Hydrocarbon Processing - November 2022 - 51

Heat Transfer
L. GLASHAN, Wood, Hampton, New Jersey;
and K. THIRUNAVUKKARASU, Wood, Trichy, India
Decarbonizing your fired heaters
with hydrogen fuel
Hydrogen (H2
) has long been considered a high-value prod's
unique
may be the " silver bullet "
uct, but not typically as a fuel for fired equipment. However, as
the world seeks cleaner, more sustainable solutions, H2
property of producing zero carbon dioxide (CO2
busted has been gaining attention. H2
for eliminating CO2
) when comemissions
in fired heaters. However, makemissions
in a 100-MMBtu/hr
H2 is a unique molecule
firing:
ing the switch does come with challenges. FIG. 1 represents the
potential to reduce yearly CO2
(fired duty) heater.
Challenges. As a basic building block of life and the most
abundant element in the universe,1
with unique challenges. However, without attachment to carbon,
it is a vastly different molecule with distinctive properties
when combusted. The following are some of the potential challenges
associated with switching to all or partial H2
* Higher flame temperature
* Higher flame speed
* Flame visibility (or lack thereof)
* Radiant/convection duty split
* Impact to radiant heat transfer
* Corrosion mechanisms
* Changes to safety and control methods
* Burner piping and fuel gas skid resizing
* Stack plume visibility.
High-temperature flame. Increasing the amount of H2
in fuel
gas has a significant impact on the flame temperature, as shown
in FIG. 2. While FIG. 2 is based on the adiabatic (i.e., theoretical)
flame temperature, it highlights that a 100% H2
) increases proporflame
can
be hundreds of degrees hotter than a flame from hydrocarbon
fuels. The primary issue with this effect is that the formation
of thermally generated nitrous oxides (NOx
tionally with flame temperature.2
Fortunately, several options can counteract the increase in
NOx emissions. Burner design modifications (i.e., retrofit) can
be considered first. External flue gas recirculation and/or steam
injection may also be implemented to lower the flame temperature.
If these methods are insufficient, selective catalytic reduction
may be required.
High flame speed. Flame speed-the rate at which a combustion
reaction takes place for a given fuel-is a critical variable in
fired heater design and operation. To use a simplified scenario, it
FIG. 1. Yearly CO2 emissions in tpy vs. vol% H2 in 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 51
can be conceptualized as the speed at which a gas will fully combust
if the gas was contained in a long tube and ignited at one
end. In actual practice, flame speed depends on several factors,
including pressure, temperature, fuel composition, excess air,
turbulence and surrounding cooling effects.3
However, in an idealized
environment consisting of laminar flow at 77°F and 14.7
psia of pressure, a tube filled with H2
and lit at one end will complete
the combustion reaction at the opposite end of the tube
roughly 7× quicker than a tube filled with methane (FIG. 3).3,4
To ensure reliable, safe combustion in a process burner, it is
vital to control the speed of the un-combusted air/fuel mixture
so it is appropriately matched to the speed of combustion or
flame speed. If the air/fuel speed is lower than the flame speed,
the combustion reaction can travel backward into the burner
and upstream equipment (i.e., flashback). Flashback potential
is especially important to consider for pre-mix burners.2
fore, before switching to H2
Therefuel
from hydrocarbon fuel, it is
essential to review the burner design to ensure it can accommodate
the air/fuel speeds necessary for safe operation.
Flame visibility. The ability to verify the presence of a stable
flame at each burner is paramount to safely operating a fired
Hydrocarbon Processing - November 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - November 2022
