Dec 8, 2012

The Dirty Business of Washing Coal

Coal gets a lot of attention when they pull it out of the ground and when they burn it, but very rarely does the industry get scrutinized by the environmental movement for what happens in between. Until, that is, something really tragic happens like what happened near Lumberport West Virginia in the North Central region of the state. On November 30, a section of embankment at the Robinson Run mine slurry pond collapsed under the weight of two pickup trucks and a bulldozer. The pile of mining refuse collapsed into the water and as of this writing, the bulldozer and its driver are still missing. What is slurry and why did someone die monitoring it?
This photo taken by WVDEP shows the collapsed slurry embankment where a mining employee  disappeared.
Whether the coal comes from underground or from a surface mine, it all has to be processed before it can get shipped and burned in a power plant boiler or in a steel mill blast furnace. No one burns raw coal plucked straight from the seam because raw coal is covered in rock dust, sulfur compounds, metals and silicates that get released when the coal is burned. State and federal laws that govern what can go up the smoke stack (and subsequently into human lungs) encourage power plant operators to buy more expensive coal that is from coal seams where the coal is naturally low in such impurities or the coal can be washed and processed. In the case of Schiller Station operated by Public Services New Hampshire in Newington, air quality regulations such as those in the Clean Air Act are strict enough that the power plant is willing to import low sulfur coal all the way from Venezuela rather than spend time and money washing local coal until it meets their exacting preferences.
Motivated by RGGI, Schiller Station in New Hampshire now burns wood chips and even cocoa husks in addition to coal.
More modern boilers that operate at high efficiencies also require fuel that is low in ash. Ash is made up of whatever impurities in the fuel that doesn't burn up like silicates and trace metals. In the case of steel manufacturing, low impurity coal is essential to producing high quality metal and typically only the finest coal is selected to be "metallurgical grade." It too is carefully scrubbed to ensure nothing gets into the finished product that doesn't belong there.

The process of turning carboniferous rocks into a clean(er), denser, more uniform and efficient fuel is complicated and fraught with its own set of environmental concerns but it is in itself a process that environmentalists should be very excited about. Increasing the cost effectiveness of coal processing that reduces air pollution is something we should all be able to get behind. In October, I was afforded the rare opportunity to tour the inside of a coal prep plant, the factory that turns raw coal pieces into a fine, combustible powder suitable for combustion in a power plant. This so called "Thermal Coal" came from the low sulfur Pocahontas seam in eastern West Virginia in Greenbrier County but I will decline to name the specific mine, company and prep plant depicted in the following images for proprietary reasons.

The coal comes out of the ground here at one of the entrances to the mine. The ceiling is about five and a half feet at the middle, but don't worry about watching your head. This particular opening isn't used by miners but when it is in operation it uses a conveyor system used exclusively for removing coal. 
The raw coal is dumped out of the mine and onto the fuel lot where it waits to be processed.  
This covered conveyor belt brings raw coal from the fuel lot into the prep plant. The plant itself is about six stories tall but it is packed with equipment. I found that I could take very few pictures inside not only because it was dimly lit but because I couldn't walk far enough away from the big machines to take any decent shots. The loud operation made it almost impossible to hear the mining engineer guiding us through and I wondered how anyone could stand to be in this damp, dusty and deafening work environment. As it turns out, the whole operation is monitored from a single office chair in a nicely air conditioned office.

First, the coal is carried by a covered conveyor belt from the fuel lot to the prep plant where anything that isn't coal is removed from the raw product. An electromagnet sweeps over the incoming rock and picks up bits of mining debris, springs and chipped cutting blades that have fallen off of machines. The rest of the steps can get more complicated. Depending on the client, a coal company might spend more or less effort cleaning the coal and getting it to the right sized particle. Older power plants don't mind a few coal chunks while the newer boilers like ultra fine, talcum-powder-like textures and the prep plant operators might do some mixing and matching with equipment to make the product just right. Check out this site to see some very cool equipment schematic diagrams of where everything goes.

