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Characterization of Ash Deposition and Heat Transfer Behavior of Coals during Combustion in a Pilot-Scale Facility and Full-Scale Utility Sushil Gupta,*,† Rajender Gupta,‡ Gary Bryant,§ Terry Wall,§ Shinji Watanabe,| Takashi Kiga,⊥ and Kimihito Narukawa# Centre for Sustainable Materials Research & Technology, UniVersity of New South Wales, Sydney, Australia, Department of Chemical & Materials Engineering, UniVersity of Alberta, Edmonton, Canada, Department of Chemical Engineering, UniVersity of Newcastle, Newcastle, New South Wales 2308, Australia, Power Plant DiVision, Ishikawajima-Harima HeaVy Industries Company, Ltd. (IHI), Tokyo, Japan, Japan Coal Energy Center (JCOAL), Tokyo, Japan, and Electric Power R&D Centre, Chubu Electric Power Company, Inc., Nagoya, Japan ReceiVed October 22, 2008. ReVised Manuscript ReceiVed February 18, 2009
Experimental measurements as well as theoretical models were used to investigate the impact of mineral matter of three coals on ash deposition and heat transfer for pulverized coal fired boilers. The ash deposition experiments were conducted in a pulverized fuel combustion pilot-scale facility and a full-scale unit. A mathematical model with input from computer-controlled scanning electron microscopy analysis of coal minerals was used to predict the effect of ash deposition on heat transfer. The predicted deposit thickness and heat flux from the model are shown to be consistent with the measurements in the test facility. The model differentiates the coals according to the deposits they form and their effect on heat transfer. The heat transfer predictions in the full-scale unit were found to be most suitable for the water wall under the furnace nose. The study demonstrates that the measurements in a full-scale unit can differ significantly from those in pilot-scale furnaces due to soot-blowing operations.
1. Introduction Ash deposition is a major problem in pulverized coal fired boilers due to their influence on heat transfer, resulting in increased operating costs. The thermal properties of the deposits depend on their structure and the adherence properties of individual ash particles.1 The adherence properties of ash particles are strongly influenced by the mineral matter present in the parent coal. The thermal properties can therefore be predicted using mathematical models and taking into consideration the mineral matter in coal.2 In the present paper we detail the results of ash deposition tests from a pilot-scale and a fullscale facility using three different coals. The study was carried out under a collaborative research project among IshikawajimaHarima Heavy Industries Co., Ltd. (IHI), Chubu Electric Power Co., Inc., and the CRC for Black Coal Utilization. In this paper we also discusses in brief the mechanistic model based on computer-controlled scanning electron microscopy (CCSEM) analysis of coal minerals to predict the thermal performance of * To whom correspondence should be addressed. Telephone: 61-2-9385 4433. Fax: 61-2-9385 4292. E-mail:
[email protected]. † University of New South Wales. ‡ University of Alberta. § University of Newcastle. | Ishikawajima-Harima Heavy Industries Co., Ltd. ⊥ Japan Coal Energy Center. # Chubu Electric Power Co., Inc. (1) Gupta, R. P.; Wall, T. F.; Baxter, L. The thermal conductivity of ash deposits: Particulate and slag structure. In Impact of Mineral Impurities in Solid Fuel Combustion; Gupta, R. P., Wall, T. F., Baxter, L., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; pp 471-484. (2) Yan, L.; Gupta, R. P.; Wall, T. F. Fuel 2002, 81, 337.
