Investigation of channel formation due to random packing in a burning waste bed

Obtaining energy from sustainable sources such as waste and biomass has req uired a significant extension of combustion technology. Many of the advance d technologies are based on thermal treatment in gas-solid packed-bed syste ms such as gasifiers, incinerators and biomass furnaces. In this paper chan nel formation in a packed bed of fuel solids as a result of the random pack ing process has been investigated. Channelling causes a severely uneven dis tribution of the primary airflow through a packed fuel bed and results in p oor combustion performance of the furnace. By assuming Furnas packing, a ge neral relationship is derived between the bed porosity and the particle siz e distribution and the proposed methodology is tested against limited exper imental data. A probability density function (PDF) of truncated Gaussian ty pe is assumed for the random size distribution at local areas within the be d and the local bed porosity is calculated accordingly. Then by solving the fluid flow equations through the porous bed, flow rate profiles are obtain ed at the top surface of the bed. Two particulate systems were investigated as a function of change in bed he ight and pressure drop through the grate. Depending on bed height and press ure drop through the grate, maximum local flow rate at the top surface of t he bed can be 1.5 similar to 2 times higher than the minimum flow rate for the particulate system with a narrower size range (2.5 mm-18 mm) while the ratio of the maximum to minimum flow rate can reach as high as 8 similar to 32 for the particulate system with a wider size range (0.677 mm-20 mm). Vi sualization of the velocity profile inside the bed reveals that flow passag es are slightly curved in some areas but straight in others. The largest ch annel observed presents a 'perfect' straight passage of airflow running fro m the very bottom of the bed to the very top of the bed. Channelling inside a burning bed of solid waste in a large-scale travelling grate incinerator plant was also investigated using a unique in-house prototype instrument. The result shows that the combustion processes within the bed were dominate d largely by the circles of formation and subsequent collapse of channels.