Management of off-specification compost by using co-hydrothermal carbonization with olive tree pruning. Assessing energy potential of hydrochar

Hydrochar and synthetic natural gas co-production for a full circular economy implementation via hydrothermal carbonization and methanation: An economic approach

Herein we study the economic performance of hydrochar and synthetic natural gas co-production from olive tree pruning. The process entails a combination of hydrothermal carbonization and methanation. In a previous work, we evidenced that standalone hydrochar production via HTC results unprofitable. Hence, we propose a step forward on the process design by implementing a methanation, adding value to the gas effluent in an attempt to boost the overall process techno-economic aspects. Three different plant capacities were analyzed (312.5, 625 and 1250 kg/hr). The baseline scenarios showed that, under the current circumstances, our circular economy strategy in unprofitable. An analysis of the revenues shows that hydrochar selling price have a high impact on NPV and subsidies for renewable coal production could help to boost the profitability of the process. On the contrary, the analysis for natural gas prices reveals that prices 8 times higher than the current ones in Spain must be achieved to reach profitability. This seems unlikely even under the presence of a strong subsidy scheme. The costs analysis suggests that a remarkable electricity cost reduction or electricity consumption of the HTC stage could be a potential strategy to reach profitability scenarios. Furthermore, significant reduction of green hydrogen production costs is deemed instrumental to improve the economic performance of the process. These results show the formidable techno-economic challenge that our society faces in the path towards circular economy societies.

Renewable Carbonaceous Materials from Biomass in Catalytic Processes: A Review

This review paper delves into the diverse ways in which carbonaceous resources, sourced from renewable and sustainable origins, can be used in catalytic processes. Renewable carbonaceous materials that come from biomass-derived and waste feedstocks are key to developing more sustainable processes by replacing traditional carbon-based materials. By examining the potential of these renewable carbonaceous materials, this review aims to shed light on their significance in fostering environmentally conscious and sustainable practices within the realm of catalysis. The more important applications identified are biofuel production, tar removal, chemical production, photocatalytic systems, microbial fuel cell electrodes, and oxidation applications. Regarding biofuel production, biochar-supported catalysts have proved to be able to achieve biodiesel production with yields exceeding 70%. Furthermore, hydrochars and activated carbons derived from diverse biomass sources have demonstrated significant tar removal efficiency. For instance, rice husk char exhibited an increased BET surface area from 2.2 m2/g to 141 m2/g after pyrolysis at 600 °C, showcasing its effectiveness in adsorbing phenol and light aromatic hydrocarbons. Concerning chemical production and the oxidation of alcohols, the influence of biochar quantity and pre-calcination temperature on catalytic performance has been proven, achieving selectivity toward benzaldehyde exceeding 70%.

Co-hydrothermal carbonization of polyvinyl chloride and lignocellulose biomasses: Influence of biomass feedstock on fuel properties and combustion behaviors

Co-hydrothermal carbonization (co-HTC) of lignocellulose biomass (LB) and chlorinated waste could produce value-added co-hydrochar while simultaneously removing inorganic metal salts and organic chlorine to the liquid phase. However, there is a lack of understanding of the influence of LB feedstocks on the fuel properties and combustion behaviors of co-hydrochars. Therefore, co-hydrochars derived from co-HTC of pine, bamboo, corncob, wheat stalk, and corn stalk with polyvinyl chloride (PVC) at the mass ratio of 9:1 under 260 °C for 30 min were tested. PVC facilitated the hydrolysis, dehydration, and polymerization of LB compositions (hemicellulose, cellulose, soluble lignin, and insoluble lignin). In turn, these LB compositions could prevent PVC aggregation and promote PVC substitution. Hydrochar fragments could coat the PVC surface and hinder its hydrolysis. Interactions between LB compositions and PVC improved the fuel properties and combustion behaviors of co-hydrochars derived from bamboo, corncob, wheat stalk, and corn stalk while decreasing the fuel properties and combustion behaviors of co-hydrochar derived from pine (HC-PPE). Except for HC-PPE, the fuel ratio (fixed carbon/volatile matter) of co-hydrochars increased to 0.90-1.18 and their HHVs reached approximately 17.5-32.45 MJ/kg without an increased risk of chlorine corrosion. The combustion of co-hydrochars was easier and more stable due to their higher ignition and burnout temperatures and lower activation energies. These findings provide comprehensive knowledge of the LB feedstocks influence on fuel properties and combustion behaviors of co-hydrochars, which would contribute to the cost-effective use of LB and chlorinated wastes.

