Table of Contents
Introduction
Biomass is one of the oldest energy sources used by humans, yet it is also an important part of modern renewable energy systems. This chapter introduces what biomass means in the context of energy, and explains the main types of biomass that are used as energy resources today. More detailed questions such as sustainability, land use, or emissions are treated in later chapters, so here the focus stays on definitions and basic classification.
What Is Biomass in Energy Terms?
In everyday language, biomass can mean any organic material. For energy studies, the meaning is more specific. Biomass is organic material of recent biological origin that contains stored chemical energy from photosynthesis and can be used as a fuel or for energy conversion.
Plants absorb sunlight and use it to convert carbon dioxide and water into carbohydrates and other organic compounds. This process stores solar energy in chemical bonds. When biomass is burned or converted in other ways, this stored energy is released as heat or transformed into other useful energy carriers.
Biomass for energy normally refers to non-fossil, recently grown material. Fossil fuels like coal, oil, and natural gas also come from ancient biological material, but they formed over millions of years and are treated as nonrenewable resources. In contrast, biomass used for energy can be renewed on human time scales if it is grown and harvested sustainably.
Basic Energy Characteristics of Biomass
Although biomass appears in many forms, it shares some common energy properties. It contains carbon, hydrogen, oxygen, and smaller amounts of other elements. When biomass is combusted completely, the main products are carbon dioxide and water, plus heat. The amount of useful energy that can be obtained from a biomass fuel per unit mass is called its heating value, or calorific value.
There are two common heating values for any fuel. The higher heating value includes the latent heat of vaporization of water, while the lower heating value assumes that water in the flue gas remains as vapor and that latent heat is not recovered. For biomass energy systems, the lower heating value is often more relevant, especially when exhaust gases are not fully condensed.
Another important characteristic is moisture content. Fresh biomass usually contains a significant amount of water. High moisture reduces the effective energy content per kilogram, because part of the released energy is used to evaporate water instead of providing useful heat.
Key concept: The useful energy obtained from biomass depends strongly on both its heating value and its moisture content. Dry, dense biomass generally provides more usable energy per unit mass than wet, light biomass.
Main Ways to Classify Biomass
Because biomass is very diverse, it is helpful to use simple categories. For energy purposes, biomass is often classified according to origin, physical form, or intended use.
By origin, one can distinguish biomass from forests, agriculture, aquatic systems, and human activities. By physical form, biomass can be grouped into solid, liquid, and gaseous types. By intended use, biomass may be grown specifically as an energy crop, or it may be a by-product or residue from another activity.
The same material can be described in more than one way. For example, sawdust is a solid biomass fuel. It is also a forestry residue and a by-product of wood processing.
Woody Biomass
Woody biomass includes material derived primarily from trees and shrubs. This is one of the most common and traditional forms of biomass for energy.
Unprocessed woody biomass consists of logs, branches, twigs, and other parts of trees. Firewood that is cut and split for heating is an example of unprocessed woody biomass prepared for direct combustion. There are also forestry residues that remain after logging, such as tops, branches, and small-diameter wood that is not suitable for lumber.
Processed woody biomass includes forms that have undergone some preparation to improve handling or combustion. Typical examples are wood chips, wood pellets, and briquettes. Chips are small pieces produced by chipping machines. Pellets and briquettes are compressed forms made from sawdust or other fine residues. These processed fuels are more uniform, easier to transport, and more suitable for automated systems than irregular logs or branches.
Woody biomass can also come from short-rotation plantations, where fast-growing tree species are planted and harvested on relatively short cycles specifically for energy use. Because these plantations are managed on purpose, they are often described as dedicated energy crops of the woody type.
Agricultural Biomass and Energy Crops
Agricultural biomass comes from crops grown for food, feed, fiber, or energy, and from the residues of these activities. It includes both herbaceous plants and crop by-products.
Crop residues are the non-edible parts left after harvest. Common examples are cereal straws such as wheat or rice straw, corn stover which includes stalks and leaves, and sugarcane bagasse, the fibrous material that remains after the juice is extracted. These residues are usually solid biomass, but they can be converted into other forms through suitable technologies.
Dedicated energy crops in agriculture are grown mainly to produce bioenergy or biofuels. They can be sugar-rich crops such as sugarcane or sugar beet used for ethanol, starch crops such as maize and wheat, and oil crops such as rapeseed, soybean, or oil palm used for vegetable oils and biodiesel. There are also perennial grasses like miscanthus and switchgrass that are grown as lignocellulosic energy crops, often for direct combustion or advanced biofuel production.
Residues from livestock, such as manure, also belong to agricultural biomass. Although manure is not burned directly in many modern systems, it is a valuable feedstock for biogas production through anaerobic digestion. In that case, the original solid or semi-liquid biomass is converted into a useful gaseous fuel.
Aquatic Biomass
Aquatic biomass refers to organic material produced in water environments. It includes macroalgae, often called seaweed, and microalgae, which are microscopic organisms that can grow in fresh or salt water. It also covers other aquatic plants such as water hyacinth.
