Hydrothermal carbonization (HTC) of fructose and urea containing solutions was conducted at 180 °C to study the influence of nitrogen‐containing compounds on the conversion process and HTC products properties. The concentration of fructose was fixed, while the concentration of urea was gradually increased to study its influence on the formation of nitrogen‐containing hydrochar (N−HC). The degradation of urea has an important influence on the HTC of fructose. The Maillard reaction (MR) promotes the formation of N−HC in acidic conditions. However, in alkaline conditions, MR promotes the formation of bio‐oil at the expense of N−HC. Alkaline conditions reduce N−HC yield by catalyzing fragmentation reactions of fructose and by promoting the isomerization of fructose to glucose. The results showed that adjusting the concentration of nitrogen‐containing compounds or the pH value of the reaction environment is important to force the reaction toward the formation of N−HC or N‐bio‐oil.

No abstract

In this study, the fate of carbon and nutrient elements, nitrogen, phosphorus, and potassium, was investigated during the hydrothermal carbonization (HTC) of three model solutions with different nutrient concentrations to mimic biogas residues from silage and cattle manure. The HTC was conducted at 180 °C, 220 °C, 240 °C, and 260 °C for 3 h reaction time. ICP-OES, GC, IC, and chemical analysis methods were utilized to measure fertilizing elements in HTC products. The distribution of nutrients between HTC product phases was significantly influenced by the compositions of the initial feedstock before HTC. The incorporation of nitrogen to the hydrochar (HC) depends on the nitrogen-containing compounds during HTC; the chemical bonding was the main mechanism for the incorporation of NH4–N into the HC during HTC, the sorption of NH4–N to the HC occurred, however, to a lesser extent (0.8% to 7.4%). Most of the NO3–N stayed in the process water (PW) during HTC (78% to 87%), and the sorption (adsorption/salts precipitation) was confirmed to be the main mechanism for the recovery of NO3–N to the HC during HTC. The uptake of N to the HC is limited and depends on the availability of the carbon network in the HC, the correlation coefficient between HC formation and the recovery of bonded N to the HC showed a high linear regression coefficient R 2 = 0.90–0.91. However, the recovery of N to the HC via sorption showed less correlation to the HC formation during HTC (R 2 = 0.35–0.54). Most of the dissolved K+ (99%) stayed in the PW during HTC. Due to the absence of metal associations in this work, most of the dissolved PO4 3– (99%) stayed in the PW during HTC, which proves that controlling metal cations in the feedstock before HTC has a considerable influence on the distribution of phosphate between the PW and HC.

The thermal treatment of an activated carbon/chars with a nitrogen precursor is not a sustainable nor an efficient method for the incorporation of N into the carbon structure. This study proposes the use of hydrothermal carbonization (HTC) as an environmentally friendly method for the incorporation of N into the bulk of carbon materials. The authors propose the following sequence for the synthesis of Nenriched carbon materials (NCM) for energy storage applications: HTC of the N precursor and biomass ! activation of N-hydrochar (N-HC). To investigate the proposed method, HTCs of spent coffee grounds (SCG) with N precursors (urea and alanine) were conducted at 220 C for 5 hours. The resulted N-HCs were subjected to a mild thermal activation via pyrolysis at 600 C for 2 hours. The results showed that HTC enhances the incorporation of N into the carbon matrix via many reactions, for example, the Maillard reaction (MR) or the Mannich reaction, which may accompany the formation of HC via the solved-intermediate pathway or the solid-to-solid pathway, respectively. However, adjusting some parameters before HTC, for example, the concentration of the N precursor and the pH value of the slurry is important to avoid a significant reduction in the N-HC yield. The proposed method led to the synthesis of NCM with a N content of 10.3wt% The X-ray photoelectron spectroscopy (XPS) confirmed the incorporation of N into the bulk of NCM and showed a significant increase in the content of heterocyclic N compounds (pyridinic N, pyrrolic N, and graphitic N) in the NCM. The incorporation of N via the proposed method significantly improved the electrochemical properties of NCM as the values of the specific capacitance and the electrical conductivity in the NCM increased by five times and four times, respectively.

This study investigates the production of biobased carbon materials from potato waste and its application in energy storage systems such as supercapacitors. Three different categories of carbons were produced: hydrochar (HC) from hydrothermal carbonization (HTC) at three different temperatures (200 °C, 220 °C, 240 °C) and two different duration times (two hours and five hours), pyrolyzed hydrochar (PHC) obtained via pyrolysis of the HTC chars at 600 °C and 900 °C for two hours and pyrochar from the pyrolysis of biomass at 600 °C and 900 °C for two hours. The carbon samples were analysed regarding their physico-chemical properties such as elemental composition, specific surface area, bulk density and surface functionalities as well as their electrochemical characteristics such as electric conductivity and specific capacity via cyclic voltammetry. N- and O-enriched carbon materials with promising specific surface areas of up to 330 m2 g−1 containing high shares of microporosity were produced. Electric conductivities of up to 203 S m−1 and specific capacities of up to 134 F g−1 were obtained. The presence of high contents of oxygen (4.9–13.5 wt.%) and nitrogen (3.4–4.0 wt.%) of PHCs is assumed to lead to considerable pseudocapacitive effects and favor the high specific capacities measured. These results lead to the conclusion that the potential of agricultural biomass can be exploited by using hydrothermal and thermochemical conversion technologies to create N- and O-rich carbon materials with tailored properties for the application in supercapacitors.

No abstract

There are increasing demands for syngas production. Fossil-based fuels are expensive and environmentally unfriendly. Affordable, sustainable, and environmentally healthy alternatives and sustainable fuel sources are needed. This study focused on producing tor-chars from two different tropical biomasses (from Nigeria, West Africa), which were torrefied and characterized to improve their properties for potential use as sustainable feedstocks in an entrained flow gasifier for syngas production. Torrefaction temperatures of 230, 240, 250, and 270 °C and residence times of 30 and 60 min were used to torrefy sundried sorghum straw (SS) and millet straw (MS) at particle size ≤5 mm. The properties of torrefied MS and SS improved significantly due to torrefaction. The optimum conditions for pretreatment of sundried MS and SS were 240 °C and 60 min, at which increased energy density significantly compensated for the severe mass loss and significant degradation of hemicellulose and cellulose. Under these conditions, the respective mass and energy yields were 53 wt% and 75% for millet straw and 49 wt% and 75% for sorghum straw. The O:C atomic ratio decreased to 0.3 in MS tor-chars and 0.2 in SS tor-chars resulting in higher heating values of 25 MJ kg −1 and 27 MJ kg −1 , respectively. Under optimum conditions, the ash content increased by 107% to be approximately 7 wt% in MS tor-char, and by 118% to about 7 wt% in SS tor-char. These properties are promising for potential feedstocks in co-firing plants or gasification systems.