Process principles and utilization methods of flue gas waste heat recovery in the field of industrial coatings

In the field of industrial coating, flue gas waste heat recovery is an energy-saving and environmentally friendly technology that reduces energy consumption and improves energy efficiency by recovering the heat in the waste gas (flue gas) generated during the coating process. The following are the process principles and main utilization methods of flue gas waste heat recovery:

Process principle

1. Calorie sources: During the coating process, drying furnaces, incinerators (such as RTO/TO) and other equipment will produce high-temperature flue gas (usually 100-400°C or even higher), which contains a large amount of heat energy.
2. Waste heat recovery: The heat in the flue gas is transferred to other media (such as air, water, heat transfer oil, etc.) through heat exchange equipment (such as heat exchanger), thereby realizing heat recovery.
3. Heat transfer mechanism:
   • Convective heat transfer: The high-temperature flue gas is in direct or indirect contact with the cold medium of the heat exchanger, and the heat is transferred by convection.
   • Radiative heat transfer: Under high temperature conditions, flue gases may transfer some heat through radiation.
4. System Design: Waste heat recovery systems usually include heat exchangers, pipes, fans, control systems, etc. to ensure efficient heat transfer and safe operation of the system.

Main use methods

1. Preheat fresh air:
   • Process: The heat of the flue gas is transferred to the fresh air entering the drying furnace or incinerator through an air-to-air heat exchanger (such as a plate or tube heat exchanger).
   • Application Scenario: Used to improve combustion efficiency or reduce fuel consumption in drying furnaces.
   • advantage：Directly reduce fuel demand and achieve significant energy-saving effect.
   • Case：In the automobile painting line, the waste heat of the incinerator flue gas is recovered to preheat the drying furnace inlet air, which can save 10-20% of fuel.
2. Heating process water:
   • Process: Through a flue gas-water heat exchanger (such as a flue gas waste heat boiler), the flue gas heat is used to heat process water or produce hot water.
   • Application Scenario: The pre-treatment cleaning, phosphating and other processes in the paint shop require a large amount of hot water.
   • advantage: Reduce the steam or electric heating demand of the boiler and reduce operating costs.
3. Heating or cooling:
   • Process：The waste heat of flue gas is used to heat the heat transfer oil or water through the heat exchanger, which is used for heating the factory building or driving the absorption refrigerator for cooling.
   • Application Scenario: Heating factories in winter or cooling workshops in summer.
   • advantage: Improve the comprehensive utilization rate of energy and reduce additional energy consumption.
4. Power Generation:
   • Process: Use high-temperature flue gas to drive the Organic Rankine Cycle (ORC) or steam turbine to generate electricity.
   • Application Scenario: Suitable for coating lines with high flue gas temperature (>300°C) and high heat.
   • advantage: Convert waste heat into electrical energy, suitable for energy-intensive enterprises.
   • limitation: The equipment investment is high and the economic feasibility needs to be evaluated based on the scale.
5. Return to incinerator (RTO/TO):
   • Process：Recover the waste heat of RTO (regenerative thermal oxidizer) or TO (thermal oxidizer) flue gas to preheat the exhaust gas entering the incinerator and reduce the use of auxiliary fuel.
   • Application Scenario: VOCs waste gas treatment system.
   • advantage: Improve the thermal efficiency of the incinerator and reduce operating costs.

Key Equipment

• Heat Exchanger: Plate heat exchanger, shell and tube heat exchanger, heat pipe heat exchanger, etc.
• Thermal storage materials: In RTO systems, ceramic thermal storage bodies are often used to store and release heat.
• Control System: Monitor flue gas temperature, flow and heat exchange efficiency to ensure stable operation of the system.

Technical considerations

1. Flue gas characteristics： Painting fumes may contain VOCs, particulate matter or corrosive substances, so the anti-corrosion and anti-clogging design of the heat exchanger must be considered.
2. Thermal efficiency: The design of the heat exchanger needs to optimize the heat transfer efficiency and avoid heat loss.
3. Economical: The investment cost and payback period of the waste heat recovery system need to be evaluated based on the actual heat scale and utilization method.
4. Environmental Compliance: Ensure that flue gas emissions meet environmental protection standards and waste heat recovery does not affect the exhaust gas treatment effect.

Actual Cases

• An automobile painting plant: Through the RTO flue gas waste heat recovery system, the flue gas heat is used to preheat the drying furnace inlet air, saving about 500,000 cubic meters of natural gas annually.
• Furniture coating line: Use flue gas waste heat boiler to produce hot water to meet the needs of pre-treatment process and reduce the cost of electric heating by about 30%.

Development Trend

• Research and application of high-efficiency heat exchange materials (such as high-temperature alloys and ceramic heat exchangers).
• Intelligent control system improves the dynamic adjustment capability of waste heat recovery.
• An integrated energy system that combines renewable energy (such as solar energy) with waste heat recovery.