Background information

Today, our atmospheric carbon dioxide concentration is 390.31 parts per million, according to the Scripps Institute of Oceanography. The concentration has been increasing at an accelerating rate annually and has surpassed the safe level of 350 parts per million, last experienced in 1988. Our increasing dependence on burning fossil fuels and deforestation have resulted in massive amounts of carbon dioxide in the atmosphere. This rise is causing erratic climate changes in the ocean and on land. I chose to study biochar carbon sequestration because sustainable biochar systems are carbon-negative. They sequester a vast amount of carbon in the soil and result in a net reduction of atmospheric CO₂ levels.

Biochar is a carbon-rich charcoal and is obtained through pyrolysis. Biochar is naturally present in soils as a result of forest fires and the ancient soil-enrichment practice of making terra preta, such as took place in the Amazon region of Brazil. The dark, fertile soil found in many countries in South America was created approximately 500 to 2,500 years ago by native peoples. The Amazonian terra preta soils discovered have up to one order of magnitude higher carbon content than other neighboring soils.

Biochar distinguishes itself from charcoal through its application. Biochar stores about 50% of the plant’s original CO₂. The aliphatic carbon in the plant is transformed into stable, aromatic carbon during pyrolysis, and decomposes at a much slower rate than biomass. Replacing the slashing and burning of forests with charring could reduce CO₂ emissions by approximately 8 parts per million in half a century. 

According to the International Biochar Initiative, biochar has multiple benefits. It provides climate benefits through carbon sequestration. It can create by-products such as bio-energy, improve water quality through a reduction in nutrient leaching, improve plant yield, and enhance water retention. It can reduce waste, soil erosion, and degradation. However, these benefits are dependent on the long-term persistence (stability) of biochar, and therefore knowledge of biochar stability is important. 

The stability of biochar determines how long the carbon will be sequestered. Biochar samples from the terra preta soils can be dated back 7,000 years. Researchers have determined that biochar has a half-life of hundreds to thousands of years. It is difficult to know the accuracy of this estimate and how this will change for biochar based on modern methods of production. A generalized, predictive knowledge of carbon storage and stability is lacking, preventing an understanding of what properties affect stability and how to optimize stability (Lehmann 2006). Current research on biochar stability is limited but includes long-term incubations.

An understanding of how controllable environmental factors affect biochar stability is lacking, but is important to optimize biochar longevity in soil. Currently, it is known that as the temperature during pyrolysis increases, total carbon content increases and volatile matter declines, leading to increased stability. Factors such as feedstock and particle size can be controlled to optimize the carbon-sequestration benefits. The longevity of biochar in the soil, when using finely powdered biochar and then tilling and plowing, is unknown. Other controllable factors, such as water-accessible volume and hydrophobicity, may also play a role in how biochar degrades.

Testable question

How do the type of biochar feedstock and particle size affect the physical and chemical stability of biochar?

Hypotheses

Hazelnut shell biochar will demonstrate greater stability than Douglas fir biochar due to its denser structure. Char in larger particle sizes (250 to 2000 µm) will be more physically and chemically stable than char in smaller particle sizes (63 to 250 µm) because of the decrease in surface area.

Methodology 

I developed the methods for experimentation because currently there is no standard protocol to implement. My goal was to assess biochar stability, and how the type of feedstock and particle size affected this. I chose to define stability as the biochar’s ability to withstand a broad variety of physical and chemical agents that occur in the surrounding environment.

Physical stability is the char’s ability to withstand physical weathering in the soil. The physical stability of the biochar was measured by replicating physical weathering through ultrasonication at increasing frequencies. Chemical stability is the char’s ability to withstand chemical weathering and decomposition in the environment. Chemical weathering was replicated through long-term chemical oxidation. These two forms of weathering were chosen with the goal that by applying stresses to the biochar and understanding its reactivity, an understanding of how biochar degrades over long periods of time could be gained.

The top-lit updraft stove operating in my backyard.

I began my research by selecting the feedstock. I made biochar out of hazelnut shells and Douglas fir wood chips. Oregon produces 99% of the nation’s hazelnuts, and shells are readily available. I used unusable timber for the Douglas fir wood chips.

