The Role of Catalysis for the Sustainable Production of Bio-fuels and Bio-chemicals

The Role of Catalysis for the Sustainable Production of Bio-fuels and Bio-chemicals describes the importance of catalysis for the sustainable production of biofuels and biochemicals, focused primarily on the state-of-the-art catalysts and catalytic processes expected to play a decisive role in the "green" production of fuels and chemicals from biomass. In addition, the book includes general elements regarding the entire chain of biomass production, conversion, environment, economy, and life-cycle assessment.

Very few books are available on catalysis in production schemes using biomass or its primary conversion products, such as bio-oil and lignin. This book fills that gap with detailed discussions of:

• Catalytic pyrolysis of lignocellulosic biomass
• Hybrid biogasoline by co-processing in FCC units
• Fischer-Tropsch synthesis to biofuels (biomass-to-liquid process)
• Steam reforming of bio-oils to hydrogen

With energy prices rapidly rising, environmental concerns growing, and regulatory apparatus evolving, this book is a resource with tutorial, research, and technological value for chemists, chemical engineers, policymakers, and students.

• Includes catalytic reaction mechanism schemes and gives a clear understanding of catalytic processes
• Includes flow diagrams of bench-, pilot- and industrial-scale catalytic processing units and demonstrates the various process technologies involved, enabling easy selection of the best process
• Incorporates many tables, enabling easy comparison of data based on a critical review of the available literature

• Language: English
• Publisher: Elsevier Science
• Release date: Mar 19, 2013
• ISBN: 9780444563323

Book preview

1

A General Introduction to Biomass Utilization Possibilities

Paul O’Connor, BIOeCON BV and ANTECY BV, Hoevelaken, The Netherlands

Outline

1.1 Introduction: Scope of This Introduction

1.2 A Short History: What Is Biomass? What Is Photosynthesis?

1.3 Chemistry of Biomass and Biomass Conversion

1.4 Drawbacks and Limitations of Biofuels 1.0: First-Generation Biofuels

1.5 Biofuels 2.0: Second-Generation Biomass Conversion Technologies

1.6 Beyond Biofuels: A Personal Future Perspective

Acknowledgments

I would like to thank and acknowledge BIOeCON and ANTECY for allowing me the time and supporting me with the writing of this chapter. In particular, I would like to thank and acknowledge Gerrit van Putten (ANTECY) for his helpful creativity and his high-quality artwork in preparing the figures.

1.1 Introduction: Scope of This Introduction

In no way should this introduction be seen as a complete review of the area of biomass utilization. There are already several good and extensive recent reviews in this field, as for instance [1–5]. The scope of this introduction, consistent with the title of this book, is limited to the utilization of biomass as fuels and chemicals, wherein the use of catalysis to convert biomass into fuels and chemicals is discussed and elucidated. Zooming in even further, the focus of this introduction is primarily on the use of inorganic catalysis.

Obviously, organic catalysis, e.g., enzymatic catalysis, can be and has been traditionally applied in the conversion of biomass, such as in the conversions of sugars into ethanol. However, the personal insights and perspectives presented by the author in this introduction point to the observation that inorganic catalysis may be more suitable for the economical conversion of unrefined raw biomass such as agricultural biomass and forestry waste. It is also important to consider here the ample availability of these waste biomass streams and the ethical consideration of not using the nonwaste edible parts of biomass for conversion into fuels and chemicals.

1.2 A Short History: What Is Biomass? What Is Photosynthesis?

The evolution of the universe can be told as the story of the evolution of energy. Energy is transformed into matter: first subatomic particles (quarks, neutrinos), then atoms and molecules, and subsequently into more complex molecules including what we now call organic molecules (containing carbon). Certain organic molecules can be found even in space, but just because they are organic molecules does not mean that they have been formed by or within living organisms. The next crucial chapter in the story of molecular development is the emergence of living organic (or biological) organisms, which can all be grouped under the term biomass. These biological creatures can grow and reproduce, increasing their (bio)mass by transforming simple small molecules (such as water and carbon dioxide) into more complex molecules using the light from the sun as the energy source. This fascinating process is called photosynthesis.

The history of photosynthesis and its discovery is a remarkable and thrilling chapter in the history of science [6–8]. Over 3000 million years ago, the first living organism, which resembled a plant, appeared. It was blue-green algae, which lived in the sea and can still be found in water today. When the plants made their first appearance on Earth, the atmosphere was unlivable for all oxygen-breathing creatures. The air was made out of carbon dioxide, a gas which to us is deadly. Then photosynthetic plants came along and, slowly over several million years, cleaned the atmosphere and filled it with oxygen. If plants had never come along and revolutionized the atmosphere, we would never have evolved and we would never have been able to think about the formation, growth, and conversion of biomass.

In 1649, Jan Baptista Van Helmont, a Flemish physician, chemist, and physicist, conducted the first biological experiment in which the ingredients were measured accurately and all changes noted precisely. For 5 years, Van Helmont waited patiently watching a tree grow, until finally he removed it from the pot, shook off all the soil, and weighed the plant. In 5 years, the willow tree had added 164 pounds to its original weight. Then, for the second part of the experiment, Van Helmont dried and weighed the soil. Had it lost 164 pounds to the weight of the tree? No. It had only lost 2 ounces! From this, Van Helmont concluded that the willow tree drew its nutrients not from the soil but from water. Accidentally, he made a mistake and concluded that the material that made up the bark, wood, roots, and leaves came from the water he had added over the 5 years!

The next big important step in the understanding of photosynthesis came in the early 1770s. Joseph Priestley, from Yorkshire, published his Experiments and Observations on different kinds of air. He was given the credit for discovering oxygen, and found that mint plants could restore the air in a container with a burning candle, so that the candle could be used again. Accidentally, one day, Joseph Priestly placed the candle in a dark corner of his laboratory. Since the mint plant could not photosynthesize, the candle’s flame extinguished. Unfortunately, Priestley never really understood the great role that light played in his experiment.

