FGD TECHNOLOGY FOR THERMAL POWER

The article gives a glimpse of need for Flue Gas Desulphurisation (FGD) implementation to curb greenhouse gases in thermal power generation. Further, it discusses various FGD technologies.

Fgd Technology For Thermal Power
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India’s dependency on coal for meeting its energy needs has resulted in the generation of huge quantities of SO2, NOx and ash over the years. Out of the total installed capacity of over 350 GW, nearly 60 per cent is coal based, the majority of which comprises of domestic coal with a high ash content of 30 – 35 per cent. Recognising the central role thermal power plays in worsening air quality, the Ministry of Environment, Forest & Climate Change (MoEF&CC) in its latest amendment issued in December 2015 tighter standards for coal-based thermal power plants. The new standards aim to drastically cut emissions of particulate matter (PM), sulphur dioxide (SO2), oxides of nitrogen (NOx) and mercury. In addition, the new norms also require power plants to sharply curtail freshwater use and to improve the fly ash utilisation.

The MoEF&CC has notified the revised standards for coal-based thermal power plants in the country, with the primary aim of minimising pollution. Ministry announced tighter standards for coal-based thermal power plants. These standards are proposed to be implemented in a phased manner. Thermal power plants are categorised into 3 categories, namely those (i) Installed before 31st December, 2003 (ii) Installed after 2003 up to 31st December, 2016 and (iii) Installed after 31st December, 2016.

The new standards are aimed at reducing emission of PM10 (0.98 kg/ MWh), sulphur dioxide (7.3 Kg/ MWh) and oxide of nitrogen (4.8 kg/MWh), which will in turn help in bringing about an improvement in the ambient air quality in and around thermal power plants. The technology employed for the control of the proposed limit of SO2 and NOx will also help in control of mercury emission as a co-benefit. Limiting the use of water in thermal power plant will lead to water conservation as thermal power plant is a water-intensive industry. This will also lead to a reduction in energy requirement for drawl of water. As per the new amendment, the thermal power plants across India from 2017 will have to cut particulate matter emissions by as much as 40 per cent and reduce their water consumption by nearly one third.

With a view to reduce greenhouse gas emission, harnessing of renewable resources to the extent possible, enhancing efficiency of the existing power plants and introduction of new technologies for power generation for enhancing efficiency and demand side management are being pursued. Since coal will continue to dominate power generation in future, Super Critical Technology has been introduced for reduction of greenhouse gases.

Need of FGD technology
SO2 is a corrosive gas created by the oxidation of sulphur-bearing materials such as coal, oil, and natural gas. SO2 emission is a particularly acute problem in the power generation industry. Sulphur oxides are generated as a result of oxidation of the sulphur present in the coal at the combustion zone. To minimise the adverse effects of sulphur oxides (SO2 and SO3) on the environment, many power plants and industrial facilities use Flue Gas Desulphurisation (FGD) scrubbers to remove SO2 and SO3 from combustion gases.

The SO2 emission levels would vary depending on the sulphur content and the composition of the coal fired. The weighted average of sulphur content in the coal is varying from 0.30 per cent to 0.55 per cent and the corresponding estimated SO2 levels works out to be around 1254 to 1650 mg/Nm3 at 6 per cent (dry basis of flue gas) therefore, the sulphur content in the coal of particular power plant is the main consideration for FGD design.

FGD is a set of technologies used to remove sulphur dioxide from exhaust flue gases of fossil fuel power plants, and from the emissions of other sulphur oxide emitting processes. It is a control device that absorbs and react using the alkaline reagent to produce a solid compound. In this system, equipment such as absorber towers, demister supports, gas outlets, recycle and process piping, process tanks, and agitators are highly exposed to corrosive and abrasive environments. Rubber linings have fundamental advantages so that neither the physical nor chemical properties of the scrubbing liquid have any major effect upon its service life.

FGD Technology selection
Different technologies are commercially available for reduction of SO2 emission in dry flue gas. Following prevalent technologies are widely used in power plants to control the SO2.emission.
• Wet limestone FGD
• Sea-water based FGD
• Ammonia based FGD
• Dry FGD.

These technologies are commercially available with proven nature and abundantly available regents for the plants. Some of the other technologies such as dry sorbent injection, ammonia based scrubbing, regenerative type, circulating fluidised bed type FGD systems etc. having lower efficiency are not commercially viable. Selection of appropriate FGD technology mainly depends on technical, economic and commercial factors that mainly involve sulphur removal ability, reliability, space requirements  and reagent availability. Economical factors like capital cost and operating cost also considered while selecting the FGD technology for particular thermal power plant. Details of the above technologies are as follows.

Wet limestone FGD

The main components used in this process are wet absorber, gas to gas heater, limestone slurry preparation system, waste water treatment system and gypsum handling system. In this technology, SO2 removes in wet phase using wet absorber. Limestone is used as regent that is ground in the mill and added with water to make slurry and sprayed on the absorber through nozzles. The flue gas coming out from the ID fan is led to absorber where it reacts and forms gypsum. The cleaned flue gas is fed to the chimney. The system is a once-through, wet type in which the SO2 gas is permanently bound by the absorbent which must be disposed of as a by-product, gypsum. The by-product is produced is wet in nature, and flue gas leaving the absorber is saturated with moisture. The by-product gypsum is further processed to remove moisture in vacuum belt filters and disposed of as solid gypsum cakes (CASO4H2O) with minimum moisture up to 10 per cent. In this process, the desulfurisation is completed in four stages like absorption, neutralization, oxidation and crystallisation.

