Paul J. Dauenhauer

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.

Chemical engineering: Hybrid routes to biofuels

Traditional methods for making fuels from biomass come in two forms — biological or chemical. The latest approach combines the best of both worlds, and heralds the advent of a second generation of biofuels. With petrol prices on the rise, biofuels are big news these days. For applications in the transportation sector, perhaps the best known liquid biofuel is biomass-derived ethanol. But ethanol has its limitations: it is highly volatile, absorbs water and has a low energy density. A team from the University of Wisconsin-Madison has developed a two-step catalytic process that can convert fructose into a potentially better liquid biofuel, 2,5-dimethylfuran (DMF). This has 40%-higher energy density and a higher boiling point than ethanol, and is not water soluble. Fructose can be made directly from biomass or from glucose and although theres some work needed before DMF production can be made commercially viable, this new catalytic process looks promising.

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.

Ultra-selective cycloaddition of dimethylfuran for renewable p-xylene with H-BEA

p-Xylene, the precursor for PET bottles, was synthesized at 90% yield by [4 + 2] cycloaddition of biomass-derived ethylene and dimethylfuran followed by subsequent dehydration with Beta zeolite.

Reactive boiling of cellulose for integrated catalysis through an intermediate liquid.

Advanced biomass processing technology integrating fast pyrolysis and inorganic catalysis requires an improved understanding of the thermal decomposition of biopolymers in contact with porous catalytic surfaces. High speed photography (1000 frames per second) reveals that direct impingement of microcrystalline cellulose particles (300 μm) with rhodium-based reforming catalysts at high temperature (700 °C) produces an intermediate liquid phase that reactively boils to vapors. The intermediate liquid maintains contact with the porous surface permitting high heat transfer (MW m−2) generating an internal thermal gradient visible within the particle as a propagating wave of solid to liquid conversion. Complete conversion to liquid yields a fluid droplet on the catalyst surface exhibiting a linear decrease in droplet volume with time leaving behind a clean surface absent of solid residue (char). Under specific interfacial conditions, conversion with large cellulosic particles on the length-scale of wood chips (millimeters) occurs continuously as generated liquid and vapors are pushed into the porous surface.

Ab initio dynamics of cellulose pyrolysis: nascent decomposition pathways at 327 and 600 °C.

We modeled nascent decomposition processes in cellulose pyrolysis at 327 and 600 °C using Car-Parrinello molecular dynamics (CPMD) simulations with rare events accelerated with the metadynamics method. We used a simulation cell comprised of two unit cells of cellulose Iβ periodically repeated in three dimensions to mimic the solid cellulose. To obtain initial conditions at reasonable densities, we extracted coordinates from larger classical NPT simulations at the target temperatures. CPMD-metadynamics implemented with various sets of collective variables, such as coordination numbers of the glycosidic oxygen, yielded a variety of chemical reactions such as depolymerization, fragmentation, ring opening, and ring contraction. These reactions yielded precursors to levoglucosan (LGA)-the major product of pyrolysis-and also to minor products such as 5-hydroxy-methylfurfural (HMF) and formic acid. At 327 °C, we found that depolymerization via ring contraction of the glucopyranose ring to the glucofuranose ring occurs with the lowest free-energy barrier (20 kcal/mol). We suggest that this process is key for formation of liquid intermediate cellulose, observed experimentally above 260 °C. At 600 °C, we found that a precursor to LGA (pre-LGA) forms with a free-energy barrier of 36 kcal/mol via an intermediate/transition state stabilized by anchimeric assistance and hydrogen bonding. Conformational freedom provided by expansion of the cellulose matrix at 600 °C was found to be crucial for formation of pre-LGA. We performed several comparison calculations to gauge the accuracy of CPMD-metadynamics barriers with respect to basis set and level of theory. We found that free-energy barriers at 600 °C are in the order pre-LGA < pre-HMF < formic acid, explaining why LGA is the kinetically favored product of fast cellulose pyrolysis.

Aerosol generation by reactive boiling ejection of molten cellulose

The generation of primary aerosols from biomass hinders the production of biofuels by pyrolysis, intensifies the environmental impact of forest fires, and exacerbates the health implications associated with cigarette smoking. High speed photography is utilized to elucidate the ejection mechanism of aerosol particles from thermally decomposing cellulose at the timescale of milliseconds. Fluid modeling, based on first principles, and experimental measurement of the ejection phenomenon supports the proposed mechanism of interfacial gas bubble collapse forming a liquid jet which subsequently fragments to form ejected aerosol particles capable of transporting nonvolatile chemicals. Identification of the bubble-collapse/ejection mechanism of intermediate cellulose confirms the transportation of nonvolatile material to the gas phase and provides fundamental understanding for predicting the rate of aerosol generation.

Kinetics and reaction chemistry for slow pyrolysis of enzymatic hydrolysis lignin and organosolv extracted lignin derived from maplewood

The kinetics and reaction chemistry for the pyrolysis of Maplewood lignin were investigated using both a pyroprobe reactor and a thermogravimetric analyser mass spectrometry (TGA-MS). Lignin residue after enzymatic hydrolysis and organosolv lignin derived from Maplewood were used to measure the kinetic behaviours of lignin pyrolysis and to analyse pyrolysis product distributions. The enzymatic lignin residue pyrolyzed at lower temperature than that of organosolv lignin. The differential thermogravimetric (DTG) peaks for pyrolysis of the enzymatic residue were more similar to the DTG peaks for pyrolysis of the original Maplewood than DTG of the organosolv lignin. The condensable liquid volatile products were collected from a Pyroprobe reactor with a liquid nitrogen trap. The primary monomeric phenolic compounds were guaiacol, syringol, and vanillic acid. However, only 14–36 carbon% of the sample could be detected by GC-MS. Over 60 carbon% of the condensable products were heavy tar molecules that are not detectable by GC-MS. These heavy tar molecules are the primary products from pyrolysis of lignin. Intermediate solid samples were also collected at various pyrolysis temperatures and characterized by elemental analysis, FT-IR, DP-MAS 13C NMR, and TOC. The methoxy groups and ether linkages decreased and the non-protonated aromatic carbon–carbon bonds increased in the solid residues as the pyrolysis temperature increased. The carbon content of the initial lignin feed (derived from enzymatic hydrolysis) and the solid polyaromatics residue (obtained at 773 K) was 58 wt% and 74 wt% respectively. This polyaromatic residue contained about 69 wt% of the original lignin feed. The solid polyaromatics undergo further slow decomposition accompanied by a constant release of carbon dioxide as the pyrolysis reaction continues. The pyrolysis of the enzymatic lignin residue was modelled by two reactions in series. In the first pyrolysis step the lignin was decomposed with an apparent activation energy of 74 kJ mol−1 and a heat of reaction of −8,780 kJ kg−1. The second pyrolysis step had an apparent activation energy of 110 kJ mol−1 and a heat of reaction of −2,819 kJ kg−1. Lignin pyrolysis has lower activation energies and higher heats of reaction than cellulose pyrolysis.

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.