**1. Introduction**

The current global energy supply is, to a large extent, based on fossil fuels (oil, natural gas, and coal) of which the reserves are finite. As a result of industrialization, population growth, and urbanization, there has been a rapid increase in global energy demand and consumption. The necessity for long-term alternative energy sources is obvious due to the increasing energy consumption, high prices and limited reserves of fossil fuels and evidence of global warming, environmental pollution, and climate change. As a result, there is renewed interest in producing and using renewable energy resources, such as biomass, wind, solar, geothermal, and tidal. Bioenergy is a sustainable form of energy derived from biomass sources [1–4]. Recently, bioenergy is getting more attention because of its potential advantages, including renewable fuel for boilers, engines, turbines, power generation and industrial processes; inexpensive and CO2 neutral; utilization of nonfood and waste second-generation biomass feedstocks; easy to store and transport as liquid fuels; high-energy density compared to atmospheric biomass gasification fuel gases [2, 5, 6]. Biomass is a promising eco-friendly alternative source of renewable bioenergy because of its abundant availability, relatively lower price, and zero greenhouse gas emissions in the context of current energy scenarios. However, the only renewable energy resource that can be used to produce transport fuels is biomass [2, 4].

Biomass is plant or animal-based organic matter that is living or was living in the recent past. Various biomass components, such as sugars, starches, and lignocellulosic (non-starch fibrous part of the plant) materials, can be converted to liquid transport fuels, reducing the use of fossil fuels. A promising alternative to reduce environmental issues related to waste disposal and management is converting biomass residues and wastes (such as crop residues, food wastes, animal manure, and municipal solid wastes) into useful bioenergy. Some of the advantages of converting biomass residues and wastes into bioenergy include (a) reducing the burden on waste management, (b) converting waste into valuable energy reduces the dependence on fossil fuels, (c) reducing decomposing waste and associated issues such as water contamination, greenhouse gas emissions, pests and insects breeding, and foul odor. [4, 7–10].

The biomass feedstocks can be transformed into biofuels through biochemical and thermal conversion processes. The thermal conversion approach, such as pyrolysis, gasification, and torrefaction, are applicable for a wide range of biomass types using different temperatures to breakdown the bonds of organic matter in a relatively short period of time, unlike the biochemical processes [2, 5, 6]. Lignocellulosic biomass, such as agricultural crop residues, wood and forestry residues, are readily available, inexpensive, and promising resources for biofuels. Biomass can be considered one of the best options for sustaining future energy demand. The more efficient biomass production and conversion processes are essential for the efficient utilization of biomass resources [11]. Biomass is a valuable fuel source that is considered renewable as it can be produced year after year. Compared to fossil fuels, biomass has the potential to reduce combustion emissions, such as CO2, SOX, and NOX [12, 13].

### **2. Biomass**

A commonly used biomass classification is based on the origin of biomass, such as agricultural crop residues, forestry and wood processing residues, purposely grown dedicated energy crops, aquatic biomass, animal, food, industrial and municipal waste, sewage sludge, digestate, and industrial crops. Various types of wastes, such as wastepaper, sewage sludge, cow manure, poultry litter, municipal, and many industrial wastes, are treated as biomass because these are a mixture of organic (and nonorganic) compounds. Biomass is also classified based on its chemical composition as carbohydrates, lignin, essential oils, vegetable oils, animal fats, natural resins (gums), etc. Lignocellulosic biomass is the most abundant biomass on the earth and it represents a major carbon source for bioenergy, biofuels, and chemical compounds [2, 4].

#### **2.1 Sources of biomass**

Agricultural crop biomasses are natural products of agriculture, including food-based and nonfood-based portions of crops. The food-based portion comprises

#### *Advances in Bioenergy Production Using Fast Pyrolysis and Hydrothermal Processing DOI: http://dx.doi.org/10.5772/intechopen.105185*

simple carbohydrates and oils from crops, such as corn, sugarcane, sugar beet, rapeseed, soybean, and sunflower. The nonfood-based portion is commonly discarded, which comprises complex carbohydrates of crops that are not harvested for commercial use or byproducts from harvesting or processing, such as corn stover, sugarcane bagasse, straw residues, waste from food processing, horticulture, and food crops [4, 7, 14]. Forestry and wood processing residues include trees that are not valuable as timber and not harvested during logging, crowns and branches from fully-grown trees that are removed during logging in commercial forests, waste from forest and wood processing (such as wood pellets, woodchips, leaves, lumps, barks, and sawdust) as well as materials removed during forest management operations. Most of the biomass used today are derived from agricultural crop, forestry, and wood biomass [7, 15].

