Influence of different oil feed rate on bituminous coal ignition in a full-scale tiny-oil ignition burner

Abstract

To reduce oil consumption during firing-up and partial-load operation, a tiny-oil ignition burner has been recommended. Through reacting-flow experiments performed on a full-scale experimental setup, the influence of different oil flow rates on bituminous coal combustion as well as flow rates without coal feed was analyzed. The ignition burner is identical to that normally used in an 800 MWe utility boiler. Under operating conditions with flow rates of 50, 100, and 150 kg/h, gas temperature distributions were measured in the burner. At the equivalent measuring points at the exits of the first and second combustion chambers, these distributions remained almost unchanged under a constant coal feed rate of 4 t/h. However on the burner centerline, distributions increased slightly with increasing flow rate. Different gas concentrations were measured at the center of the burner exit. For instance, the O₂ concentration at the burner exit varied from 0.01% to 0.31% whereas CO concentrations were more than 10000 ppm. At the same coal feed rate of 4 t/h, burner resistances are 480, 600, and 740 Pa for oil flow rates of 50, 100, and 150 kg/h, respectively.

Keywords

ignition / coal / burner / boiler / oil flow rate

Introduction

Increasing efficiency and controlling of pollutant emissions are of significant importance to utility boiler projects worldwide [1-6]. Usually a boiler is ignited by preheating with an oil gun. Once the boiler reaches its operating temperature, pulverized coal is input. As the boiler temperature increases, proportionately more pulverized coal is needed. Meanwhile, the output from the oil gun can be reduced when ignition of the coal can be sustained without oil. Consider for example a bituminous coal-fired 300 MWe utility boiler. At initial firing-up, approximately 100 t of oil fuel is consumed. With growing anxiety over the financial burden arising from oil consumption in the firing-up process and partial-load operation, developing oil-free and tiny-oil ignition burners is becoming increasingly necessary for pulverized coal-fired power stations. Different aspects of oil-free ignition burners have been researched by various investigators. Using a 10 kW plasma torch, ignition and stabilization of pulverized coal combustion by plasma assist was investigated for high, medium and low volatile matter coals. The result showed that all of the coals could be ignited. But a high temperature was needed to the low volatile coal [7]. Microwave plasma was studied on an experimental set-up for ignition and stabilization of burning of lean coal. The obtained results indicated an essential intensification of ignition and combustion processes in the microwave burner compared to those in conventional burners. The microwave energy consumption is only approximately 10% of the required expenditure of oil or gas, measured in heat equivalent. A design of an industrial microwave-plasma burner was proposed [8]. Numerical simulation of processes in air-coal dust mixture duct of pulverized coal utility boiler furnace was performed for one of rectangular air-coal dust mixture ducts with two opposite plasma torches, using a 210 MWe utility boiler firing pulverized Serbian lignite. The mass flow rate of extremely hot air-plasma, especially the mass flow rate of much colder air-coal dust mixture, strongly influenced the processes [9]. One- and three-dimensional numerical simulations, as well as laboratory and industrial measurements of coal combustion using a plasma-fuel system were reported. The system could promote early ignition, enhance stabilization, and reduce harmful emissions from power coals of all ranks (brown, bituminous, anthracite and their mixtures) [10]. The plasma ignition technology was applied to bituminous coal-fired boilers [11]. In plasma- and microwave-assisted burners, two main problems that are proving difficult to resolve are extending the capacity of the burner and lowering the maintenance frequency required for operations.

