Influence of Acid-washing Pretreatment on Biomass Pyrolysis by High-resolution Mass Spectrometry and Principal Component Analysis

As a carbon-neutral renewable energy source, biomass has received widespread attention from researchers. The main constituents of lignocellulosic biomass are cellulose, hemicellulose and lignin. In addition, there is a small amount of minerals. Minerals have a significant effect on the biomass pyrolysis process, resulting in differences in the distribution of pyrolysis products. Acid-washing is a common method of removing minerals. Therefore, it is necessary to study the effects of acid-washing on biomass pyrolysis. The effect of the acid-washing on the pyrolysis of rice husk and poplar was investigated in this work. Experimental results showed that K and Ca were the most abundant mineral elements in rice husks and poplars. The rice husks and poplars were washed with 3 wt% hydrofluoric acid before the experiment. The results showed that hydrofluoric acid could effectively remove trace minerals elements such as K, Mg and Fe from biomass. For rice husk, the removal efficiency could reach 92%. Thermogravimetric analysis was used to characterize the pyrolysis properties of biomass before and after acidwashing. The thermogravimetric curves of rice husks and poplars shifted to high temperature after acid-washing. The pyrolysis temperature in the experiment was 500 ℃ and the pyrolysis product was collected after condensation. After acid-washing, the yield of pyrolysis oil was increased by 3.40 wt% (rice husk) and 8.60 wt% (poplar), respectively. The yield of fixed-carbon was reduced by 2.67 wt% and 3.55 wt%, respectively. The chemical composition of pyrolysis oil was extremely complex and GC was incapable to analysis components with high-boiling temperature. In this experiment, the composition of pyrolysis oil was analyzed using high-resolution mass spectrometry (Orbitrap Fusion) with electrospray ionization source. The detected components in pyrolysis oil included acids, aldehydes, ketones, alcohols, phenols, furans, and pyridine (the main nitrogen-containing compounds). After acid-washing, the O₃ and O₄ components were promoted, and the O₂ and O components were inhibited. Based on mass spectrometry data, principal component analysis (PCA) method was used to track the changes in characteristic components in pyrolysis oil. The sum of the contribution rates PC-1 and PC-2 was greater than 90%, so the difference between the pyrolysis oils could be characterized by these two principal components. The PCA results showed that the N-containing compounds in the pyrolysis products were greatly affected by acid-washing. Further analysis showed the content of N-containing compounds in pyrolysis oil could be reduced by 50% after acid-washing, which potentially limited the emissions of nitrogen oxide in the subsequent utilization of bio-oil.