Enhancing the Fuel Properties of Spent Coffee Grounds through Hydrothermal Carbonization: Output Prediction and Post-Treatment Approaches

The reuse potential for the large annual production of spent coffee grounds (SCGs) is underexploited in most world regions. Hydrochars from SCGs produced via hydrothermal carbonization (HTC) have been recognized as a promising solid fuel alternative. To increase demand, optimization of the HTC and two post-treatment processes, washing and agglomeration, were studied to improve hydrochar in terms of energetic properties, minimizing unwanted substances, and better handling. HTC experiments at three scales (1–18.75 L) and varying process conditions (temperature T (160–250 °C), reaction time t (1–5 h), and solid content %So (6–20%) showed that the higher heating value (HHV) can be improved by up to 46%, and most potential emissions of trace elements from combustion reduced (up to 90%). The HTC outputs (solid yield—SY, HHV, energy yield—EY) were modeled and compared to published genetic programming (GP) models. Both model types predicted the three outputs with low error (<15%) and can be used for process optimization. The efficiency of water washing depended on the HTC process temperature and type of aromatics produced. The furanic compounds were removed (69–100%; 160 °C), while only 34% of the phenolic compounds (240 °C) were washed out. Agglomeration of both wet SCG and its hydrochar is feasible; however, the finer particles of washed hydrochar (240 °C) resulted in larger-sized spherical pellets (85% > 2000–4000 μm) compared to SCGs (only 4%)

Business and Market Analysis of Hydrothermal Carbonization Process: Roadmap toward Implementation

This study assesses the status of hydrothermal carbonization (HTC) technology and identifies barriers hindering its commercial viability. Conducting a global survey among HTC companies (with a total of 24 surveys sent), the research evaluates the current landscape, challenges, and future prospects of large-scale HTC operations. Furthermore, it presents a detailed global inventory of existing HTC facilities, illustrating geographical distribution and trends in application. Most of the companies are located in Europe, followed by Asia and North America. With substantial participation from HTC companies, exceeding 62% in the survey (15 companies), the study provides a comprehensive overview of diverse companies, their business models, regulatory challenges, and the overall state of HTC technology. The majority of companies in this study, approximately 80%, offer services in the field of waste management. This paper also explores the potential of HTC in transforming waste management practices, carbon sequestration methodologies, and the development of new materials. Employing a thorough SWOT analysis, the paper advocates for a broader adoption of HTC, emphasizing its transformative capacity in fostering sustainable management of urban, industrial, and agricultural residues, promoting circular economy principles, mitigating climate change, and offering a robust foundation for informed decision-making and sustainable development strategies

Advances in Research and Technology of Hydrothermal Carbonization: Achievements and Future Directions

Hydrothermal carbonization (HTC) has emerged as a pivotal technology in the battle against climate change and fosters circular economies. Operating within a unique reaction environment characterized by water as a solvent and moderate temperatures at self-generated pressures, HTC efficiently converts biomass residues into valuable bio-based products. Despite HTC’s potential—from the management of challenging biomass wastes to the synthesis of advanced carbons and the implementation of biorefineries—it encounters hurdles transitioning from academic exploration to industrial implementation. Gaps persist, from a general comprehension of reaction intricacies to the difficulty of large-scale integration with wastewater treatments, to the management of process water, to the absence of standardized assessment techniques for HTC products. Addressing these challenges demands collaboration to bridge the many scientific sectors touched by HTC. Thus, this article reviews the current state of some hot topics considered crucial for HTC development: It emphasizes the role of HTC as a cornerstone for waste management and biorefineries, highlighting potentialities and challenges for its development. In particular, it surveys fundamental research aspects, delving into reaction pathways, predictive models, analytical techniques, and HTC modifications while exploring HTC’s crucial technological applications and challenges, with a peculiar focus on combined HTC, wastewater integration, and plant energy efficiency

Trends and perspectives in the use of organic acids for critical metal recycling from hard-metal scraps

Hard-metal sector, strategic for the industrial economies, is suffering from the reduced availability and price volatility of its main feedstock: critical W and Co. In 2021, a 73.5 kt W and 9.2 kt Co demand for hard-metal production (65% and 5.3% of global demand, respectively), was recorded. Hard-metal scrap recycling is hence desirable for both environmental and economic reasons. A significant recovery of W and Co from manufacturing by-products and scraps is already good practice in the hard-metal industry (42% for W and 22% for Co). However, there is still a lot to do to meet the technical-economic-environmental sustainability in materials and energy enhancement for pursuing a green economy model. Indeed, Chemical Modification and Direct Recycling, which are the most widely employed industrial approaches, typically involve energy and/or harsh chemicals-intensive treatments which require expensive equipment and skilled workers. In the last decade, research efforts have been spent on implementing alternative materials reclamation processes from hard-metal scraps based on the use of bio-based organic acids with the view to increase the rate and quality of the recycled materials exploiting their peculiar metal complexing action as well as to preserve natural resources and prevent the disposal of potentially toxic/polluting substances. Despite the preliminary stage of the research, organic acids were demonstrated to be powerful but gentle agents for the selective leaching of cobalt from WC-Co-based materials as well as promising agents for WO3 dissolution. Indeed, thanks to their acid and complexing properties, they can stabilize metals in their oxidized form giving soluble products and preventing passivation phenomena. Furthermore, organic acids can be obtained by renewable biomass transformation, limiting the request for high-impact industrial chemicals. Hence they points out key features making them promising for the design of eco-friendly recovery processes. In this context, the different industrial approaches to the recovery and recycling of Hard-metal wastes, with specific reference to the role of bio-derived organic acids in hydro- and solvo-metallurgical processes, will be critically reviewed with the view of opening a discussion on the perspectives of their use in designing circular economy models in HM manufacturing as economically, technically and environmentally sustainable as possible

