Oxidative degradation and ignition of loose-packed straw beds

Oxidative degradation and ignition of straw and rain-leached straw were investigated at a laboratory scale. Leaching reduced the ash content (by about 50 wt.%), in particular chlorine and potassium, and caused some swelling. A loose-packed bed (density 50 kg/m3) was exposed to a forced flow (velocities in the range 0.34-1.9 m/s) of hot air (temperatures in the range 540-890 K). Degradation rates became significant for bed temperatures of about 500 K. However, the ignition temperatures were only slightly higher, hence oxidative degradation is a slow process, resulting in maximum conversions of about 50 wt.% (slightly lower for leached straw). Ignition temperatures increased, as the heating conditions were made more severe, and were lower for untreated straw, compared to leached straw. The ignition times were initially shorter and then longer for untreated straw, as the solid heating rate was increased. These results are due to a lower reactivity of leached straw, as a consequence of the partial ash removal, and its swelling, which modifies the heat transfer conditions across the bed.