(411e) Technology Development and Demonstration of a Low-Temperature Fluidized-Bed Biomass Dryer

At the University of Nevada, Reno, we initiated a research project to study mechanisms of drying wastewater solids (sludge) in 2007. Preliminary studies led to the development of a new low-temperature fluidized-bed sludge dryer. The sludge dryer has been tested in batch and in continuous studies, and operates at very low temperatures. We report results of a parametric study based on air flow rate and dryer temperature. The dryer produced dried solids at unusually low temperatures, between 125 ˚F and 175 ˚F, with heat provided at 210 ˚F. Collected sludge has been characterized in terms of its moisture content, particle size, and ash content. The moisture of the collected product ranges between 4% and 15%, and can be controlled by proper selection of operating conditions.

The dryer is a novel configuration, and consists of a bed of inert solids operating in the bubbling bed regime. Heat is provided to the bed by internal heat transfer tubes. Wet sludge falls into the warm bed, which is bubbling vigorously. The rapid particle movement causes the wet sludge to break apart, increasing surface area, and thereby enhancing heat transfer. As the sludge dries and fragments, it becomes relatively light dust particles. The small dry particles have specific density 1.2, and are entrained in the fluidizing air. Solid particles are removed from the air in a filter outside the bed, and collected and stored.

The capacity of the dryer is limited by the rate of air flow. Air leaving the bed can remove a finite amount of moisture, up to its saturation of humidity. Increasing too high has a deleterious effect on the performance of the fluidized bed.

We have successfully completed a demonstration of our dryer with a continuous feed of up to 20 lb/hour of wet (85% moisture) sludge onsite at the Truckee Meadows Water Reclamation Facility (TMWRF). We are currently seeking partners for a 200 lb/hour demonstration. Trials and lessons from commercialization with the support of the UNR Tech Transfer Office will be discussed.