Bioenergy Potential of Turkey’s Forest Sources, Biomass Energy Conversion Methods, Products, and Applications

Forests have been an important bioenergy source for mankind through the long ages, and they will continue as biomass feedstock sources in the future. This study aims to investigate Turkey’s forest source, biomass resource, fuel wood, and forest residue potential to discover the bioenergy potential of Turkey. How to convert this potential to energy was evaluated in terms of applications and products. Thus, the most common biomass conversion methods such as thermal processes, pyrolysis, gasification, and combustion, and biological processes, fermentation, anaerobic digestion, and biophotolysis processes, have been explained as biomass energy conversion methods. Besides, the products of biomass are explained by its energy application fields. Overall, the bioenergy potential of Turkey’s forest sources and biomass energy conversion methods will be overviewed by this study. Thus, this study will be attracted attention to forests’ biomass source the effects on economic, ecological, and socio-economic respects.


Introduction
Rapidly growing population and industrialization have increased the energy request. It is very important that this increased energy requirement is from sustainable and environmentally friendly resources. At this point, biomass energy stands out with it being sustainable, environmentally friendly, and an inexhaustible resource that can be obtained anywhere. Especially in rural areas, it is becoming the most promising energy source due to its positive effects on socioeconomic developments [1]. In addition to this, woody biomass is estimated to meet approximately 2-18% of primary energy consumption in 2050 [2].
Biomass is vegetative organisms that plants produce and store from organic matter using photosynthesis using solar energy. Bioenergy is used to define energy and energy-related products produced from biomass. Biomass is formed as a result of the combination of sunlight and carbon dioxide and water in the atmosphere with photosynthesis reaction [1,3].
( | CH 2 O| + H 2 O) + O 2 (1) where | CH 2 O| refers to biomass as carbohydrate [3]. Biomass has been the most common and crucial energy source that is used in heating and cooking for thousands of years. Thus, wood is still the most widely used and richest biomass energy source. Today, plants, agriculture and forest residues, organic household waste, industrial waste, and algae are used as biomass sources. The biomass sources can be used in wide areas such as producing heat, electricity, fuel, and some chemicals [4].
When technical, economic, environmental, and social effects of alternative energy sources such as biomass, wind, hydroelectric, solar, and geothermal energy are evaluated, it is concluded that the most suitable alternative energy is biomass energy. The most important reason for this is that its social benefit is the highest among others [5]. In addition to this, the use of bioenergy has considerably the potential to reduce emissions of greenhouse gases. Bioenergy produces approximately the same amount of carbon dioxide as fossil energy sources, but net carbon emission is zero since the plant uses carbon dioxide by photosynthesis during the day [4].
Three sides of Turkey are surrounded by seas so that it has different climates. Besides, it is located in the center of the triangle connecting the continents of Asia, Europe, and Africa [6]. In 2015 the amount of carbon absorbed by forests in Turkey is 1.9 billion tons. In addition to this, oxygen production was annually calculated as 42 million tons [7]. Turkey has a very rich fauna and plant species source due to moderate climatic conditions. For this reason, it is among the countries rich in biodiversity. Turkey's territory is covered with 27.6% of forests, 31.1% of agricultural land, 18.6% of pasture, 21.3% of other areas, and 1.4% of water. The distribution ratios of the land situation in Turkey are shown in Figure 1 [8].
The objective of this study is not only to address the current situation of Turkey's forest sources and their bioenergy potential but also to present the recent methods of biomass utilization in the applications. This book section exhibits forest bioenergy potential of Turkey and discusses the biomass conversion methods, products, and applications in terms of the production process and usage of the products in the field. This section aims to attract attention to the forests' bioenergy source and help to seek proper investments for the government and investors regarding forest biomass energy potential.

Turkey's forest biomass resources and distribution
Turkey is a transit point that connects the Asian and European continents. It is at the center of the triangle formed by the continents of Asia, Europe, and Africa. Besides, Turkey is surrounded by seas on three sides so it has a different climate. As a result, this situation makes it a rich country in terms of animal and plant diversity. Turkey's forests cover about 30% of the land area and have an equivalent of 11,000 plant species to plant diversity. Furthermore, 3708 of these plant species consist of endemic plant species. When we evaluate the forests of our country as tree species and the area they cover, the first three ranks are 18 species and 6,476,277 ha of oak (Quercus spp.), 5,420,524 ha of red pine (Pinus brutia), and 4,202,298 ha of larch (Pinus nigra) takes the forests. These are followed by beech (Fagus orientalis and Fagus sylvatica), scots pine (Pinus sylvestris), and fir (Abies nordmanniana and Abies cilicica) forests. The classification of Turkey's forests is as follows: the Black Sea Region, the North Anatolia Forests which constitute 25% of the forests in Turkey, is the most forested area in Turkey, followed by Thrace, West and Middle Black Sea Forests, Eastern Black Sea Forests, Mediterranean Forests, and Central, East, and South East Anatolia Forests [6]. Turkey's forest asset map and the distribution of the forests are given in Figure 2.

