People have used biomass energy—energy from living things—since the earliest hominids first made wood fires for cooking or keeping warm.

Biomass is organic, meaning it is made of material that comes from living organisms, such as plants and animals. The most common biomass materials used for energy are plants, wood, and waste. These are called biomass feedstocks. Biomass energy can also be a nonrenewable energy source.

Biomass contains energy first derived from the sun: Plants absorb the sun’s energy through photosynthesis, and convert carbon dioxide and water into nutrients (carbohydrates).

The energy from these organisms can be transformed into usable energy through direct and indirect means. Biomass can be burned to create heat (direct), converted into electricity (direct), or processed into biofuel (indirect).

Thermal Conversion

Biomass can be burned by thermal conversion and used for energy. Thermal conversion involves heating the biomass feedstock in order to burn, dehydrate, or stabilize it. The most familiar biomass feedstocks for thermal conversion are raw materials such as municipal solid waste (MSW) and scraps from paper or lumber mills.

Different types of energy are created through direct firing, co-firing, pyrolysis, gasification, and anaerobic decomposition.

Before biomass can be burned, however, it must be dried. This chemical process is called torrefaction. During torrefaction, biomass is heated to about 200° to 320° Celsius (390° to 610° Fahrenheit). The biomass dries out so completely that it loses the ability to absorb moisture, or rot. It loses about 20 percent of its original mass, but retains 90 percent of its energy. The lost energy and mass can be used to fuel the torrefaction process.

During torrefaction, biomass becomes a dry, blackened material. It is then compressed into briquettes. Biomass briquettes are very hydrophobic, meaning they repel water. This makes it possible to store them in moist areas. The briquettes have high energy density and are easy to burn during direct or co-firing.

Direct Firing and Co-Firing
Most briquettes are burned directly. The steam produced during the firing process powers a turbine, which turns a generator and produces electricity. This electricity can be used for manufacturing or to heat buildings.

Biomass can also be co-fired, or burned with a fossil fuel. Biomass is most often co-fired in coal plants. Co-firing eliminates the need for new factories for processing biomass. Co-firing also eases the demand for coal. This reduces the amount of carbon dioxide and other greenhouse gases released by burning fossil fuels.

Pyrolysis
Pyrolysis is a related method of heating biomass. During pyrolysis, biomass is heated to 200° to 300° C (390° to 570° F) without the presence of oxygen. This keeps it from combusting and causes the biomass to be chemically altered.

Pyrolysis produces a dark liquid called pyrolysis oil, a synthetic gas called syngas, and a solid residue called biochar. All of these components can be used for energy.

Pyrolysis oil, sometimes called bio-oil or biocrude, is a type of tar. It can be combusted to generate electricity and is also used as a component in other fuels and plastics. Scientists and engineers are studying pyrolysis oil as a possible alternative to petroleum.

Syngas can be converted into fuel (such as synthetic natural gas). It can also be converted into methane and used as a replacement for natural gas.

Biochar is a type of charcoal. Biochar is a carbon-rich solid that is particularly useful in agriculture. Biochar enriches soil and prevents it from leaching pesticides and other nutrients into runoff. Biochar is also an excellent carbon sink. Carbon sinks are reservoirs for carbon-containing chemicals, including greenhouse gases.

Gasification
Biomass can also be directly converted to energy through gasification. During the gasification process, a biomass feedstock (usually MSW) is heated to more than 700° C (1,300° F) with a controlled amount of oxygen. The molecules break down, and produce syngas and slag.

Syngas is a combination of hydrogen and carbon monoxide. During gasification, syngas is cleaned of sulfur, particulates, mercury, and other pollutants. The clean syngas can be combusted for heat or electricity, or processed into transportation biofuels, chemicals, and fertilizers.

Slag forms as a glassy, molten liquid. It can be used to make shingles, cement, or asphalt.

Industrial gasification plants are being built all over the world. Asia and Australia are constructing and operating the most plants, although one of the largest gasification plants in the world is currently under construction in Stockton-on-Tees, England. This plant will eventually be able to convert more than 350,000 tons of MSW into enough energy to power 50,000 homes.

Anaerobic Decomposition
Anaerobic decomposition is the process where microorganisms, usually bacteria, break down material in the absence of oxygen. Anaerobic decomposition is an important process in landfills, where biomass is crushed and compressed, creating an anaerobic (or oxygen-poor) environment.

In an anaerobic environment, biomass decays and produces methane, which is a valuable energy source. This methane can replace fossil fuels.

