Biofuels–Renewable Energy Sources: A Review

Abstract

Biodiesel is biodegradable and nontoxic, and it significantly reduces toxic and other emissions when burned as a fuel. The advantages of biodiesel as diesel fuel are its portability, ready availability, renewability, higher combustion efficiency, non-toxicity, higher flash point, and lower sulfur and aromatic content, higher cetane number, and higher biodegradability. The major disadvantages of biodiesel are its higher viscosity, lower energy content, higher cloud point and pour point, higher nitrogen oxide (NO_x) emissions, lower engine speed and power, injector coking, engine compatibility, high price, and greater engine wear. The technical disadvantages of biodiesel/fossil diesel blends include problems with fuel freezing in cold weather, reduced energy density, and degradation of fuel under storage for prolonged periods. The sources of biodiesel are vegetable oils and fats. The direct use of vegetable oils and/or oil blends is generally considered to be unsatisfactory and impractical for both direct injection and indirect type diesel engines because of their high viscosities and low volatilities injector coking and trumpet formation on the injectors, higher level of carbon deposits, oil ring sticking, and thickening and gelling of the engine lubricant oil, acid composition. Biodiesel is obtained by transesterifying triglycerides with methanol. A popular variation of the batch transesterification process which needs high alcohol/acid ratio (several separation problems and high corrosivity and toxicity) is the use of continuous stirred tank reactors in series. This continuous process is heterogeneous and is based on reactive distillation. The key factor is the selection of the right and effective solid catalyst which leads to reduction of energy consumption and investments at all.

Keywords:

• Biodiesel
• biofuels
• combustion
• diesel engine
• emissions
• fatty acid methyl ester (FAME)
• greenhouse gas emissions
• heavy vehicles
• process design
• reaction distillation
• renewable energy sources (RES)
• transesterification
• triglycerides
• vegetable oil

Acknowledgments

The authors want to acknowledge fruitful discussions with Drs. Jens-Emil Vindstad, Heidi Mediaas, and Knut Grande at Statoils R&D Center in Trondheim. StatoilHydro is acknowledged for financial support and for the kind permission to publish the manuscript. The authors also acknowledge the comments by Professor Ayhan Demirbas.