Quantitative Insights into the Fast Pyrolysis of Extracted Cellulose, Hemicelluloses, and Lignin

Abstract

The transformation of lignocellulosic biomass into bio-based commodity chemicals is technically possible. Among thermochemical processes, fast pyrolysis, a relatively mature technology that has now reached a commercial level, produces a high yield of an organic-rich liquid stream. Despite recent efforts to elucidate the degradation paths of biomass during pyrolysis, the selectivity and recovery rates of bio-compounds remain low. In an attempt to clarify the general degradation scheme of biomass fast pyrolysis and provide a quantitative insight, the use of fast pyrolysis microreactors is combined with spectroscopic techniques (i.e., mass spectrometry and NMR spectroscopy) and mixtures of unlabeled and ¹³ C-enriched materials. The first stage of the work aimed to select the type of reactor to use to ensure control of the pyrolysis regime. A comparison of the chemical fragmentation patterns of "primary" fast pyrolysis volatiles detected by using GC-MS between two small-scale microreactors showed the inevitable occurrence of secondary reactions. In the second stage, liquid fractions that are also made of primary fast pyrolysis condensates were analyzed by using quantitative liquid-state ¹³ C NMR spectroscopy to provide a quantitative distribution of functional groups. The compilation of these results into a map that displays the distribution of functional groups according to the individual and main constituents of biomass (i.e., hemicelluloses, cellulose and lignin) confirmed the origin of individual chemicals within the fast pyrolysis liquids.

Keywords: NMR spectroscopy; biomass; isotopic labeling; polymers; reaction mechanisms.

Scheme 1

Illustration of potential lignocellulosic biomass fractions extracted from Zea Mays.

Figure 1

Chromatogram (GC–FID) of the products of the pyrolysis of extracted biopolymers from Zea Mays: — mixture; — lignin; — hemicelluloses; — cellulose.

Figure 2

Mapping of pyrolysis regimes according to heat transport. Adapted from Ref. 33. Heat transport map for the pyroprobe (▪) and the microreactor (▪) at approximately 550 °C.

Scheme 2

Intra‐ and extra‐particle mass and heat transport events.

Figure 3

Confirmation of the product identity (furfural) and lack of scrambling by comparing experimental and predicted MS fragmentation patterns (ratio distribution vs. m/z) of FP products from mixtures of unlabeled cellulose (Cell‐¹²C), hemicelluloses (Hemi‐¹²C) and lignin (Lig‐¹²C) and ¹³C‐enriched cellulose (Cell‐¹³C), hemicelluloses (Hemi‐¹³C) and lignin (Lig‐¹³C). a) Mixture of cellulose and lignin processed by using Py‐GC‐MS; b) and c) Mixtures of cellulose, hemicelluloses and lignin processed, respectively, by using Py‐GC‐MS and the microreactor. Calc.=calculated; Exp.=experimental.

Figure 4

Yield of fast pyrolysis products for 550 °C: [▪] char yield; [▪+□] volatiles yield; [▪] total GC‐detectable product yield; [□] undetected product yield obtained by difference.

Figure 5

Relative proportions [%] of the important fractions in bio‐oil.

Figure 6

Relative portions [%] of chemical groups within enriched bio‐oils obtained by using liquid‐state ¹³C NMR spectroscopy: [▪] Cell‐¹³C; [▪] Hemi‐¹³C; [▪] Lig‐¹³C; [▪] MX¹³C (MX=mixture); [▪] Maize‐¹³C. Alkyl carbons I and II refer to primary and secondary alkyl carbon atoms, respectively.

Figure 7

Carbon source of chemical families according to the extracted biopolymers: [▪] cellulose; [▪] hemicelluloses; [▪] lignin.

Figure 8

Ratio deviation between experimental and theoretical yields of organic groups for a) cellulose, b) hemicelluloses and c) lignin.