Abstract

The principal goal of the gasification research program at the University of North Dakota Energy Research Center (UNDERC) is to develop environmental data for effluent streams from fixed-bed gasification processes using low rank coals (LRC). Current efforts involve the treatment of gas liquor wastewater streams, and subsequent reuse in process cooling towers. The relatively large volume of wastewater produced in the fixed-bed gasification of LRC’s makes the reuse of this stream in an evaporative cooling tower a desirable approach. However, the use of gas liquor as cooling tower makeup presents both operating and environmental concerns.

At UNDERC a series of cooling tower wastewater reuse tests have been performed using water pretreated to various degrees. Wastewater for each test was produced in a pilot slagging fixed-bed gasifier (SFBG) at UNDERC using North Dakota lignite from the Indian Head mine. During the first phase of testing SFBG wastewater was treated by solvent extraction and steam stripping, to reduce levels of phenolic compounds and ammonia, prior to use as makeup to a pilot mechanical draft cooling tower. In later tests, this extracted and stripped wastewater (SGL) was further treated by biological oxidation and by granular activated carbon adsorption in pilot equipment at UNDERC before being fed to the cooling tower. The final test in this series involved the use of this extensively treated wastewater with the addition of chemical corrosion and scale inhibitors as cooling tower makeup.

High carbon steel corrosion rates, biological and organic fouling rates, foaming, and volatile organic stripping rates were observed in the test using stripped gas liquor from slagging fixed-bed gasification. The problems with biofouling and organic emissions were alleviated by the biological oxidation pretreatment of the cooling tower feed, but carbon steel corrosion rates became even greater. The use of chemical inhibitors was shown to control the corrosivity of this water to the point where it could serve as a cooling tower makeup source without causing adverse environmental or operating conditions.