In-situ production of magnetic char via rapid subcritical hydrothermal carbonisation of paunch waste

The conventional hydrothermal carbonisation (HTC) is mainly tied to the high capital investment of the reactor and low product quality. In a novel approach, the rapid subcritical HTC of paunch waste (at 240 C for only 5 min) was proposed to enhance the quality of products and their utilisation via in-situ production of magnetic hydrochar. Magnetite nanoparticles were found on the surface of hydrochar with size of 10-20 nm and crystal lattice spacing of 0.25 nm, XRD characteristic peaks of magnetite, and strong FTIR peak at 580 cm -1 assigned to Fe-O stretching in the crystalline lattice of magnetite. Magnetic hydrochar could adsorb Congo Red at a capacity of 59.9 mg g-1. An organic-rich process water with a high COD of 43 g L-1 was co-generated along with magnetic hydrochar and assumed to be anaerobically digested. The estimated theoretical methane yield was found suf-ficient to run the HTC plant in an energy-neutral mode. After developing a process model in Aspen plus, the discounted cash flow analysis revealed a net present value of US$13.04 MM for 30 years of operation, which can increase to $34.79 MM if the iron precursors are sourced from locally available iron-rich sludge.