Fast Pyrolysis of Biomass: Advances in Science and Technology: Volume 50 (Green Chemistry Series) - Hardcover

Hardcover

ISBN 10: 1782626182 ISBN 13: 9781782626183

Publisher: Royal Society of Chemistry, 2017

Synopsis

Fast pyrolysis and related catalytic pyrolysis are of increasing interest as pathways to advanced biofuels that closely mimic traditional petroleum products. This book will be of interest to advanced students and researchers of the science behind renewable fuel technology, and particulary the thermochemical processing of biomass.

About this title

About the Author

James H Clark is Professor of Chemistry and Director of the Green Chemistry Centre of Excellence, The University of York, UK. He has led the green chemistry movement in Europe for the last 15 years and was the first scientific editor of the journal Green Chemistry and is Editor-in-chief of the RSC Green Chemistry book series.

From the Back Cover

Fast pyrolysis and related catalytic pyrolysis are of increasing interest as pathways to advanced biofuels that closely mimic traditional petroleum products. Research has moved from empirical investigations to more fundamental studies of pyrolysis mechanisms. Theories on the chemical and physical pathways from plant polymers to pyrolysis products have proliferated as a result.

This book brings together the latest developments in pyrolysis science and technology. It examines, reviews and challenges the unresolved and sometimes controversial questions about pyrolysis, helping advance the understanding of this important technology and stimulating discussion on the various competing theories of thermal deconstruction of plant polymers. Beginning with an introduction to the biomass-to-biofuels process via fast pyrolysis and catalytic pyrolysis, chapters address prominent questions such as whether free radicals or concerted reactions dominate deconstruction reactions. Finally, the book concludes with an economic analysis of fast pyrolysis versus catalytic pyrolysis.

This book will be of interest to advanced students and researchers interested in the science behind renewable fuel technology, and particularly the thermochemical processing of biomass.

Excerpt. © Reprinted by permission. All rights reserved.

Fast Pyrolysis of Biomass

Advances in Science and Technology

By Robert C. Brown, Kaige Wang

The Royal Society of Chemistry

Copyright © 2017 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-1-78262-618-3

Contents

Chapter 1 Prospects for Fast Pyrolysis of Biomass Kaige Wang and Robert C. Brown, 1, 
Chapter 2 Primary Reactions of Cellulose Pyrolysis J. Lédé, 12, 
Chapter 3 Lignin Depolymerization/Deconstruction Reactions During Fast Pyrolysis J. Y. Kim and J. W. Choi, 37, 
Chapter 4 Transport and Secondary Reactions of Depolymerized/Deconstructed Species Xianglan Bai and Yuan Xue, 57, 
Chapter 5 Catalytic Biomass Pyrolysis with Reactive Gases David C. Dayton, Kaige Wang, Jonathan E. Peters and Ofei D. Mante, 78, 
Chapter 6 Characterization and Separation of Bio-Oil Shurong Wang, Junhao Chen, Fan Zhang and Yurong Wang, 96, 
Chapter 7 Role of Free Radicals in Fast Pyrolysis Kwang Ho Kim, Seema Singh, Victoria Custodis and Jeroen van Bokhoven, 117, 
Chapter 8 Bio-Oil Stabilization Anja Oasmaa, 138, 
Chapter 9 Extraction of Value-Added Chemicals from Bio-Oil Products D. Meier, 160, 
Chapter 10 Catalytic Fast Pyrolysis Over Zeolites Güray Yildiz, Frederik Ronsse and Wolter Prins, 200, 
Chapter 11 Simulating Biomass Fast Pyrolysis at the Single Particle Scale Peter N. Ciesielski, Gavin M. Wiggins, Joseph E. Jakes and C. Stuart Daw, 231, 
Chapter 12 Economic Comparison of Various Pathways to Pyrolysis-Based Fuels Tristan R. Brown, 254, 
Subject Index, 273, 

CHAPTER 1

Prospects for Fast Pyrolysis of Biomass

KAIGE WANG AND ROBERT C. BROWN

Energy Technology Division, RTI International, Research Triangle Park, NC, 27709, USA; Bioeconomy Institute, Iowa State University, Ames, IA 50011, USA

E-mail: kaigewang@rti.org

1.1 Introduction

The past decade has seen increasing interest in production of fuels and chemicals from biomass. Based on the types of feedstock used, biofuels are classified as either first or second-generation. First-generation biofuels include ethanol produced from sugars and starch crops such as maize and sugarcane and biodiesel from seed oils. In contrast, second-generation biofuels are produced from cellulosic and lipid-rich plant materials that are not food crops. These include agricultural and forestry residues, dedicated energy crops like hybrid poplar and switchgrass, algae and municipal solid waste. Although the commercial production of first-generation biofuels has grown tremendously in the last decade, they have been challenged for their limited greenhouse gas reductions compared to petroleum-based fuels and concerns that their production diverts these crops from food production, the so-called food-vs.-fuel debate.Second-generation biofuels offer the prospect of overcoming both of these challenges compared to first-generation biofuels.