In 1979, a Dutch physician from Breda, Jan Ingenhousz, wanted to find out whether flowers really did help cure illnesses. After many different tests, he finally concluded that only the green parts of plants cleaned the air and that this happened only when the plants were placed in strong light. Flowers and other nongreen parts of plants used up oxygen just like animals! Ingenhousz suggested that this process of photosynthesis causes carbon dioxide to split into carbon and oxygen. Then the oxygen is released as a gas. In 1804, the Swiss scientist, Nicholas Theodore de Saussure (Recherche chimiques sur la végétation), repeated Van Helmont’s experiment but carefully measured the amounts of carbon dioxide and water that were given to the plant. He showed that the carbon in the plants came from carbon dioxide and the hydrogen from water. Then, 40 years later, a German scientist, Julius Robert von Mayer, proposed that the sun is the ultimate source of energy utilized by living organisms, and introduced the concept that photosynthesis is a conversion of light energy into chemical energy. Literally, photosynthesis means synthesis with the help of light: The Power Plant and the Chemical Factory of Life [8]

As illustrated by Figure 1-1 in a very simplified scheme, photosynthesis is a very complicated process. Photosynthesis enables biological species to utilize the energy of the sun to synthesize chemical building blocks for their growth and reproduction and to synthesize molecules in which energy can be stored for later use, namely, as fuels. The basic building blocks of photosynthesis are sugars, which can function as fuel or thereafter be transformed into polymers (cellulose, hemicellulose, and lignin) for building, growth, and structural strength. The sugars can also be transformed into higher energy content molecules such as lipids (oils). Our earliest ancestors had already utilized this ingenious chemical factory of life extensively: biomass as food, and wood as fuel for fire (heating and cooking) to survive. Biomass was also used as building blocks for housing (wood) and clothing (straw).

Figure 1-1 Photosynthesis.

Biomass energy (Figure 1-2) in the form of wood had fueled the world’s economy for thousands of years before the advent of more easily winnable coal and subsequently oil, gas, and uranium. The Industrial Revolution in England 200 years ago saw the developed world beginning to embark upon the fossil fuel era, which we now appreciate may be a limited one. Bioenergy is once more a recognized fuel supply option in the developed world, just as it has continued to be in the Third World.

Figure 1-2 Biomass energy.

While biomass energy does not affect the natural carbon-CO2 cycle (Figure 1-2), fossil fuels (Figure 1-3) do, and so add to the increase in greenhouse gasses (GHGs).

Figure 1-3 Fossil fuels.

Lewis [3] highlights the history of biomass energy and shows that biological energy production is by no means revolutionary, and that it has the potential to return again as a major energy supplier to the developed world as well as to the developing countries where it remains indispensable even today. His conclusion (in 1981) is that the second chapter in the history of biomass energy has already begun.

But before jumping to this second chapter of biomass energy, it might be worthwhile to reflect on the background of the success and origin of fossil fuels. The reason why the classical biofuels (straw and wood) were replaced by coal, oil, and gas has a lot to do with the higher concentration in terms of the availability at the source (in terms of fuel per area) and the energy density of the fuels (energy content per weight or volume), as well as the stability, storability, and transportability in the case of liquid fuels (Table 1-1).

Table 1-1

Production Rates and Properties of Biomass and Fossil-Based Fuels

The reason for this double concentration process is twofold: fossil fuels are formed by the fossilization [9,10] of biomass (terrestrial as well as aquatic). During this fossilization process, the biomass can be liquefied and locally concentrated, for instance, in the case of crude oil, which is then trapped in porous rocks under relatively impermeable formations often at very great distances away from the original sources. These oils are deoxygenated, substantially increasing the energy content of the oil. This is clearly illustrated in the Van Krevelen (Figure 1-4) diagram, which is a good starting point for a better understanding of the processes involved in the conversion of biomass.

Figure 1-4 Van Krevelen diagram.

Brooks [9] notes that the story of the formation of crude oil or petroleum (oil from rocks) and natural gas is not only an intriguing scientific puzzle but also an occurrence of the greatest practical importance to petroleum geologists. If we knew the entire story, it would enable us to understand why large continental areas are entirely without accumulations of petroleum of commercial importance and why, in certain areas, perfectly good geological structures favorable to the accumulation of petroleum have been drilled but no petroleum or natural gas whatever has been found. He reaches some very interesting conclusions based on the analysis of petroleum compositions. First of all, he concludes that the chemical complexity of petroleum, together with the evidence of low temperature history, is best accounted for by catalytic activity of active surface minerals, particularly clays, with which oil has been in contact for long periods. Petroleum reserves are not equilibrium mixtures produced by thermal action alone. The time element and evident geological history preclude oil or its source material ever having been heated to temperatures as high as 400 °C.

So, here is the evidence, or at least the indication, that catalytic conversion of biomass is nothing new and probably has been going on in Nature for millions of years already. Brooks also concludes that only methane has been formed by bacterial action and that bacterial action cannot reasonably explain the complexity of petroleum in regard to number and types of hydrocarbons found therein. Summarizing his findings, he states that the formation of petroleum appears to have taken place in two general stages or cycles: first, an early stage, in which organic matter buried in marine sediments was chemically changed to material consisting largely of carbon and hydrogen but containing few hydrocarbons; second, a later stage in which a very large number of paraffins, isoparaffins, naphthenes, and aromatics were formed by the catalytic action of active surface minerals, including clays, at relatively low temperatures.

Figure 1-5 illustrates the two major cycles of organic carbon on Earth. Organic carbon is mainly recycled in cycle 1. The crossover from cycle 1 to cycle 2 is a tiny leak that amounts to less than 0.1% of the primary organic productivity.

Figure 1-5 Cycles of organic carbon.

