Replacement of Oil-fired Boiler by Heat Medium Waste Heat Boiler

Liu Jianjun

introduction

Gansu Hualu Aluminum Company is one of the earliest domestic aluminum smelting enterprises that introduced 160KA large prebaked troughs. After 2001, it has also designed 150 new 210KA electrolyzers from Guiyang Institute. The current production capacity of aluminum ingots is 130,000 tons. The main equipment of the anode system is the dismounting equipment imported from Japan with Kikumoto carbon. Since it was put into production in 1989, the main equipment has been changed to add a φ2.2m×45m rotary kiln and an 18-room ring room roaster, and to 54 chamber roasters. The furnace type was reconstructed and a new rapid melting system for bitumen was adopted to change the asphalt batch from dry to wet. On this basis, the anode system not only completed the production of 40,000 tons of anodes for export but also increased the number of electrolytic systems. The need for productivity. However, due to the sharp increase in production capacity, the raw anode system is also facing unprecedented pressure. Especially after the asphalt melting system is put into operation in the second half of 2004, the heat medium boiler in the raw anode section is overloaded and the bottleneck problem is extremely prominent. The heat medium boiler is used. Transformation is imminent.

1 Status of heat medium boilers

1.1 The composition and principle of the heat medium boiler The heat medium boiler of Huaru Aluminum Company is a VCP-150N boiler imported from Japan. The equipment capacity is 150×104 Kal/h. It is an oil-fired boiler. It relies on burning heavy oil to heat the hot medium oil. The raw anode pitch melting system and the preheating kneading system provide heat energy. The boiler system is mainly composed of the following parts, see Table 1.

Table 1 Main composition of heat medium boiler system

Heavy oil is combusted in the boiler hearth, which is first heated by the heavy oil heater to about 140°C before combustion, and then fully atomized by steam. After the heavy oil is burned, a large amount of heat energy is transferred to the circulating medium of the boiler coil. The thermal medium oil carries the heat energy to the asphalt melting system and the preheating mixing system conducts the conduction, so that the solid asphalt is melted into liquid asphalt and the dry material is obtained. The preheating is sufficiently performed to fully mix the asphalt and the dry material, so as to obtain a paste with a certain viscosity and plasticity, and the prebaked anode raw material is produced through vibration molding.

1.2 Status and Existing Problems of Heat Media Boilers

Heat medium boilers have been introduced since the end of the 1980s and have been in operation for 16 years. The boiler system is seriously aged and the maintenance cost is high. The key spare parts such as heavy oil pumps and solenoid valves are mostly imported, which is expensive and difficult to purchase. The annual cost of spare parts materials is about 200,000 yuan. In particular, after the asphalt melting system was put into operation in September 2004, the capacity of the heat medium boiler could not meet the production needs. The design capacity of the heat medium boiler was 150×104 Kal/h, and the flow rate of the heat medium was 100 m3/h. The heat medium oil is Yanshan brand YD-325, the specific heat of heat medium oil is 0.65Kal/Kg°C, and the specific gravity is 0.95g/cm3. The temperature difference between the entrance and exit of the heat medium oil in the heat medium boiler furnace is 30°C. In the actual production, the flow rate of the heat medium system needs to be maintained at 120 m3/h. The actual required heat energy Q is calculated as:

Q real = c × ρ × △ t × qv

= 0.65 Kal/Kg °C x 0.95 g/c m3 x 1000 x 30 °C x 120 m3/h

= 222.3 x 104Kal/h

It can be seen that there is a big difference between the actual required thermal energy and the design thermal energy. The maximum temperature of the boiler outlet is 280°C. In actual production, in order to ensure normal production, the boiler outlet temperature often reaches over 300°C. Long-term over-loading of the boiler caused the furnace cover to be locally heated and the castables frequently fell off. The service life of the furnace cover was less than one month. At the same time, the life of the furnace was shortened and the key spare parts were over-consumed. Even more serious is that even if the boiler is overloaded, it is difficult to meet the production requirements. The designed capacity of the asphalt melting system is 4 t/h. The actual production of the kneading system ensures the quality of the product, and the heating medium of the asphalt melting system is deliberately compressed during production. The flow rate ensures that the heating medium flow of the kneading system reaches 70 m3/h. As a result, the liquid asphalt production capacity is less than 3 t/h, and the liquid asphalt temperature after melting is low, which is close to the production criticality and poses a great threat to the overall production stability.

