New, larger economizer ready to be installed in Plymouth, NC. Old economizer in the foreground.
Combustion efficiency and heat capture efficiency are two important factors to consider when analyzing a boiler’s overall operating efficiency.
Combustion efficiency relates to how effectively a boiler furnace and combustion system is able to combust the fuel it is firing. Boiler furnaces that produce ash with low amounts of unburned materials leaving the boiler system and low carbon monoxide (CO) emissions are considered to operate with high combustion efficiency.
Another important aspect of maximizing the energy output of a boiler relates to effectively capturing the heat released from combustion. Most boiler systems are equipped with tubular air heaters (TAHs) or economizers to serve as heat transfer devices that absorb heat from the flue gas and return it to the boiler system. But sometimes these devices are undersized, or the type of fuel being fired in the furnace is different than the original design. These factors can result in inefficient heat capture within these units. Non-optimal heat capture in the boiler back-end results in higher flue gas exit temperatures, increased fuel firing rates, lowered overall boiler thermal efficiency, and higher pollutant emissions. Other detrimental impacts of high flue gas exit temperature include greater load on the pollution control equipment and higher fan demands.
An example: Jansen recently performed an evaluation of a biomass-fired boiler facility in the Caribbean. Our evaluation identified the potential for the boiler system to increase thermal efficiency by adding a TAH. The TAH would return heat from the flue gas to the boiler system by using the flue gas to preheat combustion air. Our calculations indicated the addition of the TAH would lower the stack flue gas exit temperature by 70°F (39°C). This was shown to improve the boiler thermal efficiency by 5 percentage points and reduce fuel firing by 7%. Pollutant emissions rates and CO2 generation were predicted to be reduced by the same proportion, and overall flue gas volume exhausted through the stack was projected to lower by 20%.
This example illustrates that effectively capturing excess heat from boiler systems can not only lead to reductions in fuel firing and operating costs but also play a pivotal role in lowering the environmental impact of the boiler system. Jansen is now working with our client in the next phase of designing and incorporating the new TAH into their boiler system.
An important consideration to keep in mind when adding a new heat transfer unit to a boiler is the acid dew point of the flue gas leaving the system. Increasing the heat capture in the boiler back-end by removing heat from the flue gas opens up the potential for acid dew point corrosion to occur if the flue gas temperature is too cool. Therefore, new economizers or TAHs designed for boiler systems should have sufficient operating margins to prevent acid dew point corrosion in the flue gas system, which should be based on thorough analysis of the flue gas composition.
When optimizing heat capture in boiler systems, Jansen implements an in-depth evaluation of the heat transfer performance of these elements to identify their current deficiencies as well as developing alternate designs to improve performance.
An efficient boiler system uses the energy more effectively and does not let it escape to the atmosphere. Please contact us if you are interested in taking a deep dive into your boiler system’s heat transfer performance. We have the tools and know-how to get your boiler system to its full potential.
Authors: Morgan Silverman – Process Engineer, Samit Pethe – Manager, Process Technologies
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