Dispose of Chinese cabbage waste via hydrothermal carbonization: hydrochar characterization and its potential as a soil amendment

Landfill of waste biomass not only poses a threat to environmental protection but also leads to a great waste of biomass resources. Hydrothermal carbonization (HTC) has been considered a promising method to convert the wet biomass into hydrochar, a high-value-added product with multiple application potentials. The cabbage waste, typical wet waste biomass with a huge production per year, was hydrothermally carbonized under 190 °C and 260 °C, respectively. The results indicated that the majority of nutrients from feedstock were dissolved in spent liquor during HTC, with only a few amounts retained on hydrochar. Temperature showed a more significant impact on hydrochar properties than retention time, which enables hydrochar to be potentially used as a soil conditioner. Particularly, the hydrochar produced at 190 °C could improve plant nutrition in the short term, while that produced at 260 °C may benefit in C sequestration. Moreover, the hydrochar dominated by meso/macropores (> 90%) would be conducive to the storage of plant-available water. But both BTX and VOCs may release during hydrochar application; thus, further field experiments are needed to test the environmental risks of hydrochar when applied as a soil amendment.

Hydrothermal carbonization of the wet fraction from mixed municipal solid waste: Hydrochar characteristics and energy balance

Mixed municipal solid waste (MSW) may be pre-treated in a mechanical-biological treatment (MBT) plant to produce an exiting stream with improved combustible characteristics. The process also produces a second waste stream, which is generally separated on a size basis by industrial sieving equipment. It contains fractions with a high moisture content such as residual food waste, soiled paper and cardboard, and small fragments of other materials. Samples of this stream, collected at an existing plant, were characterized and processed by hydrothermal carbonization (HTC) at laboratory scale, at various temperatures (180, 200 and 220 °C), reaction times (1, 4 and 8 h) and solid to water ratios (0.15 and 0.07). The primary energy balance, on a hypothetical industrial scale, was performed. In brief, the results confirmed that the produced hydrochar was a brittle, hydrophobic, solid carbonaceous product which gave a better combustion performance as the residence time of the HTC process was increased. Moreover, the dewaterability of the carbonized waste was greatly improved when compared to raw, wet samples. The results of the primary energy balance confirmed that the energy contained in the produced hydrochar was higher than the energy consumption for the process itself, under all the HTC working conditions. The energy consumed in the process was in the range of 40-70 % of the energy content of the produced hydrochar.

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

Hydrothermal carbonization (HTC) provides an attractive alternative method for the treatment of high-moisture waste and, in particular, digested sludge. HTC could reduce the costs and environmental risks associated with sludge handling and management. Although it is recognized that the dewaterability of hydrochars produced from digested sludge, even at mild temperatures (180–190 °C), is highly improved with respect to the starting material, the filterability of HTC slurries for the recovery of the solid material (hydrochar) still represents a challenge. This study presents the results of an investigation into the filterability of agro-industrial digested sludge HTC slurries produced by a C-700 CarboremTM HTC industrial-scale plant. The filterability of HTC slurries, produced at 190 °C for 1 h, with the use of acid solutions of hydrochloric acid, sulfuric acid or citric acids, was investigated by using a semi-industrial filter press. The use of sulfuric acid or citric acid solutions, in particular, significantly improved the filterability of HTC slurries, reducing the time of filtration and residual moisture content. The acid treatment also promoted the migration of heavy metals and phosphorus (P) in the HTC filtrate solution. This study demonstrates that P can be recovered via the precipitation of struvite in high yields, recovering up to 85 wt% by mass of its initial P content.

Promoting Sustainability: Wastewater Treatment Plants as a Source of Biomethane in Regions Far from a High-Pressure Grid. A Real Portuguese Case Study

Wastewater treatment plants (WWTP) located in regions far from a high-pressure grid can produce renewable biomethane, which can partially substitute the natural gas locally consumed. However, the economic viability of implementing biomethane plants in WWTP has to be guaranteed. This paper uses the discount cash flow method to analyze the economic viability of producing biomethane in a WWTP located in Évora (Portugal). The results show that, under the current conditions, it is unprofitable to produce biomethane in this WWTP. Since selling the CO2 separated from biogas may result in an additional income, this option was also considered. In this case, a price of 46 EUR/t CO2 has to be paid to make the project viable. Finally, the impact of potential government incentives in the form of feed-in premia was investigated. Without selling CO2, the project would only be profitable for feed-in premia above 55.5 EUR/MWh. If all the CO2 produced was sold at 30 EUR/t CO2, a premium price of 20 EUR/MWh would make the project profitable. This study shows that the economic attractiveness of producing biomethane in small WWTP is only secured through sufficient financial incentives, which are vital for developing the biomethane market with all its associated advantages.