Boiling to the max

Boiling to the max

Boilers and combustion equipment can become expensive problems if assessments aren’t conducted regularly. We discover how you can save on unnecessary costs and extend the lifespan of this equipment.

The performance of boilers and combustion equipment depends heavily on various combustion parameters, which include:

• Air to fuel ratios;
• Stoker performance – for example, bed heights, grate speeds, flame locations and furnace pressures;
• Condition of a boiler’s internals;
• Effectiveness of fans and dampers;
• The fuel’s moisture content;
• Coal’s particle size distribution;
• The ash content in coal;
• Water quality;
• Air leakages;
• Insulation; and
• The calorific values of the fuel.

Over time the combustion variables change, which causes a loss in combustion and boiler efficiencies. One example is fire-tube boilers with coal-fired grate strokers, which have a design boiler efficiency of around 80%, which reduce to 40% (or less) of its effectiveness.

“When a boiler’s thermal efficiency is 40%, the boiler consumes double the amount of fuel to produce the same steam output. It is therefore crucial to regularly perform boiler and combustion efficiency assessments to determine if combustion variables are within specification,” says Jan-Hendrik Fourie, a senior mechanical engineer at Bureau Veritas Technical Centre Africa.

“It is essential to reassess the boiler performance following major changes and modifications or when the fuel source has changed. Even if no changes or modifications were made, it is good practice to perform boiler performance assessments on an annual basis.”

Boiler owners can also benefit from independent boiler efficiency assessments after the commissioning of a new plant or after recommissioning of a modified plant, to verify compliance with specifications or contractual obligations in terms of performance and efficiency.

Third-party assessors, like Bureau Veritas Technical Centre Africa, can also conduct further analysis and testing to determine the root causes of efficiency – and stem any production losses.

How to calculate boiler efficiencies

Two methods are available to determine boiler efficiency:

• The first is the direct method, and requires the accurate measurement of fuel consumption and steam flow
variables.
• The second is the indirect method, which is used to determine boiler efficiency by measuring all the common losses on a boiler and then subtracting all the losses from 100%. This leaves a percentage of the boiler’s efficiency.

Common boiler efficiency losses include heat loss due to:

• Dry flue gas;
• Evaporation of water (formed due to hydrogen in the fuel);
• Evaporation of moisture present in the fuel;
• Unburned combustible gases in the flue gas;
• Unburnt fuel in fly ash;
• Unburnt fuel in the bottom ash; and
• Radiation and other unaccounted losses.

To quantify most of the efficiency losses, a flue gas analysis is performed, which measures the following:

• Boiler flue gas temperature;
• Oxygen;
• Carbon dioxide; and
• Carbon monoxide.

A fuel and ash analysis must be performed, in the case of coal or other solid fuel firings, to quantify the heat loss (due to unburnt fuel in the bottom ash and to determine the moisture content in the fuel).

Samples of the fuel (before combustion) and ash (during online boiler testing) are sampled while boiler testing is performed. After testing, the samples are sent to a lab to perform ultimate and proximate analyses on these samples.

These analyses define the following:

• Fuel constituents (for example, carbon, sulphur, nitrogen, moisture and ash) based on mass fractions;
• Ash constituents (including sulphide minerals, carbonate minerals and oxide minerals) based on mass fractions;
• The inherent and surface moisture in the fuel;
• Total volatile matter;
• Net and gross calorific values of the fuel; and
• Carbon mass fraction (lost energy) in the ash samples.

Once the online flue gas analyses and the fuel and ash analyses results are available, the boiler efficiencies can be calculated.

The same techniques can be used on various boiler designs and combustion types.

The assessment process for equipment

Documents (listing the boilers that have to be tested, particulars of their designs and all past reports on these units and modifications that were made to them) have to be provided for assessment beforehand.

This is performed offsite. A plant visit, a meeting with the client and a plant walk-through follow to check out the operations on site.

Preferably, the boiler should be offline and open for internal inspection to assess the conditions of the heat transfer components to be performed. This will be conducted on a separate date from the flue gas analysis, because the boiler must be running for these tests to be performed.

At a time when the boiler is running, an online flue gas analysis is performed, which entails measuring and recording flue gas properties at various boiler locations and at different operating loads. Coal and ash samples are taken at the same time.

The ultimate and proximate coal and ash analyses are performed offsite at a Bureau Veritas laboratory, for example.

Once all results are available, indirect and direct boiler efficiency calculations based on measured plant information are performed offsite.

Finally, a comprehensive report detailing the boiler and combustion efficiencies is compiled and recommendations are made to optimise boiler performance.   


Benefits of regular boiler and combustion equipment assessments

Regular tests provide peace of mind to employees and clients alike. Other benefits include:

• Great potential for savings on fuel costs by increasing boiler efficiency. The potential savings significantly outweigh the cost of a boiler performance assessment, and the return on investment could be as fast as a few months.
• Maintenance activities to improve excessive boiler losses are identified.
• Combustion adjustment requirements to conform to the Air Quality Act or other environmental regulations are indicated.
• Finally, the implementation of recommendations may reduce boiler operating and maintenance costs.

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