COAL PREP – ART, SCIENCE, OR RECLAMATION TECHNIQUE?
by Harold Hough
Is coal preparation art or science? On one hand, it deals with the hard facts of chemistry. On the other hand, there are several ways to prep coal.
The reality is that coal prep is more. It is now a reclamation technique and a way to improve profits.
Coal prep is considered a necessity today. Not only does it remove unburnable ash and pollutants like sulfur, it makes it cheaper to move (more BTU per pound of transported coal), provides a more uniform quality, and reduces coal powered plant maintenance.
Coal preparation must balance the type of coal mined with the needs of the customer, the demands of government regulation, and even transportation issues. If the cost of diesel goes up, it makes transportation more expensive and may change the prep process to increase the number of BTUs moved per gallon of fuel. If it goes down, the trade-off of fuel cost versus coal prep cost comes into play.
The Art and Science of Coal Prep
The three top factors for coal preparation are based on the physical and chemical properties of the coal, how it will be used, and the coal prep facility. Metallurgical coal, for instance, requires a more complex coal cleaning process, while power plant coal requires less effort.
Coal prep also means different things in different countries. American coal prep has an advantage because the product coming from the coal mine is more uniform. In less developed countries like China, the raw coal coming into the plant fluctuates more in quality and size than in Western nations. Chinese coal is also very difficult to wash.
It's also important to remember that coal prep isn't one technology or field. It is several technologies that are constantly advancing as vendors produce better equipment. And, each innovation has an impact on the final product. For instance, processes that reduce the amount of unwanted ultra fine coal particles will reduce the problems associated with dewatering the coal. And, an improvement in physical separation methods based on relative density will have an impact on the need for chemical separation methods, sizing, and classification.
There are a number of coal cleaning technologies and they generally separate the coal from impurities by either physical or chemical methods. The physical methods rely on the relative densities of coal versus most of its impurities. While pure coal has a density just a bit heavier than water, the impurities are usually twice as heavy as water. Cyclones, which feed the coal into a circular current of air, force heavier impurities away from the clean coal by centrifugal force.
Chemical methods usually rely on the fact that the surface of coal is hydrophobic (repels water), while most impurities are hydrophilic (attract water to its surface). Chemical methods that rely on this method are more effective with smaller particles, while methods that use relative density require larger particles. Chemical methods frequently use the same type of floatation tanks found in other parts of the mining industry. Fine coal particles in the chemical solution attach themselves to air bubbles and float to the top, where they can be mechanically skimmed off.
Coal Prep for Profit and Reclamation
Coal prep today is also a way to improve profits for the coal company and even reclaim old slurry impoundment ponds. "For the first time in the history of coal mining, the industry can now recover much of the fine coal that has previously gone to waste due to the lack of appropriate separation technologies," said Dr. Yoon Of Virginia Tech.
A recent study conducted by the National Research Council under congressional mandate reports some 70 to 90 million tons of fine coal are being discarded to coal slurry impoundments annually by the U.S. coal industry. One company in Southwest Virginia, Pinnacle, took a coal slurry pond and invested $1.5 million in a technology called hyperbaric centrifuge. The process is described as the spin cycle in a washing machine combined with compressed air. With it, they made a $50 million profit by selling the previously unsalable coal. They also reclaimed a large piece of land worth several million too.