(199b) Sulfur Trioxide Formation during Oxy-Coal Combustion

Oxy-coal combustion technology can be a promising solution to meet the challenge of reducing carbon footprint while continuing with the fossil fuel usage for energy generation. Implementation of such technology will require the removal of N₂ (nitrogen) from oxidizer and the combustion process to occur in a CO₂ (carbon dioxide)-rich environment, which in turn, will reduce the flue gas volume to be handled for CO₂ storage and sequestration. But such volume reduction along with the change in combustion medium is to affect the reaction network and as a result, higher concentrations of SO_x (sulfur oxide) species are expected in an oxy-coal system. Presence of SO₃ (sulfur trioxide) in parts per million (ppmv) levels can cause severe damage to the valuable plant units, both at high and low temperature regions. Due to the concerns regarding the highly corrosive nature of SO₃, a comprehensive study focused on the sulfur chemistry in oxy-combustion is crucial.

This elaborate investigation intends to reveal the temporal profile of SO₃ in an oxy-coal combustion system under variable operating conditions. A simulated oxy-combustion environment; employing the temperature profile of an actual plant boiler, has been created in a unique lab-scale setup by introducing the mixtures of desired combustibles. Flue gas samples have been collected from different temperature points of the reactor and quantification of the evolved SO₃ has been performed using the salt method and FTIR (Fourier transform infrared) spectroscopy. Influences of different parameters on SO₃ evolution, e.g., equivalence ratio (0.8-0.98), O₂ (oxygen) percentage and SO₂ (sulfur dioxide) concentration in the oxidizer, have been investigated experimentally in the course of this study. To observe the effect of NO (nitric oxide), different concentrations (200-1000 ppmv) have been introduced into the system and the samples have been collected to generate the temporal profile of SO₃. In addition to the experiments, kinetic modeling employing the Chemkin software has been conducted to simulate the reactor conditions using two different combustion mechanisms. A parametric study has been performed to observe the influence of variable operating conditions, while dominating reactions for SO₃ generation have been identified for each mechanism by performing sensitivity analysis. Distinct SO₃ evolution profiles along with variable reactor exit concentrations were observed for different mechanisms and compared to experimentally collected data. This data can be utilized to improve the combustion mechanisms and to obtain more accurate prediction of SO_x concentrations in coal power plants operating under oxy-combustion conditions.