Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 157

A major focus has been and continues to be on biofuels produced from crops, such as corn, sugar cane, and soybeans, for use as renewable energy sources. Though it may seem beneficial to use renewable plant materials for biofuel, the use of crop residues and other biomass for biofuels raises many concerns related to major environmental problems, including food shortages and serious destruction of vital soil resources. Examples of feedstock for the production of biodiesel are (i) waste vegetable oil, (ii) animal fat including tallow, lard, yellow grease, chicken fat, and the by-products of the production of fatty acids from fish oil, and soybeans which are also used as a source of biodiesel (Table B-4).

Table B-4 Composition of various oils and fats used for biodiesel production.

Oil or Fat	14:0*	16:0	18:0	18:1	18:2	18:3
Corn oil	1-2	8-12	2-5	19-49	34-52	Trace
Cottonseed oil	0-2	20-25	1-2	23-35	40-50	Trace
Linseed oil		4-7	2-4	25-40	35-40	25-60
Olive oil		9-10	2-3	73-84	10-12	Trace
Peanut oil		8-9	2-3	50-60	20-30
Safflower oil – Hi linoleic acid		5.9	1.5	8.8	83.8
Safflower oil – Hi oleic acid		4.8	1.4	74.1	19.7
Rapeseed oil – Hi oleic acid		4.3	1.3	59.9	21.1	13.2
Rapeseed oil – Hi erucic acid		3.0	0.8	13.1	14.1	9.7
Soybean oil		6-10	2-5	20-30	50-60	5-11
Tallow	3-6	24-32	20-25	37-43	2-3
Yellow grease	2	23	13	44.3	7	0.7
*Indicates the number of carbon atoms in the alkyl chain and the position of the double bond.

However, for a given production line, the comparison of the feedstocks includes several issues which are (i) chemical composition of the biomass, (ii) cultivation practices, (iii) availability of land and land use practices, (iv) use of resources, (v) energy balance, (vi) emission of greenhouse gases, acidifying gases, and ozone depletion gases, (vii) absorption of minerals to water and soil, (viii) injection of pesticides, (ix) soil erosion, (x) contribution to biodiversity and landscape value losses, (xi) farm-gate price of the biomass, (xii) logistic costs such as transport and storage of the feedstock), (xiii) direct economic value of the feedstocks taking into account the co-products, (xiv) creation or maintenance of employment, and (xv) water requirements and water availability. In addition, different types of feedstocks have different types of fatty acids. The fatty acids are different in relation to the chain length, degree of unsaturation, or presence of other chemical functions, such as fatty acids.

Tropical countries have the highest potential to produce bio-fuel crops: higher energy yields, better greenhouse gas balance if properly produced, lower costs, and in some countries, large reserves of uncultivated cropland. Sugar cane and palm are the highest yielding tropical biofuel crops and consequently provide the greatest carbon offsets. Industrialized countries with biofuels targets (such as the United States and the EU countries) are unlikely to have the agricultural land base needed to meet their growing demand for current production of bio-fuels, which are largely produced from food and feed crops (e.g., maize, palm, rapeseed, soybean).

Of importance are the combustion and flow properties of biodiesel which are determined primarily by the fatty acid content of the vegetable oil which affect the fuel properties of the biodiesel. Biodiesel has lower volumetric heating values (approximately 12%) than crude oil-derived diesel fuel, but biodiesel has a high cetane number (between 46 and 70) and high flash point.

In comparison to crude oil-derived diesel, the higher cetane number of biodiesel results in (i) shorter ignition delay, (ii) longer combustion duration, (iii) low particulate emissions, and (iv) minimum carbon deposits on injector nozzles.

See also: Biochemical Platform, Biodiesel, Biofuels.

Waste cooking oil is a valuable asset for conversion to liquid fuels, and the quantity of biodiesel which could be produced from waste cooking oil can be quantified in a manner similar to that of biodiesel produced direct from agriculture. The only difference is that rather than calculating crop yields, it is necessary to quantify the amount of waste oil available.

Biodiesel is a light to dark yellow liquid that is immiscible with water and has a high boiling point, low vapor pressure, a flash point of approximately 150°C (300°F), and a density on the order of 0.88 g/cm³. Biodiesel has a viscosity similar to petrodiesel (the current term to differentiate the different forms of diesel) and can be used as an additive in formulations of diesel to increase the lubricity of pure ultra-low sulfur diesel (ULSD) fuel.

Almost any variety of oil or grease (from new food-grade vegetable oil to used cooking oil or trap grease to wastewater treatment-plant grease) can be converted into biodiesel. However, the amounts of reactants (oil, methanol, and sodium hydroxide) vary to some degree, depending on what oil is used. The amount of methanol and sodium hydroxide must be sufficient to react with the vegetable oil, but excessive amounts of these reactants should not be used. Just as an engine requires excess air to be sure that all the fuel burns, it takes excess methanol to be sure all of the oil reacts.

The chemistry of biodiesel production is known as transesterification insofar as the process involves reaction of glycoside-containing plant oil with a short chain alcohol such as methanol or ethanol. The feedstocks are typically animal, and plant fats and oils are typically made of triglycerides which are esters of free fatty acids with the trihydric alcohol, glycerol. In the transesterification reaction, the alcohol is deprotonated with a base to make it a stronger nucleophile. Commonly, ethanol or methanol is used. Usually, the reaction proceeds at a low rate; heat, as well as an acid or base, is used to help the reaction proceed more quickly.

Almost all biodiesel from waste oil is produced using the base-catalyzed technique as it is the most economical process, requiring only low temperatures and pressures and producing over 98% conversion yield (provided the starting oil is low in moisture and free fatty acids). The major steps required to synthesize biodiesel are purification, neutralization, and transesterification.

Purification: the waste vegetable oil is filtered to remove dirt, charred food, and other non-oil material often found. Water is removed because its presence causes the triglycerides to hydrolyze to give salts of the fatty acids instead of undergoing transesterification to give biodiesel. The crude oil may be stirred with a drying agent such as magnesium sulfate to remove the water in the form of water of crystallization. The drying agent can be separated by decanting or by filtration, but the viscosity of the oil may not allow the drying agent to mix thoroughly.

Neutralization of free fatty acids: A sample of the cleaned oil is titrated against a standard solution of base in order to determine the concentration of free fatty acids (RCOOH) present in the waste vegetable oil sample. The quantity of base required to neutralize the acid is then calculated.

Transesterification: While adding the base, a slight excess is factored in to provide the catalyst for the transesterification. The calculated quantity of base (usually sodium hydroxide) is added slowly to the alcohol, and it is stirred until it dissolves. Sufficient alcohol is added to make up three full equivalents of the triglyceride, and an excess is added to drive the reaction to completion. The solution of sodium hydroxide in the alcohol is then added to a warm solution of the waste oil, and the mixture is heated (typically 50°C, 122°F) for several hours (4 to 8 typically) to allow the transesterification to proceed. A condenser may be used to prevent the evaporative losses of the alcohol. Care must be taken not to create a closed system which can explode.

Workup: Once the reaction is complete, the glycerol should sink. When ethanol is used, it is reported that an emulsion often forms. This emulsion can be broken by standing, centrifugation, or the addition of a low boiling (easily removed) non-polar solvent, decanting, and distilling. The top layer, a mixture of biodiesel and alcohol, is decanted. The excess alcohol can be distilled off, or it can be extracted with water. If the latter is used, the biodiesel should be dried by distillation or with a drying agent.

See also: Biodiesel, Biodiesel Feedstocks, Biodiesel Production, Transesterification.