Autothermal and allothermal pyrolysis in a continuous fixed bed reactor

• We present methodologies for operating wood chips pyrolysis in a continuous fixed bed based on bed height control.
• Complete characterization of pyrolysis products is achieved for allothermal and autothermal operation.
• In allothermal operation, tar cracking reactions are influent below 500 °C.
• In autothermal experiments, volatile products are preferentially oxidized and organic condensates production drops to 17 wt.% (d.b.).

Pyrolysis plays a major role in the optimization of process efficiency and gas quality in two-stage gasifiers. We developed an original reactor at pilot scale to investigate continuous fixed bed pyrolysis, in either allothermal or autothermal operating mode using air injection. Instrumentation and analysis enabled thermal profiles to be established inside the bed along with fine measurement of yields, char composition, permanent gases, and compounds at the outlet of the pyrolysis reactor. Under oxidative conditions, fixed-bed pyrolysis led to smoldering and the stabilization of an ignition front between 10 and 20 cm below the bed surface. With oxidative pyrolysis, yields of organic condensates were lower than with allothermal pyrolysis, whereas the production of pyrolysis water and permanent gases increased considerably. The main conclusion is that during autothermal pyrolysis in fixed-bed conditions, the presence of oxygen promotes oxidation of volatile matter and cracking reactions, thereby increasing the production of pyrolysis water and permanent gases. We observed a significant increase in yields of CO2 and CO, while the yield of char remained comparable to that in allothermal experiments.