QRA at thermal power station -ll of NLC India Limited

Neyveli Lignite Corporation (NLC) being a Mini Ratna Company and a fully owned government enterprise has awarded iFluids Engineering to conduct QRA at its Thermal Power Station II. NLC power stations generate about 2490 MW of power. Thermal Power Station – II after meeting the needs of Second Mine has been a major source of power to all southern states of India viz., Tamil Nadu, Andhra Pradesh, Karnataka, Kerala and Union Territory of Pondicherry. The 1470Mw capacity power station consists of 7 units of 210MW each which was constructed in two stages of 630MW and 840MW.

Thermal Power Station – II (TPS-II) is grouped into two (2) stages with a total power generation capacity of 1470MW. Stage-I consists of three (3) Boiler units and Stage-II consists of four (4) Boiler units. Each Boiler has a power generation capacity of 210MW. The Boilers use low calorific value, high moisture content lignite as a fuel to generate high pressure steam. The steam is used for driving a steam turbine which in turn rotates a turbo generator connected to the steam turbine and generates power.

Lignite mined from three open cast mines in Neyveli viz. Mine-I, Mine-II & Mine-IA using fully mechanized systems is sent to the power stations TPS-I, TPS-I Expansion, TPS-II and TPS-II Expansion through conveyors. Lignite received from Mines is sent to the respective Boiler Bunkers (i.e. silos) where it is stored for daily use. Lignite from the Bunkers is transferred to Mills through Conveyors where Lignite is pulverized into fine powder. Pulverized Lignite is sent to the Boiler where it is fired as a Pulverized Fuel (PF). The Flue Gas (i.e. combustion products) from the Boiler is used for generating high pressure steam in the Steam Drum circuit that consists of Economisers (ECO), Evaporators and Reheaters (RH).

The High-Pressure steam is superheated in Superheaters (SH) and used for driving the Steam Turbine which in turn rotates the Turbo Generator and generates electric power. The power generated is stepped up in Transformers and exported to the National Power Grid. The Flue Gas from the Boiler is cooled down in  Air-Preheaters. The Fly Ash present in the Flue Gas is removed in Electro Static Precipitators (ESP) prior to discharging the Flue Gas  that meets the emission thresholds to atmosphere through a 220m tall Chimney. The Fly Ash collected in the ESP is separated and sold to third parties (e.g. cement industries) as  Dry Fly Ash.

The assessment is based on the hazards that may occur during the operations of the project according to information available. The following are the Facilities covered under QRA study of this Project.

  • Lignite from Pulverizing Mill to Furnace.
  • Light Diesel Oil from storage Tanks to Furnace.
  • Fuel Oil from storage Tanks to Furnace.
  • Ash handling Facilities.
  • Hydrogen generation Plant I and II.
  • Chlorine Tonner handling and piping.

The Main Objective of this study is to evaluate the potential risk levels for Personnel due to accidental release of hazardous materials from loss of containment scenarios at the Facilities and to demonstrate that individual risks are within the broadly acceptable region and to provide suitable control measures for gaps identified in line with PNGRB T4S Regulation 2020 Major Loss of Containment (LOC) events and associated Hazard identifications.

  • Calculation and estimation of physical effects of failure case scenarios which include that of Jet and Pool Fire heat radiation distance, Flammable gas dispersion distance, BLEVE and overpressure explosion distance.
  • Evaluation of potential risk levels for personnel due to accidental release of hazardous materials from loss of containment scenarios from the Facilities to demonstrate that individual risks are within the broadly acceptable region
  • Providing suitable control measures for the gaps identified in line with the PNGRB T4S Regulation 2020.
  • Failure Frequency evaluation
  • Quantification of Societal risk
  • Estimation of potential loss of Life
  • To confirm that risk can be reduced consistent with the ALARP principle according to the UK HSE risk acceptance criteria by performing a risk assessment.
  • Recommend risk reducing measures to ensure that all risks fall under ALARP or ACCEPTABLE region.



Jet fires:

Jet fire occurs when a pressurized release (of a flammable fluid) is ignited by any source. They tend to be localized in effect and are mainly of concern in establishing the potential for domino effects and employee safety zones rather than for community risks.

Pool fires:

This represents a situation when flammable liquid spillage forms a pool over a liquid or solid surface and gets ignited. Flammable liquids can be involved in pool fires where they are stored and transported in bulk quantities.

Flash Fire (FF):

Hydrocarbon vapor released accidentally will spread out in the direction of wind. If a source of ignition finds an ignition source before being dispersed below lower flammability limit (LFL), a flash fire is likely to occur and the flame will travel back to the source of leak. Any person caught in the fire is likely to suffer fatal burn injury.

Light Diesel Oil and Fuel Oil stored in storage tanks and transported to furnace are the most likely areas posing major risk of hydrocarbon fires.


Overpressure Explosion:

Overpressure explosion is a blast wave produced from the sudden onset of a pressure wave after an explosion. The explosion resulting from the ignition of a cloud of flammable vapor, gas or mist in which flame speeds accelerate to sufficiently high velocities to produce significant overpressure is called as the vapour cloud explosion.

The maximum damage distances for overpressure explosion were observed for catastrophic rupture of hydrogen storage cylinders.

Dust Explosion:

A dust explosion is the rapid combustion of fine particles suspended in the air within an enclosed location. Dust explosions can occur where any dispersed powdered combustible or flammable material is present in high-enough concentrations in the atmosphere or other oxidizing gaseous medium, such as oxygen. Lignite storage yard in the premises is identified as a potential dust explosion site from the study.


Toxic Dispersion:

Accidental atmospheric releases of poisonous compounds like chlorine can be detrimental to people. The concentration and length of exposures determine the extent of the release’s impact. The biggest toxic zone is seen when chlorine storage tonners break catastrophically, causing the immediate release of the entire stock.

Exposure limit considered for the study:

ComponentsConcentration (ppm)

Smoke Dispersion:

Smoke is a mixture of airborne gases and particulates released during the combustion or pyrolysis of a substance, along with the amount of air entrained or otherwise mixed into the mass. The type of fuel being burned and the combustion conditions affect the makeup of the smoke. Smoke inhalation poses a risk to all living things and can result in significant harm or even death. The maximum impact distances for the smoke dispersion effects are determined of each of the following components listed below:

  • Carbon dioxide (CO2)
  • Carbon monoxide (CO)
  • Sulphur dioxide (SO2)
  • Nitrogen dioxide (NO2)
  • Nitric oxide (NO)
  • Sulphur trioxide (SO3)

Lignite storage yard, Pulverizing mill, Ash handling facilities are identified as potential originating sites for smoke.

Exposure limit considered for the study:

ComponentsConcentration (ppm)
Carbon dioxide (CO2)4000030000
Carbon monoxide  (CO)1200400
Nitrogen dioxide (NO2)201
Nitric oxide (NO)100
Sulphur dioxide (SO2)1005
Sulphur trioxide  (SO3)0.611253.0562536.675


The Location Specific Individual Risk Per Annum for people at all locations considered for the study is found to lie in the ACCEPTABLE region. It is concluded from the study that the societal risk for the overall population considered for the study also falls in ACCEPTABLE region.

The following are the general recommendations provided:

1) Update the Plant’s PID and keep track of its operational characteristics (such as temperature and pressure).

2) Ensure that the emergency plans, both on-site and off-site, are updated..

3) “Work Permit” procedures to be followed by maintenance personnel for protection of property from damage and fire etc.

4) Ensure that the periodic inspection of static electricity discharge connections were being done. 5) Ensure that the proper evacuation route has been displayed in various locations of the Plant.