The temperature of exhaust gases from fuel-fired industrial processes depends mainly on the process temperature and the waste heat recovery method. Figure 1 shows the heat lost in exhaust gases at various exhaust gas temperatures and percentages of excess air.
Energy from gases exhausted from higher temperature processes (primary processes) can be recovered and used for lower temperature processes (secondary processes). One example is to generate steam using waste heat boilers for the fluid heaters used in petroleum crude processing. In addition, many companies install heat exchangers on the exhaust stacks of furnaces and ovens to produce hot water or to generate hot air for space heating.
Before attempting to use energy from higher temperature flue gases in lower temperature processes, engineers should take the following technical issues into consideration:
- Nature or quality of the flue gases. Flue gases from the primary processes should be clean and free of contaminants such as corrosive gases and particulates. Contaminants pose special handling problems for the gases and might affect the quality of work in the secondary process.
- Temperature of primary process flue gases. The temperature difference between the primary and secondary process should be high enough (at least 200°F[121°C]), and there should be a sufficient amount of usable waste heat.
- Matching the heat demand of the secondary process with the heat supply from the primary process. The heat supply from the primary process should be sufficiently high to meet a reasonably high percentage of the secondary process heat demand.
- Matching the timing of the heat supply from the primary process and the heat demand in the secondary process.
- Placement of primary and secondary heating equipment. The closer the primary and secondary process can be situated, the better.
Case In Point: Drying OvenAn example can help illustrate these principles. Suppose a plant uses a furnace with a firing rate of 10 MMBTU/hr, which discharges flue gases at 1,400°F (760°F) -- the primary process. The plant also has a drying oven that operates at 400°F (204°C) and requires 2.5 MMBTU/hr of heat -- the secondary process.
The recoverable heat can be estimated using figure 1. At 1,400°F, the heat content of the exhaust gases (at 10 percent excess air) is about 42 percent of the heat furnace input.
Again using figure 1, the heat content of exhaust gases at 400°F is approximately 20 percent (at 10 percent excess air). The approximate amount of heat that can be saved is 42 minus 20, or 22 percent of the heat input to the primary process.
The net heat available for the secondary process is approximately 0.22 x 10 MMBTU/hr, or 2.2 MMBTU/hr. Actual savings would be greater than this because the available heat at the 400°F exhaust gas temperature is approximately 80 percent. The actual savings for the oven are thus 2.2 divided by 0.8, or 2.75 MMBTU/hr.
In this case, there is more than enough heat to meet the heat demand for the drying oven. It would be necessary to use additional heat in the oven if the exhaust gas heat from the furnace were not sufficient to meet the oven heat demand. At a fuel cost of $8 per MMBTU, the company can save $22 in fuel costs per hour. Assuming 8,000 hours of operation per year, annual savings are $175,000.
This article was provided by BestPractices, which is part of the Industrial Technologies Program Industries of the Future strategy, a program to help the country’s most energy-intensive industries improve their competitiveness. For more information, Contact the EERE Information Center at (877) 337-3463 or visit www.eere.energy.gov/industry.
Credits1. Calculations by Richard Bennett, Janus Technology Group.
2. Figure by Richard Bennett, Janus Technology Group.