Chemical Engineering May 2020 - 30

Cover Story
Part 1
Low-Cost Techniques
for NOx Reduction
An understanding of the available NOx-reduction strategies will help engineers to select the
best technique to improve the performance of their combustion processes
Joseph
Colannino
Colannino Consultants
LLC
IN BRIEF
WHY DO INDUSTRIAL
FLAMES PRODUCE NOX?
NOX-REDUCTION
STRATEGIES
BURNERS OUT OF
SERVICE
DERATING
OXYGEN AND
COMBUSTIBLES TRIM
TRAMP AIR REDUCTION
STAGED COMBUSTION
STEAM OR WATER
INJECTION
ALTERNATIVE FUELS
FLUEGAS
RECIRCULATION
LOW-NOX BURNERS
IMPLEMENTATION
P
ollution due to oxides
of nitrogen - frequently
abbreviated
as NOx - is regulated
with varying degrees of
severity in different localities.
NOx regulations are regional
for good reason: NOx and hydrocarbons
react in a complex
way to produce smog, and
smog formation is greatly affected
by topological and meteorological
conditions. NOx
reduction to very low levels
carries economic impact, and
balancing environmental and
economic requirements has
been a mainstay of air-quality legislation since
its inception. There are many inexpensive and
historically validated NOx-reduction techniques
that can cut NOx in half, as dicsussed
in this article.
NOx regulations are complex and vary by
industry, fuel type, equipment and region.
Some parts of the U.S. require NOx emissions
at or below 9 parts per million (ppm) for
some stationary sources, such as boilers and
steam generators, while other equipment and
regions are less regulated. Remediation requirements
vary nationally from routine maintenance
and tune-ups to selective catalytic
reduction (SCR) strategies, and these differ
widely in cost. However, for many regions, stationary
NOx sources, such as furnaces, steam
boilers and generators and petroleum-refinery
process heaters, NOx reduction to around
40 ppm remains adequate (Figure 1). Before
discussing particular strategies, it is important
to understand the general principles behind
NOx formation.
Why do industrial flames produce NOx?
NOx from industrial combustion comprises
nitric oxide (NO) largely. There are three
30
FIGURE 1. Process heating units
in petroleum refineries are governed
by air-quality regulations
to control NOx pollutants
mechanisms for producing it. In the case
of fuels that contain no nitrogen in the parent
molecule (for example, natural gas), the
Zeldovich mechanism produces most of the
NOx. This may be approximated by the following
integrated rate expression:
(1)
where the brackets indicate the volume
concentration of the enclosed species, A
and b are constants, t is the total reaction
time (with θ serving as the dummy variable
in the integration), and T is the absolute
temperature. So NOx from this mechanism
depends on three primary quantities
- temperature, oxygen concentration and
reaction time - and minimizing any or all
of them will
reduce NOx. Since temperature
is exponentially weighted, the peak
flame temperature has an oversized role in
NOx formation.
If a significant number of fuel molecules
contain nitrogen bound in their structure, the
overwhelming share of NOx will be formed
from the fuel-bound mechanism, leading to
a rate equation as shown in Equation (2):
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