Detailed Reaction Mechanism To Predict Ammonia Destruction in the Thermal Section of Sulfur Recovery Units

Ammonia
and aromatics such as benzene, toluene, and xylene are
typically found in acid gas and sour water stripper gas in oil and
gas processing plants and gasification facilities. This gas is often
processed in sulfur recovery units (SRUs) to recover marketable sulfur
and thermal energy. Ammonia must be completely oxidized at high temperatures
in the furnace to prevent the plugging of catalytic reactors and the
corrosion of downstream equipment in the SRU. In this paper, a detailed
reaction mechanism is presented to capture the chemistry of ammonia
destruction in the presence of several chemically active species of
acid gas combustion in the thermal section of SRUs. The mechanism
is validated with different sets of experimental data from lab-scale
and industrial plant studies. The reaction mechanism is utilized to
simulate the furnace and the waste heat boiler (WHB) of SRUs in CHEMKIN
PRO software. Through the furnace and WHB simulations, the most suitable
operating conditions of the furnace that could lead to an effective
destruction of ammonia in the furnace is investigated, and the dominant
reaction pathways involved in the oxidation process are identified.
With increasing feed temperature and oxygen concentration in air,
the ammonia concentration was found to decrease substantially down
to an acceptable limit of <150 ppm at the exit of the thermal section
of SRUs. The decrease in NH3 occurred because of its enhanced
oxidation by several oxidants (OH, SO, and O2) at high
temperatures above 1300 °C, though it also led to a decrease
in sulfur recovery efficiency and an increase in CO production at
the exit of the thermal section. This indicates the need for optimized
furnace parameters that could lead to a reasonable trade-off between
ammonia destruction and CO emissions from the SRU. The developed reaction
mechanism provides a way to obtain optimized SRU parameters to achieve
ammonia destruction, enhanced catalyst life, and reduced emission
of harmful gases (CO and SO2).