Bioenergy 101

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• What Is Bioenergy?
• What Is Biomass?
• How Is Bioenergy Made?
• What Can We Use to Make Bioenergy?
• Common Forms of Bioenergy
• The Future of Green Energy

As the world’s population and standards of living in developing countries increase at a rapid pace, so does demand for energy. And because of our reliance on dirty fossil fuels, global average temperatures are climbing as well—making the search for a sustainable way to power our cars, heat our homes, and turn on our lights more urgent than ever.

Bioenergy, a form of energy that is ostensibly renewable and relies on the power of plants and animals, has been touted as a solution. But bioenergy’s green reputation is misleading. It requires inputs and trade-offs that are costly for our ecosystems and our climate. Here’s what you need to know.

What Is Bioenergy?

Bioenergy is an umbrella term for any energy made from living (or recently living) plants and animals. It’s been around nearly as long as we have; humans invented bioenergy when we first burned wood to keep warm.

What Is Biomass?

Biomass is bioenergy’s source material, and it varies widely. It ranges from wood and corn (most commonly) to manure, food waste, and even seaweed. But the basic principle is the same: Plants absorb energy from the sun through photosynthesis, and we can harness that energy for our own uses. (Animals absorb—and excrete—that same energy by eating plants, or eating animals that have eaten plants, and so on.)

How Is Bioenergy Made?

Methods for extracting the energy stored in biomass range from simple systems, like a backyard compost pile that generates mild heat as it decomposes, to complex modern technologies capable of processing large quantities of industrial waste and producing enough electricity to power whole towns. But all the methods rely on only a handful of physical processes:

Direct Combustion

As with the original wood-fueled fires, we can use direct combustion to simply burn biomass and generate heat. We can then use this heat directly to, say, warm buildings, as in the case of a biomass boiler room. We can also use this heat to boil water and create steam, which powers turbine generators to produce electricity.

Gasification

This method again uses heat, but with a limited oxygen supply. Biomass like wood chips or municipal waste—meaning garbage including food scraps, animal and plant products, paper, glass, metal, and plastics—is incinerated in a “gasifier,” the specifications of which depend on the type of biomass being used. Under these conditions, biomass becomes “syngas,” which can drive gas turbines. Gasification is considered more environmentally friendly than direct combustion, as it emits fewer pollutants.

Anaerobic Digestion

A fancy name for rotting, anaerobic digestion is simply the breaking down of biomass by bacteria in the absence of oxygen. That biomass is often agricultural waste—say, corncobs or plant stalks—or landfill waste, which is anaerobically digested as garbage degrades. The by-product is methane and carbon dioxide, which can be purified to make biomethane and used as fuel in vehicles. At landfills, piles of waste can be covered with clay to create an anaerobic environment and outfitted with pipes to capture and funnel the gas from inside.

Fermentation

Sugars and starches—found in crops like corn, sugarcane, and beets—can be fermented to make biofuels, such as ethanol, butanol, or methanol, to power vehicles. Cellulosic plants—like wood, grasses, and the stems and stalks of crops—can also be fermented into fuel alcohols, though it takes extra steps to do so.

What Can We Use to Make Bioenergy?

As mentioned above, biomass varies widely, especially as we continue to find cheaper and more sustainable alternatives. Here are the most common source materials:

Wood and Wood Waste

For hundreds of thousands of years, wood was our primary form of fuel for heating and cooking, making it the longest-used source of bioenergy. Today we still use wood, to produce heat and electricity. In 2017, wood and wood-derived biomass accounted for 44 percent of all U.S. biomass fuels. (In fact, people often use the term “biomass” to refer to “wood biomass” specifically.)

The wood used for bioenergy can be residue from other industries—like pellets made from the remains of the paper industry. Paper mills will sometimes use their own leftover wood to produce some of the electricity needed for production. But just as often, wood bioenergy comes from clear-cutting forests, which not only is extremely carbon inefficient but destroys a valuable carbon sequestering resource.

