Process Water Recirculation during Hydrothermal Carbonization of Waste Biomass: Current Knowledge and Challenges

Abstract

Hydrothermal carbonization (HTC) is considered as an efficient and constantly expanding eco-friendly methodology for thermochemical processing of high moisture waste biomass into solid biofuels and valuable carbonaceous materials. However, during HTC, a considerable amount of organics, initially present in the feedstock, are found in the process water (PW). PW recirculation is attracting an increasing interest in the hydrothermal process field as it offers the potential to increase the carbon recovery yield while increasing hydrochar energy density. PW recirculation can be considered as a viable method for the valorization and reuse of the HTC aqueous phase, both by reducing the amount of additional water used for the process and maximizing energy recovery from the HTC liquid residual fraction. In this work, the effects of PW recirculation, for different starting waste biomasses, on the properties of hydrochars and liquid phase products are reviewed. The mechanism of production and evolution of hydrochar during recirculation steps are discussed, highlighting the possible pathways which could enhance energy and carbon recovery. Challenges of PW recirculation are presented and research opportunities proposed, showing how PW recirculation could increase the economic viability of the process while contributing in mitigating environmental impacts.

Cascade Membrane System for Separation of Water and Organics from Liquid By-Products of HTC of the Agricultural Digestate—Evaluation of Performance

“…A multitude of reactions coinciding with the output of multiple different products can be considered typical for HTC of different types of biomass [41,42]. These reactions do not represent consecutive steps but instead form parallel networks of different reaction pathways [43]. Hydrolysis usually takes place in relatively low temperatures [44].…”

“…Decreased amount of carboxyl and carbonyl groups, due to decarboxylation, also slightly decreases solid products' O/C ratio [44,46,47]. Intermediates become substrates of polymerisation, condensation and aromatisation [43,47,48], which are also instrumental in an aggregation of carbonaceous microspheres [43]. Microspheres precipitate, thus forming secondary hydrochar [43,49].…”

“…Intermediates become substrates of polymerisation, condensation and aromatisation [43,47,48], which are also instrumental in an aggregation of carbonaceous microspheres [43]. Microspheres precipitate, thus forming secondary hydrochar [43,49].…”

“…Overall, recirculation of the liquid by-products or its subsequent use for maximum energy recovery and increased systemic efficiency offers many opportunities [43]. Several studies report the composition of liquid HTC by-products [77][78][79], with many indicating potential synergies between HTC and anaerobic digestion (AD) [52,80,81] or application of the effluent in microbial fuel cells [82].…”

Industrial-Scale Hydrothermal Carbonization of Agro-Industrial Digested Sludge: Filterability Enhancement and Phosphorus Recovery

“…During HTC, which is commonly carried out in a closed vessel at temperatures ranging between 180-250 • C and at corresponding pressures of 10-40 bars, biomass undergoes a progressive decomposition via dehydration, decarboxylation and aromatization reactions [27]. This leads to a high-energy, dense solid carbonaceous residue with enhanced hydrophobic character, known as hydrochar, and a liquid phase rich in organics [28,29], together with small amounts of gas, mainly composed of CO 2 [30]. The use of HTC technology to upgrade several different types of waste organic materials, such as agro-waste [31][32][33][34][35][36], dairy wastes [37,38], paper mill waste [39,40], and the organic fraction of municipal solid waste [41][42][43][44], has been widely investigated.…”