An Examination of the Carbon Sequestration Potential of Cattle Manure Treated with Hydrothermal Carbonization
Authors
Bever, Cordel G.
Issue Date
2024
Type
Thesis
Language
Keywords
Biochar , HTC , Hydrochar , Manure , Pyrolysis , Soil Carbon
Alternative Title
Abstract
Hydrothermal carbonization (HTC) is a thermochemical treatment process originally developed as a tool to enrich biomass for use as a solid fuel substitute. However, recent discoveries in soil science and climate mitigation suggest that processes such as HTC may also be useful for producing materials capable of sequestering carbon in soil. Some organic matter, which has been charred by fires and ultimately ends up in the soil, has been found to be extremely resilient to natural decomposition and can help improve soil health and fertility. These materials, often referred to as pyrogenic or black carbon, can act as a means for long-term carbon sequestration by first allowing plants to pull carbon from the air via photosynthesis, then stabilizing the plant carbon as char and storing it in soil. Therefore, many researchers have been interested in finding methods to produce similarly stable soil amendments out of various raw biomass feedstocks for carbon sequestration. Pyrolysis, the process used to make charcoal, is one such method. Pyrolysis has been demonstrated to create artificial pyrogenic carbon, or biochar, for use in soil, however, one of the major drawbacks of pyrolysis is that it can only be efficiently employed on dry feedstocks such as wood, corn stover, or straw. Wet agricultural wastes, including animal manure, are difficult to process using pyrolysis since a substantial amount of heat is needed to dry the biomass before pyrolysis can begin. HTC, however, excels at converting wet feedstocks into a similar material to biochar known as hydrochar, but is significantly less stable in soil, and has been found to release much of its carbon back into the atmosphere. For that reason, the research presented here aims to address the concerns surrounding hydrochar's limited soil stability and proposes the use of secondary processing to further stabilize hydrochar and wet agricultural waste for sequestering carbon. This thesis explores using HTC as the primary process step for thermochemically converting cattle manure into hydrochar before subjecting the hydrochar to a secondary stability enhancement process. The first two processes focus on using weak (hydrogen peroxide) and strong (potassium dichromate) chemical oxidizers to stabilize the hydrochar. Chemical oxidizers have the potential to remove the easily oxidizable carbon fraction, which is understood to hinder an organic material's ability to resist soil decomposition. Treatment with hydrogen peroxide was found to decrease the hydrochar's inherent soil stability and treatment with potassium dichromate provided a minimal increase to its carbon sequestration potential. The final secondary treatment process studied here was pyrolysis, which takes advantage of the increased dewaterability of hydrochar brought about by HTC, to produce biochar. Biochar produced from pyrolysis of hydrochar exhibited improved soil sequestration properties over biochar derived from pyrolysis of cattle manure. The process pairing also aided in improving the overall carbon yield over conventional pyrolysis, making for an improved method for treating high moisture agricultural waste for soil carbon sequestration.