Municipal Solid Waste

Municipal solid waste includes a wide array of materials, including food scraps, paper products, and petroleum-based plastics. More than half of the municipal solid waste in the United States winds up in a landfill. Nearly 13 percent is combusted to produce energy, as of 2015. Additionally, most large landfills in the United States are required to capture methane from decomposing organic matter; this methane can also be used for energy production.

Although these strategies sound efficient, in fact, generating power this way is an environmental misstep. Recycling, or capturing materials from the waste stream and returning them to production or nutrient cycles (in other words, using them in manufacturing or allowing their nutrients to flow back into a living organism or the environment) is much better for the planet. We should direct our focus and resources away from solid waste as an energy source and toward reducing waste in the first place, and after that, recycling or composting as much as possible.

Agricultural crops

Food crops—like corn, sugarcane, or beets—can be turned into fuel for vehicles. The most common end product is ethanol, an alcohol fuel made by fermenting the sugars and starches from plants and then distilling them. (New technologies also allow ethanol to be made from woody plants, like trees, grasses, and crop by-products.) Soybean oils can be made into another renewable transportation fuel: biodiesel.

Recycled oils and animal fat

Leftover fats—like used vegetable oils, animal fat, and grease from restaurant kitchens—can all be used to make biodiesel. The final product is made via a chemical reaction called transesterification—or the reaction of a fat molecule with an alcohol.

Common Forms of Bioenergy

Just as there are many types of biomass, bioenergy’s end products also vary widely. Here are its most common final forms:

Biofuels

Biofuels are intended as an alternative to petroleum for the transportation sector. The aim of their developers and proponents is to replace, or at least offset, petroleum-based gasoline and diesel. Compared with producing fuel from destructive oil drilling, producing it from plants like corn seems like a golden (and green) idea. But not all biofuels are created equal. When greenhouse gas emissions from the full lifecycle of different biofuels are accounted for, and when we consider their impact on water, soil, biodiversity, and food production, replacing gasoline and diesel with biofuels in most contexts becomes much less attractive as a sustainable solution.

The most common liquid biofuels are ethanol and biodiesel. Ethanol is a fuel alcohol made by fermenting sugars and starches from plants, typically corn, soybeans, and sugarcane. Most gasoline in the United States is already mixed with ethanol (typically a 10 percent blend), though mixtures with higher ethanol content can be used in certain “flex fuel” vehicles.

To address some of the environmental issues associated with biofuels, more advanced forms of ethanol that rely on nonfood biomass are being developed, but their cost is currently too prohibitive for them to be considered a viable replacement for petroleum fuel.

Biodiesel is a renewable diesel fuel made from fats like vegetable oil, animal fats, grease, and even recycled oil from restaurants. Biodiesel has about 90 percent of the energy density of petroleum diesel. It is also a cleaner-burning fuel that emits no sulfur, a by-product of burning fossil fuels that contributes to acid rain.

Biodiesel runs only in diesel engines, which are still a relatively small part of the transportation market. And it is still about 150 percent more expensive to produce than petroleum gasoline.

Biopower

Under the umbrella of bioenergy, biopower refers specifically to electricity produced by biomass—most often trees. As mentioned, wood and wood waste can be fed into biopower plants. The heat released by burning the wood material drives turbines, which produce electricity. Because of its potential impact on forest ecosystems, biopower is one of the most controversial forms of bioenergy production.

Biopower advocates make a case for biopower that’s similar to their case for other forms of bioenergy—namely, that it’s renewable and “carbon neutral.” Carbon neutral essentially means that the amount of carbon dioxide a plant or tree absorbs from the atmosphere while growing is equivalent to the amount it releases when burned as fuel. (Carbon dioxide is the primary enemy in the fight against global warming. It stays in our atmosphere and acts as a heat trap for the sun’s radiation, triggering a cascade of serious climate reactions.) Compared with dirty fossil fuels, which release carbon without capturing it, biopower certainly seems like a step up. And trees can always grow back, right?