Gaseous products

In Chap. 1, gasification (pyrolysis) of biomass, biogas, gobargas, hydro­gen, and biohydrogen were discussed in detail.

2.6.1 Liquid products

An important renewable energy resource for transportation purposes is liquid fuel based on plant oils. However, pure plant oils are generally not suitable for use in modern diesel engines. This can be overcome by the process of transesterification. The resultant fatty-acid methyl esters have properties similar to those of diesel and are commonly called biodiesel. Biodiesel presents several advantages, such as better C02 balance than diesel, low soot content, reduced hydrocarbon emissions, and low carcinogenic potential [20]. The specification standards for the European Union (EU) and the United States are EN14214 and ASTM D6751, respectively. The EU directive established a minimum content of 2% and 5.75% biodiesel for all petrol and diesel used in transport by December 31, 2005, and December 31, 2010, respectively. Biodiesel refers to the pure oil before blending with diesel fuel. Biodiesel blends are represented as “BXX,” with “XX” representing the percentage of biodiesel component in the blend (National Biodiesel Board, 2005) [21]. In the biomass-to-liquid conversion processes, biomass is broken down into a gaseous constituent and a solid constituent by low-temperature gasification. The next step involves production of synthetic gas, which is converted into fuel (termed SunFuel) by the Fischer-Tropsch synthe­sis process, with downstream fuel optimization by hydrogen after treat­ment [22]. Ethanol has already been introduced in countries such as Brazil, the United States, and some European countries. In Brazil, it is currently produced from sugar and, in the United States, from starch at competitive prices. Ethanol is currently produced from sugarcane and starch-containing materials, where the conversion of starch to ethanol includes a liquefaction step (to make the starch soluble) and a hydrolysis step (to produce glucose). There are generally two types of processes for production of bioethanol: the lignocellulosic process and the starch process. Unlike the starch-based process, the lignocellulosic process has not been as widely adopted due to techno-economic reasons.

High ethanol yield requires complete hydrolysis of both cellulosic and hemicellulose with a minimum of sugar dehydration, followed by effi­cient fermentation of all sugars in the biomass. Certain advantages of using lignocellulose-based liquid biofuels are that they are evenly dis­tributed across the globe and hence are readily available, less expensive compared to agricultural feedstock, produced at a lower cost, and have low net greenhouse gas emissions. Enzymatic processes (essentially using bacteria, yeasts, or filamentous fungi) have been considered for lignocellulosic processes. The enzymatic process when coupled with the fermentation process is known as simultaneous saccharification and fermentation. This has proved to be efficient in the fermentation of hexose and pentose sugars [23]. Genencor International (www. genen- cor. com/) and Novozymes, Inc., (www. novozymes. com) have been awarded $17 million each by the U. S. Department of Energy with a goal to reduce the enzyme cost tenfold (www. eere. energy. gov/). The Iogen Corp. (www. iogen. ca/) demo-plant is the only one that produces bioethanol from lignocellulose, using the enzymatic hydrolysis process. This plant is known to handle about 40 ton/day of wheat, oat, barley, and straw and is designed to produce up to 3 ML/yr of cellulose ethanol. Refer to Chap. 3 for bioethanol preparation, Chap. 6 for boidiesel processing, and Chap. 7 for ethanol and methanol used in engines.

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