James G. Speight - Encyclopedia of Renewable Energy

To achieve more moderate reaction conditions, further effort was made through the two-step preparation. In this method, oils/fats are, first, treated in subcritical water for hydrolysis reaction to produce fatty acids. After hydrolysis, the reaction mixture is separated into oil phase and water phase by decantation. The oil phase (upper portion) is mainly fatty acids, while the water phase (lower portion) contains glycerol in water. The separated oil phase is then mixed with methanol and treated at supercritical condition to produce fatty acid methyl esters (FAMEs) thorough methyl esterification. After removing unreacted methanol and water produced in reaction, fatty acid methyl esters can be obtained as biodiesel. Therefore, in this process, methyl esterification is the main reaction for the formation of fatty acid methyl esters, while in the one-step method, transesterification is the major one.

Reaction by supercritical methanol has some advantages: (i) glycerides and free fatty acids are reacted with equivalent rates, (ii) the homogeneous phase eliminates diffusive problems, (iii) the process tolerates great percentages of water in the feedstock catalytic process and requires the periodical removal of water in the feedstock or in intermediate stage to prevent catalyst deactivation, (iv) the catalyst removal step is eliminated, and (v) if high methanol-to-oil ratios are used, total conversion of the oil can be achieved in a few minutes. Some disadvantages of the one-stage supercritical method are clear: (i) the method operates at high pressure, (ii) the high temperatures bring along proportionally high heating and cooling costs, (iii) high methanol-to-oil ratio (usually set at 42) involves high costs for the evaporation of the unreacted methanol, and (iv) the process as posed to date does not explain how to reduce free glycerol to less than 0.02% as established in the ASTM D6584 or other equivalent international standards.

The main factors affecting transesterification are the molar ratio of glycerides to alcohol, catalyst, reaction temperature and pressure, reaction time, and the contents of free fatty acids and water in oils.

The free fatty acids and moisture content are key parameters for determining the viability of the vegetable oil transesterification process. In the transesterification, free fatty acids and water always produce negative effects, since the presence of free fatty acids and water causes soap formation, consumes catalyst, and reduces catalyst effectiveness, all of which result in a low conversion. These free fatty acids react with the alkaline catalyst to produce soaps that inhibit the separation of the biodiesel, glycerin, and wash water. To carry the base catalyzed reaction to completion, a free fatty acid value lower than 3% is needed.

The presence of water has a greater negative effect on transesterification than that of the free fatty acids. In the transesterification of beef tallow catalyzed by sodium hydroxide (NaOH) in presence of free fatty acids and water, the water and free fatty acid contents must be maintained at specified levels.

The effect of reaction temperature on production of propyl oleate was examined at the temperature range from 40°C to 70°C with free P. fluorescens lipase (Iso et al., 2001). The conversion ratio to propyl oleate was observed highest at 60°C (140°F), whereas the activity highly decreased at 70°C (158°F).

The conversion rate increases with reaction time. The transesterification of rice bran oil with methanol was studied at molar ratios of 4:1, 5:1, and 6:1. At molar ratios of 4:1 and 5:1, there was significant increase in yield when the reaction time was increased from 4 to 6 h. Among the three molar ratios studied, ratio 6:1 gave the best results.

One of the most important factors that affect the yield of ester is the molar ratio of alcohol to triglyceride. Although the stoichiometric molar ratio of methanol to triglyceride for transesterification is 3:1, higher molar ratios are used to enhance the solubility and to increase the contact between the triglyceride and alcohol molecules. In addition, investigation of the effect of molar ratio on the transesterification of sunflower oil with methanol showed that when the molar ratio varied from 6:1 to 1:1 and concluded that 98% conversion to ester was obtained at a molar ratio of 6.1. Another important variable affecting the yield of methyl ester is the type of alcohol to triglyceride. In general, short chain alcohols such as methanol, ethanol, propanol, and butanol can be used in the transesterification reaction to obtain high methyl ester yields.

Catalysts used for the transesterification of triglycerides are classified as alkali, acid, and enzyme. Alkali-catalyzed transesterification is much faster than acid-catalyzed transesterification and is most often used commercially and, quite often, for the base-catalyzed transesterification, the best yields were obtained when the catalyst was used in small concentration, i.e., 0.5% wt/wt of oil. On the other hand, during the production of free and bound ethyl ester (FAEE) from castor oil, hydrochloric acid is much more effective than sodium hydroxide

Bioenergy is renewable energy produced from organic matter – the conversion of the complex carbohydrates in organic matter to energy; organic matter may either be used directly as a fuel, processed into liquids and gasses, or be a residual of processing and conversion. Bioenergy (although not quite correct) is often used interchangeably with, biofuel and biomass. More typically, the term bioenergy refers to electricity and gas that is generated from biomass, which can be any form of plants and timber to agricultural and food waste – and even sewage. The term bioenergy also covers transport fuel (biofuel) produced from organic matter.

Biofuel is fuel derived from biological sources and biomass is the biological material used as a biofuel, as well as the social, economic, scientific, and technical fields associated with using biological sources for energy. In reality, bioenergy is the energy extracted from the biomass, as the biomass is the fuel and the bioenergy is the energy contained in the fuel.

The terms bioenergy and renewable energy are often (incorrectly used) interchangeably. However, bioenergy is specific and refers to energy produced from biological course (i.e., biomass). On the other hand, renewable energy is a more collective term that includes not only bioenergy but also nuclear energy, solar energy, tidal energy, and wind energy.

Bioenergy can offer renewable, low-carbon energy systems, sequestering atmospheric carbon as well as offer numerous environmental and socioeconomic benefits and therefore supporting global climate change targets and wider environmental, social, economic, and sustainable targets.

In addition, it is important to consider various sustainable aspects of bioenergy systems beyond carbon. Ensuring that bioenergy offers the required holistic emission reduction, context, specific and long-term approaches are necessary to understand synergies and the tradeoff of the bioenergy and related agricultural and forestry systems.

See also: Biofuel, Biomass, Nuclear Energy, Solar Energy, Tidal Energy, Wind Energy.

Bioenergy crops are low-cost and low-maintenance crops grown solely for energy production by (for example) combustion. The crops are processed into solid, liquid, or gaseous fuels, such as pellets, bioethanol, or biogas. The fuels are burned to generate electrical power or heat. The plants are generally categorized as woody or herbaceous. The former – woody plants include willow and poplar while the latter – herbaceous plants – include miscanthus and these crops, while physically smaller than trees, store (approximately) twice the amount of carbon dioxide (in the form of carbon) below ground, compared to woody crops.