With regard to the biomass origin of petroleum (crude oil), fatty oils appear to be the largest source material of petroleum, with proteins and cellulose contributing less. This indicates that the source of petroleum is probably mainly aquatic.

Figure 1-6 gives a schematic illustration of the processes involved in the formation of fossil fuels (coal, gas, and crude oil) from the original biological material (biomass). As mentioned earlier, the most evident source of liquid hydrocarbons appears to be the lipids (fatty oils) in aquatic living organisms such as algae and plankton, while coal is mainly formed from terrestrial carbohydrate (cellulosic) sources. However, there is also evidence of crude oil formation based on cellulosic sources.

Figure 1-6 Formation of fossil fuels from biomass.

Obviously, the fossil fuels found and used today are products of a very complex mix of processes, which, by the way, are still hotly debated by the specialists in this field. Still, these processes of Nature have been and can still be of great inspiration to those who are developing biomass conversion processes today.

1.3 Chemistry of Biomass and Biomass Conversion

As we have seen in Section 1.2, the composition of biomass (living organisms) consists mainly of the following:

(1) Carbohydrates (sugars, cellulose, and hemicellulose)

(2) Lignin

(3) Proteins

(4) Lipids (oils, fatty acids)

Carbohydrates and lignins are often grouped together as lignocellulosic biomass.

A rough composition of typical terrestrial biomass is given in Figure 1-7.

Figure 1-7 Terrestrial biomass composition.

Carbohydrates, such as sugars and starches, have been converted to fuel for centuries, mainly in the form of ethanol. The fermentation of sugar into ethanol is one of the earliest organic reactions that humans learned to carry out, and the history of man-made ethanol is very long. Ethanol is a powerful psychoactive substance and ethanol history is filled with accounts detailing its use as a recreational drug. Dried ethanol residue has been found on 9000-year-old pottery in China, which indicates that Neolithic people in this part of the world may already have been consuming alcoholic beverages.

In 1826, Samuel Morey (1762-1843) patented the first internal combustion engine that ran on ethanol and turpentine. Since then, ethanol has been used as a fuel in all industrialized countries.

Ethanol is mainly produced by the biological fermentation of sugars and starches. Obviously, converting sugar (as in the case of sugarcane-based ethanol) is easier and more energy-efficient than the conversion of starch as, for instance, in the case of corn ethanol.

While the primary product of photosynthesis are sugars, it is interesting to study how these carbohydrates have been converted (metabolized) into lignin, proteins, and lipids. Particularly interesting for our purpose is how a cyclic carbohydrate such as glucose can be converted into lipids (fatty acids) containing long hydrocarbon chains (Figure 1-8).

Figure 1-8 Ethanol from sugar and starch.

These long hydrocarbon chains are of prime importance in living systems because of their high energy content and excellent liquid properties.

The long hydrocarbon chains of these lipids look a lot like the long-chain (C15-C20) hydrocarbons used in high-cetane diesel fuels (Figure 1-9). No wonder that these fatty acids found in nature in oil-rich agricultural crops such as soybeans, rapeseed, and even algae are nowadays being converted via several routes into biodiesels [1]. The classical approach to the conversion of these fatty acids into biodiesel is by esterification with ethanol or methanol producing FAEEs (fatty acid ethyl esters) or FAMEs (fatty acid methyl esters) and glycerol as an undesired by-product [1,11]. In the last 10 years, several processes have been developed to produce a higher quality (lower oxygen content) biodiesel from fatty acids by hydrogenation. Well known is the NExBTL process from Neste [11,12], which is an advanced process for producing high-quality renewable diesel fuels by hydrotreating fatty acids to diesel and propane. The hydrotreated vegetable oils (HVOs) are zero-oxygen paraffinic hydrocarbons similar to gas-to-liquid (GTL) diesel fuels. They are, of course, also free of aromatics and sulfur and have high cetane numbers. HVOs can be used as a blending component in diesel fuel or as a fuel. When used as fuel on its own, significant reductions in NOx and particulate emissions can be seen. HVO is already in commercial scale production. Neste Oil is producing its renewable diesel, NExBTL, in two dedicated production plants.

Figure 1-9 Lipids (fatty acids) to diesel.

An alternative approach is hydrotreating vegetable oils in heavy vacuum oil mixtures [13]. The advantage of this approach is that existing refinery hydrotreating units can be used. An example of this approach is the Petrobras HBIO process. The vegetable oil stream blended with mineral diesel fractions is hydroconverted in hydrotreating units (HDTs), which are mainly used for sulfur content reduction in diesel and quality improvement in petroleum refineries. The HBIO technology introduces a new way to include renewable feedstocks in biofuel production. As an example, blending in 10% of soy oil into an existing heavy vacuum gas oil stream increases the cetane number by roughly 10 points [14].

As discussed in Section 1.4 of this chapter, there are some drawbacks and limitations to using vegetable oils for the production of renewable biodiesel. Fortunately, novel processes such as the NExBTL and HBIO are also suitable for the conversion of waste fatty acid streams such as used vegetable oils and animal fat waste and therefore can also contribute to second-generation biofuel production.

1.4 Drawbacks and Limitations of Biofuels 1.0: First-Generation Biofuels

There is a strong and heated debate concerning the possible negative aspects of the increasing use of bioenergy and biofuels. Different aspects have drawn attention, such as concerns about the fact that the use of biomass for bioenergy may increase the food shortage. Even the advantages of biofuels for CO2 reduction are disputed. Biomass by itself creates no additional CO2 emission and this is a positive point, but if the production of fuel from biomass requires a lot of energy (as in the case of the hydroconversion of vegetable oils to diesel), then these fuels may not economically or ecologically be a big improvement in respect to the fossil fuels already in use.