For comparison, traditional oil guns employ mechanical atomization; their output is between 600 to 1500 kg/h. The gun used in the present experiment employed joint compressed-air and mechanical atomization. To ensure an effective atomization and low oil flow rates, the gun was designed with a small nozzle. Arranged near the burner, the guns, after ignition, directly sprayed oil into the furnace to prompt combustion. When the furnace temperature increased to the appropriate level, pulverized coal could then be injected into the furnace where it was ignited. The oil gun was arranged in the second air duct using this ignition mode. The oil gun was angled to the pulverized coal burner nozzle. When the pulverized coal was injected into the furnace, some of the coal was ignited by the oil flame where the coal further away from the oil gun could not be ignited. The coal combustion efficiency was reduced during firing-up and partial-load operations. A novel tiny-oil ignition cyclone burner was designed for the Foster Wheeler (FW) Company to reduce oil consumption during the start-up and low-load operation. This burner has been employed for a 300-MW down-fired pulverized coal utility boiler in China, and the oil consumption was reduced by 70% compared with the technology of the FW Company [12].

To prompt coal combustion efficiency, tiny-oil ignition centrally fuel-rich burners were proposed (see Fig. 1). Using multiple-stage concentrations enabled a higher pulverized coal concentration in the first combustion chamber wherein the ignition temperature of bituminous coal was reduced with the aim of ignition of pulverized bituminous coal. Briefly the process is as follows. With two oil guns, a main and an auxiliary, arranged in the central pipe, a high-energy igniter ignites the atomized oil injected from the main oil gun (oil gun with igniter). The oil burns in an adiabatic chamber. Once an oil flame is formed, the atomized oil from the auxiliary oil gun (oil gun without igniter) can then be ignited. Soon afterwards, the igniter can be turned off when the oil flame can be maintained by the two oil guns and can burn steadily. To concentrate the pulverized coal into the central zone of the burner, cone separators are installed in the primary air-coal mixture duct. The fuel-rich primary air-coal mixture is ignited by the high-temperature oil flame at the place where the mixture passes into the first combustion chamber. Further downstream, the burning pulverized coal and oil flame are directed into the second combustion chamber, where the coal is ignited. During the initial firing-up, the main goal is to achieve instantaneous ignition of the coal by the oil flame and develop a steady flame of pulverized coal. During the entire process, a satisfactory flame is one that burns brightly and steadily. The costs involved in setting-up tiny-oil ignition technology are as follows: From a fuel perspective, the main point in tiny-oil ignition is coal heat rather than oil heat. Coal is consumed, but as the coal price is far below oil, tiny-oil ignition is economically sensible; From an equipment perspective, the original burner needs to be upgraded for tiny-oil ignition. An atomization air pipe and a high pressure air pipe need to be installed on the furnace. Once the boiler is fired up, the main and auxiliary oil guns are shut down and operations switch to that of a centrally fuel-rich burner, characterized by high combustion efficiency and low NO_x emission [13].

From the aspect of environmental protection, electric precipitation cannot be used because a conventional oil gun has a relatively large output and poor fuel-oil atomization. Thus, unburned oil would be deposited on the electric precipitation devices, which is potentially damaging. After the conventional oil gun ignition technology was replaced by the tiny-oil ignition technology, there is no deposition issue for the electric precipitation electrodes from unburned oil because only small quantities of fuel oil are used, whose burnout ratio is high. Thus, electric precipitation can be used in advance, greatly reducing the dust emission and avoiding environmental pollution.

Studies on the influences of coal-feed rates on bituminous coal ignition in a full-scale tiny-oil ignition burner were conducted. Gas temperatures at equivalent points in the burner increased when the coal-feed rates increased from 2 to 4 t/h and decreased at 5 t/h [14]. The influences of excess air ratios (corresponding to primary air velocities) were also investigated. It was shown that a low primary air velocity was beneficial to the ignition and combustion of bituminous coal in the burner [15]. Lean coal ignition was also done in a full-scale tiny-oil ignition burner by different coal feed rates. Gas temperatures decreased gradually when the coal-feed rates increased from 1 to 5 t/h [16]. Both the bituminous coal ignition and the lean coal ignition using the tiny-oil ignition burner, char burnout and release rates of C and H decreased as the radius increased. Increasing coal-feed rates decreased char burnout and release rates of C and H at equivalent points at the exits.

