November 2013

Heat seeking

CSIRO technology aims to convert slag heat into useable energy

By Alexandra Lopez-Pacheco

Researchers at the sustainable metal production group in Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) are getting close to commercializing a dry granulation process that turns hot slag into a money-making product. The hundreds of millions of tonnes of molten slag created during steelmaking every year are currently a byproduct the industry has to live with, with limited benefits.

“There are over a billion tonnes of steel produced in the world each year and that creates 200 to 300 million tonnes of slag, which is an enormous source of heat that is not being recovered,” says Sharif Jahanshahi, theme leader of sustainable metal production at CSIRO.

Historically, the process for treating 1,500 C molten slag was simply to let it air-cool, but that left tonnes of solidified slag waste to be broken up and hauled away. It also wasted all that heat. In fact, according to CSIRO researchers, about 1.8 gigajoules of heat is lost for every tonne of molten blast furnace slag cooled. To put this waste in perspective, the global total of energy wasted during this step is roughly equivalent to one fifth of Canada’s electricity generation sector.

The most common approach to cooling slag these days is to use water to both cool and granulate it. This way, the slag is turned into glassy granules that can be used as a substitute for Portland cement and for bringing in some revenues ($25 to $35 per tonne in Australia, for example). Morphing slag from disposing of it as waste into a product that can be sold is obviously a step forward, both environmentally and economically, but all that thermal energy in the hot slag is still lost with wet granulation, along with the fresh water used. Some 1,000 to 1,500 litres of water evaporate for each tonne of slag treated. The process can also be potentially hazardous, says Jahanshahi: “There’s the potential risk of explosions with wet granulation when the slag contains molten metals such as iron or mattes.”

The holy grail of efficiency

Since the 1980s, various re­searchers have attempted to develop a dry granulation process by breaking up molten slag into small droplets using mechanical means like air blasting, rotary drums or spinning discs, followed by solidifying the slag droplets and at the same time recovering the high-grade heat with air. But there were always snags, and none have come close to commercialization to date.

In 2002, a team at CSIRO decided to take up the challenge. They initially analyzed and evaluated previously available research and then concluded the method closest to solving this problem was dry granulation using a spinning disc or cup. This method used the least amount of energy to atomize molten slag and provided a more efficient and controlled process. But previous attempts to use a spinning disc were hindered by significant design and operating issues such as the need to suppress slag wool and handle hot droplets and granules.

“Dry granulation with a spinning disc is a very fast process that breaks up the slag to form fine droplets using centrifugal forces,” says Dongsheng Xie, project leader at CSIRO. “The slag spreads and breaks up in a fraction of a second. You want to produce small slag droplets for fast and efficient heat exchange, so that you can quench them very fast to produce the glassy product that can be used as a substitute for cement.”

But because the process occurs very quickly, things can also go awry quickly. “Even if you produce droplets, how you handle them and recover the heat quickly is not a simple task,” says Xie. By 2006, his team had made major breakthroughs to enable them to design a process to achieve both dry granulation and heat recovery at the same time. The process they developed is based on a two-step operation. The first involves putting the molten slag in a dry granulator that atomizes it into small droplets. These are quenched with air rapidly and solidified. They are then processed through a moving bed counter-current heat exchanger where they are further cooled almost to ambient temperature and the heat is captured. A key component to the process’s success is the precise use of air in both units. It has to be as minimal as possible in order to maximize the temperature of the output air stream.

Partners now crucial for success

To prove their process, the CSIRO researchers built a pilot plant with a 1.2-metre diameter granulator that processed up to 0.6 tonnes of slag per hour in 2007. Armed with good results from these tests, in 2009 they scaled it up to a semi-industrial scale plant that could process slags at up to six tonnes per hour. “The next step is to take it to industry-scale so we can process 20 to 60 tonnes per hour and demonstrate the process at a blast furnace site. That will confirm the performance and from there we can proceed to commercialization. In reality, this will likely take about two or three years,” says Xie.

Terry Norgate, a chemical engineer who was working as part of the project team before he recently retired, has carried out a detailed economic assessment of dry granulation’s benefits, which found that both the capital and operating cost of dry granulation will likely be roughly half of those for wet granulation, while also reducing greenhouse gas emissions, water use and waste.

Based on Norgate’s estimate, the operating cost of the dry granulation process would be about $4.74 per tonne of slag for a 300,000-tonne per year slag plant, with a capital cost of about $9 million, excluding any capital costs associated with use of the recovered waste heat.

CSIRO is currently in confidential negotiations with potential industry partners to scale up and conduct industrial trials at a blast furnace site. “The next stage is critical,” says Xie. “We have to find a partner who can design and build a dry granulation plant and manage all the on-site operational challenges. With the process itself we need to demonstrate the design and make sure all the material processes will continue. We are limited in the lab tests by the amount of slag available, but in the next stage we will have [the] opportunity to test our design under continuous operation. Still, from laboratory to industry, there can be a lot of variations and fluctuations, and we will be looking to work closely with our industrial partners to overcome these challenges.”

Should they succeed, those hundreds of millions of tonnes of molten slag around the globe could end up being part of a cost-effective solution that helps produce heat and power and builds foundations for buildings in a greener world, while generating revenues: a win-win-win.

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