*1.1.2 Types of biofuels*

Ethanol is the most popularly used alcoholic biofuel on the industry today. There are numerous motivations for its application as a sustainable energy, including: that it *Bioenergy Production: Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.102692*

is made from renewable agricultural feedstock such as corn, sugar and molasses, rather than non-sustainable sources, and that ethanol and its byproducts are less hazardous than other alcoholic fuels [11]. Biodiesel is a liquid fuel made from animal fats, vegetable oils, and waste cooking oil that can be used as a substitute for diesel fuel and is regarded as a viable replacement to fossil diesel [12]. It is sustainable, non-hazardous, biodegradable, sulfur- and benzene-free, may be applied in standard diesel engines without adjustment, and can be blended with fossil diesel at any ratio [7, 13, 14]. Bio-oil is a combination of organic components, primarily acids, alcohols, aldehydes, esters, ketones, and phenols. This liquid is usually dark brown in color and free-flowing, with a smoky fragrance [15, 16]. Bio-oil can be considered an environmentally benign fuel when compared to fossil fuels because it emits less CO2 and produces reduced NOx emissions than diesel oil [16]. Biogas is a gas combination mostly made up of CH4 and CO2 that is generated from agricultural residue, manure, municipal trash, plant material, sewage, green waste, or food waste while biohydrogen is produced from microalgae and bacteria metabolism. It is a form of green energy. Biogas is a diverse sustainable energy source that may be employed to substitute fossil fuels in the generation of electricity and heat, as well as a gaseous automobile fuel.

### **2. Biomass conversion technologies for bioenergy production**

Most techniques are appropriate for direct biomass conversion or intermediate conversion [17, 18]. Because the techniques are adequately mutable, gaseous, and liquid fuels that are undistinguishable to those derived from fossil feedstocks, or that are not matching but useful as fossil fuel alternatives, can be created. It's worth noting that biomass feedstocks may be used to make practically all of the fuels and commodity chemicals that are made from fossil fuels. The techniques include a wide range of thermal [18] and thermochemical technologies [19] for the conversion of biomass via combustion, gasification, and liquefaction, as well as microbial transformation of biomass through fermentative methods to create gaseous and liquid fuels. There are numerous biomass conversion pathways for creating energy haulers from biomasses. **Figure 1** depicts significant conversion pathways for producing heat, power, and transportation fuels that are now in use or under development. The accessible technologies for development in producing transportation fuels are categorized as combustion, gasification, and digestion, followed by the technologies available.

#### **2.1 Physicochemical conversion processes**

Physicochemical biomass transformation includes the generation of products employing physical and chemical conversion techniques at relatively close ambient temperatures and pressures. It is mostly linked with the conversion of fresh or used vegetable oils, animal fats, greases, tallow, and other apt feedstocks into beneficial liquid fuels and chemicals like biodiesel.

#### *2.1.1 Extraction or separation method*

There are varieties of procedures for the extraction of biomass including liquid– solid extraction, partitioning, acid–base extractions, liquid–liquid extraction, ultrasonic extraction (UE), and microwave assisted extraction (MAE) [20]. Several

#### **Figure 1.**

*Pathways for biomass conversion to finished products adapted from [19].*

extraction procedures, such as enzyme assisted extraction and solvent extraction have also been examined in the past few decades [18]. However, there are certain disadvantages to these extraction processes. Liquid–liquid extraction and liquid–solid extraction are the two most used extraction methods. Two distinct solvents are typically used for liquid–liquid extraction, one of which is unvaryingly water. Cost, toxicity, and flammability are some of the downsides of this approach [21]. A solidphase extraction (SPE) technique is also employed in separating analytes which are dissolved or suspended in a liquid mixture based on their physical and chemical properties from a wide range of matrices. Soxhlet extraction, percolation, sonication and steam distillation are examples of traditional procedures. Although these procedures are commonly used, they have numerous drawbacks: they are generally timeconsuming, requiring massive quantities of polluting solvents which are susceptible to temperature, causing thermo labile metabolites to degrade (18). For extracting analytes from solid matrices, novel extraction techniques such as supercritical fluid extraction (SFE) and pressurized solvent extraction (PSE) have been developed [22]. SFE is a comparatively recent and an operative separation technology for extracting essential oils from various plant sources. Extracts could be applied as a viable substrate for pharmaceutical medications and additives in the perfume, cosmetics, and food industries. SFE has been shown to be active for essential oil separation and its derivatives for application in the food and pharmaceutical industries. This is found to yield high-quality essential oils which have more acceptable structures other than those obtained by orthodox hydro-distillation.

