Rengasamy Ramamoorthy and Prakash P. Kumar

8.1 Introduction

Biofuels are renewable and sustainable sources of energy that can be in the solid, liquid or gas forms. A major source of biofuels is the biomass of plants rendered as bioethanol, biodiesel and biogas. Biofuels are the natural alternative sources to fossil fuels and are environmentally friendly. The concept of biofuels is not new, with firewood as the most primitive form of solid biofuel used ever since the discovery of fire. In fact, wood is still being used for cooking food and to generate heat during winter in many parts of the world. The liquid form of biofuels is either vegetable oils or ethanol derived by fermentation of plant materials. The biogas produced by anaerobic digestion of animal manure and organic household wastes into gas (methane) used for cooking is also a biofuel. Biodiesel is obtained from the vegetable oils produced from several plant species including, oil palm, canola, soybean that are also used as food oils, and more recently, from non-food sources such as Jatropha seed oil. The liquid forms of biofuels are preferred over other forms due to the ease of storage and transportation; and in many cases these can directly replace petroleum fuels. Thus, the so-called ‘‘flex fuel vehicles’’ on the road today can use gasoline blended with 15-85% of bioethanol.

The world bioethanol production in 2010 was about 86 billion liters (Renewable Fuel Association: http://www. ethanolrfa. org/news/entry/global-ethanol-production- to-reach-85.9-billion-litres-22.7-billion-ga/). Bioethanol is currently produced mainly from corn starch in the USA and from sugarcane in Brazil. The use of food crops for fuel production affects the food chain and has the potential to lead to serious socioeconomic issues as reflected in escalating food price. Therefore,

R. Ramamoorthy • P. P. Kumar (H)

Department of Biological Sciences and Temasek Life Sciences Laboratory, National University of Singapore, 10 Science Drive 4, Singapore 117543, Singapore e-mail: dbskumar@nus. edu. sg

C. Baskar et al. (eds.), Biomass Conversion,

DOI: 10.1007/978-3-642-28418-2_8, © Springer-Verlag Berlin Heidelberg 2012 cellulosic ethanol is becoming a viable alternative for corn starch and sugarcane as the feedstock. Because cellulosic ethanol is produced from plant biomass such as crop residues (straw), forestry and wood waste it does not disturb the food chain. The use of bioethanol can greatly reduce the greenhouse gas (GHG) emission, which can reach up to 94% lower than gasoline GHG emission [1,2]. Therefore, it is hoped that the use of more bioethanol in the coming decades can help to achieve the significant displacement of petroleum use mandated by the advanced energy initiative (AEI) in the USA [3, 4]. The AEI requires 30% reduction from the levels of 2005 petroleum use in the transportation sector to be replaced by domestically produced renewable bioethanol. Accordingly, numerous cellulosic ethanol pro­duction facilities are being opened or the existing facilities are expanding their capacities in the USA (Renewable Fuel Association).

Biomasses such as corn stover (stalk? leaves), rice straw and wheat straw are produced in large-scale as the by-products of food production and a large portion of it is going waste by getting burnt in the field and leading to more GHG emission. In 2009-2010, the world production of corn was about 890 million tons (mt) and at the proportion of 1:1 the corn stover produced will also be about 890 mt [5]. Similarly, around 730 mt of rice straw was reportedly produced in Africa, Asia, Europe and America, out which around 678 mt comes from Asia [6]. Also, the current global production of wheat is about 675 mt and the wheat grain to straw yield ratio is estimated at around 1:1.6 [7]. The yield of ethanol from corn grain is in the range of 400-500 liters/ton, and the yield of cellulosic ethanol from digestion of dried cellulosic biomass is (380 liters/ton) in the same range. Therefore, by not using the plant biomass from the major grain crops we are discarding an excellent renewable source of fuel. Nevertheless, it should be noted that even if the entire global non-grain biomass from the three main cereal crops (corn, wheat, rice) is used for ethanol fermentation, it can only yield about 25% of the annual use of petroleum in the world. Hence, we need to develop additional sources of lignocellulosic feedstock to generate higher amounts of bioethanol.

