There are six procedures for controlling the emissions of SOx.
They are either in-plant control measures or effluent treatment methods:
1. Natural dispersion by dilution.
2. Using alternate fuels.
3. Removal of sulphur from fuels desulphurisation.
4. Process modifications.
5. Control of SOx in the combustion process.
6. Treatment of flue gas emissions.
The different control methods are described as follows:
1. Natural Dispersion by Dilution Using Tall Stacks:
A control method involving dispersion of a pollutant from tall stacks seem archaic. Nevertheless, there is considerable controversy over this technique as opposed to flue-gas desulphurisation. The control method is based on natural dispersion at high elevation so that the ground level concentrations are acceptable at all times.
In India, minimum stack heights of 30 m are recommended. Similarly, the height of chimney, H required for effective dispersion of Q kg/hour of SOx emissions is given by, H = 14 Q0.3. This condition may demand stack heights of 400- 450m also.
For example, the TVA Cumberland Power Station in Tennesse has twin 300m concrete chimneys while International Nickel Company has built one that is 380m high, only to reduce concentrations by dilution in atmosphere. It is also a common practice to stop discharging the effluents into the atmosphere during adverse meteorological conditions. However, this technique is not possible in large scale power plants etc.
2. Using Alternate Fuels:
A switch to natural gas from the conventional high-sulphur fuels like coal and petroleum to lessen SOx emissions is an available alternative. Liquefied Natural Gas, LNG also is an effective alternative. However, for utility use, cost will be much higher than that of other alternatives. Low sulphur coal is another alternative, but obtaining low sulphur coal from the ground is neither quick nor cheap.
3. Removal of Sulphur from Fuels:
The process of removing sulphur prior to combustion is theoretically attractive but practically ineffective. Coal consists of sulphur in both organic and inorganic forms. The inorganic form of sulphur is iron disulphide (FeS2) mostly available in the forms of pyrites and marcasites.
Apparently, washing seems to be an effective process with more than 30% of sulphur being removed. But this results in a loss of combustible material and may increase the requirement of coal and thus the cost. Organic sulphur is present in the form of cystine, thiols, sulphides and some other cyclic compounds, which can only be removed by chemical processing.
Hydro-desulphurisation of coal using a solvent extraction process can remove both organic sulphur as well as inorganic sulphur. In this process finely ground coal is mixed with anthracene oil to form slurry and this slurry is heated at a very high temperature of 450°C. The ash residue consisting of both organic and inorganic forms of sulphur is eliminated by pressure Alteration.
To avoid repolymerisation a small amount of hydrogen is also added. The coal solution filtrate is sent into a chamber to remove lighter fractions. The hot liquid residue is cooled to a brittle solid fuel which can be pulverised. The product will be liquefied at about 450° C. It has a higher heating value than raw coal, and contains less than 1% of sulphur.
Coal gasification is another process widely used for the Indian coal reserves. There are several processes to convert coal into gaseous form. The earlier processes like water-gas and producer-gas processes produced gas of lower quality. The new processes are more effective which give a better quality besides removing both organic and inorganic types of sulphur.
Catalytic hydrogenation of coal suspended in tar at 100-250 bars at 450°C can achieve 75 per cent desulphurisation with the consumption of 20 kg hydrogen per tonne of feed coal. About 5 per cent of this hydrogen is transformed to hydrogen sulphide, an unwanted by-product. Similarly, impure natural gas can be freed of hydrogen sulphide by scrubbing with monoethanolamine or other amines.
The sulphur can be recovered by dry catalytic conversion when molten sulphur is obtained. Usually 0.5-5 per cent of sulphur is present in heavy fuel oils. The hydro-desulphurisation process is used for desulphurisation of fuel oil. In this process high temperatures are maintained between 400°C – 550°C. The fuel oil is treated with hydrogen in the presence of a catalyst. The residue is eliminated by pressure Alteration with pressures ranging from 35 to 70 atmospheres.
The sulphur is recovered by sending it into a flash evaporator where lighter fractions are removed. Almost all coals dissolve in solvent oils at high temperatures (550°C) and pressures (75 bars) in the presence of hydrogen which prevents polymerisation and helps in the removal of organic sulphur. Coal gasification appears to be a promising method for the abundant coal reserves in India. Oil gas can be scrubbed free of sulphur and can be used in gas-burning devices.
4. Process Changes:
Process changes involve new or modified techniques lowering atmospheric pollutant emissions. On similar lines, DCDA (Double Catalysis Double Absorption) process was developed and was found to reduce SO2 emissions from 2000-5000 ppm to less than 500 ppm in the stack gases as shown in Fig. 3.15.
