Читать книгу Liquid Biofuels - Группа авторов - Страница 34

Bioethanol is a type of biofuel that uses plants that contain starch, sugar or cellulose in its chemical content such as woods and agricultural wastes as raw material in the production process. While the fermentation process is carried out directly from the raw materials used in the production of bioethanol to the sugar-containing ones, those containing starch from the raw materials are converted to glucose by a conversion step, and then bioethanol is obtained by fermentation [82].

Ethanol is a liquid that is clean, colorless and non-toxic. Ethanol’s thermal value is lower than gasoline. Ethanol has the ability to mix with water in any proportion. Although ethanol has a high octane number, it can cause some problems in its use in diesel engines due to its very low cetane number and self-ignition resistance. The use of ethanol in gasoline engines is more advantageous, as the self-ignition resistance allows increasing the compression ratio in gasoline engines. Research is ongoing to improve the combustion quality of fuels with low cetane numbers in diesel engines. The idea of using ethanol in engines is more common in countries with large agricultural areas [83].

The conversion of lignocellulosic biomass to bioethanol consists of four main stages: pretreatment, hydrolysis, fermentation, and separation/distillation of products. In a simple way, the conversion of biomass to bioethanol is demonstrated in Figure 1.3. To produce ethanol by fermentation, cellulose and hemicellulose in lignocellulose must be hydrolyzed to sugars before fermentation. This hydrolysis process can be done with enzymes or acids. They also need to be hydrolyzed in carbohydrates such as starch. Although ethanol can be obtained from hemicellulose, it is generally commercially obtained from cellulose. Lignin is waste in these processes, either used to provide the heat of the process or used in the production of aromatic chemicals. Systems where lignin is used to provide energy are called as bio-refineries [84].

Figure 1.3 Processes used in bioethanol production from biomass [85].

The yield is very low in ethanol production because of as more than half of the carbon in cellulose remains as waste. The inability to convert the carbon in the lignin is one of the reasons, and the diluted ethanol solution obtained must be concentrated. It is possible to use biomass samples with high moisture content since the fermenter does not need to dry the raw material. Ethanol can easily be converted to ethyl tertbutyl ether (ETTE), which can be used as a gas oil addition. Also, this ethanol can be used as a fuel in vehicles [86].

Biofuel types are examined in three different generations (Table 1.4). The reason for this distinction stems from the difference of raw material sources. One of the main reasons for turning to second-generation biofuels is to use first-generation biofuels as food [87].

In addition to advancing technological developments and commercial applicability studies in the hydrolysis and fermentation of biomass, it is important to perform the necessary studies on the collection of products obtained as a result of fermentation, so that the fermentation products are mostly volatile than water and their collection is mostly done by distillation. Commercially developed distillation technology is widely used in the collection of suspension materials containing fermentation and volatile products. Bioethanol can be separated from water in liquid mixture with distillation system. The water content of unprocessed bioethanol is generally more than 80%. There is a very high energy requirement to bring ethanol to a concentration of 95.6% (the boiling mixture of ethanol with water) [91].

Butanol (butyl alcohol, 1-butanol) is the primary formula with C₉H₁₀OH and molecular weight 74.14, and it has low viscosity, colorless, flammable and banana-like odor. It can be mixed entirely with many commonly used organic solvents but less with water (70 g/L solubility in water) [92]. Butanol is mostly produced by chemical methods of petrochemicals and most of the butanol produced is converted into ester derivatives such as butylacrylate. It is mainly used in the production of plastic, acid-resistant varnishes and fast-drying automobile paints in the industry. Butanol and its derivatives are also used as extractors in the production of paint thinners and solvents, brake fluids, medicines and natural substances such as antibiotics, hormones, vitamins. A new and important application of butanol in recent years is to use it as a fuel in internal combustion engines directly or mixed with gasoline in various proportions. This fuel, known as biobutanol, is thought to play an important role in the solution of the sustainable energy problem as a new generation biofuel [93].

Table 1.4 Historical process of bioethanol production.

Biofuel Type	Time frame	Raw Material Type	Reference
1st generation	(2000 - 2010)	Agricultural products with raw materials.	[88]
2nd generation	(2010 - 2030)	Production is provided from non-food sources (wheat straw).	[89]
3rd generation	(2030 - )	It is planned to be produced from genetically modified organisms (plants and algae) that contain high levels of oil or cellulose.	[90]

Butanol is formed by the fermentation of sugar-containing materials with some bacteria. In butanol production, all substances that can be converted into fermentable sugars, except for simple sugars, can be used as substrates [94]. Sugar-containing raw materials (molasses, sugar cane, sugar beet and various fruits) can be fermented by microorganisms without any pretreatment. Raw materials containing starch and cellulose require pretreatments such as enzymatic hydrolysis and/or acid hydrolysis to convert starch and cellulose into sugars that can be used by microorganisms [95].

Biobutanol is formed by acetone, butanol and ethanol fermentation of Clostridium spp. Acetone butanol and ethanol fermentation, also called ABE or solvent, are among the earliest known fermentations that are applied industrially. One of the most important advantages of Clostridium is that it can metabolize many sugars, including decomposition products, as a source of carbon in lignocellulosic renewable agricultural wastes as ethanol-producing microorganisms cannot utilize those wastes before pretreatment step. They have high amylolytic activity; they can directly ferment starchy raw materials without the need for pretreatment, unlike yeasts. Many renewable agricultural products and wastes such as rice, barley, wheat straws, corn bran and cobs, lignocellulosic substances such as bran, corn, potatoes, starch substrates such as kassava and molasses can be used in ABE production [96].

Butanol has very important physicochemical advantages compared to ethanol as a renewable energy source. Considering it as fuel, butanol has a higher energy value than ethanol. The energy content of butanol (105,000 BTU/gallon) is close to the energy content of the gasoline (114,000 BTU/gallon). The energy content of ethanol (84,000 BTU/gallon) is quite low compared to butanol and gasoline [97].