The following points highlight the two main methods used in the treatment of sulphur from flue gas emissions. The methods are: 1. Dry Methods 2. Wet Processes.
1. Dry Methods:
The dry methods can be divided, broadly, into two types:
(i) Oxidation/reduction methods.
(ii) Metal oxides usage.
This method of oxidation is popularly known as the “Cat-Ox” process and produces sulphuric acid as shown in Fig. 3.16. In this process fly ash is first removed from the flue gas by a high temperature electrostatic precipitator. SO2 is then catalytically oxidised to SO3 and recovered as sulphuric acid. If the exit gases are at a lower temperature, they may be heated. V2O5 at 400-500°C is used as a catalyst for good conversion efficiency.
In a modified oxidation process, SO2 and oxygen present in the stack gas are absorbed on the surface of an active carbon catalyst which catalyses the oxidation of SO2 to SO3. SO3 reacts with the moisture present to form H2SO4 in the pores of active carbon. The combined effect of absorption and catalysis by the active carbon leads to a complete conversion of SO2.
This process is unique in that it utilises fluidised beds of high efficiency activated carbon and converts H2SO4 to sulphur as shown in Fig. 3.17. Flue gas is contacted with activated carbon in the absorber unit and SO2 is oxidised to SO3.
The carbon which is used as a catalyst is fed to a sulphur generator where it is contacted with H2S to form sulphur.
H2SO4 + 3H2S → 4S + 4H2
A fraction of the sulphur is recovered by vapourisation and is re-condensed as a molten product. The remaining fraction of sulphur reacts with hydrogen in a hydrogen sulphide generator to form H2S.
In the ‘Scot’ (Shell Clauss Off-gas Treatment) process, developed in the Netherlands, almost complete removal of sulphur or its compounds is possible. The gases are reduced while sulphur and its compounds are converted to H2S. In this process SO2 reacts with copper oxide at a temperature of about 400°C to form copper sulphate.
SO2 + 1/2O2 + CuO → CuSO4
Copper sulphate is then reduced at the same temperature in a hydrogen rich gas –
Cu SO4 + 2H2 → Cu + SO2 + 2 H2O
The concentrated SO2 is then sent to a Clauss sulphur recovery plant. The Clauss off gas is heated with the addition of a reducing gas (H2, CO) and passed through a reactor containing a cobalt-molybdenum catalyst. The absorber used for removal of H2S is often a diisopropanolamine (DIPA) solution SO2 concentrations of less than 250 ppm can be obtained with this process.
(ii) Use of Metal Oxides:
In this process sodium aluminate is used to remove SO2 in a fluidized bed. In this process, developed by U.S. Bureau of Mines, the dust free flue gas is fed to a reactor wherein the absorbent, a porous form of sodium aluminate (Na2O. Al2O3), adsorbs SO2 at a temperature of 300°C.
Na2O.Al2O3 + SO2 + 1/2O2 → Na2SO4 + Al2O3
The product from the above reaction is contacted with a reducing gas such as hydrogen in a regenerator at about 680°C to form hydrogen sulphide.
Na2SO4 + Al2O3 + 4H2 → Na2O.Al2O3 + H2S + 3H2O
From this the sodium aluminate is recycled and hence the process is called a cyclic adsorption process. Sulphur can also be produced as a by-product from the H2S gas by sending it into a Clauss unit.
2H2S + SO2 → 3S + 2H2O
However, maintenance of the granular strength of sorbent is the main problem relating to this process. The rigorous temperature and chemical cycling, to which the sorbent is subjected, deteriorate the sorbent by causing crystalline growth and loss of surface area.
2. Wet Processes:
The main unit responsible in a wet process is the scrubber, a spray tower, a cyclone scrubber, a venturi scrubber or a packed tower. Most of the currently available wet flue gas desulphurisation methods use slurries of compounds of calcium, magnesium and sodium.
In the wet process the treated gases are kept at low temperatures in the range of 25 – 50°C. This creates a problem in the dispersion of the flue gas. Hence reheating is required which consumes heat in the range of 3% of the heat of fuel used in the combustion chamber.