Second-generation biofuels can be produced by thermochemical or biochemical processes. Thermochemical processing utilizes heat and catalysts while biochemical processing employs enzymes and microorganisms to convert biomass into fuels and chemicals. Hybrid processing, which combines aspects of thermochemical and biochemical processing, is of growing interest. Figure 1.1 summarizes the conversion of cellulosic and lipid-rich feedstocks by biochemical and thermochemical processes into diverse products.

Thermochemical processes, operating at significantly higher temperatures than biochemical processing, are usually very fast, measured in seconds or minutes compared to hours or days for biochemical processes. On the other hand, thermochemical processes can be less selective than biochemical processes, which can unfavorably affect yields of desired products. However, this lack of selectivity often means that more kinds of feedstock molecules are converted, resulting in higher overall yields of drop-in fuels from lignocellulosic feedstocks, for example.

Thermochemical processes can be classified into gasification, pyrolysis, and solvent liquefaction. Gasification converts solid feedstocks into flammable gases known as producer gas or syngas. Pyrolysis converts solid feedstocks into mostly liquid products. Solvent liquefaction resembles pyrolysis in some respects, producing mostly liquid products, but occurs in the presence of a solvent. Of these three thermochemical technologies, pyrolysis has received the most attention in the last few years for its potential to convert lignocellulosic biomass into a liquid intermediate that can be upgraded to drop-in (hydrocarbon) fuels using technologies familiar to the petroleum industry. It also has prospects for distributed processing of biomass, which can simplify the logistics of providing feedstock to a processing plant.

1.2 Biomass Fast Pyrolysis Technology

1.2.1 Basic Concepts

Pyrolysis is the thermal decomposition of organic substances in the absence of oxygen to form liquids, solids, and non-condensable gases. The rate of pyrolysis profoundly affects product distributions. Slow pyrolysis, developed centuries ago to produce charcoal for heating purposes, occurs over periods measured in hours or even days. In contrast, fast pyrolysis both rapidly heats the feedstock and quenches the products, usually in the order of seconds, with the goal of producing an energy-rich liquid, known as bio-oil, from the vapors as the primary product. Although originally produced for use as heating oil or electric power generation, bio-oil has been increasingly regarded as an intermediate for the production of drop-in biofuels, biobased chemicals, and hydrogen fuel. To maximize bio-oil production (up to 75 wt% of biomass), several conditions must be met during pyrolysis:

• the biomass must be rapidly heated, in the order of a few seconds;

• the products of pyrolysis must be rapidly removed from the reaction zone and cooled, in the order of a few seconds;

• optimum reaction temperature is thought to be between 400–500 °C.

The solid product of fast pyrolysis, known as biochar, consists mostly of carbon but also contains ash originating from biomass. Biochar, which represents 12–15 wt% of the products of fast pyrolysis, can be used as boiler fuel but more intriguing applications include soil amendment, carbon sequestration agent, and activated carbon. Non-condensable gases from fast pyrolysis, yielding 13–25 wt%, are a flammable mixture of carbon monoxide, hydrogen, carbon dioxide, and light hydrocarbons suitable for generating process heat.

1.2.2 Fast Pyrolysis Feedstock

Many kinds of lignocellulosic biomass, ranging from agricultural residues, forestry waste, and energy crops, have been tested for suitability as fast pyrolysis feedstock. The three major components of lignocellulosic biomass, illustrated in Figure 1.2, are cellulose, hemicellulose, and lignin. Cellulose, the most abundant polymer on the planet, constitutes 30–50% of lignocellulosic biomass. It is a structural polysaccharide consisting of pyranose rings linked by glycosidic bonds. Hemicellulose is a heteropolysaccharide of random, amorphous structure cross-linked to cellulose and lignin. Lignin is a highly branched phenol-based polymer bound to cellulose and lignin to form a lignocellulosic matrix. Each of these components produce distinctive products under fast pyrolysis.

