A Beginners Guide To Coal Preparation
by Harold Hough
Okay, the title, “A Beginners Guide to Coal Preparation,” is a bit misleading. It’s sort of like saying, “A Beginners Guide to Painting the Mona Lisa in your Garage.” There’s a lot more to it than that. Coal Preparation in many ways is an art that balances the type of coal mined with the needs of the customer, the demands of government regulation, and even transportation issues.
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.
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.
Although there are any numbers of coal prep methods, the total process falls into four general processes: Pretreatment, Coal cleaning, Coal sizing and classification, and coal dewatering.
Pretreatment. Since coal prep depends to a great deal on the size of the coal, pretreatment is primarily crushing and screening. The final size is based on how the coal will be cleaned and how it will be used.
Coal Cleaning. There are a number of coal cleaning technologies, but 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.
Coarse coal, which is a declining percentage of the coal market, and is often used in older coal fired home furnaces, is cleaned with different methods like very heavy liquid solutions.
Coal sizing and classification. Once the coal has been cleaned, it is sized and classified. While vibrating screens can classify larger particles, they become less effective as the size of the coal particle declines. Manufacturers are always in search of better methods to more accurately classify coal particles.
Coal dewatering. While larger coal particles have been easy to dewater, smaller particles pose a major problem. This has posed one of the biggest problems for coal prep engineers.
There are several filtering methods currently in use around the world that remove most of the water from most coal particles. However, the smaller the coal particle, the harder and more expensive it is to dewater it. Consequently, coal prep operations are left with some coal that is uneconomical and must be stored in impoundments.
Fortunately, coal prep isn’t a static technology. Scientists at the University of Virginia have developed dewatering techniques that can make coal as fine as talcum powder a salable commodity. The process called hyperbaric centrifuge is described as the spin cycle in a washing machine combined with compressed air.