Effects of loaded-mineral elements on CO enhancement of wheat straw pyrolysis in low-concentration CO2 atmosphere

Abstract

Pyrolysis of biomass in the CO2 atmosphere can enhance CO yield (CO enhancement), which can improve flammable gas yield and promote CO2 conversion. In this work, to investigate the effects of mineral elements on CO enhancement in the pyrolysis of biomass under the low-concentration CO2 atmosphere, acid-washing wheat straw (AWS) was individually loaded with relative amounts of 3.43 wt% K, 1.75 wt% Ca, 0.75 wt% Fe, 0.72 wt% Al, 0.32 wt% Mg, and 5.70 wt% Si which were derived from elemental determinations of X-ray fluorescence analysis. Pyrolysis experiments were conducted in 5% CO2 (95% N2, Vol) and pure N2 as a reference at an optimal temperature of 715 °C. Results revealed that CO enhancement of mineral elements loaded biomass pyrolysis in a low-concentration CO2 atmosphere can be affected by mineral elements. The loaded Al, Mg, and Si have no obvious influences on CO enhancement. Significantly, the loaded K, Ca, and Fe can reinforce the CO enhancement. The CO yields increase from 61.54 mL/g (AWS) to 76.57 mL/g (K), 71.10 mL/g (Ca) and 68.89 mL/g (Fe) in N2, respectively. However, under the 5% CO2 atmosphere, the CO yields increase from 67.11 mL/g (AWS) to 133.36 mL/g (K), 112.92 mL/g (Ca), and 78.40 mL/g (Fe), respectively. Interestingly, the loaded K, Ca, and Fe shows incredible reinforcements of CO enhancement at a holding stage of 715 °C in the 5% CO2 atmosphere, increasing from 30.42 mL/g (AWS) to 71.32 mL/g (K), 74.62 mL/g (Ca), and 44.41 mL/g (Fe), respectively. The interactions between the loaded-mineral elements and CO2 were justified by the CO enhancement, crystalline phase changes, and the changed organic groups of chars from the 5% CO2 atmosphere. And we proposed the possible mechanistic effects. (1) K can promote the interactions between the biomass pyrolysis intermediates and CO2 to form more CO. (2) The synergy between CO2 and the loaded Ca promotes the thermal dissociations of Ca combing to biochar and further reactions for CO enhancement. (3) Fe catalyzed the cracking of C-O groups and provided fragments of O-containing groups (especially OH) for CO2 to improve CO yield. This study provides some references for CO2 utilization in biomass pyrolysis to increase the yield of CO.