be renewed in less than 100 years [28]. Biomasses which are carbohydrate compounds are transformed into solid, liquid and gaseous bioenergy. Bioenergy is passed through various physical, chemical and biological methods. It has commercial properties whose basic and specific properties are standardized [29].
There are four basic biomass resources: marine residue, forestry residue and crops, animal manure and industrial waste. Marine biomass – sea weeds, algae, reed plants and some microorganisms found in the seas and lakes – have high moisture content and growth rate, while forestry residues and crops are wood industry residues and wood residues consisting of woody and herbaceous plants [30, 31].
Biomass sources can be also classified as classic and modern. Classical biomass sources consist of wood obtained from forests, plant and animal residues (stalk, straw, straw, etc.) used as fuel. Modern biomass resources include energy forestry, wood and forest industry wastes, animal wastes and urban wastes. Modern biomass resources can be considered as biomass from plant, animal and industry. The raw use of classical biomass resources without any transformation creates adverse effects on the biomass energy potential and the environment. Modern biomass sources have an important biomass energy potential and with the development of these sources, it is estimated that more traditional biomass resources can be used [29, 31].
1.3.1 Marine Biomass
Marine biomass consists of microalgae, sea plants with little or no lignin content and fast-growing photosynthetic species. In the past decade, there has been an increase in the focus of research on alternative fuel production from marine biomass [32]. In addition to marine biomass, marine microorganisms have unique properties such as high osmotic tolerance, utilization of certain sugars and the production of special enzymes [33]. Seawater is a potentially important marine resource. Microalgae (seaweed) are great resources for biomass production due to their fast growing and their high lipid content in certain species. Macroalgae (seaweed) can be divided into three types, brown, red and green. When comparing sugar composition, brown seaweed contains alginate, mannitol, laminarin, fucoidin and cellulose; red seaweed contains carrageenan, agar, cellulose and lignin and green seaweed contains mannan, ulvan, starch and cellulose [34]. In general, seaweed contains 23–67% carbohydrate, 4–23% protein, 1–4% lipid and 14–42% ash content. When compared with macroalgae or terrestrial biomass, microalgae cell wall is relatively easy to break down following a lysozyme, dilute acid, therefore marine biomass is easier to obtain [35].
1.3.2 Forestry Residue and Crops
Herbal resources used for biomass energy production are in a wide range. Herbal sources include forest products, some tree species that grow very quickly, algae-like herbs, algae and energy plants grown in water. Energy sourced plants (C4) are panicum, pancitum, sugar beet, sweet millet, sweet sorghum, sugarcane, corn [36]. Ethyl alcohol and methyl alcohol are obtained from the biomass of plant origin. Energy plants absorb CO2 and use water better than other plants and become more resistant to drought. Ethanol is obtained from sugar carbonates (sugar cane, molasses and sorghum), starches (corn and potatoes) and cellulose plants (wood and agricultural residues) [37].
An average of 80-100 tons of wet or 25-30 tons of dry biomass is obtained from one hectare of field per year on a medium-yield land to produce biomass energy by making use of plant sources. It is certain that in semi-tropical regions, which are more suitable in terms of climatic conditions, the yield can reach 40 tons of biomass per hectare. The unit cost of the energy obtained from biomass can compete with other fuels [37, 38].
1.3.3 Animal Manure
The main animal sources used to obtain the large-scale energy consist of the waste of cattle and sheep and poultry. Theoretically, pigs and cattle and poultry are considered to have the highest capacity to produce biomass energy. In general, it is observed that animal wastes are mixed with straw and dried, used as fuel and agricultural fertilizer [39]. Biogas produced by fermentation of animal manure in an oxygen-free environment is very common in the world [40].
Especially animal solid wastes are seen as the ideal source for biogas (65% methane, 35% CO2) production after biological treatment. The biogas obtained provides benefits as an important energy input for both electricity and heat production. If the animal wastes are released into nature without being treated, important environmental health problems such as global warming caused by methane gas, groundwater contamination and pathogenic problems occur [40, 41].
1.3.4 Industrial Waste
The term industry wastes covers all kinds of domestic organic wastes and organic wastes that have been fabricated in industry. These wastes are easy to obtain and have almost unlimited supply. These wastes, which have been largely neglected in the past, can reach large amounts and cause important environmental problems [42]. The biomass obtained from these wastes can both eliminate environmental problems and provide resources for biofuel production. Another industrial waste is ash, which is also an important raw material for a variety of applications including building, agriculture and production of synthetic materials [43].
1.4 Methods of Conversion of Biomass to Liquid Biofuels
Technologies used to convert biomass into solid, liquid and gaseous fuels and valuable chemicals can be grouped under two main headings. These are hydrolysis conversion technologies, which includes biochemical methods and catalytic conversion, and thermochemical conversion technologies that process the decomposition of biomass by different methods. Biomass conversion technologies are shown in Figure 1.1.
Thermochemical conversion technology is based on the basis of converting biomass into products such as fuels and valuable chemicals with the effect of heat. The thermochemical conversion of biomass is one of the oldest processes used by humanity for purposes such as warming and cooking. Thermochemical conversion technologies include pyrolysis, liquefaction and gasification process [44].
Thermochemical conversion technology, which is based on obtaining thermal energy as a result of the rapid reaction of fuel with oxygen, is also a common and advanced technology to be used commercially as it is widely used in vehicles and factories. Gasification can be defined as the process of converting biomass into gaseous fuel or synthesis gas containing different amounts of CO and H2 [45]. In this process, a gasification medium such as steam, air or oxygen is needed. Oxygen is the most common active ingredient. The active ingredients interact with solid carbon and heavy hydrocarbons, turning them into lower molecular weight gases such as CO, H2. With various conversion processes applied to biomass, solid, liquid and gas fuels (easily transportable, storable and usable) with high fuel quality, equivalent properties to existing fuels, and more useful, or valuable products for the chemical industry can be obtained. The variety of fuels derived from biomass varies depending on the conversion processes applied and the characteristics of the biomass used [46].
Figure 1.1 Strategies for conversion of biomass to liquid biofuels by thermochemical and hydrolysis routes.
1.4.1 Pyrolysis and Types of the Pyrolysis Processes
Pyrolysis is the most basic thermochemical conversion process used to convert biomass into more valuable fuel. Pyrolysis is defined as the heat degradation process of biomass in an oxygen-free environment. As a result of the pyrolysis process, hydrocarbon-rich gas, oil-like liquid and carbon-rich solid product are obtained. The amount