The influence of oil-feeds without coal feed and with coal feed rate at 4 t/h on bituminous coal ignition in the full-scale tiny-oil ignition burner was investigated in the present work. Reasons for the designed rate of 4 t/h of pulverized coal are as follows. The ignition burner was identical to that used in an 800-MWe utility boiler. In practical applications, 4 t/h amounts to more than 60% of the output of the burner. In actual operations, the temperature of primary air and secondary air will increase gradually, and the combustion performance of pulverized coal will be better. Actually, in the ignition process, the pulverized coal will not total to 100% of the output.

From these measurements and subsequent analysis of specific test results, oil feed-rate influences of bituminous coal ignition were obtained. Data collected along the burner axial and radial layout and analysis of pulverized coal ignition and flame diffusion have not yet previously been reported. The purpose of this paper is to further optimize the flow rate of oil by exploring the influences of oil flow rate on pulverized coal combustion in the tiny-oil ignition burner, and to find an appropriate flow rate so as to provide references for practical boiler operations and ensure the efficiency of pulverized coal combustion.

Experimental setup

Figure 1 shows the tiny-oil ignition apparatus. In the experiment, the oil pressure was raised to 1 MPa by the oil pump, by opening the recycling oil valve. Two tubes were extended from the main oil pipe, one connected to the main oil gun and the other to the auxiliary oil gun. A manual adjustment valve was installed on the auxiliary oil gun to adjust the flow rates of the auxiliary oil gun. In this experiment, the feed rate of the pulverized coal, primary air velocity, and oil pressure was maintained the same. The oil flow rate of the auxiliary oil gun was varied; i.e., under these conditions, only this single factor was varied.

The ignition burner, identical to that used in an 800-MWe utility boiler, was equipped with 48 burners. The thermal power of a single burner was 16.7 MWe. For some 600 MW boiler units, the number of the burners was 42. The thermal power of a single burner was close to 16.7 MWe. The tiny-oil ignition centrally fuel-rich burners proposed in this paper can also be applied, whose operation was as follows. Having been supplied by the feeder, the pulverized coal was carried into the tiny-oil ignition burner by the primary air from a blower. Oil was drawn from the oil tank and sent to the main and auxiliary guns by the oil pump. Mechanical and air atomization was used on the oil guns. Administered as atomized air, compressed air entered the oil guns. A small fraction of the atomized air was also consumed in oil combustion. The main body of the air consumed in oil combustion was supplied by another blower. When traversing in the primary air duct, the pulverized coal was ignited. In the whole period of the experiments, there was no inner and outer secondary air in the experimental set-up.

All gas temperatures were measured along the axis of the burner and at the exits of the first and the second combustion chambers. Gases were sampled using a water-cooled stainless steel probe and analyzed online on a Testo 350M instrument [13]. Bracketed at the exit of the burner, the water-cooled stainless steel probe was used to cool the high-temperature gas. The water-cooled stainless steel probe consisted primarily of a water-inlet pipe, a water-outlet pipe, a sampling tube, an outer pipe and supporting components. The high-pressure cool water from the water-inlet pipe was to cool the sampling tube, and therefore, after heat change it flew out via the water-outlet pipe. The water circulation was supplied by a water pump. The gas was sampled in the sampling tube. If the gas enters this tube, temperatures would decease rapidly and the pulverized coal would stop burning. Having passed through the filtrating devices, the samples were drawn into a Testo 350M gas analyzer for subsequent analysis. The accuracy of the Testo 350M gas analyzer for each species measurement was 1% for O₂ and 5% for CO. Each sensor was calibrated before measurement. CO_max was 10000×10^-6 in this experiment. Because the pulverized coal was not fully burnt and the main focus of the research was pulverized coal combustion, NO_x was not measured in the experiments.

To measure the resistance of the ignition at the position of the straight section, a static pressure method was used (see Fig. 1). One end of a U-tube differential manometer was connected to a static pressure hole, and the other to the atmosphere. The difference between pre and post ignition is only the resistance.

