(19d) Effect of Fuel Heap Shape on Air Flow in a Side-Fed Biomass Gasifier Based on DEM-CFD Simulation

In recent years, because of the rapid reduction of other conventional energy and growing earth population, biomass usage has increased considerably. As a promising renewable energy technology, biomass gasification offers the highest efficiency among all biomass conversion routes. Gasifiers can be categorized based on the method of air supply and the position of the feeding system. The subjects in this paper are downdraft and side-fed gasifiers, which are more attractive than updraft gasifiers, for power generation in small and medium scale applications.

In this type of gasifier, as the name suggests, biomass fuel is fed via an inlet located on the side wall, and gasifying agents enter through a nozzle in the lower part of the vessel. The product gas and slag are collected from the top and at the bottom of the vessel, respectively.

From a process-related point of view, such gasifiers are considered fixed bed processes and it is assumed in most research that the fixed bed’s upper surface is even and smooth, i.e. the fixed bed is box or cylinder-shaped. However, side-fed gasifiers may experience particle blocking, channeling and bridging. Especially for fuels with low bulk density, extreme shape and wide size distribution, this results in uneven fuel heap shape, which is fairly different from an idealized fixed bed. Since the primary air enters from the bottom, the oxygen content at different locations in the fixed bed can vary due to changing accumulated filling heights of the fuel in the gasifier. In fact, inappropriate mixing of fuel and oxygen during gasification can lead to a number of issues, such as slagging, fouling, and corrosion. In this case, simulative investigation of gasification with idealized conditions may differ from real applications. Even so, studies with respect to the effect of slope form on air flow within the fuel heap aren’t found in published research.

To investigate the effect of the fuel heap shape on local gasification conditions, a small-scale fixed-bed downdraft gasifier was modeled with a laterally mounted screw feeder, with which the distribution of particles inside the gasifier was altered by using varying operating parameters. The simulation model consists of a cylindrical container as the gasifier, with a default feeding position and air supply from below. Based on CFD-DEM coupling simulations in two open source software, LIGGGHTS® and OpenFOAM®, the air flow was examined.

These simulation results were investigated qualitatively and quantitatively in terms of air flow pattern, including the uneven air distribution within the fuel heap and varying air direction at the slope surface. The results showed that the combustion efficiency at the side of the feeding system is the lowest due to insufficient air flow. Besides, the effects of operating parameters of the feeding system have been highlighted with respect to modification of the slope form. According to these first investigations, real fuel heap shape should be taken into consideration for the simulative study of gasification affected by air flow in the future.