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.1 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.1 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.2 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.3,4 Figure 1.1 summarizes the conversion of cellulosic and lipid-rich feedstocks by biochemical and thermochemical processes into diverse products.
Figure 1.1 Second-generation pathways for converting cellulosic and lipid-rich biomass into power, fuels and chemicals.
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.5 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.6,7
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:8,9
- 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.10 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.9
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.2 Each of these components produce distinctive products under fast pyrolysis.
Figure 1.2 Major components of lignocellulosic biomass.
Fast pyrolysis has also been used to thermally deconstruct other kinds of biomass feedstocks such as algae and a variety of mixed wastes including municipal solid waste, sewage sludge, manure, food processing waste, and organic by-products from manufacturing.9,11,12 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.13,14
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.15
1.2.3 Types of Fast Pyrolysis Reactors
Although fast pyrolysis was first investigated as early as 1875,16 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.8,17 Key features of these different classes of fast pyrolysis reactors are summarized in Table 1.1.
Table 1.1 An overview of different fast pyrolysis reactor technologies (reproduced from ref. 17 with permission from Dr Anthony V. Bridgwater)
| Reactor type | Development Status | Max. yield wt% | Complexity | Feed size specification | Inert gas requirements | Specific reactor size | Scale-up |
| Bubbling fluidized bed | Commercial | 75 | Medium | High | High | Medium | Easy |
| Circulating fluidized bed | Commercial | 70 | High | High | High | Medium | Easy |
| Rotating cone | Commercial | 70 | High | High | Low | Low | Easy |
| Auger | Pilot | 60 | Medium | Medium | Low | Low | Medium |
| Entrained flow | Laboratory | 60 | Medium | High | High | Medium | Easy |
| Ablative | Laboratory | 75 | High | Low | Low | Low | Difficult |
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 lignocellulo...
