**2. Importance of biomass**

Non-renewable sources produce a significant portion of current energy supplies globally and their use is associated with the emission of CO2 into the atmosphere. However, about 10–15% of this demand is covered by renewable resources, making biomass by far the most important renewable energy source used to date. Biomass contributes to 9% - 13% of the total energy supplies in industrialized countries. However, biomass energy is the primary energy source in many developing nations, contributing to about 20–30% of the total energy supplies. In some developing countries, biomass covers 50 to 90% of the total energy demand. In industrialized countries, biomass is used as a modern bioenergy source for industrial (heat, electricity), transportation (biofuels) and domestic (mainly heat) applications [9–11]. A significant part of the traditional use of biomass (firewood, sometimes animal waste) in developing countries is non-commercial and used for cooking and heating, generally by the poorer part of the population. The non-commercial use of biomass is poorly mapped and as a result, the contribution of biomass to the energy supply is not accurately known [12]. Solid biomass is one of the primary energy sources (mainly used for cooking) in many developing countries, especially in rural sub-Saharan Africa and South Asia. Traditional biomass use is not sustainable because it may result in soil quality degradation by depriving soil nutrients and burning it in inefficient cooking stoves can cause high levels of indoor air pollution. Most households in sub-Saharan Africa rely on the traditional use of biomass to meet their daily cooking needs. In countries such as the Democratic Republic of Congo, Ethiopia, Tanzania, Nepal and Nigeria, more than 80% of the total energy

### *Recent Advances in Thermochemical Conversion of Biomass DOI: http://dx.doi.org/10.5772/intechopen.100060*

demand is met through biomass energy sources. In developing countries, biomass is used to some extent in industries such as brick manufacturing [13, 14].

Crude oil is an excellent source of energy, it is easily transported and energyrich, it is one of the most energy-dense fuels. The energy density of crude oil and methane are 42 MJ/kg and 55 MJ/kg, respectively. Even coal has a good energy density (27–32 MJ/kg). Compared to these fossil fuels, the energy density of biomass ranges from 8 MJ/kg for greenwood to 20 MJ/kg for dry plant matter. Biomass has low bulk density and causes a major problem during storage, handling and transportation for further processing. The bulk density of biomass varies from around 40 kg/m3 for loose straw and bagasse, in the range of 80–100 kg/m3 for agricultural residues and 150–200 kg/m3 for woody biomass to the highest levels of around 250 kg/m3 for some wood residues. This translates to needing more biomass to produce for the same amount of heat or power and higher transportation costs, processing costs, etc. [15, 16].

Most of the biomass used today is derived from agricultural and forestry biomass. Agricultural biomass includes the food-based portion and the non-foodbased portion of crops. The food-based portion comprises oils and simple carbohydrates of crops such as corn, sugarcane and beet. The non-food-based portion comprises complex carbohydrates of crops such as the leaves, stalks, seed hulls, orchard trimmings, nutshells, rice husk, bagasse, coffee pulp and straw. Forestry biomass includes wood residues associated with the production of timber in the forest, as well as the processing of timber into their final products [17, 18].

The comparatively low energy density of biomass means that issues associated with land use must be taken into consideration. Expansion of land use for biomass production can lead to a high potential for environmental damage due to deforestation, erosion, nutrient runoff, emissions, etc. This reduces any potential benefit of using biomass. Large-scale cultivation of non-food perennial energy crops for bioenergy feedstock is feasible when sufficient land areas are available. The best land for agriculture must be used to grow food crops. To avoid food versus energy conflict, it is important to use infertile/marginal lands for energy crop cultivation with little use of fertilizer or pesticides and potentially needing minimal water. Energy crops should not be grown at the expense of biodiversity [15]. For productive agricultural systems, prospects of biomass production based on the factors provided by nature such as light, soil, water and nutrients with soil and water are considered as the most crucial natural resource constraints. Identifying land areas with minimal disturbance to food production is critical for technically and economically feasible biomass production. To achieve sustainable large-scale biomass production, marginal or abandoned agricultural land has been widely considered as important. Energy crops are adaptive to marginal or abandoned agricultural land. Compared with food crops, energy crops such as switchgrass and miscanthus generally require much less water to grow and are suitable to partially replace the dryland crops [19]. Beyond the vast areas of land needed to grow energy crops, the long-term impact of soil quality due to repeated removal of biomass is a concern. Water usage is another major concern. Biomass may have a moderate carbon footprint, but its water footprint is enormous. Only a small percentage of the biomass produced by photosynthesis is currently being cultivated, harvested and used, but how much can be used sustainably? As with any approach to energy generation, the massive demand for energy stresses the need to be careful in considering the use of biomass for energy generation [15].

