By Gavin du Venage
South Africa enjoys the best power generation capacity on the African continent, thanks to the large coal reserves that underpin energy production. With the abundance comes coal fines — all 50 million metric tons (mt) of it per year.
High in water content — up to 50% — it is held in containment dams where it sits malevolently until it can be disposed of. Until this happens, the slurry is a risk to surface and groundwater, as dams have a nasty habit of leaking; it is coal slurry that is at least partly responsible for the Olifants River, which runs through Mpumalanga’s coal region, having the unfortunate distinction as South Africa’s most polluted river.
A study sponsored by the Mail & Guardian in 2013 showed that pools of water that build up between mines and slurry dams had a pH of 2, or pure acid. Levels of sulfates, or salts, reach as high as 6,000 milligrams per liter(mg/L) in some waterways of the town of Emalahleni, east of Johannesburg. Internationally, levels above 400 mg/L are considered potentially hazardous to human health.
A team working under professor Ben Zeelie at the University of Port Elizabeth made the remarkable discovery that coal dust can be transformed by adding a little more than the humble algae. It turns out the bane of pool owners everywhere can help solve clean-up problems that have defeated mines for the better part of a century.
“Algae does extraordinary things to coal fines,” Zeelie said in an interview. Zeelie is a professor of chemistry at the university’s Summerstrand campus and one of the country’s leading voices on industrial chemistry.
Zeelie explained that, when added to coal slurry, algae binds to the carbon, which makes it relatively simple to separate it out from the more unpleasant elements that usually accompany it, in a process the team has trademarked as “Coalgae.” Coal inevitably has concentrations of arsenic, mercury and selenium, minerals that add considerably to the toxicity of coal.
Also excluded is clay, another common property of coal. The significance of clay is that it is one of the complicating factors in drying and processing fines into briquettes for re-use. “What we have left is coal of the very highest quality — export level quality that can be used for energy or for steel-making,” Zeelie noted.
Pressing discarded coal into briquettes to re-use is not a new idea. Most power stations will make some use of refashioned waste material. Other methods, however, require additives or binders that may carry pollution hazards of their own. Cost is another factor that has hindered binding coal fines.
Also, moisture contained in the slurry diminishes the heat value of the coal being used, and recovery is low when discard is re-treated.
The scale of the problem is significant. According to the Department of Mineral Resources, around 2 billion mt of discard coal has been generated over the past 20 years. It’s a pile that is steadily growing — another 50 million mt of coal slurry is added each year.
All this adds up to a stupendous waste of material that is potentially useable. Some companies, such as Anglo American, are experimenting with alternative uses for waste coal. Anglo’s thermal coal operations in the Emalahleni area have resulted in more than 137 million mt of discard coal, increasing at about 4 to 4.5 million mt a year through additions from continuing operations.
Anglo is looking at fluidized bed combustion (FBC), in which fuel particles are suspended in a hot, bubbling fluidized bed of ash and other particulate materials like sand and limestone, etc. Jets of air are blown to provide the oxygen required for combustion. The resultant mix of gas and solids promotes rapid heat transfer and chemical reactions within the bed.
“Anglo American is looking at the feasibility of setting up a plant in Witbank that will burn coal discard from our mines,” Anglo spokesman Hulisani Rasivhaga confirmed.
FBC is an energy intensive process, and costly. It also tends to be more efficient with larger particles, rather than the sub-250 microns size coal fines typically found in the mining region.
This is an ideal size for Coalgae, said Zeelie. When pressed into briquettes, the resulting burn is virtually smokeless and produces very little sulfur. “In nature, algae naturally binds with carbon. It rejects other components that are found in coal.”
The process, like many scientific advances, was discovered by chance. The university had been exploring algae for biofuel purposes — the subject is one of intense international scrutiny now — when they noticed something peculiar.
“We had been growing algae in plastic bags suspended above the ground. Some leaked out, and we saw it formed clumps as it mixed with dust. We thought we’d look and see what it did with coal fines. The results were way beyond what we were expecting.”
The successful binding of algae and coal fines opens up other possibilities as well. Zeelie said the technology also has the potential to take solidified coal/algae and convert it into oil. Through a process called pyrolysis, a high-temperature, oxygen-free process that converts biomass waste directly into carbon fuel, Coalgae can be turned into oil. With the pyrolysis process, biomass is treated with water at high temperature and pressure (300-350°C & 120-180 bar) to produce bio-crude.
Unlike the Fischer Tropsch process that South African synthetic fuel producer Sasol uses to turn coal into fuel or gas, pyrolysis produces almost no emissions. The trick is to do so in a commercially practical way. Pyrolysis is now one of the hottest topics of research internationally, because it can be applied to any biomass — agricultural waste, for instance — to produce crude oil.
What’s needed now is a real-world demonstration plant — something that is already in the works. The plan is to have a one-hectare site of bioreactors set up in Mpumalanga by the end of next year.
“The idea is to set up a plant to prove this technology,” said Dr. Jacobus Swanepoel, regional director, technology development, Africa, for Hatch Goba, the project managers for the plant’s design and build. “We want it done on-site, where the coal will be used.”
The pilot plant will cost around $6 million to build, which includes research equipment not required for a commercial facility.
“Export quality coal sells for around $90 a mt; we estimate our costs at producing grade five — export quality coal — at around $25 a mt,” Swanepoel said. “This project will take waste that is now worth nothing and turn it into something that has value.”
He expects the pilot plant to produce around 2,000 mt of briquettes a year — insignificant in the context of the sheer volume of available coal fines, but once scaled up, the impact would become apparent.
Keeping the algae alive and healthy is not as simple as it seems. They need water, although not a large amount, but also sunlight and carbon dioxide. Like all living things, algae also need nitrogen. These elements must be kept at the right mix in a bioreactor if the algae are to thrive.
“The idea is to take everything we learn during the pilot phase — the water use, bioreactors and so on — to come up with an estimate of the running cost and capital cost. This will be used for a cost projection for a larger site, of perhaps 100 hectares,” said Swanepoel.
It may be some time before this, or any other solution to South Africa’s waste coal stockpiles, becomes a commercially viable solution. Given the size of the problem, however, it is encouraging that work is now actively being done in this field. Eskom has made it clear that although it wants to reduce the country’s coal fixation, this is unlikely to change dramatically in the future.