Hydrothermal carbonization of various paper mill sludges: an observation of solid fuel properties

Authors: SAHA, NEPU - Ohio University; SABA, AKBAR - Ohio University; SAHA, PRETOM - Ohio University; MCGAUGHY, KYLE - Ohio University; Franquivillanueva, Diana; Orts, William; Hart-Cooper, William; REZA, TOUFIQ - Ohio University

Submitted to: Energies
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2019
Publication Date: 3/5/2019
Citation: Saha, N., Saba, A., Saha, P., McGaughy, K., Franqui-Villanueva, D.M., Orts, W.J., Hart-Cooper, W.M., Reza, T.M. 2019. Hydrothermal carbonization of various paper mill sludges: an observation of solid fuel properties. Energies. 12(5):858. https://doi.org/10.3390/en12050858.
DOI: https://doi.org/10.3390/en12050858

Interpretive Summary: Each year the pulp and paper industries generate enormous amounts of effluent treatment sludge. The sludge is made up of various fractions including primary, secondary, deinked, fiber rejects sludge, etc. Hydrochars derived from paper and pulp mill sludge were created by thermally treated different sludge samples in a controlled reactor at ~260 °C. Hydrochars were characterized for their application as a “green” fuel source for displacing coal, a fossil-fuel that contributes directly to greenhouse gas emissions.Performance as a coal substitute depended on the quality of feedstocks; however, when combusted with coal, the resulting mixture exhibited a similar combustion index to coal alone. Primary and secondary sludge was identified as a possible fuel source to co-combust with coal.

Technical Abstract: Each year the pulp and paper industries generate enormous amounts of effluent treatment sludge. The sludge is made up of various fractions including primary, secondary, deinked, fiber rejects sludge, etc. The goal of this study was to evaluate the fuel properties of the hydrochars produced from various types of paper mill sludges (PMS) at 180 °C, 220 °C, and 260 °C. The hydrochars, as well as the raw feedstocks, were characterized by means of ultimate analysis, proximate analysis, moisture, ash, lignin, sugar, and higher heating value (HHVdaf) measurements. Finally, combustion indices of selected hydrochars were evaluated and compared with bituminous coal. The results showed that HHVdaf of hydrochar produced at 260 °C varied between 11.4 MJ/kg and 31.5 MJ/kg depending on the feedstock. This implies that the fuel application of hydrochar produced from PMS depends on the quality of feedstocks rather than the hydrothermal carbonization (HTC) temperature. The combustion indices also showed that when hydrochars are co-combusted with coal, they have similar combustion indices to that of coal alone. However, based on the energy and ash contents in the produced hydrochars, Primary and Secondary Sludge (PPS2) could be a viable option for co-combustion with coal in an existing coal-fired power plant.