Graduate Thesis Or Dissertation

Stability of Biochar Under Storage Conditions and Evaluating its Potential as a Carbon Sequestration Tool in the United States

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  • Biochar has shown promise as a soil amendment for improvement of agricultural and forest productivity, remediation of heavy metals, and sequestration of carbon, but has not yet been adopted for widespread use in any of these applications. By assessing the production potential as a carbon sequestration tool, and the stability of biochar products, it is hoped to foster greater interest in research and large-scale application of these products. Previous studies have observed differences between the effects of freshly produced and aged biochar on soil, but the nature of these differences has not been explored. Here, the chemical and thermal changes of biochar under storage were measured using Fourier-transform Infrared Spectroscopy (FTIR) and Differential Scanning Calorimetry / Thermogravimetric Analysis (DSC/TGA) on several distinct biochars stored in glass under air, in glass under nitrogen, or in plastic under air, to assess the influence these storage conditions have on thermal and chemical properties over time. An understanding of these changes is vital to scaling up biochar implementation so that delivered products will perform reliably. Results indicate that biochar produced at 700°C is minimally impacted by a storage time of 7 months, but that biochar produced at 300°C does exhibit greater changes both in chemical and thermal properties which appear to correlate with a loss of cellulose and hemicellulose structures over the storage duration. Storage of biochar within air-permeable plastic was associated with greater adsorption of water and volatile compounds, although no benefit from storing samples under nitrogen was observed. The production and application of biochar to soil as part of a carbon sequestration strategy has been advocated by several researchers to mitigate the effects of climate change. This study estimates the quantity of biochar that could be produced annually in the United States from crop residues, forestry residues, and municipal waste, without requiring changes to land use. Annually, an estimated 836 Mt of these materials are available, composed of 77% crop residue, 19% forestry residue, and 4% municipal waste. Conversion of this into biochar would prevent approximately 224 Mt of carbon (819 Mt CO2) from entering the atmosphere as a result of natural decomposition, which would offset 12.6% of annual U.S. greenhouse gas emissions. Beneficial rates of biochar application to agricultural and forest soils found in literature were used to determine the soil biochar capacity, which is estimated at over 70 times the feasible annual production, therefore even high production of biochar will result in secondary benefits to soils. Modeling of feedstock decomposition was conducted at various decomposition rates and harvest frequencies to estimate the time required for conversion to biochar to enhance carbon storage under different scenarios. The results of this model suggests that the use of crop residues is favorable in less than five years, while conversion of forest residues may require anywhere from 4 to 200 years for decomposition to equal the carbon lost during the biochar production process. Selection of forest residues to avoid ecosystems or residue fractions with low decomposition rates would decrease the total available mass and maximum carbon sequestration capacity but also ensure that benefits are realized within the appropriate time scale. The quantity of material that would fall into this range is unable to be determined with the current research available, and would likely need to be determined on a site-by-site basis.
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