Fast pyrolysis has also been used to thermally deconstruct other kinds of biomass feedstocks such as algae and a variety of mixed wastes including organic by-products from manufacturing. These feedstocks often contain relatively large fractions of starch, lipids and proteins compared to lignocellulosic biomass, as well as significant ash. The compositional complexities of alternative feedstocks adds to the difficulties of pyrolyzing them.

Drying biomass feedstocks to less than 10 wt% moisture and comminution to less than 3 mm particle diameter are important steps to successful fast pyrolysis. Despite this preprocessing, the low bulk density, irregular particle shapes and cohesive/adhesive behavior of many kinds of biomass can lead to bridging, blockage, and other feeding difficulties.

1.2.3 Types of Fast Pyrolysis Reactors

Although fast pyrolysis was first investigated as early as 1875, significant progress in developing it for bio-oil production only dates from the 1980s. A variety of reactors were investigated with the goal of heating biomass to temperatures exceeding 400 °C in a few seconds. Suitable reactors include bubbling fluidized beds, circulating fluidized beds, rotating cone reactors, auger reactors, entrained flow reactors and ablative reactors. Key features of these different classes of fast pyrolysis reactors are summarized in Table 1.1.

Among the various kinds of reactors, fluidized beds have received the most attention for fast pyrolysis due to excellent heat and mass transfer characteristics, simplicity of operation, and relative ease of scale-up. Bio-oil yields of 60–75% from fast pyrolysis of lignocellulosic biomass have been reported. Canadian-based Dynamotive commercialized the bubbling fluidized bed fast pyrolysis technology with a capacity of 200 tons of biomass per day (ton day-1). Another Canadian company, Ensyn Technologies Inc. has developed and commercialized its Rapid Thermal Processing (RTP) process since the early 1980s, which is based on circulating fluidized bed technology. Several RTP™ biomass pyrolysis plants are in commercial operation, with capacity ranging from 1 to 200 ton day-1.

Rotating cone reactors were originally developed at Twente University, with the aim of eliminating carrier gas requirements while maintaining a high throughput and rapid heat transfer similarly to fluidized bed reactors. Instead of using inert gas, biomass is mechanically mixed with a solid heat transfer medium in a rotating cone. The rotating cone reactor concept has been commercialized by Biomass Technology Group (BTG) in the Netherlands, building several reactors with capacities of up to 120 ton day-1. The auger reactor, which also features mechanical mixing of biomass with a solid heat transfer medium, has been developed by ABRI-Tech of Canada and the Karlsruhe Institute of Technology (KIT). A pilot scale twin-screw auger reactor with 12 ton day-1 capacity is now operational at KIT. Other types of reactor, including entrained flow, vacuum moving bed, ablative reactor, and microwave pyrolysis reactors are still in relatively early stage of technology development.

1.2.4 Bio-Oil from Fast Pyrolysis

Quenching of the condensable vapors and aerosols from fast pyrolysis produces bio-oil. The rapid removal and cooling of pyrolysis products to minimize secondary reactions that can crack vapors are critical to achieving high liquid yields. Traditionally this is accomplished by spraying cold liquid hydrocarbon or recycling bio-oil into the pyrolysis vapor stream. The bio-oil collected in this fashion is an emulsion of lignin-derived phenolic compounds in an aqueous phase containing mostly carbohydrate-derived compounds. Unfortunately, the high reactivity of the bio-oil makes it difficult to distill into separate components, limiting opportunities for upgrading it to diverse value-added products. More recently, efforts have been made to develop fractionating bio-oil collection systems based on boiling points of bio-oil constituents. One such system consists of pairs of temperature-controlled condensers and electrostatic precipitators operated in series to remove vapors and the aerosols as heavy ends (anhydrosugars and phenolic oligomers), a middle fraction (furans and phenolic monomers), and light ends (water and light oxygenates).

Although the dark-brown liquid collected from fast pyrolysis is called "bio-oil" or "bio-crude", it has few similarities with petroleum. Unlike the hydrocarbons found in petroleum, it contains very high levels of oxygen, comparable to that of the biomass from which it is derived. On a mass basis, its heating value is comparable to biomass. In fact, it is sometimes characterized as "liquid biomass". Bio-oil is composed of a complex mixture of water, volatile oxygenates, anhydrosugars, and non-volatile oligomers. Many of these compounds are extremely reactive, leading to polymerization of bio-oil even at ambient temperatures. These physiochemical properties make bio-oil unsuitable as transportation fuel without upgrading. A number of upgrading processes have been explored, including catalytic cracking, hydroprocessing, aqueous phase reforming, and fermentation, to produce transportation fuels from fast pyrolysis process.

