Energy Use and Efficiency of Industrial Boilers

Energy Use and Efficiency of Industrial Boilers

Industrial Boilers typically operate to meet a desired heating load, which is to say that the output of the boiler can be easily adjusted to meet a load that is not constant. Industrial boilers operate to meet a process load that is driven by the production output for a product. 

Industrial boiler applications are usually a multiple boiler configuration that include a lead boiler that meets the base load requirements supported by a lag boiler that that supports the load that is in excess of the lead boiler capacity. In some case there is also a standby boiler that is ready for operation in the case of a failure of the lead or lag boiler or to support the process when one of the other boilers is out of service for scheduled routine maintenance. 

Note that during the course of the year, the designation of lead, lag and standby rotates through the boilers so that they have similar hours of operation over the course of year. 

While industrial boilers can load follow, they cannot be frequently cycled on and off as thermal cycling can damage the units. To accommodate this requirement, the lag and standby boilers are typically in a warm standby mode where the boiler is maintained at a temperature that will allow it to be placed into service in a short period of time. Note that in a warm standby mode of operation, fuel is consumed and no usable output is produced.

Industrial boiler efficiency:

The efficiency of industrial boilers is the ratio of heat output to the fuel input. The heat output is in the form of steam or hot water. Both forms of these output streams can be measured to calculate the quantity of energy produced by the boiler. The input fuel in the form of oil or natural gas can also be accurately measured to calculate the quantity of energy input into the boiler. When calculating the input energy value for the fuel, one must note the assumption for the value of the heat of combustion of the fuel as it typically expressed in the form of higher heating value and lower heating value. Higher Heating Values for a fuel include the full energy content as defined by bringing all products of combustion to 77°F (25° C). Natural gas typically is delivered by the local gas company with values of 1,000 - 1,050 Btu per cubic foot on this HHV basis. Since the actual value may vary from month to month some gas companies convert to Therms. A Therm is precisely 100,000 BTU. These measures all represent higher heating values. Lower heating values neglect the energy in the water vapor formed by the combustion of hydrogen in the fuel. This water vapor typically represents about 10% of the energy content. Therefore the lower heating values for natural gas are typically 900 - 950 Btu per cubic foot.

Losses in boilers that affect the efficiency include heat loss through the walls of the boiler, heat loss through the exhaust and not combusting 100% of the fuel provided to the system. Steam boilers also have a blow-down requirement. Boiler feed water often contains some degree of impurities, such as suspended and dissolved solids that accumulate inside the boiler during normal operation. The increasing concentration of dissolved solids can lead to carryover of boiler water into the steam, causing damage to piping, steam traps and even process equipment. The increasing concentration of suspended solids can form sludge, which impairs boiler efficiency and heat transfer capability. To avoid boiler problems, water must be periodically discharged or “blown down” from the boiler to control the concentrations of suspended and total dissolved solids in the boiler. Surface water blow-down is often done continuously to reduce the level of dissolved solids, and bottom blow-down is performed periodically to remove sludge from the bottom of the boiler. The blow-down water has been heated inside the boiler and therefore uses some of the fuel but does not result in useful heat output. Note, there are devices that can be incorporated into a boiler to recover some of the blown-down energy but these are not standard equipment at this time.