### *2.1.2 Trans-esterification*

Both homogeneous and heterogeneous catalysis have been used to trans-esterify biomass such as microalgal oils for biodiesel synthesis. Because it catalyzes the reaction at low temperature and atmospheric pressure and can produce a significant conversion yield in a short period, homogeneous alkaline catalysis has been the most widely utilized method for biodiesel production. Alkaline catalysts including sodium hydroxide (NaOH) and potassium hydroxide (KOH) are extensively employed; however, because of the high free fatty acid concentration in microalgal oils, alkaline catalysts cause the free fatty acids in oils to generate soap and are not suited for

microalgal biodiesel generation. As the content of free fatty acids is greater than 1%, acid catalysts are utilized to overcome the constraint of high free fatty acid content [23]. Sulfuric acid (H2SO4) and hydrochloric acid are the most used acid catalysts (HCl). In comparison to alkaline catalysts, they require longer response times and a higher temperature. Initially, an acid catalyst is utilized in some research to convert free fatty acids into esters by esterification. After the free fatty acid content in the oils has been decreased to less than 1%, the oils undergo a second transesterification phase employing an alkaline catalyst. Regardless of the excellent conversion yields achieved by homogeneous catalysts, catalyst loss occurs after the process. In this regard, heterogeneous catalysts are known to contribute significantly to the future for their advantages in terms of recovery and reuse [24].

#### **2.2 Thermochemical conversion processes**

This is a cost-effective technology. Dry (non-aqueous) and hydrothermal techniques are two types of dry (non-aqueous) procedures [20]. Biomass undergoes structural breakdown which degrades to condensable vapors, and eventually disintegrating to gaseous molecules in a dry thermochemical transformation method as the temperature rises. A better understanding of everything from the process of decomposition of a single component to the technoeconomic evaluation of the biofuel sector is needed to achieve commercial synthesis of biofuels via thermochemical transformation of biomass [25].

#### *2.2.1 Conventional combustion*

This is defined as the oxidative chemical reaction that produces light, heat, smoke, and gases in a flame front when combustible elements (hydrogen and carbon) are ignited in fuels. Nitrogen is relatively inert, though it burns endothermically with oxygen at high temperatures to generate the undesirable NOx pollutants [26]. Combustion techniques now provide a significant amount of biomass-based renewable energy [27]. Wood, dry leaves, hard vegetable husks, rice husks, and dried animal manure are all examples of biomass that can be burned in combustion plants. An exothermic chemical reaction occurs during the combustion process. When biomass is burn't in the presence of oxygen, chemical energy is released. At about 800 to 1000° C, combustion occurs inside the combustion chambers. It's worth noting that the biomass utilized to produce biofuels by combustion must have a moisture content of less than 50%. Traditional wood use is inefficient (sometimes as low as 10%) and causes pollution with dust and soot. The adoption of considerably improved heating systems, such as those that are automated, have catalytic gas cleaning, and use standardized fuel, has resulted from technological developments [25].

Effective biomass-to-electricity/heat conversion is achievable because of fluidized bed technologies and better gas purification. Biomass co-combustion, particularly in coal-fired power plants, is considered a single most rapidly developed biomass conversion route in numerous EU countries (including Spain, Germany, and the Netherlands). The benefits of co-firing are clear, with features such as improved total electrical efficiency (often about 40%) because of existing plant economies of scale, and little to non-existent investment costs when high-quality fuels such as pellets are utilized [26, 28]. Furthermore, direct avoided emissions are significant due to the direct substitution of coal. Since several coal-fired power plants are completely depreciated, co-firing is generally a very beneficial greenhouse gas (GHG)

countermeasures alternative. Additionally, biomass combustion reduces sulfur and other emissions. Because many plants currently have some co-firing capability, there is a growing need for increased co-firing shares (up to 40%) [21].

#### *2.2.2 Carbonization*

Carbonization is the process of converting waste biomass into high-carbon, highenergy charcoal [10]. It redefines renewable energy and power producing principles. Char is made through a pyrolysis process in which biomass is burned to high temperatures in an inert atmosphere until the absorbed volatiles are released, hence increasing its heating value and energy content. Carbonization is an old process that is still employed today, but the increasing interest in it, particularly with biomass, stems from the fact that it opens new commercial and scientific opportunities. The carbon in the created char may be removed to make the valuable graphite and graphene. On a weight basis, the efficiency of these archaic systems is regarded to be quite low. For such operations, the wood to charcoal conversion rate is predicted to be between 6 and 12 tonnes of wood per tonne of charcoal [29]. Carbonization, also known as "dry wood distillation", removes most the wood's volatile components. Carbon accumulates mostly as the oxygen and hydrogen levels in the wood decline. The wood experiences a variety of physico-chemical changes as the temperature rises. The majority of water evaporates between 100 and 170°C, and gases, including condensable vapors like CO and CO2, between 170 and 270°C. Following that, condensable vapors (those with long carbon chain molecules) produce pyrolysis oil, which is used to generate chemicals or fuels. Exothermic reactions are defined as those that occur between 270 and 280°C and are characterized by the spontaneous creation of heat.

