The technology used for recycling and utilizing the radiation heat loss on the surface of the rotary kiln cylinder mainly involves installing an arc-shaped heat collection cover on the rotary kiln cylinder, using water as the working fluid, and using a waste heat boiler to discharge wastewater. The insufficient part is supplied by the power generation softening water treatment system, and then sent to the heat collection cover by a circulating water pump. The hot water temperature outside the heat collection cover reaches 97 ℃ or above. Firstly, it exchanges heat with the feed water of the kiln head boiler to improve the thermal efficiency of the waste heat power generation system. Then, it exchanges heat with tap water for bathing in the bathhouse. Finally, a booster pump is added for winter heating in the production and office area. The cooled water is then returned to the circulating water tank; Stop heating in summer and exchange heat with tap water before directly circulating back to the 24 ring water tank.
Design of Radiant Heat Recovery for Rotary Kiln Barrel
Rotary kiln is a high energy consuming equipment. During the clinker calcination process, the waste gas heat consumption accounts for about 30% of the dry process kiln, and the simplified radiation heat consumption accounts for about 6% of the dry process kiln. Under the call of national energy-saving and emission reduction policies, projects to recover waste heat from rotary kiln exhaust gas have been widely launched, achieving considerable economic and environmental benefits, such as the configuration of waste heat power generation projects. The surface temperature of the rotary kiln is mostly between 230 and 350 ℃, and heat is dissipated into the environment through radiation and natural convection.
At present, this part of the heat has not been recovered and utilized, and even with a small amount of utilization, it is not very complete. The surface heat dissipation of static equipment such as kiln tail preheater, decomposition furnace, and grate cooler can be minimized by using new high-quality insulation materials.
Thermal radiation is like light. When it hits an object, it has the characteristics of reflection, penetration, and absorption, and the absorbed part is converted back into thermal energy. In order to recover the radiation heat dissipation loss of the rotary kiln cylinder, we analyzed the temperature distribution on the outer surface of the cylinder during normal operation of the equipment. There are 6 axial flow fans on the outer surface of the firing belt for forced air cooling, and the transition belt for natural cooling. The surface temperature of the cylinder is monitored by the cylinder temperature scanner for 24 hours in real time. In case of local high temperature, on-site patrol personnel use infrared temperature measurement guns to measure it for emergency treatment.
Install a heat collection hood on the periphery of the rotary kiln to recover radiant heat. The heat collection unit is arranged between the two wheels of the rotary kiln cylinder, with a 900mm inspection and maintenance distance left on both sides of the wheels. The curvature of the arc-shaped heat collection unit is the concentric circle of the rotary kiln cylinder, and the inner diameter of the heat collection cover is 200mm away from the surface of the cylinder. The heat exchange carrier is softened water, which is supplied by the fixed discharge wastewater and power generation water treatment system of the kiln tail SP waste heat boiler. The circulating water is equipped with a 30m ³ water storage tank and a circulating water pump; The heat absorption pipe group is composed of multiple seamless steel pipes for medium pressure boilers, evenly distributed in a 360 ° circumference. To ensure heat exchange efficiency, the surface of the heat absorption pipes is treated with high-temperature resistant black paint for anti-corrosion, increasing blackness and reducing reflectivity; Install a radiation energy reflector plate on the outer side of the heat absorbing tube, which is made of δ 0.5mm stainless steel plate. It has high diffuse reflectivity, allowing the back of the heat absorbing tube to also be reflected by the radiation energy, further increasing the heating area of the heat absorbing tube and effectively recovering the heat radiated outward from the surface of the cylinder. The reflector plate is fixed on the steel keel on the inner side of the heat collecting cover shell, and also serves as a protective plate for the insulation material inside the shell.
The water from the circulating water pump enters from the bottom of the heat collection hood and exits from the top, aiming to smoothly discharge the air and steam inside the heat exchange tube. All heat collection units are installed in series or parallel. For rotary kilns that use infrared scanning thermometers for temperature measurement, this fully enclosed recycling method makes it impossible to detect the surface temperature of the kiln body. Therefore, the design has reserved an infrared scanning longitudinal inspection port and a manual temperature measurement window, providing convenience for production operations without affecting the existing operating conditions of the rotary kiln and the detection of surface temperature outside the cylinder. The main body of the heat exchanger is installed on a bracket welded with channel steel.
