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.