There is also the question whether it is ethical to use high-quality foods, such as sugar and corn, for conversion into fuels, when in some parts of the world, people are still facing starvation and malnutrition. Even more shocking is the deliberate cultivation of nonedible plants at the expense of food crops so that this ethical point of view can be circumvented.

The fact is, however, that less than 30% of the world’s cultivated biomass is fit for human consumption. The nondigestible, low-value part of the biomass, often more than 70% (see Figure 1-7), is usually not used and is simply burned. Bioenergy from biomass will be really interesting only when it is possible convert this difficult-to-digest, nonedible part (animal waste, agri waste, wood, fibers, etc.) into a useful energy source.

The advantages of applying nonedible biomass wastes are summarized as follows:

Ethical

Biofuels from biomass waste do not compete with the normal food supply.

Ecological

If produced with a minimum use of energy, these fuels will generate lower net CO2 than the current fossil fuels.

Economical

The world has enough biomass waste to supply energy to a large part of it. Energy scenarios indicate the possibility to collect 20-30% of the energy from biomass in the twenty-first century [15].

Biofuels from biomass waste can compete with crude oil gas and coal if the process of converting biomass waste into useful biofuels is not too energy intensive. There are already several processes that convert biomass waste into fuels. Usually they apply gasification and GTL technology to convert biomass waste into liquid fuels. Another route is making use of enzymes. Special enzymes (biocatalysts) are being developed that are also capable of digesting wood-based biomass waste. A third alternative was presented by BIOeCON (the e for economic, ecologic, and ethical) formed in 2006 [16,17] as a think tank bringing together an international network of creative scientists to develop novel ways to produce biofuels out of biomass waste. The biomass waste is directly converted into a liquid phase with the use of selective catalysis.

The most important factors hindering the growth of biofuels are economical: the availability and costs of raw biomass that can be processed to renewable fuels. The guiding principles for the development of biofuels 2.0 therefore are as follows:

The first generation of biofuels (biodiesel from vegetable oils and ethanol from sugar, starch, or corn) make use of raw materials that are rather limited in supply and therefore costly. Furthermore, as discussed earlier, one may raise the question whether it really makes sense to downgrade these scarce and high-value edible materials into transportation fuels. The story is different for the second-generation of biofuels, biofuels 2.0, which make use of the more abundantly available cellulosic biomass waste. Several new technologies are being developed to unlock these large and low-cost sources of biomass energy. Cellulosic ethanol can, for instance, be produced via enzymatic conversion once the solid cellulose is separated from lignin, and the structure opened up and hence made more accessible to the enzymes. There are several developments going on in the area of improving pretreatment processes, such as acid and/or steam heat treatments. Unfortunately, the separation of ethanol from water still remains a costly factor, and ethanol volatility may limit the quantity that can be blended into gasoline.

An alternative route is to convert the solid biomass into a gas and produce a synthesis gas (CO + H2), which can then be converted to a liquid via the Fischer-Tropsch process. This route is often called BTL (biomass-to-liquid) via GTL. While this technology has been proven on a lab scale, it does require several complex process steps and is quite capital- and energy-intensive.

A more simple and robust (in terms of feedstock flexibility) approach is to convert the solid biomass into a liquid (BTL) by direct liquefaction. Several thermal and thermocatalytic processes are under development in this area. A drawback is that the quality of the bio-oil produced is often rather poor and extensive treatment and upgrading is required to produce the right components for transportation fuels and/or chemicals. An interesting, new approach in this respect is the catalytic pyrolysis of biomass, whereby catalytic technology is used to achieve the liquefaction of the solid biomass under milder conditions and at a lower cost. The technology is similar to FCC (fluid catalytic cracking) and therefore requires less time to commercialize than most other schemes. This opens the way for an ethically and ecologically justified raw material, ready for further processing in existing petrochemical refineries instead of fossil-based crude oil. Economically, this is an interesting development because it uses a major part of the existing infrastructure in oil and/or petrochemical refineries. This means that only limited additional investment will be required for the production of durable fuels and biologically degradable polymers from biomass.

1.5 Biofuels 2.0: Second-Generation Biomass Conversion Technologies

Second-generation biomass conversion technologies are those that convert nonedible biomass streams such as agricultural and animal waste into useful chemicals and/or renewable transportation fuels (Biofuels 2.0). The nonedible biomass consists mainly of cellulosic materials (see Figure 1-7: cellulose, hemicellulose, and lignin) and is also often called (ligno)cellulosic biomass. Reviewing the several emerging technologies to convert lignocellulosic biomass, we can distinguish between two main routes:

(1) Indirect Conversion

(2) Direct Conversion

In the case of indirect conversion, the biomass is first converted into an appropriate intermediate before performing the next step of converting the intermediate into the final product, which is a biomass-based chemical or fuel. As discussed earlier, one approach is to first break down the cellulosic biomass into sugars and/or starches, and then to convert these sugars with the conventional enzymatic technology into ethanol. Alternatively, these sugars can be used to produce several other chemicals and/or fuels, as illustrated in Figure 1-10.

Figure 1-10 Indirect conversion via sugar.

Reviewing the various processes and companies active in this field, one can see that there is quite a lot of research and development going on in utilizing sugars to produce various products such as butanol (BP, Dupont, Gevo), octanol (Codexis), isoprenoids (Amyris), and other paraffins as diesel compounds (LS9, Amyris, Solazyme).

Unfortunately, the first step, which is the conversion of cellulose (and hemicellulose), remains the principal bottleneck. Development in this field has been much less dramatic, possibly with an exception to the process being developed by HCL CleanTech.

