Dry cleaning of coal has not found acceptance in commercial applications because of the lack of methods suited to separation and removal of non-combustible minerals generally smaller than ¼ inch. With the advent of modern magnet technology it is now feasible to remove ferruginous minerals including iron pyrite, a major source of sulfur in coal, in sizes down to nominally 100 - 200 mesh. The MagMill is one such innovation for the dry cleaning of pulverized coal at the point of use such as coal-fired power plants.
The MagMill combines pulverization and dry separation, including magnetic separation, at the point of use to remove mineral contaminants from coal efficiently and economically. 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 and HAP's removal. 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. Other uses of the MagMill include preparation of a clean feed to gasification and pelletization processes.
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 except anthracite from across North America were tested. . The DTEES CE Raymond 352 skid-mounted pulverizer is shown in the left photo. EXPORTech's magnetic separators are enclosed in the orange structure shown in the right. The pulverizer is to the left of the separators.
For more information contact Dr. Robin Oder or +1-724-325-4431. An overview of the MagMill and results of testing are given below
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.
Ten to twenty tons of each of seven coals including Eastern US bituminous coal, Western US sub-bituminous and PRB coals and Canadian lignite were processed at the Vicksburg site.
Pennsylvania Upper Freeport Coal:
Percent reductions in ash, sulfur and mercury expressed as pounds per Million Btu or pounds per Tera Btu achieved for five MagMill runs processing Upper Freeport raw bituminous coal from western Pennsylvania are shown in the visual. The feed coal contained 27% ash, 1.65% sulfur, and 0.26 ppm mercury. The Btu/Lb level increased from 10,986 in the feed to 12,219 in the product. At 95% Btu recovery, the reductions in ash, sulfur and mercury were 34%, 24% and 31%, respectively. The MagMill system preferentially removes the hard to grind elements that contribute to loss of plant efficiency through unscheduled outages caused by pressure part wear in the pulverizer, boiler and coal transport piping.
Powder River Basin Coal: Twenty tons of Decker Seam PRB coal were processed at Vicksburg. The feed coal contained 21.55% moisture, 5.6% ash, 0.38% sulfur, 0.012 ppm mercury and had a dry heating value of 12,641 Btu/lb. Operating at 98% Btu recovery, the MagMill achieved 65% reduction in Mercury, 55% reduction in Arsenic, 60% reduction in Pyritic Sulfur, 17% reduction in total Sulfur, and 7% reduction in ash. These results would allow a power station to achieve its goal of reducing total mercury and sulfur using less sorbent in its existing ESP's without increasing total particulate emissions.
For reasons of economy the hot air was recirculated through the pulverizer at the Vicksburg facility, resulting in moistures higher than would be expected at commercial coal-fired power plants where the air is not recirculated. For this reason the Vicksburg results represent the lower bound of results achievable in commercial units.
Canadian 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 North American lignites. The feed ash and sulfur were 16.3% and 0.80% respectively. Pyritic sulfur was reduced by 55% and hematite by 37%, resulting in a 44% reduction in Fe2O3 in the coal and a 26% reduction in SO2 emissions.
Approximately one ton each of the feed lignite and the MagMill product were subjected to combustion testing at an independent testing laboratory. The effects on elements and minerals show that
· Magnetic separation significantly reduced iron, titanium, phosphorus, sodium, and potassium concentrations and increased calcium, magnesium, sulfur, barium, and strontium concentrations of the coal ash;
· Increases in calcium and magnesium contents enhanced sulfur absorption capacity of the coal ash;
· As expected, the magnetic separation process significantly reduced the hematite and pyrite contents of the coal;
· The magnetic process was effective in removing 13% of the quartz that is a major contributor to erosion at the Power Station.
The combustion tests also showed that the MagMill has a positive effect on the plant operation.
· Steam tube erosion index values decreased on average by 48% when firing the magnetically processed coal;
· Steam tube erosion index for Unit 1 decreased from the medium-erosion-severity range of 34 to 66 to the low-severity (<33) range as a result of the magnetic treatment;
· Firing of the magnetically processed coal reduced SO2 emissions by 26% but did not significantly affect NOx emissions;
· The deposition rate during firing of the magnetically treated coal was about one-half of what occurred during combustion of the untreated coal.
The MagMill technology is specific to removal of abrasive minerals and elements from the feed to the burner. The effect can be a dramatic reduction of maintenance costs, especially for plants of supercritical design.