Hydrothermal carbonization (HTC or wet torrefaction) is a treatment process which converts moist feedstocks into homogenized, carbon rich, and energy dense solid fuel, called hydrochar. One of the main advantages of HTC compared to other thermochemical treatment processes is the use of residual moisture as reaction medium and catalyst. Thus, there is no need for expensive drying prior to HTC treatment. Thermodynamic properties of water change greatly in the subcritical region from 180-280 °C, and as a result, subcritical water behaves as a non-polar solvent and mild acid and base catalyst simultaneously. Biomass, when subjected to HTC, releases oxygen-containing volatiles and hydrochar becomes highly hydrophobic.
Although HTC offers a relatively simple and straightforward solution to process diverse biomass feedstocks, the requirements of high pressure and high temperature make the process complex and costly to design and operate. The lab-scale batch process has already been demonstrated in various laboratories around the world, but batch process is not cost-effective for industrial-scale deployment. The batch process requires loading, heating, cooling, and unloading in sequence for each batch, thus, heat recovery is compromised and scale-up is not feasible. Meanwhile, a continuous process would offer a relatively smaller footprint, higher energy recovery hence efficiency and economics of scale. An ideal HTC process should contain a continuous feeding and product recovery, and also should be able to operate continuously with precise temperature and pressure control.
In this study, a bench-scale continuous HTC reactor system has been designed, commissioned, and operated with various feedstocks including glucose, cellulose, and dairy manure. The throughput of the reactor system was maintained at 5 gal/h, while the reaction time was maintained at 5 min. The maximum temperature and pressure were tested for this study was 250 °C and 40 bar. Both solid and liquid product were tested for their physico-chemical properties and compared with the corresponding products from batch process produced in a Parr reactor. It was found that temperature and pressure were stable during operation and products are relatively similar to that of batch process.
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