Waste heat
Introduction
Waste heat is the heat contained in the products and by-products of a process, which raises its temperature to levels greater than those suitable for emission or storage. This heat can be harnessed so that two objectives are met simultaneously:
Collect and distribute heat for reuse in the same equipment or in others. Decrease the fluid emission temperature in a way that reduces thermal pollution of the plant.Waste heat in the effluents of industrial processes represents asignificant loss of thermal energy in industry. The useof this heat significantly increases the energy efficiency of the equipment and the overall efficiency of the plant.
The higher the temperature of the waste heat source, the greater the capacity to harness this heat.
In general, in a plant, the equipment that can be improved with waste heat recovery measures are multiple:
Electric and gas furnaces. Boilers of all kinds (gas, diesel, biomass, etc.). Dryers. Evaporators. Compressors. Cooling systems. Turbines. Engines. Cogeneration plants.Exploitation lines
The waste heat use lines are basically two:
Recovery of waste heat from flue gases. Approximately, a decrease of 20ºC in the emission temperature of these gases implies an increase in the energy efficiency of a boiler of 1%. Since the flue gases come out very hot, the possible reduction in temperature is great, achieving significant savings.
Recovery of waste heat from other fluids. This includes the use of heat from hot wastewater from equipment cooling processes. The possibilities of use are lower, and the temperatures are much lower than in the case of flue gases.
Flue gas recuperators
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 the heat of the gases heats water. Recuperators or air heaters, which transfer the heat of the gases to an air stream, usually used as an oxidizer of another combustion process.Air heater
It is an important auxiliary equipment in boilers and industrial furnaces and works by recovering the sensitive heat of the exhaust gases, both to reuse it in the equipment itself, and to take it to another area of the plant.
Preheating the air produces the following effects:
Heat losses in flue gases are reduced, saving approximately 1% of fuel for every 20 °C reduction in the temperature of these gases. It increases the flame temperature in the combustion zone, increasing the heat transferred by radiation and the amount of steam produced with less excess oxidizing air. Some fuels can only be burned with preheating of the air, as is the case with coal.Acid spray
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 droplets of high concentration, due to the balance.
The phenomenon of acid spray makes it necessary to reach an optimal value of the reduction of the temperature of the gases, balancing the energy savings that suppose to recover more heat, with the expense consequence of reducing the useful life of the tubes by corrosion.
Sensitive heat and latent heat
Sensitive heat is the heat used to vary the temperature of a body, in this case water or thermal fluid. It is related to the specific heat, which in the case of liquid water is 1kcal/kg ºC.
Latent heat is the heat used to produce a change of state in a body, such as the vaporization of water, whose latent heat of vaporization/condensation is 540kcal/kg (at 100ºC).
Boiler economizers
They raise the temperature of the water that feeds the boiler obtaining fuel savings at home.
They are composed, basically, by a bundle of conveniently grouped tubes, inside which the water to be heated circulates, while the flue gases bathe the outer surface.
The tubes have four fundamental conformations, which make them suitable for one or another function:
AESS tubes, made of finless stretched steel. They are generally used when the temperature of the gases is very high, with large thermal jumps between the walls of the tube and large heat transfer. AA tubes, made of steel drawn with transverse steel fins. They are used in any heat transfer process provided that there is no risk of corrosion by acid spray. F-tubes, made of cast iron with fins. They are obtained by direct casting of the piece. Higher corrosion resistance than steel (up to twenty times greater). They were the most widely used in the past, but their use is in decline due to their low mechanical strength, which makes them unsuitable for economizers with vaporization. AF tubes, of stretched steel coated with cast iron parts. They combine the mechanical strength of steel with the corrosion resistance of iron coating. They are the most used today.
To avoid the phenomenon of acid spray, it should be operated with the following minimum temperatures:
Minimum water and gas temperatures for different tubes Type of tube Temperature min. gases Temperature min. water Steel tubes (AESS, AA) 160-170ºC 120-130ºC Iron coated tubes (F, AF) 150-160ºC 70-80ºC
Bi-transfer systems
In these systems, heat does not flow from the gases to the water directly, but a carrier fluid is used that transfers heat from one to the other.
They are very indicated when, due to the physical location of the two cold and hot fluids, direct contact between them is not possible. Adequate pipe insulation and a pump are sufficient to transport waste heat over long distances.
The carrier fluid will be water as long as the operating conditions allow it (less than 200ºC, since higher temperatures imply a pressure difficult to maintain). When this is not the case, a thermal oil will be used.
Thetubes used for the exchange are AF, in the event that the fuel contains sulfur, andAA, if it is exempt from it.
Condensation of wastewater vapors in dryers
Gases from dryers come out at high temperature and carrying a high degree of humidity in a vapor state, so that both sensitive heat and latent heat of condensation can be recovered from them.
The exchange can be carried out in two ways:
Direct method. The gases and water to be heated are physically mixed, reaching thermal equilibrium. The exchange is done through tubes, where the moisture of the drying gas partially condenses.In both cases, a condensed water collection system is needed.
Thermal tubes
A thermal tube is the set of two concentric tubes sealed, so that the fluid it contains inside cannot leave it. This carrier fluid is carried from one end to the other by the effect of capillarity, and can be both water and organic fluids, even metals such as mercury or sodium for special applications.
If heat is supplied at one end, the carrier fluid evaporates and passes to the central part of the tube by capillarity, then flows by the effect of pressure to the other end, where it condenses by yielding its latent heat to another fluid, to finally return to the space between tubes by capillarity.
Therefore, thebanks of thermal tubes must be placed with one end in the hot focus (flue gas) and another in the cold focus (water or air to be heated).