Distribution of forest land in Turkey
Turkey has an ecologically rich diversity due to its geographical location and climatic diversity. The effects of forests on this ecological diversity and wealth are very important. Turkey has 78 million hectares of surface area. In addition to this, forest areas cover by 28.6% percentage except for treeless forest areas [7]. The ratio of land area and the amount of woodland in Turkey is given in Figure 3.
Forest areas can be divided into two classes as grove and coppice according to their operation types. Turkey's forests are composed of 88% of grove forest areas (19.6 million hectares) and 12% of coppice forest areas (2.7 million hectares) [7]. The rates of the forest areas according to the operation types are given in Figure 4. Turkey's forest lands' main function distribution is composed of 50% economic, 42% ecological, and 8% sociocultural [7]. Distribution rates according to the main functions of forest areas are given in Figure 5.
According to Turkey's forest lands taken into consideration for 42 years, the field of forest area size and change of forest wealth have increased through the years. Forest areas increased by 2.1 million hectares in 42 years. Activities such as protection, development, afforestation, and precautions for forests have been effective on this increase [7]. The amount of the forest area of Turkey through the years and the rate of the country land are given in Table 1.

Distribution of forest wealth
Turkey's forest assets are 20.2 million hectares in 1972 and reached 22.3 million hectares in 2015. In parallel with this, the wood wealth in forests increased from 0.9 billion m 3 in 1972 to 1.2 billion m 3 in 2003, to 1.6 billion m 3 in 2015. In respect to this, between 1973 and 2015, there has been an increase of 700 million m 3 in the tree wealth of the country's forests. In this increase, afforestation studies, migration of citizens living around the forest, and improvement of forest areas have been very effective [7]. The amount of coniferous, broad-leaved, mixed grove, and coppice forest areas of the forest asset in 2012 is given in Table 2.   According to the forest renovation plan in 2013-2015 in Turkey, the amount of forest area in 2015 was estimated to be 22.3 million hectares. The amount and rates of the distribution of the forest areas according to the operation types, the forest area, tree wealth, and annual current increase status are given in Table 3. Turkey's average annual amount of revenue derived from forests planted in 2015 is calculated as the volume-shelled body. This value was calculated as approximately 15.94 million m 3 from grove forests and 2.37 million m 3 from coppice forests. As a result, it was calculated as 18.31 million m 3 of total forest area [7].
Turkey's 13.9 million hectares of forest area (62%) is pure forest. In this amount, the rate of tree species entering the mixture is less than 10%. Besides, approximately 8.4 million hectares of forest (38%) is mixed forest [7]. The distribution of forest ratio of species to general forest area is given in Figure 6.  Turkey's forest areas consist of 33% broad-leaved forests (oak, beech, alder, chestnut tree species such as beech), 48% coniferous forests (tree species such as Turkish pine, crimean pine, scots pine, fir, spruce, cedar), 19% coniferous + broadleaved mixed forests. Oak occupies the largest area in the forests (5.9 million ha), followed by Turkish pine, crimean pine, beech, scots pine, juniper, fir, cedar, spruce, stone pine, alder, chestnut, hornbeam, poplar, lime tree, ash tree, and eucalyptus [7]. Distribution values of forest areas by tree species are given for 2018 in Table 4.
Turkey's forest wealth distribution, the current value increment distribution, and distribution of forest areas by the year 2005-2018 are given as follows. Figure 7 shows forest wealth distribution in 2018. The forest was composed of 95%  Table 3.
The situation of forest areas according to their operation types [7].

