MagMill
The MagMill combines dry pulverization and magnetic separation at a power plant to remove mineral contaminants from coal more efficiently and economically than can be done at a prep plant. Developed to be installed as a retrofit in pulverized-coal-fired power plants today, the MagMill offers the opportunity to change the design of new coal combustion or gasification plants which will have lower capital and operational costs. The technology has both environmental and operational advantages. First, it removes ash forming minerals containing sulfur and hazardous trace metals such as mercury, arsenic, selenium and others. This lowers the cost of post-combustion control including fly ash disposal. Secondly, by selectively removing hard and abrasive minerals from the feed coal before they are over ground, deleterious sources of abrasive and erosive wear and of slagging and fouling are removed before the coal enters the combustor. This is vitally important in modern supercritical power plant design where steam tube tolerances are especially small.
A 3 ton per hour (TPH) MagMill demonstration unit was located at the DTE Energy Services' pet coke processing and shipping facility in Vicksburg, MS, from 2007 - early 2011. The DTEES CE Raymond 352 pulverizer and MagMill LLC's separators are shown below. Coals of all ranks from across North America were tested. For more information contact Dr. Robin Oder or call him at 724-325-4431. An overview of the MagMill and results of testing are given below.
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What is a MagMill?
The MagMill removes the hard to grind (low value HGI), dense (heavier than the hydrocarbon component of coal) and abrasive minerals (iron carbonate, iron pyrite, etc.) from coal while reducing grinding energy and abrasive wear and improving the quality of coal by lowering mineral content, sulfur, and hazardous trace metals.
The MagMill can treat all coals where the particles to be removed are at least feebly magnetic and are “liberated” or “semi-locked” in the nominal 8 mesh to 200 mesh size range. Ash, sulfur and hazardous trace metals must be associated with magnetic particles. Liberation is the function of the pulverizer. Removal is the function of the magnetic separator.
The MagMill as applied to a B&W MPS pulverizer is shown here but can be used with any vertical mill.
Coal feed falls onto the pulverizer grinding table from above and is slung outward as the table rotates beneath large metal tires which crush the coal as they roll over it. Hot air swirls upward around the outer circumference of the table and carries the fine coal released in the milling upward to the classifier at the top of the mill where oversize particles are separated and returned to the grinding table. The concentration of hard and abrasive minerals is significantly greater on the surface of the grinding table than in the feed coal because these minerals require more passes through the grinding zone to reach product specification than does the soft hydrocarbon component of the coal . The MagMill withdraws a stream of these concentrated minerals from the lower regions of the mill and passes it to a dry magnetic separator outside the pulverizer. The separator recovers the carbon for return to the mill and rejects minerals that otherwise would have gone to the burner.
Test Results
Pennsylvania Upper Freeport Coal: At 95% Btu recovery, reductions in concentrations of ash, sulfur and mercury were 34%, 24% and 31% respectively for an Upper Freeport coal.
Powder River Basin Coal: Reductions in concentrations of mercury, arsenic, and pyritic sulfur ranging between 55% and 65% on a mass/MBtu basis were achieved at 98% Btu recovery for a Powder River Basin coal.
Western Lignite Coal: The percentage reductions in ash, sulfur, mercury and pyritic sulfur given on a mass/MBtu basis illustrate the extraordinary ability of the MagMill process to separate feebly magnetic iron pyrite, even from coals with low concentrations of ash and sulfur, such as the Western coals. In this case, almost 66 percent of the mineral sulfur was separated by the MagMill. Independent combustion testing showed that steam tube erosion index values decreased by 48% on average when firing magnetically processed lignite.
