Application of Heat Pipe Exchangers in Coking Plants and Energy Saving Analysis

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1. Working principle and characteristics of heat pipe exchangers. A heat pipe is an efficient heat transfer element; the type of heat pipe widely used in industry is the gravity heat pipe, which features a simple structure, low cost, and reliable performance (referred to simply as a heat pipe). Inside a sealed container (such as a steel tube), there is a certain amount of working fluid (such as water), with this amount accounting for about 20% of the internal volume of the steel tube. Due to the high vacuum level inside the heat pipe, the working fluid within the shell exists in a saturated gas-liquid two-phase state. The fluid inside the tube absorbs heat from the hot fluid (e.g., flue gas) in the heating section (evaporation section), which causes it to vaporize and turn into water vapor. Due to a slight pressure difference, the water vapor rises within the tube, passes through the adiabatic section, and reaches the cooling section (condensation section) at the upper part of the heat pipe, where it releases its latent heat to the cold fluid outside the tube (e.g., air), thereby condensing again on the inner wall. Due to gravity, the liquid droplets flow down along the wall to one end of the heating section at the lower part of the heat pipe, thus completing one cycle. Through such repeated and continuous cycles, the heat pipe transfers heat continuously from the hot fluid side (e.g., flue gas) to the cold fluid side (e.g., air). Gravity heat pipes have the following characteristics: 1.1 Safety. The heat transfer characteristics of the secondary partition in heat pipes are an important guarantee for achieving safe, reliable, and long-term operation. It can effectively reduce shutdowns in continuous industrial production. Modern large-scale production seeks efficiency. Production shutdowns caused by equipment failures result in significant losses; with traditional partitioned heat exchange equipment, any damage to one of the heat exchange elements requires a shutdown for maintenance. Heat pipe systems are different; they involve secondary wall heat transfer. Even if a single heat pipe in a heat exchange system composed of multiple heat pipes is damaged, the two types of heat transfer fluids will not mix with each other. As a result, this does not affect the overall heat exchange efficiency, and there is no need to shut down the system for maintenance. This provides a reliable guarantee for efficient, modern continuous large-scale production. 1.2 Corrosion resistance The heat flow conversion and self-cleaning capabilities of heat pipe exchangers are important technical guarantees for preventing dew point corrosion and dust blockage in operational heat exchange equipment. It has been proven that problems such as reduced equipment efficiency and even forced shutdowns caused by blockages and dew point corrosion in large power station boilers, various industrial boilers, industrial waste heat boilers with high dust content, and other dust-containing heat exchange devices can be prevented and avoided by switching to heat pipe exchangers. A misconception that arose in the development of heat pipe technology was that using finned tubes as a secondary heat exchange surface would improve heat exchange efficiency; however, this approach is not suitable for heat exchange with dusty gases, as it can lead to dust blockages that result in the failure of the equipment. Yet, through continuous industrial applications, it has been found that finned tube heat exchange is not equivalent to dust blockage. On the contrary, by simply achieving an optimal wind speed, not only will dust not accumulate, but the scale layer on the heat exchange surface can also be continuously removed, thus maintaining efficient heat exchange. During heat exchange with gas containing a dew point, it is possible to cleverly use adjusting the fin spacing on both sides of the heat pipe to keep the tube wall temperature above the gas dew point, thereby preventing tube wall perforation and blockage due to dew point corrosion. 1.3 Adjustability of thermal resistances on both sides: The thermal resistances on either side of the heat pipe can be adjusted. Not only can the internal thermal resistance (length and diameter) be modified, but when the hot or cold fluid is gas, fins can be added to enhance heat exchange. The geometric dimensions and spacing of these fins can be varied flexibly, thereby allowing adjustment of the external thermal resistance for heat exchange between the heating or cooling section and the gas. 2. Application example: A coking plant emits flue gas at a rate of 150,000 Nm³/h, with a flue gas temperature of 600 °C. The waste heat in this flue gas is recovered to supply heating for nearby residential buildings (approximately 350 households, with each household having an area of around 100㎡). Due to the poor insulation properties of these residences, a heat load of 1200 W/㎡ is assumed, leaving a sufficient margin. 2.1 Heat pipe waste heat recovery unit (gas – water): The heat pipe waste heat recovery unit is installed at the flue outlet of the coke oven to recover the waste heat from the flue gases and use it to heat water for heating or domestic use. Its structure: the lower part is the flue, the upper part is the water tank, and there is a partition in the middle. The top is equipped with a safety valve, pressure gauge, and temperature gauge connections, while the water tank has inlet and outlet ports as well as a drain port. During operation, the flue gas flows through the flue of the heat pipe waste heat recovery unit, washing the lower end of the heat pipe; after absorbing heat, the heat pipe transfers this heat to its upper end, where it releases heat to heat the water. To prevent ash deposition and corrosion, the flue gas temperature at the outlet of the waste heat recovery unit is generally kept above the dew point. 2.2 Structural Characteristics According to the principle of energy balance, by reducing the flue gas flow rate from 40,000 Nm³/h from 6000 # to 180 #, the hot water generated through heat recovery (at 95 #) is sufficient to meet the heating requirements. 3. Energy-saving benefits: Once this project is completed, only 27% of the flue gas volume will be used, allowing for the effective recovery of 17,280,000 kj of energy; this amount represents the energy-saving benefits of this project. Here, assuming an annual operating time of 3600 hours (5 months during the heating season) and a coal price of 400 yuan per ton, the energy-saving benefits are as follows: Heat recovered per hour = ρV•CP•Δt = 1.295 × 40000 × 1.1 × 420 = 23,930,000 kJ. The amount of coal saved per year = Heat recovered per hour × Annual operating time / Calorific value of standard coal = 23,930,000 × 3600 / 29,171.2 / 1000 = 2,953 tons per year. The energy-saving benefit = Amount of coal saved per year × Coal price = 2,953 × 400 / 10,000 = 1,181,000 yuan. By utilizing waste heat for heating, air pollution caused by residents using coal for heating is also reduced. The amount of flue gas emitted into the atmosphere is reduced as follows: Flue gas emissions = Emissions per ton of coal × Amount of coal burned per year. With 310 m³ of coal burned per ton, 2953 tons/year equals 915,000 m³/year; thus, the environmental benefits are significant. Payback period = Total investment in the project / Energy-saving benefits = 59/118.1 = 0.5 (a) 4. Conclusion: Heat pipe waste heat recovery units can recover heat from flue gas, and the recovered heat can be used to heat water for various purposes as needed. It can save fuel costs, reduce production expenses, lower exhaust emissions, and achieve both energy savings and environmental protection. The investment in renovation can be recouped within 3–10 months, yielding significant economic benefits.