NOx Reduction in Lightweight Aggregate Kilns

by Kiln Flame Systems Ltd.

NOx is a major air pollutant emitted from all stationary and non-stationary combustion sources and, as such, has historically been subject to ever more stringent regulation worldwide and across industries. Regulations have typically been applied to larger industries and sources first and then trickled down to smaller sources, including smaller rotary kiln operations, such as those in the lightweight aggregate industry.

NOx is formed through three major mechanisms:

  • thermal NOx – high temperature oxidation of nitrogen in combustion air
    to NOx;
  • fuel NOx – oxidation of nitrogenous components in fuels and fuel
    char/ash to NOx; and
  • prompt NOx – a complex series of reactions leading to oxidation of
    nitrogen to NOx.

In most combustion systems, such as boilers, the dominant mechanism NOx generation is fuel NOx. This mechanism accounts for up to 75% of all NOx emissions. Thus, moving to low nitrogen fuels (e.g. natural gas) is advantageous. However, for high temperature rotary kiln processes, such as production of lightweight aggregate (LWA), thermal NOx is by far the most dominant of the three mechanisms, responsible for up to 70% of NOx emissions.

Since the nature of the LWA process and products means that nothing can be done to reduce the required high temperatures responsible for NOx generation, emissions reduction has to be achieved through application of low NOx kiln burner technology.

Kiln burners operate by using the momentum from a small amount of the total combustion air, known as primary air (typically anywhere between 15-40% of total combustion stoichiometric air requirement) and exiting the burner nozzle at relatively high velocity, to draw in (entrain) the surrounding remaining combustion air, known as secondary air, which is preheated via passage through the product cooler.

Thus, the burner primary air is not only responsible, via its momentum leaving the burner nozzle, for achieving the required fuel/air mixing, flame shape and heat transfer to the product bed, but is also a key factor in generating NOx due to its presence in the core of the flame. Therefore, a careful balance needs to be attained between minimizing the amount of primary air whilst maintaining sufficient nozzle exit momentum to achieve the correct combustion conditions. It is also noted that in many solid fuel system, the amount of primary air is defined by the coal mill operation.

Resolving the multiple contradictory and competing needs of combustion, process operation and reduced NOx emissions is the purpose of a correctly designed low NOx kiln burner. These designs typically minimize primary air levels and/or combustion intensity through a combination of techniques incorporated into the burner nozzle, such as bluff-body, separate axial/swirl primary air channels, and separate adjustable axial/swirl gas channels.

In order to successfully design and construct a low NOx burner for LWA applications, it is crucial that the current kiln combustion parameters and operation are fully understood. This is achieved through assessment of process data and measurement of non-instrumented parameters during a site survey. The representative dataset can then be used as input information into Computational Fluid Dynamic (CFD) modeling, which provides a “window” into the process of understanding current combustion conditions and assisting with the design of an optimized low NOx burner for the specific kiln conditions and fuel(s).

One such successful NOx reduction project is that undertaken by KFS for Trinity Lightweight at their Frazier Park, California facility. NOx reduction was required in order to meet the requirements of a forthcoming Ventura County Emissions Rule governing smaller unit operations, such as the Trinity LWA kilns.

Kiln combustion and process data was used as the basis for CFD modeling (see Figure 1), which resulted in the design of a new optimized low NOx burner (see Figure 2). The burner was installed in April of 2017 and achieved NOx reductions beyond the guarantee levels and well within the requirements of the new emissions Rule. It is noted that an important element of the success of any new low NOx burner installation is thorough and detailed operator training, such that the new constraint of working with reduced NOx emissions is fully understood and that benefits from an optimized kiln burner can be fully realized.