Volume 22, number 1, march 2008

Four ways to optimize the energy performance of commercial and industrial boilers

The Quebec environmental and economic context constantly impels us to review our methods so that we remain competitive in terms of our competitors and our values. The reduction of boiler room running expenses has become a good way to lower corporate operating costs. Here are four simple means to achieve this goal.

1. Using high-performance burners with a wide modulating range (modulation, air-gas mixture)

The burner is fundamental to achieving good boiler room efficiency. The modulation range and the ratio required between the quantity of combustion air and natural gas must be analyzed when a burner
is selected. A good quality, fan-assisted burner can mix combustion air and natural gas to obtain complete and clean combustion with a minimum quantity of excess air (10% to 15%), throughout its modulation range. Some burners can reduce their power by up to 10% of their maximum power (10:1 modulation rate) without generating a loss due to a poor air-gas mixture ratio or a poor combustion.

The next figure clearly shows the impact of a high modulation ratio on natural gas consumption.

2. Improving the combustion efficiency of your burners (micro–modulation, O2 sensor, VFD)

It is possible to add very interesting auxiliary systems to a burner to make it even more efficient.

The old lever, tie-rod and crank mechanisms are often imprecise and out of kilter. Their replacement with fully independent servomotors helps optimize the air-natural gas mixture for several points of the burner power range. In this way, the excess air is minimized, repeatable and stabilized for all power levels.

Effect of the variation of air density on excess air

Air combustion temperature (°C) Percentage excess air (%)
4.5 25.5
10.0 20.2
26.7 15.0
37.8 9.6
48.9 1.1

Once micromodulation is installed on the burner, the optimum proportion of the mixture is maintained only when the boiler room temperature is identical to the temperature that prevailed during startup of the micromodulation system. This is due to the fact that the combustion air fan delivers a constant air volume. The air mass delivered to the burner thus varies according to temperature and pressure. The following table shows the effect of this density variation on the excess combustion air.

To counter this, it is possible to install an automatic correction system for the air-natural gas mixture. Due to the addition of asensor detecting the excess air level in the combustion products, it is possible to inform the combustion controller and vary the position of the servomotor modulating the combustion air, thus minimizing the excess at all times. No additional air intake is required to cover the system against an air shortage when the combustion air is at a very high temperature. To generate electrical saving related to the combustion air fan, it is possible to install a frequency variator, which controls the fan motor speed, instead of using a damper on the air inlet. The potential saving related to this measure is about 2% and can vary greatly depending on the boiler.

3. Using economizers, when possible (direct or indirect contact)

Traditional steam boilers normally cannot operate at very high efficiency, because the combustion products leaving the boiler technically cannot be colder than the temperature of the steam they produce. Typically, the temperature of the combustion products at the boiler outlet is about 55°C hotter than the saturation temperature of the steam produced. However, the boiler feed water coming from the condensate tank is at about 82°C. The addition at the boiler outlet of an indirect heat recuperator, known as an “economizer,” allows the transfer of a certain quantity of this available energy to the feed water and thus reduces natural gas consumption. It is also possible to recover this residual energy with a direct-contact recuperator and transfer it to another energy system that runs at a lower temperature. This is an interesting application, because it becomes possible to push overall system efficiency to 100%. The potential saving related to the use of a direct-contact economizer can be as much as 20%. The potential saving from an indirect-contact economizer may reach 5% and can vary greatly according to the boiler.

PURGE RATE (%) VS. MAKE-UP WATER Energy recovered, millions of Btu/h (MMBtu/h)
  Boiler operating pressure (psig)
  50 100 150 250 300
2 0.45 0.5 0.55 0.65 0.65
4 0.9 1.0 1.1 1.3 1.3
6 1.3 1.5 1.7 1.9 2.0
8 1.7 2.0 2.2 2.6 2.7
10 2.2 2.5 2.8 3.2 3.3
20 4.4 5.0 5.6 6.4 6.6

Source: U.S Department of Energy. Improving steam system performance

4. Recovering the energy contained in the steam boiler purge water

Although ideally it has been minimized, purge water contains a significant quantity of recoverable energy. The table above presents the recovery potential according to the boiler operating pressure and the purge rate for a boiler producing 100,000 lb/h of steam.

This energy can be recovered with a simple heat exchanger to preheat the make-up water or with an evaporation tank allowing production of steam at lower pressure. This steam can then be used to feed other building systems, such as the deaerator or the steam heating coils.

These are four simple approaches that help reduce your energy bill while respecting the environment.

Martin Blanchet, Eng., CEM
Technical Advisor