2 Retrofit of Heat Medium Fuel Boiler

2.1 Conception of Retrofit of Heat Medium Fuel Boiler

The heat medium boiler has become a major bottleneck restricting the raw anode production of Honglu Aluminum Co., Ltd. due to its severe lack of capabilities. At the end of 2004, one year after the asphalt melting system was put into operation, our company decided to retrofit the heat medium oil-fired boilers on the basis of inspecting relevant domestic aluminum plants and considering the practical considerations of our company.

Hualu Aluminum Corporation uses two φ2.2m×45m rotary kilns for calcining petroleum coke. During the calcining process, a large amount of high-temperature exhaust gas is generated. The single large kiln is approximately 40000N m3/h and its temperature is 700°C to 1000°C. The tail gas is mainly supplied to the waste heat furnace for the production of steam for the company's production and living needs. Due to the increase of production pressure, the two large kiln operations are always in long-term cycle, and a large amount of steam is excessively discharged to form unnecessary waste. Making full use of the heat energy of large kiln exhaust gas, using waste heat boilers instead of oil-fired boilers, has become the best way to reform the heat medium boiler system.

2.2 Implementation of Retrofit of Heat Medium Fuel Boiler

Based on the extensive argumentation, Honglu Aluminum Corporation entrusted Tianjin Zhongjian Thermal Carrier Equipment Co., Ltd. in February 2005 to reform the heat medium boiler system and design and manufacture a flue with a heating capacity of 250×104Kal/h. The waste heat boiler replaces the original oil-fired boiler, and the connecting flue and flue damper are installed between the flue outlets of the two rotary kiln dust chambers to introduce the high-temperature flue gas of the rotary kiln into the waste heat boiler, and the heat conduction in the waste heat boiler. The oil is heat exchanged, and the temperature of the flue gas is reduced from about 1000°C to about 350°C, and then exhausted through the chimney to the atmosphere through a high-temperature induced draft fan. The main design parameters of the waste heat boiler are shown in Table 2.

Table 2 Main design parameters of waste heat boiler

New waste heat boilers and original oil-fired boilers are connected to the grid, and some auxiliary equipment (oil injection pumps, heat storage tanks, etc.) and heat medium pipe systems in the original system are used. In order to ensure the heat supply capacity of the waste heat boiler, the new system has added two 280 m3 heat-medium circulation pumps, and at the same time, the collection and sub-tanks have been installed in the total supply and return pipes. A communication pipe is installed between the collection and sub-cylinder to adjust the heating temperature of the heat-conducting oil. In addition, the original heat-medium system main line was partially modified, and three circuits were formed by dividing and collecting oil cylinders, in which the asphalt melting system was changed from single-circuit to double-circuit, and the asphalt high-position tank, loss-of-weight scale, overflow tank, etc. formed a separate circuit. In order to change the pipelines of the asphalt melting system, there are many equipments and the poor circulation of the thermal oil. Taking into account the extension of the heat medium system pipeline, the total amount of heat medium oil increased from 16 tons before the transformation to 36 tons. To this end, a new V=5 m3 heat medium expansion tank and a V=24 m3 heat medium oil tank were added. To improve the thermal stability and safety of the system.

High-temperature induced draft fan choose W5-47No9C type, its design flow rate is 30000m3/h, as the core equipment of the system, in order to facilitate the control and adjustment of the production process, to ensure the continuous and stable production, the use of variable frequency control. The reconstruction project of the heat medium waste heat boiler system lasted for five months and was successfully completed in early July 2005. After the transformation of the heat medium system process shown in Figure 1.