the coals in PF combustion systems.3,4 The predicted ash deposit properties and heat flux in both facilities were compared with the experimental measurements. 2. Experimental Section 2.1. Coal Samples. The ash deposition experiments were part of a comprehensive testing of three coals that were of particular interest to the Japanese utility. Two Australian coals and one Indonesian coal were selected for this study. Coal analysis was carried out using the feed supply of the Hekinan power station, Japan. Table 1 shows the ash composition and other relevant properties of the coals. Coal PC-1 has a low ash content (4.9%) and moderate levels of basic oxides (16%), whereas coals PC-2 and PC-3 have medium to high ash contents but low levels of basic oxides. The coal PC-1 can be seen to have a slightly higher sulfur content. The pilot-scale and full-scale experiments were conducted at the Pulverized Coal Combustion Facility (PCCTF) at Chubu Electric Power, Nagoya, and at unit no. 3 of the Hekinan Power Station in Japan, respectively. After completion of the experiments, the ash and deposit samples were analyzed for chemistry and morphology using scanning electron microscopy at the University of Newcastle, Australia. 2.2. Ash Deposition Probe. A specially designed stainless steel probe shown in Figure 1 was used to collect the ash deposit samples. The inside (T2) and fireside (T1) temperatures of the probe are (3) Yan, L. CCSEM Analysis of Minerals in Pulverised Coal and Ash Formation Modeling. Ph.D. Thesis, University of Newcastle, Newcastle, Australia, 2000. (4) Gupta, R. P.; Yan, L.; Gupta, S. K.; Wall, T. F. Effect of minerals and macerals on thermal performance of boilers. Presented at the Engineering Foundation Conference on Effect of Coal Quality on Power Generation, Park City, UT, 2000.
10.1021/ef800913d CCC: $40.75 2009 American Chemical Society Published on Web 04/14/2009
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Table 1. Proximate Analysis and Coal Ash Properties PC-1
PC-2
PC-3
Proximate Analysis (Air-Dry Basis) moisture content (wt %) 4.50 2.40 ash content (wt %) 4.90 14.40 volatile matter content (wt %) 40.30 26.00 fixed carbon content (wt %) 49.60 56.80 fuel ratio 1.23 2.19 total S content (wt %) 0.47 0.39
5.00 12.30 40.50 42.50 1.05 0.48
Oxide Analysis of Ash (wt %) 54.86 53.30 23.56 35.30 1.09 1.40 7.04 4.00 3.06 3.77 3.18 0.50 1.83 0.50 1.58 0.30 3.39 0.40 0.57 1.30
62.95 28.03 1.90 1.93 1.73 0.90 0.93 0.82 0.52 0.19
SiO2 Al2O3 TiO2 Fe2O3 CaO MgO K2O Na2O SO3 P2O5
Other Properties 2.33 SiO2/Al2O3 ratio basic oxide content (wt %) 16.70 base/acid (B/A) ratio 0.21 ash fusibility (°C), HT 1382
1.51 9.07 0.10 1552
Figure 2. Schematic of the combustion chambers of the Pulverised Solid Fuel Combustion Test Facility (1.3 MW) at Chubu Electric Power, Japan.
2.25 6.31 0.07 1600
measured by two thermocouples embedded into the two sides of the probe. The change in the temperature difference between T1 and T2 is assumed to indicate the change in the instantaneous heat flux. The inner probe surface is maintained at 323 K in the test furnace using water flow and at 873 K in the full-scale unit with the help of airflow. It may be noted that differences of probe surface temperatures would influence deposition. However, other parameters would also influence the ash character such as the heat flux and flame temperatures, and are much higher in the case of the fullscale furnace. The primary aim of the probe was to monitor changes in the heat flux and collect ash deposit samples. Therefore, the difference in inner probe surface temperatures in two cases is not expected to influence the ranking of the thermal performance of the three coals, which was the main aim of this study. 2.3. Pilot-Scale Tests. The Pulverised Solid Fuel Combustion Test Facility at Chubu Electric Power was used for assessing the ash deposition behavior of the three coals. Figure 2 shows the schematic of this furnace, which is 5 m long, cylindrical in shape, and 1 m in diameter. Section A of the furnace consists of refractory walls with a small fraction of the water jacket, while section B contains most of the water tubes. Consequently, most of the heat is extracted in section B of the furnace. The inlet and outlet temperatures of the water tubes, and the furnace exit gas temperatures (FEGTs), were continuously monitored. The flue gases leave the furnace at an average temperature of 1173 K, while the actual values may vary with the coal type. However, as the FEGT value is not used in the model, it will not affect the predicted heat transfer trends by the model. The deposition experiments continued for about 72 h, while deposits were collected on the probe for about 8 h. The deposit
Figure 1. Schematic of the steel probes used for the deposit collection and the temperature measurements.
