Hydrocarbon Processing - December 2022 - 59

Heat Transfer
J. POTH, IGS, Schleiden, Germany
Cut emissions and fuel while increasing capacity
in fired heaters
According to a recent digital refining
article,1
fired heaters are responsible for
), with at least
emissions
an estimated 400 MMtpy-500 MMtpy
of carbon dioxide (CO2
73% of average refinery CO2
coming from combustion. Refineries and
petrochemical plants are under increasing
pressure to reduce emissions while also
looking for ways to reduce fuel consumption
amid rising fuel prices.
Fired heaters have the greatest running
cost in a refinery or petrochemical
site. If any heater is inefficient by 1%-2%,
it can consume an additional $1 MM/
yr in fuel or result in millions of dollars
in lost revenue. High fuel consumption
also translates
to increased emissions.
Units of particular interest include catalytic
reformers, steam methane reformers
(SMRs) and ethylene furnaces.
This article will discuss solutions to
issues in the radiant and convection sections
of fired heaters that affect performance
and profitability.
RADIANT SECTION
EFFICIENCY
Process fluids are heated in the radiant
sections of fired heaters. The fluids travel
through steel alloy tubes, which are heated
principally by radiant heat generated
by burners within the refractory lined
box. The configuration and condition of
the process tubes and the refractory surfaces
affect heat transfer efficiency in the
radiant section.
How does process tube oxidation
occur? When used, steel alloy tubes oxidize
and scale layers continuously grow
on the external surfaces. The developing
scale insulates the tube surface, hindering
conductive heat transfer to the process
and decreasing radiant section efficiency.
Extra heat is needed, and the firing rate
is increased to overcome the insulating
effect. This results in increased flue gas
temperature (bridgewall temperature).
Consequently, CO2
(NOx
and nitrogen oxide
) emissions are increased. As the
scale grows and further increases in firing
rate are required, boiler water treatment
limitations are encountered and production
rates are threatened.
Carburization is an industry trend
used to operate fired heaters more efficiently-at
lower excess oxygen levels-
to save fuel and reduce CO2
emissions.
The increased potential for the carburization
of external surfaces of radiant section
tubes leads to the grain boundary penetration
of carbon, carbide formation, surface
embrittlement, crack formation and
metal loss. The result is a reduced service
life of the radiant tubes.
The author's company offers online
tube descaling. This will improve the radiant
section heat transfer efficiency and
reduce the bridgewall temperature. However,
the effect is temporary since oxidation
and scale formation will continue.
High-emissivity ceramic coatings provide
a protective thin-film layer on the
outer surfaces of process tubes, which
prevents metal oxidation, corrosion and
carburization. This maintains the thermal
conductivity coefficient close to new tube
conditions. The coatings may be applied
to existing tubes during a shutdown or to
new tubes at a remote facility where the
surface preparation, coating and curing
occurs. The average benefit of catalytic
reformer heaters is to increase the radiant
section efficiency by 6.6%, with a corresponding
6.6% reduction in CO2
emissions
and approximately 20% reduction
in NOx
emissions.
Refractory surface emissivity: Why
does it matter? A significant portion
FIG. 2. After-convection section robotically
cleaned.
Hydrocarbon Processing | DECEMBER 2022 59
of the radiant energy interacts with the
refractory surfaces, and the mechanism of
this interaction has an appreciable effect
on the overall efficiency of radiant heat
transfer. A major factor in determining
radiant efficiency is the emissivity of the
refractory surface.
The ultimate radiant heat transfer efficiency
is achieved where the enclosure is
a black body, where all the surfaces have
the maximum emissivity factor of 1.0.
Ceramic coatings with emissivity values
> 0.9 have been designed to supplement
the refractory surfaces' radiation
characteristics. Benefits of up to 5% in
radiant section efficiency improvement
with corresponding CO2
duction of up to 5% and NOx
emissions reemissions
reduction
in ethylene and SMR up to
30% can be achieved.
FIG. 1. Heavy fouling on a convection section.
Hydrocarbon Processing - December 2022

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