1.4.5 Biochemical Methods
Biomass is already broken down by biochemical processes in nature because biomass consists of natural substances. These processes, which can be listed as aerobic and anaerobic degradation, fermentation, enzymatic hydrolysis, are realized by breaking down the biomass of bacterial enzymes or microorganisms [79].
CH4 and CO2 gases are formed together with the solid product in the anaerobic disintegration performed by the bacteria using oxygen in the biomass instead of oxygen in the air. In aerobic decomposition, microorganisms that break down the biomass produce CO2, heat and solid products by using oxygen in the air. The process in which biomass is converted to sugar and then to ethanol and other chemicals using yeast with acid or enzyme is also called fermentation. The product obtained from fermentation is liquid, unlike anaerobic breakdown. Although the conversion of starch and sugar-based raw materials to ethanol by fermentation is used for commercial purposes, it is difficult and expensive to break down lignocellulosic biomass, such as wood, into sugars. It is not commercially preferable to apply fermentation to lignocellulosic biomass as a hydrolysis pretreatment is required to convert cellulose and hemicellulose into sugar [80].
1.4.6 Co-Conversion Methods of Pyrolysis (Copyrolysis)
The pyrolysis of raw materials with two or more sources of biomass is called copyrolysis. The copyrolysis method is similar to normal pyrolysis processes that do not require any solvent or catalyst. Efficiency obtained from copyrolysis varies between 1.42% and 22% compared to normal pyrolysis depending on the substances used. At the same time, copyrolysis is more advantageous than pyrolysis in terms of the quality and quantity of bio-oil obtained. The synergistic effects of biomass used in copyrolysis with each other are the main factors causing an increase in the amount of oil and quality obtained [81].
1.5 Bioethanol and Biobutanol Conversion Techniques
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