Table 1 lists the equipment used and the technical characteristics. Table 2 tabulates the operating parameters. Note that the oil flow rate of the main oil gun is 35 kg/h whereas for the auxiliary oil gun this is variably set to 15, 65, and 115 kg/h. Table 3 presents the ultimate analysis and other characteristics of 0# light diesel oil used in the experiments. Table 4 records the characteristics of the bituminous pulverized coal used in the experiments. Calorific values were measured with the methods in proximate and ultimate analyses in accordance to 213-2003, 212-2001 and 476-2001 GB (Chinese Standard). The pulverized coal fineness is R₉₀ = 9.2%, i.e. 90.8% of the particles pass through a sieve with 90 μm apertures. The test was performed on only one kind of bituminous coal. The volatile and gross calorific value was high. For similar coal characteristics of bituminous pulverized coal used in the experiments, the results could provide reference.

Results and discussion

Gas temperature distribution

Figure 2 depicts the profiles of the gas temperature measured along the burner centerline; here x is the measured distance from the central pipe exit (see Fig. 1). Using the two oil guns without coal feed, the gas temperature gradually decreased as the distance increased. Most of the oil fuel from the main oil gun and part of the oil fuel from the auxiliary oil gun burnt out in the central pipe, where high temperature gas was then formed. Because the oil flow rate was reduced, the heat released was thus limited, and cold air quantities were large. As the gas flew toward the burner nozzle, cold air diffused into the gas, resulting in a gradual decrease in the gas temperature. As the oil flow rate increased from 50 to 150 kg/h, the heat released from oil combustion increased, which delayed the temperatures from decreasing. When the oil rate increased from 50 to 100 and then 150 kg/h, the gas temperature at the exit center of the burner increased from 577 to 856 and then 1007°C. During firing-up, the flame from the central pipe ignited the pulverized coal. The coal continuously released heat as it burnt. The heat released in the center of the burner had two components: one from the oil and the other from the coal, the latter being the larger of the two because the coal feed rate was large. The coal continuously released heat as it burnt. As a consequence, the gas temperature continuously increased along the direction of the burner centerline. As the oil flow rate increased, the heat released from oil combustion increased. With the oil flow rate increasing from 50 to 150 kg/h under a steady coal feed rate of 4 t/h, the temperature at the exit center increased from 1194 to 1265°C. With different oil feed rates, the changes in temperature were small.

Figure 3 demonstrates the gas profile measured at the exits of the first and second combustion chambers; where r₁ and r₂ are the diametric distances in the exit cross-sections of the first and second combustion chamber, respectively. During firing-up with and without coal feed, gas temperatures were highest near the centerline. As diametric distance increased, gas temperatures decreased gradually. The wall temperatures of the first and second combustion chamber exits were less than 94°C and 227°C respectively. The low temperatures near the burner wall could effectively prevent wall burnout. As the oil flow rate of the auxiliary oil gun was higher than that of the main oil gun, the gas temperatures on the side of the auxiliary oil gun were higher than that on the side of the main oil gun at the exit of the first combustion chamber. Because high-temperature gas was mixed with cold air, the gas temperatures at the second combustion chamber exit were lower than those at the first combustion chamber exit.

During the firing-up without coal feed, the heat released in the first combustion chamber came mainly from the oil combustion of the main oil gun. The oil flow rate for the main oil gun was the same for the three cases studied, so the temperature distributions for the three cases without coal feed were similar at the exit of the first combustion chamber. For these three cases, the oil flow rates from the auxiliary gun were 15, 65 and 115 kg/h, respectively. The oil from this gun burnt mainly in the second combustion chamber and after the burner exit. Hence, as the oil flow rate was changed from 50 to 100 and then 150 kg/h, the heat released from oil combustion increased and the peak value of temperature increased from 577 to 889 and then 1007°C.

