Hydrocarbon Processing - November 2022 - 53

Heat Transfer
the flame. This may also increase heat transfer in the radiant
section. While a reduction in luminosity may reduce direct
radiation from the flame, increased emissivity of flue gas molecules
and increased refractory temperatures will likely offset
this effect, suggesting that current methods for radiant heat
transfer calculation are adequate for H2
firing. The authors'
fuel. The
company recently performed a study of a petrochemical heater
designed for and operating with up to 90 vol% H2
study found that the predicted firebox temperature and the
field measured temperature matched very closely (within 5°F).
Potential corrosion mechanisms. The industry standard
for corrosion mechanisms (API 571: " Damage mechanisms affecting
fixed equipment in the refining industry " ) mentions the
three major corrosion mechanisms involving H2
* H2 embrittlement (HE)
:8
* H2 stress cracking from exposure to
hydrofluoric acid (HSCHF)
* High-temperature H2 attack (HTHA).
HSCHF can be ruled out under the assumption that no
hydrofluoric acid is involved in this application. That leaves
HTHA and HE: HTHA typically occurs at high temperature
and pressure (refer to API 941 for temperature and pressure
limits for various materials); while HE typically occurs at lower
temperatures (< 300°F) and pressures at or near atmospheric.
In both mechanisms, atomic H2
(as opposed to molecular
, which can create pressure
H2) forms and diffuses into steel. Once inside the steel, it can
react with the carbon to form CH4
and cracking.8
Fuel gas delivery systems typically do not operate at high
enough pressure for HTHA to be a concern. High-strength
steels with high hardness numbers can be susceptible to HE.
According to API 571, the effect is pronounced at temperatures
from ambient to about 300°F, corresponding to the range
of most fuel gas systems.8
NEW VERSION
While the softer carbon-steel piping
typically used is generally not susceptible to HE, welds, bolted
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FIG. 5. Flue gas flowrate vs. vol% H2 in the fuel for a 100-MMBtu/hr
(fired duty) heater. This assumes the balance of fuel is CH4
, with
15% excess air.
Hydrocarbon Processing | NOVEMBER 2022 53
connections and burner internals comprised of high-strength
steel may be areas of concern. Careful review of the fuel gas delivery
system should be performed before switching to H2
fuel.
Safety and controls. H2
ity in air, approximately 4 vol%-75 vol%.9
has a wide range of flammabilThese
properties
give H2 the National Fire Protection Association's (NFPA's)
highest flammability rating of 4.9
While CH4
also has a flammability
rating of 4, it requires more energy to ignite and is
easier to detect.
Due to its low molecular size, H2
may be susceptible to
leakage in fuel gas piping designed for hydrocarbon fuels. H2
gas is colorless and odorless. Due to its high diffusion rate in
air, H2
mercaptans that are added to CH4
fore, H2
cially if switching between H2
is in operation. When firing H2
fuel gas system before switching to 100% H2
leak detection instruments may need to be added to a
firing.
H2 fuel also requires significantly less air to combust stoichiometrically
vs. CH4
, the heater controls should be
adjusted to reduce the combustion air. However, if a switch
back to CH4
ments to increase combustion air, the CH4
fuel is made quickly, without sufficient adjustwill
not fully combust
and creates a potentially unsafe, fuel-rich environment. A
fuel gas system that accurately measures the fuel gas composition
and controls the combustion air accordingly is critical.
. This is a key point to remember, espeand
CH4
fuels while the heater
does not blend well with the comparatively heavier
to give it an odor. There
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