Matthew S. Mettler

Top ten fundamental challenges of biomass pyrolysis for biofuels

Pyrolytic biofuels have technical advantages over conventional biological conversion processes since the entire plant can be used as the feedstock (rather than only simple sugars) and the conversion process occurs in only a few seconds (rather than hours or days). Despite decades of study, the fundamental science of biomass pyrolysis is still lacking and detailed models capable of describing the chemistry and transport in real-world reactors is unavailable. Developing these descriptions is a challenge because of the complexity of feedstocks and the multiphase nature of the conversion process. Here, we identify ten fundamental research challenges that, if overcome, would facilitate commercialization of pyrolytic biofuels. In particular, developing fundamental descriptions for condensed-phase pyrolysis chemistry (i.e., elementary reaction mechanisms) are needed since they would allow for accurate process optimization as well as feedstock flexibility, both of which are critical to any modern high-throughput process. Despite the benefits to pyrolysis commercialization, detailed chemical mechanisms are not available today, even for major products such as levoglucosan and hydroxymethylfurfural (HMF). Additionally, accurate estimates for heat and mass transfer parameters (e.g., thermal conductivity, diffusivity) are lacking despite the fact that biomass conversion in commercial pyrolysis reactors is controlled by transport. Finally, we examine methods for improving pyrolysis particle models, which connect fundamental chemical and transport descriptions to real-world pyrolysis reactors. Each of the ten challenges is presented with a brief review of relevant literature followed by future directions which can ultimately lead to technological breakthroughs that would facilitate commercialization of pyrolytic biofuels.

Revealing pyrolysis chemistry for biofuels production: conversion of cellulose to furans and small oxygenates.

Biomass pyrolysis utilizes high temperatures to produce an economically renewable intermediate (pyrolysis oil) that can be integrated with the existing petroleum infrastructure to produce biofuels. The initial chemical reactions in pyrolysis convert solid biopolymers, such as cellulose (up to 60% of biomass), to a short-lived (less than 0.1 s) liquid phase, which subsequently reacts to produce volatile products. In this work, we develop a novel thin-film pyrolysis technique to overcome typical experimental limitations in biopolymer pyrolysis and identify α-cyclodextrin as an appropriate small-molecule surrogate of cellulose. Ab initio molecular dynamics simulations are performed with this surrogate to reveal the long-debated pathways of cellulose pyrolysis and indicate homolytic cleavage of glycosidic linkages and furan formation directly from cellulose without any small-molecule (e.g., glucose) intermediates. Our strategy combines novel experiments and first-principles simulations to allow detailed chemical mechanisms to be constructed for biomass pyrolysis and enable the optimization of next-generation biorefineries.

The chain length effect in pyrolysis: bridging the gap between glucose and cellulose

Despite the potential for biomass pyrolysis to produce renewable fuels, the governing chemical reactions are largely unknown due to the complexity of biopolymers, such as cellulose. In this work, we use isothermal pyrolysis experiments to reveal the chain length (or end-group) effect that controls the distribution of pyrolysis products from linear β-1,4-glucan polymers (e.g., cellulose). Finally, we show that a simplified end-group/interior monomer model is largely incapable of predicting product yields from cellodextrin pyrolysis.

Pyrolytic conversion of cellulose to fuels: levoglucosan deoxygenation via elimination and cyclization within molten biomass

Fast pyrolysis of biomass thermally cracks solid biopolymers to generate a transportable liquid (bio-oil) which can be upgraded and integrated with the existing petroleum infrastructure. Understanding how the components of biomass, such as cellulose, break down to form bio-oil constituents is critical to developing successful biofuels technologies. In this work, we use a novel co-pyrolysis technique and isotopically labeled starting materials to show that levoglucosan, the most abundant product of cellulose pyrolysis (60% of total), is deoxygenated within molten biomass to form products with higher energy content (pyrans and light oxygenates). The yield of these products can be increased by a factor of six under certain reaction conditions, e.g., using long condensed-phase residence times encountered in powder pyrolysis. Finally, co-pyrolysis experiments with deuterated glucose reveal that hydrogen exchange is a critical component of levoglucosan deoxygenation.

Tuning cellulose pyrolysis chemistry: selective decarbonylation via catalyst-impregnated pyrolysis

Widespread adoption of biomass pyrolysis for lignocellulosic biofuels is largely hindered by a lack of economical means to stabilize the bio-oil (or pyrolysis oil) product. In this work, impregnation of supported metal catalysts provides a new approach to selectively decarbonylate primary pyrolysis products within intermediate cellulose liquid to targeted gasoline-like molecules with enhanced energy content and stability. Selective deoxygenation of hydroxy-methylfurfural (HMF) and furfural (F) to 88% yield of stable furans occurred over carbon-supported Pd, with negligible loss in overall bio-oil yield or furanic content.