In addition to landfills, anaerobic decomposition can also be implemented on ranches and livestock farms. Manure and other animal waste can be converted to sustainably meet the energy needs of the farm.

Biofuel

Biomass is the only renewable energy source that can be converted into liquid biofuels such as ethanol and biodiesel. Biofuel is used to power vehicles, and is being produced by gasification in countries such as Sweden, Austria, and the United States.

Ethanol is made by fermenting biomass that is high in carbohydrates, such as sugarcane, wheat, or corn. Biodiesel is made from combining ethanol with animal fat, recycled cooking fat, or vegetable oil.

Biofuels do not operate as efficiently as gasoline. However, they can be blended with gasoline to efficiently power vehicles and machinery, and do not release the emissions associated with fossil fuels.

Ethanol requires acres of farmland to grow biocrops (usually corn). About 1,515 liters (400 gallons) of ethanol is produced by an acre of corn. But this acreage is then unavailable for growing crops for food or other uses. Growing enough corn for ethanol also creates a strain on the environment because of the lack of variation in planting, and the high use of pesticides.

Ethanol has become a popular substitute for wood in residential fireplaces. When it is burned, it gives off heat in the form of flames, and water vapor instead of smoke.

Biochar

Biochar, produced during pyrolysis, is valuable in agricultural and environmental use.

When biomass rots or burns (naturally or by human activity), it releases high amounts of methane and carbon dioxide into the atmosphere. However, when biomass is charred, it sequesters, or stores, its carbon content. When biochar is added back to the soil, it can continue to absorb carbon and form large underground stores of sequestered carbon—carbon sinks—that can lead to negative carbon emissions and healthier soil.

Biochar also helps enrich the soil. It is porous. When added back to the soil, biochar absorbs and retains water and nutrients.

Biochar is used in Brazil’s Amazon rainforest in a process called slash-and-char. Slash-and-char agriculture replaces slash-and-burn, which temporarily increases the soil nutrients but causes it to lose 97 percent of its carbon content. During slash-and-char, the charred plants (biochar) are returned to the soil, and the soil retains 50 percent of its carbon. This enhances the soil and leads to significantly higher plant growth.

Black Liquor

When wood is processed into paper, it produces a high-energy, toxic substance called black liquor. Until the 1930s, black liquor from paper mills was considered a waste product and dumped into nearby water sources.

However, black liquor retains more than 50 percent of the wood’s biomass energy. With the invention of the recovery boiler in the 1930s, black liquor could be recycled and used to power the mill. In the United States, paper mills use nearly all their black liquor to run their mills, and the forest industry is one of the most energy-efficient in the nation as a result.

More recently, Sweden has experimented in gasifying black liquor to produce syngas, which can then be used to generate electricity.

Hydrogen Fuel Cells

Biomass is rich in hydrogen, which can be chemically extracted and used to generate power and to fuel vehicles. Stationary fuel cells are used to generate electricity in remote locations, such as spacecraft and wilderness areas. Yosemite National Park in the U.S. state of California, for example, uses hydrogen fuel cells to provide electricity and hot water to its administration building.

Hydrogen fuel cells may hold even more potential as an alternative energy source for vehicles. The U.S. Department of Energy estimates that biomass has the potential to produce 40 million tons of hydrogen per year. This would be enough to fuel 150 million vehicles.

Currently, hydrogen fuel cells are used to power buses, forklifts, boats, and submarines, and are being tested on airplanes and other vehicles.

However, there is a debate as to whether this technology will become sustainable or economically possible. The energy that it takes to isolate, compress, package, and transport the hydrogen does not leave a high quantity of energy for practical use.

Biomass and the Environment

Biomass is an integral part of Earth’s carbon cycle. The carbon cycle is the process by which carbon is exchanged between all layers of Earth: atmosphere, hydrosphere, biosphere, and lithosphere.

The carbon cycle takes many forms. Carbon helps regulate the amount of sunlight that enters Earth’s atmosphere. It is exchanged through photosynthesis, decomposition, respiration, and human activity. Carbon that is absorbed by soil as an organism decomposes, for example, may be recycled as a plant releases carbon-based nutrients into the biosphere through photosynthesis. Under the right conditions, the decomposing organism may become peat, coal, or petroleum before being extracted through natural or human activity.

Between periods of exchange, carbon is sequestered, or stored. The carbon in fossil fuels has been sequestered for millions of years. When fossil fuels are extracted and burned for energy, their sequestered carbon is released into the atmosphere. Fossil fuels do not reabsorb carbon.

