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Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals, and Power

Xiaolei Zhang1 and Robert C. Brown2

1School of Mechanical and Aerospace Engineering, Queen's University Belfast, Belfast, BT9 5AH, UK

2Department of Mechanical Engineering, Iowa State University, Ames, IA, USA

1.1 Introduction

Thermochemical processing of biomass uses heat and catalysts to transform plant polymers into fuels, chemicals, or electric power. This contrasts with biochemical processing of biomass, which uses enzymes and microorganisms for the same purpose. In fact, both thermochemical and biochemical methods have been employed by humankind for millennia. Fire for warmth, cooking, and production of charcoal were the first thermal transformations of biomass controlled by humans, while fermentation of fruits, honey, grains, and vegetables was practiced before recorded time. Despite their long records of development, neither has realized full industrialization in processing lignocellulosic biomass. While petroleum and petrochemical industries have transformed modern civilization through thermochemical processing of hydrocarbons, the more complicated chemistries of plant molecules have not been fully developed.

Ironically, the dominance of thermochemical processing of fossil resources into fuels, chemicals, and power for well over a century may explain why thermochemical processing of biomass is sometimes overlooked as a viable approach to bio-based products. Smokestacks belching pollutants from thermochemical processing of fossil fuels is an indelible icon from the twentieth century that no one wishes to replicate with biomass. However, as described in a report released by the US Department of Energy in 2008 [1], thermal and catalytic sciences also offer opportunities for dramatic advances in biomass processing. Actually, thermochemical processing has several advantages relative to biochemical processing, as detailed in Table 1.1. These include the ability to produce a diversity of oxygenated and hydrocarbon fuels, reaction times that are several orders of magnitude shorter than biological processing, lower cost of catalysts, the ability to recycle catalysts, and the fact that thermal systems do not require the sterilization procedures demanded for biological processing. The data in Table 1.1 also suggest that thermochemical processing can be done with much smaller plants than is possible for biological processing of cellulosic biomass. Although this may be true for some thermochemical options (such as fast pyrolysis), other thermochemical options (such as gasification-to-fuels) are likely to be built at larger scales than biologically based cellulosic ethanol plants when the plants are optimized for minimum fuel production cost [2].

Table 1.1 Comparison of biochemical and thermochemical processing.

Source: Adapted from Reference [1].

The first-generation biofuels industry, launched in the late 1970s, was based on biochemically processing sugar or starch crops (mostly sugar cane and maize, respectively) into ethanol fuel and biochemically processing oil seed crops into biodiesel. These industries grew tremendously in the first 15?years of the twenty-first century, with worldwide annual production reaching almost 26?billion gallons of ethanol [3] and 5.3?billion gallons of biodiesel in 2016 [4]. The development of first-generation biofuels has not been achieved without controversy, including criticism of crop and biofuel subsidies, concerns about using food crops for fuel production, and debate over the environmental impact of biofuels agriculture, including uncertainties about the role of biofuels in reducing greenhouse gas (GHG) emissions [5]. Many of these concerns would be mitigated by developing second-generation biofuels that utilize high-yielding nonfood crops that can be grown on marginal or waste lands. These alternative crops are of two types: lipids from alternative crops and lignocellulosic biomass.

Lipids are a large group of hydrophobic, fat-soluble compounds produced by plants and animals. They are attractive as fuel for their high energy content. The most common of these are triglycerides, which are esters consisting of three fatty acids attached to a backbone of glycerol. Triglycerides can be converted into transportation fuels in one of two ways. Biodiesel is produced by transesterification of the triglycerides to methyl esters, which are blended with petroleum-derived diesel. Renewable diesel is produced by hydrotreating triglycerides to yield liquid alkanes and co-product propane gas (see Figure 1.1). Biodiesel has dominated most lipid-based fuel production because of the relative simplicity of the process, which can be done at small scales. Biodiesel is not fully compatible with petroleum-derived diesel, an advantage of renewable diesel. However, hydrotreating requires higher capital investment, with economics favoring larger facilities that may be incompatible with the distributed nature of lipid feedstocks [6].

Figure 1.1 Simplified representation of hydrogenation of triglyceride during hydrotreating.

Soybeans were originally thought an attractive feedstock for biodiesel production, reducing GHG emissions by 41% compared to conventional diesel and producing 93% more energy output compared to corn ethanol [7,8]. However, use of soybeans and other edible oils for fuel has been criticized as competing with their use as food [8,9]. Soybeans are also an expensive energy source, representing 85% of the cost of producing biodiesel [8]. For this reason, most first-generation biodiesel and renewable diesel have been produced from low-cost waste fats and oils.

Wider use of biodiesel and renewable diesel will require alternatives to traditional seed crops, which only yield 50-130?gal/acre [10]. Suggestions have included jatropha (200-400?gal/acre) [11] and palm oil (up to 600?gal/acre) [12], but the most promising alternative is microalgae, which are highly productive in natural ecosystems with oil yields as high as 2000?gal/acre in field trials and 15?000?gal/acre in laboratory trials [13]. Lipids from algae also have the advantage of not competing with food supplies. However, the process is currently challenged by the high costs associated with harvesting and drying algae and the practical difficulties of cultivating algae with high lipid content [14]. Considerable engineering development is required to reduce capital costs, which are as high as $1 million/acre, and to reduce production costs, which exceed $10/gal. The challenge of lipid-based biofuels is producing large quantities of inexpensive lipids rather than upgrading them to fuels.

Lignocellulosic biomass is a biopolymer of cellulose, hemicellulose, and lignin (Figure 1.2) [16]. Lignocellulosic biomass dominates most terrestrial ecosystems and is widely managed for applications ranging from animal forage to lumber. Cellulose is a structural polysaccharide consisting of a long chain of glucose molecules linked by glycosidic bonds. Glycosidic bonds also play a vital role in linking pentose, hexose, and sugar acids contained in hemicellulose. Breaking these bonds releases monosaccharides, allowing lignocellulosic biomass to be used for food and fuel production. Biochemical processing of lignocellulosic biomass employs a variety of microorganisms that secrete enzymes that catalyze the hydrolysis of glycosidic bonds in either cellulose or hemicellulose. Many animals, such as cattle and other ruminants, have developed symbiotic relationships with these microorganisms to allow them to digest cellulose. Thermal energy and catalysts can also break glycosidic bonds, usually more inexpensively but less selectively than enzymes.

Figure 1.2 Three main components of lignocellulosic biomass: cellulose, hemicellulose, and lignin [15].

Lignin, a complex cross-linked phenolic polymer, is indigestible by most animals and microorganisms. In fact, it protects the carbohydrate against biological attack. Thus, even ruminant animals that have evolved on diets of lignocellulosic biomass, such as grasses and forbs, can only extract 50-80% of the energy content of this plant...