The combustion behavior of Hot-Water-Dried slurries of selected low rank coals was investigated during a three year program, using the 550,000 Btu/hr ash fouling furnace at the University of North Dakota Energy Research Center. Combustion of the slurries presented no additional problems of handling, feeding, flame stability or carbon burnout compared to powdered coals or inburning commercial bituminous coal slurries. Heating values of the slurries ranged from 5600 to 7400 Btu/lb, with solids loadings of 55 to 60%. Essentially complete (99% ) carbon burnout was achieved in all tests. Furnace operating conditions included fuel residence time of three seconds, and an exit furnace temperature of 2000/sup 0/F. Compared with powdered coal, the flame produced in slurry combustion is distributed further through the furnace combustion zone, with lower peak temperatures than observed with powdered coal combustion. For three of the four coals studied, the weight of ash deposits on simulated boiler tubes per weight of total ash input was lower for slurry combustion than for powdered coal. In the case of one subbituminous coal, however, the amounts of the deposits were greater than the deposits produced in powdered coal combustion. There was no significant difference in the combustion behavior of slurries prepared at different temperatures by hot water drying. However, x-ray diffraction analyses of deposits produced in combustion of powdered coal, indicates that mineral species present were predominantly simple oxides and sulfates, while for deposits produced in combustion of slurry, tube fouling deposits were primarily of complex aluminosilicates. This evidence indicates that the combustion characteristics of slurries do have some differences and effects on character of the ash deposits. 38 refs., 19 figs., 4 tabs.