The Benefits and Limitations of Bioenergy

As the human population continues to grow, so does our need for reliable energy sources. There is also an increased focus on finding energy sources that are not just reliable but also renewable. In addition to advances in solar and wind power as sustainable energy sources, some industries have turned to bioenergy as a means of producing fuels for transportation, heat, and electricity. While bioenergy hardly represents a “silver bullet” to our society’s energy woes, there are benefits to improving bioenergy industries and expanding research in the field.

What is bioenergy and biomass?

 Bioenergy is derived from organic matter that is directly or indirectly produced via photosynthesis, referred to as biomass. Derived from plant- and algae-based materials, biomass is typically converted into a fuel (biofuel) or  another form of energy (biopower) or product (bioproduct). In addition to being comparable to many fossil-based energy sources, converting biomass into energy allows for the reuse of carbon waste products to become a renewable power source.

All resources that provide a renewable source of biomass that is then converted into bioenergy, either through biofuels or other products, is called a “feedstock.” The primary feedstocks are: 

1. Dedicated energy crops (crops grown specifically to provide biomass) 
2. Agricultural crop residues (agricultural crop waste such as stalks) 
3. Forestry residues (waste from logging timber such as limbs and dead unmerchantable trees) 
4. Algae (harvested microalgae and cyanobacteria) 
5. Wood processing residues (waste from wood processing such as sawdust) 
6. Municipal waste (commercial and residential garbage) 
7. Wet waste (commercial and residential food waste). 

Once collected, biomass can be directly converted into liquid biofuel, the most common of which is ethanol and biodiesel.  

How do biofuels work?

Made from plant material, ethanol is an alcohol that can be blended with gasoline at different ratios to decrease harmful carbon emissions in most conventional, gasoline-powered vehicles. The most common method for extracting ethanol from biomass is through the fermentation of plant starches and sugars; however, scientists are developing ways to also use the non-edible cellulose of plants for this process. 

After the conversion is complete, the ethanol is mixed with traditional fossil fuels, i.e., gasoline, at various concentrations. This typically ranges from 10 percent ethanol and 90 percent gasoline to 15 percent ethanol and 85 percent gasoline but can be as high as 50 to 80 percent ethanol in special cases. Mixing ethanol with gasoline is still a necessary step since ethanol is roughly 30 percent less efficient than gasoline, meaning more pure ethanol is required to fuel the same milage as gasoline. Additionally, using pure ethanol can require extensive modifications that most engines don’t yet have, such as adjustments to the carburetor and intake manifold as well as installing a cold starting system⁶. While ethanol may be less efficient than gasoline, by adding it, users increase the octane number of the fuel, which yields higher performance in combustion engines.  

Biodiesel is the other main form of biofuel that is made via a process called transesterification of vegetable oils and animal fats, converting them into a fuel similar to diesel. Biodiesel is more commonly used as a direct replacement to diesel fuel in vehicles than ethanol is in gasoline powered vehicles. Despite this, biodiesel output is still lower than that of gasoline and, much like ethanol, the more biodiesel in the fuel blend, the more modifications need to be made to the engine. Pure biodiesel can also cause long-term maintenance issues.

In addition to being converted into biofuels for transportation, biomass can also be converted into biopower via burning, bacterial decomposition, or conversion into a liquid or gas fuel. The direct burning of biomass to produce high-pressure steam is used to drive turbine generators either on their own or as a substitute for a portion of coal in existing power plants. Alternatively, organic waste biomass, such as sewage, can be decomposed using bacteria to provide a renewable form of methane gas that can then be purified and used to generate electricity. Finally, by taking advantage of gasification and pyrolysis, biomass can be converted into a gaseous or liquid fuel to be used in conventional boilers or furnaces.

Benefits and limitations

As with most technologies, there are strengths and weaknesses to utilizing bioenergy. The main benefits are that it is renewable, helps with waste reduction, and has high reliability as an energy source. 

The abundance of biomass and how quickly it gets replenished means that, unlike fossil fuels, it is a highly renewable energy source. Additionally, much of the biomass that gets used to convert into bioenergy is typically a waste product from other industries. This means that by diverting it away from landfills to be converted to energy, we can help decrease the size of landfills and help eliminate some of the risks associated with the decomposition of organic matter in uncontrolled areas. Finally, unlike many other renewable energy sources, bioenergy is not intermittent or variable. Energy plants that use biomass can be turned on or off depending on power need rather than source availability. These strengths have helped make bioenergy one of the main sources of renewable energy. 

However, there are downsides to bioenergy. In addition to the typical costs of getting a plant up and running, there are other expenditures around the extraction, transportation, and storage of biomass. Most other renewable power sources rely on onsite resources and therefore do not incur these added costs. Another concern is space. Bioenergy plants require a large amount of land to operate effectively. This combined with transport costs can severely limit where these plants can be placed. Lastly, bioenergy does have some environmental drawbacks. These can range from the environmental costs associated with building the plants to the damage caused by growing crops to use as biomass. Even though bioenergy is more environmentally friendly than burning fossil fuels, it also expels pollutants into the air, such as carbon dioxide and volatile organic compounds. 