Seaweed can be harvested from natural marine areas or cultivated on purpose. It contains carbohydrates, proteins, and other compounds that can be converted into biogas, bioethanol, or other energy carriers. Microalgae can have very high growth rates and some species contain large amounts of lipids. These fats can be extracted and processed into liquid biofuels. Aquatic biomass may also be used to produce biogas after suitable pre-treatment.
Aquatic biomass has different physical and chemical properties from terrestrial biomass. It often has high moisture and sometimes high mineral content. These characteristics affect which energy conversion pathways are suitable. For this reason, aquatic biomass is sometimes treated as a distinct category within biomass energy resources.
Biomass Residues and Wastes
A large amount of potential biomass for energy does not come directly from fields or forests, but from residues and wastes generated during industrial, commercial, and municipal activities.
Industrial biomass residues arise from processes such as sawmilling, furniture production, paper manufacturing, food processing, and textile production. These residues can take many forms, including sawdust, bark, wood offcuts, rice husks, nut shells, fruit pits, and other by-products rich in organic matter. Many of these materials are suitable for combustion, gasification, or other conversion processes.
Municipal biomass wastes are components of municipal solid waste that are biodegradable. This includes food waste from households, restaurants, and food retailers, as well as garden and park waste like grass clippings, leaves, and small branches. Sewage sludge from wastewater treatment plants also contains organic material that can be used for biogas production.
Some energy systems focus specifically on biogenic fractions within mixed waste, and there are methods to estimate what portion of a waste stream is biomass based. The idea that waste can serve as a resource is central to biomass energy from residues and wastes. It connects to broader concepts of circular economy that will be explored later.
Solid, Liquid, and Gaseous Biomass Fuels
From a practical energy perspective, it is also useful to classify biomass according to its physical state at the point of use. Most original biomass resources are solid. However, through various processes, biomass can be transformed into liquid or gaseous fuels.
Solid biomass fuels are the most direct type. Firewood, charcoal, pellets, briquettes, crop residues, and many industrial by-products fall into this category. They are mainly used for heat and power generation through combustion. Some solid biomass may also be gasified, which partially converts it into a combustible gas through controlled reactions at high temperatures.
Liquid biomass-derived fuels are commonly called liquid biofuels. Typical examples are bioethanol, which is usually produced from sugars or starch, and biodiesel, which is produced from vegetable oils or animal fats. There are also more advanced liquid fuels such as hydrotreated vegetable oil and synthetic fuels derived from biomass feedstocks. These liquids can replace or blend with conventional gasoline, diesel, or jet fuel.
Gaseous biomass fuels are often grouped under the term biogas or biomethane. Biogas is generated mainly through anaerobic digestion, a biological process in which microorganisms break down wet organic matter without oxygen. Landfills and wastewater treatment plants can also produce biogas. When biogas is upgraded by removing carbon dioxide and other impurities, the result is biomethane, which has a composition similar to natural gas. Biomethane can be injected into gas grids or used as a vehicle fuel.
Key distinction: Solid biomass is the original physical form of most resources, while liquid and gaseous biofuels are usually produced by converting solid or wet biomass through physical, chemical, or biological processes.
Primary Biomass and Processed Biofuels
Another helpful distinction is between primary biomass and processed biofuels. Primary biomass refers to raw or only slightly processed materials that come directly from natural or agricultural systems. Examples include firewood, straw, and unprocessed residues. These are often used close to where they are produced.
Processed biofuels are products that have undergone more substantial transformation to create fuels with specific properties or to separate particular energy-rich components. Wood pellets, charcoal, ethanol, biodiesel, and biomethane are all processed fuels. In many cases, processing improves energy density, storage stability, or compatibility with existing energy infrastructure such as boilers, engines, and pipelines.
The transition from primary to processed biomass does not change the biological origin of the energy. However, it affects how efficiently that energy can be transported, stored, and used in modern systems. The details of these conversion processes are considered in other chapters.
Biogenic Versus Non-Biogenic Fractions
When dealing with complex waste streams, it is important to separate biogenic and non-biogenic parts. Biogenic materials come from recently living organisms, such as food waste, paper, cardboard, natural textiles like cotton or wool, and wood products. Non-biogenic materials include plastics derived from fossil fuels, metals, glass, and inert minerals.
From an energy classification viewpoint, only the biogenic fraction of waste qualifies as biomass. When mixed wastes are burned for energy, the energy released from biogenic components is often counted as renewable, while the energy from non-biogenic components is often treated as nonrenewable. This separation is relevant for greenhouse gas accounting and renewable energy statistics.
Summary
Biomass, in the context of renewable energy, is organic material of recent biological origin that can be used as a source of energy. It stores solar energy in chemical form and can be harnessed through combustion or other conversion pathways. Biomass can be organized into several main types, including woody biomass from forests and plantations, agricultural crops and residues, aquatic biomass from marine and freshwater systems, and a variety of residues and wastes resulting from industrial and municipal activities.
From a usage perspective, biomass appears as solid fuels, and through processing, as liquid and gaseous biofuels. It can be used either in relatively raw, primary forms, or as more uniform and specialized processed fuels. These basic definitions and classifications provide the foundation for exploring traditional and modern bioenergy systems, their sustainability, and their role in future energy transitions.