Hydrocarbon Processing - November 2022 - 52

Heat Transfer
heater. Usually, this can be done by visual inspection or with
conventional flame scanners when firing hydrocarbon fuel gases.
However, when 100% H2
is used as a fuel source, the flame
can be seen. In contrast, FIG. 4B
becomes virtually invisible. This is demonstrated in FIG. 4A, in
which a burner firing 100% H2
depicts nearly all H2
natural gas.5
combustion with a small percentage of
Clearly, a small quantity of natural gas in the fuel
can dramatically enhance the ability to see the flame.
In situations where hydrocarbon fuel gas is unavailable or
not allowed, alternate methods of detecting the flame, such as
specialized ultraviolet flame scanners or sound monitors, may
be potential solutions.
Radiant/convection duty split. As represented in FIG. 5, increasing
H2
quantity of flue gas generated.
For an existing heater being revamped to run on H2
content in the fuel can significantly decrease the
fuel, this
reduction in thermal mass traveling through the convection
section decreases the heat absorbed. In a single service heater
(i.e., the same service in the convection and radiant section),
this may be counteracted by increasing the heat absorbed in
the radiant section, with careful attention paid to the increased
radiant flux, bridgewall temperature and tube metal temperatures.
However, if there is a second, independent service in the
convection section, the convection coil must be reconfigured
to maintain the original heat absorbed. The effects on draft and
fan operation, as applicable, should also be considered. Excess
air may be increased to generate more thermal mass; however,
this will also increase NOx
emissions and reduce efficiency.
Impact to radiant heat transfer in the firebox. In the firebox
(radiant section), heat is transferred to the process tubes by
three main methods of radiation:2
* Direct radiant from the flame (i.e., flame burst)
* Reradiation from flue gases
* Reradiation from refractory surfaces.
The radiation from the flame is related to its luminosity and
may impact heat transfer. Luminous radiation is the radiation
from solid particles suspended in the flame.6
pure H2, luminosity is reduced to virtually zero, which may decrease
the radiant heat transfer slightly.
However, contribution from direct flame radiation is typically
low vs. other forms of radiation in the firebox, as evidenced
by combination oil and gas burners that have been in
operation with little change in performance noted between
fuels fired.2
Reradiation from flue gases occurs primarily from CO2
H2O and sulfur dioxide (SO2
ecules [nitrogen (N2
,
) molecules.7
), O2
Symmetrical molconsidered
" transparent " and do not reradiate heat.7
H2 flame produces significantly more H2
emissivity than CO2
flue gas and is essentially replaced with H2
tendency to increase radiant heat transfer.
As mentioned previously, a H2
ture than a methane (CH4
and carbon monoxide (CO)] are
An allO,
which has higher
. Since CO2 is eliminated entirely from the
O, this will have the
flame has a higher tempera)
flame, which may increase the surface
temperature of the refractory that is in close proximity to
For example, an
For a flame from
oil flame has three to four times more flame radiation due to increased
luminosity created by soot content.2
FIG. 2. Adiabatic flame temperature (AFT) in °F vs. vol% H2
This assumes the balance of fuel is CH4
in fuel.
, with 15% excess air.
FIG. 3. Graphic depicting the approximate laminar flame speed of
hydrogen vs. CH4
at 77°F and 14.7 psia of pressure.3
52 NOVEMBER 2022 | HydrocarbonProcessing.com
FIG. 4. Picture (A) of a process burner firing 100% H2
, and a process
burner (B) firing mostly H2 with a small percentage of natural gas.3
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