4.2.1. Formation of sulphur compounds in combustion environments

4.2.1.a. Sulphur monoxide (SO)

O + S₂ SO + S reac 40

hydrogen sulphide

O + H2S OH + SH	reac 41
O + SH SO + H	reac 42

carbonyl sulphide

O + COS SO + CO reac 43

carbon disulphide

O + CS₂ CS + SO reac 44

SO thus formed is highly reactive and has a lifetime of about a few milliseconds.

4.2.1.b. Sulphur dioxide (SO₂)

O + SO + M SO₂ + M + hv reac 45

Some authors postulate that the formation of SO₂ is actually a process in two steps (Cullis and Mulcahy (1972)) :

O + SO + M + M	reac 46
SO2 + hv	reac 47
+ M SO2 + M	reac 48

SO₂ is formed in the flame at rates comparable to H₂O, and faster than CO₂ (Harris (1990)), its concentration rapidly rising to between 200 and 2,000 ppm.

SO₂ may undergo reaction with water in order to form sulphurous acid H₂SO₃ (Harris (1990)):

SO₂ + H₂O H₂SO₃ reac 49

However, the amount of H₂SO₃ formed does not represent a problem as its solubility in water is very small at low temperatures and it decreases at higher temperatures (10 % at 20 °C and only 0.58 % by weight at 90 °C). Its solutions are mainly composed of hydrogen sulphite ions () and a small percentage of sulphite ions ().

4.2.1.c. Sulphur trioxide (SO₃)

In gas turbines the concentration of sulphur trioxide in the exhaust gases is normally between 7 and 11 ppm (Gills (1973)) due to high dilution. However, cases have been reported (Hunter (1982)) of conversion up to 20 %.

Thermodynamic calculations show that the theoretical conversion of SO₂ into SO₃ at flame temperatures is less than 0.1 %. The direct formation of SO₃ by oxygen attack

SO2 + O SO3	reac 50
SO2 + O2 SO3 + O	reac 51

is almost completely displaced to the left at typical combustion temperatures (over 1,000 °C), thus conversion of SO₂ into SO₃ is very small. In addition, the reaction is extremely slow at such high temperatures, (Hunter (1982)) although it can be catalysed by oxides of W, V, Mo, Cr, Ni and Fe.

However, in practical systems the amount of SO₃ produced in the flame is greater than predicted by the equilibrium between SO₂ and oxygen at flame temperatures. The reaction is thus assisted by labile species in the flame.

The non-linear dependence of the reaction rate on SO₂ concentration suggests that a collisional reaction would be involved with the formation of an activated sulphur trioxide intermediate (Cullis and Mulcahy (1972)), which eventually yields SO₃:

O + SO2	reac 52
+ M SO3	reac 53

The three-body character of this reaction is demonstrated by its dependence on pressure.

SO₃ is also involved in concurrent formation and disappearance reactions (Glassman (1986), Hunter (1982)). Apart from thermal decomposition, SO₃ is reduced by:

O + SO3 O2 + SO2	reac 54
H + SO3 OH + SO2	reac 55
SO3 + H2 SO2 + H2O	reac 56

thus reducing radical concentrations. Simultaneous combustion of CO in the presence of the oxidation of SO₂ to SO₃ enhances such a process (Hedley (1967)) . Every time one CO molecule is burned one oxygen atom disappears but two of them are formed, thus a surplus of oxygen atoms is created.

The conversion of SO₂ into SO₃ is affected by several factors (Harris (1990)) :

• Excess oxygen (Hedley (1967)) : At equivalence ratios above stoichiometric no SO₃ is formed, whereas increase of oxygen from 0 to 1 % excess causes a sharp increase and no significant rise after that (Cullis and Mulcahy (1972)) .

• Sulphur content in the fuel: The lower the S content, and thus the lower the SO₂ content, the higher the conversion to SO₃. The SO₃ conversion is not a function of the initial SO₂ concentration.

