Bioenergy is one of many diverse resources available to help meet our demand for energy. It is a form of renewable energy that is derived from recently living organic materials known as biomass, which can be used to produce transportation fuels, heat, electricity, and products.
When we use plants and other organic material to generate energy we call it bioenergy. Bioenergy is a form of renewable energy generated when we burn biomass fuel. Biomass fuels come from organic material such as harvest residues, purpose-grown crops and organic waste from our homes, businesses and farms.
Types of bioenergy include biogas, bioethanol, and biodiesel which may be sourced from plants (corn, sugarcane), wood, agricultural wastes, and bagasse. Bioenergy is considered renewable because its source is inexhaustible, as plants obtain their energy from the sun through photosynthesis which can be replenished.
How does biomass generate energy?
When biomass is used as an energy source, it’s referred to as ‘feedstock’. Feedstocks can be grown specifically for their energy content (an energy crop), or they can be made up of waste products from industries such as agriculture, food processing or timber production.
Dry, combustible feedstocks such as wood pellets are burnt in boilers or furnaces. This in turn boils water and creates steam, which drives a turbine to generate electricity.
Wet feedstocks, like food waste for example, are put into sealed tanks where they rot and produce methane gas (also called biogas).
The gas can be captured and burnt to generate electricity. Or it can be injected into the national gas grid and be used for cooking and heating.
Is bioenergy environmentally friendly and sustainable?
Burning biomass does release carbon dioxide. But, because it releases the same amount of carbon that the organic matter used to produce it absorbed while it grew, it doesn’t break the carbon balance of the atmosphere.
In comparison, burning fossil fuels releases carbon dioxide that has been locked away for millions of years, from a time when the earth’s atmosphere was very different. This adds more carbon dioxide into our current atmosphere, breaking the carbon balance.
The overall sustainability and environmental benefits of bioenergy can depend on whether waste feedstocks or energy crops are being used.
Waste feedstocks & Energy crops
Waste biomass gives off gases naturally when it rots. If this happens in a place where there’s no oxygen, such as food waste buried deep within landfill, it can generate methane which is a much stronger greenhouse gas than carbon dioxide.
Instead of allowing methane to vent into the atmosphere, breaking it down in a sealed tank allows it to be captured and burnt. Burning methane leaves you with carbon dioxide and water, which are better for the environment.
Energy crops are grown specifically for generating energy. So, unlike capturing methane from waste, there isn’t an argument that burning them reduces greenhouse gases which would have been given off anyway.
However, energy crops can still be low carbon if they are managed sustainably. For example, when energy crops are burnt, equivalent crops should be planted that will absorb the same amount of carbon that was released by burning.
A RENEWABLE ENERGY RESOURCE
Biomass resources that are available on a renewable basis and are used either directly as a fuel or converted to another form or energy product are commonly referred to as “feedstocks.”
#1. BIOMASS FEEDSTOCKS
Biomass feedstocks include dedicated energy crops, agricultural crop residues, forestry residues, algae, wood processing residues, municipal waste, and wet waste (crop wastes, forest residues, purpose-grown grasses, woody energy crops, algae, industrial wastes, sorted municipal solid waste [MSW], urban wood waste, and food waste).
#2. DEDICATED ENERGY CROPS
Dedicated energy crops are non-food crops that can be grown on marginal land (land not suitable for traditional crops like corn and soybeans) specifically to provide biomass. These break down into two general categories: herbaceous and woody. Herbaceous energy crops are perennial (plants that live for more than 2 years) grasses that are harvested annually after taking 2 to 3 years to reach full productivity. These include switchgrass, miscanthus, bamboo, sweet sorghum, tall fescue, kochia, wheatgrass, and others. Short-rotation woody crops are fast-growing hardwood trees that are harvested within 5 to 8 years of planting. These include hybrid poplar, hybrid willow, silver maple, eastern cottonwood, green ash, black walnut, sweetgum, and sycamore. Many of these species can help improve water and soil quality, improve wildlife habitat relative to annual crops, diversify sources of income, and improve overall farm productivity.
#3. AGRICULTURAL CROP RESIDUE
There are many opportunities to leverage agricultural resources on existing lands without interfering with the production of food, feed, fiber, or forest products. Agricultural crop residues, which include the stalks and leaves, are abundant, diverse, and widely distributed across the United States. Examples include corn stover (stalks, leaves, husks, and cobs), wheat straw, oat straw, barley straw, sorghum stubble, and rice straw. The sale of these residues to a local biorefinery also represents an opportunity for farmers to generate additional income.