Oxidizing air heaters in boilers, furnaces and dryers
There are numerous types of heaters, which can work in different temperature ranges:
- Smooth steel tube heaters. It consists of a bundle of tubes contained in an outer shell. Gases circulate inside the tubes and air outside against the current.
- Steel pipe heaters with extended surfaces. The use of fins makes it possible to reduce the total volume of the heater.
- Cast iron heaters with fins. Rectangular section tubes placed horizontally are used. Air flows inside.
- Borosilicate glass heaters. For low gas temperatures these heaters have a number of advantages such as high heat transfer (no scale occurs due to low roughness), corrosion resistance and ease of cleaning. Disadvantages include glass fragility and thermal expansions.
- Refractory steel heaters. They are used for high temperatures and are bundles of tubes that can transmit heat by convection, radiation or a combination of the two. In the case of radiation recuperators, care must be taken that the temperature of the gases is constant, since the heat emitted is proportional to the fourth power of this temperature, and small variations would result in large changes in the temperature of the heated air. Therefore they are not suitable for discontinuous furnaces.
- Regenerative. They consist of a rotating metal structure with an extended surface to accumulate heat more quickly. The rotation puts the cells in contact with the hot and cold bulbs alternately, thus transporting the heat.
- Ceramic regenerators. They consist of two areas filled with ceramic bricks. At first, the gases pass through one of the zones heating the bricks, passing the air through the other area. Then the flow is reversed and it is the air that crosses the area of hot bricks, absorbing heat and cooling them. The process is repeated alternately. They can handle corrosive gases at high temperatures.
Preheating of the product of the ovens
The exhaust gases from the furnaces can be recirculated countercurrent so that they preheat the product in a preheating chamber, recovering their sensitive heat to a greater extent before escaping through the chimney.
Heat pump
It serves to recover heat from hot water from cooling processes. Temperatures do not usually exceed 55ºC-65ºC, so the recovery for industrial use is not very relevant, being its main application the production of domestic hot water.
Design-related efficiency measures
Size the gas ducts according to the maximum gas flow and build them according to the usual recommendations to avoid pressure losses due to excessive friction. Adapt the type of recovery boiler according to the suspended solids content of the flue gases. Use economizers in recovery boilers. Use superheaters in recovery hips whether the temperature of the gases is sufficient.Efficiency measures relating to the operation
Reduce thetemperature of the flue gases as much as possible, avoiding corrosion by acid spray formation. Perform combustion with the minimum of air compatible with environmental conditions and the state of the facilities. Use additives that prevent fouling in water pipes and equipment, as they hinder heat transmission. Preheat oxidizing oxidizing air in combustion processes.Efficiency measures relating to cleaning and maintenance
Keep exchange surfaces clean of soot and scale, by blowing steam, compressed air or washing with hot water (to avoid thermal shocks). Perform maintenance of closures and joints to avoid pressure losses in the pipes. Heat the following parts to minimize heat losses (insulation with ceramic fibres recommended): Ducts and pipes. Economizers. Exterior of heat exchangers in general.Additional information: see Data Sheets Electric ovens, Gas ovens, Industrial dryers and Waste heat recovery catalogue
Recovery boilers for steam production
They are boilers in which there is no combustion chamber, since all the heat transferred to the water comes from the combustion gases of other equipment such as a furnace.
These gases have a lower temperature than those produced in the equipment itself, so the transmission of heat by convection takes more weight than that of radiation. For convection to be good, an intimate mixture between the molecules of the gas is needed.
Adequate convection is ensured by turbulent gas circulation. This is achieved in different ways depending on the type of boiler:
In water-tube boilers: with a cross-flow design and with a spacing between tubes causing good contact with the gases. In pyrotubular boilers: circulating the gases at high speed inside the tubes, and using turbulators that cause violent collisions of the molecules of the gases with the walls. In special boilers : special shapes are sought, such as curvature of the tubes, baffle partitions or others that also guarantee violent shocks.Water can circulate naturally or by forced convection, which allows the use of tubes of smaller diameter and thickness.
Criteria for choosing the right type of boiler
1) Selection criteria for pyrotubular boilers:
They are used to take advantage of the heat of hot gases or liquids at high temperatures. They are used for gases at high pressures, up to 25atm. They are usually used in cases where it is possible to send the complete and finished boiler from the factory. They are, in general, easier to clean and less susceptible to scaling than water-tubes. If both types can be used, pyrotubeular is cheaper. When the arrival temperature of the gases is high (500ºC to 1,000ºC), the inlet of the tubes is quite susceptible to deterioration. Generally there is a greater loss of charge in the gases than in the water tubes.2) Selection criteria for water-tube boilers:
They can be designed for any pressure. Smooth tubes or tubes with fins or extended surfaces can be used. When gases create a low transfer or when only a small loss of load is possible, they give greater performance than pyrotubular. They are more useful for gases at high temperature and low pressures, since they can be assembled according to needs and availability at the factory. They are not recommended when the impurities of the gases are very fouling due to the difficulty of blowing. There are fewer mechanical failures in them than in pyrotubular ones. It is possible to build them with natural circulation and with forced circulation. Certain designs of water bottles can be built with superheater, air heater and economizer.Advantages and disadvantages of recovery boilers
The use of these recovery boilers has a number of important advantages, among which can be mentioned:
The heat exchange process in recovery boilers is one of the highest performance. Recovery boilers require a lower investment than in other heat recovery systems, as they do not have a burner hearth. Control in these boilers is easily carried out based on steam demand and pressure.However, recovery boilers also have some disadvantages:
The use of water/steam as a carrier fluid requires a high quality of feed water. In general, it is not possible to sufficiently lower the temperature of the gases; consequently, an additional system is required to recover this last sensitive heat in the gases leaving the boiler.
