Technologies for the Control of Sulphur Dioxide Emissions From Coal /Pet Coke Fired Boiler

Technologies for the Control of Sulphur Dioxide Emissions From Coal /Pet Coke Fired Boiler

Prof .Veena. A Shinde a, Mr.Kushal V. Kadam b

a Professor of Chemical Engineering Department, Bharati Vidyapeeth College of Engineering, Pune, Maharashtra

b PG Student of Chemical Engineering Department, Bharati Vidyapeeth College of Engineering,Pune,Maharashtra

Emissions of sulfur dioxide after burning the coal / pet coke in boiler causes serve damage not only to the environment, historical monuments like TajMahal but also to the human health. Because of the ecological and human health impacts of sulfur dioxide regulatory standards have been set to bring down the SO2 emission into the atmosphere. This paper presents a review of methods of flue gas desulphurization ( FGD) processes for the reduction of the emission of SO2 with recovery of an economical by-product ,selection of flue gas desulphurization technology and provides a description of results of the limestone based flue gas desulphurization installed on the pet coke fired boiler . Among the various flue gas desulphurization processes the most widely used one is the limestone based flue gas desulphurization process because reagent limestone is easily available and cheap also which produces saleable by-product gypsum.

Keywords: – flue gas desulphurization, gypsum, limestone, SO2.

1. Introduction

Sulphur dioxide is the major pollutant which causes air pollution in urban areas which in turn contribute to acid deposition that results in influencing climatic changes. Most of Asian sulphur emissions originate from coal combustion, which satisfies at present about 80% of the energy demand in the region[1]. flue gas desulphurization is widely applicable as means of controlling SO2 emissions from power stations. The flue gas desulphurization together with measure to reduce SO2 emissions from power stations will significantly reduce sulphur emissions to meet central pollution control board norms for SO2 [2].Emissions of SO2 can be controlled in several ways. It may be possible to switch to a fuel or ore that has

lower sulphur content, or improve the efficiency of the industrial process so that less fuel is required. The sulphur in the fuel or ore can in principle be removed before use however, in practice it is uneconomic to remove more than a small percentage of the sulphur. The sulphur can also be removed during use. However, in many applications, the most efficient means of controlling SO2 emissions is to remove the SO2 from the flue gases before they are released to the atmosphere by using flue gas desulphurization technology[3].

1. Central pollution control board norms for SO2

   The Central Pollution Control Board of India has set three different standard for SO2 in the ambient air

   120 µg/ m3 for industrial areas, 80 µg/ m3 for

   residential areas and 30 µg/ m3 for sensitive areas as an annual average (Annual Arithmetic mean of minimum 104 measurements in a year taken twice a week 24 hourly at uniform interval.) Annually the average of these areas should not exceed 80 µg/ m3,60 µg/ m3, and 15 µg/ Nm3 as an 24 hrs.(24 hourly/8 hourly values should be met 98% of the time in a year. However, 2% of the time, it may exceed but not on two consecutive days) [4].

2. Fundamentals of FGD

   All commercial FGD processes are based on the fact that SO2 is acidic in nature and remove the SO2 from flue gases by reaction with a suitable alkaline substance. The commonly used alkaline materials are limestone (calcium carbonate). Because of limestone is an abundant and relatively cheap material than other alkalis such as sodium carbonate, magnesium carbonate and ammonia which is expensive than limestone .The alkali used reacts with SO2 in the flue gas to produce a mixture of sulphite and sulphate salts (of calcium, sodium, magnesium or ammonium, depending on the alkali used). The proportions of sulphite and sulphate are depending on the process conditions. The reaction between the SO2 and the alkali can take place either

   in solution called wet flue gas desulphurization processes or at the wetted surface of the solid alkali called dry and semi-dry flue gas desulphurization processes)[3].

   In wet flue gas desulphurization systems, the alkali usually in a solution or more slurry form and flue gas are contacted in a spray tower. The SO2 in the flue gas dissolves in the water to form a dilute solution of acid that then reacts with alkali. The sulphite and sulphate salts produced precipitate out of solution, depending on the relative solubility of the different salts present. Calcium sulphate for example is relatively insoluble and readily precipitates out. Sodium and ammonium sulphates are very much more soluble[3].