Figure 6.
Proportion of forest areas by tree type [7].       productive high forest. Figure 8 shows distribution of forest wealth between 2005 and 2018. The productive high forest percentage increased by 88-95% in 13 years. Figure 9 indicates distribution of increment in 2018. Figure 10 indicates distribution of increment between 2005 and 2018. Figure 11 shows distribution of forest areas in 2018. Figure 12 shows change of forest areas by years 2005-2018 [9]. In the last 30 years, an increase of approximately 990.000 ha has been achieved in forest areas with afforestation studies and increasing environmental awareness. Thus, not only superficial increase but also quality increase in forest areas was observed [6].

Turkey's forest bioenergy potential
Annual increment in volume of forests can be explained by the increase in total height and diameter of the tree in a cubic meter (m 3 ) during the growth period of the trees. Thus, the annual current increment was 28.1 million m 3 in total and 1.4 m 3 in a hectare in 1973. In addition to this, the annual current increment was calculated as 45.9 million m 3 in total and 2.1 m 3 in hectare in 2015. The reason for this increase is due to the increment in tree wealth and forest areas with the maintenance to forests [7]. In Figure 13, the wood biomass source that can be obtained from forests is given as a model.
Revenue in forestry is the annual revenue amount and is calculated in m 3 . The amount of revenue in 2015 was determined as 15.942.459 m 3 in grove forests and 2.372.162 m 3 from coppice forests, with a total of 18.314.621 m 3 [7]. The change in forest revenue amounts by years is given in Figure 14.

Production amounts of fuel wood
The total amount of trees and annual revenue growth of forest areas can be considered as the biomass potential of forests. Thus, the amount of production and unprocessed wood production according to the tree species shows the potential of the production of firewood. Tables 5 and 6 show the amount of wood that can be produced as fuel according to the forest area and tree types.

Forest waste bioenergy potential
It is estimated that available biomass energy potential from waste is about 8.6 million tons of oil equivalent (toe) in Turkey. Furthermore, it is anticipated that these waste biomass have a biogas potential of 1.5-2 Mtoe [10].  Model of wood biomass source that can be obtained from forests [2].
Production amounts of the tree species for volume of wood per species in 2018 [9].
The total amount of waste originating from forests was calculated as 4.8 million tons (1.5 Mtoe) in Turkey. The gasification plant capacity that can be installed is estimated to be 600 MW [11]. The energy value of forest waste is estimated to be 859.899 toe/year in Turkey. The number of biomass electricity generation plant in Turkey is 128 units [12]. Wood biomass potential in forests depends on factors such as forest biomass increase, forest area, and forest growth [2]. Therefore, the bioenergy potential is also highly dependent on factors such as forest biomass increase, obtained from forest wastes, forest area, and growth of the forest. Figure 15 shows the distribution of the amount of Turkey's forest wastes based on the amount of biomass.

Biomass energy conversion methods, products, and applications
The majority of biomass energy is used for cooking and heating in households. Approximately 6.5 million houses use wood as the main fuel for heating purposes in Turkey. Moreover, in the paper industry, approximately 60% of the factories' energy needs are obtained from waste wood [5].
There are main processes such as direct combustion, gasification, alcoholic fermentation, pyrolysis, liquefaction, anaerobic digestion, hydrogasification, and transesterification where energy is obtained from biomass. These processes have their own advantages according to the biomass source and the type of energy obtained. If biomass is converted using modern technologies and energy conversion efficiency is ensured, biomass energy could be a primary energy source in the future [4].
Demirbaş [13] classified wood as a second-generation biofuel in the study. Besides, examples of these biofuels are bio-alcohols, bio-oil, bio hydrogen, and bio Fischer-Tropsch diesel. In addition to these, using alternative fuels from biomass as fuel additives can improve fuel properties such as cetane and octane number, viscosity, and density in diesel and gasoline engines. Thus, fuels produced from biomass can be used as alternative fuels in internal combustion engines [14].
Biomass conversion techniques can be applied on biomass materials to obtain solid, liquid, and gaseous fuels. After the conversion process, fuels can be produced with the main products such as biodiesel, biogas, bioethanol, and pyrolytic gas. Besides, by-products such as fertilizer and hydrogen can be also obtained [15].  Alternative biofuels such as biogas and bioethanol fuels can be obtained by the biomass conversion process. Bioethanol can be used instead of oil, and biogas can be used instead of natural gas [16]. Conversion techniques using biomass sources, fuels obtained using these techniques, and application areas are given in Table 7.
The biomass processing process can be divided into two classes: thermal and biochemical. It can be divided into three subtitles as direct combustion, gasification, and pyrolysis in the thermal process. The biochemical process can be classified under two subtitles as fermentation and anaerobic digestion. Figure 16 shows the methods and products obtained in the processing processes of biomass [16].

