SO2

Copper I) chloride, CuCl. White solid (CuClj plus HCJ plus excess copper or SO2). Gives carbonyl and phosphine complexes. 

Karl Fischer reagent A mixture ofU and SO2 dissolved in pyridine - MeOH used as a titrant for water with which HI is liberated and the pH determined with a meter. 

Ditrosonium hydrogen sulphate, chamber crystals, NOHSO4. White solid m.p. 73°C (decomp.). Prepared SO2 and fuming nitric acid. Used in diazotization. 

SiOs] [0aSS(0)2] ions. See sulphur oxyacids. Formed from sulphites, [SOj] , and SO2. 

Selenious acid, H2Se03. Formed water on Se02. More stable than H2SO3, oxidized to H2Se04, reduced to Se by SO2. Gives selen-ates(IV) with oxides, hydroxides or carbonates. 

Na2S20j. Obtained NaOH solution saturated SO2 at 100 C. Forms 7 and 6 hydrates. Used in photography. 

Na COj and Na2S with SO2 or from Na2S03 plus sulphur. Forms many hydrates. Used-in photography ( hypo ) because it dissolves silver halides. Also used in tanning, preparation of mordants, as a fermentation preventative in dyeing and in chemical manufacture. 

Sulphur monochloride, S2CI2, m.p. —80 C, b.p. I38"C. Yellow liquid hydrolysed by water to SO2, HCl and S. Lower halides S CIj (jt up to 5) are formed S2CI2 plus H2 at a hot surface with freezing of products. S2CI2 is used in the rubber industry as a solvent for S. 

Sulphur dioxide, SO2, m.p. — 72-7°C, b.p. — I0"C. Colourless gas with characteristic smell. Formed by burning S, metal sulphides, H2S in air or acid on a sulphite or hydrogen sulphite. Powerful reducing agent, particularly in water. Dissolves in water to give a gas hydrate the solution behaves as an acid - see sulphurous acid. Used in the production of SO3 for sulphuric acid. 

Sulphur trioxide, SO3, m.p. 17 C, b.p. 49 C. Formed SO2 plus O2 over a catalyst (contact process - see sulphuric acid). The solid exists... 

Vanadium dioxide, VO2 is dark blue (V2O5 plus SO2) but is readily reduced further to Vo.i86-V,.6a. VO2 gives the (VO) ion with acids and vanadates(IV) with alkalis and as mixed metal oxides. 

ZnS204. Made SO2 on aqueous suspension of Zn dust. Used in bleaches and the vat dyeing process. 

A knowledge of these compounds is important because they often have undesirable attributes, e.g., unpleasant odor, the SO2 formed by combustion, catalyst poisoning. There are a number of refining processes to eliminate sulfur compounds. 

All these methods begin with combustion of the sample resulting in the sulfur being oxidized to SO2 and SO3. Table 2.3 summarizes the different analytical methods with references to the corresponding standards. 

These are called high temperature induction furnace methods which differ only as to the kind of furnace used and employ the same ASTM procedure. The sample is heated to over 1300°C in an oxygen stream and transformed to SO2 which is analyzed with an infra-red detector. 

Methods Giving SO2 and SO3 but that Measure Total Sulfur... 

Emission problems of SO2 and NO, linked to the presence of sulfur and nitrogen in heavy fuels will be examined later. 

In the past, reducing the sulfur content was mainly concerned with the heaviest products, most particularly the fuel oils. This development is explained by a legitimate concern to reduce SO2 emissions, notably in areas around large population centers. This is how low sulfur heavy fuels —having a maximum of 2% sulfur— and very low sulfur ( % sulfur) came into being. Currently the whole range of petroleum products, particularly motor fuels, should be strongly desulfurized for reasons we will explain hereafter. 

These developments will reduce the total emissions of SO2 but the effect will remain limited. The reduction in sulfur levels in diesel fuel from 0.2 to 0.05% Is admitted to reduce yearly emissions of SO2 in France to only 10 to 12%. 

The main justification for diesel fuel desulfurization is related to particulate emissions which are subject to very strict rules. Part of the sulfur is transformed first into SO3, then into hydrated sulfuric acid on the filter designed to collect the particulates. Figure 5.21 gives an estimate of the variation of the particulate weights as a function of sulfur content of diesel fuel for heavy vehicles. The effect is greater when the test cycle contains more high temperature operating phases which favor the transformation of SO2 to SO3. This is particularly noticeable in the standard cycle used in Europe (ECE R49). 

Desulfurization will become mandatory when oxidizing catalysts are installed on the exhaust systems of diesel engines. At high temperatures this catalyst accelerates the oxidation of SO2 to SO3 and causes an increase in the weight of particulate emissions if the diesel fuel has not been desulfurized. As an illustrative example, Figure 5.22 shows that starting from a catalyst temperature of 400°C, the quantity of particulates increases very rapidly with the sulfur content. 

The European regulations have set SO2 emission limits for industrial combustion systems. They range from 1700 mg/Nm for power generation systems of less than 300 MW and to 400 mg/Nm for those exceeding 500 MW between 300 and 500 MW, the requirements are a linear interpolation (Figure 5.24). To give an idea how difficult it is to meet these requirements, recall that for a fuel having 4% sulfur, the SO2 emissions in a conventional boiler are about 6900 mg/Nm this means that a desulfurization level of 75% will be necessary to attain the SO2 content of 1700 mg/Nm and a level of 94% to reach 400 mg/Nm. ... 

There are, however, technological means available to burn incompletely desulfurized fuels at the same time minimizing SO2 emissions. In the auto-desulfurizing AUDE boiler developed by IFF, the effluent is treated in place by an absorbent based on lime and limestone calcium sulfate is obtained. This system enables a gas desulfurization of 80% it requires nevertheless a relatively large amount of solid material, on the order of 200 kg per ton of fuel. 

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