In addition to the agricultural by-products, fast growing grasses such as switchgrass (Panicum virgatum L.), Miscanthus X giganteus, reed canary and trees such as willows and hybrid poplar have been identified as dedicated biofuel crops. Of these, switchgrass and Miscanthus are the most favored candidates due to their low input needs and high yield that can be harvested with existing agricultural methods [8, 9]. There are varieties suitable for different ecosystems [10] with estimated net energy yield of over 60 GJ/hectare/year [1]. Similarly, Miscanthus has been shown to yield harvestable biomass between 30 and 60 t/hectare/year [4]. At the 30 t/hectare yield, it was estimated that 12 million hectares of US cropland can yield adequate volumes of ethanol (133 x 109 l) corresponding to about 20% of the annual gasoline used in the USA, and in comparison, corn starch grown in a similar land area would yield only about 49 x 109 liters of ethanol with much higher fertilizer needs and other inputs accounting for significantly higher GHG emission [4]. Hence, it is clear that the net GHG release will be highly reduced by using switchgrass and Miscanthus as feedstock for bioethanol.

To get sustainable amount of biomass for the future biofuel production needs it is important to enhance the biomass yield of these dedicated biofuel crops. In this chapter we will discuss some of the possible molecular and genetic strategies to enhance plant biomass.

Contrary to S. cerevisiae, P. stipitis is able to naturally utilize L-arabinose and/or D-xylose and efficiently ferments xylose to ethanol, being the gene donor of the xylose catabolic pathway successfully expressed in S. cerevisae. It has also been considered for fermentation of hemicellulose hydrolysates to ethanol [78-80]. Several auxotrophic mutants with higher fermentation capacities and improved xylose utilization have been developed in order to obtain suitable P. stipitis strains for further hemicellulose-to-ethanol metabolic engineering [78]. P. stipitis is, however, unable to grow anaerobically and is more sensitive to ethanol and inhibitors than S. cerevisiae. The S. cerevisiae gene that confers the ability to grow under anaerobiosis (URA1, encoding the dihydroorotate dehydrogenase) was successfully expressed in P. stipitis, allowing anaerobic fermentation of glucose to ethanol [170]. In addition, the disruption of the cytochrome c gene increased xylose fermentation and consequently, ethanol yield [169]. In an evolutionary engineering approach, P. stipitis was adapted in hemicellulose hydrolysate con­taining glucose, xylose, and arabinose, improving tolerance to acetic acid and pH [131]. In a CBP perspective, xylan conversion into ethanol was enhanced by the heterologous expression of fungal xylanases in P. Stipitis [38]. The recent progress in genomic and transcriptomic characterization of P. stipitis [80] opened new perspectives for metabolic engineering towards efficient hemicellulose fermentation.

Algae can be cultivated in open ponds, photobioreactors, or in closed and hybrid systems. Open pond systems for cultivation of algae are cheap and economical, but suffer from the obvious disadvantages. In that, they require a large expanse of land and water and are susceptible to contamination by other microorganisms, and to climatic changes.

Photobioreactors are closed tank systems, where most of the disadvantages stated under open pond systems can be obviated. These systems, though they involve higher infrastructure costs, are more efficient and offer higher biomass concentrations, high surface-to-volume ratios, and shorter harvest times. A major advantage with these reactors is that along with cultivation of algae, they can be used for simultaneous scrubbing of power plant flue gases and removing nutrients from wastewater. A variety of designs is available to give higher productivity and reproducibility as better control of cultivation conditions is possible. Demirbas et al. [30] have described the different types of systems available for cultivation of algae and the comparative costs involved for each system.

Harvesting of algae can be done by a number of different methods such as centrifugation, foam fractionation, flocculation, membrane filtration, and ultra­sonic separation. The harvesting costs may contribute to about 20-30% of the total cost of cultivation.

The percentage of moisture in the feed biomass to extruder machine is a very critical factor. In general, it has been found that when the feed moisture content is 8-10%, the briquettes will have 6-8% moisture. At this moisture content, the briquettes are strong and free of cracks and the briquetting process is smooth. But when the moisture content is more than 10%, the briquettes are poor and weak and the briquetting operation is erratic. Excess steam is produced at higher moisture content leading to the blockage of incoming feed from the hopper, and sometimes it shoots out the briquettes from the die. Therefore, it is necessary to maintain optimum moisture content.