• Catalytic substances: It has been suggested (Glassman (1986)) that in combustion of residual oils heterogeneous catalysis by steel surfaces and vanadium pentoxide plays an important role in SO₂ oxidation to SO₃. Experimental evidence (Hedley (1967)) has been collected of increased SO₃ formation by ash deposits and other solid particulates. Cunningham Error! Reference source not found. reports that SO₃ formation in large boilers is catalysed by ash and coke deposits on the heating tube banks (900 - 1,200 °C). The fact that conversion drops when such deposits are removed provides evidence of the former.

• Combustion and mixing processes

• Dwell time of the flue gases in the system

• Combustor design.

where:
: mole fraction of SO₃
: mole fraction of SO₃ initially present, at t = 0
: equilibrium SO₃ mole fraction, which is a function of equivalence ratio, pressure and temperature, so that at high temperature its values are low.
: relaxation time =
k_-50: reverse reaction constant for reaction 50 = 7.41×10²⁰ T^-1 exp(-82,689/RT), cm³ mol^-1 s^-1
k₅₄: reaction constant for reaction 54 = 1.2×10¹² exp(-9,500/RT), cm³ mol^-1 s^-1
X_O: O-atom concentration

Typical residence times in the dilution zone of a gas turbine combustor are in a range from 1 to 100 ms, and measured values of SO₃ range from 2 to 8 % of SO_X. These values are similar to those calculated by Hunter's model: For relaxation times between 0.3 and 30 ms equilibrium levels of SO₃ range from 2 to 8 % of SO_X.

Hedley (Hedley (1967)) performed experiments in a monodimensional boiler and found SO₃ concentrations in the flame stage to be much higher than theoretical equilibrium values, decaying afterwards to remain lower than those of theoretical equilibrium.

By assuming that an activated SO₃ molecule was formed and decomposed by means of

reac 57

a maximum concentration of sulphur trioxide was calculated

which is reached at a time

However this model is only applicable to isothermal conditions as the rate coefficients are temperature dependent.

Formation of SO₃ in combustion systems is undesirable because it forms H₂SO₄ with H₂O:

SO3 + H2O H2SO4	reac 58
H2SO4 + 2 H2O 2 H3O+ +	reac 59

As the combustion gases exit from the combustor their temperature decreases and H₂SO₄ is formed which eventually condenses at a suitable temperature, the acid dew point. Sulphuric acid causes corrosion on Ni-based alloys used in boilers and in other equipment located downstream from the combustor, such as that in combined systems.

The acid dew point is defined (Harris (1990)) as the highest temperature at which H₂SO₄ vapour (SO₃ + H₂O) is in equilibrium with H₂SO₄ liquid at a given pressure. Typical values are around 130 °C, and they increase (ie condensation is more easily attained) as the amounts of SO₃ and moisture in the exhaust increase. Also dust and solid particulates in the gas exhaust (Cunningham (1978)) can increase the acid dewpoint.

An expression for calculation of acid dew points was given by Verhoff and Banchero (Harris (1990)):

where
T_DP: dew point
P: partial pressure

SO₃ is a well-known promoter of carbon formation when added in small amounts (approximately 0.1 %) (Cullis and Mulcahy (1972)). However, if added in larger proportions (several percent) along with hydrogen sulphide it helps to decrease carbon formation.

If carbonaceous deposits are present, sulphuric acid may condense in corrosive particles forming acid smuts, especially below 1 % excess oxygen (Cunningham (1978)) where particulate emissions are high. A threshold of 3 ppm SO₃ has been found for the formation of acid smuts. Additives like ammonia, dolomite (CO₃CaMg) and oxides (MgO), hydroxides and carbonates of magnesium have been used to neutralise H₂SO₄, as emissions of Mg or MgSO₄ have little environmental effects.

Experimental trials with sorbents like dolomite, hydrated lime and pressure hydrated dolomite lime for sulphur control were made by Cowell et al. (Cowel et al. (1992)). The amount of reduction achieved at a Ca/S ratio of 3 ranged from 55 % for dolomite, 82 % with pressure hydrated dolomite and 90 % with hydrated dolomite. Dolomite is economically most favoured among the three.

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