   In dry and semi-dry systems, the solid alkali is brought into contact with the flue gas, either by injecting or spraying the alkali into the gas stream or by passing the flue gas through a bed of alkali. In either case, the SO2 reacts directly with the solid to form the corresponding sulphite and sulphate. The solid produce quite porous and finely divided. In semi-dry systems, water is added to the flue gas to form a liquid film on the particles in which the SO2 dissolves, promoting the reaction with the solid. [3]

3. Selection of FGD Process

   The selection of FGD processes by differentiating the parameter as sorbent used, by-products produced, removal efficiency and capital cost. Selection of the most appropriate FGD process for a particular application will normally be made on economic grounds, i.e. the process with the lowest overall through-life cost. However, there are many different factors that affect the overall cost.

   These include:

   • Technical Consideration.

   • Economic Issues

     • Operating costs

     • Capital costs.

   • Commercial Consideration

Technical considerations include the efficiency of desulphurisation process that can offer the flexibility of the process, the space availability that the FGD plant requires and the technical risks.

Economic issues include the capital and operating costs, including the cost of the plant, the costs of the sorbent used any revenues or expenses arising from disposal of the by-products and maintenance costs.

Commercial considerations include the commercial risk, the maturity of the technology, the number and size of units already in operation and performance of process and suppliers guarantees. [2]

2. Methods of Flue gas desulphurization

1. Wet Flue gas desulphurization

   1. Limestone Process

      Process Description:-

      In the wet limestone process, the incoming flue gas from boiler after ESP / Bag Filter is brought into contact with aqueous slurry of limestone in a scrubber tower. Sulphur dioxide in the gas reacts with the slurry to form sulphite of calcium and then oxidizes in oxidation tank to produce Gypsum, which is continuously removed from the oxidation tank in the form of slurry. This slurry is passed through hydro cyclones which separates heavier gypsum particles which are further sent to filtration plant where gypsum is removed in the form of flakes ( 10 -15 % moisture) and filtrate is recycled back to the process [5].

      Chemical Reactions:-

      CaCO3+SO2+½ H2O CaCO3. ½ H2O + CO2 CaSO3.½H2O +3/2 H2O + ½ O2 CaSO4.2H2O

      Flue Gas Inlet

      Flue Gas

      Outlet

      Flue Gas Inlet

      Flue Gas

      Outlet

      Limeston

      Water

      Limeston

      Water

      Gypsum

      Gypsum

      Figure 1. Limetone process

   2. Sodium Process

      Process Description:-

      In the Sodium process, the incoming flue gas from is brought into contact with an aqueous slurry of NaOH / NA2CO3 in tower. Sulphur dioxide in the gas reacts with the slurry to form Sodium Bisulphite which is continuously removed from the tank. In large scale systems, the by-product is often sent directly to evaporation ponds. In smaller industrial plants, the by-product is frequently sent to a wastewater treatment plant or discharge after neutralization and oxidation [7].

      Chemical Reactions:-

      Na2CO3 + 2SO2 + H2O 2NaHSO3 + CO2

      OR

      2NaOH + SO2 Na2SO3 + H2O Na2SO3 + SO2+ H2O 2NaHSO3

   3. Ammonia Process

      Process Description:-

      The ammonia/ammonium sulphate or ammonium scrubbing process works in a similar way to the limestone gypsum process except that aqueous ammonia is used as the scrubbing agent. SO2 is removed from the flue gas by reaction with ammonia, and the final product is ammonium sulfate[8].

      Chemical Reactions:-

      2NH3 + SO2 + H2O (NH4)2SO3 (NH4)2SO3 + ½ O2 (NH)2SO4

   4. Seawater Process

Process Description:-

There are two basic seawater FGD process concepts: one uses the natural alkalinity of the sea water to neutralize absorbed SO2 and other uses added lime. All Commercial Sea Water FGD processes rely on the alkalinity on the bicarbonate in sea water to neutralize the SO2 there by producing sulfite or sulfate [2], [10].