Another expanding and potentially larger source of biomass is dedicated energy crops that are grown specifically for their fuel value on marginal land unsuitable for agriculture. These are high-yield and low-maintenance crops that produce maximum energy yield. There are two types of energy crops, herbaceous and short-rotation woody crops. Herbaceous energy crops include perennial grasses, such as switchgrass, miscanthus, bluestem, elephant grass, bamboo, and wheatgrass, that are harvested annually after maturity. In 2–3 years, herbaceous energy crops reach complete production and do not require replanting for 15 years or more. The drawback of most nonwoody energy crops is that their chemical properties (high ash and salt content) make them less suitable for combustion. Woody crops are grown on short rotations, generally with more intensive management than timber plantations. These fast-growing hardwood trees include poplar, willow, maple, cottonwood, black walnut, and sweetgum. The woody crops are harvested within 5–8 years of planting [4, 14–17].

Aquatic biomass includes different types of algae, plants, and microbes found in water, such as aquatic plants, water hyacinth, seaweed, kelp, macroalgae, and microalgae [18]. Another primary biomass source is municipal, industrial, food, and animal waste. Municipal solid waste includes waste from commercial, industrial, and residential sectors containing a significant amount of biomass with energy content. The industrial waste includes waste from textile and food processing industries and waste from various industrial and manufacturing processes, such as sugar cane residues and paper sludge. Food waste includes postconsumer waste, animal fat, used cooking oil, residues from food and drink manufacturing, preparation and processing, etc. Animal and human waste includes cooked or uncooked food, fruits, paper, manure of different animals, and waste from farm and processing operations. The problem of disposing of waste is reduced to a certain extent when waste materials are treated and converted to useful energy products. Primarily, animal and human waste are free of harmful materials. In contrast, industrial waste may contain different harmful additives and toxic chemicals [4, 14, 15, 19].

Plant biomass has a carbon-to-oxygen (C/O) ratio of almost one. Because of the high level of oxygen, the energy density of biomass is relatively lower than fossil fuels, which means that issues associated with land use must be considered. The potential benefit of biomass can be reduced by environmental damage due to the expansion of land use for biomass production, leading to a high potential for deforestation, emissions, erosion, nutrient runoff, etc. When sufficient land areas are available, large-scale cultivation of energy crops for bioenergy is feasible. The agricultural lands must be used to grow food crops. Land for energy crops needs to be selected carefully to avoid food versus energy conflict. Identifying lands with minimal disturbance to food production is critical for technically and economically feasible biomass

production. To achieve sustainable large-scale biomass production, infertile/marginal or abandoned agricultural land with little fertilizer or pesticides and potentially needing minimal water has been widely considered important. Energy crops are adaptive to infertile/marginal or abandoned agricultural land. Energy crops, such as switchgrass and miscanthus, generally require much less water to grow and are suitable to replace dryland crops partially. Energy crops should not be grown at the expense of biodiversity. Beyond the vast land areas needed to grow energy crops, the long-term impact of soil quality due to repeated removal of biomass and water usage are major concerns [4, 20, 21].

Plants absorb atmospheric CO2 and produce carbohydrates in photosynthesis that form the building blocks of biomass. Water and sunlight are the other two key ingredients of photosynthesis. The burning of biomass does not add to the total CO2 inventory of the earth as it releases CO2 back into the atmosphere that the plants have absorbed recently in photosynthesis producing biomass. Therefore, biomass is considered the most important carbon-neutral or green carbon fuel source. But the overall biomass chain needs to be considered for true carbon neutrality of biomass. Significant cost, energy needs, and CO2 emissions account for biomass harvesting, drying, handling, transportation, processing, and storage, which need to be considered in life-cycle analysis for sustainability. Biomass plays an integral part in the overall sustainable energy solution. Biochar facilitates the conversion of marginal lands to lands suitable for agriculture by improving soil quality. The impacts of adding biochar to soils may include reduced land area required for food production as a result of increased productivity and making marginal lands economically productive [4, 12, 20, 22].