In order to separate the coal particles from any non-coal particles like rock bits, the powder gets dumped into a water tank. One of the interesting properties of coal is that it is hydrophobic and tends to resist wetting, while many coal impurities like metals readily dissolve in water and are considered hydrophillic and this property can be used to separate the two. The problem is that coal and rock are both denser than water, so it all sinks. By increasing the density of the water through the addition of magnetite, a naturally magnetic mineral, the water becomes dense enough to cause the coal to float and the rock to sink. For good measure, the mixture is sent through a centrifuge machine where the liquid is shot through a cyclonic vortex that hurls the densest material to the outside and letting the lighter coal collect in the middle. Think of it like those tornado tubes you used to make by taping two liter bottles together. To help the heavy impurities separate out, detergents and flocculents are added and after all of the solids are removed, the detergents are recycled with a clarifier tank like you might see at a water treatment plant. Sometimes this water is injected into abandoned mines to be stored forever, or as it did in 2004 and several other cases, until it eventually leaks into the water supply.

Recycling the magnetite for the next batch, on the other hand is as easy as switching on an electromagnet.

To recover the finest of coal dust, coal residues are pumped into vats with tiny bubbles blown into them, causing the coal to rise to the top on a thin, frothy foam where it is skimmed off and collected as a ridiculously fine, low sulfur powder that is very valuable. And wet.

The coal dust is now scrubbed and pure, but of  it is soaking wet and far heavier than it needs to be. It has the consistency of a coarse, melted slushie and to reduce shipping costs, as much of that moisture as possible has to be removed. To squeeze out the moisture, the coal, which now has the texture of a melted slushie, is forced between two rollers that squeeze the water out. When the coal emerges dewatered, it is a continuous inch-thick sheet that is crumbly like a pancake of wet sand. This end product is unceremoniously dumped onto another fuel lot by a different conveyor belt and watching heavy coal mud slop to the ground is somewhat mesmerizing. 

So the coal gets cleaned of impurities and debris, but where does all of that waste go? While the coal gets dumped into rail cars through a tipple, the wet leftovers, called slurry, just get dumped. The resulting slurry ponds are toxic pools of detergents, metals and whatever else that has been separated out and stored behind dams. The number of active prep plants in the US is 267, of which 75 are in West Virginia. That number has a tendency to change quickly as business ebbs and flows and the plants temporarily shut down and reactivate frequently, but the number of coal slurry impoundments is a broader and more permanent problem that affects 21 states. These 596 impoundments (114 of which are scattered across West Virginia) are persistent risks. Monitoring the site is done frequently by the company and by state and federal inspectors because any movement of the slurry could mean disaster. Unfortunately, criminal penalties and fines don't seem to have many teeth in states where the coal industry is vociferously defended and standards remain low in some respects and far too high in others (on this, more soon).

The most notable was the Buffalo Creek spill that released 130 million gallons of slurry onto 16 towns downstream in 1972, killing dozens. The company got away with paying a $1 million settlement while the state footed a $13 million bill to clean up. The families of the victims received a pittance measured in thousands. For the working class West Virginians in the Southern Coal Fields region, this was just another hard edge to a tough life they had long since gotten used to.

The accident that took a man's life last week won't be the last. Critics point out that solid slurry was unstable and built on a wet, shifting base layer that should never have been allowed to happen, but industry push back to regulation has been gargantuan. Small slurry spills are a regular occurrence throughout Appalachia and Southern Ohio where clean up costs paid by the state vastly exceed the costs to the industry paid in fines. Many of the contaminants like chromium, lead, mercury, arsenic and copper will persist in the area long into the future.

So long as "clean coal" is focused only on reducing the burned emissions, we will always overlook the high cost of washing it before it ever reaches the power plant. We can do better and we should, if not for the sake of the incredible landscape of Wild and Wonderful West Virginia, then the tough people who don't deserve to be buried alive for living in the land they love.


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