Secondary Heat Recovery from Exhausts and Oxidizers
Does it make sense to add secondary heat recovery systems to your thermal process to capture and repurpose the BTUs you have created before they escape out the top of the stack? An economic analysis can help you determine whether it is justified at your plant.
Secondary heat recovery systems capture excess energy in the exhaust stream of processes or oxidizers. On the surface, heat recovery makes good sense: Capture the waste energy and repurpose it rather than throwing it out an exhaust stack. The concept sounds relatively straightforward. As with most things, however, the proof lies in the details.
The decision to install heat recovery typically is a question of dollars and cents. The economic case for heat recovery can be a multi-faceted formula involving energy savings as well as the costs for equipment, installation, operation and maintenance. From a technical standpoint, here are five questions that should be considered:
- What are the heat sinks/users in the plant that could be a candidate for heat recovery?
- What are the available heat recovery fluids and temperatures involved?
- Based on the temperatures and mass flows involved, how much energy is available for heat recovery?
- Can the plant use all the available energy?
- Are there any contaminants that may plug or foul the heat exchanger?
This article will take a closer look at how the answers to those five questions can help you determine whether secondary heat recovery makes sense on your thermal processing equipment.
1. How Will Heat Users Affect Recovery?
This question may seem trivial. After all, most plants already have a process in mind before marching down the path to heat recovery. However, this question is an important first step that sets the stage for any heat recovery system. Some heat users are not as apparent as others, and there could be low hanging fruit otherwise overlooked. Common heat recovery options include dryer/oven makeup air, boiler feedwater, burner combustion air, reboiler/evaporator and building heat.
2. What Heat Recovery Fluids and Temperatures Are Involved?
The fluid used to recover heat from an oxidizer will set the overall performance for the heat recovery system. Lower temperature fluids will allow more available heat to be recovered, but they also offer a lower grade of heat. Energy will only flow from hot to cold. If the ultimate heat sink is boiler feedwater at 225°F (107°C), for instance, a hot water/glycol loop at 150°F (65°C) cannot be used as an intermediate heat transfer fluid.
Capital cost also should be considered when selecting a heat recovery fluid. It may be cheaper and simpler to directly transfer heat from the oxidizer exhaust to the end user. However, this approach may not always be the most economical.
Take the example of dryer makeup air. Directly transferring heat would require an air-to-air heat exchanger. Because air is a poor medium for heat transfer, the overall effectiveness of the exchanger is lower and exchanger cost is higher compared to an air-to-liquid heat exchanger. The necessary infrastructure such as fans and ductwork also may be more expensive than a comparable liquid system when large distances are involved.
Water and oil systems have infrastructure costs such as expansion tanks, piping and pumps as well. It is typically more cost effective to select a heat transfer fluid already available because the core infrastructure already exists.
3. How Much Energy Is Available for Heat Recovery?
Performing a system mass and energy balance will help determine how much energy and, therefore, savings is really on the table. Be sure to consider the number of operating hours for the equipment. Heat recovery on an oxidizer that is only running 3,000 hours a year will, of course, offer less energy savings when compared to one operating continuously.
The oxidizer also will need to operate at the same time as the equipment using the heat. An ideal scenario would have the oxidizer treating the exhaust of the same equipment it is supplying with heat. Integrating oxidizer exhaust with an upstream dryer is one example (figure 1). Because the oxidizer is treating the dryer exhaust, it always will be online with the oven. The equipment also will likely be in close proximity, making heat recovery more affordable and easier to integrate.
Air management throughout an entire process — from plant makeup air heating systems to pollution control system exhaust — can provide many opportunities to save energy and cut operating costs. However, the viability of process heat recovery should begin with answering fundamental questions to determine if it is at all appropriate to explore alternative energy recovery applications.
4. Can the Plant Use All the Available Energy?
For every BTU of energy that is recovered, a BTU of energy must be used at the heat sink. If the plant only needs 2 million BTU/hr of energy, it does not matter if there is 10 million BTU/hr of available heat at the oxidizer. Sometimes, there is simply more energy available in the oxidizer exhaust than can effectively be used elsewhere.
Of course, it is possible to convert heat into other forms of useful work such as driving a motor or a generator. These solutions, however, are rarely if ever economical on an industrial scale. In these situations, secondary heat recovery may not be the best avenue for energy savings.
In applications where more energy is available than can be repurposed, the focus then should shift to the energy usage at the oxidizer. Replacing the oxidizer media or internal heat exchanger, adding catalyst or adding natural gas injection are potential options to lower energy costs at the oxidizer.
5. What Contaminants May Plug or Foul the Heat Exchanger?
Any potential savings from a heat recovery system can quickly evaporate if equipment is constantly down due to cleaning and other maintenance tasks. Fouling will reduce the performance and capacity of any heat exchanger. It is important to identify any particulate or contaminants that can foul heat transfer surfaces.
If heat exchanger fouling is a potential outcome, several design approaches can help mitigate its impact:
- Apply a suitable fouling factor to the heat exchanger design. This factor will increase the heat exchanger area and, therefore, cost. However, it also will increase the time needed between cleanings.
