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This week’s column assumes that the reader is familiar with the fact that Duke Energy has been tasked with closing or upgrading its coal ash lagoons across the state of North Carolina following a massive coal ash spill into the Dan River near to Eden, NC. If you are not familiar with the background, I recommend you read my previous column, A Tale of Two Spills, before proceeding.

When you burn coal to make electricity, just like burning wood in a fireplace, some ash is left behind. Coal ash is hazardous since it contains a number of toxic, heavy metals such as lead, arsenic, and mercury. As a result, human beings and other animals are well advised to not ingest coal ash or, even more importantly, breathe it into their lungs. Since the greatest health risk from coal ash arises from breathing it in, coal companies immerse the ash in water in on-site lagoons to keep coal ash dust out of the air.

While it is possible to recycle coal ash into concrete and several other building products, it is not an economically attractive alternative. Therefore, electric power companies all across the country have been accumulating a staggering amount of coal ash – estimates range in the trillions of tons – over the past half a century or so. Duke Energy has 108 million tons of coal ash stored in 32 lagoons across North Carolina. Since coal-fired power plants are nearly always next to rivers and lakes, so are these lagoons. In an attempt to prevent more incidents like the Dan River spill, Duke Energy has been ordered to develop a plan to remove the coal ash from the lagoons and store it “dry” in lined landfills far from rivers or lakes. Every time I read that, I am struck with the question of, “What do they mean by dry?” Let me explain why that is an important question starting with a thought experiment which will help to outline the key issues.

Let’s say that you are at the beach and have a bucket of sand with a layer of water above it and I assigned you the task of drying the sand in the most time and energy efficient manner possible. First, you should decant off the water above the sand. That’s easy to do and not energy intensive. Next you could get more water out of the sand by squeezing it. For example, if you had another bucket of the same size, you could press down on the wet sand and this would coax out some additional water that you could then decant off. At this point, you would be left with a wet cake of sand of which water would constitute approximately 25% of the total weight.

In order to remove the rest of the water you would need to evaporate it. Therefore, you will need to apply heat, a substantial amount, and you will need to mix the sand during the process. If you do not mix the sand, the water in the interior of the cake would take a very long time to diffuse the surface of the cake were it can evaporate and escape. Your possible heat sources are the sun, provided it is not cloudy or night time, or you could build a fire. As you have likely inferred, the point of this thought exercise is to illustrate that removing water trapped within cake of small particles is not a simple task.

A lagoon of coal ash shares has quite a bit in common with a bucket of wet sand. At the bottom of the lagoon is a sludge of wet coal ash particles beneath layer of water. The water can be decanted off without difficulty. Just as was the case with the sand, more water could be removed from the coal ash sludge by squeezing it. There are many known technologies that could be used such as a belt press. However, running tens of millions of tons of wet coal ash cake through a belt press would take a very, very long time. So perhaps Duke Energy would choose to skip this step.

I was unable to find physical property data for coal ash, so I need to make an assumption before proceeding. Based on similar materials, I estimate that pressing coal ash sludge would create a wet cake containing approximately 20% water.  Therefore, the 108 million tons of coal ash in Duke Energy’s lagoons would correspond to 135 million tons of wet cake containing 27 million tons of water.

So this brings us to my question. Is this wet cake of coal ash what Duke Energy is describing as dry? To my engineering mindset, the answer should be “no.” However, I suspect this is what Duke Energy is describing as dry coal ash and, despite my quibbles with word usage, I hope this what they mean. There are two important reasons for this. The first is energy consumption. Evaporating 27 million tons of water to truly dry the coal ash would require 53,000,000,000,000 BTUs of energy. To give you an idea of how much that is, it is equal to the energy required to provide electricity to 1.4 million average American households for a year. The second problem is dust. If Duke Energy thoroughly dried the ash, especially since you have to mix it to get it to dry, they would generate a lot of dust. The best way to control the resulting dust would be to spray it with water, which would be an absolutely ridiculous thing to do after just drying it. So what are they really doing?

I have been trying to figure this out for a while. Duke Energy has a section on its website about its coal ash clean up program complete with several videos. However, the question of what constitutes dry coal ash is not addressed. There is some footage of a bulldozer scooping up some coal ash from a lagoon that has no standing water. No dust is begin generated by the bulldozer with suggests that the operator is scooping up wet cake rather than dry ash, which suggests that my theory is correct. If so, then Duke Energy is being prudent in not expending the energy necessary to dry the ash and safety conscious in not generating a risk of dust exposure. However, if this is true, they are shying away from discussing the science underlying this effort. I’m not sure why that is but, not to worry, Common Science® is not similarly shy.

Jeff Danner spoke with Aaron Keck on WCHL Monday.

 

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