Abstract
Hydrothermal carbonization (HTC) allows the conversion of organic waste into a solid product called hydrochar with improved fuel properties. Olive tree pruning biomass (OTP), a very abundant residue in Mediterranean countries, was treated by HTC to obtain a solid fuel similar to coal that could be used in co-combustion processes. Three different reaction temperatures (220, 250, and 280 °C) and reaction times (3, 6, and 9 h) were selected. The hydrochars obtained were extensively analyzed to study their behavior as fuel (i.e., ultimate, proximate, fiber and thermogravimetric analysis, Fourier-transform infrared spectroscopy (FTIR), activation energy, and combustion performance). The concentrations of cellulose, hemicellulose, and lignin in the samples depict a clear and consistent trend with the chemical reactions carried out in this treatment. Regarding O/C and H/C ratios and HHV, the hydrochars generated at more severe conditions are similar to lignite coal, reaching values of HHV up to 29.6 MJ kg−1. The higher stability of the solid is reflected by the increase of the activation energy (≈60 kJ mol−1), and ignition temperatures close to 400 °C. With this, HTC is a proper thermal treatment for the management of raw OTP biomass and its further conversion into a solid biofuel.
Highlights
Lignocellulosic biomass has a huge potential for the production of fuel, heat, and electrical power.This along with the need of alternative energy sources with less environmental impacts than fossil fuels, has caused its use to increase lately [1]
We conclude that Hydrothermal carbonization (HTC) is a proper treatment for the management of raw Olive tree pruning biomass (OTP) since a predrying step is not necessary, being as well a good way to homogenize different biomasses
The study of hydrochar production from OTP via HTC under different reaction temperature and time conditions led to the conclusion that the less severe the treatment was, the less carbon conversion existed, making this solid less attractive for its use as a fuel
Summary
Lignocellulosic biomass has a huge potential for the production of fuel, heat, and electrical power. This along with the need of alternative energy sources with less environmental impacts than fossil fuels, has caused its use to increase lately [1]. Biomass was usually utilized for direct combustion and co-combustion with low-rank coals as an easy way to benefit from the biomass energy [2]. Direct combustion is less efficient due to some biomass inherent properties such as low carbon content and hygroscopic nature. Raw biomass is difficult to ignite and the combustion is incomplete and unstable, some pretreatments may be carried out before its use as fuel [3,4]. Co-combustion of hydrochar with coal has been previously studied by Gao et al paying attention to the improvement of Processes 2020, 8, 1201; doi:10.3390/pr8101201 www.mdpi.com/journal/processes
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