What Are The Different Ways That Biomass Can Be Converted To Fuel While Capturing Co2?

Biomass refers to any organic material that comes from plants or animals which can be used as an energy source. It is considered a renewable energy source because plants and trees can regrow over a relatively short period of time compared to fossil fuels which take millions of years to form. Biomass sources include wood, crops, algae, and waste from animals, municipalities and industries. When biomass is burned or otherwise converted into energy, it releases carbon dioxide – however, the plants that are the source of biomass absorb a nearly equivalent amount of carbon dioxide as they grow. Therefore, biomass can provide a carbon-neutral or even carbon-negative method of energy production.¹

There is significant interest in developing methods to convert biomass to energy fuels like ethanol, biodiesel and biogas, while also capturing the carbon dioxide emissions. This provides a process that can 1) produce renewable fuels to displace fossil fuels, and 2) remove carbon dioxide from the atmosphere, helping to reduce the impacts of climate change. Converting biomass with CO2 capture has the potential to provide carbon-negative energy.^{2, 3}

Types of Raw Biomass Materials

There are three main types of raw biomass materials that can be used for energy production:

Plant Materials – This includes wood from forests and trees, agricultural crops and their residues, and aquatic plants like algae. Common examples are wood chips, straw, corn stover, soybean residues, and fast growing energy crops like switchgrass or poplar trees. Algae can also be grown and harvested specifically for energy production.

Animal Wastes – Manure from livestock operations is a source of biomass. Other animal-derived materials are fats, oils, and greases that can be rendered from meat production and food service facilities.

Municipal Solid Waste – Garbage, food waste, plastics, paper, yard trimmings and other waste materials can be converted to energy at waste-to-energy plants. Recyclables are typically removed first.

The wide variety and abundance of biomass sources is an advantage for biomass energy production. The most common feedstocks depend on regional agricultural practices and waste streams.

Sources:

[1] https://www.eia.gov/energyexplained/biomass/

[2] https://www.need.org/wp-content/uploads/2019/10/BiomassAtAGlance_11x17.pdf

Thermochemical Conversion Methods

Thermochemical conversion breaks down biomass feedstocks using heat. There are several thermochemical processes that can convert biomass into energy products like biofuels while capturing CO2 emissions:

Direct combustion involves burning biomass directly to produce heat and electricity. The biomass fuels are burned in a boiler or furnace to generate high-pressure steam that powers a turbine to generate electricity. The process releases CO2 emissions, but they can potentially be captured using flue gas treatment technologies before being released into the atmosphere (1).

Gasification converts biomass into a gaseous fuel called syngas through partial oxidation at high temperatures. The syngas is then cleaned and filtered before being combusted to produce electricity, processed into fuels like ethanol or diesel, or used to manufacture products. Gasification enables more efficient CO2 capture compared to direct combustion (2).

Pyrolysis uses heat in the absence of oxygen to thermally decompose biomass into liquid bio-oil, syngas, and charcoal. The bio-oil can be upgraded into transportation fuels. Pyrolysis occurs at lower temperatures than gasification, allowing more precise control over the breakdown process and products. Condensing the vapors enables CO2 capture (2).

These thermochemical processes provide flexible options to convert diverse biomass materials into energy products while integrating CO2 capture systems. However, challenges remain in terms of improving conversion efficiencies and optimizing CO2 capture from syngas cleaning.

Biochemical Conversion Methods

Biochemical conversion involves using microorganisms or enzymes to break down biomass into fuels and chemicals. There are two main biochemical conversion processes: fermentation and anaerobic digestion.

Fermentation uses microorganisms like yeast and bacteria in an oxygen-limited environment to convert sugars from biomass into ethanol and other alcohols that can be used as fuels. The biomass feedstocks are pretreated to release the sugars, which are then fermented by the microbes. For example, the sugars in corn starch or sugarcane juice can be fermented to produce corn ethanol or sugarcane ethanol, respectively.

Anaerobic digestion relies on a microbial community in an oxygen-free environment to break down biomass and produce biogas, which is composed of methane, carbon dioxide and trace gases. Organic material like livestock manure, food waste, and crop residues are placed in a digestion tank where the microbes anaerobically break down the material. The biogas can then be captured and used to generate electricity and heat or can be upgraded to renewable natural gas.

CO2 Capture Methods

There are three main approaches for capturing CO2 from biomass power plants or biofuel production facilities: pre-combustion capture, oxyfuel combustion, and post-combustion capture.

Pre-combustion Capture

Pre-combustion capture involves converting the biomass into syngas (a mixture of hydrogen and carbon monoxide) through gasification. The syngas then undergoes a water-gas shift reaction to produce hydrogen and CO2. The CO2 can then be separated from the hydrogen before combustion takes place. This approach results in a relatively pure stream of CO2 that can be readily captured, compressed and transported for storage.

Oxyfuel Combustion

Oxyfuel combustion uses pure oxygen rather than air for the combustion process. This results in a flue gas that has high concentrations of CO2 and water vapor, allowing the CO2 to be easily separated. The water vapor is condensed through cooling and compression. Oxyfuel combustion requires an air separation unit to produce pure oxygen, which leads to additional energy requirements. However, it avoids the need for capture equipment post-combustion.