HCL CleanTech claims to have improved the economics of an old, industrially proven German process for converting biomass to fermentable sugars that are usable as a feedstock for biofuels and bioproducts. The Rheinau (or Bergius) process [18,19] was one of the means by which Nazi Germany obtained sufficient fuel for their war machine, even though they did not capture the oil fields of the Middle East. They were able to convert wood from their extensive forests into alcohol, which could serve as fuel in combustion engines. Ultimately, as other means of energy production were more efficient (including the better known Bergius process for converting coal to fuel oil) and as food became more and more scarce in war-torn Germany, the Bergius process began to be used mostly to produce yeast, as a food supplement for both cattle and people. The process was not a simple one, as Bergius himself refers in his article of 1937 [19] to The problems in the Industrial use of concentrated hydrochloric acid. HCL CleanTech has developed a technology to fully recover the concentrated hydrochloric acid (HCl) used as a catalyst in breaking the lignocellulose into sugars. HCL CleanTech’s technology removes the acid from the water to another medium (amines), which later releases the acid at the desired concentration. The process yields 97-98% of the theoretical sugars contained in any lignocellulose. A clean, concentrated (45-55%) stream of fermentable sugars can then be converted into ethanol.

Recently, HCl CleanTech has announced a demonstration project with Virent. The project funded by the U.S. Department of Energy combines HCL CleanTech’s lignocellulosic conversion technologies, which produce nonfood sugars, with Virent’s BioForming technology, which converts plant sugars into hydrocarbon molecules such as those now refined from petroleum. These sugars can be utilized by Virent’s process to make hydrocarbons that can be used as chemicals or blended to make drop-in fuels for transportation [20]. The Virent technology is based on the work done by Dumesic et al. [21] on the production of alkanes by aqueous-phase reforming of biomass-derived oxygenates.

Because of the many difficulties in converting cellulose into sugars, the companies operating in the field of sugar conversion to bioproducts need to use the relatively expensive and scarce sugar as their base material; therefore, for economical reasons, they are focusing on the production of higher value specialties and hence have not yet been able to successfully penetrate the very competitive transportation fuels market.

An alternate indirect approach to producing biofuels is to first convert the solid biomass into a gas by gasification, and then to produce synthesis gas (CO + H2), which can then be converted into a liquid via the Fischer-Tropsch process. As mentioned earlier, this route is often called the BTL via GTL route. The most common application of this route is to produce GTL-like high-quality biodiesel, but many other chemicals, such as methanol, DME (dimethyl ether), and ethanol, can also be produced via this route (Figure 1-11).

Figure 1-11 Indirect conversion via syngas.

Two of the most visible efforts in this BTL via GTL area were discontinued in 2011, partly due to technical problems and partly due to economical reasons [22], namely, the CHOREN venture (funded by Shell) to produce high-quality diesel from biomass, and Range Fuels, which aimed to produce ethanol from biomass-based synthesis gas.

The evident drawback of any indirect conversion is that extra processing steps are required leading to additional investment and operating costs while reducing the overall yield of biofuels. Even if the yield within a single step is high, combining several process steps can reduce the overall yield dramatically.

The question, therefore, arises whether it is possible to convert cellulosic biomass directly into a liquid fuel, avoiding the intermediate stage of forming and separating sugars and/or synthesis gas. This is not a new question, and it has already been addressed by researchers as early as in the 1920s. L.C. Swallen [23] claimed the production of organic acids, such as acetic, formic, oxalic, and succinic, by converting waste corncobs in water under pressure at 160-250 °C with NaOH. In the early 1930s, Bergstrom et al. [24] from wood-rich Sweden investigated the conversion of wood chips in water in an autoclave at 220-360 °C using calcium hydroxide as a catalyst. Alcohols and ketones were produced, and calcium carbonate was regenerated back into calcium hydroxide.

The flow scheme of this process is shown in Figure 1-12.

Figure 1-12 Wood chip conversion in water.

A similar process has been patented by Urison et al. [25]. They claimed to have distilled wet vegetable matter with caustic and lime. As far as known to the author, none of these processes has had a broad commercial application.

Much later, in the 1980s, the hydrothermal route to the direct conversion of solid biomass into liquid fuels was revived by Shell’s R&D [26,27] in the form of the HTU process: hydrothermal upgrading of solid biomass. In this process, it is claimed that, with biomass in water at above 300 °C, oxygen can be removed from the high-oxygen-containing biomass (~ 45% weight oxygen) without adding hydrogen, producing a liquid hydrocarbon stream with a relatively low-oxygen-containing stream (~ 10% weight oxygen) similar to that of vegetable oils. This low-oxygen bio-oil can be further reduced in oxygen content via conventional gasoil hydrotreating. In the HTU process itself, oxygen is removed in the form of carbon dioxide. This mechanism seems very similar to the Bergstrom process described earlier, where calcium hydroxide is converted into calcium carbonate and acids are converted into ketones.

In one of the embodiments described in the HTU patent [26], sodium carbonate is introduced as a catalyst, resulting in an improved oil yield at the expense of carbon (coke) and gas.

What is most amazing about the HTU process is that cellulose is converted into hydrocarbons in the gasoline and diesel range. This is remarkable, as the longest carbon chains in cellulose are C5s and C6s, while it seems the HTU is producing oil in the C5-C30 range.

It seems as if at least three different types of reactions are taking place:

(1) Solid biomass (cellulose) is being decomposed and liquefied forming smaller (C5-C6) molecular units, most probably organic acids as also claimed by Swallen [23];

(2) These organic acids are being decarboxylated forming CO2 and even smaller (C4-C5) molecules containing lower oxygen

(3) These molecules containing lower oxygen are recombining (polymerizing and/or alkylating) into larger molecules in the C5-C30 range.

Probably the reactions (2) and (3) are taking place in the same reaction cycle whereby ketones are formed from the reaction of multiple organic acids, as illustrated by Figure 1-13.

Figure 1-13 Catalytic cycle of fatty acids.

The reaction of fatty acids forming larger ketones is well known as in the production of high-quality saturated base oil or base oil components used for lubrication oils. Koivusalmi et al. [28] describe how unsaturated carboxylic acids are oligomerized and decarboxylated in the presence of a cationic clay and/or zeolite catalyst.