Figure 3. Comparison of the deposit structures at probes during combustion of coals PC-1 (A), PC-2 (B), and PC-3 (C) in the pilotscale facility. Ash particles were incident from the right-hand side. White regions on the left-hand side show the steel surface, while dark regions on the right-hand side show epoxy.
samples were collected at two different locations simultaneously for 8 h, namely, opposite the burner and the refractory sidewalls. Figure 3 shows SEM images of ash particles deposited on the steel probe located in front of the burner. Figure 4 compares ash deposits formed on the sidewalls of section A of the furnace due to the three coals. 2.4. Full-Scale Tests. The full-scale deposition experiments were carried out at unit no. 3 of the Hekinan Power Station in Hekinan city, which is a 700 MW opposite wall fired furnace manufactured by IHI. Figure 5a shows the schematic of the boiler and location of the test probes. Coal was fired at a rate of approximately 250 tons/h using IHI design swirl stabilized type burners. Figure 5b shows the physical appearance of ash deposits on probes from four locations after completion of the tests with the three coals. On the basis of only visual examination, it was difficult to make a clear
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Figure 4. Comparison of the nature of ash accumulation on the walls of the combustion chambers during combustion tests of coals PC-1 (a), PC-2 (b), and PC-3 (c) in the pilot-scale facility.
Figure 5. (a) Illustration of probes (P1 to P4), burners, and air port locations in unit no. 3 of the Hekinan Power Station (700 MW), Japan. (b) Steel probes with deposits from four locations using the three coals.
distinction among the nature of the deposits on different probes. Therefore, probe deposits were carefully examined using SEM to provide a qualitative comparison of the nature of the deposits. A detailed description of these deposits is provided elsewhere.5
3. Results and Discussion 3.1. Characteristics of Ash Deposits in the Pilot-Scale Test Facility (1.3 MW). 3.1.1. Heat Transfer. The heat transfer data from the probe measurements are not used in the current paper due to noise and discontinuity in the data.5,6 Alternatively, the effects of ash deposition were estimated by the changes in the heat absorption through the water tubes in the furnace. The heat absorption is considered to be 100% at clean furnace walls and is considered to remain the same for up to 10 h from the start of the experiment, during which a thermal equilibrium is achieved in the furnace. The heat balance in the test facility indicated that (5) Gupta, R. P.; et al. The Thermal Properties of Ash Deposits: DeVelopment of a Model Based on Measurements at the Chubu Electric Solid Fuel Combustion Facility and Hekinan Unit No. 3; IHI Project Report; University of Newcastle: Newcastle, Australia, 2000. (6) Gupta, S.; Gupta, R.; Bryant, G.; et al. Presented at the Engineering Foundation Conference on Power Production in the 21st Century: Impacts of Fuel Quality and Operations, Snowbird, UT, Oct 28 to Nov 2, 2001.
Figure 6. Comparison of the reduction of heat absorption during combustion tests in the pilot-scale facility based on water tube measurements.
