Modelling and experiments of straw combustion in a grate furnace

A two-dimensional mathematical model for the combustion of straw in a cross-current, moving bed was developed as part of a tool for optimizing operating conditions and design parameters. To verify the model and to increase the understanding of straw bed combustion, laboratory fixed-bed experiments were performed in a 15 cm diameter and 137 cm long vertical reactor. Air was introduced through the bottom plate and the straw was ignited from the top with an electrical radiation source. The temperature was measured at eight positions within the bed and gas analysis was performed for O2, CO2, CO, CxHy, NOx, NH3 and SO2. The inlet air flow rate and air preheat temperature were varied. The experimental data for ignition front temperatures and front propagation velocity compared reasonably well with the modelling results. A higher air inlet temperature resulted in lower maximum bed temperatures, which was successfully modelled. A limited parameter analysis study was performed. Measurement of NO from the bed showed that, within the air inlet flow rates investigated, the conversion of fuel nitrogen to NO decreased with increasing air flow rate during ignition front propagation, while the conversion increased during the char burnout phase.