#4. FORESTRY RESIDUES
Forest biomass feedstocks fall into one of two categories: forest residues left after logging timber (including limbs, tops, and culled trees and tree components that would be otherwise unmerchantable) or whole-tree biomass harvested explicitly for biomass. Dead, diseased, poorly formed, and other unmerchantable trees are often left in the woods following timber harvest. This woody debris can be collected for use in bioenergy, while leaving enough behind to provide habitat and maintain proper nutrient and hydrologic features. There are also opportunities to make use of excess biomass on millions of acres of forests. Harvesting excessive woody biomass can reduce the risk of fire and pests, as well as aid in forest restoration, productivity, vitality, and resilience. This biomass could be harvested for bioenergy without negatively impacting the health and stability of forest ecological structure and function.
#5. ALGAE
Algae as feedstocks for bioenergy refers to a diverse group of highly productive organisms that include microalgae, macroalgae (seaweed), and cyanobacteria (formerly called “blue-green algae”). Many use sunlight and nutrients to create biomass, which contains key components—including lipids, proteins, and carbohydrates— that can be converted and upgraded to a variety of biofuels and products. Depending on the strain, algae can grow by using fresh, saline, or brackish water from surface water sources, groundwater, or seawater. Additionally, they can grow in water from second-use sources, such as treated industrial wastewater; municipal, agricultural, or aquaculture wastewater; or produced water generated from oil and gas drilling operations.
#6. WOOD PROCESSING RESIDUES
Wood processing yields byproducts and waste streams that are collectively called wood processing residues and have significant energy potential. For example, the processing of wood for products or pulp produces unused sawdust, bark, branches, and leaves/needles. These residues can then be converted into biofuels or bioproducts. Because these residues are already collected at the point of processing, they can be convenient and relatively inexpensive sources of biomass for energy.
#7. SORTED MUNICIPAL WASTE
MSW resources include mixed commercial and residential garbage, such as yard trimmings, paper and paperboard, plastics, rubber, leather, textiles, and food wastes. MSW for bioenergy also represents an opportunity to reduce residential and commercial waste by diverting significant volumes from landfills to the refinery.
#8. WET WASTE
Wet waste feedstocks include commercial, institutional, and residential food wastes (particularly those currently disposed of in landfills); organic-rich biosolids (i.e., treated sewage sludge from municipal wastewater); manure slurries from concentrated livestock operations; organic wastes from industrial operations; and biogas (the gaseous product of the decomposition of organic matter in the absence of oxygen) derived from any of the above feedstock streams. Transforming these “waste streams” into energy can help create additional revenue for rural economies and solve waste-disposal problems.
ENERGY FOR HEAT AND ELECTRICITY
#1. BURNING
Most electricity generated from biomass is produced by direct combustion. Biomass is burned in a boiler to produce high-pressure steam. This steam flows over a series of turbine blades, causing them to rotate. The rotation of the turbine drives a generator, producing electricity.
Biomass can also serve as substitute for a portion of coal in an existing power plant furnace in a process called co-firing (combusting two different types of materials at the same time).
#2. BACTERIAL DECOMPOSITION (ANAEROBIC DIGESTION)
Organic waste material, such as animal dung or human sewage, is collected in oxygen-free tanks called digesters. Here, the material is decomposed by anaerobic bacteria that produce methane and other byproducts to form a renewable natural gas, which can then be purified and used to generate electricity.
#3. CONVERSION TO A GAS OR LIQUID FUEL
Biomass can be converted to a gaseous or liquid fuel through gasification and pyrolysis. Gasification is a process that exposes solid biomass material to high temperatures with very little oxygen present, to produce synthesis gas (or syngas)—a mixture that consists mostly of carbon monoxide and hydrogen. The gas can then be burned in a conventional boiler to produce electricity. It can also be used to replace natural gas in a combined-cycle gas turbine.
Pyrolysis uses a similar process to gasification but under different operating conditions. In this scenario, biomass is heated at a lower temperature range but in the complete absence of oxygen to produce a crude bio-oil. This bio-oil is then substituted for fuel oil or diesel in furnaces, turbines, and engines for electricity production.
EVERYDAY COMMODITIES MADE FROM BIOMASS
#1. INTEGRATED BIOREFINERIES
A key component of developing a diverse, robust, and resilient bioeconomy is the establishment of integrated biorefineries, where biomass is converted into fuels, power, and chemicals. Chemicals and materials produced alongside biofuels can improve the overall economics of the refinery process.
For example, in the petroleum industry, almost 75% of a barrel of crude oil goes towards making fuels, corresponding to approximately $935 billion in revenue. In contrast, only 16% of a barrel of oil goes towards making petrochemicals, generating nearly as much revenue ($812 billion) as fuels, despite the much smaller volume. Applying this same strategy to the bioenergy sector could enhance the long-term economic viability of the industry.