THERMODYNAMICS OF SULFUR CAPTURE

A convenient (and simple) point of departure is Sandler's chemeq.bas program (Chemical Engineering Thermodynamics, 1989). The original data file (react.dta) does not contain the parameters for CaSO₄ and Na₂SO₄, while the NaHCO₃ and Na₂CO₃ data need to be updated (to take into account their temperature dependence). The following graph is then obtained to make the point that nahcolite decomposition,

2NaHCO₃ —> Na₂CO₃ + CO₂ + H₂O,

is much more thermodynamically favorable than limestone decomposition,

CaCO₃ —> CaO + CO₂.

A similar analysis is made in the graph below for the corresponding sulfur capture reactions,

Na₂CO₃ + SO₂ + 0.5O₂ —> Na₂SO₄ + CO₂,

CaO + SO₂ + 0.5O₂ —> CaSO₄.

An often encountered practical issue is that reducing conditions prevail at certain times or in certain portions of the reacting system. The possibility of CaS formation should thus be examined. This is best represented on a CaO/CaS/CaSO₄ phase diagram, as illustrated in the graph below (see Energy & Fuels 12, 905-912, 1998).

lrr3@psu.edu (revised 10/18/98)