What Are The Best Plants For Biomass Energy?
Biomass energy is renewable energy derived from organic matter, including plants, animals, and their byproducts. It is considered a renewable resource because the energy it contains comes from the sun. Through the process of photosynthesis, plants absorb energy from the sun and use it to convert water and carbon dioxide into carbohydrates and other compounds (National Geographic, 2023 https://www.nationalgeographic.org/encyclopedia/biomass-energy/). When biomass is burned, the chemical energy stored in plants is released as heat which can then be used to generate electricity.
Biomass has historically been an important source of energy and currently accounts for around 5% of U.S. energy consumption (EIA, 2023 https://www.eia.gov/energyexplained/biomass/). It is considered a clean, renewable fuel source that can help reduce dependence on fossil fuels and lower greenhouse gas emissions. This article will explore the top sources of biomass energy, properties that make plants good for biomass, and the best plant options for generating biomass power.
How Biomass Energy Works
Biomass energy starts with photosynthesis. Plants absorb sunlight and carbon dioxide, and through photosynthesis convert it into stored chemical energy in the form of cellulose and lignin. This stored energy can be released by burning the plant matter in a process called direct combustion. The heat from combustion is used to boil water, creating steam that spins a turbine to generate electricity.
The process starts by growing vegetation like trees, grasses, agricultural residues, or aquatic plants. After harvest, the biomass feedstock is transported to a power plant. It is fed into a boiler or furnace, where it is burned to produce heat. The heat turns water into steam, which spins turbines to generate power – just like a coal, natural gas, or nuclear power plant. The steam can also be used for heating or industrial processes. In some cases, the biomass may be converted to a gas before combustion. Technologies like anaerobic digestion and fermentation produce biogas, which can replace natural gas. The solids left after anaerobic digestion, called digestate, can be used as fertilizer.
The energy potential depends on the type of biomass. Woody plants, grasses, and waste products generally produce energy through direct combustion. Algae and microbial biomass need additional processing via thermochemical or biochemical conversion to optimize energy output. All biomass contains stored solar energy from photosynthesis, which gets released as heat during combustion.
Top Sources of Plant Biomass
The most common plant sources used for biomass energy include:
Wood – Wood from trees and shrubs is the largest source of biomass today. Sources of wood biomass include residues from forestry and industry, scrap wood from construction, and dedicated energy crops such as short-rotation woody plants like willow and poplar (https://www.nrel.gov/research/re-biomass.html).
Grasses – Herbaceous energy crops like switchgrass, miscanthus, and tall fescue are fast growing grasses that can produce high biomass yields while enhancing land cover. Other grass crops include wheatgrass, bamboo, and kochia (https://www.energy.gov/eere/bioenergy/biomass-resources).
Agricultural residues – Crop residues like corn stover (stalks, leaves, husks), wheat straw, and sugarcane bagasse provide abundant agricultural biomass. Processing residues like sawdust and rice husks also contribute (https://www.nationalgeographic.org/encyclopedia/biomass-energy/).
Algae – Algae can be grown specifically for energy production. Macroalgae like giant kelp, microalgae like spirulina, and algal biofuels are emerging sources of biomass (https://www.energy.gov/eere/bioenergy/biomass-resources).
Ideal Properties of Biomass Plants
For plants to be considered ideal for biomass energy applications, certain desirable traits are sought after, including:
Fast growth rate – Plants like switchgrass and willow trees that can quickly regrow after harvest are preferred. Fast regrowth allows more frequent harvests and higher overall yields. Switchgrass can produce over 15 tons per acre annually.
Low moisture content – Woody plants like trees and shrubs often have moisture contents of 20-30%, while herbaceous grasses are 50% or higher. Lower moisture is better for combustion. Moisture content can be reduced by harvesting at certain times or through drying/curing methods.
High caloric content – The heating value of various biomass materials ranges widely from less than 10 million BTU per ton for green wood to over 20 million BTU for high grade pellets. Higher caloric values yield more energy per unit weight.
Low ash and low nitrogen – Biomass high in ash (inorganic material) and nitrogen can lead to slagging, fouling, and NOx emissions when burned. Ideal plants contain less than 5% ash and 0.5% nitrogen content.
Easy harvesting – Plants that require low energy inputs for harvesting and have easily mechanized processes are preferred. Tall grasses can be easily mowed and baled for example.
Resistance to pests/disease – Plants susceptible to major pest and disease problems require more management and have lower, less predictable yields.
High yields per acre – Plants that produce the greatest amount of harvestable biomass per unit land area have an obvious advantage. Miscanthus can produce up to 25 tons per acre for example.
Other factors like soil adaptation, drought/cold tolerance, and multiple harvests per year are also important.
