Modelling of tar formation and evolution for biomass gasification: A review

The pyrolysis of 2-phenethyl phenyl ether (PPE, C(6)H(5)C(2)H(4)OC(6)H(5)) in a hyperthermal nozzle (300-1350 °C) was studied to determine the importance of concerted and homolytic unimolecular decomposition pathways. Short residence times (<100 μs) and low concentrations in this reactor allowed the direct detection of the initial reaction products from thermolysis. Reactants, radicals, and most products were detected with photoionization (10.5 eV) time-of-flight mass spectrometry (PIMS). Detection of phenoxy radical, cyclopentadienyl radical, benzyl radical, and benzene suggest the formation of product by the homolytic scission of the C(6)H(5)C(2)H(4)-OC(6)H(5) and C(6)H(5)CH(2)-CH(2)OC(6)H(5) bonds. The detection of phenol and styrene suggests decomposition by a concerted reaction mechanism. Phenyl ethyl ether (PEE, C(6)H(5)OC(2)H(5)) pyrolysis was also studied using PIMS and using cryogenic matrix-isolated infrared spectroscopy (matrix-IR). The results for PEE also indicate the presence of both homolytic bond breaking and concerted decomposition reactions. Quantum mechanical calculations using CBS-QB3 were conducted, and the results were used with transition state theory (TST) to estimate the rate constants for the different reaction pathways. The results are consistent with the experimental measurements and suggest that the concerted retro-ene and Maccoll reactions are dominant at low temperatures (below 1000 °C), whereas the contribution of the C(6)H(5)C(2)H(4)-OC(6)H(5) homolytic bond scission reaction increases at higher temperatures (above 1000 °C).

Direct detection of products from the pyrolysis of 2-phenethyl phenyl ether

The conversion of lignin by catalytic fast pyrolysis into useful fine chemicals is a promising route to fuel production, however selectivity and conversion are still not optimal. Here, the authors investigate the reaction mechanism by detection of reactive intermediates r…

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Understanding the mechanism of catalytic fast pyrolysis by unveiling reactive intermediates in heterogeneous catalysis.

https://hal.archives-ouvertes.fr/hal-00991550/document

Detailed kinetic study of anisole pyrolysis and oxidation to understand tar formation during biomass combustion and gasification

A micro-reactor system (approximately 0.5–1 mm inner diameter by 2–3 cm in length) coupled with photoionization mass spectrometry and matrix isolation/infrared spectroscopy diagnostics is described. Short residence time flow reactors (roughly ≤ 100 μs) combined with suitable diagnostic tools have the potential to allow observation of unimolecular decomposition processes with minimum interference from secondary reactions. However, achieving the short residence times desired requires very small micro-reactors that are difficult to characterise experimentally because of their size. In this article the benefits of using these micro-reactors are presented along with some details of the systems employed. This is followed by some general flow considerations and then some simple analyses to illustrate particular features of the flow. Finally, computational fluid dynamics simulations are used to explore the flow and chemical behaviour of the reactors in detail. Some findings include: (1) The reactor operates in th...

The properties of a micro-reactor for the study of the unimolecular decomposition of large molecules

The pyrolyses of anisole (C6H5OCH3), d3-anisole (C6H5OCD3), and d8-anisole (C6D5OCD3) have been studied using a hyperthermal tubular reactor and photoionization reflectron time-of-flight mass spectrometer. Gas exiting the reactor is subject to an immediate supersonic expansion after a residence time of approximately 65 μs. This allows the detection of highly reactive radical intermediates. Our results confirm that the first steps in the thermal decomposition of anisole are the loss of a methyl group to form phenoxy radical, followed by ejection of a CO to form cyclopentadienyl radical (c-C5H5); C6H5OCH3 → C6H5O + CH3; C6H5O → c-C5H5 + CO. At high temperatures (Twall = 1200 °C − 1300 °C) the c-C5H5 decomposes to propargyl radical (CH2CCH) and acetylene; c-C5H5 → CH2CCH + C2H2. The formation of benzene and naphthalene is demonstrated with 1 + 1 resonance-enhanced multiphoton ionization. Propargyl radical recombination is a significant benzene formation channel. However, we show the majority of benzene is fo...

Radical chemistry in the thermal decomposition of anisole and deuterated anisoles: an investigation of aromatic growth.

