This news story originally provided by Mining Weekly

October 1, 2005

Coal-fines utilisation future

An environmental-impact assessment for a full-scale coal-fines utilisation demon- stration plant at Eskom’s Hendrina power station is now under way.

This was reported at the recent South African Coal Preparation Conference (organised by the South African Coal Processing Society and held at Secunda, in Mpumalanga) by Mike Blenkinsop, of the company Waste Energy Recovery and Management (Werm).

The project to use coal fines in feeding power stations began in 1999, when Eskom and Werm started research and development into the feasibility of an economically-viable process to use the large amounts of fine coal existing in this country in the electricity utility’s power stations.

Hendrina was chosen as the site for the project, and the first phase was the erection of a laboratory-scale test plant.

This was followed, in 2001, by the building of a 10 t/h pilot plant.

This was used for a test programme which sought to establish whether fine coal could be dried properly using waste heat from the power station and then effectively added to the power station in a way that reduced its effects on the materials-handling plant.

In addition, the process used had to be eco-nomically feasible and environmentally beneficial.

The process developed by Werm for Eskom involves a two-stage drying process for the fines.

These are intially dumped into slimes dams in the form of slurry with a moisture content of some 60%; they are then dried, partially by natural processes and partly using specially-adapted equipment employing methods developed by Werm, to a moisture content of between 14% and 18%.

(Any drier would make the material more difficult to handle and create a serious and environmentally-unacceptable dust problem.) Interestingly, Blenkinsop reported that rain affected the drying process less than might have been expected.

The material is then transferred to a ‘fine-coal plant’ (FCP) where it is dried by waste heat in the form of flue gas from the power station.

The pilot plant has the capacity to feed one of the Hendrina mills with 50% of its feedstock in the form of fine dry coal, and it was integrated into the power station’s system to allow a determination of the effects of the process on the mill’s operation.

The test programme was completed in April 2002.

Its results demonstrated that large quantities of fine coal can be successfully dried using flue gas, that the power station produces enough flue gas to dry realistic quantities of fine coal for use by that power station, that mills can operate successfully with up to 50% of their feedstock in the form of fine coal and that mill performance benefits significantly from the addition of fine coal as a feedstock; the upper limit of fine-coal feed to the mill for its safe operation was not determined.

As a result, a technoeceonomic study was commissioned to ascertain if it would be beneficial to apply this technology on a commercial scale at Hendrina and to assess the feasibility of building a full-scale demonstration plant at the power station.

This study was completed in early 2004; it reported that investment in the technology was justified, assuming certain financial parameters (especially the coal price), and the Eskom board subsequently gave its approval for the project to proceed, under the assumptions made in the study.

The commercial-scale FCP will comprise a drying plant, a dust-separation plant and a medium-phase pneumatic transport plant.

The fine coal, its mositure content already reduced at the slimes dam, will be fed into fluidised-bed dryers, where it will be dried by hot flue gas; as the flue gas is inert, the drying process is inherently safe from coal-dust explosions.

When the fine coal particles are sufficiently dry, the flue-gas stream will lift them up and carry them to the (coal) dust-separation, or fabric filter, plant where they will be separated from the flue-gas stream – the spent flue gas will then be returned to the common flue-gas duct by means of a set of induced-draught fans (these fans power the entire system).

The dried fine coal will be fed from the fabric-filter hoppers into the medium-phase pneumatic transport system through a set of rotary valves; at the end of the pneumatic conveyor line (there will be one per power station unit) the fine coal will be separated from the air stream in a fabric-filter-fitted receiver vessel, and the clean air released into the atmosphere by an explosion duct.

The fine coal will then be fed to each mill through a gravity-fed piping system that will systemically feed the hoppers above each mill; the mills will have dual feeders, one for the fines and one for the current supply.

The fine-coal feeder will supply what it can, up to a maximum of 50%, and the other feeder will make up the difference.