Top 5 Biomass Plants
There are several plants that are well-suited for biomass energy production due to their fast growth, high yield, and low maintenance requirements. Here are 5 of the top plants used for biomass:
1. Switchgrass
Switchgrass (Panicum virgatum) is a hardy, fast-growing tall grass native to North America. It can produce high yields even on marginal land with limited inputs. Switchgrass is very efficient at converting sunlight into biomass due to its photosynthetic pathway. It’s one of the dominant species used in perennial bioenergy crops.1
2. Miscanthus
Miscanthus (Miscanthus x giganteus) is a tall, fast-growing perennial grass that can grow up to 13 feet tall. Native to Asia, it produces very high yields with low inputs and limited maintenance. Miscanthus is very resilient, withstanding cold winters and dry periods. The dense root structure also prevents soil erosion.2
3. Poplar
Poplar trees and hybrids (like poplar x canadensis) are fast-growing trees well-suited for biomass production in temperate regions. Poplar produces high biomass yields from marginal lands without excessive inputs. The wood is low in extractives, making it favorable for bioenergy applications.3
4. Willow
Willow trees produce high biomass yields and grow rapidly when coppiced (cut back). Their dense root systems prevent soil erosion and require limited maintenance. Willows thrive in wet conditions, making them ideal for riparian buffers and phytoremediation sites.3
5. Sweet Sorghum
Sweet sorghum (Sorghum bicolor) is a tall, fast-growing grass that thrives in hot, dry environments. All parts of the plant can be used – the juice for biofuels, leaves and stalk for biomass, and grain for food. Sweet sorghum produces high biomass yields with low inputs of water and fertilizer.1
Growing and Harvesting Considerations
There are several important factors to consider when growing and harvesting biomass plants for energy production. Proper cultivation practices help maximize biomass yield and optimize the energy content of the plants. According to the Department of Energy, some best practices include using high-yielding plant varieties, planting at optimal spacing, and applying proper fertilization and irrigation.1
The timing of harvest is also critical. Most herbaceous energy crops should be harvested at peak maturity to achieve maximum dry matter and energy yield per acre. Woody crops are best harvested during dormant seasons to minimize moisture content and maximize energy density. Leaving some residual biomass after harvest can help conserve nutrients and organic matter in the soil.2
Efficient harvesting operations are vital for optimizing biomass supply logistics. Using properly maintained and adjusted equipment can help maximize harvest efficiency and minimize soil compaction, runoff, and erosion. On-farm storage infrastructure allows flexible delivery scheduling and preservation of biomass quality characteristics. Overall, sustainable agronomic practices coupled with efficient harvest and storage operations are key to reliable, quality biomass feedstock production.2
Processing the Biomass
Biomass needs to be processed before it can be used as an energy source. The main processing methods include drying, pelletizing, and extracting oils.
Drying reduces the moisture content of raw biomass like wood chips, agricultural residues, or animal manure. This makes the biomass lighter to transport and allows it to burn more efficiently. Rotary drum dryers or flash dryers are commonly used.
Pelletizing compresses dried biomass into dense pellets by forcing it through a die using high pressure. Wood pellets are a common example. Pelletizing increases the density of biomass so it can be handled and transported like a coal product.
Extracting plant oils from crops like soybeans and canola produces biodiesel, a liquid fuel. The raw vegetable oil is processed to remove impurities and improve fuel properties. The resulting biodiesel can replace conventional diesel in vehicles and generators.
Other common processing methods include chipping, grinding, screening, pulverizing, and briquetting. These prepare the biomass into formats that can be easily utilized for generating energy and heat.
Environmental Impact
Using biomass for energy has various environmental impacts that must be considered for sustainability. While biomass is considered a renewable energy source since plants can be regrown, there are concerns around emissions from burning biomass and impacts on land use.
When biomass is burned, carbon dioxide is released which contributes to greenhouse gas emissions and climate change. However, the key difference from fossil fuels is that the carbon in biomass comes from carbon dioxide removed from the atmosphere by the plants during photosynthesis. So biomass can be considered carbon neutral if the biomass is replanted at the same rate it is burned (EIA). Still, the timing of carbon dioxide emissions versus absorption must be considered.
Burning biomass also produces air pollutants like particulate matter, carbon monoxide, nitrogen oxides, and sulfur dioxide. These can impact local air quality but are lower than emissions from coal. Proper equipment can help control biomass emissions (EnergySage).
Large-scale biomass energy requires significant land to grow the fuel crops. This can potentially displace other uses for the land and impact biodiversity and soil health. Sustainable practices around crop rotation and organic techniques can help mitigate these effects.
The Future of Biomass
Biomass energy has the potential to play a major role in the transition to renewable energy sources in the coming decades. New technologies and applications are emerging that can improve the sustainability and viability of biomass long-term.
According to research from North Carolina State University, biomass will likely remain an important sustainable energy source over the next 20-30 years, with more efficient production processes and uses (https://cnr.ncsu.edu/news/2021/01/biomass-sustainable-energy-future/). The pulp, paper, and timber industries in particular are well-positioned to utilize biomass while improving environmental practices.
There is a need for better governance and sustainability practices in large-scale biomass production and supply chains to ensure responsible sourcing and usage. Markets and incentives for sustainable biomass energy development will also play a key role (https://www.hpprocess.com/blog/the-future-of-biomass/).
Advanced biofuels, biochemicals, bioplastics and other bioproducts are emerging from biomass feedstocks and may drive future demand. Continued technology innovation around pyrolysis, gasification, anaerobic digestion and other processes can maximize efficiency and sustainability.
While questions remain around carbon neutrality and competing uses for land and feedstocks, biomass will likely continue as a renewable, dispatchable energy source. With responsible policies and practices, biomass can play a key role in the global energy transition.
Conclusion
In summary, certain plants have emerged as top sources of biomass for energy due to their high yields, low costs, and sustainable properties. Energy crops like switchgrass, Miscanthus, and fast-growing trees can produce abundant biomass to be converted into solid, liquid, and gaseous biofuels through various processes. While promising, plant biomass does have some limitations and drawbacks to consider regarding land use, transportation, and full life cycle emissions.
Overall, plant biomass represents a renewable, carbon-neutral energy source that can supplement fossil fuels in the transition to a more sustainable energy future. With continued research and development, biomass plants and conversion technologies will become even more efficient and available. While not a total solution, bioenergy from plant biomass can form one part of a diversified energy mix based on local resources and infrastructure.
With proper planning and management, plant biomass energy can provide clean power and fuels while supporting rural economies and environmental goals. Though the full potential has yet to be unlocked, plant biomass remains one of the most versatile and practical sources of renewable energy available today.