A Case Study of Implementation of Circular Economy Principles to Waste Management: Integrated Treatment of Cheese Whey and Hi-Tech Waste

In a global context characterized by severe environmental problems and increasing resource scarcity, waste represents both a challenge and an opportunity. This study aims to demonstrate with a real case the potential for optimizing the waste valorization action attainable through the synergic application of different treatments to residues of equally different nature and origin. In particular, bio-chemical (dark fermentation), chemical-physical (selective leaching) and thermo-chemical (hydrothermal carbonization) treatments were applied for the integrated valorization of whey from sheep cheese production and Hi-Tech waste (discarded electrical and electronic equipment). The treatments were applied at a laboratory scale on real samples of these residues. The organic acids used for selective leaching of valuable metals from Hi-Tech waste were obtained by dark fermentation of the cheese whey, while hydrothermal carbonization was used to convert the waste from previous stages into hydrochar feasible as solid fuel or soil improver. The dark fermentation tests have highlighted the possibility of recovering ≈ 100 g of organic acids from 1 L of whey; furthermore, it is also possible to recover bio-hydrogen depending on the operating conditions applied and the type of targeted organic acids. The leaching tests have demonstrated how the organic acids from whey fermentation have selective and quantitative mobilization capacities comparable to those of the same acids available on the market. The carbonization tests produced carbon-enriched hydrochar with promising fuel properties, as well as process waters suitable for anaerobic digestion with methane production. The results of the project led to the filing of an international patent

Valorisation of organic residues through hydrothermal carbonization

Hydrothermal carbonization (HTC) is a thermochemical process which can directly convert wet organic materials producing a carbon-rich solid (hydrochar, HC) and a liquid phase (process water, PW). Hydrochar has chemical and physical properties that make it similar to natural peats and coals. Depending on the process conditions, mostly temperature (180-250°C) and residence time (few hours), this material can be enriched in its carbon content, modifying its structure and providing it interesting characteristics that make it possible to be used for several applications, like energy production, as a soil ameliorant, adsorbent material for different pollutants, and some others reported in literature. In this work, the HTC process was applied to different organic materials such as hemp, grape marc, spent coffee grounds, AD (anaerobic digestion) digestate from hemp, from cow manure, and from agricultural residues, cigarette butts, surgical masks and gloves. Feedstocks and resulting products were physically and chemically characterised to be tested for different applications (e.g., energy production by combustion or AD, soil ameliorant, peat substitute in growing media or fertigation, or to be biologically treated in aerobic wastewater treatment plant). Compared to the feedstocks, hydrochars showed for all the tested materials strong physical and chemical modification (e.g., carbon content increased, and ash reduced). Due to the increased HHV, hydrochar may be used as solid fuel, which entails a higher energy recovery compared to the feedstock. Following HTC, all the tested materials showed a reduced volume (over 70%, in some cases) and an increased density. The possibility to use HC as peat substitute in growing media was tested for cow manure digestate, spent coffee grounds, and grape marc, showing that low amount of HC can partially substitute peat with similar or even better results. However, the addition of higher amount showed inhibition. Pre- and post-treatments tested (extraction, drying, washing) seem to have positive effect on seed germination and plant growth, removing some phytotoxic compounds. However, since these steps can be expensive, further tests should be carried out to understand which compounds cause inhibition, how to remove them or whether it is possible to destroy or avoid producing them by adequately setting the HTC process parameters. Process water (PW) should be properly treated since it contains many different compounds which can become an environmental problem. In this work, the PW valorisation was tested through characterising the liquid. The high amount of nutrient in PW may suggest the use in fertigation; however, germination tests showed inhibition effect. The aerobic treatability was tested through the bioassay on nitrifying bacteria. The high amount of VFAs may make PW a suitable substrate for AD to produce biogas. This work demonstrated that hydrothermal carbonization is a suitable process for the treatment of a wide range of organic materials. Depending on the feedstock, the resulting products may have different properties and characteristics which make them feasible for diverse applications. In general, all the feedstock tested may be used as solid fuel with several advantages, such as higher energy content, reduced volume, possible reduced emission during combustion, ease of dewatering, etc. The use on soil is promising when low amount of HC is used. However, pre- or post-treatments may increase the concentrations. Process water may be biologically treated: anaerobically, to produce biogas and remove some organic compounds and aerobically, but an acclimatation phase may be necessary. HTC proved to be a promising process in the biorefinery concept, especially when integrated with other processes (AD, aerobic degradation, etc.). Materials and energy can be recovered by HTC and re-introduced in the productive cycle and in the market, promoting the circular economy and the zero-waste concept

