As the death toll rose to 29 on Thursday, initial reports from the site of the accident at NTPC Ltd’s Unchahar plant in Rae Bareli — the first of its kind in the Indian thermal power sector — indicate that safety norms were violated.
The reports point to a practice that is regularly resorted to across coal-fired plants in the country. The practice involves de-choking the bottom hopper — the funnel-shaped bottom end of the boiler — by using rods to push out ash clinkers while the boiler is in operation, in violation of safety norms. On Wednesday, this exercise seems to have gone horribly wrong at Unchahar.
According to information gathered from multiple sources, the Unchahar 500 MW Unit No. 6 was running at a load of around 200 MW at 1536 hours on Wednesday. Sources said the bottom hopper de-ashing was being done by opening the manhole door at around 10 metres height on the boiler platform, by an NTPC operations and maintenance team. The boiler was in manual mode of operation then.
Suddenly, the furnace pressure increased and hot clinkers of ash fell in a splash, injuring those in the vicinity. This was reportedly followed by a blast due to the increase in pressure, resulting in the release of flue gas at very high temperature from a duct at 20 metres height in the boiler, which caused the burns and casualties.
The Indian Express sent a detailed email to NTPC on the initial reports, with specific questions on whether the bottom hopper of the Unit 6 boiler was choked with ash clinkers ahead of the accident, if a manhole (at 10 metres height) was opened to inspect the choking of the bottom ash hopper, if workers resorted to poking the bottom hopper through the manhole of a running boiler, and if workers also attempted to poke from the bottom opening of the hopper. In response, an NTPC spokesperson said: “…we feel it is too premature for us to conclude the exact causes, for which a committee has been constituted. We request you to bear with us and not go by hearsay and unauthenticated information being circulated around.”
In coal-fired thermal power plants, steam is produced in high pressure in the steam boiler due to burning of fuel (pulverised coal). This steam is further heated in a superheater, and then enters the turbine and rotates the blades. The formation of clinker is a usual problem in coal-fired boilers of thermal power plants, which could result in forced outage of a short duration and associated generation loss. The causes of clinker formation are low-quality coal having low gross calorific value, more ash content, high mineral content and over-firing of the molten slag. Also Read: NHRC issues notice to UP government, magisterial probe ordered
In the normal course, the resultant clinker is small and falls down the funnel-shaped boiler bottom due to gravity, through a 6-8 inch opening in the bottom hopper. A steel conveyor belt which runs beneath the opening continuously removes the clinker, which is sprinkled with water to cool it, then crushed and moved out as ash slurry.
The problem occurs when the clinkers are bigger in size and end up blocking this opening. The solution then is to shut down the boiler, open a manhole and put the poking rod in to remove the clinker blocks. If that doesn’t work, the optimal solution is to wait for a day for the boiler to cool down, then enter through the manhole and break the clinker pieces blocking the bottom opening, according to technical experts involved in the exercise.
Across thermal projects in India, though, this is a rare practice. Loss of generation results in monetary loss, since the boiler has to be shut down for a day and then restarted. So the managements tend to push operators to undertake a short-cut for de-choking the bottom hopper, in violation of safety procedures, and thereby risking lives.
Even as NTPC’s official probe report is still underway, officials in the know indicated that workers at the Unchahar plant had opened a manhole at 10 metres height, and were in the process of inserting poking rods to clear out the bottom of the hopper. Typically, upon poking, the clinkers fall down, sometimes in big lumps, and the steel conveyor belt carts it out. But when the clinker is stuck, workers could be deployed to poke it from the bottom opening also. The conveyer belt is shifted for this.
“Workers go below the hopper, sometimes hoisted on scaffoldings, to poke with rods from the bottom. The clinker conveyor is switched off. The process is called double poking, wherein the threat of big lumps falling on the heads of workers is a clear and present danger. The danger in doing it when the boiler is operational is that tonnes of clinkers or solid ash may fall down suddenly,” said a source.
Also, when the blockage is cleared, a massive amount of air is sucked in by the boiler due to huge draft, both natural and induced, given that there is a huge chimney at the other end. If the boiler vacuum control system is not able to respond adequately to the sudden increase in pressure, the boiler, designed to operate under negative pressure, will get pressurised suddenly. The fireball inside the operational boiler will spread and temperature will rise, resulting in potential for a blast of hot fuel gases and ash.
In its response to The Indian Express, NTPC, which otherwise has a stellar safety record, did not corroborate or deny the sequence of events, despite the specific queries.Meanwhile, speaking to mediapersons in Lucknow, Ravindra Rathee, Regional Executive Director (North), NTPC, asserted that there was no “explosion” in the boiler, but the “economiser hoppers” (used to capture large ash particles from flue gas) located at 20 metres height got opened due to extra pressure, and the resultant release of flue gas caused the burns. “We talked to the officials and, prima facie, what it looks like is that there was some problem in the evaporation of ash. There was some hindrance in the process of ash coming out of the bottom of the furnace. Therefore, production was decreased from 400 MW to 190 MW. There was ash stuck at the bottom of the furnace, and after it was removed, some disturbance was created,” said Rathee.