Section 14. COAL, COKE AND POWER PLANT WASTES
STEAM STRIPPING OF COAL LIQUEFACTION WASTE
FOR AMMONIA AND SULFIDE REMOVAL
Kuo-Chun Tsai, Assistant Professor
Mark A. Kamer. Graduate Student
Department of Chemical and Environmental Engineering
University of Louisville
Louisville, Kentucky 40292
John H. Gray, Manager
Environmental Protection and Industrial Hygiene
Ashland Synthetic Fuels, Inc.
Catlettsburg, Kentucky 41129
Ammonia-nitrogen and sulfide are the two most important pollutants contained in
coal liquefaction waste. If not removed, these two pollutants will present a great deal of
nuisance to the environment. For instance, if left in the treated effluent, ammonia-nitrogen
will cause nutrient enrichment (eutrophication) and dissolved oxygen depletion in the
receiving water, and sulfide, if discharged to a concrete pipe, will produce severe pipe corrosion (crown corrosion). More important, both ammonia-nitrogen and sulfide are considered to be toxic to biological systems above a certain level [ 1 ]. Therefore, if biological
waste treatment such as an activated sludge process is to be used to treat coal liquefaction
waste, it is of utmost importance that both ammonia-nitrogen and sulfide in the wastewater be reduced to nontoxic levels prior to entering the aeration tank.
Several nonbiological (i.e., physicochemical) treatment methods are currently available
for removing high-level ammonia-nitrogen and sulfide from wastewater. Notable among
them are breakpoint chlorination, selective ion exchange and air/steam stripping for removing ammonia-nitrogen. To remove sulfide, chemical oxidation and air/steam stripping
can be used. For high-strength ammonia and sulfide wastes, such as those from chemical,
fertilizer and coal-conversion plants, air/steam stripping is a viable process because of its
high removal efficiency and competitive cost.
The principle of air/steam stripping for ammonia and sulfide removals can be best
demonstrated by:
PH>7
NHj +OH"   Z     NH3   I   +H,0 ...
- — ' l (1)
pH<7
H2S t
pH>7
pH<7
HS" + H+ (2)
Equation 1 clearly indicates that an alkaline pH favors the conversion of ammonium
ions (soluble ammonia) to free ammonia (gaseous ammonia). It has been shown that at a
pH of 11.0 and a temperature of 20°C, approximately 95% of ammonia-nitrogen contained
in the domestic sewage will be in the form of free ammonia. The free ammonia produced
may then be stripped off by allowing wastewater droplets to contact with large volumes of
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