In contrast to fossil fuels, biomass comes from recently living organisms. The carbon in biomass can continue to be exchanged in the carbon cycle.

In order to effectively allow Earth to continue the carbon cycle process, however, biomass materials such as plants and forests have to be sustainably farmed. It takes decades for trees and plants such as switchgrass to reabsorb and sequester carbon. Uprooting or disturbing the soil can be extremely disruptive to the process. A steady and varied supply of trees, crops, and other plants is vital for maintaining a healthy environment.

Algal Fuel

Algae is a unique organism that has enormous potential as a source of biomass energy. Algae, whose most familiar form is seaweed, produces energy through photosynthesis at a much quicker rate than any other biofuel feedstock—up to 30 times faster than food crops!

Algae can be grown in ocean water, so it does not deplete freshwater resources. It also does not require soil, and therefore does not reduce arable land that could potentially grow food crops. Although algae releases carbon dioxide when it is burned, it can be farmed and replenished as a living organism. As it is replenished, it releases oxygen, and absorbs pollutants and carbon emissions.

Algae takes up much less space than other biofuel crops. The U.S. Department of Energy estimates that it would only take approximately 38,850 square kilometers (15,000 square miles, an area less than half the size of the U.S. state of Maine) to grow enough algae to replace all petroleum-fueled energy needs in the United States.

Algae contains oils that can be converted to a biofuel. At the Aquaflow Bionomic Corporation in New Zealand, for example, algae is processed with heat and pressure. This creates a “green crude,” which has similar properties to crude oil, and can be used as a biofuel.

Algae’s growth, photosynthesis, and energy production increases when carbon dioxide is bubbled through it. Algae is an excellent filter that absorbs carbon emissions. Bioenergy Ventures, a Scottish firm, has developed a system in which carbon emissions from a whiskey distillery are funneled to an algae pool. The algae flourishes with the additional carbon dioxide. When the algae die (after about a week) they are collected, and their lipids (oils) are converted into biofuel or fish food.

Algae has enormous potential as an alternative energy source. However, processing it into usable forms is expensive. Although it is estimated to yield 10 to 100 times more fuel than other biofuel crops, in 2010 it cost $5,000 a ton. The cost will likely come down, but it is currently out of reach for most developing economies.

People and Biomass

Advantages
Biomass is a clean, renewable energy source. Its initial energy comes from the sun, and plants or algae biomass can regrow in a relatively short amount of time. Trees, crops, and municipal solid waste are consistently available and can be managed sustainably.

If trees and crops are sustainably farmed, they can offset carbon emissions when they absorb carbon dioxide through respiration. In some bioenergy processes, the amount of carbon that is reabsorbed even exceeds the carbon emissions that are released during fuel processing or usage.

Many biomass feedstocks, such as switchgrass, can be harvested on marginal lands or pastures, where they do not compete with food crops.

Unlike other renewable energy sources, such as wind or solar, biomass energy is stored within the organism, and can be harvested when it is needed.

Disadvantages
If biomass feedstocks are not replenished as quickly as they are used, they can become nonrenewable. A forest, for instance, can take hundreds of years to re-establish itself. This is still a much, much shorter time period than a fossil fuel such as peat. It can take 900 years for just a meter (three feet) of peat to replenish itself.

Most biomass requires arable land to develop. This means that land used for biofuel crops such as corn and soybeans are unavailable to grow food or provide natural habitats.

Forested areas that have matured for decades (so-called “old-growth forests”) are able to sequester more carbon than newly planted areas. Therefore, if forested areas are not sustainably cut, re-planted, and given time to grow and sequester carbon, the advantages of using the wood for fuel are not offset by the trees’ regrowth.

Most biomass plants require fossil fuels to be economically efficient. An enormous plant under construction near Port Talbot, Wales, for instance, will require fossil fuels imported from North America, offsetting some of the sustainability of the enterprise.

Biomass has a lower “energy density” than fossil fuels. As much as 50 percent of biomass is water, which is lost in the energy conversion process. Scientists and engineers estimate that it is not economically efficient to transport biomass more than 160 kilometers (100 miles) from where it is processed. However, converting biomass into pellets (as opposed to wood chips or larger briquettes) can increase the fuel’s energy density and make it more advantageous to ship.

Burning biomass releases carbon monoxide, carbon dioxide, nitrogen oxides, and other pollutants and particulates. If these pollutants are not captured and recycled, burning biomass can create smog and even exceed the number of pollutants released by fossil fuels.