Catalytic decarboxylation by cationic clays has also been claimed by Marquez Moreira et al. [29] in the Petrobras-developed process to reduce the acid content of high TAN (total acid number) crudes. Acidic crudes are probably oil streams that have not completely matured (i.e., not completely deoxygenated) and hence need a helping hand, in this case by application of specific clays as catalysts. This may not be so unexpected considering the earlier mentioned conclusions from Brooks [9]: The chemical complexity of petroleum, together with the evidence of low temperature history, are best accounted for by catalytic activity of active surface minerals, particularly clays, with which oil has been in contact for long periods of time.

The fact that most of the crudes found up to now are not so acidic and contain only traces of oxygen may imply that the clay-catalyzed decarboxylation of biomass-based crude precursors is not the limiting step in their maturation. On the other hand, the fact that most of the fossilized hydrocarbons are coal and gas may indicate that the selectivity toward liquid fuels (crude oil) has not been very good. Obviously, other factors, for instance, long residence times at adverse conditions causing overcracking of the oil to coke (i.e., coal) and gas, may also have played an important role.

The HTU process has also not become a commercial success. The scaling up of this process is very complex because of the combination of high temperatures (350 °C) and high pressures (the saturated water pressure being more than 150 bars) involved. Also, as the reaction intermediates (high-oxygen-containing unsaturated compounds) are very reactive, side products such as gum, char, and tar are formed, which can easily and dangerously block the high-pressure reactor system. Reducing the reaction time (normally on the order of 5-10 min), the temperature, and hence, pressure of the HTU process would be a possible way to reduce reactor plugging and the costs (strongly related to the pressure) of the process. O’Connor et al. [30] proposed a catalytic version of the HTU process using certain layered clays as catalysts to reduce reaction time and restrict the reaction temperatures to the 250 °C range. Unfortunately, it appears that even at these lower temperatures the problem of gum and tar formation is very much present, leading to very short run lengths before plugging-up of the reactor.

The sugar-like reaction intermediates formed, for instance, maltose, easily degrade to char and tar at temperatures above 150 °C [31].

Considering the fact that during the conversion of biomass, it will always be necessary to deal with high-oxygen-containing molecules, which yield very reactive reaction intermediates, it becomes very important to strive for milder reaction conditions, that is, lower reaction temperatures and/or shorter residence times.

Based on this crucial observation, BIOeCON has focused R&D on biomass-to-fuel conversion in two distinct directions [17,32,33]:

(1) Biomass catalytic cracking (BCC): Biomass conversion catalytic pyrolysis takes place at very short contact times (seconds) and in an FCC-like reactor, followed by regeneration configuration with a dedicated catalyst. The biomass is liquefied and decarboxylated to an acceptable level, while any char (i.e., coke) formed is captured on the catalyst and the catalyst is continuously regenerated by burning off the coke as in a classical FCC unit. The stable low-oxygen bio-oil formed can be hydrotreated and blended into existing transportation fuels (Figure 1-14).

Figure 1-14 BCC.

(2) Biomass hydroconversion in a dedicated solvent (BiCHEM): Here, the biomass is converted in a liquid phase process. Cellulose and hemicellulose are dissolved at moderate temperatures in an inorganic ionic liquid (molten salt) and subsequently converted into less polar and more stable components that can be separated and applied as platform molecules for the chemical industry and/or as high-performance fuel additives [34] (Figure 1-15).

Figure 1-15 BiCHEM.

The BiCHEM process can be considered as a further evolution of the C-HTU concept except for the fact that the medium (water) needs to be changed to make dissolution of the cellulosic material possible at temperatures below 150 °C. On the other hand, the BiCHEM process also builds on the older ICI process as described by Ragg et al. [35]. Molten metal salts are applied to dissolve cellulose to produce glucose, which is separated and subsequently converted by conventional enzymatic methods. The BiCHEM process deviates fundamentally from the ICI process, as the dissolved cellulose is in situ (in the presence of the molten salts) converted into components other than glucose. These components are more stable than glucose and also easier to separate from the molten salt.

The BCC process is a fundamental improvement of the well-known (catalytic) pyrolysis technology for the conversion of solid biomass wastes into liquid bio-oils. Traditionally, these bio-oils are very acidic, and have a high oxygen content (~ 40% weight). Hence they are thermally unstable and polar, and so, difficult to separate from the water that is also amply produced during the pyrolysis process. The literature on thermal pyrolysis of biomass is extensive (see for example [1,2]). More recently, the use of catalysis in the pyrolysis process has also been studied. Unfortunately, as reported by Samolada and Lappas [36,37], amongst others, conventional FCC conditions and catalysts do not substantially reduce the oxygen content, meaning that the desired decarboxylation reactions are not prevalent. Williams et al. [38] achieved substantial decarboxylation rates and oxygen content reductions of the bio-oils produced in a fluidized-bed reaction system, meaning substantially longer contact times than with conventional FCC. However, this comes at the cost of much higher coke and gas yields.

Therefore, the challenge in the BCC process is to realize very selective decarboxylation rates, so that a low-oxygen-content bio-oil is produced while minimizing the undesired side products such as coke and gas [32]. As in the case of HTU [26,27], and for that matter, C-HTU [30], the CO2 to CO selectivity is crucial. Obviously, decarboxylation yielding CO2 will be more effective and result in a higher oil yield than the conversion to CO. Reaction temperature has a major effect on the CO2 to CO ratio, as illustrated Figure 1-16 (adapted from [39]).