### **2.2 Composition of biomass**

The chemical composition of biomass is different from fossil fuels. Lignocellulosic biomass is a complex mixture of biopolymers consisting of three key elements, carbon (C), oxygen (O), and hydrogen (H). The percentages in dry matter of C, O, and H are 42–47%, 40–44%, and 6%, respectively, whose total content reaches typically above 95%. In addition, depending on the plant species and environment, plant biomass also contains various macronutrients, micronutrients, trace elements, and other heavy metals [4, 18]. The non-starch fibrous part of the plant (lignocellulosic) material is the major component of plant biomass. Three major constituents of lignocellulosic biomass comprising the cell wall of plants are cellulose, hemicellulose, and lignin. Cellulose, the main component of the plant cell wall, provides structural support. The second most abundant polymer in lignocellulosic biomass is hemicellulose. The third most abundant polymer in lignocellulosic biomass is lignin. Usually, cellulose is the major component in wooden biomasses, whereas hemicellulose is the key component in leaves and grasses and lignin in shells. Hemicellulose is thermally less stable than cellulose. Lignin is the most stable of all three. Knowledge of biomass composition in terms of cellulose, hemicellulose, and lignin can be helpful in controlling the product chemistry [2, 4, 23, 24].

The other compounds present in biomass include inorganic compounds and organic extractives. These nonstructural components include fats, waxes, proteins, terpenes, simple sugars, gums, resins, starches, and essential oils that do not constitute the cell walls or cell layers. Often these compounds are responsible for the smell, color, flavor, and natural resistance to decaying of some species. The inorganic compounds constitute less than 10% by weight of biomass, forming ash in the pyrolysis

#### *Advances in Bioenergy Production Using Fast Pyrolysis and Hydrothermal Processing DOI: http://dx.doi.org/10.5772/intechopen.105185*

process. Depending on the type of biomass, the cellulose, hemicellulose, and lignin content fall in the range of 40–60%, 15–30%, and 10–25%, respectively. Fermentable sugars produced by hydrolyzing carbohydrates (cellulose and hemicellulose) can be converted into fuels and chemicals. The content of cellulose, hemicellulose, and lignin in wood biomass is high (~90%), while more extractives and ash are present in agricultural and herbaceous biomass [2, 4, 24].

Analysis of biomass feedstock is an essential part of understanding the behavior of biomass in energy use. The proximate analysis, ultimate analysis, and higher heating value (HHV) of biomass feedstock can provide a clear understanding of its thermochemical conversion characteristics. The proximate analysis gives information on biomass composition in terms of volatile matter (VM), fixed carbon (FC), ash content, and moisture (M) content. VM is the condensable and non-condensable vapors/ gases released from biomass during heating. The amount of VM depends on the heating rate and the final biomass temperature. FC is the solid carbon (nonvolatile) that remains in the char after devolatilization. FC and VM indicate the percentage of biomass burned in solid and gaseous states, respectively. Ash is the noncombustible solid residue remaining after biomass is completely burned. These are of fundamental importance for bioenergy use. These data provide the essential information for the furnace design, including sizing and location of primary and secondary air supplies, refractory, ash removal, and exhaust handling equipment. [4, 25, 26].

The ash contains mostly inorganic residues and its composition depends on the biomass type. The inorganics in ash include silica, calcium, iron, aluminum, and small amounts of potassium, sodium, magnesium, and titanium. The content of ash in biomass is generally small. But if biomass contains alkali metals or halides, ash may play a significant role in biomass combustion or gasification. Agricultural residues, grasses, and straw generally contain potassium compounds and chlorides are particularly susceptible to this problem and can cause severe corrosion, fouling, and agglomeration in boilers or gasifiers. Burning biomass at lower temperatures mitigates the problems of corrosion and slagging. The ash produced during biomass conversion does not necessarily come from biomass itself but also from other sources like contamination as well. Biomass can pick up dirt, soil, rock, and other impurities during collection and handling, partly contributing to ash content [4, 14, 25].

