generation. Generally coal, petroleum-based materials and wastes containing carbon can be used in gasification. In these reactions, carbon combines with water to form CO2, CO and H2 [107]. Figure 1.5 shows the conversion of natural gas into a synthesis gas; a mixture of hydrogen, carbon monoxide and carbon dioxide. The quantities of these compounds in the mixture vary according to the selected natural gas conversion process and the type of synthetic fuel to be obtained [108].
The pyrolysis method, which is the thermal degradation of the raw material in an oxygen-free environment with the thermal conversion methods applied to the biomass, the liquid product obtained is an option for petrochemical derivative fuels and petrochemical industry input with its high thermal value, measurable, movable and storable properties [109]. The main parameters obtained in pyrolysis studies, which affect liquid, solid, gas product yields are heating rate, pyrolysis temperature, particle diameter, dripping gas flow rate and particle size. In addition, the effect of secondary reactions in which the heating rate is over 100 °C min-1, occurring under static pyrolysis and static retorting conditions, limiting the yield and quality of the liquid is not reduced and it is possible to obtain a high-yield liquid product [110].
Figure 1.5 Production of synthesis gas from natural gas flow chart [108].
Gasification of biomass is one of the recommended methods for direct hydrogen production from renewable energy sources, although natural gas costs more than conventional vapor reformation [111]. Advanced and current technology is an integrated gasification combined cycle (IGCC) system in which hydrogen and electricity are produced from coal together. In the gasification section, the main system unit is a gasifier. Steam and oxygen are combined with coal under a certain temperature and pressure. Only a small part of the raw material burns out (also called partial oxidation). As a result of the process, a mixture of hydrogen, carbon monoxide and other gases called synthesis gases are produced. The minerals in the raw material are removed from the bottom of the gasifier, substances such as sulfur and ammonia are removed in the later stages of the process. The combined cycle part of the process refers to the use of the combustion turbine followed by the steam turbine used to generate electricity from synthesis gas, which increases system efficiency. The benefit of this integrated system is to use synthesis gas as a source of hydrogen. This can be used electrically by means of transportation fuel or fuel cells [112].
1.7 Advantages and Drawbacks of Biofuels
Biomass energy has a great potential among alternative energy sources and it is a resource that can provide continuous energy, not discrete like wind and sun. The fact that biomass energy is easily stored provides an advantage over other renewable energy sources. Biomass does not cause an increase of CO2 in the atmosphere [113]. Biomass is a fuel that does not contribute to the greenhouse effect theoretically in case of renewal of forest and plant existence; therefore it released as much carbon as takes carbon dioxide from the atmosphere if it burns. Biomass energy is divided into biodiesel, bioethanol and biogas, and each has its own advantages and disadvantages [114].
Advantages of biodiesel are being safer, easily biodegradable and non-toxic. It is a renewable resource and can be produced with local facilities. It contributes economically and strategically because it reduces dependence on oil. It contributes to agriculture and industry with by-products such as fertilizer, pulp and glycerin [115, 116]. On the other hand, disadvantage of biodiesel is having heat value slightly lower than petrodiesel. This situation causes a low power drop due to combustion in the engine. It is affected by cold weather conditions more quickly than petrodiesel. Especially clouding is seen earlier. This is a limiting factor for the use of biodiesel in cold climates. NOx emissions are somewhat higher than petrodiesel [115-117].
The advantage of bioethanol is that it can be shown to decrease the emission values of the gases harmful to the environment; this is caused by a more efficient and cleaner combustion due to the oxygen it contains. It contributes to the reduction of air pollution. The value of plants used as raw materials will increase with the increasing demand for bioethanol, which will improve the economy and create wider market opportunities in the agricultural field. It will contribute to the economy as it can be replaced with gasoline as much as its usage rate. The octane of bioethanol is higher than gasoline. High octane creates more pressure and increases the thermal efficiency of the engine [117, 118]. On the other hand problems can occur in long-term storage of bioethanol. Bioethanol and water in the tank, which remain immobile, can decompose from gasoline and settle to the bottom of the tank. To prevent this, the fuel system must be completely emptied and cleaned before storage. Also, a gasoline stabilizer can be placed in the fuel tank after draining. In vehicles using bioethanol, while it is difficult to work in cold weather, there is also the possibility of steam plug formation in hot weather [119].
Biogas technology allows both energy recovery and waste to be obtained from organic waste/residual substances. It is a cheap, environmentally friendly source of energy and fertilizers; it provides waste recovery. It ensures that the disease factors that threaten human health and groundwater caused by animal fertilizers largely lose their effectiveness [120]. Considering the drawbacks of biogas, the nitrogen in the feed material might turn into an organic form of ammonium. Ammonium turns into ammonia or nitrate. If the storage conditions of the fermented fertilizer are not suitable, nitrogen and ammonia emissions occur. This causes health problems arising from waste, biogas risk of poisoning, explosion and fire. Pathogens and parasites cannot be completely removed in mesophilic and thermophilic reactors. For removal, energy pasteurization is required. Investment costs of biogas plants are high [121, 122].
1.8 Applications of Biofuels
Wood (energy forests, wood waste), oilseed plants (sunflower, rapeseed, soy, cotton, etc.), carbohydrate plants (potatoes, wheat, corn, etc.), fiber plants (flax, hemp, etc.), vegetable residues (branch, stalk, straw, root, shell etc.), animal fats, animal wastes and urban and industrial wastes are evaluated in biofuel technology [123]. Biomass is renewable and can be grown anywhere, providing socioeconomic development and an environmentally friendly, strategic energy source. Many liquid, solid or gas biofuels are obtained by burning the biomass directly or by physical processes (size reduction-crushing and grinding, drying, extraction and accumulation) and conversion processes (biochemical and thermochemical processes). Biofuels are utilized in two basic areas, namely bioelectric generation and engine biofuel, other than traditional methods for heating purposes (wood, waste-waste, incineration, etc.) [123, 124].
Vegetable oils, biogas, landfill gas and solid biofuels are used in heating systems and bioelectric production. Vegetable oils are used in heating systems by blending directly, fuel biodiesel and fossil fuels [125].
Biogas is a mixture of organic substances in an anaerobic (oxygen-free) environment, in the presence of different groups of microorganisms, by biomethanization processes [126]. Biogas can be used as an alternative gas to natural gas, in direct combustion-heating and heating, as engine fuel and in electricity generation. Biogas technology is an integrated facility application that produces energy (electricity, heat-cold) and organic (liquid and solid) fertilizers [127].
The most widely used fuel in biofuels in the world is bioethanol and more than 95% of bioethanol production is obtained by processing agricultural products. In many countries of the world, the use of bioethanol in vehicles has been made compulsory and its rate has been varied according to its own production size in each country. Ethanol is used by mixing with gasoline at different rates in order to reduce air pollution or reduce the consumption of petroleum products [128].
1.9 Future Prospects
The
