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This contribution reports the effects of product recovery methods on the yields and properties of hydrochars produced from hydrothermal carbonization (HTC) of algal biomass. A slurry of Chlorella vulgaris with 10 wt% of solid loading was hydrothermally carbonized at 180 – 220 °C with holding time of 15 and 60 min. The resulting hydrochars were recovered by two prevailing methods, namely direct filtration and dichloromethane (DCM)-aided filtration. The results indicate that, under identical HTC conditions, compared with DCM-aided filtration, direct filtration always results in higher hydrochar yield because of the biocrude retained on its surface. The presence of residue biocrude in the hydrochars from direct filtration further leads to considerable differences in their properties, as compared with their counterparts from DCM-aided filtration. Specifically, under identical HTC temperature and holding time, DCM-aided filtration yields hydrochars of lower volatile matter contents but higher fixed carbon and ash contents, as compared with the hydrochars from direct filtration. Direct filtration generally recovers hydrochars of greater higher heating values and energy-based yields as compared to DCM-aided filtration. The product recovery methods show considerable impacts on the concentrations of Na, K, Mg, and Ca and their retentions in the hydrochars at 220 °C for 60 min, with higher concentrations and retentions of these elements being observed in the hydrochars from DCM-aided filtration. The hydrochars at 220 °C for 15 and 60 min from DCM-aided filtration show higher specific reactivity of 0.040 – 0.058 min−1 and 0.066 – 0.096 min−1 at hydrochar conversions of ≤ 82 % and ≤ 78 %, respectively, as compared with their direct-filtration counterparts, due to their higher concentrations of catalytic alkali and alkaline earth metals. These findings highlight the importance of considering product recovery methods when comparing the yields and properties of hydrochars from HTC

This paper proposes a method of preparing high-strength formed coke from woody biomass without binder. Chipped and pre-dried Japanese cedar was heat-treated in an inert atmosphere (i.e., torrefied) at 225–325°C (Tt), pulverized to sizes in three different ranges, molded into briquettes (in the form of thick disk with diameter/thickness ≈ 2.5) at temperature up to 200°C by applying mechanical pressure of 128 MPa. The torrefied/briquetted cedar (TBC) was then converted into coke by heating to 1000°C in an inert atmosphere at normal pressure. This process sequence enabled to prepare coke having indirect tensile strength (St) of 8–32 MPa, which was much higher than that without torrefaction, below 5 MPa. The torrefaction greatly improved pulverizability of the cedar, which was further promoted by increasing Tt. St of TBC and that of coke both increased as the particle sizes of TBC decreased, but this explained only a minor part of significant effect of Tt on St of the coke. St was maximized at Tt = 275°C regardless of the degree of pulverization. The Tt effects on physicochemical properties of TBC and coke were investigated in detail. The difference in St of coke by Tt was mainly due to that in the increment of St along the carbonization at 500–1000°C. Fracture surfaces of the coke had particular morphologies that had been inherited from the original honeycomb structure of the cedar.

This contribution evaluates the impacts of reaction conditions on the yields and properties of biocrude and aqueous phase from hydrothermal carbonization (HTC) of Chlorella vulgaris (solid loading: 10.0 wt%) at 180 – 220 °C for 15 and 60 min. The results indicate that increasing HTC temperature (THTC) from 180 to 220 °C enhances biocrude yield from 8.8 to 34.6 wt%, for the holding time (th) of 60 min, and from 4.9 to 26.0 wt% for 15 min. The carbon recovery and higher heating value (HHV) of biocrude elevate with increasing both THTC and th, with the highest values of 50.6% and 34.0 MJ/kg, respectively, observed at 220 °C and 60 min. Among inorganic species, Na, Fe, and Zn display reasonable retentions in the biocrude with a pronounced effect of THTC observed at 15 min. The yield of aqueous phase increases first when THTC rises from 180 to 200 °C but decreases with further increasing THTC to 220 °C due to the re-polymerization of water-soluble organics into biocrude and/or the formation of gaseous products. The aqueous phase experiences a reduction of total organic carbon with increasing reaction severity because of the transfer of carbon to biocrude and/or gases. The aqueous phase is rich in organic nitrogen, dihydrogen phosphate, acetate, and potassium, which are required for its recycling as cultivation media. The concentrations of heavy metals (Fe, Cr, Mn, Co, Ni and Zn) in the aqueous phase are all below 9 mg/kg under the studied HTC conditions.

