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Review Paper, Hydrothermal carbonization (HTC) is a proven cost-effective and energy-efficient method for waste management and value-added product recovery. There are, however, several issues that require further improvement or research. Identifying the strengths and weaknesses of HTC in comparison to traditional pyrolysis is crucial for scientists to choose between them or use both (complementary) to achieve specific product properties. Additionally, sharing information on diverse modeling approaches and scales is crucial to enhance the robustness and universality of HTC process models. In addition, the study on the applicability of hydrochars on target applications such as soil amendment is crucial to give back nutrients to soils and face the dependence on finite specific feedstocks in this field. Also, proper management of the process by-products, especially process water, must be addressed to improve the carbon and hydric footprint of the process. Reviewing the suitability of HTC to treat specific challenging wastes, whose strength is not related to their calorific value but to their nutrient composition (i.e., manures), is also an appealing topic for HTC research. This paper aims to tackle the above-mentioned issues through an updated review and discussion of research gaps that require further investigation.

Herein, the valorization of vegetable and fruit waste was assessed via hydrothermal carbonization (HTC) and anaerobic digestion (AD) in terms of product characterization and energy requirements. HTC was conducted at reaction temperatures between 150 ºC and 190 ºC, and residence times between 20 min and 40 min. The increase in the process severity resulted in hydrochars with higher carbon contents and higher energy densification ratios. AD was performed in two different ways. i.e., batch and semi-continuous reactions. From the batch experiments a methane yield of 300 L CH4/kg VS was obtained, while for the semi-continuous, the average specific methane production estimated (for HRTs from 75 to 50 days) was 213 ± 32 L CH4/kg VS. To estimate the energy requirements, mass and energy balances were performed considering the basic stages of each process to obtain a suitable biofuel material. In this sense, it was concluded that for this specific waste, AD was a more suitable process with a positive energy net balance. On the contrary, HTC presented a negative energy net balance being required 1.29 MJ/kg of fresh food waste. A combined HTC-AD treatment may be an efficient method to take advantage of both technologies leading to higher energy efficiencies and other valuable products.

The use of phytoremediation to remove metals of contaminated soils is an interesting technique that is usually limited by adverse physical and chemical properties of this type of soils. The addition of biochar produced from manure waste could improve soil properties due to its nutrient content, high cation exchange capacity or water holding capacity. However, the high water content of manure wastes precludes its pyrolysis treatment use for biochar production, without a previous drying step. Indeed, hydrothermal carbonization (HTC) of manure wastes could be an adequate treatment method as it takes place in water solution. The product obtained after HTC of biomass, named hydrochar, exhibits different physiochemical properties than biochar that could affect their potential application. The objective of the present work is to study the effect of chars obtained by pyrolysis or HTC of manure wastes in mining soil phytoremediation with Sinapis alba. Two selected mining soils (PORT and GAM) were treated with two manure biochars prepared at 450 °C (BMW450) and 600 °C (BMW600), two hydrochars prepared by HTC of manure at 190 °C (HWM190) and 240 °C (HMW240) and raw manure waste (MW) at a rate of 10% in mass. Later, different soil samples were incubated with or without Sinapis alba growth. Experimental results shown that properties of chars have a great influence on the efficiency of the use of Sinapis alba in the phytoremediation of mining soils. The addition of BMW600 and raw material (MW) increased the production of aerial and root biomass for GAM soil. For PORT soil, HMW190, two biochars and MW increases root and aerial biomass whereas HMW240 only produces an increment on aerial biomass. Addition of two hydrochars and MW had a positive effect on the biochemical soil activities and the highest microbial biomass carbon of GAM and PORT soil samples was observed after addition of HMW190. Finally, the addition of biochars and hydrochars could improve the phytoremediation of mini

An activated carbon manufacturing process from winemaking waste is analyzed. In that way, vine shoots conversion is studied as a basis for plant designing, and mass and energy balances of hydrothermal carbonization and physical activation are fulfilled. To develop an energy-integrated plant, a network of heat exchangers is allocated to recover heat waste, and a cogeneration cycle is designed to provide electricity and remaining heat process demands. Furthermore, thermoeconomic analysis is applied to determine the thermodynamic efficiency and the economic viability of the plant. Energy balance indicates that heat exchangers energy integration covers 48.9% of the overall demands by crossing hot and cold streams and recovering heat from residual flue gas. On the other hand, the exergy costs analysis identifies combustion of pruning wood as the main source of exergy destruction, confirming the suitability of the integration to improve the thermodynamic performance. Attending to economic costs analysis, production scale and vineyard pruning wood price are identified as a critical parameter on process profitability. With a scale of 2.5 ton/h of pruning wood carbonization, a break-event point to compete with activated carbons from biomass origin is reached. Nevertheless, cost of pruning wood is identified as another important economic parameter, pointing out the suitability of wet methods such as hydrothermal carbonization (HTC) to treat them as received form the harvest and to contribute to cutting down its prices.