- Use a more open heat exchanger design by increasing fin spacing or using larger tubes. Again, this approach will increase exchanger cost, but it also will make the exchanger more resistant to fouling.
- Incorporate the ability to clean and access the heat exchanger surfaces.
- Consider online cleaning via a steam or soot blower in particularly challenging environments.
- Incorporate a bypass around the heat exchanger. Not only does an exchanger bypass allow for balancing heat supply with demand, it also allows the oxidizer to run regardless of heat exchanger status. The last thing an operator needs is for a heat recovery system to impact production because a fouled heat exchanger is restricting airflow.
Two brief case histories will help illustrate how selecting a right-sized heat recovery solution can help improve operations in specific applications.
Window-Film Industry: Oxidizer Reduces Gas Usage by 93 Percent
A manufacturer in the window-film industry produces thin solar-control laminated films used to screen UV rays, block heat, reduce glare and increase occupant comfort. The company also produces multi-ply safety and security films that reduce hazards when glass shatters. The products are used in vehicles, homes and buildings.
Using a decades-old recuperative thermal oxidizer to destroy solvents from the drying ovens on its three process lines, the company was spending excessively to operate its coating lines. Thermal energy from the oxidizer exhaust was recovered by heat exchangers and used to operate the process ovens and other in-plant systems. Unfortunately, the operating efficiency and capacity limitations of the oxidizer created problems.
The plant was using more than 100,000 therms of gas monthly. The 20,000-scfm oxidizer used a single two-speed exhaust fan to serve three processes. The fan caused capacity and continuity conflicts between lines. This demanded continuous monitoring and adjustments by operators and caused 25 percent lower line speeds at times.
In addition to productivity issues, air temperatures from the exchangers exceeded 800°F (426°C) — much hotter than the dryers needed. The oxidizer yielded more heat than the plant could use, and valuable energy had to be vented to the atmosphere.
The proposed solution was a 40,000-scfm regenerative thermal oxidizer (RTO). The unit supplied has a proprietary switch valve that delivers high VOC destruction without a flush system. As part of developing the heat recovery solution, the oxidizer supplier performed an engineering study and determined that the regenerative thermal oxidizer exhaust could be used safely in the drying ovens. The oxidizer manufacturer also proposed a new heat recovery system and operating sequence that would run more efficiently without wasting energy. Upgraded supporting equipment also was supplied, including T-dampers, exhaust fans and the hot and cold fans for the heat recovery system.
The customer agreed with the recommendations and installed the regenerative thermal oxidizer with proprietary valve switch. Once installed, the oxidizer achieved VOC destruction efficiencies above 99 percent. The fan design enabled the three production lines to run independently without operator involvement. In addition, the oxidizer’s capacity eliminated the need to reduce line speeds, which helped the plant see higher production.
The regenerative thermal oxidizer provides process air at 400°F (204°C), a more useful temperature for the window-films manufacturer. When all processes are online, the oxidizer spends approximately 40 percent of the time in self-sustain mode, which does not use any gas at all.
For the window-films manufacturer, the oxidizer and heat recovery system reduced typical monthly gas consumption by 93 percent. The energy savings provided full return on investment in 18 months.
Food Processing:Heating Boiler Feedwater
When designing a heat recovery system, it is tempting to try to recover as much energy as possible. However, the lower-cost option can sometimes be to let that precious energy out the stack.
As an example, consider a food-products company looking to install a heat recovery system. The goal of this system was to use exhaust from a regenerative thermal oxidizer (RTO) to heat high pressure boiler feedwater at 215°F (101°C).
In this application, the heat recovery question was complicated by the presence of sulfur compounds in the process exhaust. Sulfur compounds will convert to SO2 and SO3 in the oxidizer. Under the presence of water, SO3 will condense into sulfuric acid if the heat exchanger metal temperature drops below the acid dewpoint. Dilute sulfuric acid is aggressive and will corrode the heat exchanger, which can lead to regular coil replacement if not properly managed or engineered into the design.
In this application, it was determined that the sulfuric acid dewpoint was 300°F (149°C). In order to aid in the design, the heat recovery equipment designer calculated the impact of the heat recovery fluid temperature on the available heat (table 1). A lower fluid temperature meant a lower temperature at the stack and, therefore, more available heat for energy recovery.
Based on the amount of energy savings, the food processor first considered using a heat transfer fluid for the boiler feedwater at 215°F (101°C). Running a fluid at this temperature would capture 1.175 million BTU/hr — the most available.
At the same time, this selection would mean that the heat exchanger would operate below the acid dewpoint of 300°F (149°C). The exchanger coil would corrode rapidly unless it was constructed of an expensive alloy.
After considering all of the options, the food processor opted to use a thermal oil operating around 325°F (163°C). Although energy is left unrecovered, the overall cost of ownership would be lower because a cheaper coil with a longer operating life can be used.
In conclusion, having a keen focus on plant management throughout an entire process — from plant heating systems to pollution control system exhaust — provides many opportunities to save energy and cut operating costs. But, to make these benefits truly valuable, heat recovery should be addressed only after the process and oxidizer systems have been properly optimized and understood. The process should begin with answering fundamental questions such as those outlined above to determine the best and lowest overall cost approach.