Development and Evaluation of a Low-Density Biomass Feeding System for Fluidized Bed Gasifiers

Abstract

For efficient operation of a biomass gasifier, the biomass material must be fed continuously to the system. A feeding system for chopped straw and rice husk was designed, constructed and evaluated. It consisted of: a frame, a hopper, an auger, two agitators, a drive system and a power unit. Initial testing showed that wheat straw and rice husk, being highly cohesive materials, created tunnel flow and piping conditions. This occurs when the pressure above an impending dome of material is too small resulting in the creation of a stable dome and blockage of the discharge. In order to achieve good flow conditions, it was concluded that the hopper must operate under "mass outflow" and the material should not be allowed to build up along the flow channels. These objectives were achieved by the proper redesign of the hopper configuration, the installation of agitators in the hopper and use of an auger in the outlet duct leading into the gasifier. However, as the augur was used to move the biomass material from hopper to the gasifier, it was observed that hot gases leaked from the gasifier into the hopper and heat was also transmitted from the gasifier to the hopper though the augur shaft by conduction resulting in burning of biomass material in the hopper. Therefore, the augur shaft was fitted with copper tubing to serve as a water cooling system and the tapered section of the augur was fitted with a stainless steel section with water inlet and outlet to serve as a secondary cooling system. After, the system has been successfully modified for feeding wheat straw and rice husk, it was tested to determine the optimum operating conditions. Mass flow tests were performed with four sprocket combinations and four auger speeds. Increasing the auger speed and/or the lower agitator speed increased the straw output of the feeding system. However, increases in the upper agitator speed resulted in reduced mass flow of the material due to the mixing effect created by the upper agitator which reduced the down movement of the straw toward the auger housing. The sprocket B combination (30 tooth sprocket on the lower agitator shaft and a 50 tooth sprocket on the upper agitator shaft with a lower agitator speed of 1/3 auger speed and an upper agitator speed of 1/15 auger speed) gave the greatest straw output. The feed rate at this combination ranged from 0.74 to 6.76 kg/minute (12.58 to 114.92 GJ/minute). By adjusting the dial controller, the output could be easily matched with the gasifier’s required straw input.