1.3 Recent Advances in Fast Pyrolysis Research and Development

The pace of research and development in fast pyrolysis has accelerated considerably in the past decade. As shown in Figure 1.3, scientific publications increased from a dozen or so per year in the early 1980s to over 700 per year in 2016. Most of this growth occurred in the last decade in a period when development of advanced biofuels became a priority in the United States and many other countries around the world.

This rapid growth in research and development has not yet translated into notable successes in the commercial deployment of fast pyrolysis technologies. A number of factors have contributed to the slow pace of commercialization. The meteoric rise in petroleum prices in the first decade of the twenty first century was followed in the next decade by a dramatic price slump, hurting the economics of advanced biofuels. Failure of many markets to assign a cost to carbon emissions from fossil fuels has also diminished the incentive for the commercialization of low carbon fuels. However, in some respects, commercialization efforts launched by the U.S. Energy Security Act of 2005 got ahead of the state-of-the art technology in advanced biofuels: crucial scientific questions and engineering challenges related to pyrolysis as well as other advanced biofuels technologies had not been resolved. Much has changed in the past decade. This book covers recent advances in fast pyrolysis science and technology, with chapters on reaction chemistry of thermal deconstruction, computational modeling, product utilization, catalytic pyrolysis and economic evaluation of fast pyrolysis technology.

1.3.1 Reaction Chemistry of Fast Pyrolysis

As the most abundant polymer on the planet and the major component of lignocellulosic biomass, the pyrolysis of cellulose has been studied since the early 20th century. Although it is generally accepted that levoglucosan is the primary product of cellulose fast pyrolysis, the mechanism by which it forms and its role in producing secondary products continues to be debated. It has been reported recently that cellulose pyrolysis proceeds through the formation of a liquid intermediate, from which volatile products are formed. Multiphase reactions are involved in cellulose pyrolysis, including solid–liquid, liquid–liquid, liquid–gas, and gas–gas reactions. Moreover, reaction intermediates may exist for only fractions of a second, further complicating experiment investigations of cellulose pyrolysis. Chapter 2 is a historical review of the many studies of cellulose pyrolysis covering the several theories and unresolved questions around the primary reactions of cellulose depolymerization.

As described in Chapter 3, lignin is the most complex and difficult to pyrolyze component of lignocellulosic biomass. Lignin melts before it depolymerizes. The liquid products of lignin pyrolysis include volatile phenolic monomers and non-volatile phenolic oligomers with molecular weights ranging up to 2500 Da. These phenolic compounds are extremely reactive, often condensing and dehydrating to char and light gas before they can volatilize. These characteristics of lignin can cause agglomeration in reactors and clogging of condensers.

Due to the inherent reactivity of pyrolysis products, various secondary reactions could occur during biomass pyrolysis, leading to a lower yield of bio-oil with undesirable properties. Chapter 4 reviews recent studies related to secondary reactions of pyrolysis of cellulose, lignin, and whole biomass. Heat and mass transfer phenomena during fast pyrolysis are also addressed in this chapter.

The role of free radicals in pyrolysis has been the subject of speculation for many years. The short half-life of radicals is among the challenges in studying them. Recent advances in analytical techniques such as in situ Electron Paramagnetic Resonance have been employed to probe free radical chemistry during pyrolysis. Recent studies have revealed that free radicals participate in a variety of reactions, especially the depolymerization of lignin. Chapter 7 reviews studies that have detected free radicals in bio-oil and biochar and investigated their role in fast pyrolysis chemistry. The role of radicals in bio-oil stability is also discussed.

1.3.2 Computational Modeling of Fast Pyrolysis

Improved understanding of the physics and chemistry of fast pyrolysis arising from experimental studies has encouraged multi-scale modeling of pyrolysis processes. Intra-particle chemistry and transport processes are receiving particular attention from the scientific community as their role in determining overall yields and selectivity of pyrolysis products becomes increasingly evident. Chapter 11 describes recent advances in modeling pyrolysis in individual particles and provides readers the state-of-the-art in predicting pyrolysis kinetics. Limitation of current modeling efforts and future opportunities are also discussed.

(Continues...)

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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 2017
• Language: English
• ISBN 10: 1782626182
• ISBN 13: 9781782626183
• Binding: Hardcover
• Number of pages: 290
• Editor: Robert C Brown