Manufacturers Best Practice Guide

Steam: Boiler Efficiency Tips


1. Measure boiler efficiency

  • Analyze flue gas to measure the amount of oxygen and the stack gas temperature, and then calculate boiler efficiency. Adjustments can then be made to optimize the level of excess air and incoming air temperatures.

2. Ensure appropriate air-to-fuel ratio
  • Excess air consumes energy as it is heated. Energy is then wasted as this heated air is released up the stack.
  • Without sufficient air, not all fuel will burn. This wastes fuel and may even be dangerous. Unburned fuel will continue to move through the system, leaving behind soot. Deficient air levels may also result in the formation of carbon monoxide and smoke. Soot and carbon monoxide can accidentally combust in areas not designed for this, which will shorten equipment life.

3. Reduce/Remove feedwater impurities
  • Feedwater is treated before it is pumped into the boiler in order to remove impurities that might cause deposits or corrosion. Without proper treatment, impurities build up and reduce boiler efficiency.
  • Soot buildup also acts as an insulator, blocking some of heat transfer between combustion gas and steam generation. To keep the fire-side surfaces clean, ensure there is adequate excess air to avoid soot build-up. A soot blower used periodically (boiler blowdown) helps remove soot from the fire-tubes.
  • Besides blocking efficient heat transfer, impurities lead to energy waste because of the need for periodic boiler blowdown. Although blowdown effectively removes impurities, it is also one of the largest energy wastes since it involves discharging boiler water. This wastes both the thermal energy embodied in the water as well as the chemical additives used to treat the feedwater.
  • Electronic controllers can automate bottom blowdown so that it is done as infrequently as possible without allowing excessive build up of impurities. With continuous blowdown, discharged water can be directed through a heat exchanger to recover some of the thermal energy that would otherwise be lost. This heat can then be used to pre-heat boiler feedwater. However, the best option is to properly treat feedwater correctly and minimize blowdown.
  • Scale is the most common problem. Scale acts like insulation between the combusted fuel and the boiler water, in effect blocking some of the heat transfer. Scale can reduce a boiler's efficiency by as much as 12%. Besides scale, a number of other impurities, such as calcium, magnesium, silica, iron oxide, oxygen and suspended solids, need to be removed from boiler feedwater.
  • Most methods for removing impurities involve the addition of chemical treatments. Treatments should be balanced to minimize the use of chemical additives as much as possible (and thus, reduce costs) without interfering with the removal of impurities. Clarifiers and filters also help eliminate impurities, and are worthwhile investments. Other water treatment methods include reverse osmosis, electrodialysis and ion exchange demineralization.

4. Utilize a Computer-based distributed control system (DCS)

  • DCS automatically controls the fuel burner to "trim" oxygen levels as needed, keeping the air-to-fuel ratio optimized. Trim systems are particularly useful when stack temperatures are high or when fuel composition is highly variable. Payback on investment is usually less than one year.

5. Check and repair air leaks

  • Air leak repair should be part of routine maintenance, as recommended by the boiler manufacturer. Air infiltration can occur in the furnace, burner or stoker, and steam generation section.

6. Address incoming air temperature changes with oxygen trim control
  • Higher temperatures result in better combustion efficiency. Seasonal variation in air temperatures affects excess air and boiler efficiency. This is important as seasons change and ambient air temperatures fluctuate. An oxygen trim control is beneficial to accurately control excess air when combustion air temperature changes.
  • To improve efficiency, air can be pre-heated using an air economizer. One low-cost method is to modify the combustion air inlet so that it draws hot air released by the stack near the top of the boiler room or use flue gas recirculation.

7. Prevent Heat Loss
  • Heat loss can reach as much as 35% of fuel input. Most of this waste heat goes straight up the stack, like heat lost up the chimney of a fireplace. The lower the temperature of these gases, the more efficient the system. In other words, a 100% efficient system would, theoretically, expel flue gases at the same temperature as ambient air. In reality, the lowest temperature possible is roughly 300º F, unless a condensing heat exchanger or heat economizer is utilized.
  • An economizer recovers waste heat from the stack. The heat is then usually directed to boiler feedwater for preheating. Be sure to have the boiler system cleaned and tuned before installing an economizer so that an accurate measurement of stack gas temperature can be taken. Additional heat can be extracted from the flue gas (below 300º F) by use of a condensing economizer. To keep the fire-side surfaces clean, ensure there is adequate excess air to avoid soot build-up. A soot blower used periodically helps remove soot from the fire-tubes. Water-side maintenance depends on water treatment protocols (discussed below).

8. Condensate Recovery

  • Condensate drained and recovered from steam traps can be collected and reused as boiler feedwater. This helps improve overall system as well as reducing boiler operating costs. In most cases, reusing condensate is more cost effective than using new utility water.
  • Less obvious than water savings, energy savings are realized because it takes substantially less fuel to convert recovered condensate into steam. Utility water typically enters the boiler-steam system at temperatures between 50ºF and 80ºF, and is then heated to more than 212ºF (the boiling point of water). The temperature of recovered condensate, on the other hand, can exceed 200ºF.


Resources

For more information and resources on efficiency in steam systems visit
The Department of Energy's Web Site

And for updates and current information on rebates and incentives visit Flex Your Power's Rebate Finder