Chemical Reactions:-

– +

– +

– 2 – +

– 2 – +

SO2 + H2O HSO3 + H HSO3 + ½ O2 SO4 + H

Flue Gas Inlet

Flue Gas Inlet

Flue Gas

Outlet

Flue Gas

Outlet

Sodium Hydroxide /

Soda Ash Water

Sodium

Bisulphit

Sodium Hydroxide /

Soda Ash Water

Sodium

Bisulphit

Figure 2. Sodium process

Flue Gas

Inlet Flue Gas

Outlet

Water

Disposal

To Spray Tower

To Spray Tower

Ammonium Sulfate

Figure 3. Ammonia process

1. Semi Dry Flue Gas Desulphurization

   1. Spray Dryer Process Description

      In spray dryer processes, sulfur dioxide is removed from the flue gas by contact with an Atomized spray of reactive absorbent such as lime slurry or sodium carbonate solution. The

      Sulfur dioxide reacts with the absorbent while the thermal energy of the flue gas vaporizes

      The water in the droplets without saturating the flue gas to produce a fine powder of spent Absorbent. The dry product, consisting of sulfite and sulfate salts, unreacted absorbent, and fly ash, is collected in a fabric filter or electrostatic precipitator (ESP)[6],[2].

      Chemical Reactions:- CaO + H2O Ca(OH)2

      Ca(OH)2 + SO2 CaSO3 + H2O

2. Dry Flue Gas Desulphurization

   1. Furnace Sorbent Injection

Process Description:-

In the furnace sorbent injection is a technique in which lime or limestone is injected directly into the section of the furnace where temperature ranges between 950 oC to 1000oC.Hydrated lime or limestone decomposes when exposed to furnace temperature and becomes porous solid with high surface area. The reactive sorbent captures SO2 in suspension to form calcium sulfate and remaining unreacted sorbent are Aeration oxidizes all the calcium sulfite to calcium sulfate and forces precipitation to

occur on existing gypsum crystals in the reaction tank. This minimizes tendency for gypsum to precipitate on surfaces in the absorber and cause plugging of pipes and nozzles by maintaining gypsum concentration in absorber.

carried out of the furnace by the flue gas and collected in a fabric filter or electrostatic precipitator (ESP) [6], [5].

Chemical Reactions:- CaCO3 CaO + CO2

CaO + SO2 + ½ O2 CaSO4

3.Experimentation

Wet Limestone Flue Gas Desulphurization Process

The wet limestone flu gas desulphurization process as demonstrated at Shree Cement 44 MW power plant A simplified explanation of the SO2 absorbed in there circulated slurry reacts with dissolved limestone (CaCO3) in the slurry to form calcium sulfite hemihydrate (CaSO3 · ½H2O) according to the following reaction:

SO2 + CaCO3 + ½ H2O CaSO3 · ½H2O + CO2

Carbon dioxide formed from reaction of limestone with SO2 is released into the flue

gas. Oxidation air is bubbled through the slurry to convert CaSO3·½H2O to gypsum

(CaSO4·2H2O) according to the following reaction:

CaSO3 ·½H2O + ½O2 + 3/2 H2O CaSO4 · 2H2O

Flue Gas Outlet

Sea Water

Flue Gas Inlet

Air

Discharge to Sea

Air

Flue Gas Outlet

Sea Water

Flue Gas Inlet

Air

Discharge to Sea

Air

Water

Water

Figure 4 . Seawater process

Lime

Flue Gas

Outlet

Lime

Flue Gas

Outlet

Flue Gas

Inlet

Flue Gas

Inlet

Disposal

Disposal

Figure 5. Spray dryer process

1. The individual steps involved in the removal of SO2 from gas streams by the limestone process may be summarized as follows:[2].

   1. Transfer of SO2 in the gas phase to the gas liquid interface.

   2. Dissolving SO2 into water at the interface.

   3. Ionization of dissolved SO

      2.

