13.2.1 Overview

Sulfur exists in coal as organic sulfur, pyretic and sulfates [ 1], and exists in liquid fuels mostly in organic form [ 235], with mass fractions ranging from 0.5% to 3%. All SOx emissions are produced because of the oxidation of fuel-bound sulfur. During the combustion process, fuel sulfur is oxidized to $SO_2$ and $SO_3$. A portion of the gaseous SOx will condense on the particles, attaching an amount of water and thus forming sulfuric acid, or may react further to form sulfates. While SOx emissions are the main cause of acid rain, $SO_3$ also contributes to particulate emissions, and is responsible for corrosion of combustion equipment. Furthermore, there is a growing interest in the interaction of sulfur species with the nitrogen oxide chemistry [ 235], as NO levels are affected by the presence of sulfur species. The evidence to date indicates that thermal NO levels (Section  13.1.3) are reduced in the presence of $SO_2$. However, the effect of sulfur compounds on the fuel NOx formation is yet to be clarified.

Sulfur emissions are regulated from stationary sources and from automotive fuels. Sulfur pollutants can be captured during the combustion process, or with after treatment methods, such as wet or dry scrubbing. Coal fired boilers are by far the biggest single SOx emissions source, accounting for over 50% of total $SO_2$ emissions [ 55].

For higher sulfur concentrations in the fuel, the SOx concentration field should be resolved together with the main combustion calculation using any of the ANSYS FLUENT reaction models. For cases where the sulfur fraction in fuel is low, the post-processing option can be used, which solves transport equations for $H{_2}S$, $SO_2$, SO, SH, and $SO_3$.

The Formation of SOx

The SOx model incorporates the following stages:

1.   Sulfur release from the fuel

For liquid fuels, one can conveniently assume that sulfur is released as $H_2$S [ 235]. However, the process is more complicated in the case of coal; here some of the sulfur is decomposed into the gas phase during devolatilization as $H{_2}S$, COS, $SO_2$ and $CS_2$, while part of the sulfur is retained in the char to be oxidized at a later stage. The percentage of sulfur retained in char is rank dependent [ 1].

2.   Sulfur reaction in the gas phase

In oxygen rich flames the predominant sulfur species are SO, $SO_2$ and $SO_3$. At lower oxygen concentrations $H{_2}S$, $S_2$ and SH are also present in significant proportions, while $SO_3$ becomes negligible [ 235]. In PCGC-3 as well as in the works of Norman et al. [ 251] the gas phase sulfur species are assumed to be in equilibrium.

3.   Sulfur retention in sorbents

Sulfur pollutants can be absorbed by sorbent particles, injected either in situ, or in the post flame region.

For low sulfur fuels, we can assume that sulfur is mainly released as $H{_2}S$. The rate of release can be determined similarly to that of fuel-bound N. For the char S it can be assumed that $SO_2$ is produced directly at the same rate as that of char burnout. Transport equations for $H{_2}S$, $SO_2$, SO, SH, and $SO_3$ species are incorporated and an appropriate reaction set has been developed as described in the ensuing sections.