Guide to Waste Heat Recovery Systems for Industrial Boilers
For operations looking to reduce product production costs or operational costs in general maximizing system energy efficiency via waste heat recovery systems is a solid and provable answer. These systems retrieve waste heat from multiple sources such as hot exhaust gases, boiler blowdown, and steam condensate. By reusing this waste heat, plants can significantly reduce energy costs and CO2 emissions, while enhancing overall system energy efficiency.
This blog post delves into various waste heat recovery systems for boilers, their benefits, and ways to capitalize on waste heat recovery in your boiler room.
Understanding the Principles of Steam Boiler Waste Heat Recovery
In general, waste heat is available for recovery when there is a process stream (steam, water, air, etc.) at a higher temperature than another process stream. For example, the exhaust gases leaving a high pressure steam boiler are around 400-500°F, while the boiler’s feedwater is 212- 227°F (depending on the source of the feedwater). This temperature difference gives us the opportunity to transfer heat from the hotter flue gas to the cooler feedwater. Some common, commercially available options to take advantage of this type of heat recovery are: Feedwater Heaters (Economizers), 2-Stage Economizer, and Continuous Blow Down Heat Recovery units.
Economizers
Economizers, also known as a Feedwater Heater, are often employed to recover waste heat by heating boiler feedwater. On high pressure steam boilers, the initial flame temperature is around 2,000oF, as this hot exhaust gas works its way through the boiler, the temperature is reduced as the gas gives up its heat to the boiler water (heating the water and making steam). By the time the exhaust gas gets to the stack, it will be down to 400-500°F (the exact temperature depends on the boiler’s pressure/operating temperature, and it’s efficiency). The boiler’s feedwater, is typically 212-227°F. This temperature difference allows us the opportunity to tap into the otherwise wasted heat from the boiler’s exhaust.
Boiler economizers are the means to transfer the heat from the boiler’s exhaust to the feedwater. They do this, by passing feedwater through finned tubes, inside of the economizer, while the exhaust gasses pass around the tubes. This allows heat transfer to occur from the exhaust gases into the feedwater, increasing the boiler’s efficiency.
Per the US Department of Energy, incorporating a basic non-condensing boiler feedwater economizer can boost a boiler’s thermal efficiency and cut fuel consumption by 2%-5% on average. The exact reduction in fuel cost savings (efficiency increase) will depend on the temperature of the exhaust gases, the water temperature, boiler operating hours, and fuel costs. The greater the operating hours, the higher the fuel costs, the greater the return. For example, a heavily loaded high pressure boiler can see a return on investment on an Ecomizer in ~3 years or less, a more lightly loaded boiler will take longer.
2-Stage Economizers and Condensing Economizers
If your process or boiler room has other streams of cold water, it may make sense to look at condensing or 2-stage economizers. 2-stage Economizers are standard Economizers/Feed Water Heaters with a condensing section heating some additional process flow(s).
So, what is a condensing economizer? A condensing economizer is one designed to drop the temperature of the boiler’s flue gas low enough to cause water vapor in the flue gas to condense on the heat recovery surfaces. As Hydrogen containing fuels are burned with Oxygen, a byproduct will be H2O, water, in its vapor form due to the temperature of the reaction. This water vapor contains ~1,000BTU/lb, if it can be converted back to its liquid form on a heating surface, that ~1,000BTU/lb can be recovered (transferred into the process fluid). This condensing efficiency can be ~10% or more, higher than non-condensing.
To utilize a condensing economizer effectively, you need a process fluid stream which needs to be heated, sufficient to utilize the waste heat. One option is to use the boiler plant’s own makeup water (water used to make up for water losses from the steam/condensate system). But, any cool water source can work, whether it is for process, or some other use, if it needs to be heated, the boiler’s waste heat can be used.
Continuous Blow Down Heat Recovery Units (CBHR)
High pressure steam boilers have two sources of blow down, either bottom blow down, or surface blow down. Bottom blow down is an intermittent (on/off), manual , to remove sludge from the bottom of the boiler. Surface Blow Down is either continuous or intermittent. Intermittent Surface Blow Down is controlled by conductivity readings activating a solenoid valve (known as an Automated Surface Blow Down Control). Conductivity is used to measure suspended solids in boiler water. Note: this Intermittent mode can be combined with a parallel, continuous flow, where the continuous flow is set to where suspended solids are well controlled, but the Automatic Surface Blow Down Control is available to further control conductivity readings.
With a steady stream of hot, surface blowdown, we have another source of energy about to be lost from the steam system, unless we can use that energy to heat another process fluid. Without a CBHR, this hot condensate must be cooled before going to drain, typically by going through a blow down separator, where some of the Surface Blow Down flashes off as steam (can be used in the deaerator) and the remaining liquid is further cooled by mixing with a cold water source. With a CBHR system, the Surface Blowdown still goes through a separator (where some flashes to steam, for use in the deaerator), then the liquid goes through a heat exchanger where it gives up heat to the cooler makeup water. Payback time on CBHR systems, on high pressure boilers are quick, typically under 12 months.
Upgrading Your Industrial Boiler
Waste heat recovery systems for industrial boilers are a powerful tool for businesses looking to increase efficiency, reduce costs, and minimize their environmental footprint. By understanding how these systems work and what benefits they offer, you can take the first step towards a more sustainable and cost-effective future for your facility.
Ready to tap into the power of waste heat recovery? Check out our blog about increasing your industrial boiler’s efficiency, or contact WC Rouse today to learn about how we can help you optimize your industrial boiler system.
