Boiler Efficiency
Determining your boiler efficiency is effectively determining all of the different inefficiencies resulting from the process of burning fuel to create steam energy. Standards and testing organizations such as the American Society of Mechanical Engineers (ASME) and Deutsches Institut für Normung (DIN) have similar but different metrics for calculating efficiency losses, but from a general standpoint they can be grouped into the following categories.
Sensible Heat Loss.
Sensible heat losses can be thought of as heat you can sense directly with a thermometer. For example, combustion air enters your power plant at ambient conditions, and flue gas is exhausted from the cold end of the boiler air heater at some elevated temperature. The closer the exhaust gas is to ambient temperature, the less sensible heat is lost to the environment.
Other sensible heat losses include the heat contained in bottom and fly ash removed from the boiler and pyrites and rock that are rejected from coal mills. The quantity of excess air used for combustion has a significant effect on this loss, as every pound of excess air that travels through the boiler carries with it potentially usable energy.
Latent Heat Loss.
Latent heat losses are not easily detectable by a thermometer and are energy losses associated with a phase change of water. When a fuel is burned in a boiler, not only does all moisture contained within the fuel vaporize into steam, but all hydrogen contained within the fuel combusts to form water, which also is vaporized into steam. Unless the temperature of the exhaust gas leaving the boiler air heater is below the boiling point of the water contained within the gas, all of that latent heat of vaporization will exit the boiler and be lost to the environment.
Because latent heat losses are primarily fuel-related, they cannot be easily changed without switching or drying your fuel. (See “Improve Plant Efficiency and Reduce CO2 Emissions When Firing High-Moisture Coals” in the November 2014 issue.)
Unburned Combustible Loss.
Unburned combustible losses are efficiency losses from incomplete combustion of fuel in the boiler. This is primarily measured in the form of carbon residue in the ash, but it also includes carbon monoxide (CO) production. These losses are generally influenced by both fuel properties (fuel volatility) and operations practices (excess air level, fuel fineness, and the like). It is important to note that unburned combustible loss is not the same as loss-on-ignition (LOI), as unburned combustible loss is an energy loss, whereas LOI is calculated on a mass basis in the ash.
Radiation and Convection Loss.
Utility boilers are enormous equipment systems, with numerous penetrations for tubes and instruments, and a very large surface area exposed to the environment. As a result, no matter how well-designed the insulation is and how diligent plant personnel are in fixing air leaks, energy will still be lost via radiation and convection.
Margin and Unknown Losses.
Due to the large size and complexity of the boiler, it is often not practical to measure every single possible source of energy loss from the power plant. As a result, a “margin” or “unknown loss” value is typically used to estimate these losses. Typical values range from 0.5% to 2.0%.
When all of these efficiency losses are taken into account, a typical utility boiler can utilize fuel energy with an efficiency ranging from 83% to 91%.
Improving Boiler Efficiency.
Sensible heat losses can be reduced by installing improved combustion controls to allow fine-tuning the excess air level in the furnace operators to reduce the excess oxygen level in the furnace. Preheating combustion air with waste heat from the plant will also increase efficiency, and some plants are considering schemes to use solar thermal collectors as air preheaters during daylight hours.
As latent heat losses are strongly tied to fuel quality, and current boiler designs do not allow for condensing air heaters, outside of switching to a dryer fuel, there is little that can practically be done to reduce latent heat losses.
Unburned combustible losses can be reduced by improved boiler and burner tuning, with some plants able to gain more than 1% in net efficiency as a result of a minor amount of tuning or capital investment.
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