Fig. 1 Flow chart of the reformed thermal media system

3 effects

Huaru Aluminum Company's heat medium waste heat boiler alternative fuel oil boiler system has been put into production for nearly a year since mid-July 2005. Accumulative production of anodes and anodes totaled 68,000 tons during the period. It has achieved good production results and has solved the heat medium. The oil-fired boiler restricts the bottleneck problem of production, and it also improves the relevant technical conditions of raw anode production. After the transformation, the production capacity of the asphalt melting system has been significantly improved. However, the temperature of the actual heating medium has been reduced considerably. In the production, the temperature and flow of the heat medium are all lower than the design level, and the capacity of the heat medium system has a certain surplus. At the same time, it is worth noting that the heat medium waste heat boiler system has achieved the goal of equipment failure since it was put into operation. In order to prevent the failure of the rotary kiln, the high-speed operation of the induced draft fan (about 1600 rpm) poses a hazard to the equipment in the workshop. Adjusting its motor frequency to a maximum of 40 Hz provides optimal protection of the equipment while ensuring the stability of production technology. However, the original heat-oil-fired boilers do not need to be used anymore. They are temporarily idle. See Table 3 for the comparison of the technical conditions in the actual production before and after the transformation.

Table 3 Comparison of actual technical conditions in the actual production before and after the reconstruction of the heat medium boiler

The heat medium waste heat boiler replaces the oil-fired boiler and effectively uses the calcined rotary kiln exhaust gas. The use of less than one-third of the exhaust gas of a single large kiln can satisfy the production of the raw anode workshop, which not only relieves the pressure of the steam waste heat furnace, but also effectively saves resources. . At the same time, the heat medium waste heat boiler replaces the oil-fired boiler to save a lot of heavy oil consumption. The fuel oil boiler is produced from heavy oil as fuel, and the quality of the heavy oil is strict. The heat value must be ≥8500Kg/Kg and the impurity content is ≤0.25%. After 2000, with the development of the petroleum refining and chemical industry, heavy oil (also known as residue oil) at the end of its production chain has failed to meet the requirements of raw anode production. Instead of residual oil, the production of raw anodes in Honglu Aluminum Corporation uses raw slurry. As a heavy oil use, after the asphalt melting system is put into operation, the raw anode heavy oil unit consumption is about 16 Kg/t. Calculated at an annual anode production capacity of 75,000 t, the annual heavy oil consumption is 1200 tons, and the slurry price has now exceeded 3000 yuan/t, and the procurement More difficult. The heat medium waste heat boiler uses rotary kiln exhaust gas as a heat source and no longer burns heavy oil, so it has considerable economic benefits.

At the same time, due to the simple design of the heat-medium waste heat boiler system, all equipments are made domestically, the equipment performance is good, and the maintenance is simple. The cost of spare parts materials required for the system is less than 30,000 yuan per year. After the system was put into normal operation, the daily operation of the boiler was turned over to the control personnel of the rotary kiln, and the five full-time production personnel of the original heat medium system were all transferred.

4 Conclusion

Gansu Hualu Aluminum Co., Ltd.'s raw anode production process replaces oil-fired boilers with heat-medium waste heat boilers, which is a vivid implementation of the national energy conservation and emission reduction strategy and the implementation of sustainable development strategies. The successful implementation of the transformation project not only effectively solved the bottleneck problem of restricting production, and achieved the goal of equipment failure management, but also achieved good economic benefits. Since the project was put into operation, annual savings of spare parts, materials, heavy oil and labor costs were nearly 4 million. Yuan, at the same time, changed the surrounding operating environment and eliminated atmospheric emissions of sulfur-containing materials after heavy oil combustion, which has certain environmental significance. The transformation practice of this project shows that it has the value of further promotion in the industry.