Residual heat is the heat contained in the products and by-products of a process, which raises its temperature to levels higher than those suitable for its emission or storage. This heat can be used so that two objectives are met simultaneously:
Collect and distribute heat for reuse on the same or other equipment.
Reduce the temperature of fluid emission so that the thermal pollution of the plant is reduced.
The residual heat in the effluents of the industrial processes supposes an important loss of thermal energy in the industry. The use of this heat significantly increases the energy efficiency of the equipment and the overall efficiency of the plant. The higher the temperature of the residual heat source, the greater the capacity to use this heat. In general, in a plant, the equipment that can be improved with residual heat recovery (värmeåtervinning in Swedish) measures is multiple:
- Electric and gas ovens
- Boilers of all types (gas, diesel, biomass, etc.)
- Cooling systems
- Cogeneration facilities
The lines of use of waste heat are fundamentally two: Recovery of waste heat from flue gases. Approximately a 20°C decrease in the emission temperature of these gases implies an increase in the energy efficiency of a 1% boiler. Since the combustion gases come out very hot, the possible reduction in temperature is large, achieving significant savings. This simply means that heat recovery (varmegenvinding in Danish) from other fluids. This section includes the use of heat from hot sewage from equipment cooling processes. The possibilities of exploitation are lower, and the temperatures are much lower than in the case of flue gases.
Flue gas recovery
They are the equipment designed for the recovery of waste heat from flue gases. Basically they can be divided into two types:
- Economizers, in which with the heat of the gases water is heated.
- Air recuperators or heaters, which transfer the heat of the gases to an air stream, generally used as combustion of another combustion process.
It is important auxiliary equipment in boilers and industrial furnaces and works by recovering the sensible heat of the exhaust gases, both to be reused in the equipment itself, and to take it to another area of the plant. The preheating of the air produces the following effects:
Heat losses in combustion gases are reduced, saving approximately 1% of fuel for every 20°C of temperature reduction of said gases.
The flame temperature in the combustion zone increases, increasing the heat transferred by radiation and the amount of steam produced with a smaller excess of combustion air.
Some fuels can only be burned with preheating of the air, as is the case with coal.
In the combustion process, the sulfur contained in the fuel is oxidized to SO2, which can be further oxidized to SO3. Sulfur trioxide has a great tendency to combine with water to form sulfuric acid, which remains dissolved in water, with great corrosive power. If the temperature drops too low, the dew point of this solution can be reached, condensing drops of high concentration due to equilibrium. The phenomenon of acid dew makes it necessary to reach an optimum value of the reduction of the temperature of the gases, balancing the energy savings of recovering more heat, with the consequent expense of reducing the useful life of the pipes by corrosion.