Rice Straw
Rice (Oryza sativa — Asian rice, and Oryza glaberrima — African rice) is one of the most important staple foods for a large part of the world’s human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies. According to 2009 data, it is the grain with the third highest worldwide production, after corn and wheat. Rice is normally grown as an annual plant, although in tropical areas it can survive as a perennial and can produce a ratoon crop for up to 30 years. The rice plant can grow to 1-1.8 m (3.3-5.9 ft) tall, and occasionally more, depending on the variety and soil fertility. It has long, slender leaves 50-100 cm (20-39 in) long and 2-2.5 cm (0.79-0.98 in) broad. The small wind pollinated flowers are produced in a branched arching to pendulous inflorescence 30-50 cm (12-20 in) long. The edible seed is a grain (caryopsis) 5-12 mm (0.20-0.47 in) long and 2-3 mm (0.0790.12 in) thick.
Rice cultivation is labor-intensive, well-suited to countries and regions with low labor costs and high rainfall, as its cultivation requires ample water. Rice can be grown practically anywhere, even on a steep hill or mountain. The traditional method for cultivating rice is flooding the fields either during or after setting the young seedlings. As rice is a major global crop, rice straw is one of the abundant lignocellulosic waste materials in the world. Every kilogram of grain harvested is accompanied by production of 1.5 kg of rice straw [71]. It gives an estimation of about 700-800 Tg of rice straw produced per year globally, a large part of which is going as cattle feed and the rest as waste. The options for the disposition of rice straw are limited by the low bulk density, slow degradation in the soil, harboring of rice stem diseases, and high mineral content. One of the major practices for removing rice straw is burning it right on the rice field where it was grown, but this practice increases air pollution and consequently affects public health [72].
Rice straw has several characteristics that make it a potential feedstock for fuel ethanol production. It has a high cellulose and hemicellulose content that can be readily hydrolyzed into fermentable sugars. In terms of chemical composition, the straw predominantly contains cellulose, hemicellulose and lignin. The pentoses are dominant in hemicellulose, in which xylose is the most important sugar. Rice straw is high in inorganic salts, resulting in higher ash content compared to wheat straw, and also has a high silica content in ash (SiO2 is 75% in rice and 55% in wheat). On the other hand, rice straw as a feedstock has the advantage of having a relatively low total alkali content (Na2O and K2O typically comprise less than 15% of total ash), whereas wheat straw can normally have more than 25% alkali content in ash [73].