   4. Transfer of H+, HS0 -, and SO 2- ions

      3

      from the interface interior.

      3

      into the liquid

      Table 1. Operating parameters

   5. Dissolving and ionization of Ca(OH)2 or CaCO3 to form Ca2+.

      Sr. No	Part Description	MOC
      A ]	Slurry preparation and transfer circuit
      1	Lime Stone slurry preparation tank	Carbon Steel+ Chlorobutyl Rubber (Thickness 5 mm)
      2	Lime Stone slurry transfer Pump	Alloy Steel
      B ]	Flue gas circuit
      1	Diverter Valve	Carbon Steel
      2	Booster Fan	Carbon Steel
      C]	Desulphurisation circuit
      1	Spray Tower	Carbon Steel + Chlorobutyl Rubber (Thickness 3 mm)
      2	External Structural Supported Chimney tower	Carbon Steel + Natural Hard Rubber ( Thickness 3mm)
      4	Spray Headers	Carbon Steel + Chlorobutyl Rubber (Thickness 5 mm)
      5	Slurry Spray Nozzles	Silicon Carbide
      6	Mist Eliminator	PP
      7	Mist Eliminator Washing System	Carbon Steel + Natural Rubber ( Thickness 3mm)
      8	Slurry recirculation Pump	High Chromium Alloy
      9	Oxidation Tank	RCC + Chlorobutyl Rubber (Thickness 10 mm)
      10	Oxidation Pipe	Carbon Steel + Chlorobutyl Rubber (Thickness 5 mm)

      Sr. No	Part Description	MOC
      A ]	Slurry preparation and transfer circuit
      1	Lime Stone slurry preparation tank	Carbon Steel + Chlorobutyl Rubber (Thickness 5 mm)
      2	Lime Stone slurry transfer Pump	Alloy Steel
      B ]	Flue gas circuit
      1	Diverter Valve	Carbon Steel
      2	Booster Fan	Carbon Steel
      C]	Desulphurisation circuit
      1	Spray Tower	Carbon Steel + Chlorobutyl Rubber (Thickness 3 mm)
      2	External Structural Supported Chimney tower	Carbon Steel + Natural Hard Rubber ( Thickness 3mm)
      4	Spray Headers	Carbon Steel + Chlorobutyl Rubber (Thickness 5 mm)
      5	Slurry Spray Nozzles	Silicon Carbide
      6	Mist Eliminator	PP
      7	Mist Eliminator Washing System	Carbon Steel + Natural Rubber ( Thickness 3mm)
      8	Slurry recirculation Pump	High Chromium Alloy
      9	Oxidation Tank	RCC + Chlorobutyl Rubber (Thickness 10 mm)
      10	Oxidation Pipe	Carbon Steel + Chlorobutyl Rubber (Thickness 5 mm)

   6. Reaction of Ca2+ with SO 2- and HSO –

      3.3 Material of Construction Major Equipments

      3 3

      to form CaSO3 in solution.

   7. Precipitation of CaSO3 4H O.

      . 2

   8. Dissolving O2 in water at the interface.

   9. Transfer of dissolved O2 from the interface into the liquid interior.

   10. Oxidation of sulfite ions to sulfate ions.

   11. Reaction of Ca2+ with SO2 – to form CaSO4 in solution.

   12. Precipitation of CaSO4.2H2O.

2. Operating Parameters

4. Results and Discussions

We had got the following result after successful trial on limestone based flue gas desulphurization plant :-