The use of phytoremediation to remove metals of contaminated soils is an interesting technique that is usually limited by adverse physical and chemical properties of this type of soils. The addition of biochar produced from manure waste could improve soil properties due to its nutrient content, high cation exchange capacity or water holding capacity. However, the high water content of manure wastes precludes its pyrolysis treatment use for biochar production, without a previous drying step. Indeed, hydrothermal carbonization (HTC) of manure wastes could be an adequate treatment method as it takes place in water solution. The product obtained after HTC of biomass, named hydrochar, exhibits different physiochemical properties than biochar that could affect their potential application. The objective of the present work is to study the effect of chars obtained by pyrolysis or HTC of manure wastes in mining soil phytoremediation with Sinapis alba. Two selected mining soils (PORT and GAM) were treated with two manure biochars prepared at 450 °C (BMW450) and 600 °C (BMW600), two hydrochars prepared by HTC of manure at 190 °C (HWM190) and 240 °C (HMW240) and raw manure waste (MW) at a rate of 10% in mass. Later, different soil samples were incubated with or without Sinapis alba growth. Experimental results shown that properties of chars have a great influence on the efficiency of the use of Sinapis alba in the phytoremediation of mining soils. The addition of BMW600 and raw material (MW) increased the production of aerial and root biomass for GAM soil. For PORT soil, HMW190, two biochars and MW increases root and aerial biomass whereas HMW240 only produces an increment on aerial biomass. Addition of two hydrochars and MW had a positive effect on the biochemical soil activities and the highest microbial biomass carbon of GAM and PORT soil samples was observed after addition of HMW190. Finally, the addition of biochars and hydrochars could improve the phytoremediation of mini

In many countries, sewage sludge is directly used for energy and agricultural purposes after dewatering or digestion and dewatering. In recent years, there has been a growing interest in additional upstream hydrothermal carbonization (HTC), which could lead to higher yields in the energetic and agricultural use. Twelve energetic and agricultural valorization concepts of sewage sludge are defined and assessed for Germany to investigate whether the integration of HTC will have a positive effect on the greenhouse gas (GHG) emissions. The study shows that the higher expenses within the HTC process cannot be compensated by additional energy production and agricultural yields. However, the optimization of the HTC process chain through integrated sewage sludge digestion and process water recirculation leads to significant reductions in GHG emissions of the HTC concepts. Subsequently, nearly the same results can be achieved when compared to the direct energetic use of sewage sludge; in the agricultural valorization, the optimized HTC concept would be even the best concept if the direct use of sewage sludge will no longer be permitted in Germany from 2029/2032. Nevertheless, the agricultural valorization concepts are not generally advantageous when compared to the energetic valorization concepts, as it is shown for two concepts.

In the present study, the influence of substrate pre-treatment (grinding and sieving) on batch anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) was first assessed, then followed by co-digestion experiments with the liquid fraction from hydrothermal carbonization (LFHTC) of dewatered sewage sludge (DSS). The methane yield of batch anaerobic digestion after grinding and sieving (20 mm diameter) the OFMSW was considerably higher (453 mL CH4 STP g−1 VSadded) than that of untreated OFMSW (285 mL CH4 STP g−1 VSadded). The modified Gompertz model adequately predicted process performance. The maximum methane production rate, Rm, for ground and sieved OFMSW was 2.4 times higher than that of untreated OFMSW. The anaerobic co-digestion of different mixtures of OFMSW and LFHTC of DSS did not increase the methane yield above that of the anaerobic digestion of OFMSW alone, and no synergistic effects were observed. However, the co-digestion of both wastes at a ratio of 75% OFMSW-25% LFHTC provides a practical waste management option. The experimental results were adequately fitted to a first-order kinetic model showing a kinetic constant virtually independent of the percentage of LFHTC (0.52–0.56 d−1) and decreasing slightly for 100% LFHTC (0.44 d−1).