Combustion behavior of animal-manure based hydrochar and pyrochar

Authors: Ro, Kyoung; LIBRA, JUDY - Leibniz Institute; BAE, SUNYOUNG - Seoul Women'S University; BERGE, NICOLE - University Of South Carolina; FLORA, JOSEPH R - University Of South Carolina; PECENKA, RALF - Leibniz Institute

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/28/2018
Publication Date: 12/3/2018
Citation: Ro, K.S., Libra, J.A., Bae, S., Berge, N., Flora, J.V., Pecenka, R. 2018. Combustion behavior of animal-manure based hydrochar and pyrochar. ACS Sustainable Chemistry & Engineering. 7:470-478. https://doi.org/10.1021/acssuschemeng.8b03926.
DOI: https://doi.org/10.1021/acssuschemeng.8b03926

Interpretive Summary: Massive consolidation of animal feeding operations has been taking place over the past decades, resulting in more and more concentrated animal feeding operations (CAFOs) in the U.S. and worldwide. With these CAFOs, new, more effective manure management systems are required to make the animal feeding operations economically viable and environmentally benign. The amount of manure produced often exceeds the local demand for nutrient recycle to agricultural land, leading to pollution and nuisance problems. Hydrothermal carbonization (HTC) is an emerging thermochemical conversion technique that can be used to produce carbon-rich char called hydrochar from wet manure with less energy requirements than traditional dry pyrolysis used to produce pyrochar. There were substantial differences in combustion characteristics between hydrochar and actual coal samples. It suggested that the fossil coals used by existing coal power plants should not be replaced entirely with hydrochar, but preferably as a blend. Small percentages (10% or less) of hydrochar blended with coal did not change combustion characteristics significantly. In contrast, the combustion characteristics of animal-manure pyrochars were not substantially different from that of coals. However, blending high percentages of animal-manure pyrochars would substantially increase ash contents, complicating downstream ash management practices. Although more scale-up and ash characterization study is needed before implementation, the results of this study suggest high potential of co-combusting small percentages of animal-manure based hydrochar and pyrochar with coal in existing coal power generation facilities.

Technical Abstract: Wet animal manures are potential feedstocks for energy production, however, pre-processing may be necessary before use in combustion facilities. The thermal processing of such two such wet manures, swine manure and poultry litter, via hydrothermal carbonization and pyrolysis resulted in chars with reduced volatile matter content and higher uniformity than the raw manures. The changes in energy density were dependent on the process conditions. Results from a thermogravimetric analysis showed that hydrochars produced from swine manure and poultry litter underwent two combustion stages - active and char combustion, while the pyrochars and four local coals showed only one stage. The activation energies calculated for the hydrochars in the first stage were similar to those of the four coals. All hydrochars required higher activation energies in the second stage. The substantial differences in characteristic combustion temperatures and the kinetic parameters between hydrochar and actual coal samples suggest that the fossil coals used by existing coal power plants should not be replaced entirely with hydrochar, but preferably as a blend. Small percentages (10% or less) of hydrochar blended with coal did not change combustion characteristics significantly. In addition, there were only small increases in ash contents. In contrast, the combustion characteristics of animal-manure pyrochars were not substantially different from that of coals. However, blending high percentages of animal-manure pyrochars would substantially increase ash contents, complicating downstream ash management practices. Although more scale-up and ash characterization study is needed before implementation, the results of this study suggest high potential of co-combusting small percentages of animal-manure based hydrochar and pyrochar with coal in existing coal power generation facilities.