The pyrolyses of the guaiacols or methoxyphenols (o-, m-, and p-HOC6H4OCH3) have been studied using a heated SiC microtubular (μ-tubular) reactor. The decomposition products are detected by both photoionization time-of-flight mass spectroscopy (PIMS) and matrix isolation infrared spectroscopy (IR). Gas exiting the heated SiC μ-tubular reactor is subject to a free expansion after a residence time of approximately 50–100 μs. The PIMS reveals that, for all three guaiacols, the initial decomposition step is loss of methyl radical: HOC6H4OCH3 → HOC6H4O + CH3. Decarbonylation of the HOC6H4O radical produces the hydroxycyclopentadienyl radical, C5H4OH. As the temperature of the μ-tubular reactor is raised to 1275 K, the C5H4OH radical loses a H atom to produce cyclopentadienone, C5H4═O. Loss of CO from cyclopentadienone leads to the final products, acetylene and vinylacetylene: C5H4═O → [CO + 2 HC≡CH] or [CO + HC≡C–CH═CH2]. The formation of C5H4═O, HCCH, and CH2CHCCH is confirmed with IR spectroscopy. In separat...

Thermal decomposition mechanisms of the methoxyphenols: formation of phenol, cyclopentadienone, vinylacetylene, and acetylene.

The pyrolyses of phenol and d5-phenol (C6H5OH and C6D5OH) have been studied using a high temperature, microtubular (μtubular) SiC reactor. Product detection is via both photon ionization (10.487 eV) time-of-flight mass spectrometry and matrix isolation infrared spectroscopy. Gas exiting the heated reactor (375 K–1575 K) is subject to a free expansion after a residence time in the μtubular reactor of approximately 50–100 μs. The expansion from the reactor into vacuum rapidly cools the gas mixture and allows the detection of radicals and other highly reactive intermediates. We find that the initial decomposition steps at the onset of phenol pyrolysis are enol/keto tautomerization to form cyclohexadienone followed by decarbonylation to produce cyclopentadiene; C6H5OH → c-C6H6 = O → c-C5H6 + CO. The cyclopentadiene loses a H atom to generate the cyclopentadienyl radical which further decomposes to acetylene and propargyl radical; c-C5H6 → c-C5H5 + H → HC≡CH + HCCCH2. At higher temperatures, hydrogen loss from the PhO–H group to form phenoxy radical followed by CO ejection to generate the cyclopentadienyl radical likely contributes to the product distribution; C6H5O–H → C6H5O + H → c-C5H5 + CO. The direct decarbonylation reaction remains an important channel in the thermal decomposition mechanisms of the dihydroxybenzenes. Both catechol (o-HO–C6H4–OH) and hydroquinone (p-HO–C6H4–OH) are shown to undergo decarbonylation at the onset of pyrolysis to form hydroxycyclopentadiene. In the case of catechol, we observe that water loss is also an important decomposition channel at the onset of pyrolysis.

Unimolecular thermal decomposition of phenol and d5-phenol: direct observation of cyclopentadiene formation via cyclohexadienone.

The unimolecular thermal decomposition mechanisms of o-, m-, and p-dimethoxybenzene (CH3O-C6H4-OCH3) have been studied using a high temperature, microtubular (μtubular) SiC reactor with a residence time of 100 μs. Product detection was carried out using single photon ionization (SPI, 10.487 eV) and resonance enhanced multiphoton ionization (REMPI) time-of-flight mass spectrometry and matrix infrared absorption spectroscopy from 400 K to 1600 K. The initial pyrolytic step for each isomer is methoxy bond homolysis to eliminate methyl radical. Subsequent thermolysis is unique for each isomer. In the case of o-CH3O-C6H4-OCH3, intramolecular H-transfer dominates leading to the formation of o-hydroxybenzaldehyde (o-HO-C6H4-CHO) and phenol (C6H5OH). Para-CH3O-C6H4-OCH3 immediately breaks the second methoxy bond to form p-benzoquinone, which decomposes further to cyclopentadienone (C5H4=O). Finally, the m-CH3O-C6H4-OCH3 isomer will predominantly follow a ring-reduction/CO-elimination mechanism to form C5H4=O. Electronic structure calculations and transition state theory are used to confirm mechanisms and comment on kinetics. Implications for lignin pyrolysis are discussed.

Adam M. Scheer

Papers

Radical chemistry in the thermal decomposition of anisole and deuterated anisoles: an investigation of aromatic growth.

Thermal decomposition mechanisms of the methoxyphenols: formation of phenol, cyclopentadienone, vinylacetylene, and acetylene.

Unimolecular thermal decomposition of phenol and d5-phenol: direct observation of cyclopentadiene formation via cyclohexadienone.

Unimolecular thermal decomposition of dimethoxybenzenes