Plants grow through photosynthesis by absorbing atmospheric CO2 and producing carbohydrates that form the building blocks of biomass. Water and sunlight are the other two key ingredients of photosynthesis, which typically convert less than 1% of the energy available in sunlight to chemical energy. When biomass burns,

it releases CO2 back to the atmosphere that the plants had absorbed recently. i.e., the burning of biomass does not add to the total CO2 inventory of the earth. Therefore, biomass is considered the most important source of green carbon or carbon-neutral fuel. In order to decide the true carbon neutrality, the overall biomass chain needs to be considered, including cultivation (for energy crops), harvesting, drying, storage, transportation and processing. These represent a significant cost, energy needs and CO2 emissions sources [7]. All these factors must be taken into consideration in life-cycle analysis for sustainability. Biomass plays an integral part in the overall sustainable energy solution, but it is not a panacea. Biomass for biochar production makes land usage more complex. The effect is not only the land usage for biomass supply but also the impacts of adding biochar to soils. The impacts may include increased productivity and, hence, reducing the land area required for food production as well as the potential for biochar to make previously unmanaged or marginal land economically productive, thereby facilitating the conversion of marginal land to agriculture [15, 20].

Besides heat and electricity generation, biomass can be used as a feedstock for biofuel production with technologies already available on the market. Biofuels are liquid or gaseous fuels produced from biomass and can be used as a replacement or blended with fossil-based fuels for different applications. This makes biomass very valuable within future energy systems based on renewable/sustainable sources of energy. The biomass potential for the energy markets needs to be evaluated without affecting the demands for food and fodder as well as for raw materials. The use of biomass has been debated critically on the background of the ongoing environmental and sustainability discussions.

### **2.1 Biomass classification**

Biomass can be classified into different groups depending upon the origin where it is produced, including agricultural biomass, forestry and wood processing residues, dedicated energy crops (crops cultivated solely for energy), aquatic biomass, sewage sludge, digestate (remains of anaerobic digestion), industrial crops, animal, industrial, municipal and food waste. Biomass is also classified based on the chemical composition as carbohydrates, lignin, essential oils, vegetable oils, animal fats and natural resins (gums) [21–24].

Agricultural biomasses are natural products of all agriculture. These include a wide range of agricultural crop residues (the non-food based portion of crops) that are not harvested for commercial use or byproducts from harvesting or processing, such as corn stover (leaves, stalks, husks and corn cobs left in a field after harvest), sugarcane bagasse, straw residues (barley, oats, rice, rye, wheat) from grain production, waste from other food crops, horticulture and food processing. Other plant residues include husks of grains and seeds, coconut shells, fruit stones and nut shells. Straw and other agricultural residues usually have a high ash content and contain chlorides and potassium compounds, which can cause high levels of corrosion in boilers. The problems of corrosion and slagging can be mitigated by burning biomass at lower temperatures [1, 18].

Forestry and wood processing residues include trees not harvested during logging (trees that are not valuable as timber, such as imperfect commercial trees, dead wood and other non-commercial trees), biomass removed during logging (such as crowns and branches from fully-grown trees) in commercial forests, waste from forest and wood processing (such as palm kernel shells, wood pellets, woodchips, leaves, barks, lumps and sawdust) as well as materials removed during forest management operations (such as trunks of smaller trees removed during thinning, dead and dying trees removed during forest control) [1, 25].

Dedicated energy crops are another expanding and potentially larger source of biomass. Energy crops are low maintenance and high yield crop species that give the maximum energy yield. These are grown specifically for their fuel value (energy applications) on marginal land unsuitable for agriculture. Several crops can be readily used as energy sources. There are two types of energy crops, herbaceous and short-rotation woody. Herbaceous energy crops include perennials that are harvested annually after reaching maturity. It takes 2–3 years to reach complete production. These are grasses such as switchgrass, miscanthus, bluestem, elephant grass, bamboo and wheatgrass. They do not require replanting for 15 years or more. The drawback with most non-woody energy crops is that their chemical properties generally make them less suitable for combustion due to the high ash and salt content [18, 26, 27]. The woody crops are grown on short rotations, generally with more intensive management than timber plantations. These fast-growing hardwood trees are harvested within 5–8 years of planting. These crops include poplar, willow, maple, cottonwood, black walnut and sweetgum [25, 27].

Different kinds of algae, plants and microbes found in water form another class of biomass called aquatic biomass. Aquatic biomasses include macroalgae, microalgae, seaweed, kelp, water hyacinth and aquatic plants [24]. Animal, industrial, municipal and food waste and sewage sludge are other important sources of biomass. Animal and human waste biomass includes waste resulting from farm and processing operations, manure of different animals, cooked or uncooked food, fruits, paper and pulps. When these waste materials are treated and converted to useful energy products, not only energy is being produced, but the problem of disposing of these materials is also reduced to a certain extent. Industrial waste involves waste from various manufacturing and industrial processes like paper sludge from paper industry, sugar cane residues from sugar mills, waste from food processing industry, waste oils, textile industry waste and others. Animal and human waste biomass and industrial biomass are categorized differently because industrial biomass may contain different types of toxic chemicals and harmful additives. In contrast, animal and human waste are primarily free of these types of harmful materials [28]. Municipal solid waste (MSW) includes waste from residential, commercial and industrial sectors that contains a significant amount of biomass (such as paper, cardboard, wood, food, leather, textiles and yard trimmings) with energy content. Food waste contains residues from food and drinks manufacture, preparation and processing, post-consumer waste, animal fat, used cooking oil etc. Other biomasses include Industrial crops (crops developed to produce specific chemicals or feedstocks such as kenaf), construction and demolition waste, building material waste, abandoned furniture etc. [18, 25].