Wet limestone technology can be used to control SO2 for wide range of sulphur in fuel and have high SO2 removal efficiency. In this process, limestone used as regent is easily available. This technology has high capital cost and high auxiliary consumption.

Sea-water based FGD
The main components used in this process are wet absorber, gas to gas heater and oxidation basin. Wet absorber is the zone where the flue gas is scrubbed against the flow of seawater. SO2 gets converted into liquid state and is collected at the bottom of the absorber. The purpose of gas to gas heater is to transfer the heat from hot flue gas to the scrubbed gas to ensure the temperature of gas at stack is greater than ADP temperature. The function of oxidation or aeration basin is to remove the CO2 produced during the scrubbing and maintain the pH of the system to ensure the optimum operability of the system.

In this technology, seawater is used as absorbent and removes SO2 in wet phase using wet scrubber and discharge the liquid waste seawater containing sulfate. In this system, flue gas is scrubbed with seawater in absorber. The flue gas from the ID fan outlet is led to the absorber where the sea water is sprayed through nozzles. The absorption of SO2 takes place in the absorber, where seawater and flue gas are brought into close contact in a counter-current flow.

Sea water contains significant amounts of HCO3 and other alkaline compounds that help sulfur dioxide in flue gas to dissolve in water. The scrubber effluent flows to the treatment plant where it is combined with raw seawater and then aerated to recover pH value and reduce chemical oxygen demand before being discharged to the sea. Thus, the absorbed SO2 is converted into sulfate before discharge. The sulfate is completely dissolved in seawater, so as a result there is no waste product to dispose.

The main advantage of this technology is less capital cost and auxiliary consumption however, no regent is required and no formation of byproduct.

Ammonia based FGD
This technology uses anhydrous ammonia as reagent. The anhydrous ammonia is diluted with process water to achieve the desired concentration. The diluted ammonia solution is sprayed in an absorber or scrubber through which flue gas is passed. The solution captures the SO2 in the flue gas to form ammonium sulfate which is then oxidised to ammonium sulfate solution by introduction of air into the absorber. The ammonium sulfate solution becomes concentrated and partially crystallised during the gas contact, forming slurry which is pumped to a hydrocyclone is then dehydrated in a centrifuge and dryer to generate ammonium sulphate pellets with negligible water content. The byproduct ammonia sulfate is further packed and processed into product begs by packing machine. The byproduct ammonia sulfate is commonly used as a fertilizer and has other uses. This method has lower water consumption and auxiliary power consumption.

Dry type FGD
This technology use lime slurry used as regent. The main components in this process are dry scrubber, ESP/fabric filter, reagent slurry preparation system, by-product collection and recirculation system and by-product handling system. The lime stone is mixed with water at a controlled rate to maintain a high slaking temperature that helps to generate fine hydrates of lime with high surface area. Process makeup water is added to the slacker to produce solid slurry. The slurry is fed to the absorber by reagent feed pump. The reagent slurry is atomised through rotary cup spray atomisers or through dual fluid nozzles. The flue gas post air preheater enters the spray dryer absorber where gas stream is cooled by the regent slurry spray. The mixture then passes through the fabric filter for removal of particulate before entering the ID fan. A portion un-reacted lime, gypsum and the other reaction products collected in the fabric filter is mixed with water and returned to the process as high solid slurry. The remaining solids are directed to a storage silo for by-product. The by-product is semi-dry/dry in nature and flue gas leaving the absorber is not saturated with moisture. This technology of desulphurization requires less capital cost for smaller capacity units. It has lower water consumption and auxiliary power consumption. There is no visible moisture plume in stack, as the flue gas leaving absorber is not saturated with moisture.

Beside the advantages, cost of lime storage is higher. The technology is limited to low sulphur fuel because of high reagent cost. It has limited reagent utilisation.

Recommendations
With regulatory changes and technology developments, SO2 emission management has evolved significantly over the years and continues to improve. FGD technologies such as dry type and wet type systems improve the overall efficiency of plants and provide monetary benefits to operators.  They also help operators conform to emission regulations and standards. Based on the Indian coal quality generally available for thermal power Electrical India | April 2020 31 stations and bring down the SO2 emission level less than the norms, Dry type FGD and Wet limestone type FGD could be feasible. Through, the capital cost of dry FGDs are lower when compared to wet limestone FGD, based on the lifecycle cost comparison, the wet limestone FGD has lesser operating cost. The dry type FGD systems have a lesser water requirement in the absorber as compared to wet limestone FGD. However, the high cost of expensive slaked/quick lime required as reagent in dry FGDs significantly increases operating cost of the systems. Hence, wet limestone FGD has been found to be optimum technology.

Initiatives and way forward
The Ministry of Power (MoP) has outlined certain initiatives to monitor the SO2 emission from thermal power plants and ensure compliance with environmental directives. To this end, the thermal power plants are required to upload the details of SO2 emission available with them on monthly basis. A quarterly report of this data needs to be also submitted to NITI Aayog. Central Electricity Authority (CEA) has also provided tentative cost estimate for FGD.

Conclusion
Government has notified the key performance standards for power stations to ensure environmental protection. The government yet to take concrete steps to actually implement these standards due to lack of resources that might assist in performing their functions most notably, enough professional staff and appropriate information technology systems. In order to maintain the revised environmental norms, the government is required to develop a strong monitoring mechanism and strengthen the regulatory bodies to monitor these norms. For maintaining the revised environmental norms, substantial capital investment needs to be required by the power companies. The regulators are required to take care in this regard.


Fgd Technology For Thermal Power Ashok
Ashok Upadhyay,
Dy. Director (Generation),
M.P. Electricity Regulatory Commission,
Bhopal

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