In the past, it has more or less been a general rule that any time the total product is being washed, both mechanical and thermal drying was required. The reason for this was that a product moisture of 3 to 4% was required and thermal drying was and is the only practical method by which this can be accomplished.
Some of the items which had to be considered in the economic study were the following:
- Present and future thermal dryer operating costs.
- Product moisture required by customer.
- Could mechanical dryers be used to replace the thermal dryers.
- The operating cost of mechanical dryers.
The first step was to determine what the moisture would be if centrifugal dryers were used on some or all size fractions and the use of thermal drying was discontinued.
The following is a breakdown of the drying of each major fraction before the change: The plus ¼” material was being dewatered by use of vibrating screens to a moisture of 4. 2%, The ¼” x 28 mesh material was being mechanically dried in centrifuges to a moisture of 6.8% and then thermal dried. The minus 28 mesh material was being dewatered by a vacuum disc filter to a moisture of 34% and was then being thermal dried.
After considering the different alternatives, it was determined that by adding Bird screen bowl centrifugal dryers on the minus 28 mesh coal, Bird 1100 dryers on the 1-¼” x ¼” coal and discontinue the use of the thermal dryers, the total product moisture, after these changes, would theoretically be 6.8%. Note: The actual moisture after change was 6. 8 to 7%.
An existing plant was reviewed to determine if the scrubbers on the thermal dryers should be upgraded or the dryers shut down. The plant was producing a 3.5 to 4% moisture product using the following drying procedures: The 6″ x ¼” was dewatered only on vibrating screens giving a moisture of 3.7%; the ¼” x 28 mesh product was dewatered in Bird 110 centrifugal dryers giving a moisture of 8.9%; and the 28 mesh x 0 coal was dewatered by vacuum disc filtration with a resulting moisture of 26%.