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By G.D. Nigudkar
The Trombny thermal power station in Mumbai, India, owned by Tata Power Co. Ltd. includes both coal- and gas-based units. Unit -5, for example, is a 500-megawatt coal-fired power plant that consumes about 6,000 tons of coal per day.
Over the course of 32 years of operation, the Trombny station had consumed only high-ash coal, mined in India and delivered to the plant by train in “box wagons.”
Not long ago, however, as a lack of suitable ash-disposal areas and complex transportation raised costs, Tata Power switched to importing low-ash coal, after first finding the best way to bring it into the plant. It worked with Tata Consulting Engineers Ltd. (TCE) to develop and install a new coal-conveying system.
In the past, at an unloading station in the plant’s coal yard, trains of box wagons — each holding 58 tons of coal — were decoupled and emptied using a “wagon tippler,” discharging the coal into an underground hopper. The hopper discharge gates were capable of feeding up to 500 tons per hour of coal onto a belt-feed conveyor, dependent on plant requirements. The system included dust suppression.
Once on the belt conveyor, coal was moved either to a crusher or conveyed to a stockyard for storage. From the crusher, coal was conveyed to boiler bunkers for immediate use, or temporarily stored.
Enough ash already
With the coal’s stored energy expended, resulting coal ash was removed for disposal. But an “ash disposal area” used for many years was totally filled. Its contents needed disposal at a distant place. With all the coal ash generated, ash reclamation and disposal costs were too high.
Costs were also high for fuel and labor to operate the train and unload the box wagons. Rail and mechanical equipment demanded upkeep. Finally, coordinating rail traffic on the busy main line was challenging and sometimes led to delays and late deliveries, as did inefficient wagon unloading.
To find a low-ash coal source and the best way to bring it to the plant, Tata Power worked with Tata Consulting Engineers Ltd (TCE), which, like Tata Power, is a member of the Tata Group. TCE’s expertise is in bulk-handling systems for the chemical, industrial, nuclear and power plant industries.
As is well known, Tata Group is a privately held conglomerate headquartered in Mumbai, Maharashtra, India. Founded in 1868, Tata Group today encompasses seven business groups: IT, engineering, materials, services, energy, consumer products and chemicals. Its annual revenues have been estimated at more than $100 billion.
Tata Power decided to import Indonesian coal containing only from 2% to 6% ash. Tata Power planned to receive the Indonesian coal by shipments of 60,000 dead-weight tonnage.
To receive the coal, after study, TCE proposed two main alternatives.
First was construction of an offshore coal berth. Ships would berth at an outer anchorage, in deep sea about three kilometers offshore from Tata Power Trombay, “self-unloading” into hoppers on a coal berth that would be constructed at the end of a jetty extending about three kilometers into the sea. Belt and pipe conveyors would bring the coal to the plant. A multi-fuel berth option would allow unloading liquid natural gas and other gas and oil fuels besides coal. However, this alternative was not pursued.
The way forward
Instead, plans were approved for construction of a 160-meter long “finger berth” at the Trombay plant’s sea end. Thus, ships are unloaded at sea, with off-loaded coal transferred to barges that carry up to 2,200 metric tons and that are unloaded at the finger berth. A pipe-conveying system moves the bulky commodity to the coal yard, where a “stacker/reclaimer” first spreads it around the yard and then later retrieves it for use.
Unloading and conveying equipment was provided by FLSmidth, a well-known Danish engineering firm focused on the minerals and cement industries.
Equipment includes a barge un-loader, belt conveyor, two KOCH pipe conveyors, three junction towers, computer controller and various ancillaries. The barge un-loader with telescopic loading chute runs on rails and can move the barge’s length and width, ensuring complete load-out.
Pipe conveyors eliminate dusting and spillage as coal is moved. The belt runs flat over the head pulley but is troughed by idlers at the feeding point. Then after being loaded, the belt moves through a series of idlers until it formed into a 450-millimeter-diameter “pipe” carrying material to discharge. There the pipe opens back out onto a trough belt and runs flat over the tail pulley as the material falls. On return, the belt is formed back into a pipe, for material return, if required.
The way today
After a barge is docked at the 160-meter-long coal berth, the barge un-loader removes the coal at up to 1,500 tons per hour and transfers it to the 1,400-millimeter-wide belt conveyor. The belt conveyor moves the coal a short distance to the first junction tower where it is transferred to the pipe conveyor. A dust collection system in each tower removes dust generated during discharge. The junction tower also houses the incoming conveyor's drive motor, gear box, coupling, and pulleys.
The first pipe conveyor moves the coal about 440 meters to the second junction tower, where it is discharged to the second pipe conveyor or diverted to another area of the power plant. The second pipe conveyor moves the coal a little more than 1,000 meters to the third junction tower, where it is either discharged onto a belt conveyor for crusher loading or moved to a storage area.
Conveyors and all ancillary equipment are connected to a central computer, allowing for remote monitoring and control of the entire system. Safety sensors and switches throughout the conveying system ensure that proper conveying. Shut down is automatic upon problem detection. Several watchtowers along the conveying path allow workers to monitor operations.
Tata Power at present operates the unloading and conveying system 16 to 18 hours a day, depending on the tide, and can unload and convey around 16,000 tons of coal at an average rate of 1,550 tons per hour, as per its requirements.
The old system of the train and box wagons is still used to meet about 20% of the plants requirements, based on indigenously purchased coal.
The new system allows importing low-ash coal, which has reduced ash reclamation costs, improved the plans power-generation efficiency and, because use of the pipe conveying is dust free, helps meet stringent pollution regulations.
G.D. Nigudkar, General Manager, TATA Consulting Engineers Ltd.-India, is a graduate from Jiwaji University Gwalior. He has pursued post-graduate studies at the University of Roorkee-Roorkee, India. He has more than 35 years experience working with conveying and handling, compressed air and other type systems in power, chemical, nuclear and other type industrial plants.
The author would like to express his gratitude to N.V. Parulekar of Tata Power for his help in preparing this article, which otherwise wouldn’t have been possible.
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