Increasing efficiency by boiler house components? – Yes we can!

The life expectancy of a boiler system is between 20 and 40 years. Typical efficiency gains achieved by replacing or modernising old systems is between 10 and 30 percent, depending on the initial situation. At current fuel costs, even extensive measures often pay for themselves in just a short time. In part one we'll show you more about the possibilities of boiler/burner combinations and reducing flue gas losses.

Energy-efficient optimisation potentials of the boiler/burner combination

If a steam boiler at a given load is balanced over the incoming and outgoing material and energy flows, the proportion of unusable energy soon becomes evident. Fuel, combustion air, feed water and electrical power (pumps and fans) are fed. Besides the usable thermal energy contained in the steam, other variables are flue gases at a specific temperature and with a certain oxygen content, possibly unburned fuel components, desalting and blow-down losses, losses from thermal radiation and thermal conduction at the boiler surface. These losses can be minimised by employing suitable measures.

Reducing flue gas loss

Economizer and calorific heat exchangers

Energy is supplied to a conventional steam boiler system by combusting a fuel-air mixture. The heating surfaces (flame tubes and smoke tubes) emit heat to the water inside the boiler via thermal radiation, thermal conduction and convection. Not all the energy contained in the fuel is transferred to 100 %. Consequently, the flue gas temperatures are higher than the medium temperature of the boiler.

In order to use the considerable heat potential, economizers and often flue gas condensers can be connected downstream of the boilers. These units cool the hot flue gases to a certain temperature and, in return, preheat the feed water or other low-temperature water.

In the case of dry operation, the flue gases are cooled only to a temperature above the condensation temperature of the flue gases. As a result, the condensation energy contained therein is not used. Fuel cost savings of up to 7 % can be realised at full load.

If the flue gas temperature is reduced to below the condensation temperature, the condensation energy can also be used. Under the right framework conditions, a savings potential of up to 7 % more is possible in practical applications (figure 1).

If the condensate rate of the steam boiler system is low (< 50 %), the requisite, cold make-up water flow rate is usually sufficient for using the flue gas condensation. At high condensation rates, the requisite make-up water flow rate is very low. However, as long as a hardness-free low-temperature water circuit is provided, the condensing technology can still be used. The released condensation heat can be used to heat up process water or support the heating system, for example.

Feed water cooling module

As an alternative to the condensing heat exchanger, the use of a lower-cost feed water cooler may also be sensible (firure 2). Particularly in those cases, where the payback appears too low in the absence of a sufficiently large heat sink.

The feed water cooling module is used to transfer heat from the feed water (103 °C after full deaeration) to the cold make-up water by means of a heat exchanger. As a result of the consequential cooling down of the feed water, there is a larger temperature difference between the feed water and flue gas in the economizer. This enables a larger heat extraction from the flue gas to be achieved, so that the firing efficiency increases by up to 1.8 %. The feed water temperature control prevents feed water that is too cold from entering the economizer, so that no flue gas condensation arises there.

Air preheating

In the case of new systems with an economizer, air preheating is an ideal efficiency-increasing measure if a flue gas condenser cannot be integrated for process-related reasons. With these systems, the flue gas temperature is reduced by preheating the combustion air.

Various versions are available on the market. Bosch offers a standardised air preheating system for single-flame or doubleflame tube boilers with duoblock burners. This system makes economic sense from boiler outputs of approx. five tonnes of steam per hour (figure 3).