Characterization of free radicals by electron spin resonance spectroscopy in biochars from pyrolysis at high heating rates and at high temperatures

Abstract

a b s t r a c tThe concentration and type of free radicals from the decay (termination stage) of pyrolysis at slow and fast heating rates and at high temperatures (above 1000 C) in biomass char have been studied. A roomtemperature electron spin resonance spectroscopy study was conducted on original wood, herbaceous biomass, holocelluloses, lignin and their chars, prepared at high temperatures in a wire mesh reactor, an entrained flow reactor, and a tubular reactor. The radical concentrations in the chars from the decay stage range up between 7$10 16 and 1.5$10 18 spins g À1. The results indicated that the biomass major constituents (cellulose, hemicellulose, lignin) had a minor effect on remaining radical concentrations compared to potassium and silica contents. The higher radical concentrations in the wheat straw chars from the decay stage of pyrolysis in the entrained flow reactor compared to the wood chars were related to the decreased mobility of potassium in the char matrix, leading to the less efficient catalytic effects of potassium on the bond-breaking and radical re-attachments. The high Si levels in the rice husk caused an increase in the char radical concentration compared to the wheat straw because the free radicals were trapped in a char consisting of a molten amorphous silica at heating rates of 10 3 e10 4 K s À1. The experimental electron spin resonance spectroscopy spectra were analyzed by fitting to simulated data in order to identify radical types, based on g-values and line widths. The results show that at high temperatures, mostly aliphatic radicals (g ¼ 2.0026e2.0028) and PAH radicals (g ¼ 2.0027e2.0031) were formed.

Formation and characterization of mechanochemically generated free lignin radicals from olive seeds

“…Cryomilled sample represents much higher intensities in the spectra due to increased number of radicals generated during cryomilling. The gvalue-of the cryomilled sample shifts from 2.0047 to 2.0025 which indicates the formation of new mechanoradicals probably due to bond breakage on the main chain as reported for hydrocarbons[35][36][37]. The observed singlet spectra at the g-value of 2.0025 were assigned to phenoxy radicals, which were generated by homolytic scission of the alkyl phenyl ether, alkyl-O-phenyl bonds connecting the three-dimensional networks of the lignin molecules[31, 34].…”

The Nature of Stable Char Radicals: An ESR and DFT Study of Structural and Hydrogen Bonding Requirements

“…During fast pyrolysis the elemental potassium is unlikely to be released into the gas phase at low temperatures. It is known that the remaining alkali metal ions might catalytically enhance the oxidation reactivity and increase the char yields . Sancier discussed the mechanism of potassium in relation to the radical concentration in catalytic coal gasification.…”

“…It is known that the remaining alkali metal ions might catalytically enhancet he oxidation reactivity and increase the char yields. [19,32] Sancier [33] discussed the mechanism of potassium in relationt ot he radical concentration in catalytic coal gasification. The high potassium levels can weaken carbon-carbon bonds in aromatic rings andt herebyf acilitateb ond breaking at high temperatures.…”

“…The compositional analysis of original feedstocks is shown in the Supporting Information (Table S‐1). The char samples were obtained in separate pyrolysis experiments performed at a slow heating rate (10 °C min −1 ) in the tubular reactor, and at a fast heating rate (1000 °C s −1 ) in the wire mesh reactor, previously described by Trubetskaya et al …”

Termólisis de Clorooxoacetato de Etilo en Fase Gas: Comparación con Oxoacetato de Etilo y Detalles del Sistema Estático para el Estudio Cinético

“…Efecto del inhibidor. Para impedir que la reacción siguiese un mecanismo por radicales libres en cadena [14][15][16][17][18] en lugar de la vía molecular, se utilizó tolueno como inhibidor de radicales [20][21][22][23][24][25] en diferentes proporciones. En la tabla 3 puede observarse cómo la invariabilidad del coeficiente de velocidad en presencia y ausencia de tolueno es indicio de que esta reacción en estudio no estuvo inhibida, lo que permite sugerir un mecanismo molecular.…”