Rawel Singh, Thallada Bhaskar, Bhavya Balagurumurthy

Biofuels Division, CSIR-Indian Institute of Petroleum, Dehradun, India

11.1 INTRODUCTION

Concerns over energy supply security, global climate change as well as local air pollution, and the increasing price of energy services are having a growing impact on policy making throughout the world. Today’s energy and transport system, which is based mainly on fossil energy carriers, can in no way be evaluated as sustainable. The search for a sustainable and environment-friendly source of hydrocarbons is the need of the hour. Research efforts di­rected toward the conversion of biomass into a liquid transportation fuel have their origins in the first U. S. energy crisis of October 1973, a consequence of the Yom Kippur War and the Organization of Petroleum Exporting Countries (OPEC) oil embargo. Subsequently, the 1979 Iranian revolution and more recent concerns about the security of imported petroleum and the contribution of carbon dioxide (CO2) emissions to global warming trends have led to renewed efforts to provide an essentially CO2-neutral supply of transportation fuel (Blanch, 2012).

It has been long expected that biofuels and biorefineries can at least partially mitigate these problems and create more sustainable and balanced economies. To date three generations of biofuels have been developed. The first-generation biofuels were made from edible feedstock such as corn, soybean, sugarcane, and rapeseed. Biofuel production from these resources was, rightfully or not, blamed for the subsequent surge in food prices. Second-generation biofuels produced from waste lignocellulosic biomass and dedicated lignocellulosic feedstock such as miscanthus, switchgrass, or poplar have advantages over those of the first generation. The main advantages are higher yields and lower land requirement (in both quality and quantity).

The concept of using algae to make fuels was already being discussed 50 years ago (Oswald and Golueke, 1960), but a concerted effort began with the oil crisis of the 1970s. Large research
programs in Japan and the United States focused on developing microalgal energy produc­tion systems. Third-generation biofuel feedstock, micro-, and macroalgae can have an edge over the previous two generations. These marine organisms show the prospect of high bio­mass yields without requiring any arable land and have the potential to be cultivated in con­tainment off-shore (Trent, 2012). Moreover, some algal species grow well in saline, brackish, and waste water, which makes them more promising feedstock than terrestrial crops that rely exclusively on fresh water. These features, along with successful methods for large-scale algae cultivation and processing, can make third-generation feedstock superior to that of pre­vious generations (Daroch et al., 2012).

The typical differences between lignocellulosic biomass and algal biomass are depicted in Figure 11.1. The transition from first — and second-generation to third-generation biofuels offers a reduction in land requirements. This is due to higher energy yields per hectare as we move along this transition as well as utilization of nonagricultural land (Fenton and O hUallachain, 2012). In addition, algae do not deplete any soil nutrients that could aid ag­riculture. Green and blue-green (cyanobacteria) microalgae have been on Earth for millions of years and differ substantially from higher plants. They are single-celled microorganisms that live in aquatic environments, and all components necessary for life and procreation are lo­cated within a single cell. In higher terrestrial plants, specialized cells with specific functions are required, making up roots, stems, flowers, and other functional parts. Cellulose, hemicel — lulose, and lignin often provide structural support for these specialized cells and are present in significant quantities. In contrast, microalgae and cyanobacteria are not lignocellulosic in

hemicellulose, lignin

composition but are composed of proteins, lipids, noncellulosic carbohydrates, and nucleic acids (Heilmann et al., 2010).

Today, efforts are being made to maximize the productivity of biomass and identify new species of plants and processes to fulfill future demands for food, fodder, materials, and en­ergy. The utilization of algae is seen as one of the possible alternatives (Kroger and Muller — Langer, 2012). Algae are a feedstock that has certain advantages over land-based feedstock. Under favorable conditions, the growth rate of algae is estimated to be 5-10 times higher than land-based crops, implying a higher production rate of theoretically convertible biomass. Additionally, certain species may have a high fraction of lipids or carbohydrates of up to 70-80 wt% (Chisti, 2007). There are several reasons for the high production rate. One of them is the higher photosynthetic efficiency.

Many commercial efforts are underway to maximize economic return and improve energy balances in algal cultivation. Currently, much work is focused on extracting high-value chemicals (e. g., nutraceuticals) and energy-dense lipids (e. g., for biodiesel) from algae, but this still leaves behind a large residual of "defatted" biomass. Effective utilization of defatted algal biomass will be necessary to achieve favorable energy balances and production costs (Pan et al., 2010).

The use of macroalgae for energy production has received less attention for the production of fuels/chemicals, despite the fact that macroalgae have long been cultivated for several pur­poses (food production, chemical extraction) in China, Korea, the Philippines, and Japan. The productivity is in the range of 1-15 kg m-2 y-1 dry weight (10-150 tdw ha-1 y-1) for a seven — to eight-month culture. Either brown algae (Laminaria, Sargassum) or red algae have been used so far for such purposes (Aresta et al., 2003).