Figure 1-16 CO2-CO-gas.

et al. [40] compared the kinetics of hydrothermal conversion (HTU) and pyrolysis and concluded that they are very similar in terms of gas composition (CO2, CO, CH4, etc.) at constant temperature. The decarboxylation selectivity in HTU at 300-350 °C is very good (optimal as can be extrapolated from Figure 1-16). Unfortunately, the biomass conversion rates are low and so,long reaction times are required (minutes instead of seconds), resulting in more opportunity for undesired side reactions and, hence, more coke and gas. To avoid this would require much faster reaction rates and/or running the operation at much lower reaction temperatures where the intermediate reaction products are stable (so at lower than 150 °C). At these low temperatures, water is no longer a suitable medium for the dissolution of lignocellulosic materials, and hence, other media such as concentrated acids [19], ionic liquids [41], or molten salts [35] would be more appropriate.

The ideal situation for pyrolysis, on the other hand, would be to achieve a high decarboxylation selectivity, meaning a high CO2/CO ratio, at the lowest possible pyrolysis temperature. This needs to be combined with high reaction rates, minimizing the contact time and consequently any undesirable side reactions involved, which is clearly a great challenge for inorganic catalysis.

1.6 Beyond Biofuels: A Personal Future Perspective

BIOeCON was formed in 2006 with the vision to work on breakthrough innovations in the area of converting nonedible biomass into fuels, chemicals, and/or electrical power.

BIOeCON has been quite successful and, working with an international network of creative scientists, has developed essential breakthrough concepts laid down in more than 100 patents, which are now being scaled up and commercialized. In November 2007, BIOeCON and Khosla Ventures founded KiOR to develop and commercialize the biomass fluid catalytic cracking (BFCC) process; KiOR is presently constructing the first commercial BFCC plant in Columbus, Mississippi. In 2010, BIOeCON and Petrobras announced a partnership to develop the BiCHEM technology for the selective conversion of agricultural wastes, such as sugarcane bagasse, into high-value chemicals that can be used to produce green plastics or further transformed into advanced fuel and food additives. BIOeCON is also developing a biomass-based fuel cell to convert cellulosic waste into electricity with high efficiency. This project is code-named BiCEPS.

Looking beyond biofuels, a new venture called ANTECY was formed in 2010 with the vision and mission to convert solar energy directly into a storable and transportable liquid, also suitable as fuels and/or as feedstock for the chemical industry.

At a recent guest lecture to university students on renewable energy, one student asked if this means that we no longer believe in biofuels. Why the leap away from biofuels to solar fuels? Since the author’s graduation as a chemical engineer, he has worked with Shell, with Akzo Nobel, and, most recently, with Albemarle. In all three cases, he has been active in developing catalysts and/or processes to convert low-value heavy oils into useful clean products such as lead-free gasoline and/or low-sulfur diesel. The focus at the time was to make as good use as possible of the existing resources on Earth, primarily fossil crude oil.

As discussed in the early sections of this chapter, crude oil comes basically from biomass, which, over millions of years, and under certain conditions, has been naturally aged and converted (fossilized) into oil, gas, and coal. The original biomass was formed by the process of photosynthesis whereby CO2 and water are converted with the energy of the sun into biological building blocks (sugars, cellulose, oils, etc). So, in crude oil is solar energy that was captured via photosynthesis millions of years ago and, after rotting and fossilization of the biomass, has been converted into liquid oil. One could call it a time-delayed solar fuel. With the amount of fossil crude becoming less abundant, or at least more difficult and expensive to extract from the earth, it has become important to search for alternative means to produce our fuels and chemicals in a way that would also contribute less to the increase of GHGs: CO2, and CH4.

The conversion of biomass is one of these alternatives. In agriculture and in forestry, a lot of waste biomass (mainly cellulosic material) is produced that is either burned, producing CO2, or is rotting away producing CH4 and CO2. It is to be kept in mind that CH4 is an even worse GHG than CO2! With biomass conversion processes, we can make use of this biomass waste and convert it into transportation fuels (BCC) and/or chemicals (BiCHEM). In fact, we are speeding up or bypassing Nature’s fossilization process to produce the desired product in seconds or minutes instead of millions of years. The trick we apply to do this is called catalysis! In the BFCC and BiCHEM processes, we are applying inorganic catalysts, which are active and robust enough to handle adverse feedstocks and reaction conditions.

The time frame for biomass-based fuels and chemicals is starting TODAY; we have enough biomass waste available, and the processes to convert the biomass are available online. As was confirmed by several major oil companies recently, biomass is going to be the only significant source of renewable fuels and chemicals in the next 10-15 years.

But what about the longer term? While the present biomass conversion processes speed up or bypass the natural fossilization of the biomass, they do not address the fact that the conversion of solar energy into biomass (photosynthesis) remains a very slow and inefficient process. The fastest growing terrestrial biomass (e.g., sugarcane) captures only about 1% of the available solar energy. In the case of aquatic biomass (algae, seaweed), 5-10% seems possible at the laboratory scale, but the costs of growing and harvesting are still prohibitive if the end products are commodities such as transportation fuels. Converting algae to higher value food additives and/or cosmetics seems to be economically feasible (Figure 1-17).

Figure 1-17 Aquatic biomass.

At BIOeCON, we struggled to find a way to produce low-cost algae-based fuels but, although the conversion of the algae or seaweed is quite promising, growing and harvesting it remain the stumbling blocks.

One way to go is to consider genetic modification (GM) of the biomass. According to Craig Vetter (the Nobel Prize-winning genomics scientist), biofuels made from algae that will be able to scale and compete with oil will have to be synthesized and will not come from Nature. Venter and his research team successfully created the first synthetic bacterial cell, which was controlled completely by a synthetic genome: the first cell to have a computer for a parent, or designed DNA on a living system. Venter now says he has realized that a fully synthetic cell is the way to go to create competitive algae fuel. When it comes to tweaking naturally occurring algae cells, he says, you’ll never get there with that. We need a fundamental change to how we approach all this. This is definitely an interesting approach, which is also being supported by, among others, ExxonMobil. Exxon Mobil is investing $600 million in Venter’s venture.