Thermal process
The majority of modern bioenergy plants are use biomass for obtaining heat and power. Developing gasification and pyrolysis bio-oil technology offers much more efficient energy conversion with turbine and combined cycle technologies [3]. Thermal process can be examined under three main headings: burning, gasification, and pyrolysis.  Table 7.

Pyrolysis
Pyrolysis process is the simplest and oldest method for biomass to gas from. Pyrolysis process is a physical and chemical situation that occurs by heating organic substances up to 500-600°C without oxygen. In this process, gas components, volatile condensates, charcoal, and ash are released. When it rises to high temperature, wood gas and components gas are released by heating the wood up to 900°C in an oxygen-free environment. As a result of pyrolysis, substances such as gases, water, organic compounds, tar, and charcoal are obtained [15,18].
Pyrolysis is the method of obtaining solid, liquid, and gas products by breaking down the biomass with heat. Slow pyrolysis is a well-known method widely used in the production of charcoal. Fast pyrolysis is the method in which biomass converts more than 75% of liquid bio-oil at high temperatures. This bio-oil chemical composition obtained is very similar to biomass. This bio-oil can be used as renewable fuel in gas turbines, diesel engines, or boilers. Bio-oil has about 60% calorific value of conventional fuel oils by volume [3].
Bio-oil is a liquid fuel obtained by the thermochemical process of biomass. Bio-oil obtained from wood is liquid and dark brown-colored. Its density is 1200 kg/m 3 and it is more than the density of biomass and fuel oil. Bio-oil water content is 14-33 wt% by mass and cannot be removed by traditional methods such as distillation. Higher heating value (HHV) is 27 MJ/kg, and it is lower than traditional fuel oil (43-46 MJ/kg) [13]. The conversion of biomass into product by pyrolysis and the process steps of the products obtained are given in Figure 18.

Gasification
Gasification technology is one of the oldest conversion processes, and it has been used for more than 200 years [19]. The gasification process is the method for achieved combustible gas by dissolving solids like carbon-containing biomass at high temperatures. The process up to approximately 500°C in the gasification of organic substances is the pyrolysis phase. Here, carbon, gases (calorific value can be up to 20 MJ/m 3 ), and tar are obtained. When heating up to 1000°C, carbon reacts with water vapor to produce CO and H 2 . Depending on the variable oxygen rate in the raw material, additional oxygen input may not be required for the gasification process. Gasification takes place in a reducing atmosphere with low air oxygen or steam injection. During this process, biomass is burned with the air supplied to the fuel cell under control, and the resulting products include combustible gases such as hydrogen, methane, as well as carbon monoxide, carbon dioxide, and nitrogen. Thus, combustible gases such as carbon monoxide, hydrogen, methane, and low Figure 17. Transformation of biomass in thermal process [17]. amounts of other gases with low or medium calorific value are obtained. After cleaning the gas, it can be used as fuel in gas turbines, gasoline engines, dual fuel diesel engines, or in fuel cells after purification [3,15,18].
Biomass gasification technology provides the opportunity to convert renewable biomass resources into clean gaseous fuels or synthesis gases. Heat or electricity is produced from these produced gases. In addition to these, there is the potential to produce liquid transportable fuel, hydrogen, or chemicals from them. Gasification Conversion of biomass to products by pyrolysis [13].

Figure 19.
Transformation processes of biomass into products by gasification [19].
is a promising energy conversion technology with its flexible, efficient, and environmentally adaptable features [17]. Besides, the most important feature of gasification is its high electrical efficiency. In the future, it is expected to be used instead of natural gas or diesel fuel in gas turbines or fuel cells, industrial boilers, and furnaces, to replace gasoline or diesel in internal combustion engines [19].

