Cost and Energy Efficiency

1. Efficiency, Safety, and Life of the Equipment

It is impossible to change the efficiency without changing the safety of the operation and the resultant life of the equipment, which in turn affects maintenance cost. An example to illustrate this relation between efficiency, safety, and life of the equipment is shown in the figure below.

The temperature distribution in an efficient-operated boiler is shown as the solid line. If fouling develops on the waterside due to poor water quality control, it will result in a temperature increase of the hot gases on the fireside as shown by the dashed line. This fouling will result in an increase in stack temperature, thus decreasing the efficiency of the boiler. A metal failure will also change the life of the boiler, since fouling material will allow corrosion to occur, leading to increased maintenance cost and decreased equipment reliability and safety.

2. Results Best Practices

In a study conducted by the Boiler Efficiency Institute in Auburn, Alabama, researchers have developed eleven ways to improve boiler efficiency with important reasons behind each action.

• Reduce excess air – Excess air means there is more air for combustion than is required. The extra air is heated up and thrown away. The most important parameter affecting combustion efficiency is the air/fuel ratio.
– Symptom – The oxygen in the air that is not used for combustion is discharged in the flue gas, therefore, a simple measurement of oxygen level in the exhaust gas tells us how much air is being used. Note: It is worth mentioning the other side of the spectrum. The so called “deficient air” must be avoided as well because (1) it decreases efficiency, (2) allows deposit of soot on the fire side, and (3) the flue gases are potentially explosive.
– Action Required – Determine the combustion efficiency using dedicated or portable combustion analysis equipment. Adjustments for better burning
• Cleaning
• Swirl at burner inlet
• New tips/orifices
• Atomizing pressure
• Damper repair
• Fuel temperature
• Control repair
• Burner position
• Refractory repair
• Bed thickness
• Fuel pressure
• Ratio under/overfire air
• Furnace pressure
• Undergrate air distribution.

• Install waste heat recovery – The magnitude of the stack loss for boilers without recovery is about 18% on gas-fired and about 12% for oil- and coal-fired boilers. A major problem with heat recovery in flue gas is corrosion. If flue gas is cooled, drops of acid condense at the acid dew temperature. As the temperature of the flue gas is dropped further, the water dew point is reached at which water condenses. The water mixes with the acid and reduces the severity of the corrosion problem.
– Symptom – Flue gas temperature is the indicator that determines whether an economizer or air heater is needed. It must be remembered that many factors cause high flue gas temperature
(i.e., fouled waterside or fireside surfaces, excess air, etc.).
– Action Required – If flue gas temperature exceeds minimum allowable temperature by 50˚F or
more, a conventional economizer may be economically feasible. An unconventional recovery device should be considered if the low-temperature waste heat saved can be utilized in heating water or air. Cautionary Note: A high flue gas temperature may be a sign of poor heat transfer resulting from scale or soot deposits. Boilers should be cleaned and tuned before considering the
installation of a waste heat recovery system.
• Reduce scale and soot deposits – Scale or deposits serve as an insulator, resulting in more heat from the flame going up the stack rather than to the water due to these deposits. Any scale formation has a tremendous potential to decrease the heat transfer.
– Symptom – The best indirect indicator for scale or deposit build-up is the flue gas temperature. If at the same load and excess air the flue gas temperature rises with time, the effect is probably due to scale or deposits.
– Action Required – Soot is caused primarily by incomplete combustion. This is probably due to deficient air, a fouled burner, a defective burner, etc. Adjust excess air. Make repairs as necessary to eliminate smoke and carbon monoxide.
Scale formation is due to poor water quality. First, the water must be soft as it enters the boiler. Sufficient chemical must be fed in the boiler to control hardness.
• Reduce blowdown – Blowdown results in the energy in the hot water being lost to the sewer
unless energy recovery equipment is used. There are two types of blowdowns. Mud blow is designed to remove the heavy sludge that accumulates at the bottom of the boiler. Continuous or
skimming blow is designed to remove light solids that are dissolved in the water.
– Symptom – Observe the closeness of the various water quality parameters to the tolerances stipulated for the boiler per manufacturer specifications and check a sample of mud blowdown to ensure blowdown is only used for that purpose. Check the water quality in the boiler using standards chemical tests.
– Action Required – Conduct proper pre-treatment of the water by ensuring makeup is softened. Perform a “mud test” each time a mud blowdown is executed to reduce it to a minimum. A test should be conducted to see how high total dissolved solids (TDS) in the boiler can be carried without carryover.
• Recover waste heat from blowdown – Blowdown contains energy, which can be captured by a waste heat recovery system.
– Symptom and Action Required – Any boiler with a significant makeup (say 5%) is a candidate for blowdown waste heat recovery.
• Stop dynamic operation on applicable boilers
– Symptom – Any boiler which either stays off a significant amount of time or continuously varies in firing rate can be changed to improve efficiency.
– Action Required – For boilers which operate on and off, it may be possible to reduce the firing rate by changing burner tips. Another point to consider is whether more boilers are being used than necessary.
• Reduce line pressure – Line pressure sets the steam temperature for saturated steam.
– Symptom and Action Required – Any steam line that is being operated at a pressure higher than the process requirements offers a potential to save energy by reducing steam line pressure to a minimum required pressure determined by engineering studies of the systems for different seasons of the year.
• Cogenerate – This refers to correct utilization of steam pressure. A boiler provides steam to a
turbine, which in turn, is coupled to an electric generator. In this process, all steam exhaust from
the turbine must be fully utilized in a process requirement.
• Operate boilers at peak efficiency – Plants having two or more boilers can save energy by load management such that each boiler is operated to obtain combined peak efficiency.
– Symptom and Action Required – Improved efficiency can be obtained by proper load selection, if operators determine firing schedule by those boilers, which operate “smoothly.”
• Preheat combustion air – Since the boiler and stack release heat, which rises to the top of the boiler room, the air ducts can be arranged so the boiler is able to draw the hot air down back to the boiler.
– Symptom – Measure vertical temperature in the boiler room to indicate magnitude of stratification of the air.
– Action Required – Modify the air circulation so the boiler intake for outside air is able to draw
from the top of the boiler room.
• Switch from steam to air atomization – The energy to produce the air is a tiny fraction of the energy in the fuel, while the energy in the steam is usually 1% or more of the energy in the fuel.
– Symptom – Any steam-atomized burner is a candidate for retrofit.
– Action Required – Check economics to see if satisfactory return on investment is available.

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