Current research

Much of the current research around bioenergy is focused on improving its efficiency and making it more competitive with fossil fuels. Research done by Chang et al. has been exploring how to develop highly efficient direct ethanol fuel cells^1,2 and has achieved record-setting power density. This would allow providers to circumvent the mixing step of developing biofuels, eliminating the need for gasoline. In a recent press release, co-author and associate professor Yang Yang said: “Our research enables direct ethanol fuel cells to compete with hydrogen-fuel cells and batteries in various sustainable energy fields, which have not yet been achieved before our invention.” Yang added: “Ethanol is a clean and safe biofuel in the liquid phase, which is much easier and safer for storage and transport than pure hydrogen. Compared to the technology to extract hydrogen from ethanol and then convert hydrogen to electricity, our technology can directly convert ethanol into electricity, so it is an overall positive energy balance and negative emission technology.” 

The US Department of Energy’s (DOE) Argonne National Laboratory has also been working toward building better biofuels in recent years. Partnering with the National Renewable Energy Laboratory, Pacific Northwest National Laboratory, and Idaho National Laboratory, the DOE is trying to develop new blends of conventional and biofuels to make them more cost-competitive and climate friendly. In a press release Troy Hawkins, Argonne’s group manager, said: “Our goal was to develop new biofuels blended with conventional fuels to improve engine performance. This means a gasoline-powered car or truck could go further on the same amount of fuel. Or a diesel vehicle could meet more stringent emissions standards.” 

Other research has focused on feedstock characterization and bioengineering^{4, 5, 9}. Some of the most current work looks to assess the feasibility of long-term uses of bioenergy^{3, 7}. This research finds that bioenergy offers promising short-term benefits with regards to climate change mitigation and renewable energy but cautions policymakers to not focus on bioenergy as an exclusive long-term solution and instead keep investing in a variety of next-generation renewable energy technologies.

Biomass is a massive source of renewable energy that shows a lot of promise as a means of controlling or even eliminating waste, limiting greenhouse gas pollution, and improving green energy reliability. These strengths do not mean that bioenergy is the end goal, however. The limitations of the technology indicate that other renewable energy sources may still provide greater benefits. For now, bioenergy is an important industry that is likely to see extensive growth over the coming decades.

References

1. Chang, Jinfa, Guanzhi Wang, Cheng Li, Yaqi He, Yuanmin Zhu, Wei Zhang, Muhammad Sajid, Abdelkader Kara, Meng Gu, and Yang Yang. 2023. "Rational design of septenary high-entropy alloy for direct ethanol fuel cells." Joule 7 (3): 587-602.

2.Chang, Jinfa, Guanzhi Wang, Xiaoxia Chang, Zhenzhong Yang, Han Wang, Boyang Li, Wei Zhang, et al. 2023. "Interface synergism and engineering of Pd/Co@N-C for direct ethanol fuel cells." Nature Communications. 

3. Creutzig, Felix, N H Ravindranath, Goran Berndes, Simon Bolwig, Ryan Bright, Francesco Cherubini, Helena Chum, et al. 2014. "Bioenergy and climate change mitigation: an assessment." GCB Bioenergy: Bioproducts for a Sustainable Bioeconomy 7 (5): 916-944.

4. Demartini, Jaclyn D, Sivakumar Pattathil, Jeffrey S Miller, Hongjia Li, Michael G Hahn, and Charles E Wyman. 2013. "Investigating plant cell wall components that affect biomass recalcitrance in poplar and switchgrass." Energy & Environmental Science (3).

5. Eudes, Aymerick, Yan Liang, Prajakta Mitra, and Dominique Loque. 2014. "Lignin bioengineering." Current Opinion in Biotechnology 26: 189-198.

6. Lippman, R. 1982. How to modify your car to run on alcohol fuel: guidelines for converting gasoline engines with specific instructions for air-cooled volkswagens. Technical Report, Seattle, WA: OSTI.

7. Reid, Walter V, Mariam K Ali, and Christopher B Field. 2019. "The future of bioenergy." Global Change Biology 26 (1): 274-286.

8. Roberts, Linda G, and Thomas Smagala. 2022. "Biofuels." Reference Module in Biomedical Sciences. 

9. Shen, Hui, Charleson R Poovaiah, Angela Ziebell, Timothy J Tschaplinski, Sivakumar Pattathil, Erica Gjersing, Nancy L Engle, et al. 2013. "Enhanced characteristics of genetically modified switchgrass (Panicum virgatum L.) for high biofuel production." Biotechnology for Biofuels and Bioproducts.