What is Desulphurisation

45	INTRODUCTION The concept of ‘dry flue gas desulphurization process’ utilizing spray drying technology is relatively new. The acceptance of this technology can be associated with its cost effectiveness, simple operation, and ability to use standard carbon steel as material of construction, high desulphurization efficiency, and absence of any secondary pollution involving the dry waste product. The spray dryer absorber produces in the desulphurization process a free flowing powder and hence offers an excellent alternative to the sludge type waste products produced by wet scrubbers. The temperature of flue gas should be sufficiently high to achieve free flowing powder from liquid alkaline solution. This process can treat flue gas generated from either low or high sulphur content fuels (0.2% to 6%). The degree of SO₂ removal can be adjusted by the amounts of absorbent used and removal efficiency upto 95% is achievable. Various absorbents can be used; choice depends upon what is locally available and minimum cost to site. Lime (CaO) has been preferred for many plants. Limestone (CaCO₃) is a cheaper alternate. Other possibility include dolomite (CaCO_3. MgCO₃), soda ash (Na₂CO₃), sodium hydroxide (NaOH), and magnesium oxide in a form of magnesite (MgOMgCO₃). Usually the dried powder is used as a raw material in some application to make this process commercially viable. The dried product can be further treated to obtain sellable product like gypsum. The dried product can be used as a landfill for landscaping, a strip mine reclamation etc. since it is environmentally acceptable.
	WORKING PRINCIPLE OF SPRAY DRYER (DRY PRECESS) DESULPHURISATION PLANT See Figure No. 5 Refer figure 5 for schematic diagram of spray chamber desulphurization plant. The flue gases to be scrubbed are introduced in the chamber from the bottom to have counter-current flow pattern. An alkali liquid solution is used as an absorbent to remove sulphur based gases. The alkali solution is pumped and sprayed through nozzle in a typical spray dryer technology fashion to form a cloud of fine droplets. The atomization process creates an extensive droplet surface area for effective chemical reaction (absorption). The flue gas, if it contain fly ash particle having any alkali content, will further enhance the desulphurization process. Evaporation of moisture in the absorbent droplets occurs simultaneously with the gaseous pollution absorption. The atomization process is controlled so that the droplet cloud creates the optimum surface area for absorption, but evaporates to dryness giving sufficient time to allow the absorption process to be completed. The resulting dry powder of alkali salts, fly ash, and a little unreacted absorbent is collected at the bottom of the chamber. The fines get conveyed with the flue gases to the dust collector for final collection. The outlet temperature of flue gases is maintained by controlling the absorbent feed rate. The ‘SO₂ control loop’, monitors the SO₂ level in the exhaust stack and regulates the amount of absorbent passing to the atomizer. This ensures the minimum use of absorbent to achieve the desired degree of desulphurization while securing a dry chamber continuous operation. The discharge is easy to handle powdered waste product. Back pressure control loop ensures constant pressure for smooth running of the equipment on which desulphurization system is installed. The I.D. Fan provides the necessary gas flow at designed pressure drop.
	MAIN COMPONENTS OF SPRAY DRYER (DRY PRECESS) DESULPHURISATION PLANT 1. DRYING CHAMBER : The flow pattern is counter-current i.e. gases are introduced at the bottom of chamber and flow upwards while the solution is sprayed from the top and travel downwards. It gives the necessary volume for spray to get developed and provides adequate residence time for reaction & drying to complete. The bottom cone has included angle of 45° for easy flowing of the powder. Material of construction can be carbon steel. 2. BAG FILTER : To arrest the fines escaping to atmosphere bag filter is provided as dust collector. Typical sketch of bag filter assembly is shown in figure *. For more details please see write-up of ‘Pollution Control Equipments’. 3. SPRAY SYSTEM : It consists of following- FEED PUMP : This is a pump developing required pressure & flow for proper atomization. Standby pump is provided with pressure gauge & switch at the outlet. Pressure indicator is provided at the inlet that indicates chocking of the filter. Pressure gauge is provided at the outlet of the pump. Isolation valves are provided for both the pumps & filters. Needle valves are provided for pressure gauges and switches. NOZZLE ASSEMBLY : This is a hollow cone spray single fluid nozzle. It is used to form fine spray at the throat of venturi. The nozzle is made of tungsten carbide with stainless steel holder. ALKALI FEED TANK : This acts as a tank for alkali solution. The alkali solution can be prepared by dissolving the alkali in solid form in water. The tank is provided with a stirrer to facilitate the solution preparation. The tank is provided with top up facility for water or ready solution. 4. pH CONTROL LOOP : The SO₂ is monitored in the exhaust and the pH of the alkali is adjusted. Higher the SO₂, higher is the pH level. The alkali solution is transferred to the alkali feed tank by pump. To maintain the correct pH metered quantity of solution should be pumped. The pH control loop through solenoid valve controls the amount of alkali fed to the feed tank. 5. BACK PRESSURE CONTROL LOOP :** For smooth working of any equipment, back pressure cannot be exceeded beyond a particular limit. Hence continuos pressure monitoring system is provided at inlet of the desulphurization plant. Pressure transmitter senses the pressure continuously and gives signal to motorized damper valve. The proportionate movement of damper maintains the pressure constant.
	EXCLUSIVE FEATURES OF SPRAY DRYER (DRY PRECESS) DESULPHURISATION PLANT Produces dry waste. There is no sludge or wastewater. Flexible operation, adaptable to varying SO₂ levels and flue gas flows. Material of construction can be Carbon steel. No need for special alloys to prevent corrosion. No risk of flue gas being obstructed by deposits in chamber. High reliability- few moving parts – low maintenance cost. System has low pressure drop. Humidification and close control of flue gas outlet temperature gives higher particulate collection efficiency in the bag filter. Applicable to both low and high sulphur fuels. Automatic system. ‘Back pressure control loop’ provided ensures smooth operation of the upstream equipment. Filters are provided at inlet of pump to avoid clogging of pump. Two filters are provided one working one stand-by with isolation valves, to clean filters without stopping the plant. Level controllers on tank maintain the liquid level constant. No manual fill up required.
	APPLICATION The applications of both Venturi Scrubber & Spray Chamber Desulphurization Plants are given below. To scrub SO₂ from various fuel fired equipments like- Boilers D. G. Sets Furnaces Incinerators Thermic fluid heaters Hot water / air generators To scrub gases other than SO₂. The liquid solution used for scrubbing depends upon the gas to be scrubbed. Hence by changing the liquid solution various gases viz. HCl fumes, CO₂, P₂O₅, etc. can be scrubbed by a similar system.