Some of the wet processes are described as follows:
i. Calsox Process (Lime and Limestone Scrubbing):
Flow diagram of this process is shown in Fig. 3.18. The flue gas is scrubbed with a 5 to 15 per cent slurry of calcium sulphite/sulphate salts which also contains amounts of lime (CaO) and limestons (CaCO3). The SO2 reacts with the slurry to form additional sulphite and sulphate salts.
The solids are continuously separated from the slurry and discharged into a settling pond. The remaining liquor, at a pH of 6 to 8, is recycled to the scrubbing tower after fresh lime or limestone has been added. A schematic representation of the process is shown in figure 3.18.
Although overall mechanism for the process is complicated, the following reactions are likely to occur:
The problems in this process include scaling, corrosion, erosion and solid waste disposal. A sizeable disposal area adjacent to plant is required. Another drawback of the process is the necessity for reheating the cleaned flue gas.
Reheating is accomplished by installing a gas cooler before the scrubber and a gas stack heater after the scrubber. Thus two additional units must be provided to the gas flow system.
ii. Chemico Process (Magnesium Oxide Scrubbing):
In this process, known as the “Chemico Process”, magnesium oxide acts as a venturi scrubber by absorbing SO2 and generating magnesium sulphite and sulphate.
The mixed sulphite/sulphate slurry along with unreacted MgO is separated from liquid phase in a centrifuge.
MgSO3 → 800°C → MgO + SO2
This process is a regenerative process as the mother liquor is regenerated. Concentrated SO2 (10 to 15%) evolved from the flue gas is used in making elemental sulphur or reprocessed to manufacture H2SO4 as shown in Fig. 3.19.
iii. Welman Lord (Single Alkali) Process:
This process removes sulphur dioxide by washing fuel gases with an aqueous solution of sodium sulphite. It is quite a common practice in chemical industries, in which 90% of desulphurisation is possible. In this process sulphite is converted to bisulphite as the SO2 from flue gases is absorbed by a saturated solution of sodium sulphite as shown in Fig. 3.20.
Na2O3 + SO2 + H2O → 2 NaHSO3
The sodium bisulphite formed is led to a double effect evaporator-cum-crystalliser where it decomposes into sodium sulphite at a temperature of around 100°C. This results in the ejection of SO2 and steam.
2 NaHSO3 → 100°C → Na2SO3 + SO2 + H2O
Fly-ash is removed before the SO2 scrubbing to keep the rate of its accumulation in the scrubbing-liquid low. SO2 and water vapour released from the evaporators are passed into a condenser and the product goes to the dissolving tank for dissolution of Na2SO3 crystals and the rich SO2 gas is processed. Sodium sulphate is produced in this reaction which is removed and substituted by an equivalent amount of NaOH.
Clear solutions of either sodium or ammonia are excellent absorbers of SO2. The regeneration step can be carried out at a relatively low temperature in a liquid system. The one advantage that sodium scrubbing has over ammonia is that the cation is non-volatile. Fume development is another problem in almost all ammonia scrubbers. Both processes produce an unavoidable side product, sodium sulphate in one case and ammonium sulphate in the other.
iv. CuO/CuSO4 Process:
This process removes NOx and SOx simultaneously by using copper oxide (CuO) supported on stabilised alumina placed in two or more parallel passage reactors. The reactions, which characterise process operation, can be expressed as –
Flue gas is introduced at about 100°C into one of the reactors where SO2 resets with CuO to form copper sulphate (CuSO4). The CuSO4 and to a lesser extent, the CuO act as catalysts in the reduction of NOx with NH3.
When the reactor is saturated with CuSO4 flue gas is switched to a fresh reactor for acceptance of the flue gases and the spent reactor is regenerated. In the regeneration cycle hydrogen (H2) is used to reduce CuSO4 to copper (Cu), yielding an SO2 steam of sufficient concentration for conversion to sulphur or sulphuric acid.