A reader of our recent Watergy post pointed out to me that in China, other than for making steel, coal used in China (including those used for power) is seldom washed clean of its ash content before combustion. A recent op-ed in China Daily by Dr. Chuck Wells, Chief Technologist of OSIsoft, Inc., provides great insight on the value of washing coal at the mouth of coal mines prior to transporting it to power stations as a simple “cleaner coal” strategy.
What is coal washing?
From the BBC:
Coal washing involves grinding the coal into smaller pieces and passing it through a process called gravity separation. One technique involves feeding the coal into barrels containing a fluid that has a density which causes the coal to float, while unwanted material sinks and is removed from the fuel mix. The coal is then pulverised and prepared for burning.
On the benefits of washed coal, Dr. Wells says:
A recent full scale test of a 550 MW plant sponsored by the US Department of Energy demonstrated a 1 percent increase in the output of the plant when burning washed coal. The benefits of burning washed coal include: reducing transportation costs, increasing availability of the plant, increasing its actual capacity to produce power, lowering the power consumption of process equipment, and decreasing the amount of CO2, NOx, and mercury emitted from the plant. Clearly, burning washed coal results in large returns for the small increases in the price of deep cleaned coal.
In China, that percentage increase in output would potentially be much higher if coal were washed before combustion for producing power. This is because the ash content of coal can be as high as 46%, as Dr. Wells observes at one large plant in Inner Mongolia. Thus, washing coal at the coal mine can dramatically reduce transportation costs if such ash content is washed away. Dr. Wells elaborates:
During the 2008 winter snow storms, the China rail transportation system was unable to deliver coal. Transporting washed coal would have effectively nearly doubled the transportation capacity of the railroads to deliver energy to the coal fired plants.
(GLF Note: This doubling of transportation capacity assumes, of course, that coal throughout China contains similar ash content levels–46%–as the one at the large power plant in Inner Mongolia.)
Reducing ash content is also a way to go easier on the expensive capital equipment:
…ash is typically much harder than coal and hence the pulverizers use more energy. Equipment component wear is also increased due to the harder material and higher velocities necessary to deliver the equivalent energy. One plant in US reported a 17 percent reduction in pulverizer power by shifting to cleaned coal.
Finally, the rejected ash can also be used to recondition the mine and rehabilitate the environmental landscape by depositing the ash into the mine mouth.
In light of watergy, how are we going to get all that water to was that coal?
The water consumption in these plants is low since most is treated and re-used.
If some form of closed-loop washing is used, it does suggest the need for energy to treat the water before it cycles back to the water treatment process. My question is–Where does that energy come from, especially considering the remote locations of some of these coal mines?