The Role of Sample Dimension and Temperature in Cellulose Pyrolysis

Abstract

Fast pyrolysis of biomass is a next-generation biofuels production process that is capable of converting solid lignocellulosic materials (in their raw form) to a transportable liquid (bio-oil) which can be catalytically hydrogenated to fuels and chemicals. While biomass fast pyrolysis has enormous potential to produce renewable fuels, an understanding of the fundamental chemistry that converts biomass components, such as cellulose, to bio-oil is not available in the literature. In this work, we use thin-film pyrolysis to reveal the effect of temperature under transport-free reaction conditions and then evaluate the effect of sample dimension (i.e., characteristic length scale) by comparing product distributions of conventional powders and thin films. In the first part of the work, we show that the yield of total furan rings (i.e., all products containing a five-membered furan ring) does not change significantly with increased reaction temperature compared to other pyrolysis products, such as light oxygenates and anhydrosugars. However, we find that the functional groups bound to the furan ring (e.g., alcohols and aldehydes) are easily cleaved to produce smaller furans. In the second part of the work, we show that sample dimension is a key descriptor for product yields. For example, levoglucosan (the most abundant product of cellulose pyrolysis) yield differs significantly between conventional powder (millimeter-sized samples which are transport-limited) pyrolysis and thin-film (micrometer-scale thin-films which are isothermal) pyrolysis (49% for powder; 27% for thin-film at 500 °C).

Fast pyrolysis of glucose‐based carbohydrates with added NaCl part 1: Experiments and development of a mechanistic model

“…A frequency factor of 1.0 3 10 8 M 21 s 21 is used for Na 1 binding to all the neutral species. 53 Mayes et al 25 estimated the Gibbs free energy change DG 773k for the binding of isolated Na 1 and glucose to form a Na 1 -glucose complex to be 10.0 kcal mol 21 , with similar values found for many intermediates and products of glucose pyrolysis included in that study. The values relevant for the pyrolysis environment are the difference in free energy as the ions migrate in the melt phase, which will not be as entropically unfavorable as calculated in the quantum chemical calculations for isolated species.…”

“…With NaCl present in fast pyrolysis of glucose-based carbohydrates at 500 8C in this study, chloride would be efficiently volatilized while sodium would stay in the melt phase, based on the experimental findings from the literature. 14,21,22 Furthermore, Na 1 would be likely to bind with oxygen atoms in the reactive species. 26,[28][29][30] In this work, Na 1 ions were assumed to be able to bind with all the neutral species including polymeric chains and LMW species and form Na 1 -complex species (Na 1 -complexes).…”

“…Armentrout and coworkers reported that, generally, there is no activation barrier in excess of the reaction endothermicity for the loss of the ligand from metal-ligand complexes. 54,60,61 Therefore, the model used 0 kcal mol 21 for the E a of Na 1 ion binding to neutral species. A frequency factor of 1.0 3 10 8 M 21 s 21 is used for Na 1 binding to all the neutral species.…”

“…However, recent experimental work on fast pyrolysis showed that more pyrolysis products could be more accurately identified and quantified. [21][22][23][24][25] Moreover, the influence of NaCl on other glucose-based carbohydrates such as maltohexaose and cellobiose has not been studied yet. Experimental data over a range of molecular weight would allow a better assessment of the influence of chain length on the product distribution from fast pyrolysis and rationalization of reaction intermediates and mechanisms involved.…”

“…14 For example, the addition of Na 1 decreased the activation energy (E a ) from 67.4 to 60.0 kcal mol 21 and increased the reaction rate by a factor of 30 times for 1,2-dehydration of ethanol to ethylene. A density functional theory study of Zhang and Liu on pyrolysis of b-D-glucopyranose discovered that Na 1 can lower the energy barrier of dehydration reactions and accelerate the formation of small molecules.…”