Gas analyser helps calm furnaces

The Tasmanian Electro Metallurgical Company (TEMCO) is a wholly owned subsidiary of BHP Billiton and the only manganese ferroalloy plant in Australia. TEMCO supplies this critical steel additive to more than 50 companies around the world. Through the use of its four electric arc furnaces, TEMCO can produce more than 250,000 tonnes of manganese alloys per year, of which 80% is exported.

For a company like TEMCO, safety is of critical importance. One part of the plant, where four electric arc furnaces are housed, can experience explosions without warning due to the furnaces’ sporadic operating nature. This type of submerged arc furnace is identified as among the most dangerous in the world.

Process Technology Supervisor at TEMCO Paul Dennis says: “Sudden explosions are known to arise in these furnaces due to the temperature control mechanism, which pumps water through a series of tubes inside the furnace to maintain the internal temperature. Over time, the water erodes these steel tubes and, consequently, causes the water to leak into the furnace.” The extremely high furnace temperatures and increasing water leakage are the main causes of the eruptions. At 400-600°C, water that enters the furnace through the eroded steel tubes is instantly converted into steam.

As this erosion continues, more steam accumulates inside the furnace and the internal pressure builds until it eventually explodes.

Apart from the significant and, in some cases, life-threatening health and safety consequences of any explosion, companies also need to absorb the expense associated with the plant’s closure and downtime. To solve this increasingly difficult challenge, an off-gas furnace solution was employed to assist in monitoring water pressure.

“A crucial component of my research was my ability to collect accurate and reliable data from the furnace off-gas. However, given the complexity of the environment, combined with the high levels of dust involved, finding an appropriate instrument was quite challenging,” Dennis explained.

After consulting with industry experts, it was recommended to employ the use of the Siemens LDS6 gas analyser because of its accurate and high-speed gas monitoring properties and its flexible application. The LDS6 is an in situ diode laser gas analyser with a measuring principle based on the specific light absorption of different gas components. The device is suitable for fast and non-contact measurement of gas concentrations or temperatures in process or flue gases. But the LDS6 had never been used in a submerged arc furnace before. Therefore, in order for TEMCO to trial this instrument, a customised analyser specifically modified to suit TEMCO’s plant environment was developed. Extensive consultation occurred between Siemens manufacturers in Germany, a Siemens authorised reseller and BHP Billiton to develop a suitable analyser.

Siemens Australia Product Manager Brendan Welsh was one of several Siemens employees involved in coordinating discussions between Germany and Australia.

“As this was a first-time application for the LDS6, much caution had to be taken in order to guarantee the analyser’s accuracy, sensitivity, reproducibility, reliability and maintainability. One of the major modifications required for this application was to counteract the extremely high levels of dust present at the facility. Around 100 grams of dust per cubic metre was recorded, which was much too high for the laser analyser to work accurately.

“Our authorised reseller built an air chamber that was used to blow off the dust from the laser every few minutes. This allowed the laser to gain an accurate scan of the water levels and, consequently, deliver consistent and reliable readings,” said Welsh.

The initial trial of the Siemens LDS6 in TEMCO’s submerged arc furnace delivered promising results. Small leaks in the furnace that had traditionally been undetected were able to be monitored with 100% accuracy. The leaks also were confirmed several hours before any conventional methods. As a result, the data collected from this trial was instrumental in establishing a set of control limits to forewarn of any future explosions. Using this analyser, Mr Dennis was able to observe the water level trends and take action accordingly. In one case, he noticed abnormally high increases in a furnace’s water levels and, consequently, ordered a shutdown of the furnace.

“The decision to shut down the furnace was difficult because I was aware of the high costs involved as well as the production downtime it would cause. However, in hindsight, it was worth all the trouble because further examination estimated that an explosion was likely to happen within the next two hours if nothing had been done.”

Dennis was hopeful that by using the LDS6 analyser, submerged arc furnaces would be less of a health and safety concern at BHP Billiton.

“Since its commissioning, we have been able to work with the LDS6 to develop a safety procedure around this instrument. This is a big step towards achieving zero fatalities in an environment that has been considered highly volatile and unpredictable for many years.”

Dennis has since won a BHP Billiton Excellence in Safety award for his contribution towards understanding the sporadic explosive nature of submerged arc furnaces.