As coal production continues to grow at a rapid pace globally, driven by the enormous growth of coal-fired power generation in India and China, the presence of low-rank coals with high moisture, in the overall mix of thermal coals, is on the rise. Decidedly, the generation cost per KwH has been the main driver of the growing share of LRCs in low-income countries but, also, the highest quality, most readily minable coals are now less abundant. A de-watering technology, alongside of a “clean coal” process are the global industry’s clearest technology priorities.
CCTI’s process addresses three fundamental challenges that have proved elusive to the majority of participants in the industrial coal-drying segment. These are the challenge of a product that will not re-absorb moisture; the challenge to produce a product of low friability that can be safely transported with minimal risk of spontaneous combustion; and the challenge of a process that is inexpensive and economically viable. We have succeeded in all three areas. As is the case in the Pristine™ processes, we do not pulverize the feed coal. The raw coal suffers almost no degradation and, consequently, briquetting or pelletizing is not a part of our process.
CCTI’s Pristine-M™ is a continuous process that is comprised of three separate components. Based on the original Pristine™ design, the Pristine-M™ process utilizes a devolatizer to produce gases that are used for process heat as well as to stabilize the dry coal.
Only a small portion of the feed coal (typically less than 7%) is devolatized. Process parameters are optimized so as to produce only enough volatile gases for the mentioned purposes. Liquid byproducts are not desirable in this process. Excess devolatized coal is blended back with the dry and stabilized coal at the end of the process and, thus, is not sacrificed.
A second component of the process is comprised of Carrier-designed dryers that are manufacture throughout the world. Drying takes place at 250° F, a temperature that is adequate to drive off inherent moisture with the degree of removal, e.g., down to 15% or 10% or 5%, being a function of residence time, bed depth and temperature. Certain coal types have a tendency to degrade into fines as a function of the degree to which the raw coal is dried. In such cases, the decision to remove less moisture or install a small briquetting line may be make sense. It is an economics decision, in the main.
Pristine-M™ is designed to remove fines at various stages of the process, making them available for combustion (process heat), if desired. The few fines that enter the third phase of the process tend to agglomerate and harden on the surface of the dry and stabilized coal.
In the third stage of the Pristine-M™ process, namely, the stabilization/ Vapor Phase Deposition phase, the volatile matter is caused to be absorbed into the pores of coal from which the moisture has been driven off. To achieve the desired result, stabilization parameters are established based on the chemical profile of the feed coal.
By using the Pristine-M™ process, the coal is rendered impermeable; structural integrity is maintained; and the heat value of the coal may be enhanced beyond what would occur from the removal of moisture alone. Tests indicate the same Hardgrove Grindability Index (HGI) of the product coal as of the feed coal. The Pristine™ coal does not have a tendency to break in transport and also has a shelf life long enough to sustain long ocean or surface transportation and long periods in coal yards exposed to the elements. Samples of Pristine™ have survived for well over 6 months outdoors, unsheltered, without re-absorption of moisture.
Like the Pristine process, the Pristine-M™ process is modular. It is based on a commercial module designed to feed through 30 tons per hour that, handling 50% moisture coal, would produce about 160,000 tons of dry coal per year. A one million ton per year plant would be comprised of 6 modules. The process is continuous with resident times estimated to be about than 15 minutes, depending on the degree of moisture removal and the inherent moisture in the coal. The plant operates at atmospheric pressure.
When there are significant variations in the feed coal (structural, chemical, or inherent moisture), the CCTI drying plant is designed to handle the different coals. In our system, contains a control system that will automatically adjust for differences, within a limited range. It is also possible to run coals of different characteristics through the process simultaneously, using separate modules able to address the differences appropriately.