Coldry technology for low rank coal drying
Coldry technology is being developed by Environmental Clean Technologies (ECT) Limited, Australia. It is a patented process which changes the naturally porous form of low rank coals to produce a dry and dense pellets by a process which is called as ‘brown coal densification’(BCD). It consists of expelling of water from a wide range of lignite and sub-bituminous coals containing upto 70% moisture into high calorific value (CV) black coal equivalent (BCE) pellets with a moisture content of around 10%. BCE means that the net energy value of the Coldry pellets is similar to that of many black coals.
Coldry process combines two unique aspects namely
(i) brown coal densification, and
(ii) waste heat utilization.
The process stimulates a natural chemical reaction within the coal, which polymerizes active sites in the coal compounds and expels chemically bound water.
Fig 1 Chemistry of Coldry technology
BCD is a natural phenomenon whereby the physical structure of the coal is transformed from a wet, soft, friable raw material to a dense, dry, hard material. It takes a very specific type of processing to apply shear-stress over time to trigger BCD. The primary processing equipment design and operating parameters are tailored to the characteristics of the raw coal.
Application of right amount of mechanical shear to the raw coal results in a soft and malleable coal ‘paste’ which enables low pressure extrusion of the paste to form pellets. Fundamental here is that the physically trapped moisture is mobilized and, as this moisture migrates to the surface of the pellets and evaporates, the porous structure of the pellet collapses and densifies.
Control of drying rate within a predictable timeframe is important aspect of the Coldry process. Further, since generation of heat through traditional methods is relatively expensive, Coldry process harnesses waste energy resources and directs the heat to low temperature drying of the pellets. BCD proceeds ideally in the range of 40-70 deg C.
Coldry technology has following three distinct process stages:-
- Mechanical shearing – It is for release of physically trapped moisture, which is achieved through destruction of the porous structure of the coal. Mechanical shearing process results into coal slurry which is of suitable consistency for extrusion.
- Extrusion – Extrusion is carried out of the coal slurry to produce pellets of optimal dimension for subsequent drying.
- Drying – Drying is carried out with waste energy from an adjacent PPto evaporate the mobilized moisture within the pellets to obtain a finished product having moisture content <15%. Waste energy from any other source can also be used.
Process of coal drying
- Harnesses low grade waste energy as its main source of energy and hence the process is economical and reduces the CO2 footprint.
- Reduces evaporative water loss at the adjacent PP.
- Enables recovery of upto 95% of the water expelled in the drying of the raw coal.
- Process is simple and mechanical providing high reliability and easier maintenance.
- Process takes place at low temperature and low pressure thus reducing energy consumption and increasing equipment life.
- Process is modular and consists of pre-fabricated component for easier installation
- Produces high quality water as by-product which is ready for immediate industrial use without expensive treatment, and becomes potable with minor filtering.
- Pellets have enhanced calorific value.
- Pellets do not reabsorb atmospheric water.
- Pellets have low risk of spontaneous combustion and are suitable for transportation.
- Pellets retain high value volatile matter of the raw coal thus ideal feedstock for downstream processes such as gasification, coal to liquids and other coal derived chemicals.
Low ash levels derived from the raw coal (similarly with sulphur).
Process flow sheet of Coldry process
Fig 3 Integration of Coldry process with power plant