Dual-Purpose Valorization of Olive Pomace: Energy Production and Agricultural Application via Hydrothermal Carbonization

This study explores the hydrothermal carbonization (HTC) of olive pomace under varying conditions of temperature (180, 200, and 220 °C) and holding time (0 and 1 h) to identify the best valorization path for the resulting by-products. The hydrochars produced displayed a higher carbon content and lower oxygen and ash content compared to the feedstock, resembling the characteristics of solid fuels and suggesting their potential as sustainable energy sources. The liquid by-product (process water), was assessed for its effects on the germination of cress seeds (Lepidium sativum L.), revealing that while lower concentrations enhance seedling growth, higher concentrations exhibit significant phytotoxic effects. The study concludes that with appropriate dilution, process water can be effectively utilized in agriculture, aligning with sustainable waste management practices, and contributing to a circular approach

Airborne microbial contamination in indoor environments - comparison of sampling technologies for enumeration by cultivable method

International audienceThe aim of this study was to compare different sampling methods to investigate the presence of airborne cultivable microorganisms in indoor environments: impaction, impingement and filtration, before enumeration by cultivable method. Microbial aerosol measurements were carried out in 3 different indoor environments: classroom A in a high-efficiency building, classroom B in a low-efficiency building and office C. Three air sampling devices were used: an impaction-based device, the MAS-100NT (MBV), an impinger-based, the BioSampler (SKC) and two filters, cellulose nitrate filter (Sartorius) and fibrous filters (Lydall) requiring microorganisms liquid extraction from the filter before counting. Concentrations of the total cultivable airborne bacteria and fungi ranged from 20 to over 1000 CFU/m3. The highest airborne microbial concentration was observed in classroom A with ventilation off. The results showed that the impaction-based method was the most suitable for microbial aerosol sampling in indoor environments when followed by CFUs count. It enables to collect a greater number of cultivable microorganisms, compared with impingement or filtration with fibrous filter. Avoiding the transfer to liquid phase, more microorganisms are collected and can grow. Good results were achieved with filtration using cellulose filters without extraction phase

Benefits and Limitations of Using Hydrochars from Organic Residues as Replacement for Peat on Growing Media

New technologies for the production of peat-substitutes are required to meet the rising demand for growing media in horticulture and the need to preserve natural peatlands. Hydrothermal conversion of organic residues into char materials, hydrochars, with peat-like properties may produce such substitutes, reducing environmental impacts and CO2 emissions from improper management. To assess their potential as a component in growing media, cress seed germination tests are used to assess hydrochars from digestate (D), spent coffee grounds (SCG), and grape marc (GM). Pre and post-treatments (extraction, washing, and drying) are applied to remove phytotoxic compounds associated with process waters retained on the hydrochars, and a nitrification bioassay with process water is used to predict their toxicity. All hydrochars achieve similar or better germination results compared to their feedstock, showing a potential to replace at least 5% of peat in growing media. SCG and GM hydrochars show inhibition above 5%, while all post-treated D-hydrochar mixtures produce >3 times longer roots than the control. The nitrification test shows a high sensitivity and good agreement with the high inhibition trends found in the germination tests with process water. Such tests can be a good way to optimize process combinations for the hydrothermal production of peat replacements

Valorization of Face Masks Produced during COVID-19 Pandemic through Hydrothermal Carbonization (HTC): A Preliminary Study

The COVID-19 pandemic has led to the increased use of disposable face masks worldwide, resulting in a surge of potentially infectious waste. This waste must be safely managed and disposed of to prevent the spread of the virus. To address this issue, a preliminary study explored the use of hydrothermal carbonization (HTC) as a potential method for converting surgical mask waste into value-added carbonaceous materials. The HTC treatments were conducted at 220 °C for 3 h with or without the addition of acetic acid. The resulting hydrochar was characterized using several techniques, including thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and N2-physisorption analyzers. The study found that the masks formed a melt with reduced mass (−15%) and volume (up to −75%) under the applied conditions. The carbon content and higher heating value (HHV) of the produced hydrochars were higher than those of the original masks (+5%). Furthermore, when acetic acid was added during the HTC experiment, a new crystal phase, terephthalic acid, was produced. This acid is a precursor in surgical mask production. The study suggests that hydrothermal carbonization could potentially achieve sanitization and volume reduction in non-renewable and non-biodegradable surgical masks while also producing a solid fuel or a raw material for terephthalic acid production. This approach offers an innovative and sustainable solution to manage the waste generated by the increased use of disposable face masks during the pandemic