Biomass Boundary Layer Turbine Power System

A “Boundary Layer Turbine” (BLT), with a specially designed multiple-disk rotor consisting of a number of closely packed parallel disks fixed to the shaft, was used to demonstrate direct conversion of biomass for small-scale distributed power generation. The turbine operates under the effect of skin friction drag exerted on the parallel plates, resulting from the flow of hot gases between the parallel plates. This concept is well known for its resistance to erosion when pumping viscous fluids, and the technology has been developed for commercial pump applications but not for a turbine. The turbine based on this concept is capable of encountering particle-laden gas and can accept ash-containing biomass fuels. In the present experiments, wood-derived sawdust (particle size ∼1 mm) and natural oats were fired separately as the test fuels. These fuels were injected directly into the stream of vitiated hot air downstream of the combustor. The location of injection was based on a 1- to 3-second residence time for complete combustion. This paper discusses a performance study and assessment of deposition, erosion, and corrosion (DEC) effects on the working components of the BLT. The potential for cost-effective electricity production from biomass in distributed-generation applications is also explored. The BLT was operated for 40 hours, consuming 68 kg of biomass fuel. The testing included initial firing of 10% biomass (by heating value), increasing to 100%. Documented performance shows isentropic turbine efficiencies of 11% at 3.2 kW and 6284 rpm. Turbine inlet conditions averaged 2.8 bar and 645 K. Over the course of testing, no significant component degradation was observed. The hot components were coated with a small amount of soot, but no deposits were formed that would lead to plugging or buildup in the turbine housing. The results of the study represent the first step toward development of a biomass BLT. It has been demonstrated that no significant barriers should hamper the use of biomass fuels in the rotor; however, isentropic efficiencies will have to be improved to at least 50% to achieve meaningful overall cycle efficiency.