Post-combustion Capture

Post-combustion capture involves separating CO2 from the flue gas after biomass has been combusted in air. Methods include chemical absorption using solvents, physical absorption by high pressure CO2, membrane separation, and adsorption using solid sorbents. Chemical absorption using amines is the most mature approach, but consumes significant energy for solvent regeneration. Post-combustion capture can be added to existing biomass power plants.

Environmental Benefits

Converting biomass to fuel while capturing CO2 can provide important environmental benefits. One key benefit is reducing net CO2 emissions. Since biomass absorbs CO2 as it grows, the CO2 released when the biomass is burned can be balanced out by the CO2 absorbed during growth. By capturing and sequestering some of the CO2 emissions from biomass energy, net emissions can actually be negative, resulting in CO2 removal from the atmosphere.

Another benefit is utilizing waste material as an energy source rather than letting it decompose and release methane, which is a more potent greenhouse gas than CO2. Agricultural residues, forest debris, animal manure, and municipal solid waste can all serve as feedstocks for biomass energy and fuels. This prevents these materials from being landfilled and converting them to useful energy.

In addition, some biomass conversion processes like gasification generate fewer air pollutants like sulfur oxides, nitrogen oxides, and particulate matter compared to directly burning biomass or fossil fuels. This can improve local air quality and public health.

Economic Considerations

The cost of producing biofuels with carbon capture can be high compared to fossil fuels. Building the infrastructure for biomass conversion and CCS requires major upfront capital investments. According to a 2021 study, the levelized cost of electricity for bioenergy with CCS ranges from $100-150 per MWh, 2-3 times higher than fossil fuels without CCS.

Government incentives like tax credits and carbon pricing can help improve the economics of biofuel production with CCS. The MIT Joint Program on the Science and Policy of Global Change found that a carbon price of $50/ton would allow bioenergy with CCS to become cost competitive with coal by 2050 [1]. However, prices for biofuels can be subject to commodity price volatility compared to relatively stable fossil fuel prices.

Case Studies

There are several notable case studies of successful biomass to fuel with CCS projects around the world. One example is the bioenergy with carbon capture and storage (BECCS) project at Drax Power Station in the UK (source). Drax converted its coal-fired units to run on biomass and has installed pilot BECCS facilities to test capturing CO2 from biomass flue gases. They have successfully demonstrated 95% CO2 capture efficiency from biomass feedstocks like wood pellets.

Another case study is the waste-to-energy and CCS project by Fortum Oslo Varme (FOV) in Norway (source). FOV uses municipal solid waste as a feedstock for its combined heat and power plant and has an amine-based CO2 capture system to capture 90% of the flue gas CO2. The CO2 is then sent via ship for permanent geological storage. This demonstrates CCS integration with a waste biomass feedstock.

These projects highlight the technical viability of BECCS using diverse biomass sources like wood, agricultural residues, and municipal solid waste. When combined with geological storage, BECCS provides carbon negative renewable energy and fuels.

Challenges and Limitations

While converting biomass to fuel while capturing CO2 has significant potential benefits, there are also some key challenges and limitations to overcome:

Cost barriers are a major challenge. The technologies involved such as gasification, pyrolysis, and CO2 capture and storage have high capital and operating costs. More research and development is needed to improve efficiencies and bring down costs (Babin, 2021).

Scaling up the technologies from pilot to commercial scale also poses engineering and logistical difficulties. The equipment is complex and moving to larger scales introduces new technical hurdles (Freitas, 2021).

Feedstock availability can be a limitation, as large quantities of biomass are needed to produce meaningful quantities of fuel. Sourcing sufficient non-food biomass sustainably requires significant land use changes (Babin, 2021).

Future Outlook

The future looks bright for biomass conversion technologies that capture CO2. With improving efficiencies and new innovations, these technologies are poised for significant growth.

One key area is continuing to improve the technologies and processes involved in biomass conversion and CO2 capture. Research is ongoing into more efficient gasification, pyrolysis, torrefaction, and other thermochemical processes that maximize fuel production while minimizing CO2 emissions.^[1] New catalysts and reactor designs could further increase yields and reduce costs. Biochemical conversion processes are also gaining in efficiency, with research into advanced fermentation techniques and genetically engineered microbes to boost biofuel output.^[2]

Strong policy support will be needed for biomass conversion with CCS to reach its full potential. Government incentives like tax credits or carbon pricing programs could help drive investment and deployment. Setting clear CO2 emissions reduction targets in energy policy is also important to create long-term market certainty. Public investment into pilot projects and commercialization support would accelerate technological gains.^[3]

Most projections estimate substantial growth for biomass conversion with CCS in the coming decades. One estimate suggests BECCS systems could capture 10 billion metric tons of CO2 per year by 2050, making a significant dent in emissions.^[1] Realizing this potential will require continued technology evolution, policy backing, and private sector investment. If all key elements align, biomass conversion with CO2 capture appears set to play a major role in the clean energy transition.

^[1] https://iea.org/energy-system/carbon-capture-utilisation-and-storage/bioenergy-with-carbon-capture-and-storage
^[2] https://utilitiesone.com/the-future-of-bioenergy-innovations-in-biomass-conversion
^[3] https://cmclinnovations.com/use-cases/energy/biomass-to-ccs/