Investigation of the role of feedstock properties and process conditions on the slow pyrolysis of biomass in a continuous auger reactor

Abstract

Slow pyrolysis is a complex process that can convert biomass and waste into valuable products. An improved understanding of the influence of process conditions and feedstocks is required to inform process design and optimisation. A continuous auger reactor (300 g/h) was used to process wheat straw under slow pyrolysis conditions, to study the influence of pyrolysis temperature (400, 500, 600 °C) and solid residence time (3, 6, 10 min) on the product yields and char properties. Production of char decreased with pyrolysis temperature (35.0–26.7 wt%), while gas production increased (15.8–29.7 wt%). The yield of liquid product achieved a maximum of 46.8 wt% at 500 °C. Solid residence time did not affect the product yields significantly. Increasing the pyrolysis temperature produced an increase in the calorific value of the char, mainly through an increase in carbon and fixed carbon (FC) contents and a decrease in oxygen and volatile matter (VM) content. Higher solid residence time caused a reduction in the volatile matter of the char. The surface areas from nitrogen porosimetry at 77 K were very low for all the chars tested (below 15 m2 g−1), suggesting that the pores of the char are blocked or narrowed during operation. FTIR data showed a significant reduction in surface chemical functionalities with pyrolysis temperature, such as C-O and CO, and the solid residence time had no significant effect. The produced chars have the potential to be used as solid fuel, as a soil amendment, or as adsorbents for aqueous phase contaminants. Multiple linear models were developed using literature data to estimate the product yields and properties of the char, including volatile matter, fixed carbon, ash content and higher heating values. The predicted values had a good agreement with the actual values with R2 values in the range of 0.75–0.95. Thus, the models can be used for the design and optimisation of char production by slow pyrolysis.