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

Abstract

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 NH 3 occurred because of its enhanced oxidation by several oxidants (OH, SO, and O 2 ) 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 SO 2 ).