Effect of feedstock water content and pyrolysis temperature on the structure and reactivity of spruce wood char produced in fixed bed pyrolysis

The influence of initial water content and temperature on the pyrolysis product distribution as well as on the structure and reactivity of the pyrolysis char was investigated. Spruce wood chips with 2.4%, 16.4%, and 55.4% initial water content were pyrolyzed in a tubular batch reactor at 500 and 800 degree C with a heating rate between 4 and 12.6 degree C/min. The structural features of the char samples were examined with scanning electron microscopy, Brunauer-Emmett-Teller (BET) method, and mercury porosimetry. The reactivity in CO(2) was investigated using thermo-gravimetric analysis and a fixed bed quartz reactor. It could be seen that higher water content led to a higher yield of condensable products and a lower amount of char. At a pyrolysis temperature of 500 degree C the CO-content of the product gas did increase significantly with increasing water content. Moreover, initial water content had no significant effect on the microscopic structure of wood chars. The specific char surface area did increase with increasing initial water content up to the fiber saturation point. It was also observed that the specific char surface area was strongly influenced by the pyrolysis temperature. When the pyrolysis temperature increased from 500 to 800 degree C, the BET surface area became at least 200 times smaller and the average size of micropores became about 10 times smaller. Most likely, pyrolysis at 800 degree C induced more secondary reactions that were responsible for the occlusion of the micropores within the char [1]. Finally, it was found that reactivity in CO(2) significantly decreased with increasing pyrolysis temperature. However, initial wood water content did not have a significant effect on char reactivity in CO(2).