Reduction and Control Technologies
Overview
Principles of NOx Reduction
Fuel Switching
Combustion Control
Flue Gas Treatment
Fuel Reburning
Choosing the Best NOx Technology for the Job
Summary and References
Choosing the Best NOX Technology for the Job
Introduction
Turndown
Capacity
Efficiency
Excess Air
Carbon Monoxide (CO) Emissions
Introduction
Certain NOx controls can worsen boiler performance while other controls can appreciably improve performance. Aspects of the boiler performance that could be affected include turndown, capacity, efficiency, excess air, and CO emissions. Failure to take into account all of the boiler operating parameters can lead to increased operating and maintenance costs, loss of efficiency, elevated CO levels, and shortening of the boiler’s life.
Selecting the best low NOx control package should be made with total boiler performance in mind. Investigate all of the characteristics of the control technology and the effects of the technology on the boiler’s performance. The newer low NOx technologies provide NOx reductions without affecting total boiler performance.
The following section discusses each of the operating parameters of a boiler and how they are related to NOx control technologies.
Choosing a low NOx technology that sacrifices turndown can have many adverse effects on smaller boilers that cycle. When selecting NOx controls, the boiler should have a turndown capability of at least 4:1 or more, in order to reduce operating costs and the number of on/off cycles.
Every time the boiler cycles off, before it comes back on, it must go through a specific start-up sequence for safety assurance. It can take between one to two minutes to get the boiler back on line. If there is a sudden load demand, the response cannot be accelerated. Keeping the boiler on line assures a quick response to load changes.
Frequent cycling also deteriorates the boiler components. The need for maintenance increases, the chance of component failure increases, and boiler downtime increases. So, when selecting NOx control, always consider the burners turndown capability.
Capacity and turndown should be considered together because some NOx control technologies require boiler derating in order to achieve guaranteed NOx reductions. For example, flame shaping (primarily enlarging the flame to produce a lower flame temperature – thus lower NOx levels) can require boiler derating, because the shaped flame could impinge on the furnace walls at higher firing rates.
The boiler’s capacity requirement is typically determined by the maximum load in the steam/hot water system. Therefore, the boiler may be oversized for the typical load conditions that may occur. If the boiler is oversized, its ability to handle minimum loads without cycling is limited. Therefore, when selecting the most appropriate NOx control, capacity and turndown should be considered together for proper boiler selection and to meet overall system load requirements.
Some low NOx controls reduce emissions by lowering flame temperature, particularly in boilers with inputs less than 100 MMBtu/hr. Reducing the flame temperature decreases the radiant heat transfer from the flame and could lower boiler efficiency. Depending upon the boiler design, this may be partly compensated by increased convective heat transfer. Any potential energy efficiency loss due to the lower flame temperatures can be partially offset by utilizing external components, such as an economizer.
A boiler’s excess air supply provides for safe operation above stoichiometric conditions. A typical burner is usually set up with 10-20% excess air (2-4% O2). NOx controls that require higher excess air levels can result in fuel being used to heat the air rather than transferring it to usable energy. Thus, increased stack losses and reduced boiler efficiency occur.
Carbon Monoxide (CO) Emissions
High flame temperatures and intimate air/fuel mixing are essential for low CO emissions. Some NOx control technologies used on industrial and commercial boilers reduce NOx levels by lowering flame temperatures by modifying air/fuel mixing patterns. The lower flame temperature and decreased mixing intensity can result in higher CO levels.