Next, I made a top-lit updraft stove to make biochar. I learned how to make this stove at a stove workshop at SeaChar, a nonprofit organization that puts the stoves into rural Costa Rican villages. Using a Vernier thermocouple, I determined that the stove operates between 360° and 420ºC during a single run. I contacted the founder of the Pacific Northwest Biochar group, John Miedema, who owns a Pacific Class Fluidyne Gasifier, an industrial pyrolysis unit. Miedema made biochar for my research out of the same feedstock at 370°C, 500°C, and 620°C.

Fluidyne Pacific Class Gasifier

After the biochar was made, I crushed it into two particle sizes using a coffee grinder and sieves of 63, 250, and 2,000 micrometers. The two particle sizes used in the experiment were 63–250 micrometers and 250–2,000 micrometers. With two feedstocks, four different temperatures of pyrolysis, and two particle sizes, a total of 16 different char samples were used.

hazelnut char

A proximate analysis was done to characterize the chars. This was done following ASTM standards using a laboratory oven. I calculated the ash content, the volatile matter, and the moisture content of the chars.

crushing the biochar to the different particle sizes

Next, the physical stability of the chars was assessed. I ultrasonicated each biochar sample at 60, 250, 450, and 644 J/ml, with each sample having three replicates. The samples were ultrasonicated in a thermos flask to minimize heat loss. After ultrasonication, the samples were filtered, and the filtrate was analyzed in a total organic carbon analyzer to measure how much carbon had leached into the water.

Next, the chemical stability of the char was assessed. I progressively oxidized all samples for 70 hours using 3% hydrogen peroxide. One gram of each char was weighed into a flask to which 50 ml of 3% hydrogen peroxide was added. The samples were placed in a water bath at 75°C for two hours, dried for 24 hours at 105°C, and weighed. Then, the 3% hydrogen peroxide was added again to the flasks, and the samples remained in the water bath for four hours and were dried and weighed. The process was continued, with the samples remaining in the water bath for eight eight-hour intervals, creating 10 data points of oxidation breakdown for each char and 70 cumulative hours of oxidation. The data is represented as the percentage of mass lost over time.

Samples in the water bath (left) and hazelnut char residue after oxidation (right).

Flow Chart of Procedure

DATA:

Preliminary Tests: Characterization of the Char—Proximate Analysis of Biochar (following ASTM international procedure)

Legend:
HZ= Hazelnut
DF = Douglas Fir
S= made in stove (360°-420°C) 

Chemical Stability Results (Chemical Oxidation) 

Douglas Fir Biochar Chemical Stability Results
*Error bars on data points represent standard error

Hazelnut Biochar Chemical Stability Results

Physical Stability Results (Ultrasonication)

Table 5: % Initial Carbon Composition in Chars 

*These numbers were obtained through analysis using a Carlo Erba Carbon Analyzer 

Physical Stability Results for Douglas Fir Char

Physical Stability Results for Hazelnut Shell Char 

Table 6: Amount of Energy Required for 100% Carbon Breakdown 

* These values were determined by taking the equations for the trend lines for the physical stability data and solving for when x when y = 100%. 

Discussion:

Physical Stability of Biochars after Ultrasonication (Figures 1-8, Table 3)

• As the energy output of ultrasonication increased, the percentage of total carbon lost increased.
• The temperature of pyrolysis did not affect the percentage of total carbon lost after ultrasonication.
• Particle size and feedstock did not affect the percentage of total carbon lost (p-value = 0.47199).
• All chars lost less than 0.25% total carbon after ultrasonication at 644 J/ml for 30 minutes.
• Hazelnut char of 63–250 µm lost the most carbon after ultrasonication.
• Douglas fir char of 250–2,000 µm made at 620°C lost the least total carbon (0.01%) after ultrasonification at 644 J/ml.
• Hazelnut char of 250–2,000 µm made at 500°C had the slowest rate of breakdown as energy output increased. It would theoretically require 151,979 J/ml of energy to release 100% of the carbon, which is one order of magnitude larger than other estimates.
• The char that required the least energy (22,484 J/ml) to release 100% carbon was Douglas fir char of 63–250 µm made at 620°C.

All the biochars demonstrated extreme physical stability and robustness. This suggests that the degradation of biochar would not affected by agricultural disturbances and soil compaction. The stability of the biochars was not dependent on particle size, feedstock, or the temperature of pyrolysis. This suggests that biochar may be physically stable irrespective of these factors, and that physical stability is a function of the biochar’s chemical structure rather than controllable outside factors.