In the briquetting process water also acts as a film type binder by strengthening the bonding in briquettes. In the case of organic and cellular products, water helps in promoting bonding by van der Walls’ forces by increasing the true area of contact of the particles. In fact, the surface effects of water are so pronounced that the success or failure of the compaction process depends solely upon the moisture content of the material. The right amount of moisture develops self-bonding properties in ligno — cellulose substances at elevated temperatures and pressures prevalent in briquetting machines. It is important to establish the initial moisture content of the biomass feed so that the briquettes produced have moisture content greater than the equilibrium value, otherwise the briquettes may swell during storage and transportation and disintegrate when exposed to humid atmospheric conditions.

An ammonia pretreatment prior to IL dissolution improved delignification of biomass and enhanced recyclability. The rice straw (particle size 2-5 mm) was first treated with ammonia (10%) at 100°C for 6 h. After filtering and drying steps, it was dissolved in [EMIM][OAc] at 130°C for 24 h. The ammonia pretreatment step reduced the time for complete dissolution in IL from 24 to 6 h. It increased slightly the amount of regenerated cellulose after the IL treatment for <24 h. The major improvement was the significant increase in the glucose conversion rate of 97% (compared to the amount of regenerated cellulose) with the ammonia pretreatment, compared to the 78% rate without ammonia pretreatment. This improvement allowed for a significant reduction in the amount of cellulases necessary for cellulose hydrolysis: despite a reduction of the cellulase concen­tration by a factor 10, the cellulose conversion rate remained at 83% [46].

Deoxygenation of fatty acids is a broad term that covers HDO, decarboxylation, decarbonylation, or even cracking. Temperature of the process is close to 300°C or higher. Therefore, independently on deoxygenating catalyst selection, the fol­lowing reactions could occur:

• HDO (hydrotreating)

• Decarboxylation

• Decarbonylation

• Hydrogenation of double bond in unsaturated fatty acids

• Cracking

In this case selectivity toward desired reaction mechanism should be division criteria, which is important to understand different approaches for deoxygenation, there straight and weaknesses.

Rice straw, a waste from paddy processing, has several characteristics that make it a potential feedstock for fuel ethanol production. It has high cellulose and hemi — cellulose content that can be readily hydrolyzed into fermentable sugars. The chemical composition of feedstock has a major influence on the efficiency of bioenergy generation. The low feedstock quality of rice straw is primarily deter­mined by a high ash content (10-17%) compared to wheat straw (around 3%) and also high silica content in ash (SiO2 is 75% in rice and 55% in wheat) [205]. On the other hand, rice straw as feedstock has the advantage of having a relatively low total alkali content (Na2O and K2O typically comprise <15% of total ash), whereas wheat straw can typically have >25% alkali content in ash [12].

In terms of chemical composition, the straw predominantly contains cellulose (32-47%), hemicellulose (19-27%), and lignin (5-24%) [48, 116, 159, 204] as shown in Table 9.4. The pentoses are dominant in hemicellulose, in which xylose is the most important sugar (14.8-20.2%) [149].

Bioethanol can be produced from the processing industry waste rich in sugar/ starch by the microbial technology that may evolve an alternative to our limited and non-renewable resource of energy. Increasing environmental regulations for controlling waste disposal will further enhance the possibilities of ethanol pro­duction from waste.

9.7.1 Sugar Molasses

A process has been developed for the preparation of power alcohol from molasses on pilot scale with immobilized whole cells. Ethanol production from molasses has also been scaled up with addition of 15% total sugar content using Z. mobilis [39]. A scheme of fuel ethanol production from sugarcane bagasse has been shown in Fig. 9.15. Ethanol production by Z. mobilis can be increased by addition of cal­cium carbonate in high sugar medium and at higher fermentation temperature (43°C) [175].

Batch fermentations of sugarcane blackstrap molasses to ethanol using pressed yeast as inoculum, demonstrated an exponential relationship between the time necessary to complete fermentation and the initial concentrations of sugar and the yeast cells [18]. Fed-batch alcoholic fermentation of sugarcane blackstrap molasses (at 32°C, pH 4.5-5.0) without air and compressed yeast enhanced the average yeast yields and average yeast productivities without affecting the ethanol yield.

