When you think of renewable energy, photovoltaic panels and wind turbines are likely the images that come to mind. But there's more to renewable energy than solar and wind. Biomass is another earth-friendly source of energy that could help replace environmentally harmful fossil fuels like oil and coal. But what is biomass, and how can it change our energy future for the better?

In short, biomass is organic material made by living organisms that contains stored energy from the sun. Plants absorb radiant energy from sunlight and then convert it into chemical energy in the form of glucose, or sugar. This energy is passed on to people and animals that consume the plant matter. The chemical energy from biomass is released as heat when burned.

Types of biomass include wood, crops, landfill gas, alcohol fuels and trash. Biomass can either be a waste product or grown specifically for energy in the form of crops like hemp, corn, poplar, willow, sorghum, switchgrass and sugarcane.

According to the U.S. Energy Information Administration, biomass fuels provided about 4 percent of the energy used in the United States in 2010. About 46 percent of that amount was from wood or wood-derived biomass such as wood chips and sawdust; 43 percent came from biofuels like ethanol, and 11 percent was sourced from municipal waste.

Biomass boiler in SeattleHow biomass energy works

Biomass is converted to clean, efficient "biopower" through a variety of processes including direct combustion, co-firing, re-powering, combined heat and power (CHP), gasification and anaerobic digestion.

Direct combustion is the simplest and most obvious means of obtaining energy from biomass; our ancestors have been doing it since the dawn of humanity in the form of wood fires. Other methods, however, are more efficient and less likely to pollute the air. Co-firing mixes biomass with coal at coal-fired power plants, which may offer a transitional means of somewhat cleaner energy until infrastructure for truly renewable energy is in place. "Re-powering" is when coal plants are converted to run entirely on biomass.

When direct combustion is used both to heat a building and to produce energy, that process is called "combined heat and power." Biomass gasification involves heating biomass under pressure in the presence of a very small, tightly controlled amount of oxygen and converting it to a mixture of hydrogen and carbon monoxide called "syngas;" this gas can be run through a gas turbine or burned and run through a steam turbine to create electricity.

Finally, anaerobic digestion utilizes microorganisms to break down biomass in a controlled environment to produce the greenhouse gases methane and carbon dioxide. Used to process sewage, animal manure and landfill waste, this biomass production method uses the resulting methane for heat and power and prevents it from escaping to the atmosphere.

Pros and cons of biomass

The main cons of biomass lie in how it's used. It does come with environmental risks. The Union of Concerned Scientists explains that biomass produced for energy can potentially be harvested at unsustainable rates, cause damage to ecosystems, produce harmful air pollution, consume large amounts of water and produce net greenhouse gas emissions. However, these risks are mitigated when biomass is managed properly. Energy crops should never compete with food crops for land, and emissions of biomass carbon should be taken up or recycled by subsequent plant growth.

Most scientists believe that there are a wide range of beneficial biomass resources that will reduce overall carbon emissions. Growing beneficial biomass crops can maintain or even increase the stocks of carbon stored in soil or plants. Energy crops, particularly those that are native to the region in which they are grown, can be produced on marginal land. Many varieties, like switchgrass, grow quickly and are therefore highly renewable.

Byproducts like manure, methane gas from landfills, wood pulp from sawmills and paper mills and urban waste including tree trimmings and biodegradable household trash can be used to generate biomass energy. Such a use takes these products out of the waste stream, giving them value.

Have other thoughts on biomass? Leave us a note in the comments below.

Photo: Joe Mabel/Wikimedia Commons

See also:

What is carbon capture?

What is switchgrass?

What is clean coal?