During firing-up, the pulverized coal burnt gradually and released heat in the process. The gas temperatures at the second combustion chamber exit were higher than those at the first combustion chamber exit. As the oil flow rate increased, the amount of heat released by oil combustion increased and the gas temperature increased, too. In the first combustion chamber, as the coal was injected into the burner, the pulverized coal absorbed heat and gradually released volatile; the volatile first fired and then released heat. At the exit of the first combustion chamber, the temperatures for the three cases studied, with different oil flow rates and a steady coal feed rate of 4 t/h, are higher than those without coal feed. As the high-temperature flame flew from the first to the second chamber, the char in the pulverized coal combusted gradually and released large amounts of heat. O₂ concentrations at the burner exit were low at 0.01%, 0.05%, and 0.31% for these three cases (see Table 5). In the second combustion chamber, the combustion depended on O₂ concentrations. For different oil feeds, the oxygen supply within the burner was the same, so the temperature distributions for the three cases studied here were similar. The values of the temperatures at equivalent measuring points were almost identical. The influences of the oil-feed rate were smaller for bituminous coal ignition than the coal-feed rates [16]. From the analysis on the temperature under the conditions prevailing here, the oil rate of 50 kg/h was sufficient enough to ignite the pulverized coal.

During firing-up with the oil flow rate set at 100 kg/h and without coal feed (see Fig. 4(a)), the oil was ignited instantaneously, burnt steadily, and a small bright flame was formed. During firing-up with an oil flow rate of 100 kg/h and with a coal feed rate of 4 t/h (see Fig. 4(b)), the oil flame achieved instantaneous ignition and a steady burn of the pulverized coal developed. The big flame formed by the two oil guns and pulverized coal was bright and steady during the whole process under oil flow rates of 50, 100 and 150 kg/h. Figure 4 displays the oil fuel and coal flame.

Gas compositions and burner resistance

Table 5 lists the gas compositions at the center of the burner exit as well as the burner resistance. For example, 0.31% and 480 Pa respectively represents the oxygen in the center of the burner exit and the increased resistance when the coal was ignited in the condition that the coal feed rate was 4 t/h and the oil feed rate was 50 kg/h. For oil feed rates of 50-150 kg/h, O₂ concentrations were in the range of 0.01% to 0.31% and CO concentrations exceeded 10000×10^-6. Because oil was injected from the center, the O₂ at the center of the burner exit was almost exhausted. Even if the flow rate of oil increased, oil consumption in the burner still remained steady. The O₂ concentrations were in the range of 0.01% to 0.04% and 1.79% to 7.18% when the bituminous coal feed rates were 2 to 5 t/h [16] and the lean coal feed rates were 1 to 5 t/h [15]. The bituminous coal burnt well in the tiny-oil ignition burner. Therefore, based on the experiments, the oil flow rate of 50 kg/h was recommended. With a coal feed rate of 4 t/h and a primary air temperature of 15°C, the burner resistances, with two oil guns in operation and coal feed, were 480, 600, and 740 Pa for oil feed rates of 50, 100 and 150 kg/h, respectively.

Conclusions

During firing-up without coal feed, the gas temperatures at equivalent measuring points at the exits of the first chamber and along the burner centerline increased significantly as the oil flow rate increased from 50 to 150 kg/h. With a coal feed rate of 4 t/h, the gas temperatures at the equivalent measuring points at the exits of the first and second combustion chambers was almost unchanged and along the burner centerline increased slightly, as oil flow rates increased. This is because primary air only provides the oxygen needed for combustion of the volatile; the lack of O₂ limited coal combustion within the center of the burner. With oil flow rates of 50, 100 and 150 kg/h, the tiny-oil ignition burner could ignite the experimental bituminous coal successfully. The flame of the pulverized coal at the burner exit was bright and steady. To save oil, an oil flow rate of 50 kg/h is recommended in actual operations.

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