What is Biomass?


Biomass is biological material derived from living, or recently living organisms. In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material.

Chemical composition
Biomass is carbon based and is composed of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, often nitrogen and also small quantities of other atoms, including alkali, alkaline earth and heavy metals.  These metals are often found in functional molecules such as the porphyrins which include chlorophyll which contains magnesium.

Plant material
The carbon used to construct biomass is absorbed from the atmosphere as carbon dioxide (CO2) by plant life, using energy from the sun. Plants may subsequently be eaten by animals and thus converted into animal biomass. However the primary absorption is performed by plants. If plant material is not eaten it is generally either broken down by microrganisms or burned: If broken down it releases the carbon back to the atmosphere, mainly as either carbon dioxide (CO2) or methane (CH4), depending upon the conditions and processes involved If burned the carbon is returned to the atmosphere as CO2. These processes have happened for as long as there have been plants on Earth and is part of what is known as the carbon cycle.

Fossil fuels
Fossil fuels such as coal, oil and gas are also derived from biological material, however material that absorbed CO2 from the atmosphere many millions of years ago.

As fuels they offer high energy density, but making use of that energy involves burning the fuel, with the oxidation of the carbon-to-carbon dioxide and the hydrogen to water (vapor).  Unless they are captured and stored, these combustion products are usually released to the atmosphere, returning carbon sequestered millions of years ago and thus contributing to increased atmospheric concentrations.

The difference between biomass and fossil fuels
The vital difference between biomass and fossil fuels is one of time scale. Biomass takes carbon out of the atmosphere while it is growing, and returns it as it is burned.  If it is managed on a sustainable basis, biomass is harvested as part of a constantly replenished crop. This is either during woodland or arboricultural management or coppicing or as part of a continuous program of replanting with the new growth taking up CO2 from the atmosphere at the same time as it is released by combustion of the previous harvest. This maintains a closed carbon cycle with no net increase in atmospheric CO2 levels.

Categories of biomass materials
Within this definition, biomass for energy can include a wide range of materials. The realities of the economics mean that high value material for which there is an alternative market, such as good quality, large timber, are very unlikely to become available for energy applications.  However there are huge resources of residues, co-products and waste that exist in the UK, which could potentially become available, in quantity, at relatively low cost, or even negative cost where there is currently a requirement to pay for disposal.

There are five basic categories of material:
• Virgin wood, from forestry, arboricultural activities or from wood processing
• Energy crops: high yield crops grown specifically for energy applications
• Agricultural residues: residues from agriculture harvesting or processing
• Food waste, from food and drink manufacture, preparation and processing, and post-consumer waste
• Industrial waste and co-products from manufacturing and industrial processes.

Source: The BIOMASS Energy Centre & UK Forestry Commission. biomassenergycentre.org.uk

How Biomass Energy Works

To many people, the most familiar forms of renewable energy are the wind and the sun. But biomass (plant material and animal waste) is the oldest source of renewable energy, used since our ancestors learned the secret of fire.

Until recently, biomass supplied far more renewable electricity or bio-power than wind and solar power combined.

If developed properly, biomass can and should supply increasing amounts of bio-power. In fact, in numerous analyses of how America can transition to a clean energy future, sustainable biomass is a critical renewable resource.

Sustainable, low-carbon biomass can provide a significant fraction of the new renewable energy we need to reduce our emissions of heat-trapping gases like carbon dioxide to levels that scientists say will avoid the worst impacts of global warming. Without sustainable, low-carbon bio-power, it will likely be more expensive and take longer to transform to a clean energy economy.

But like all our energy sources, bio-power has environmental risks that need to be mitigated. If not managed carefully, biomass for energy can be harvested at unsustainable rates, damage ecosystems, produce harmful air pollution, consume large amounts of water, and produce net greenhouse emissions.

However, most scientists believe there is a wide range of biomass resources that can be produced sustainable and with minimal harm, while reducing the overall impacts and risks of our current energy system. Implementing proper policy is essential to securing the benefits of biomass and avoiding its risks.

Based on our bio-energy principles, UCS work on bio-power is dedicated to distinguishing between beneficial biomass resources and those that are questionable or harmful in a practical and efficient manner so that beneficial resources can make a significant contribution to our clean energy future.

Note: This page addresses using biomass to generate bio-power. For more information on bio-fuels, go to the UCS Clean Vehicles Programs bio-fuels pages.

Biomass is a renewable energy source not only because the energy it comes from the sun, but also because biomass can re-grow over a relatively short period of time. Through the process of photosynthesis, chlorophyll in plants captures the sun's energy by converting carbon dioxide from the air and water from the ground into carbohydrates complex compounds composed of carbon, hydrogen, and oxygen.

When these carbohydrates are burned, they turn back into carbon dioxide and water and release the energy they captured from the sun. In this way, biomass functions as a sort of natural battery for storing solar energy. As long as biomass is produced sustainable meeting current needs without diminishing resources or the lands capacity to re-grow biomass and recapture carbon the battery will last indefinitely and provide sources of low-carbon energy.

Source: The Union of Concerned Scientists ucsusa.org

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