A large amount of the heat supplied by most fuel-fired heating equipment is wasted as exhaust or flue gases. In furnaces, 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 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 Recovery 101: Exhaust gas loss or waste heat depends on flue gas temperature and its mass flow, or in practical terms, excess air resulting from combustion air supply and air leakage into the furnace. The excess air can be estimated by measuring oxygen percentage in the flue gases.
Waste Heat Recovery
Heat losses must be minimized before waste heat recovery is investigated. The most commonly used waste heat recovery methods are preheating combustion air, steam generation and water heating, and load preheating.
Preheating Combustion Air
A recuperator is the most widely used heat recovery device. It is a gas-to-gas heat exchanger placed on the stack 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.
Steam Generation and Water Heating
These systems are similar to conventional boilers but are larger because the exhaust gas temperature is lower than the flame temperature used in conventional systems. Waste heat boilers can be used on most furnace applications, and special designs and materials are available for systems with corrosive waste gases. Plants that need a source of steam or hot water can use waste heat boilers, which may also work for plants that want to add steam capacity. However, the waste boiler generates steam only when the fuel-fired process is operating.
If exhaust gases leaving the high temperature portion of the process can be brought into contact with a relatively cool incoming load (the material being heated), energy will be transferred to the load, preheating it and reducing the energy consumption. Load preheating has the highest potential efficiency of any system that uses waste gases. Load preheating systems can be difficult to retrofit and are best suited for continuous rather than batch furnaces.
Waste heat recovery should generally be considered if the exhaust temperature is higher than 1,000°F, or if the flue gas mass flow is very large. Contact us today to learn more about how a FLU-ACE direct contact condensing heat recovery system can help your business lower its fuel costs and reduce its carbon emissions.
Heat recovery technologies have wide applications in the food and beverage industry. From breweries and bottling operations, to food processing, packaging, and other food and beverage operations, heat recovery technologies can help companies reduce their energy costs while lowering their greenhouse gas emissions. Here is a case study that looks at one of our FLU-ACE heat recovery systems installed at one of a leading cereal manufacturer's plants.
Thermal Energy International implemented a FLU-ACE Condensing Heat Recovery System on the plant’s boiler exhaust. The system was designed to recover up to 5 MMBtu/h of waste heat energy that would otherwise be exhausted to the atmosphere.
The recovered heat, in the form of water at 60°C, is used to heat and preheat:
This project was implemented on a turn-key basis, and was completed on budget and on schedule.
FLU-ACE uses direct contact gas-to-liquid mass transfer and heat exchange. It condenses almost all of the water vapour (steam) from the exhaust, and this latent heat is the source of the bulk of the waste heat available in a boiler flue gas.
“Sensible” heat refers to energy that can be released through a temperature change. Heating water from 32°F to 212°F (0°C to 100°C) is a change in “sensible” heat. “Latent” heat refers to energy stored or released in a phase change, such as the heating that is done when steam changes from vapor to water, without any temperature change. As it turns out, the energy released when a pound of steam turns into a pound of water, all happening at 212°F (100°C) (no temperature change, so this is latent heat) is some five times the energy released when that same pound of water is cooled from 212°F to 32°F (100°C to 0°C). All this to say there is a lot of energy released in the phase change from water vapor to liquid water.
Therefore, condensing heat recovery technology, which can capture latent heat, is much more efficient than the typical feedwater economizer, which can only capture sensible heat. When it comes to overall boiler plant efficiency, FLU-ACE is able to provide a 10% to 15% improvement, while a typical feedwater economizer provides an improvement of 2% to 4%.
Heat Recovery 101: What’s the difference between a direct-contact and an indirect-contact economizer?
Condensing heat recovery can be applied in two ways: direct-contact and indirect-contact systems. An indirect-contact system is simply a standard heat exchanger: water passes through tubes and the hot exhaust passes outside the tubes. Sensible heat and latent heat are transferred through the tube walls from the hot side to the cold side. By contrast, direct contact systems bring the cold water medium in direct contact with the hot gas in an open spray tower or packed spray tower to recover the heat directly by heating the water.
Neither of these technologies is a "one size fits all" solution," so it’s best to take a look at the pros and cons of each type before making the right choice for your facility.
Some advantages and disadvantages of the two types of systems include:
Direct Contact Economizer
Indirect Contact Economizer
Clearly the selection of either direct contact or indirect contact is dependent upon the specific process heating application, both on the characteristics of the heat source and the heat sink.
For more on this topic see our white paper entitled Condensing Heat Recovery – The Final Step towards the 95% Efficient Boiler Plant, as published in Process Heating, Volume 22, Number 2.