Comparisons of Formation Characteristics of NOx Precursors during Pyrolysis of Lignocellulosic Industrial Biomass Wastes

Abstract

Lignocellulosic industrial biomass wastes (IBWs) are dominant biomass resources in China while their thermal reutilization may bring serious environmental issues due to high nitrogen content. Investigation of the formation of NOx precursors during their pyrolysis is significant. Based on the pyrolysis of three typical IBWs, medium-density fiberboard waste (MFW), Chinese herb residue (CHR), and tea stalk waste (TSW), in a horizontal tubular quartz reactor, similarities and differences between the formation characteristics of NH3 and HCN were investigated with the help of chemical absorption–spectrophotometry, X-ray photoelectron spectroscopy, and thermogravimetric analysis technologies. The results indicated that amide-N was the overwhelming nitrogen functionality in lignocellulosic IBWs, determining the dominance of NH3 among NOx precursors. However, the ratio and total yield of HCN-N and NH3-N could be changed by affecting intrinsic formation pathways owing to pyrolysis conditions as well as physicochemical properties. Joint effects of thermal conditions on each NOx precursor yield were sequenced as rapid pyrolysis at high temperatures > slow pyrolysis at high temperatures > rapid pyrolysis at low temperatures ≈ slow pyrolysis at low temperatures. Meanwhile, the heating rate during slow pyrolysis had an ignorable impact. As a result, during rapid pyrolysis at high temperatures, larger particle size (0–900 μm) could significantly decrease the total yield by 16–17 wt % as well as favor NH3-N yield, while both pyrolysis atmosphere and moisture content presented limited effects. Furthermore, different thermal stability of amide-N type together with distinctive fuel components in three lignocellulosic IBWs led to their differences on the ratio of TSW > CHR > MFW at all temperature ranges and the total yield of MFW > CHR > TSW at low temperatures. However, total yield at high temperatures was observed to be 20–45 wt %, which had no relationship with fuel types. These observations will provide some helpful guidance regarding clean thermal reutilization of lignocellulosic IBWs.