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ABSTRACT
In conventional rotary drying process, lignite is fed into the drum with rotary-valve feeder and hot flue gas is injected concurrently at the bottom of the feeder. It is called single-stage drying since lignite is passed through one-step drying process. This study presents dual-stage drying process in which the primary dried coal is reintroduced into the rotary drum which results in secondary dried coal. Its effect on particle size distribution and total moisture at various feed mass flow rates ranging from 325 to 500 kg/h were investigated. The results show that total moisture of lignite decreased significantly for both single-stage (23.9%) and dual-stage drying (11.7%) without any spontaneous combustion even at excessive temperature as high as 188°C. Recirculation of flue gas creates less oxidative air which ranged from 16.5% to 19.7% inside the rotary drum. Standard deviation of moisture content implies that dual-stage drying process is able to reduce variation in moisture content which means better drying consistency.
Acknowledgements
The authors gratefully acknowledge the support from the Head R&D Center for Mineral and Coal Technology and the Head R&D Agency of Ministry of Energy and Mineral Resources for their support and assistance in carrying out this study. The authors would also like to acknowledge all technicians, researchers, and engineers of the Coal Drying and Briquetting team.
Nomenclature
| MRF | = | mass feed rate (kg/h) |
| HGI | = | Hardgrove Grindability Index |
| PSD | = | particles size distributions (%) |
| kcoal | = | thermal conductivity of coal (W/mK) |
| kw | = | thermal conductivity of water (W/mK) |
| ka | = | thermal conductivity of ash (W/mK) |
| kcv | = | thermal conductivity of carbon and volatile matter (W/mK) |
| Hev | = | heat of evaporation (MW) |
| Lev | = | latent heat evaporation (kJ/kg) |
| Cw | = | specific heat capacity of water (kJ/kg K) |
| Tamb | = | ambient temperature (K) |
| Rf | = | fragmentation rate (%) |
| Wf | = | amount of fragmented lignite (%) |
| W0 | = | amount of initial lignite before drying (%) |
Greek letters
| = | volume fraction of water | |
| = | volume fraction of ash | |
| = | volume fraction of carbon and volatile matter |