Enhancing fuel properties of napier grass via carbonization: a comparison of vapothermal and hydrothermal carbonization treatments

Authors: PAIAGUA, DANIELA - Technical University Of Berlin; LIBRA, JUDY - Leibniz Institute; ROTTER, VERA SUSANNE - Technical University Of Berlin; Ro, Kyoung; FISCHER, MARCUS - Leibniz Institute; LINDEN, JULIA - Technical University Of Berlin

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/17/2023
Publication Date: 11/23/2023
Citation: Paiagua, D.M., Libra, J.A., Rotter, V., Ro, K.S., Fischer, M., Linden, J. 2023. Enhancing fuel properties of napier grass via carbonization: a comparison of vapothermal and hydrothermal carbonization treatments. Agronomy. 13(12). https://doi.org/10.3390/agronomy13122881.
DOI: https://doi.org/10.3390/agronomy13122881

Interpretive Summary: Hydrothermal carbonization (HTC) uses liquid water as the reaction medium while vaporthermal carbonization (VTC) uses steam to thermochemically convert wet organic residues and wastes from numerous sources to a carbonaceous solid product called hydrochar. In this research, we produced chars from HTC and VTC of Napier grass: Napier grass tends to have high ash contents and other inorganic elements that cause corrosion, fouling, and slagging problems when combusted. We evaluated the biofuel properties of HTC and VTC hydrochars along with the raw Napier grass using two international standards for biofuel quality. Although HTC produces chars with higher fuel quality, VTC offers higher solid, energy and carbon yields. If the VTC char is to be co-combusted with other high-quality fuels, the low fuel quality of VTC char may not be relevant and VTC process may be preferred due to its higher solid and energy yield. Further studies on co-combustion and economics are recommended to advance the science of these processes and for actual field application in the future.

Technical Abstract: Napier grass is a fast-growing herbaceous biomass that can be used as biofuel. However, herbaceous biomasses tend to have high ash content, potassium, and chlorine that cause corrosion, fouling and slagging problems when combusted. In this study, the Napier grass was submitted to vapothermal carbonization (VTC) and hydrothermal carbonization (HTC) processes at 190 and 220°C to find out if its fuel properties could be improved. The net calorific value of the chars, produced from both processes VTC and HTC, improved when compared to that of the raw Napier grass (up to 20.6% more VTC220 and up to 29.8% more with HTC220). The sulphur (S) content in the chars was additionally reduced compared to that of the raw Napier grass (in average up to 15.3% less with VTC190 and 28.5% less with HTC190). The biofuel properties of the HTC chars were better than both VTC chars’ and Napier grass’ properties in terms of, ash contents (mean values were 5.6% for Napier grass, 7.1% for VTC chars and 3.3% for HTC) and chlorine (Cl) contents (mean values were 1.08% for Napier grass,1.24% for VTC chars and 0.19 % for HTC chars). HTC process is preferred over the VTC process when trying to avoid corrosion: when HTC process is performed, high percentages of Cl (up to 80%), S (up to 70%), sodium (Na, up to 80%) and potassium (K, up to 90%) were distributed into the process water. This will prevent fouling and slagging problems when burning HTC char. The biofuel qualities of the raw Napier grass, VTC, and HTC chars were evaluated by two standards: the Korean regulation for biomass solid recovered fuels (Bio-SRF) and the International Organization for Standardization (EN ISO 17225). According to the Bio-SRF, neither Napier grass nor VTC chars could be used as Bio-SRF since Cl was already high in the raw feedstock and VTC process accumulated it in the char. The HTC chars fulfilled all limit values and could be considered as potential recovered fuels according to the Bio-SRF. Regarding the EN ISO17225 assessment, Napier grass and VTC chars did not satisfy the Cl requirement. Both VTC, HTC chars along with the raw Napier grass fulfilled the requirements for heavy metals (Pb, Ni, Cr, and Cd) except for copper (Cu).