The advancement of industries such as the charcoal industry has resulted in significant improvements in production efficiency, with commercial synthesis, particularly in Brazil, currently with efficiency levels of >30%. The three main methods of generating charcoal are internally heated (by controlled burning of the raw material), externally heated (using fuelwood or fossil fuels), and hot circulating gas. Internally fired charcoal kilns are the prevalent type of kiln. It is estimated that these kilns waste 10–20% of the wood (w/w), with another 60% (w/w) lost in the transformation to, and emission of gases into the atmosphere [29]. Externally heated reactors fully eliminate oxygen, yielding higher-quality charcoal on commercial scale. They do, however, need the application of an external fuel source, which can be obtained from "producer gas" once pyrolysis has started.

#### *2.2.3 Liquefaction*

Thermochemical transformation of biomass to liquid fuels in a hot, pressurized water environ long enough to disintegrate the solid biopolymeric framework into predominantly liquid constituents is known as biomass hydrothermal liquefaction [30]. Hydrothermal processing temperatures range from 523 to 647 K, with working pressures ranging from 4 to 22 megapascal (MPa). The technique is meant to treat wet materials without the necessity for drying and provide access to ionic process parameters using a liquid water processing medium. The temperature is high enough to trigger pyrolytic process in biopolymers, and the pressure is high enough to control the liquid water processing phase. Hydrothermal method is classified into three distinct stages based on the severity of the working conditions. At temperatures <520 K, hydrothermal carbonization happens [17]. Hydrochar is the main product, and it

*Bioenergy Production: Emerging Technologies DOI: http://dx.doi.org/10.5772/intechopen.102692*

resembles low-rank coal in qualities. The hydrochar from microalgae is largely made up of the carbohydrate and protein fractions, with the lipid fraction remaining intact, allowing the lipids to be recovered during hydrothermal carbonization.

At intermediate temperatures between 520 and 647 K, this process is called hydrothermal liquefaction (HTL), a promising thermochemical liquefaction technique and it produces a liquid fuel called bio-crude. Biocrude is like petroleum crude, and it may be used to make all the petroleum distillate fuel products. Gasification reactions take control at temperatures above 647 K, and the process is known as hydrothermal gasification, which creates synthetic fuel gas. One of the merits of hydrothermal gasification over liquefaction stems from the fact that the water phase that follows gasification contains less organic carbon, resulting in improved carbon efficiency [31]. In each case, the underlying goal is to remove oxygen to produce a final product which has a higher energy density. Unlike HTL, thermochemical liquefaction of biomass has received recognition in recent years as it provides a greater energy density and has a faster reaction time, and it can be used on a wider range of materials. HTL can efficiently treat wet and dry biomass without lipid content limitations, from lignocellulosic to organic waste. The product created in this process is known as bio-crude, which is the renewable analog to oil, because it is an energy-dense intermediate that may be refined to a fuel [22].

#### *2.2.4 Pyrolysis*

By adding heat to a feedstock in the absence of oxygen, long chain molecules are broken down into short chain molecules through pyrolysis [32]. **Figure 2** depicts different bioenergy production routes of pyrolysis. Pyrolysis occurs at temperatures between 300°C and 700°C while the mild pyrolysis know as torrefaction (of wood chip) is evident at temperatures below 300°C [9]. The process is used in the manufacturing of syngas from biomass or waste as input (a mixture of hydrogen, volatile organic compounds, and carbon monoxide). By modifying the process settings, it is necessary to synthesize fluids similar to diesel and a variety of various products. Because of a greater understanding of the physical and chemical parameters that control pyrolytic reactions, the optimisation of reactor settings required for certain forms of pyrolysis has been made possible. More research is currently ongoing

**Figure 2.**

*Pathways of pyrolysis processes for bioenergy production adapted from [32].*

to produce hydrogen using high-pressure reactors and producing alcohol from pyrolytic oil using low-pressure catalytic techniques (which require zeolites) [20]. The advantages of pyrolysis and gasification is the conversion of their solid materials into vapor which are further burnt in turbines, providing fuel flexibility and security. The heat required to drive the chemical reactions that generate syngas is a key disadvantage of both technologies. As a result, some fuel must be used in the syngas production process.
