Condensation Heat Recovery
It has long been desired to lower the temperature out of waste heat streams to below the 250°F to 350°F range. With existing technology, this has been considered to be the minimum exhaust temperature range due to the condensation of acid from combustion products.
Condensation heat recovery cools the exhaust gases below the water dewpoint, and can recover a large percentage of the water-vapor latent, and sensible heat. While the Company has done very little of this to date, it is an evolving heat recovery type. In Europe, these systems have been used for several years, in hundreds of applications. There are two approaches, both discussed below.
This brings water directly into contact with the flue gases to remove the heat. The water becomes acidic and a secondary heat exchanger is used to transfer the recovered heat in a recirculating loop to a clean water stream.
Direct contact is also referred to as Non-condensing waste heat recovery. Direct contact usually reduces the waste heat recovery stream temperature to around 150°F. The acid condition prevails at a pH of about two, and there is little water condensation from the flue gases. Therefore, there is little latent heat recovery. This approach saves an additional 3% to 4% of sensible heat from a waste heat recovery stream, (for either an oil or gas fired exhaust stream), when cooling it from around 300°F to 150°F.
In this approach, a corrosion-resistant heat exchanger is placed directly in the flow of the waste heat flue gas. There is no contamination of the fluid receiving the heat.
This fluid can be a process stream or a utility stream that needs the heat. It can also be fluid that is transferring the heat to other users in the plant in a recirculating closed system.
Indirect contact is also referred to as Non-contact or Condensing. Condensing is usually done to a temperature below 100°F. The acid condition prevails and the water in the flue gases condenses, diluting the acid.
For this type of heat recovery, we can use the following guidelines. How many additional efficiency points could be gained if we cooled the hot fluid in the waste heat recovery unit all the way to ambient temperature? For this condition, we are looking at the absolute maximum recovery. By reducing the temperature from 150°F, to ambient, we could save the following additional fired unit efficiency points:
• On gas: 2% of sensible heat and about 10% latent heat
• On oil: 2% of sensible heat and about 5% latent heat
The above variation in efficiency for gas and oil is due to the lower hydrogen content in oil (less water vapor in the gas).
Condensation systems are reported to reduce particulates and sulfur dioxide emissions.
Manufacturers of glass, pyrex, borosilicate, and teflon are actively trying to develop corrosion-resistant heat exchangers for this type of waste heat recovery.
It will be some time before industry adopts this “almost ambient” stack philosophy.
Note that when stack temperatures become cooler, the penalty for firing with excess air is decreased.