DEVELOPMENT OF BLAST-FURNACE GAS FIRING BURNER FOR COFIRING BOILERS WITH PULVERIZED COAL

Transcription

1 DEVELOPMENT OF BLAST-FURNACE GAS FIRING BURNER FOR COFIRING BOILERS WITH PULVERIZED COAL Takashi Kiga, Takehiko Ito, Motoya Nakamura and Shinji Watanabe Combustion Engineering Dept. Ishikawajima-Harima Heavy Industries Co., Ltd , Toyosu, Koto-ku, Tokyo 135 JAPAN 1. INTRODUCTION To utilize surplus gases such as blast-furnace gas (BFG), Linz Donawitz gas (LDG) and coke-oven gas (COG) as fuels in power plants is one of the effective energy-saving measures in iron and steal companies. For the purpose, a 145 M We, wall firing boiler was designed and constructed by Ishikawajima-Harima Heavy Industries Co., Ltd. ( I H I ) to fire three kinds of surplus gas, BFG, LDG and COG other than pulverized coal and heavy oil. Among these fuels, BFG assisted by COG or heavy oil and pulverized coal were main fuels with whichever the full load could be obtained. Commissioning tests have started with pulverized-coal firing, and satisfactory performances have been obtained with the coal alone. However, BFG burners and LDG burners were found to be subjected to a serious ash trouble on their burner mouth and gas nozzle after 2 weeks coal firing. A lot of amount of ash was deposited on the burner mouth and it was plugged the gas nozzle partially or entirely as shown in Fig. 1. In addition, as shown in Fig. 2, which shows the status of the gas nozzle of the BFG burner after removing ash, it was deformed. In order to investigate the cause of the trouble and to propose the method for burner modifications, experimental and analytical studies were carried out. 2. SPECIFICATIONS OF THE BOILER The subject boiler was constructed in the iron and steal company as a steam generator of a 145 MWe thermal power plant and it started the commercial operation in The unit is equipped with flue gas treatment system such as selective catalytic reactor (SCR), flue gas desulphurization (FGD) and electrostatic precipitator (ESP). Figures 3 and 4 show the sectional side view of the boiler and the burner arrangement, respectively. Twenty four (24) burners are arranged in three (3) stages with four (4) Impact of Mineral Impurities in Solid Fuel Combustion, edited by Gupta et al. Kluwer Academic / Plenum Publishers, New York,

2 286 T. Kiga et al. rows for each opposed wall. Upper two (2) stages of front wall are for BFG, an upper stage of rear wall is for LDG, and lower three (3) stages are for pulverized coal from three (3) roller mills. COG nozzles and heavy fuel oil burner are also installed in the BFG and LDG burners. So they are called as combination burners. The IHI NOx Preventing Advanced Combustion Technology (INPACT) [Kiga, 1989] is applied so that a very low NOx operation can be realized; that is, over fire air ports (two-stage combustion air ports) are settled up in consideration of the residence time in the furnace. The main specifications of this boiler are shown in Table 1. Typical properties of surplus gases are tabulated in Table 2. Since the BFG has a very low heating value of around its flame is stabilized by a pilot flame of COG or heavy fuel oil. On the other hand, properties of coal used in the commissioning test are presented in Table 3. The coal was Australian bituminous coal and it was pulverized by the mills into over 90% through 200 mesh sieve [Tamura, 1996]. 3. EXPERIMENTAL Several kinds of half-scale size burner model were manufactured and they were utilized both for flow pattern testing using cold air and for combustion testing Test Facilities These tests were conducted in the 12 MW(thermal) pilot-scale combustion test facilities in IHI s Aioi Works. The specifications of the test facilities are listed in Table 4 and the general arrangement of its furnace is schematically shown in Fig. 5. The furnace comprises of a multi-burner furnace and a single-burner one. The single burner furnace was used in the test.

3 Development of Blast-Furnace Gas Firing Burner for Cofiring Boilers with Pulverized Coal Test Burners The tests were conducted for three (3) kinds of BFG burner model, Type I (original burner model), Type II, and Type III, where Type II was a model of a temporally modified burner and Type III was that based on the final modification design. The photographs of these burners tested are shown in Fig. 6.

4 288 T. Kiga et al. The BFG is provided from an annulus nozzle, and multi-spud type COG nozzles are arranged in the inside of the inner barrel of the BFG nozzle. The heavy fuel oil burner is mounted in the center of the burner and a swirler is attached near an atomizer for flame stabilizing. The primary air which is divided from the combustion air at the inlet of the burner is supplied in the center of the burner, and it is minimized when the BFG is not provided. The secondary air is supplied from multiple rectangular nozzles arranged in the BFG nozzle for Type I and Type II, and it is supplied from around the BFG nozzle for Type III. Type II and Type III models are equipped with multiple openings on the outer barrel of the BFG nozzle to lead secondary air into BFG nozzle. In addition, Type II model has a baffleplate at the outlet of the secondary air nozzle so as to avoid the reverse flow toward the secondary air nozzle. Type III model has an baffleplate at downstream of the openings to change the direction of the air flow from the openings into a suitable way to avoid the reverse flow toward BFG nozzle when the burner is out of service.

