Nascent Biomass Tar Evolution Properties under Homogeneous/Heterogeneous Decomposition Conditions in a Two-Stage Reactor

Abstract

In order to study the mechanism of biomass tar formation and elimination in a two-stage downdraft gasifier, the nascent rice straw pyrolysis tar evolution properties under homogeneous/heterogeneous decomposition conditions have been investigated in a constructed lab-scale two-stage reactor by varying factors as temperature, concentration and reforming agents of CO 2 /H 2 O/O 2 , and char bed heights. The nascent tar was produced in the first stage reactor and then decomposed in the second stage with different reforming agents or char beds. In the first stage, the results showed that nascent pyrolysis tar yields increased with increasing pyrolysis temperature, tar was mainly produced during 200À400°C, and 400À500°C would be a proper pyrolysis temperature range in commercial operation due to little effect on tar yields in higher temperature. In the second stage, it can be observed that nascent biomass tar was converted into polycyclic aromatic hydrocarbons (PAHs) (even soot), thermally stable one ring aromatics, and noncondensable gases in homogeneous conditions with increasing temperature. Different effects were obtained in varying tar species under different homogeneous reforming agents. However, benzene, toluene, styrene, phenol, and naphthalene are the most typical compounds, accounting for 50À75% in total tar concentration at 900°C in all decomposition conditions. Char bed can selectively reduce PAH species remarkably and increase the toluene yields. As for the three reforming agents, steam showed the highest efficiency in tar elimination, while CO 2 and O 2 present will induce OH, H, and O radicals formation, which increases hydrocarbon conversion. The mechanism of tar destruction in a two-stage downdraft gasifier can be concluded as follows: nascent tar yields from the pyrolysis stage will be first reformed into PAHs, thermally stable one ring aromatics and noncondensable gases in the throat region, and then PAHs species are almost completely decomposed by the char bed, which are the main troublesome tar components in syngas, and finally the syngas with low tar was obtained.

Catalytic Cracking of Toluene as a Tar Model Compound Using Sewage-Sludge-Derived Char

“…The yields of H 2 showed no significant differences in tests 4−12, which indicated that water was the dominant factor of the H 2 yield. Wu et al 32 also found that steam showed the highest efficiency in tar elimination, while the presence of CO 2 and O 2 induced the formation of OH, H, and O which promoted hydrocarbon conversion.…”

Catalytic reforming of pyrolysis tar over metallic nickel nanoparticles embedded in pyrochar

“…The liquid products derived from biomass pyrolysis were determined by weighing, mainly including tar and water [30]. Biomass tar is a complex organic mixture composed of hundreds of condensable hydrocarbon compounds with other oxygenated hydrocarbons, which is determined by weighing [31].…”

“…In the reforming zone, the homogeneous partial oxidation and heterogeneous char conversion are two significant factors to ensure that the low-tar syngas is achieved. The selectively properties of RHC Ni in eliminating tertiary tars greatly increased the application prospects of the gasifier [30]. In this work, the major tar compounds came from tar Class 4; sometimes referred as tertiary tars containing non-oxygenated organic compounds with light PAHs.…”

Characteristics and Mechanism of Soot Formation during the Fast Pyrolysis of Biomass in an Entrained Flow Reactor

“…However, phenol was not detected in this study, which can probably be attributed to the relative longer residence time. The aggravation of the pyrolysis process, such as increasing temperature or extended reaction time, was able to remove the oxygen functionalities completely. , The primary reaction of thermal decomposition of phenol is proven to be the production of CPD and CO through: Once CPD forms, it dissociates unimolecularly to the cyclopentadienyl (CPDyl) radical: Two CPDyl radicals combine and rearrange to form naphthalene: Similar to the naphthalene formation process, a phenanthrene can also be formed based on the combination of CPDyl with indenyl (R1072). Therefore, large aromatics can originate from these resonance stabilized radicals.…”

“…The aggravation of the pyrolysis process, such as increasing temperature or extended reaction time, was able to remove the oxygen functionalities completely. 6,46 The primary reaction of thermal decomposition of phenol is proven to be the production of CPD and CO through: 47 (R764)…”