Waste heat recovery
Thermal efficiency of process heating equipment, such as furnaces and kilns is the ratio of heat delivered to a material and heat supplied to the heating equipment. For most heating equipment, a large amount of the heat supplied is wasted in the form of exhaust or flue gases. These losses depend on various factors associated with the design and operation of the heating equipment.
A large amount of the heat supplied by most fuel-fired heating equipment is wasted as exhaust or flue gases. In furnaces and kilns air and fuel are mixed and burned to generate heat, some of which is transferred to the heating device and its load. When the heat transfer reaches its practical limit, the spent combustion gases are removed from the furnace/kiln via a flue or stack. At this point, these gases still hold considerable thermal energy. In many systems, this is the greatest single heat loss. The energy efficiency can often be increased by using waste heat gas recovery systems to capture and use some of the energy in the flue gas.
Heat losses must be minimized before waste heat recovery is investigated. The most commonly used waste heat recovery methods are preheating combustion air.
Recuporators are the most widely used heat recovery device. It is a gas-to-gas heat exchanger placed on the flue of the furnace that preheats incoming air with exhaust gas. Designs rely on tubes or plates to transfer heat from the exhaust gas to the combustion air and keep the streams from mixing
Another way to preheat combustion air is with a regenerator, which is an insulated container filled with metal or ceramic shapes that can absorb and store significant thermal energy. It acts as a rechargeable storage battery for heat. Incoming cold combustion air is passed through the regenerator. At least two regenerators and their associated burners are required for an uninterrupted process: one provides energy to the combustion air while the other recharges.
Reducing waste heat losses brings additional benefits, among them:
- Lower energy consumption by up to 35%
- Improved furnace productivity
- Lower emissions of carbon monoxide (CO), nitrogen oxides (NOx) and unburned hydrocarbons (UHCs)
- Contribute to more consistent product quality and better equipment reliability
Waste heat recovery should generally be considered if the exhaust temperature is higher than 550°C, or if the flue gas mass flow is exceptionally large. CONTACT US today to learn more about how we can help your business lower its fuel costs and reduce its carbon emissions.