Catalyzed Hydrothermal Carbonization with Process Liquid Recycling

Catalyzed hydrothermal carbonization (CHTC) was used to produce hydrochar biofuel from wood chips at 240 degrees C in 1 h batches that included recycling of the process liquid. Infrared spectra showed changes in the chemical structure consistent with dehydration and decarboxylation. The CHTC hydrochar had higher heating values (HHV) of 28.3 MJ/kg, energy yield of 64%, and hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios similar to those of coal. The same process without the catalyst (HTC) produced a hydrochar with HHV of 27 MJ/kg, energy yield of 57%, and H/C and O/C ratios similar to those of lignite. Partial recycling of the CHTC process liquid resulted in a 5% increase in the energy yield; elemental composition, HHV, and scanning electron microscopic images of the CHTC hydrochar for different recycles were indistinguishable. Densified CHTC hydrochar pellets were 97% durable and hydrophobic when compared with wood pellets and torrefied-wood pellets, which was shown by water ingress measurements using an electrochemical cell with pellet electrodes. The CHTC process with recycling has the potential to provide a green hydrochar biofuel with excellent handling, storage, and transportation properties, that could be a suitable direct replacement for coal.