This study proposes a new process that treats pyrolytic wastewater (PWW) from biomass pyrolysis with char-assisted drying. It can be operated at two modes: (I) one-stage drying for PWW treatment; and (II) two-stage drying and condensation for both treating PWW and recovering valuable chemicals (e.g., acetic acid). The process is proved via drying the mixture of a char and a synthetic PWW solution containing acetic acid, acetol, furfural, and phenol at different temperatures (60, 80, and 105 °C) and char-to-PWW mass ratios (1.0, 1.5, and 2.0). The results, supported by density functional theory (DFT) calculations, demonstrate that under Mode I, the char captures over 97.1% of the organic compounds in the PWW at 60 °C and a char-to-PWW ratio of 2.0, while evaporating almost all the water. The evaporated stream can be potentially discharged into the atmosphere without condensation or condensed into wastewater with a significantly reduced content of total organic carbon. Under Mode II, the char-PWW mixture is sequentially dried at 60 and 105 °C, with the exhaust gases being separately condensed. This yields a stream of purified acetic acid solution (concentration: ∼76 wt%) concentrated by a factor of ∼6.3 (compared with the PWW), with a recovery rate of ∼29%.

The removal of alkali and alkaline-earth metallic (AAEM) species from biomass and/or its pyrolysis-derived char is often required to mitigate ash-related issues during their combustion. Leaching of biomass with an acidic aqueous phase (AP) produced from its pyrolysis is a promising strategy but encounters several issues, including high moisture content in wet biomass after dewatering and loss of organic carbon from biomass. Here, we propose a new process that leaches char with pyrolytic AP for the removal of AAEM species from and the uptakes of water-soluble organics by char. The char and pyrolytic AP produced from the pyrolysis of wheat straw at 450 °C in an auger reactor were subjected to time-dependent leaching at a liquid-to-solid mass ratio of 20. The results demonstrate that the majority of K, Mg, and Ca are leached within the first hour. Nonacidic compounds in the pyrolytic AP slightly hinder the leaching of K but have no impacts on that of Mg and Ca. Except for acetol, the uptakes of other organic compounds (e.g., furfural and phenolic compounds) by char are rapid in the first hour, become slower in 1–2 h, and then reach equilibrium. The results from density functional theory (DFT) calculations suggest the presence of chemisorption for the interactions between these organic compounds and char. Repeated leaching runs at the optimized leaching time (1 h) suggest that the pyrolytic AP is sufficient for leaching the char from the same run, achieving removal rates of ∼63.7 wt % for K, ∼43.7 wt % for Mg, and ∼60.7 wt % for Ca. After repeated leaching, there are only four organic compounds, including 2,3-butanedione, acetol, 2(5H)-furanone, and the remaining acetic acid, left in the leachate, simplifying the compositions of the pyrolytic AP. Leaching the char, instead of wheat straw, with the pyrolytic AP offers significant advantages, including the reduced moisture content in the wet solid after dewatering, negligible loss of organic carbon, and enhanced capture o

This contribution develops an analytical procedure for the proximate analysis of algal biomass, using Spirulina platensis and Chlorella vulgaris as model algal samples. The procedure consists of three steps. First, thermogravimetric analyzer (TGA)-based method designed from ASTM E870-82 was employed for the proximate analysis of the two algae, at 950 °C in argon for the determination of volatile matter (VM) and fixed carbon (FC) and at 600 °C in air for estimating the ash content. Visual inspection of the resulting solid residues suggested the presence of black unburnt carbon, indicating the inapplicability of the method. Nevertheless, the contents of moisture, VM, and the sum of FC and ash in the two algae were determined in this step. In the second step, Spirulina platensis and Chlorella vulgaris were directly oxidized at 600 °C in air, in the same TGA as above, to avoid or minimize the effect of stable quaternary nitrogen formed during pyrolysis at 950 °C in the first step. While Spirulina platensis was successfully ashed in this step, the solid residue of Chlorella vulgaris was virtually black. This necessitated the third step, in which Chlorella vulgaris was ashed in the air at 600 °C in a tubular reactor, with the aid of hydrogen peroxide. Nearly complete oxidation of carbonaceous material in Chlorella vulgaris was achieved when the holding time at 600 °C was 4 h. Based on the proposed analytical procedure, the contents of VM, FC, and ash are 82.3, 10.4, and 7.3 wt %, respectively, for Spirulina platensis and 84.3, 10.4, and 5.3 wt % for Chlorella vulgaris, expressed on a dry basis.