Operating your boiler with an optimum amount of excess air will minimize heat loss up the stack and improve combustion efficiency. Combustion efficiency is a measure of how effectively the heat content of a fuel is transferred into usable heat. The stack temperature and flue gas oxygen (or carbon dioxide) concentrations are primary indicators of combustion efficiency. Given complete mixing, a precise or stoichiometric amount of air is required to completely react with a given quantity of fuel. In practice, combustion conditions are never ideal, and additional or “excess” air must be supplied to completely burn the fuel.
The correct amount of excess air is determined from analyzing flue gas oxygen or carbon dioxide concentrations. Inadequate excess air results in unburned combustibles (fuel, soot, smoke and carbon monoxide) while too much results in heat lost due to the increased flue gas flow -- thus lowering the overall boiler fuel-to-steam efficiency. Table 1 relates stack readings to boiler performance.
On well-designed natural gas-fired systems, an excess air level of 10 percent is attainable. An often-stated rule of thumb is that boiler efficiency can be increased by 1 percent for each 15 percent reduction in excess air or 40°F (22°C) reduction in stack gas temperature.
Minimize Boiler Blowdown. Minimizing your blowdown rate can substantially reduce energy losses, as the temperature of the blown-down liquid is the same as that of the steam generated in the boiler. Minimizing blowdown also will reduce makeup water and chemical treatment costs.
As water evaporates in the boiler steam drum, solids present in the feedwater are left behind. The suspended solids form sludge or sediments in the boiler, which degrades heat transfer. Dissolved solids promote foaming and carryover of boiler water into the steam. To reduce the levels of suspended and total dissolved solids (TDS) to acceptable limits, water is periodically discharged or blown down from the boiler. Mud or bottom blowdown is usually a manual procedure done for a few seconds on intervals of several hours. It is designed to remove suspended solids that settle out of the boiler water and form a heavy sludge. Surface or skimming blowdown is designed to remove the dissolved solids that concentrate near the liquid surface. Surface blowdown is often a continuous process.
Insufficient blowdown may lead to carryover of boiler water into the steam, or the formation of deposits, but excessive blowdown will waste energy, water and chemicals. The optimum blowdown rate is determined by various factors, including the boiler type, operating pressure, water treatment and quality of makeup water. Blowdown rates typically range from 4 percent to 8 percent of boiler feedwater flow rate, but they can be as high as 10 percent when makeup water has a high solids content.
Automatic Blowdown Control Systems. These systems optimize surface blowdown by regulating water volume discharged in relation to amount of dissolved solids present. Conductivity, TDS, silica or chlorides concentrations, and/or alkalinity are reliable indicators of salts and other contaminants dissolved in boiler water. A probe provides feedback to a controller driving a modulating blowdown valve. An alternative is proportional control -- with the blowdown rate set proportional to the makeup water flow.
Recover Heat from Boiler Blowdown. Heat can be recovered from boiler blowdown by using a heat exchanger to preheat boiler makeup water. Any boiler with continuous blowdown exceeding 5 percent of the steam rate is a good candidate for the introduction of blowdown waste heat recovery.
Larger energy savings occur with high-pressure boilers. Table 2 shows the potential for heat recovery from boiler blowdown.
Return Condensate to the Boiler. The energy in the condensate can be more than 10 percent of the total steam energy content of a typical system. When steam transfers its heat in a manufacturing process, heat exchanger or heating coil, it reverts to a liquid phase called condensate. An attractive method of improving your plant's energy efficiency is to increase the condensate return to the boiler.
Returning hot condensate to the boiler makes sense for several reasons. As more condensate is returned, less makeup water is required, saving fuel, makeup water and chemicals and treatment costs. Less condensate discharged into a sewer system reduces disposal costs. Return of high-purity condensate also reduces energy losses due to boiler blowdown. Significant fuel savings occur as most returned condensate is relatively hot (130 to 225°F [54 to 107°C]), reducing the amount of cold (50 to 60°F [10 to 15°C]) makeup water that must be heated.
Sidebar: 10 Tips: Boilers
- Periodically monitor flue gas composition and tune your boilers to maintain excess air at optimum levels.
- If there is a continuous blowdown system in place, consider installing a heat recovery system.
- If there is a non-continuous blowdown system, then consider converting it to a continuous blowdown system coupled with heat recovery.
- Reduce operating costs through maximizing the return of hot condensate to the boiler.
- If a condensate return system is absent, estimate the cost of a condensate return and treatment system (as necessary) and install one if economically justified.
- Repair steam distribution and condensate return system leaks.
- Insulate condensate return system piping to conserve heat and protect personnel against burns.
- Review your blowdown practices to identify energy saving opportunities.
- Examine operating practices for boiler feedwater and blowdown rates developed by the American Society of Mechanical Engineers (ASME). Considerations include operating pressure, steam purity, and deposition control.
- Consider an automatic blowdown control system.
BestPractices is part of the Industrial Technologies Program's Industries of the Future strategy, which helps the country's most energy-intensive industries improve their competitiveness. The Industrial Technologies Program is part of Energy Efficiency and Renewable Energy, U.S. Department of Energy, Washington, D.C. For more information from EERE, call (877) 337-3463 or visit www.eere.energy.gov.