• Energy Research

The role of microalgae cultivation in wastewater treatment and reclamation has been studied extensively, as has the potential utility of the resulting algal biomass. Most methods for processing such biomass generate solid residues that must be properly managed to comply with current sustainable resource utilization requirements. Hydrothermal carbonization (HTC) can be used to process both individual wet feedstocks and mixed feedstocks (i.e., co-HTC). Here, we investigate co-HTC using microalgae and digested sewage sludge as feedstocks. The objectives were to (i) study the material's partitioning into solid and liquid products, and (ii) characterize the products' physicochemical properties. Co-HTC experiments were conducted at 180-250°C using mixed microalgae/sewage sludge feedstocks with the proportion of sewage sludge ranging from 0 to 100 %. Analyses of the hydrochar composition and the formation and composition of secondary char revealed that the content of carbonized material in the product decreased as the proportion of sewage sludge in the feedstock increased under fixed carbonization conditions. The properties of the hydrochars and the partitioning of material between the liquid phase and the hydrochar correlated linearly with the proportion of microalgae in mixed feedstocks, indicating that adding sewage sludge to microalgae had weak or non-existent synergistic effects on co-HTC outcomes. However, the proportion of sewage sludge in the feedstock did affect the secondary char. For example, adding sewage sludge reduced the abundance of carboxylic acids and ketones as well as the concentrations of higher molecular weight cholesterols. Such changes may alter the viable applications of the hydrochar.

This work focuses in the application of the hydrothermal carbonization (HTC) technology as a possible moist agro-industrial waste management treatment. Through this technique, olive mill, canned artichoke and orange wastes (OMW, CAW and OJW, respectively) were carbonized in a lab-scale high pressure reactor at different temperatures (200–250 °C) and durations (2, 4, 8 and 24 h) in order to obtain useful bioenergy feedstocks. The effect of the residence time and temperature on the properties of the bio-char obtained was studied through different characterization techniques. Material and energy balances were also performed to determine the potential energy saving of hydrothermal carbonization versus dry thermal treatments like torrefaction (TF). It is found that the moisture content of HTC-hydro-chars decreases as the temperature and duration increase, which implies that wet biomass can be upgraded and, at the same time, dewatered through HTC. The best results are found for the OMW, whose moisture content decreases from over 70% to less than 30% for the experiments carried out under the more severe conditions. Consequently, it is be possible to reach energy savings over 50% by using HTC instead of TF technologies. Regarding the hydro-char properties, the hydrothermal carbonization of the three organic wastes treated leaded to hydro-chars that present carbon contents and heating values closed to those of brown coal and great energy densifications, depending on the type of waste. Accordingly, it can be concluded that it is feasible to manage moist agro-industrial wastes via HTC, which is ostensibly more efficient than TF in terms of energy consumption. Authors kindly wish to thank the Spanish Ministry of Economy and Competitiveness (INNPACTO Project IPT-2012-0565-310000) and the Spanish Ministry for Education and Science (contract grant number 8850828) for financial support. The author V. Benavente also thanks the Conselleria d’Educación, Cultura i Esport, for a Ph.D. grant (contract grant number ACIF/2014/275).

Abstract Hydrochars produced from agro-industrial wastes contain significant amounts of alkali metals and other inorganic elements that lead to different ash-related issues in energy systems. This work aims to modify the composition of hydrochar ash by adding rice hulls and/or mineral additives (i.e., lime, bentonite and kaolin) to improve their fusion behavior. Two series of samples were prepared to analyze the effect that the use of additives before or after the HTC processing of olive mill waste at 225 °C for 2 h had on the hydrochar properties. Hydrochars were then ashed, in agreement with UNE-EN 18122:2016, and the corresponding ashes subsequently characterized through XRF and ash fusion tests to determine their ash composition and ash characteristic fusion temperatures (AFTs), respectively. Among the studied additives, rice hulls led to the best results, as their use increased the initial ash deformation temperature to >1500 °C and resulted in the lowest hydrochar ash content. Using rice hulls before or after the HTC process did not affect the ash behavior, but it did change the hydrochar properties as a fuel. The joint HTC of rice hulls and feedstock led to a less carbonized solid with 13% higher moisture content and 15% lower energy density. Accordingly, the addition of rice hulls to ready-produced hydrochars is recommended to maintain the energy benefits of HTC as possible. The use of rice hulls as a low-cost additive to improve the fusion behavior of hydrochar ash opens new avenues for hydrochars in energy conversion processes with limited ash-related operational problems.

The effects of hydrothermal treatment on the drying properties of sludge were determined. Sludge was hydrothermally treated at 180-260 °C for 0.5-5 h using NaOH and HCl as additives to influence reaction conditions. Untreated sludge and attained hydrochar samples were then dried under identical conditions with a laboratory microdryer and an X-ray microtomograph was used to follow changes in sample dimensions. The effective moisture diffusivities of sludge and hydrochar samples were determined and the effect of process conditions on respective mean diffusivities evaluated using multiple linear regression. Based on the results the drying time of untreated sludge decreased from approximately 80 min to 37-59 min for sludge hydrochar. Drying of untreated sludge was governed by the falling rate period where drying flux decreased continuously as a function of sludge moisture content due to heat and mass transfer limitations and sample shrinkage. Hydrothermal treatment increased the drying flux of sludge hydrochar and decreased the effect of internal heat and mass transfer limitations and sample shrinkage especially at higher treatment temperatures. The determined effective moisture diffusivities of sludge and hydrochar increased as a function of decreasing moisture content and the mean diffusivity of untreated sludge (8.56·10(-9) m(2) s(-1)) and sludge hydrochar (12.7-27.5·10(-9) m(2) s(-1)) were found statistically different. The attained regression model indicated that treatment temperature governed the mean diffusivity of hydrochar, as the effects of NaOH and HCl were statistically insignificant. The attained results enabled prediction of sludge drying properties through mean moisture diffusivity based on hydrothermal treatment conditions.

