Emission norms
Indian coal typically possesses a high ash content ranging from 35 to 42 per cent, accompanied by an average calorific value of 4000 kcal per kg and an average sulphur content of 0.35 per cent. Emissions of particulate matter (PM) from thermal power plants (TPPs) play a significant role in air pollution, with estimates indicating that coal-based TPPs are responsible for approximately 60 per cent of total PM emissions.
TPPs are subject to rigorous emission regulations for PM established by the Ministry of Environment, Forest and Climate Change. For coal-based TPPs, the PM emission standards are specified as follows: 100 mg per Nm³ for plants installed before December 31, 2003; 50 mg per Nm³ for plants installed between January 2014 and December 2016; and 30 mg per Nm³ for plants commissioned from January 2017 onwards.
Maintaining an equilibrium between the rising need for electricity and acceptable environmental standards underscores the necessity for ongoing upgrades in pollution control measures and enhancements to environmental management systems. Power plants can access various emission control devices, encompassing electrostatic precipitators (ESPs), fabric filters, cyclone separators.
Electrostatic precipitators
ESPs are filtration devices designed to eliminate smoke and dust particles from exhaust emissions. By electrically charging particles, they attract them from the gas stream on to collector plates with opposite charges. Some strategies to improve ESP efficiency include enhancing electrode technologies, incorporating plate enhancements with rigid discharge electrodes, improving collection efficiency, monitoring ESP operations and employing modern control algorithms.
Notably, NTPC Limited has successfully completed the retrofitting of ESPs, covering over 13 GW capacity, aimed at mitigating particulate emissions at its stations. The company has also completed the renovation and modernisation of ESPs across 40 units.
Fabric filters
Fabric filters also known as dust collectors, are utilised to capture PM from process gas streams before their release into the atmosphere. These systems exhibit high efficiency in collecting submicro-meter-sized particles. In baghouses, the dust-laden gas enters a compartment where smaller particles accumulate on filter bags, forming a layer that allows clean gas to pass through while larger particles settle in the hopper.
These baghouses typically achieve a removal efficiency of 99 per cent for fine particles and 99.5 per cent overall. For instance, Haryana’s Jhajjar Power Limited has installed fabric filters and ESPs at its 1,320 MW coal-based power station to reduce PM emissions. Similarly, Vedanta has integrated industrial internet of things technology into fabric filters at its 2,400 MW thermal power station to enhance emission control.
Wet particulate scrubbers
Wet particulate scrubbers are designed to capture both fly ash and sulphur dioxide from flue gas emissions. The venturi scrubber, one of the most commonly used wet scrubbers, introduces water into the flue gas stream at the venturi throat, creating droplets. Fly ash particles interact with these droplets to produce a wet by-product, which is subsequently disposed of. The majority of wet scrubbers are engineered to control both sulphur dioxide and PM by employing alkaline fly ash as a sorbent.
Cyclone separators
This is another apparatus that utilises centrifugal force to effectively separate suspended particles from the flue gas stream. Known for their affordability, compact size, straightforward design and ability to withstand high temperatures and pressures, these devices are widely employed across various industries.
Mercury emission control
Systems for controlling NOx and SOx, along with technologies like ESP and sorbent injection, provide the added benefit of controlling mercury emissions. Mercury control typically involves oxidising metallic mercury to ionic mercury to facilitate its removal from flue gas desulphurisation systems.
A common method involves injecting activated carbon into the plant’s exhaust stream, where mercury is removed through chemical adsorption on powdered activated carbon. Another widely used technology involves halogen injection into coal, utilising substances such as calcium bromide or sodium iodide. This process is cost-effective and achieves high levels of mercury oxidation.
At NTPC, mercury analysers have been installed for emission and air monitoring, with environmental monitoring data readily available to central and state pollution control boards as per their requirements. Additionally, mercury emissions at many NTPC stations are below the minimum detectable limit.
In conclusion, managing PM emissions from TPPs is imperative, requiring strict adherence to government emission norms and the deployment of digital solutions for monitoring and controlling PM emissions in real time. With the technologies explained above, there exists significant potential within the power sector to effectively mitigate both the emissions.
Aastha Sharma