Still, the genomics approach seems rather complicated and may not be without great safety and environmental risks: what if, for instance, these oil-producing mutated species escape the laboratory or the GM-algae oil production plant and start polluting our oceans with oil?

Is there not an easier and safer way to convert solar energy into liquid fuels?

What if we could skip the biomass altogether and do the photosynthesis ourselves?

Yes! What if we achieve the holy grail of chemistry: artificial photosynthesis?

Artificial photosynthesis here implies the direct conversion of CO2 and water into a liquid carbohydrate or hydrocarbon, making use of solar energy. This is not a new subject and many academics are working on it, although most of them are trying to simulate exactly what Nature is doing in the so-called artificial leaves.

It may be better to use Nature only as our example and not to imitate it completely, but to invent simpler, cleaner, and robust ways to do the same job that Nature teaches us in the art of photosynthesis. So, what if we can capture the solar energy and use it in a simple catalytic process to convert GHGs such as CO2 and CH4 into a liquid fuel? (Figure 1-18).

Figure 1-18 Direct solar energy to fuels.

I believe it will be feasible in the near future. First, the cost of capturing energy from the sun is dropping fast. The projections are that the costs of electricity from photovoltaics will be the same as or lower than the cost of electricity from coal or gas, by the year 2020.

If we now use this solar electricity to produce hydrogen from water or methane and use the hydrogen (H2) produced to convert CO2 into methanol, then we have our first solar liquid fuel or chemical.

This is not a new process, and it has already been invented; so why aren’t we doing this already? The answer is that the existing processes to produce hydrogen from water and to capture and convert CO2 are very inefficient and costly.

Knowing this, we founded ANTECY with the objective of increasing the efficiency by at least 100 times!

An impossible task? Just as impossible as turning waste biomass into oil and chemicals?

So now back to the question: why the leap away from biofuels to solar fuels?

I do not see it as a leap away from biofuels; I see it more as the logical next step in the history and continuous development of our energy resources, whereby the sun has always been the provider and we have been reaping his fruits in different ways: first as fossil fuels, now as biofuels, and in the future as direct solar fuels. I also do not see any competition; we will need all (energy) hands on deck. Definitely,we will need fossil fuels for the next 50-100 years, and, hopefully, biofuels will start picking up at least 10-20% of the load soon to contain the present rise of GHGs. The next step will be the emergence of direct solar fuels, which in time will be able to replace all fossil fuels and open up several new opportunities in the way we live and work with our energy resources.

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Chapter 2

Biomass Composition and Its Relevance to Biorefining

Daniel J.M. Hayes, Carbolea Research Group, Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Ireland

Outline

2.1 Introduction

2.2 Chemistry of Biomass Materials

2.2.1 Carbohydrates

2.2.2 Lignin

2.2.3 Lignocellulose Macrostructure

2.2.4 Extractives

2.2.5 Protein

2.2.6 Ash

2.2.7 Triglycerides

2.3 Biomass Types

2.4 Biorefining Technologies

2.4.1 Effects of Biomass Composition on Hydrolysis Technologies

2.4.2 Effects of Biomass Composition on Thermochemical Processing

2.5 First-Generation Versus Second-Generation Biomass

2.6 Feedstock Logistics

2.7 Lignocellulosic Feedstocks

2.7.1 Energy Crops

2.7.2 Agricultural Residues

2.7.3 Wastes

2.8 Advances in Lignocellulosic Feedstocks

2.9 Summary

Acknowledgments

The Author wishes to acknowledge the advice provided by Prof. Michael H.B. Hayes and Dr. J.J. Leahy (both at the University of Limerick) and the funding provided by the Irish Department of Agriculture, Fisheries and Food, the EPA, and the EU 7th Framework Programme (DIBANET project). A database of the compositional data of samples analyzed at Carbolea is available on the Web site of the research group (www.carbolea.ul.ie).

2.1 Introduction

Fossil fuels satisfy the majority of our current energy and chemical needs [1]. However, there have been calls for the role of these fuels in providing for these sectors to be decreased in order to improve the security of supply of energy/chemicals and to reduce the anthropogenic carbon dioxide emissions associated with their combustion [2,3]. The use of biomass resources has been put forward as a sustainable substitute for fossil fuels, and oil, in particular, in the provision of energy, fuels, and chemicals. In developed countries, there has been a particular focus [4,5] in recent years on developing biomass conversion technologies for the large-scale production of biofuels that can be mixed with, or substituted for, conventional petroleum-based fuels and used in regular engines. It is considered that this approach offers the greatest near-term potential for mass oil substitution in the transport sector, given that the infrastructural developments needed for hydrogen or electric vehicles may take decades [6].

The term biomass covers a wide range of plant and plant-derived materials, including biodegradable wastes. The chemical compositions of these myriad feedstocks vary greatly, and this has an influence on the type of biomass conversion technologies that will be appropriate for any given material. The biofuel industry often refers to technologies/biofuels as first or second generation. First-generation biofuels (1GBs) are considered to be those that are obtained from sugar, starch, or oil-based crops and wastes. To date, nearly all of the biofuel produced commercially has come from these feedstocks. Second-generation biofuels (2GBs) are produced from the conversion of lignocellulosic materials, biomass that predominately contains cellulose, hemicellulose, and lignin [7–12]. As yet, 2GBs have not been produced commercially in large quantities.

This chapter discusses the important chemical components of biomass feedstocks and also outlines the technologies available for exploiting them for the production of biofuels and/or chemicals. While 1GB feedstocks are mentioned, the focus is on 2GB biomass since this is forecast to have the most potential for the future development of the biomass industry [13]. However, the chemical composition of a feedstock is not the only important characteristic in determining its suitability for conversion. There are numerous other important factors such as cost, sustainability, and seasonality of supply. These factors are also considered, and there is also a discussion of each of the main biomass types along with examinations of some of the most important lignocellulosic