Disadvantages and Next Steps to Solve
1. The collector is installed on the outer side of the rotary kiln casing, which is beneficial for the recovery of thermal radiation from the casing. However, it is a stationary equipment and can be suspended for installation. The rotary kiln is also a rotary equipment, making installation and maintenance inconvenient.
2. As the temperature distribution of the cylinder is not a constant parameter, it is necessary to automatically detect the outlet (or steam) temperature, adjust the inlet flow rate, and maintain the outlet (or steam) temperature to be basically constant.
3. Due to the large heat exchange area, the generated heat energy has not been fully utilized.
4. Exploring other pathways for heat utilization.
project investment
The design, processing, and installation of the new practical technology are all completed on site, with a steel consumption of 110t. The civil engineering project includes a water supply pump station (indoor circulating water tank and pump, bathhouse heating water tank and pump, electrical room), outdoor pipe trenches, pipe support foundations, and partial heating installation, with a cost of 325000 yuan. The comprehensive cost is 1.495 million yuan.
Application effect
The technology of recovering and utilizing the radiation heat and waste heat from the rotary kiln shell completely seals the kiln shell 360 °, which to some extent recovers the heat dissipation on the surface of the shell. However, the increase in air temperature between the shell and the collector cover in this recycling method leads to poor heat dissipation of the shell, which has a negative impact on the firing of the kiln skin. The temperature difference between the outer surface of the shell and the inner side of the refractory brick decreases, and the kiln skin is prone to detachment. Remove the lower 180 ° collector cover of the calcination belt and only retain the upper 180 ° collector cover, with the transition belt fully enclosed at 360 °. After rectification, it has been in operation for 3 years without any adverse effects.
The original intention of the design of this utility technology is to provide heating water for office areas in winter, employee canteens, bathhouses, and bathhouses in summer. However, the surface radiation heat recovery device of the rotary kiln shell in this project has a heat collection area of 375 m ². It was only during practical application that it was discovered that water from the circulating water pump at a temperature of 30-35 ℃ was supplied at a rate of 30 m ³ per hour. After heating, the temperature rose to above 97 ℃, generating a large amount of steam. In this case, the 30 m ³ heat exchange water tank in the bathhouse cannot absorb so much heat at all. The return water temperature rises, the circulating water tank temperature rises, and after entering the heat collection hood again, the steam volume increases and a large amount evaporates. Subsequently, a tubular heat exchanger was added, and the first stage tubular heat exchanger increased the water supply temperature of the kiln head boiler from 40 ℃ to 75 ℃, while the circulating water temperature decreased to around 75 ℃; The secondary tubular heat exchanger heats the tap water, reducing the circulating water temperature from 75 ℃ to 50 ℃, and raising the tap water temperature from around 10 ℃ to 50 ℃ for use by factory workers for bathing and daily life; The third level heat exchange is used for heating the production area office building, central control building, some duty rooms, cafeteria and bathhouse. The circulating water temperature is reduced from 50 ℃ to 30-35 ℃ and then returned to the circulating water tank. Stop heating in summer and directly return to the circulating water tank after exchanging heat with tap water.
The application of this new practical technology has improved the efficiency of waste heat power generation, saved fuel costs, power costs, labor costs, annual maintenance costs, and heating electricity costs caused by heating and bathing hot water, reduced dust and carbon emissions from boilers and heating coal stoves, saved electricity, and is a new method for clean production, low-carbon living, energy conservation, and environmental protection in enterprises.
Although excess heat is provided to the waste heat power generation system, the heat dissipation of the kiln shell accounts for 5.68% of the total heat, and the recovered radiation heat is very small; And because this kiln is processing carbide slag, the content of carbide slag in the raw material reaches about 45%, The outlet temperature of the C-shaped cylinder is kept at 350-400 ℃ for a long time, and compared to the waste heat from the exhaust gas, the increase in power generation caused by the radiation heat recovery of the cylinder is negligible. If the heating and heating effect is not achieved through the technology of utilizing the radiation heat recovery and utilization of the rotary kiln cylinder, but through other means, the company will spend approximately 1.3 million yuan annually. The total investment of this project is 1.495 million yuan. If the contribution to waste heat power generation is added, this technology can achieve investment recovery in the same year.
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