The question remaining, of course, is: could gypsum wallboard, in regions of the WTC buildings that were exposed to fires, have released a significant amount of sulfur as SO
2?
Chemists have investigated the thermal decomposition of gypsum, CaSO
4.2H
2O or anhydrite, CaSO
4, since the early 1900s because of its potential for making sulfuric acid by the liberation of SO
2 or SO
3 from a plentiful and inexpensive starting material. It was known at this time that the direct reaction:
CaSO4 -> CaO + SO3 + O2
(followed by: SO
3 + H
2O -> H
2SO
4), only proceeds at an acceptable rate at temperatures ~ 1400
oC. However, early research showed that the above reaction could be accelerated by additives such as clay and, more importantly, the reduction of CaSO
4 to CaO and SO
2 by reaction with solid carbon or gaseous carbon monoxide was found to be possible at temperatures well below 1000
oC. In these cases CaSO4 was decomposed by two novel reactions:
2CaSO4 + C -> 2CaO + CO2 + 2SO2
and,
CaSO4 + CO -> CaO + CO2 + SO2
Since the 1980s there been renewed interest in these reactions because of their role in the removal of SO
2 from combustion gases by contact with lime (CaO) in so-called flue gas desufurisation processes, (See, for example /9/ and references contained therein.). As a consequence, the chemistry of calcium sulfate reduction has been investigated over a wide range of conditions. Thus, for example, R. Kuusik and co- workers (See Ref/10/), have reported details of the thermal decomposition of calcium sulfate in carbon monoxide/nitrogen mixtures and note that in 10 - 20 % CO/ N2, calcium sulfide, CaS, and carbon dioxide are formed at temperatures in the range 780 - 850
oC, while at CO concentrations below 10 %, calcium oxide, CaO, carbon dioxide and sulfur dioxide are formed above 900
oc. Kuusik et al. also note that the presence of impurities such as SiO2 in the calcium sulfate lower the decomposition temperatures by up to 100
oC.