5 Development of Blast-Furnace Gas Firing Burner for Cofiring Boilers with Pulverized Coal 289 Two (2) types of burner throat were prepared; one was made by styrene foam used in the flow pattern testing in order to measure the velocity in the burner by removing it and the other was made by steal and refractory used in the combustion testing Test Conditions The ash troubles were occurred during pulverized-coal firing when the BFG burners were out of service. The flow pattern tests were therefore carried out in the condition that

6 290 T. Kiga et al. the air flow rate was settled so that the air velocity through the burner throat was simulated for actual conditions when the BFG burner was out of service. In the tests the distributions of the air velocity were measured by a hot-wire anemometer at the section of the edge of burner tile and throat. On the other hand in the combustion tests an oil stabilizer was operated to simulate the condition in which the metal temperature measured on the BFG nozzle took the highest value in the actual boiler. The light oil was used in the tests. 4. STRESS AND DEFORMATION ANALYSIS The thermal stress and deformation analysis was performed for the real size burners of Type I (original) in order to examine the reason of the deformation of the gas nozzle of the BFG burner. The analysis was carried out using the finite element program MSC/NASTRAN code. Analytical conditions were given based on the metal temperature measured in the actual operation. 5. RESULTS AND DISCUSSIONS Figure 7 illustrates distributions of air velocity obtained from the flow pattern tests. A reverse flow coming from the outside into the inside on both the secondary air nozzle and the BFG nozzle is found for the original burner (Type I). Such a reverse flow conveys

7 Development of Blast-Furnace Gas Firing Burner for Coliring Boilers with Pulverized Coal 291 hot flue gas and fly ash onto the burner throat and into the nozzles and will cause ash depositions on them. On the other hand Type II (temporally modified burner) and Type III (finally modified burner) have positive air velocity at the secondary air nozzle and BFG nozzles and near the burner throat. Especially for Type III burner the strong cleanup and cooling effects on the burner throat are expected owning to the secondary air flow along the throat. The secondary air flow through the BFG nozzle also make the BFG nozzle and the burner tile clean and cool. The modifications of flow pattern enables

8 292 T. Kiga et al. to prevent these burners from ash troubles. Furthermore Type III burner forms so-called a strong internal circulation flow which will make the flame stable although the test conditions are to simulate the status of burner s out-of-service. The flames observed in the combustion tests are shown in Fig. 8. As is expected from the flow pattern test results good flame stability is obtained about Type III burner. As for the metal temperature of the BFG nozzle it was confirmed that that of modified burners (Type II and Type III ) was clearly lower than that of original burner as shown in Fig. 9.

9 Development of Blast-Furnace Gas Firing Burner for Cofiring Boilers with Pulverized Coal 293 The result of the thermal deformation analysis is shown in Fig. 10. It is obviously found that the deformation pattern obtained in the analysis quite resembles to that observed in the actual boiler as previously shown in Fig. 2. From the analytical results the rigid structure of the secondary air nozzle and the BFG nozzle regardless of thermal deformation are the cause of such damage. In the modification at the actual boiler therefore the swirl plates for secondary air and BFG were not fixed and the BFG nozzle was slit to prevent it from deformation by stretch in addition to making the nozzle cool as mentioned above. Based on the above-mentioned studies all of gas burners including LDG burners in the subject boiler were modified into new type ones. After the modification the plant has been operated without such troubles. CONCLUSIONS Against a serious ash trouble on BFG burners and LDG burners experimental studies and thermal stress analysis were carried out and the followings were obtained. Ash deposits on gas nozzles and burner mouth were caused by the reverse flow conveying hot flue gas and fly ash toward the gas nozzles and along the burner mouth. The deformation of the BFG and secondary air nozzles was caused by its rigid and cooling-less structure. The studies offered ash-free BFG and LDG burners for cofiring with pulverized coal and the subject boiler has been free from such troubles. Since to utilize surplus gases in power plants is one of the effective energy-saving and measures in iron and steal companies we would like to proceed to spread our technologies widely.

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11 Development of Blast-Furnace Gas Firing Burner for Cofiring Boilers with Pulverized Coal 295 ACKNOWLEDGEMENTS We would like to acknowledge the assistance and suggestions of Mr. Yutaka Takamatsu of Volcano Co. Ltd. a manufacturer of the combination burners in the tests and modifications. REFERENCES Kiga T Miyamae S. Makino K. and Suzuki K. (1989). Low NOx Combustion Technologies for Coal- Firing Boilers and Its Application Results to Commercial Boilers. AIChE 1989 Summer National Metting Philadelphia PA. Tumura M. Kiga T. Endo Y. Murakami H. and Tanaka T. (1996). Practical Studies on Combustion Technologies of Micro-Pulverized Coal. IHI Engineering Review 29(3)