the flue gases carried approximately half of the total input heat. The heat absorption by the cooling tubes dropped by 15% due to ash deposition, resulting in an equivalent amount of increase in heat carried away by the flue gases. Figure 6 compares the reduction in heat transfer due to the three coals as summarized in Table 2. An upward arrow indicates an increase in the parameter value while a downward arrow indicates a decrease in the value compared to that in clean probe conditions. Coals PC-1, PC-2, and PC-3 indicated 8%,
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Table 2. Heat Flux and FEGT Variation during Combustion Tests of Three Coals in the Pilot-Scale and Full-Scale Facilities pilot-scale furnace
PC-1 PC-2 PC-3
full-scale furnace: heat flux
heat flux (%): water tubes (V)
FEGT (°C) (v)
P1
P2
P3
P4
8 12 15
90 120 200
V V
V V V
-
V V v
12%, and 15% reduction in heat transfer, respectively, during a testing operation of approximately 12 h. Figure 7 illustrates that the decrease in heat flux is also associated with an increase in the FEGT values. 3.1.2. Deposit Structure and Heat Transfer. The deposit samples were analyzed by using SEM to obtain the morphology and composition of deposit-forming particles. The thickness and porosity of the deposits were also estimated from the SEM images (Figure 3). The SEM/EDS analysis of the deposits showed that aluminosilicate is the major deposit-forming constituent. Coals PC-1 and PC-2 contained few iron and calcium silicates, respectively. The deposit particles of coal PC-1 differ from those of coals PC-2 and PC-3 by the irregular shape of the particle, which suggests that the particles were not molten before arriving at the probe surface. Figure 3 compares the nature of the deposit on the probe from the three coals. The measured deposit thickness at the test furnace probe is observed to be the highest for coal PC-1 followed by coals PC-2 and PC-3. Figure 3A shows the deposit for coal A, which contained some pyrites, resulting in a strong deposit due to stickiness. In the case of coal B, only larger particles hit the probe and stuck loosely due to momentum as shown in Figure 3B. In the case of coal C, the loose deposits can be clearly seen in Figure 3C. The average porosity of the probe deposit samples from the three coals is on the order of 50% and is not significantly distinct. Figure 4 compares the deposition on the test furnace walls, and shows that this is not consistent with the probe deposit thickness, which is found to be the least for coal PC-1 followed by coal PC-2 and coal PC-3. The deposition at the probe occurs by direct impaction and is different from the way deposition occurs at the furnace walls. Therefore, ash deposition at the furnace walls is considered to be more relevant. Coal PC-2 deposits are expected to have a lower porosity due to the presence of higher amounts of iron- and calcium-rich species when compared to those from coal PC-3. The study highlights the inadequacy of the probe arrangement in its present form to simulate the ash deposition on the furnace walls. 3.2. Characteristics of Ash Deposits in the Full-Scale Furnace (700 MW). The heat transfer data are estimated from the airflow rate and the probe temperatures. The relative heat flux changes with time have been used for comparison of
Figure 7. Comparison of FEGT variation with time during combustion tests in the pilot-scale facility.
thermal performance. The relative heat flux is given by q/qmax, where q and qmax are the instantaneous and maximum heat fluxes, respectively. q ) Cpm∆T/1000 where q ) heat flux through the probe (kW), Cp ) specific heat of air (1002 J/kg), M ) mass flow rate of cooling air in a probe (kg/s), and ∆T ) outlet air temperature - inlet air temperature. The heat transfer at different probe locations represents the heat flux in different zones of the furnaces and is summarized in Table 2. Figure 8 shows that the reduction in heat transfer at burner zones (P1 and P2) is significantly higher compared to the rest of the water wall (P3 and P4) for each of the coals. The reduction in heat transfer was the least at the surfaces opposite the furnace nose (P3). In general, reduction in heat transfer decreases as the flue gas passes from the lower combustion zone (P1 and P2) to the upper furnace zone (robes P3 and P4), which may be related to the arrangement of the soot blower system and differences in soot-blowing frequency. The average soot-blowing cycle was of the order of 8 h, which could not be maintained to the same frequency due to operational problems. The effect of coal quality appears to be most significant at the water walls under the furnace nose (P4). Coals PC-1 and PC-2 indicate a reduction in heat transfer while coal PC-3 shows an increase in heat transfer at probe P4. On the basis of analysis of the PC-2 and PC-3 samples, the chemical composition of the probe deposits was not significantly different from those of deposit samples from the pilot-scale facility. Probe deposit samples of coal PC-1 were not available as the samples were not intact during transportation to Newcastle. Therefore, SEM analysis of the probe deposits was available only for coal samples PC-2 and PC-3. SEM analysis further indicated that the porosity of the deposit samples at various locations varies from 50% to 75%. At the burner regions, the particles forming the deposit are spherical and fine in size (10 µm) are transported to heat transfer surfaces by inertial impaction, while small particles (