The relationship between FC and char yield in biomass is positive, while VM and ash relate negatively to char yield. The greater biomass VM is expected to lead to greater gas production instead of the solid phase. Moisture content has a significant impact on the biomass conversion process. High moisture content is a major concern in biomass conversion. Thermochemical conversion processes generally require biomass with low moisture content. However, biochemical conversion processes can use biomass with high moisture content. Some moisture is required in the gasification process to produce hydrogen and with increasing moisture content, the amount of hydrogen increases. The moisture content can be very high (>90%) in some wet biomass (such as water hyacinth). As the energy used in the evaporation of moisture is not recovered, moisture drains much of the deliverable energy during conversion [4, 25–27].

The ultimate analysis provides the composition of biomass on a gravimetric basis, including major elements (C, H, O, S, and N), moisture, and ash. The ultimate analysis is usually reported on a dry and ash-free basis. These are useful for performing mass balances on biomass conversion processes. Elemental chemical composition, volatiles, moisture, and ash are essential for thermochemical conversions of biomass. Additionally, information on the polymeric composition of biomass is required for

conversions, such as torrefaction, pyrolysis, and gasification. The ultimate analysis helps calculate the quantity of combustion air needed to sustain the combustion reactions. Usually, the sulfur and nitrogen content of biomass is very low and produces minimal pollutants SOX and NOX [25, 27].

### **3. Biomass conversion**

Biomass can be converted to end products (such as heat, biofuels, or chemicals) through chemical, biochemical, and thermochemical conversion processes. Selection of the conversion process depends on number of factors, such as the desired form of end products, biomass feedstock available, environmental standards, policy, economic conditions, and specific factors related to the project. In most situations, the selection of the conversion process is based on two factors, the desired form of end products and biomass feedstock available. The moisture content of biomass primarily determines the biomass conversion process. Dry biomasses (such as wood or straw) are required for thermochemical conversions, such as pyrolysis, gasification, or combustion. Low-energy density due to higher moisture content makes wet biomass unsuitable for these processes. Transportation and energy costs significantly increased due to the high moisture content. Hydrothermal and biochemical processing are wet conversion processes that have gained growing attention and are more suitable for processing high moisture content biomass, including aquatic biomass, sewage sludge, food waste, and manure. Compared with thermochemical conversion, biochemical conversion consumes less energy but requires more time. Consequently, cost-effective hydrothermal processing has been given more attention than thermochemical conversion (with drying). If moisture content lies between wet and dry regions, additional parameters (such as cost and feasibility of drying) need to be considered in selecting a suitable conversion process [1, 2, 4, 10, 28–30].

Thermochemical conversion processes usually offer many advantages over biochemical conversion processes, including better conversion efficiency, handling a wide variety of feedstocks, shorter reaction times, and high-energy efficiency. As a result, thermochemical conversion processes have recently received greater attention for biofuel production. Many thermochemical conversion processes are available to convert biomass into products (solid, liquid, and gaseous). Thermochemical conversion processes use high temperatures to breakdown the bonds of biomass organic matter. These are classified according to the oxygen content used in the process, including combustion (complete oxidation), gasification (partial oxidation), and pyrolysis (thermal degradation in the absence of oxygen). Torrefaction, a mild form of pyrolysis, is also performed in the absence of oxygen. Hydrothermal processing, a thermal degradation in the absence of oxygen, is an alternative route to process wet biomass. The typical products of the thermochemical conversion of biomass are biochar (carbon-rich solid residue), bio-oil (liquid fraction, condensable vapors), and non-condensable gases. The distribution of products (biochar, bio-oil, and gases) depends primarily on the conversion process [2, 4, 9].

### **4. Pyrolysis**

Pyrolysis is one of the thermal decomposition processes conducted in the absence of oxygen to convert biomass into three distinct product fractions—solid residue (biochar), condensable vapors resulting in liquid product fraction (bio-oil), and
