Mechanical engineers work to securely sequester coal emissions

A lump of coal is a lump of pollutants, including sulfur, mercury, and, of course, carbon. Still, because coal is cheap, abundant, and chock full of energy, the sooty stuff is going to be burned all over the world for years to come. Mechanical engineering Associate Professor Chris Edwards figures Stanford engineers might as well try to make it less harmful, and he thinks they might just have a way.

“The joke is that coal has the whole periodic table in it, but resources like fossil fuels are incredibly high density in terms of energy,” Edwards says. “Unfortunately more than 85 percent of our power comes from carbon-containing fuels.”

If that’s the reality, then the task is to redirect the nasty byproducts from the air to somewhere else, such as giant, sealed aquifers of brackish water a mile deep in the earth. Edwards, mechanical engineering Associate Professor Reggie Mitchell, and graduate students J.R Heberle and Paul Mobley are not the first people to propose such an idea, but they have conceived of a new method for doing so that could be more effective than other techniques.

In the September issue of the International Journal of Greenhouse Gas Control they describe a power plant that extracts energy from coal without releasing anything into the air. In essence, the flame that burns the coal is embedded within water pumped up from the aquifer, and is therefore never exposed to surrounding air. Some elements of the coal end up in a kind of ash that can be trucked away to disposal sites, and the rest becomes securely trapped in briny water that is pumped back into the aquifer.

This month, the researchers will begin the next step: physical testing. In a lab trimmed with stainless steel and bullet-proof glass, they’ll fire up a miniature version of their plant’s combustor. Success in translating the theory and calculations in the paper into a working model would be a step that is both encouraging and fulfilling.

“The combustor is one of many pieces that have to be researched heavily for the full concept to be realized in an operating plant,” Heberle says. “It’s encouraging that we’ll have this one part well on its way.”

The trouble with bubbles

The concept is designed to address problems that earlier aquifer storage proposals haven’t solved.

The ‘traditional’ way of stuffing coal emissions into an aquifer is to capture carbon dioxide at the smokestack and then pump it down into the water, somewhat like carbonating water to make seltzer. The hope—and this is being tested by teams at a few sites around the world— is that the gases will either be mixed into the water, or at least remain sealed in by the surrounding rock. Eventually, after a century or so, theory predicts that they will become permanently absorbed.

The potential problem is that the mixing isn’t reliable. Much like seltzer is prone to going flat, the gases pumped into an aquifer have a similar tendency to escape. If there are any cracks in the surrounding rock, they could seep back out into the atmosphere.

Another approach is to isolate the carbon emissions of coal burning and then to mix them at very high pressure into water pumped up from the aquifer. Then the carbonated water is pumped back down. In this method, Edwards says, the gases remain more reliably mixed into the water, but the process of isolating the carbon dioxide in the first place is a significant drain on resources, and therefore the economic and energy efficiency of a power plant.

Fire in the water

The Stanford group takes these ideas a few big steps further. They burn the coal completely inside aquifer water that has been subjected to so much heat and pressure that it’s in a ‘supercritical’ state. The water seals the combustion process off from any contact with outside air.

Supercritical water is normal water that has been heated to more than three times the boiling point (705 degrees Fahrenheit) and subjected to about 220 times atmospheric air pressure (221 bar). While many fuels are not soluble in regular water (e.g. oil slicks float), they blend well with supercritical water.

What the Stanford plant would do is grind the coal into particles, and heat those to separate out the inorganic minerals. The minerals go into an ash. What’s left is mostly the fuel, which becomes mixed into a soupy fluid, or slurry, with temporarily desalinated, supercritical aquifer water.

The combustor, where the fuel is burned, is a long tube. Oxygen separated from surrounding air is injected into the fuel, creating a stream that burns at the interface (typical combustion essentially involves the rapid, heat-releasing combination of oxygen with a fuel), which is entirely inside the surrounding flow of aquifer water.

From the combustor, the now very hot water ( roughly 2,420 degrees Fahrenheit) flows through a heat exchanger, providing energy that can be transferred to an electrical generator (e.g. it can boil water to make steam to turn a turbine). Heberle estimates that under properly managed conditions the plant can be about 42 percent efficient, which is comparable to other plants that have carbon capture technologies. The Stanford plant would not only securely sequester the carbon, but also many of the other nasty elements (sulfur, mercury, etc.)  that could have otherwise escaped into the air.

Such a process could retain most of the economic advantages of coal, but markedly decrease the environmental disadvantages, Edwards says.

“Engineering is about trying to provide options for people to improve their lives,” Edwards says. “Coal is everywhere. I’d like to give people, particularly in developing countries, a way to use it that is economic and safer for them and the environment.”