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This last Friday I had to head out to the plant to meet a vendor. We have been trying to control fugitive dust from the coal handling operations. As part of the new EPA rules, we are having to burn more powder river basin (PRB) coal out of the Wyoming and Montana area rather the the coal from El Sugendo mine located in Northern New Mexico. The prb coal has lower sulphur content but with higher ash. The result is that it emits lower amounts SO2 and SO3 and SO4, but we have to deal with the effects of more abrasion in the duct work. At any rate, the prb coal is more friable and breaks apart very easily. The result is that there is lots more dust during handling. Part of the coal handling at the plant is the rail cars come in and drop the coal in a belly dump car into the rail car unloader. From the rail car unloader it goes down into 9 track hoppers and then is fed onto a conveyor belt underground. This belt elevates the coal up to the coal transfer tower. |
| From the coal transfer tower (seen here in the back ground) it transfers from conveyor #1 to conveyor #2. This second belt takes the coal mostly horizontal to the different coal stock piles. There is a tripper car located on this belt that is movable, allowing the coal to be dropped at the desired location. Once the coal drops off of the tripper conveyor (#2) it falls onto the ready piles. We typically maintain 3-4 different ready piles. This drop of 90 feet or so really stirs up the dust around the drop zone. The coal handling system was originally built to handle up to 5,000 tons of coal per hour. This was fine when we were taking delivery of McKinley coal (from a Peabody mine in Northern New Mexico), but that mine has shut down due to running out of easy to access coal. The coal seam went too deep and they didn't want to invest the money to extract it out of the ground. So this facilitated a shift in the coal the plant purchased (oddly enough we had to deal with this same coal supply problem when I was in Snowflake - same supplier). |  |
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Long story short, the prb coal is harder to deal with due to the dust. It also has a very high tendency to spontaneously combust which results in coal fires at random places in the coal pile at a rate of about 3 per week. One of the vendors that I had talked to about supplying dust control equipment was Company Wrench. They are primarily a demolition company, but have recently started making and selling dust control equipment. They make the dust destroyer, which is pictured here. This is a unit with a large fan (37,500 cfm) with two different nozzle arrays around the discharge chute. It is powered by a 80 hp diesel motor. The diesel also powers a booster pump the can deliver an additional 150 psi to the nozzle array. This results in a fine mist fog being projected up to 250 feet from the unit. The air coming out of the nozzle is upwards of 120 mph. We thought we would give this thing a try (we are open to all options currently) as we are trying to comply with the AZDEQ. We are limited to a 40% opacity from any dust generated at the plant, and this drop currently exceeds this limit. |
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Here is what the air looked like just after we started to unload coal from the conveyor. Since we didn't have a train scheduled to be on site for unloading during the window the Dust Destroyer was able to be on site, we took a loader and loaded the rail car unloading hoppers with coal that was sitting on the pile. We wanted to get a good feel for how it would work. |
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As the photos show, this product clearly is not capable of taming the dust generated from this PRB coal. The salesman (shown here) recommended many units to simultaneously spraying to try to control the dust. The problem is that the coal dust doesn't want to be blended with water. You would need to add a surfactant to the water to aid in this process, but at $100,000 per dust destroyer, I don't think that we will be pursuing that route anytime soon. |
| In the background you can see the coal dust around the boilers after we quit unloading. We are currently trailing a GE product to add foam to the coal dust, and it seems to be controlling the fugitive dust pretty well. The last test was successful, but we want to make it successful on two trains just to verify that the first train didn't drive through a thunderstorm on its way down to our facility. Currently the chemical costs are estimated to be about $120,000 per year for dust control. Just one more cost that we are having to add to process coal to help out on environmental factors. |  |