Energy and chemicals group Sasol received four stainless steel rotating grate castings from South Africa-based steel castings manufacturer Steloy in July 2013.

Steloy sales and marketing director Wynand Labuschagne notes that although certain components of the gasifiers, such as the grate ploughs, are replaced, most are reconditioned, and this is the first time that Sasol has replaced the rotating grates of its gasifiers at the Secunda plant, in Mpumalanga, since they were installed in the 1970s.

The gasifiers heat coal, and collect what Sasol terms as raw synthesis gas, produced during this process. This gas is used to manufacture petrochemical products.

The rotating grate is located at the bottom of the gasifier and is turned by mechanical gears, which extract the spent ash. The 4-m-diameter grate comprises 39 core components, stands about 2.5 m high and weighs about 20 t. The grate is effectively a scraping mechanism that collects ash and dumps it in an ash lock.

“The grate is made of heat-resistant stainless steel castings and comprises inner and outer components. The top or external portions of the grates have wear blocks welded onto them to protect them from being damaged.

At the bottom of the grate is the drive gear, with the internal mechanics of the grate, which is made of chrome moly steel, mounted on top of it. The external structure is bolted onto the internal structure, which then forms the final rotating grate,” Labuschagne explains.

He mentions that manufacturing the casting for the bottom gear of the gasifier represented a challenge, owing to the gear comprising two equal pieces of heavy section and the high-integrity materials used to manufacture the unit needing to undergo induction hardening afterwards.

“Casting gears is challenging to start with, but when a gear is going to be used in this kind of environment and adhere to very strict quality standards, the challenges increase,” he adds.

The castings were completed at the end of 2012, which was followed by the final machining and assembly of the finished components.

Labuschagne notes that the machining process took about six months and was followed by assembly of the grates by South Africa-based company Machine Tool Promotions.