Abstract This study investigated the effect of lipid extraction of microalgae feedstocks subjected to hydrothermal carbonization (HTC) with regard to the carbonization degree, chemical composition and phytotoxicity of hydrochars produced under different reaction temperatures and residence times. Special attention was given to the formation and composition of secondary char, as this part of the hydrochar may be of particular importance for environmental and technical applications. A microalgae polyculture grown in municipal wastewater was extracted to retrieve lipids, and both unextracted (MA) and extracted microalgae (EMA) were used to produce hydrochars at 180–240 °C for 1–4 h. The composition of the hydrochars was thoroughly characterized by elemental analysis, thermogravimetric analysis and pyrolysis–gas chromatography/mass spectrometry analysis. MA exhibited a greater carbonization degree than EMA and contained higher amounts of secondary char under the same processing conditions. During the carbonization of EMA, more decomposition products remained in the liquid phase and less polymerization occurred than for MA, which explained the lower solid yield of EMA-derived hydrochars in comparison to MA hydrochars. Consequently, although they contained potentially toxic substances (i.e., carboxylic acids, aldehydes and ketones), the EMA-derived hydrochars exhibited a lower phytotoxic potential. This indicates that low-temperature hydrochars containing less than 10% of extractives might be suitable as soil amendments, whereas extractive-rich hydrochars would be more appropriate for other long-term applications, such as adsorbents for contaminant removal, energy storage and composite materials. Detailed characterization of microalgae-derived hydrochars is required to enable the most suitable application areas to be identified for these materials, and thereby make full use of their function as carbon sinks.

Although hydrothermal carbonization of biomass components is known to be mainly governed by reaction temperature, consistent reports on the effect and statistical significance of process conditions on hydrochar properties are still lacking. The objective of this research was to determine the importance and significance of reaction temperature, retention time and solid load on the properties of hydrochar produced from an industrial lignocellulosic sludge residue. According to the results, reaction temperature and retention time had a statistically significant effect on hydrochar ash content, solid yield, carbon content, O/C-ratio, energy densification and energy yield as reactor solid load was statistically insignificant for all acquired models within the design range. Although statistically significant, the effect of retention time was 3-7 times lower than that of reaction temperature. Predicted dry ash-free solid yields of attained hydrochar decreased to approximately 40% due to the dissolution of biomass components at higher reaction temperatures, as respective oxygen contents were comparable to subbituminous coal. Significant increases in the carbon contents of hydrochar led to predicted energy densification ratios of 1-1.5 with respective energy yields of 60-100%. Estimated theoretical energy requirements of carbonization were dependent on the literature method used and mainly controlled by reaction temperature and reactor solid load. The attained results enable future prediction of hydrochar properties from this feedstock and help to understand the effect of process conditions on hydrothermal treatment of lignocellulosic biomass.

Two-phase olive mill waste (TPOMW) was converted via torrefaction into a carbon rich solid interesting as bioenergy feedstock. TPOMW was characterized and torrefied in an oven at temperatures ranging from 150 to 300 °C for 2 h. Mass and energy losses occurred during torrefaction were measured and the torrefied products were characterized including ultimate analysis, heating value measurements, accelerate solvent extraction (ASE) and FTIR in order to assess the effects of torrefaction on the physicochemical properties of TPOMW. Additionally, ash fouling evaluation was also performed through XRF analysis. The weight fraction of C, defined in percentage as wt.%, improved from 56 to 68 wt.% and the high heating value rose from 26.4 to 30.0 MJ·kg−1 as torrefaction temperature increased, reaching typical values of subbituminous coal and finding the best results at 200 °C in terms of maximizing the heating value and minimizing the energy losses. Accordingly, from FTIR analysis it was observed that the degree of coalification increased during torrefaction of TPOMW. ASE results shown that the residual olive oil in TPOMW was removed during torrefaction, being completely eliminated at 300 °C. The alkali index for TPOMW was found to be 0.66 kg alkali·GJ−1, which implied a high fouling tendency that could be mitigated through co-firing. Finally, t-TPOMW briquettes with good mechanical strength and energy density of 26.7 GJ·m−3 were produced using a hydraulic piston press. Results demonstrated that torrefaction allows transforming TPOMW into a coal-like material, which would imply a profitable way to manage these wastes. Authors kindly wish to thank the Spanish Ministry of Economy and Competitiveness (INNPACTO Project IPT-2012-0565-310000) for financial support.

Mixed sludge from a pulp and paper mill was hydrothermally carbonized at 180-260°C for 0.5-5h with the use of HCl or NaOH for determining the effect of acid and base additions during sludge carbonization. Based on the results carbonization was mainly governed by dehydration, depolymerization and decarboxylation of sludge components. Additive type had a statistically significant effect on hydrochar carbon content and carbon and energy yield, of which especially energy yield increased through the use of HCl. The theoretical energy efficiencies of carbonization increased with decreasing reaction temperature, retention time and the use of HCl and suggested that the energy requirement could be covered by the energy content of attained hydrochar. The BOD5/COD-ratios of analyzed liquid samples indicated that the dissolved organic components could be treated by conventional biological methods.