Most of the conventional sulphuric acid plants till 1970 were designed for about 95% conversion of SO2 in four stages and proved to be an effective process as this was the optimum level of oxidation. However, this led to concentration of SO2 in stack gases in the region of 2000 ppm and more. The present desirable concentration of 500 ppm can easily be achieved with a double stage contact conversion. This technique is known as Double Catalysis Double Absorption (DCDA) process.
Earlier, in the conventional contact processes SO2 produced by the burning of sulphur with pre-heated air was converted to SO2 by passing over vanadium pentoxide catalyst in four stages. The gas was cooled and absorbed with high efficiency in a circulating stream of 98 to 99% sulphuric acid. This results in the effluent gas with high SO2 concentrations which is usually above the permitted standards. Hence tail gas scrubbing was further necessary for bringing the SO2 concentration to acceptable limits.
This problem led to a modern modification of contact plant which is called the DCDA process. In this process sulphur is burnt with air in a horizontal spray – type combustion chamber. The emitted SO2 gas from the sulphur burner has a very high temperature. It is cooled in the waste heat boiler, which recovers surplus heat as the by-product, steam.
From the waste heat boiler, the gas flows to the converter system in different stages. The gases from the converter, after 90% of SO2 has been converted to SO3, are interrupted and passed at an intermediate stage to an absorber to remove SO3. The gases are then reheated and returned to the converter for further conversion.
They then pass through the additional catalyst, are cooled, pass through a second absorber and then to the atmosphere. The unconverted gases, after being heated by the gases entering the absorber, are returned to the next stage of converter. As a result, the overall SO2 conversion efficiency increases.
It is also possible to use higher inlet concentrations of SO2 (10-12%) as against the usually employed 8% concentration. The thermal efficiency of the system can further be improved with suitable waste heat recovery methods like utilising heat from sulphur burners or heat from oxidation of SO2 to SO3.
At high pressures of about 22 atm. and 9% SO2, conversion efficiencies of upto 99.7% with SO2 concentration as low as 40 ppm can be achieved. The use of oxygen instead of air further decreases the plant size and volume of gases. Thus the DCDA process requires just one extra absorption tower than the conventional plant.
It has been established that any plant of capacity 50 tonne of acid per day and above can be converted into a double absorption system economically. Another advantage of DCDA process is that it can be adopted to a wide range of SO2 concentrations. This process is proved to be economical and may be strongly recommended for almost all the H2SO4 manufacturing units.
5. Control of SOx in the Combustion Process:
Here, finely powdered limestone is injected directly into the conventional combustion chamber. The limestone is calcinated to CaO by the heat of combustion and it reacts with SO2 contained in the flue gas to form sulphites and sulphates. The unreacted materials and fly-ash are removed by dry collectors.
The formation of CaSO4 is most favoured at temperatures above 1000°C. Due to the smaller residence time in combustion zone limestone does not completely react with SO2 to produce a stoichiometric yield of CaSO4. At temperatures above 1200°C, CaSO3 is unstable and SO2 removal by sulphite formation is inhibited.
As a result, the process removes less than half of the sulphur oxides. Due to this, the dry limestone injection process, although relatively simple and easy to operate is not very attractive from the point of view of control technology. Research is going on to increase the efficiency of this process and other widely used processes like the fluidised bed combustion process.
In this process limestone and crushed coal together form the fluidised bed and oil is used as the fluidising medium. The operating temperatures are of the order of 700 – 1000°C since the fluidised beds are capable of transferring high heat. This process proves to be quite effective as it removes more than 90% of sulphur.
In addition to high SO2 removal efficiencies, the fluidised bed combustion process prevents the onset of ash fusion and as a result, the fouling and corrosion of boiler tubes associated with the molten slag are considerably reduced. Also the formation of nitrogen oxides by the nitrogen fixation reaction is reduced.
The method also has some drawbacks. It requires design modification for boilers and also additional installations for the preparation of limestone. If the limestone is ground to the same size as coal then a practically inseparable mixture of ash and lime is produced. The fine ash can be elutriated and partially sulphated lime is regenerated.
2 CaSO4 + C → 2 CaO + 2 SO2 + CO2
This regeneration, which requires temperatures in excess of 1000°C, would substantially suppress the consumption of limestone.
6. Treatment of Sulphur from Flue Gas Emissions:
Progress in developing satisfactory desulphurisation processes for flue gases has been extremely slow because of the complexity and magnitude of the problem. The technical and economic feasibility of most desulphurisation processes are closely related to plant size and location. It seems unlikely that a single desulphurisation method will be developed that is capable of controlling effluents from all types of sources.
The control techniques to be employed depend upon such factors as boiler size, configuration, load pattern, geographical location and the like. Nearly fifty flue gas desulphurisation processes have been proposed, but as such no ideal process exists. The general classification of these processes may be wet and dry.