Burner technology has improved greatly in recent years. Older Burners had a series of cams and levers that were calibrated to provide specific amounts of air for a particular rate of firing. Being mechanical items, these wore and sometimes required calibration. They, therefore, had to be regularly serviced and calibrated.
Generally, new burners use frequency controlled motors and cams to open air vents and run fans at optimum speeds that can improve performance by up to 3%. Improvements in burner design have also shown that burners older than five years are typically 1-2% less efficient than new burners. It should be noted that any air leaks in the boiler combustion chamber will have an adverse effect on the accurate control of combustion.Energy costs today are the highest in recent history. Today’s economic and environmental demands dictate that we get the greatest practical efficiencies from our plants. To do this we must have a basic understanding of what those efficiencies are and how we may implement them. The boiler house is the engine of the modern steam system, so it is important to make sure yours is in good working order by ensuring that vital maintenance and testing is carried out regularly.
Steam plant optimisation is the overall improvement of the plant’s operation. The most common strategies used to accomplish this task include and generally focus on, the improvement of primary equipment operating efficiency, i.e. fuel and energy savings. In heavy commercial and industrial boiler applications these efficiencies are normally found in the application of waste heat recovery equipment, systems and process automation as well as improved operating practices.
VIABILITY OF EXISTING BOILER VESSELS
The departure point for any improvements concerning the boilers, with regard to adding economisers or improvements in burners, has to be the physical condition of the boiler vessels themselves. Some of the available Boiler Inspection Techniques that can be utilised are:
Ultrasonic Thickness Surveys – Typically these use software and experience to gather large amounts of thickness data and transform it into a colourised report to detect wear patterns or localised loss in tube wall thickness.
Radiography - Conventional and Computed Radiography can be used for information purposes such as Stress-Assisted Corrosion Investigation or code compliance for new installations
Visual Inspection (VT) - Visual examination is often used to detect mechanical damage or wear due to harsh service conditions experienced by certain boiler types.
Burner efficiency is based upon the combustion efficiency of the burner and is an indication of the burner's ability to burn fuel. The amounts of unburned fuel and excess air in the exhaust are used to assess a burner's combustion efficiency. Burners are considered efficient that have low levels of unburned fuel while operating at low excess air levels.
Boilers that have no flue gas economisers, as general rule of thumb, are typically only 88% efficient in burning fuel.? A large element of the burner’s efficiency is concerned with the control of air mixing and combustion. The latest burners re-circulate flue gases to ensure optimum combustion, with minimum excess air.
They are also fitted with sophisticated electronic control systems that monitor all the components of the flue gas, and make adjustments to fuel and air flows to maintain conditions within specified parameters. These greatly improved turndown ratios (the ratio between maximum and minimum firing rates), which enable efficiency and emission parameters to be satisfied over a greater range of operation.
NON-CONDENSING FLUE GAS ECONOMISERS
The most common economisers are non-condensing economisers. They are heat exchanger coils‚ usually finned‚ that are located in the flue gas ducting at the exit of the boiler, through which feedwater is pumped. The feedwater is fed into the boiler at a higher temperature than would be the case if no economiser were fitted. Less energy is then required to produce steam. Alternatively, if the same quantity of energy is supplied, then more steam can be produced. This results in a higher efficiency. In broad terms a 10°C increase in feedwater temperature will give an efficiency improvement of 2%.
They are designed and operate to maintain the flue gas temperature above the flue gas condensing temperature to prevent corrosion of the flue gas ducting.
CONDENSING FLUE GAS ECONOMISERS
The application difference between a non-condensing economiser and a condensing economiser is that non-condensing economisers are primarily used to heat a smaller volume of water to a high temperature for boiler feed water, and condensing economiser heat a larger volume of water to a lower temperature. Condensing economisers can be more efficient because they can have a lower outlet exhaust temperature and take advantage of the energy in condensed flue gasses (the Latent Heat of Vaporisation).
A conventional feedwater economiser reduces steam boiler fuel requirements by transferring heat from the flue gas to the boiler feedwater. For natural gas-fired boilers, the lowest temperature to which flue gas can be cooled is about 120°C to prevent condensation and possible stack or stack liner corrosion.
The condensing economiser improves waste heat recovery by cooling the flue gas below its dew point, which is about 60°C for products of combustion of natural gas. The condensing economiser reclaims both sensible heat from the flue gas and latent heat by condensing flue gas water vapour.
When gas is combusted, heat energy in the form of evaporated water vapour is released up the boiler stack. This latent heat represents approximately 9% of the initial fuel energy content. The bulk of this latent heat can be recovered by cooling the exhaust gas below its dew point using a direct contact or indirect condensing economizer. It is possible to heat water to about 90°C with an indirect economiser or 60°C with a direct contact economizer. The volume of hot water produced is dependant upon the volume of steam produced and the amount of gas required to produce this steam.
The fully condensing economiser requires more additional construction than a non-condensing economizer however as virtually all of the gas energy is recovered paybacks are also similar to the non-condensing economiser and are typically 2 years according to manufacturers.
Conclusions and recommendations
1.Evaluate your boiler capacity, average steam generation, combustion efficiency, flue gas temperature, annual hours of operation and annual fuel consumption
2.Determine where heated water may be used, for example: boiler makeup water heating, reheating, or domestic hot water or process water heating requirements.
3. Estimate the thermal requirements that can be serviced through installation of a condensing or non-condensing economiser. Calculate annual fuel energy and cost savings.
4.Obtain an installed cost quotation and determine the cost-effectiveness of:
a. A non-condensing economiser.
b. A condensing economiser.
Review boiler condensate return system ensuring correct installation of all piping.