TREATMENT OF BLOWDOWN FROM A COAL PREHEATER
Yeun C. Wu, Associate Professor
J. Skubak, Graduate Research Assistant
Richard Scholze, Graduate Research Assistant
Department of Civil Engineering
University of Pittsburgh
Pittsburgh, Pennsylvania 15146
Chiu Yang Chen, Associate Professor
Department of Environmental Engineering
National Chung Hsing University
Taichung, Taiwan
Republic of China
A pulverized coal preheating facility is used to heat coal to 500 F prior to coke oven
charging. This system ultimately reduces coking time and enables a higher production rate
as compared to conventional coking. This operation also has an ancillary effect of eliminating various transfer steps, because the coal is charged to the oven via pipeline.
However, emissions from this type of coal preheating facility must be controlled. The
facility uses a water venturi scrubber to clean the blowdown air stream, which contains fine
particles of coal not collected in the cyclone recovery system. This scrubbing system, in alleviating an air emission problem, produces an aqueous effluent which is not acceptable for
discharge.
Normally the blowdown from the scrubbing system is discharged to a coke quenching
sump. The water in this sump is pumped to the coke quench, and evaporated. Solids which
settle to the bottom of the sump are removed regularly with a clam bucket. Unfortunately,
the quench sump can overflow, whenever there is a pushing delay. Installation of additional
surge capacity for the quench sump would be prohibitive in most cases due to lack of space
in the working area. Also, in the near future, regulations for coke quenching may prohibit
quenching with polluted water.
As a consequence of these existing problems and projected regulations, it is important to
understand whether this blowdown need be treated to allow direct discharge to natural
rivers/streams. This study proceeded with testing to determine settling characteristics of the
blowdown to obtain adequate data for engineering design of clarification facilities. In addition, the adsorption ability of activated carbon was studied as a polishing step in treatment. Of course, prior to the performance of these studies already mentioned, the general
properties of the blowdown were first determined.
DESCRIPTION OF THE COAL PREHEATER SYSTEM
The method of preheating coal commonly employed by steel industry is to feed coal into
a stream of hot gases and convey it in the gas stream to centrifugal cyclone separators. A
primary cyclone removes larger particles, while secondary cyclones in parallel, remove
smaller particles. A portion of this clean gas stream continues through a recycle fan and back
into the combusted gas stream. The remainder of the gas is ducted to a venturi scrubber
where it must be cleaned prior to discharge. This system is illustrated in Figure 1.
The venturi scrubber utilizes water treated in a recycle loop. After scrubbing particulates
the water is separated in a flotation cell: the underflow is recycled and the floating material
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