Chemical Stability Under Oxidation by Hydrogen Peroxide (Figures 9–16, Table 4, 5)

• Douglas fir chars made at higher pyrolysis temperatures (500°C and 620°C) lost less mass than hazelnut chars.
• Douglas fir char made at 620°C in the larger particle size (250–2,000 µm) lost the least mass (40.8%) after 70 hours.
• All samples of chars made at lower temperatures were completely oxidized at the end of 70 hours.
  • Hazelnut chars made at low temperatures completely oxidized in 30 to 40 hours and then leveled off, leaving a white residue. The chars oxidized in a linear fashion. 
• For both feedstocks, particle size did not affect the oxidation of the chars made at low temperatures.
• Particle size did affect the rate of oxidation for the chars made at high temperatures (p-value = 0.05).
  • Chars in smaller particles oxidized at a faster rate than chars in larger particles.
  • Douglas fir chars made at 500°C and 620°C in small particles lost 10% more mass after 70 hours of oxidation than chars in large particles.
  • Douglas fir char made at 500°C and 620°C in small particles lost 23% more mass after 70 hours than chars in large particles (p-value = 0.04).

For chars made at high temperatures (500°C and 620°C), particle size significantly affected the mass lost after 70 hours of oxidation. Tilling and draining reduces the particle size of biochar, which reduces its sequestration abilities 10% to 25% over hundreds of years because it becomes more susceptible to oxidation. The importance of particle size in chemical oxidation may be especially pertinent in biochar applications to topsoil, in which gas exchange takes place at a higher rate.

Looking at feedstocks, Douglas fir char was more chemically stable than hazelnut shell char. This may be attributed to the fact that chars made from different feedstocks have different surface chemistry and pore structure. Pore structure determines the area of char exposed to oxygen and available for microbial degradation.

Chars created at higher temperatures were more chemically stable than chars made at lower temperatures because of increased aromaticity and stronger bonds at higher temperatures. The biochar oxidized at a steadily increasing rate due to a smaller biochar-to-oxidant ratio with each successive oxidation period (Zimmerman 2009). This may also be a result of the biochar converting from hydrophobic to hydrophilic during the treatment (Lehmann and Joseph 2009).

Potential Sources for Error

The two particle sizes used were a single sieve fraction apart. Some particles in both samples sizes were potentially very close. The concentration of particles of different sizes was not regulated.

The chemical oxidation was not absolutely progressive because the entire samples were dried and weighed every eight hours while the concentration of oxidant remained the same.

During ultrasonication, water was used as a solvent, which potentially influenced the leaching of nutrients. In addition, the temperature increased during ultrasonication, which likely influenced the results.

Conclusions

• The chemical stability of biochars made at high pyrolysis temperatures was affected by particle size, with small particles being less stable than large particles. Douglas fir char in small particles lost 10% more mass than char in large particles after oxidation. Hazelnut char in small particles lost 25% more mass.

• The oxidation of chars made at low temperatures (370°C and stove char) was not affected by particle size.

• Douglas fir char lost less mass after oxidation in comparison to hazelnut shell char, demonstrating that the type of feedstock does affect the rate of oxidation and the chars’ stability.

• Particle size and feedstock did not affect the physical stability of the char. All chars lost less than 0.25% total carbon after oxidation at 644 J/ml, reflecting stability under physical stress for long time periods.

• Biochar is physically stable but is susceptible to degradation by oxidation.
• Longevity of the biochar carbon-sequestration benefits depends on environmental factors and can be optimized through manipulation of particle size and type of feedstock. Therefore, carbon sequestration benefits can be maximized with further research.

Future Research

Future research would include refining my methods in order to more accurately predict the duration of biochar breakdown based on environmental factors. Furthermore, a study on how microbial activity in the soil contributes as a biological factor in biochar degradation would be beneficial. Next steps could include a study of biochar’s surface properties, specifically its hydrophobicity, as an important factor in oxidation. Future research will include a field study to understand if chars that demonstrate stability and good carbon sequestration are also valuable in multiple soil types.  

Acknowledgements

My thanks to Markus Kleber and Myles Gray from Oregon State University for providing guidance in methodology and lab equipment, and to John Miedema for providing me with biochar.

Bibliography

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