Power ploy. IIT-Bombay’s superalloy for ultra-efficient boilers

Researchers at IIT Bombay have tested an oxidation-resistant, nickel-based superalloy that is able to stand the extreme temperatures and pressures of the most efficient boilers in coal-fired thermal power plants.

While further work is required to be done for validation of ‘Alloy 617’, the conclusions of the research work mark a milestone in the evolution of the Advanced Ultra Super Critical (AUSC) technology.

In boiler technology, efficiency—a measure of how much of energy contained in the fuel is converted into useful energy—increases as the temperature and pressure of steam go up. In the subcritical boilers, these are about 570 degrees and 175 bars, with 33-37 per cent efficiency. Then come the supercritical boilers with 570 degrees, 250 bars and 38-42 per cent efficiency. Supercritical boilers are the norm in the industry today. Ultra supercritical boilers (620 degrees, 275 bars, 42-46 per cent) are just around the corner. But the most efficient boilers are the AUSCs, of 710 degrees, 320 bars, with more than 50 per cent efficiency.

As India gears up to add another 87 GW of coal-fired power plants by 2032, it is imperative to go in for the best technologies, which produce more energy from the same amount of coal.

For some time now, there is a talk about bringing in the Advanced Ultra Super Critical (AUSC) boilers, where the ultra-hot steam zips through the turbines with more energy, generating more electricity. The AUSCs have about 30 per cent less carbon footprint than the subcritical boilers.

The problem in developing these very high efficiency boilers is ‘materials’. You need to make boilers whose walls and tubes can stand such tortuously high temperatures and pressure.

The IIT Bombay research team, headed by Prof VS Raja of the Department of Metallurgical Engineering and Material Science, tested the commercially available alloy, called Alloy 617, for boiler applications. This heat-resistant alloy of nickel, iron and cobalt can withstand high temperatures. The team performed tests in a simulated Advanced Ultra Supercritical (AUSC) environment, which was designed to replicate the extreme temperature and pressure conditions that exist in a coal-fired power plant.

“IIT Bombay was invited by the government of India to undertake the study. It was a national project in mission mode and the Government of India wanted to go ahead with thermal power plants with higher thermal efficiency,” says Prof Raja.

New territory

Until now, Alloy 617 has remained relatively unexplored for potential use in AUSC technology due to difficulties in creating a test loop that simulates the extreme conditions of an AUSC steam oxidation test loop, says an article on IIT-B’s website. Previous studies have been limited and inconclusive, with the steam temperature and pressure conditions examined not exceeding 670^oC and 27 MPa, respectively and water chemistry not controlled precisely.

Speaking to Quantum, Prof Raja said that the objective of the research was to understand the alloy for AUSM applications. While the alloy has been found to be able to withstand harsh temperatures and pressures, more work would need to be done to make a functional boiler, he said.

The researchers also tested another alloy, Alloy 740, which showed better mechanical strength, but oxidised earlier. Asked whether India has the capability to manufacture Alloy 617, Prof Raja says the public sector alloy company, MIDHANI (Mishra Dhatu Nigam) might be able to do it.

The alloy for the experiments was locally fabricated by Symec Engineers and this is a first-of-its-kind study in India. “Very few such setups exist across the world and this is the only setup in India,” says Prof Raja.

Reacting to this development, boiler design expert M R Ganesan, a former Executive Director at BHEL, said that the IIT Bombay research findings are very significant and useful for the development of better boilers. Ganesan also wanted BHEL to revive the ‘magneto hydro dynamics’ project, which can handle coal gas at temperatures of 4000^oC and generate power with much less carbon footprint.

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