxygen from a pair of nozzles facing each other below the hole. Through simulations and gasification tests conducted in the gasifier, the following four results were obtained: (1) The nozzles located close to the hole can effectively heat the undersurface of the hole. (2) For practical gasification tests, a new monitoring method observing the average gas temperature measured at two spots just below the hole is developed. The monitoring method makes it possible to figure out the average gas temperature at the bottom surface of the hole. (3) A criterion newly introduced for yielding the molten slag from the hole enables the optimal quantity of oxygen to be supplied from the nozzles. The average gas temperature of the bottom surface of the hole is required to be heated higher than the ash softening temperature. (4) In a 75 h gasification test extracted from a 368 h continuous operation, stable operation was confirmed and the measured gas temperature fell within the range of ±150 K, in contrast to the designed values obtained by simulations. This indicates that the design procedure and the monitoring method suggested in this study are effective for practical use. G
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