Chemical Engineering May 2020 - 35

hydrocarbons - a potentially explosive
and toxic mixture. To avoid this
calamity, it is best to monitor both
oxygen and combustibles. The technology
is well practiced and inexpensive.
An oxygen trim system will reduce
oxygen to a target level, and the
combustibles trim will act to increase
or reduce the oxygen setpoint to assure
minimal levels of combustibles.
These actions increase efficiency
and minimize NOx formation and are
superior to purely mechanical fuel-air
ratio systems.
Tramp air reduction
An old gardening adage states that
" a weed is a plant out of place. " In a
similar vein, tramp air is air admitted
out of place. In general, combustion
engineers desire all the air to pass
through the air register so that it will
fully participate in the combustion
process. Tramp air is air admitted
through leaks in the boiler or furnace
casing, broken seals or weldments,
holes due to corrosion and so on.
This air does not participate efficiently
in the combustion process.
The result is that boiler operators are
forced to add more air through to
the burner in order to reduce flame
length, to be sure that all the fuel is
oxidized and so on. The results are
high global levels of excess oxygen
and higher NOx. For negativepressure
units, one may find tramp
air using a hollow probe comprising
¼-in. (6-mm) tubing connected via
a flexible hose to a pressure gage
designed to indicate a vacuum of
around ¼ in. of water column. Then,
one uses the probe to check seams,
penetrations and other areas of interest.
A deflection in the gage indicates
a leak, which may often be
sealed on the spot with high-temperature
silicone caulking. Reducing
tramp air will force more air through
the burner, resulting in better behaved
flames and lower NOx. The
probe may also be used for positively
pressurized furnaces as well,
though the source of such leaks is
often obvious even without a probe,
as evidenced by discoloration on
the casing and other similar signs.
Repair is usually not possible during
operation for positive-pressure units.
Nonetheless, one may mark the location
with high-temperature ink or
paint to signal its repair for the next
unit outage.
Staged combustion
If air and fuel are more gradually
mixed in a burner, the flame tends
to lengthen and transfer more of its
radiant heat to the process. Since radiative
heat transfer is a fourth-order
process, the hot spots in the flame
will give up their heat more quickly
than their cooler counterparts. The
result is a reduction in NOx via radiative
cooling of the flame. One
may stage fuel, air, or both. In fuel
staging, air is added incrementally in
two or three stages. The initial portion
is used to stabilize the combustion
and reduce prompt NOx, while
the remainder is added after some
combustion has taken place, resulting
in lower local oxygen concentrations
and reduced Zeldovich NOx.
We Have EHS Compliance
Down to a Science
Client Sectors
Chemical
Petrochemical
Petroleum Refining
Pharmaceuticals
Pulp & Paper
Food & Beverage
Trinity Consultants helps organizations overcome complex,
mission-critical challenges in EHS, engineering, and science.
f EHS regulatory and compliance support
* MON and BWON regulatory reporting
* Ethylene Oxide emissions monitoring and reporting
* Leak Detection and Repair Support
* Chemical Hazard Communication and GHS Services
* Process Safety Management and Risk
Management Program
* Industrial Hygiene
f BREEZE® air emission, fire, and explosion
modeling software
f Broad EHS and Science professional training program
f Engineering and EHS staffing support
Stay current on regulatory developments, subscribe at
trinityconsultants.com/subscribe.
trinityconsultants.com / +1 972.661. 8100
North America / Europe / Asia / Australia
For details visit adlinks.chemengonline.com/76990-27
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM MAY 2020
35
http://www.trinityconsultants.com/subscribe http://www.trinityconsultants.com http://adlinks.chemengonline.com/76990-27 http://WWW.CHEMENGONLINE.COM
Chemical Engineering May 2020

Table of Contents for the Digital Edition of Chemical Engineering May 2020
