Volume 28, number 1, June 2014

Evaluating the efficiency of a steam boiler plant: A crucial step in evaluating cost-effective energy-savings measures.

A large majority of the natural gas distributed in Québec is used for producing steam. While it is true that this is an excellent method for the distribution and displacement of energy at low cost, steam boiler plants unfortunately get bad press. It is often said that they are not efficient. But what do we mean exactly by efficiency when it comes to steam production? How is that efficiency evaluated, and what are the principal measures that would help improve it?

Gaz Métro asked the Natural Gas Technologies Centre (NGTC) to produce a report including the equipment in a steam boiler plant, a calculation of its efficiency, the various energy-loss points, and improvement measures.

This article presents summary of the results of that study.

Energy contained in water versus steam

The thermal energy produced by a natural gas boiler can heat water up to evaporation. There is much more energy in saturated steam that in water at the same temperature. For example, at atmospheric pressure, the energy required to heat water from 0 to 100ºC represents only 20% of the energy required to vaporize it. Superheated steam can also be produced which, since it attains a higher temperature than that of a phase change, is very useful in limiting condensation from cooling in the pipes and in protecting equipment such as turbines.

Components of a steam boiler plant

There are several types of boilers: fire tubes, water tubes, coils, or vertical without tubes. The choice of the type of boiler for a boiler plant depends on needs, as well as on the operating conditions: space available, steam pressure required, pace of load changes, start-up mode, etc.

Figure 1 illustrates the principal components of a steam boiler plant: steam boiler and burner, deaerator, softener, and chemical products.

Figure 1: Components of a steam heating system

To limit problems of scaling, sludge and scum formation and corrosion due to dissolved oxygen, the water that supplies the boiler has to be treated. The softening reduces the presence of minerals like calcium and magnesium. The precipitation of minerals with chemical components, the use of ion exchangers or inverse osmosis, are some of the methods currently used to soften water. The deaerator helps reduce the level of dissolved oxygen in the water. The dissolved gases are harmful since they can cause corrosion on the internal surfaces of the boiler.

Evaluating the eficiency of a steam boiler plant

In a steam boiler plant, energy losses occur at various points: in the products of combustion, in radiation on the boiler’s external surface, in auxiliary equipment that requires steam, etc. There are three methods for evaluating the energy performance of a boiler plant.

  1. The direct method is based on data on steam production and fuel consumption.

  2. The indirect method takes into account the temperature of the flue gases and the level of oxygen in the flue gases (combustion test).

  3. The preparation of a detailed energy profile to identify the various energy-loss points.

As part of the project to develop a Good practices guide: Improving energy efficiency in steam heating systems, carried out in collaboration with the NGTC, the last method was chosen.

For each boiler in the heating system, the following energy-loss points were considered:

  • The boiler, including the burner for the combustion of natural gas.

  • A deaerator for each boiler. Generally, there is only one deaerator for all the boilers in a plant, but this simpler calculation method has a very little impact on the results of the energy profile.

  • Treatment systems and contaminant purges.

The characteristics of the boiler, both those specified by the manufacturer and the operating conditions, are essential in calculating energy losses. Table 1 shows the characteristics inherent in the boiler plant and the energy-loss points.

Table 1: Characteristics of the boiler plant and energy-loss points

Characteristics

Energy-loss points

Burner power and steam production – data plate

Radiation and convection

Temperature of products of combustion, combustion air and natural gas

Flue gases – Dry and humid losses

Excess air percentage

Flue gases – Dry and humid losses

Purge percentage

Purges

Temperature of water feed – before treatment – mixture of condensate and make-up water

Deaerator vent  – Calculation of water flow

Quality of steam and pressure produced

Deaerator vent

Lastly, the composition of natural gas and its calorific value are very important data in evaluating the losses in the flue gases. For this project, the natural gas consisted of 96% methane, 2% ethane, 1% carbon dioxide and 1% nitrogen. The superior calorific value was thus established at 37.4 MJ/m³.

The heating efficiency can thus be calculated based on these data, that is, the quantity of energy useable for producing steam. This is defined as follows:

% Q heat = 100% – %QS – %QH – %QRadConv ,

where:

  • %QS = Dry losses in the flue gases, which come from heating combustion air at the temperature of the flue gases;

  • %QH = Humid losses in the flue gases, that, is, the energy in the water vapor that escapes in the flue gases;

  • %QRadConv = Losses through radiation and convection.

Also, the efficiency of steam production corresponds to the ratio between the energy contained in the steam that is available to users and the energy from the fuel that supplies the boiler. To complete the calculation, the steam lost through the deaerator vent also has to be evaluated. It is much easier to identify energy efficiency measures when the losses are calculated fully. As a general rule, the most significant losses can be found in the products of combustion.

Three energy savings measures that are worth the cost

Several energy-savings measures can be implemented in a steam boiler plant and each one could certainly be the subject of an Informatech article! To begin, here are three measures currently implemented.

The first measure is the installation of an economizer, which recovers the heat lost through the products of combustion to pre-heat the make-up water supplying the boiler. Different models are available on the market, depending on needs and the operating conditions of the boiler plant. The potential energy savings from this measure may easily vary from 5-13% of the boiler’s effective rated output.

The second measure is controlling the purges to reduce losses. Rather than going down the drain, the purged water could be collected through an energy recovery system (shell and tube type exchanger or plate heat exchanger) to pre-heat the boiler’s make-up water. Depending on the rate of purge and the pressure of the steam produced, savings varying from 0.5% to more than 6% can be realized, in relation to the quantity of energy supplied to the boiler.

The third, but not the least useful measure is to control excess air better in order to improve combustion efficiency. In fact, the greater the excess air, the greater the flue gases flow and the more the energy produced by combustion is diluted. To improve combustion, adjusting excess air based on the boiler’s operating state is highly recommended. The savings should be between 3-5%.

Conclusion

Following the production of a detailed report on the evaluation of the energy efficiency of a steam boiler plant, the Natural Gas Technologies Centre developed a calculator that is very simple to use. Under given operating conditions, it can calculate the heating efficiency and steam production of a boiler plant with four steam boilers. Based on the results related to losses, the potential energy savings from the different measures can also be evaluated: economizer on the products of combustion, reduction and recovery of heat from purges, micro-modulation and recovery of heat on the deaerator vent.

Watch for future announcements: the calculator will soon be available on Gaz Métro’s Internet site.

Marie-Joëlle Lainé, Eng.
DATECH Group