Sulfur exists in coal as organic sulfur, pyretic and sulfates [ 1], and exists in liquid fuels mostly in organic form [ 249], in compositions 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 and . 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, 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 [ 249], as NO levels are affected by the presence of sulfur species. The evidence to date indicates that thermal NO levels (Section 20.1.3) are reduced in the presence of . 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 emissions [ 60].
For higher sulfur concentrations in the fuel, the SOx concentration field should be resolved together with the main combustion calculation using any of the FLUENT reaction models. For cases where the sulfur fraction in fuel is low, a generalized post-processing option could be developed based on the solution of transport equations for , , SO, SH, and .
The Formation of SOx
The SOx model will incorporate the following stages:
For liquid fuels, one can conveniently assume that sulfur is released as S [ 249]. 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 , COS, and , 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].
In oxygen rich flames the predominant sulfur species are SO, and . At lower oxygen concentrations , and SH are also present in significant proportions, while becomes negligible [ 249]. In PCGC-3 as well as in the works of Norman et al. [ 265] the gas phase sulfur species are assumed to be in equilibrium.
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 . The rate of release can be determined similarly to that of fuel-bound N. For the char S it can be assumed that is produced directly at the same rate as that of char burnout. Transport equations for , , SO, SH, and species are incorporated and an appropriate reaction set has been developed as described in the ensuing sections.