Neutral spirits and ethanol are the major fermentation products from citrus molasses [21, 51]. In Florida only, 1 million L of alcohol is produced from citrus molasses annually. The process includes dilution of molasses to 25°B followed by fermentation yeast. The alcohol is recovered by distillation. Enzymatic digestion of citrus peel, solubilizing of 85% total peel solids with 65% hexose sugar [133] made available more sugar for fermentation, thus increasing the yield of alcohol. However, reduced yield of alcohol has been reported from molasses produced by

heat evaporators (30-50°B) where some loss of fermentable sugar during handling and storage might have taken place [21].

Conventional resources, mainly fossil fuels, are becoming limited because of the rapid increase in energy demand. This imbalance in energy demand and supply has placed immense pressure not only on consumer prices but also on the environment, prompting mankind to look for sustainable energy resources. Biomass is one of the few resources that has the potential to meet the challenges of sustainable and green energy systems. Biomass can be converted into three main products such as energy, biofuels and fine-chemicals using a number of different processes. Today, its a great challenge for researchers to find new environmentally benign meth­odologies for biomass conversion, which are industrially profitable as well.

This book aims to offer the state-of-the-art reviews, current research and the future developments of biomass conversion to bioenergy, biofuels, fatty acids, and fine chemicals with the integration of multi-disciplinary subjects which include biotechnology, microbiology, energy technology, chemistry, materials science, and engineering.

The chapters are organized as follows: Chaps. 1 and 2 provide an overview of biomass conversion into energy. Chapters 3 and 4 cover the application of ionic liquids for the production of bioenergy and biofuels from biomass (Green chem­istry approach towards the biomass conversion). Chapter 5 focuses on the role of catalysts in thermochemical biomass conversion. This chapter also describes the role of nanoparticles for biomass conversion. Chapter 6 gives an overview of catalytic deoxygenation of fatty acids, their esters, and triglycerides for production of green diesel fuel. This new technology is an alternative route for production of diesel range hydrocarbons and can be achieved by catalytic hydrogenation of carboxyl groups over sulfided catalysts as well as decarboxylation/decarbonylation over noble metal supported catalysts, and catalytic cracking of fatty acids and their derivatives.

The common examples of biofuels are biobutanol, bioethanol, and biodiesel. Biobutanol continuously draws the attention of researchers and industrialists because of its several advantages such as high energy contents, high hydropho — bicity, good blending ability, and because it does not require modification in present combustion engines, and is less corrosive than other biofuels.

Unfortunately, the economic feasibility of biobutanol fermentation is suffering due to low butanol titer as butanol itself acts as inhibitor during fermentation. To overcome this problem, several genetic and metabolic engineering strategies are being tested. In this direction, Chap. 7 outlines the overview of the conversion of cheaper lignocellulosic biomass into biobutanol.

Chapter 8 discusses some of the strategies to genetically improve biofuel plant species in order to produce more biomass for future lignocellulosic ethanol pro­duction. Chapter 9 describes the production of bioethanol from food industry waste. Hydrogen is an attractive future clean, renewable energy carrier. Biological hydrogen production from wastes could be an environmentally friendly and eco­nomically viable way to produce hydrogen compared with present production technologies. Chapter 10 reviews the current research on bio-hydrogen production using two-stage systems that combine dark fermentation by mixed cultures and photo-fermentation by purple non-sulfur bacteria.

Organosolv fractionation, one of the most promising fractionation approaches, has been performed to separate lignocellulosic feedstocks into cellulose, hemi — celluloses, and lignin via organic solvent under mild conditions in a biorefinery manner. Chapter 11 focuses particularly on new research on the process of or — ganosolv fractionation and utilization of the prepared products in the field of fuels, chemicals, and materials. Production and separation of high-added value com­pounds from renewable resources are emergent areas of science and technology with relevance to both scientific and industrial communities. Lignin is one of the raw materials with high potential due to its chemistry and properties. The types, availability, and characteristics of lignins as well as the production and separation processes for the recovery of vanillin and syringaldehyde are described in Chap. 12.

The production of consistent renewable-based hydrocarbons from woody bio­mass involves the efficient conversion into stable product streams. Supercritical methanol treatment is a new approach to efficiently convert woody biomass into bio-oil at modest processing temperatures and pressures. The resulting bio-oil consisted of partially methylated lignin-derived monomers and sugar derivatives which results in a stable and consistent product platform that can be followed by catalytic upgrading into a drop-in-fuel. The broader implications of this novel approach to obtain sustainable bioenergy and biofuel infrastructure is discussed in Chap. 13.