• SO2

Absorption Efficiency -90 %

Table 2. Material of Construction Major Equipments

1. Running Cost

   Sr. No.	Description	Unit	Value
   1	Limestone
   i	Limestone Consumption		kg/hr	4245
   ii	Cost of Limestone		Rs/kg	0.25
   iii	Cost of Limestone		Rs/hr	1061
   2	Fresh water
   i	Fresh Water consumption		m3/hr	18
   ii	Cost of Fresh Water		Rs/m3	25
   iii	Cost of Fresh Water		Rs/hr	450
   3	Electricity consumption
   i	Power Consumption		kW	1078
   ii	Cost of Electricity		Rs/kWh	2.84
   iii	Cost of Electricity		Rs/hr	3062
   4	Running Cost for FGD Plant [1(iii)+2(iii)+3(iii)]		Rs/hr	4573
   5	Gypsum production
   i	Wet Gypsum ( 15 % Moisture & 78 % purity )		kg/hr	7576
   ii	Dry Gypsum ( 78% Purity )		kg/hr	6440
   iii	Cost of Gypsum production from FGD Plant (78% purity )		Rs /ton	710
   6	Landed Cost of Mined Gypsum (78% Purity)		Rs/ton	1908
   7	Savings to due to Gypsum Production		Rs/ ton	1198

   Table 3. Running Cost

   • Limestone Consumption- 4245 kg/hr

   • Gypsum Generation -7576 kg/hr

   • Gypsum Purity ( Min ) 78 %

Graph 1.One of the Result of month Nov 2012 shows the relation between gypsum generation, SO2 absorption and gypsum purity

5. Future market

In India rely heavily on thermal power plant for power supply. India has large number of coal or pet coke fired units burning pet coke or indigenous coal. These coals or pet coke content sulphur and the emphasis on environment. There will be a larger market for flue gas desulphurisation new plant. The massive increase in electrical generating capacity required to keep place with increasing power demand means that the emphasis for flue gas desulphurization units.

6. Conclusion

The wet limestone flu gas desulphurization process as at Shree Cement 44 MW power plant uses a counter current scrubbing process with in force oxidation to produce gypsum and achieving a high degree of SO2 emissions reduction when burning high-sulfur coals (pet coke) because of easy availability of limestone at low cost .

The consumption of gypsum in cement making so therefore saving due to in house gypsum production and achieve less payback period for limestone based flue gas desulphurization system.

The cost of installing a flue gas desulphurization unit depends on various factors such as scale of process, sulphur content in the coal or pet coke, availability and cost of reagents. In India it has large natural reservoirs of limestone and hence limestone process is better than other processes.

7. References

1. J. Cofala,M. Amann, F. Gyarfas, W.Schoepp,

   J.C. Boudri, .Hordijk,C. Kroeze, Li Junfengc, Dai Lin, T.S. Panwar, S. Gupta,Cost effective control of SO2 emissions in Asia, Journal of Environmental Management, 6 April 2004,pp.-149

2. Arthur Kohl, Richard Nielsen, Gas Purification Gulf Publishing Company

   ,Texas.

3. Boward W L,Brinkman ,Singer J G ,Flue Gas Desulphurisation,Department of Trade and Industry,London,SWIH OET,March 2000,pp.3-5

4. Dr.B.Senagupta,Dr.S.K.Paliwal, Environmental Standards for Ambient Air,Automobiles,Fules,Industries and Noise,Central Pollution Control Board Ministry of environment and forest, Delhi,July 2000.

5. Ravi K.Srivastava,Controlling of SO2 emissions: A Review of Technologies, EPA (U S Environmental Protection Agency, Washington, November 2000, pp.5-9.

6. R.A.Pandey,R.Biswas,T.Chakrabarati, S.Devotta, Flue Gas Desulphurization: Physicochemical and Biological Approaches, Environmental Science and Technology,Nagpur,3 March 2010,pp.571- 581.

7. www.yosemite.epa.gov.com, Flue Gas desulphurization (Acid Gas Removal Systems)

8. Amy P.Evans,Ammonium Sulfate WFGD Technology,MARSULEX Environmental Technologies,Lebanon,July 2007

9. D.S Henzel ,B.A Laseke,E.O Smith,D.O Swenson, Limestone FGD Scrubbers : Users Handbook,EPA (U S Environmental Protection Agency,Kanas City,Aug 1981.

10. Wu Zhao Xia,The Flakt Hydro process : flue gas desulphurization by use of sea water,International Journal Environmental and pollution,Vol 12 ,No1 ,pp 67-72