Combustion
The process of biomass giving a fast chemical reaction with oxygen is called burning. As a result of combustion, heat, carbon dioxide, water vapor and some metal oxides are given to the environment [15]. The biomass and full combustion components are given in Figure 21.
Industrial and commercial combustion plants can burn a wide variety of fuels, from tree biomass to urban solid waste. Furnaces are the simplest combustion technology, and biomass burns in a combustion chamber. Combustion technology can be divided into two main categories as grate burner and fluid bed burner. In biomass combustion plants, a high-temperature and high-pressure steam is obtained as a result of combustion. This steam is passed through the turbine and converted into electrical energy with efficiency in the range of 17-25%. It can be increased up to 85% with efficient cogeneration systems [3,19].
Pellets are generally solid wood particles with a cylindrical diameter of 10 mm and a length of less than 35 mm. Pellets produced from wood or waste wood are used to generate electricity in cogeneration systems, for heating in residences and industry. Wood pellets are the fuel with the highest thermal value after coal [22]. A comparison of the higher heat values of biomass and coal are given in Table 8.

Fermentation
It contains hemicellulose and lignin in different amounts in the biomass. Glucose can be obtained from cellulose using enzymes with chemical hydrolysis or after enzymatic hydrolysis with chemical processes. This process must be done with extreme care, as glucose can sometimes degrade during chemical hydrolysis. By fermentation of glucose, many chemical products can be obtained such as ethanol, acetone, and butanol which are equivalent to products from crude oil [15].

Digestion
Anaerobic digestion is a biological process and can take place in a completely oxygen-free environment. It is done by microorganisms that can live in an oxygenfree environment. The process is given in Eq. (2): Organic matter + bacteria + water heat ⎯ → methane + carbon dioxide + hydrogen, sulfur + stable, fertilizer + bacteria (2) Biomass can be separated by microorganisms through fermentation in an oxygen-free environment. End of the fermentation process, a valuable fertilizer, and gases such as methane and carbon dioxide products can be obtained [15]. The components of the biomass [21].
Anaerobic digestion (AD) is a process of producing flammable gas, consisting of methane and carbon dioxide at a rate of 60:40 using microbes in an oxygen-free environment. Therefore, the biogas production process is a complex and sensitive process that contains many microorganism groups. Biogas is a flammable gas formed by decomposing biological wastes in an oxygen-free environment. Biogas approximately contains 50-60% methane gas. Biogas is a colorless, flammable gas. In addition to this, biogas consists of its main components such as methane and carbon dioxide. Besides, it contains a small amount of hydrogen sulfide, nitrogen, oxygen, and carbon monoxide. Generally, 40-60% of organic matter is converted to biogas. The general composition of biogas consists of 60% CH 4 and 40% CO 2 , and its thermal value is 17-25 MJ/m 3 . The remaining waste is an odorless solid or liquid waste suitable for use as fertilizer. After producing methane gas, methane gas can be used instead of LPG with very small changes. This gas can be used in spark ignition engines, gas turbines, and fuel cells [3,16,23,24]. The components of biogas are given in Table 9.
Biogas is a gaseous fuel as an alternative to natural gas. Thus, it can be used in the following fields: direct heating, motor fuel, turbine fuel power generation, fuel cells, additives for natural gas, and in the production of chemicals [23]. Flow diagrams of biogas production facilities are given in Figures 22 and 23.

Biophotolysis
Hydrogen and oxygen can be obtained by the biophotolysis process using some microscopic algae. These algae use solar energy in seawater, so they can work as a kind of solar cell. Thus, the microscopic algae can separate seawater photosynthetically to hydrogen and oxygen [15].

Conclusions
The study concludes that biomass energy in Turkey is seen as one of the most sustainable and promising renewable energy sources. Forest bioenergy potential can be converted to alternative biofuels. This process consists of the most common biomass conversion methods such as thermal processes, biological processes, and biophotolysis processes. The thermal processes consist of pyrolysis, gasification, and combustion, while the biological processes are fermentation and anaerobic digestion. Thus, forest bioenergy potential can be used for producing energy. In this regard, forest wastes or forest biomass can be turned into pellets and used in electricity generation in power plants. In addition to this, pyrolysis, gasification, fermentation and anaerobic digestion methods, alcohol, and biogas can be produced from forest wastes and used in the residential industry and transportation. Especially, bio-oils and bio-alcohols can be used in internal combustion engines, furnaces, or boilers as fuel. Besides, biogas also is used as a fuel in households or industry. Thus, Turkey can be reduced to its dependence on foreign energy demand due to the advantage of the rich forest resources. Besides, it is obvious that rich forest resources will contribute to both the ecological and socioeconomic structures of countries. Overall, the rich forest biomass potential is not only contributed to countries' economic field but also the ecological and socio-economic.

Acknowledgements
The author would like to thank Amasya University.  Flow diagram of dry fermentation [24].