Industrialization and globalization is causing numerous fluctuations in our ecosystem including increased level of heavy metals. Bioextraction is an alter­native to the existing chemical processes for better efficiency with least amount of by-products at optimum utilization of energy. The last chapter provides an over­view of bioextraction methodology and its associated biological processes, and discusses the approaches that have been used successfully for withdrawal of heavy metals using metal selective high biomass transgenic plants and microbes from contaminated sites and sub grade ores.

This book is intended to serve as a valuable reference for academic and industrial professionals engaged in research and development activities in the emerging field of biomass conversion. Some review chapters are written at an introductory level to attract newcomers including senior undergraduate and graduate students and to serve as a reference book for professionals from all disciplines. Since this book is the first of its kind devoted solely to biomass conversion, it is hoped that it will be sought after by a broader technical audience. The book may even be adopted as a textbook/reference book for researchers pursuing energy technology courses that deal with biomass conversion.

All chapters were contributed by renowned professionals from academia and government laboratories from various countries and were peer reviewed. The editors would like to thank all contributors for believing in this endeavor, sharing their views and precious time, and obtaining supporting documents. Finally, the editors would like to express their gratitude to the external reviewers whose contributions helped improve the quality of this book.

February 2012 Dr. Chinnappan Baskar

Dr. Shikha Baskar Dr. Ranjit S. Dhillon

The chemical industry today is coming a full circle on its raw material resources. Till the early twentieth century, most industrial products were made from vege­table plants and crops. This situation changed after the 1970s when most of these natural products started being replaced by petroleum-based organic chemicals, and petroleum refineries acquired unprecedented importance. However, with rapid depletion of fossil fuels and the imminent danger of running out of fossil fuels completely, the biomass resources are once again gaining importance globally. Biorefineries, the counterpart of petroleum refineries, for generation of transpor­tation fuels and other chemicals are being set up and technologies for their improvement are being developed. However, there are some fundamental differ­ences between the two (Table 1.13), which need to be clearly defined and understood, if the future biorefineries are to completely replace the petroleum refineries.

The first and foremost difference is in the nature of the raw material used as feedstock. Raw material for an oil refinery, i. e., crude oil is usually rich in hydrocarbons and consists of mixture of different organic hydrocarbons, but has essentially no oxygen. Biomass, the raw material for biorefinery, on the other hand, consists of too little hydrogen, too much oxygen, and lower fraction of carbon compared to the crude oil. The presence of oxygen reduces the heat content of molecules and gives them high polarity, which makes blending with fossil fuel difficult. This becomes important while considering the power requirement and the cost efficiency of the processes used. Also, the composition of biomass varies with the source of feedstock. This has an advantage in that, this variety in composition

can be exploited to facilitate formation of more classes of products compared to those that can be obtained from an oil refinery. However, an associated disad­vantage is that a larger range of processing technology is needed for a biorefinery. Thus it is essential that a biorefinery be equipped to cope up with such drastic changes in the feedstock composition. The integrated biorefineries already dis­cussed above are a step toward this objective. The second fundamental difference lies in the availability of the feedstock. Feedstock for a petroleum refinery is available throughout the year whereas the biorefinery feedstock, especially that required for the first — and second-generation biofuels, is seasonal. Thus, a petro­leum refinery can be operated throughout the year, whereas a biorefinery has to essentially operate in a seasonal time frame. Again, the integrated biorefineries are a remedy to this limitation. Biorefineries which can switch over from one feed­stock to another, depending on its availability, without compromising on the efficiency and cost-effectiveness need to be developed. A third aspect, which goes in favor of biorefineries, is the fact that it is possible to set up these biorefineries in rural areas, and as dispersed industrial complexes, so that the feedstock is locally available, thus avoiding the complex logistics of feedstock transportation and associated costs. Petroleum refineries, on the other hand, are essentially large industrial complexes set up at locations distant from the oil resources, making the transportation costs of its raw material to the refinery location, indispensable. Lastly, though the products of both the refineries are almost comparable, the intermediate products, or the chemical and biorefinery platforms, which enable further processing of the intermediate products to other value-added chemicals differ [56]. Figures 1.31 and 1.32 give an overview of the chemical and biorefinery platforms and the products obtained from them.

Table 1.13 Comparison between a biorefinery and a petroleum refinery Biorefinery Petroleum refinery