Hydrothermal Carbonization Pretreatment Research Articles

Enhancing biomethane production from citrus waste: An integrated approach of hydrothermal carbonization and anaerobic digestion for sustainable waste management

This study investigates the energy recovery potential of bio-oil derived from hydrothermal carbonization (HTC) of citrus waste through anaerobic digestion (AD). The bio-oil, a complex mixture containing 30-50% of the original carbon from biomass, serves as a valuable substrate for AD. Leveraging the HTC pretreatment, the hydrolysis step in AD becomes more efficient, facilitating faster degradation rates. Anaerobic digestion of the bio-oil was conducted in a high-loading hybrid anaerobic reactor. The reactor underwent stabilization using tomato liquid fraction, followed by bio-oil feeding with an applied volumetric loading of 5 g COD/L-d under mesophilic conditions. Remarkably, COD removals exceeded 90% when utilizing the tomato fraction and surpassed 80% in the bio-oil feed. Additionally, methane yield approached theoretical levels, highlighting the effectiveness of combining HTC and AD technologies. The study demonstrates that the integration of HTC and AD offers a promising alternative for the sustainable utilization of citrus industry wastes, showcasing high removal efficiencies and methane production. This approach aligns with circular economy principles, providing a pathway for efficient waste valorization and renewable energy generation.

Sugar Extraction from Secondary Agricultural Waste Biomass Using Hydrothermal Carbonization and Direct Contact Membrane Distillation

Sustainable and renewable sources of liquid and solid fuels are essential to prevent fossil fuel use from damaging the environment. Secondary agricultural residues, which are already transported to food processing centers, have great potential to be converted into biofuels. The wastes from coffee roasting, sugar production, and rice milling have been investigated using hydrothermal carbonization (HTC) to produce aqueous products containing monosaccharides alongside solid biofuels. These sugar-laden liquid products were characterized after pretreating coffee silverskins, sugarcane bagasse, and rice husks with HTC. They were then concentrated using direct contact membrane distillation (DCMD), a low-energy process that can use waste heat from other biorefinery processes. The higher heating value of the solid products was also characterized by bomb calorimetry. The liquid products from HTC of these wastes from food production were found to contain varying concentrations of glucose, xylose, galactose, and arabinose. DCMD was capable of concentrating the liquid products up to three times their original concentrations. Little difference was found among the higher heating values of the solid products after 180 °C HTC pretreatment compared to 200 °C pretreatment. HTC of waste from food processing can provide solid biofuels and liquid products containing sugars that can be concentrated using DCMD.

Torrefaction and hydrothermal carbonization pretreatments of rice impact nature of biochar from subsequent pyrolysis

Torrefaction and hydrothermal carbonization (HTC) as important pretreatment methods for processing starch-rich food might affect pyrolysis behaviors of the food waste. This was investigated herein by torrefaction and HTC of rice at 200 ºC and subsequent pyrolysis of the pretreated rice at 600 ºC for probing the influence on evolution of the products. The results indicated that the torrefaction could not remarkably affect yield of the pyrolytic products, but made the torrefied rice more hydrophobic via removing some polar functionalities. The HTC at 200 ºC significantly enhanced overall yield of bio-oil from rice. The HTC also enhanced aromatization degree, making the hydrochar more difficult to crack, reducing the tar yield significantly, increasing the activation energy of 162.3 kJ/mol in pyrolysis of rice to 301.6 kJ/mol in pyrolysis of the hydrochar, and improving combustion performance of the resulting biochar. The HTC could form dominantly microsphere morphology in the hydrochar and biochar. The in-situ IR characterization of the pyrolysis process showed that the torrefaction could induce some deoxygenation reactions to form some aromatics while the HTC pretreatment removed a significant portion of aliphatic structures. Life cycle assessment indicated that HTC pretreatment of rice lowered environmental pollution and fossil fuel depletion during pyrolysis.

Analytical evaluation of the coupling of hydrothermal carbonization and pyrolysis (HTC-Py) for the obtainment of bioavailable products

Fir sawdust was firstly pre-treated through hydrothermal carbonization (HTC) at two different temperatures: 150 and 200 °C. The resulting hydrochars were then subjected to intermediate pyrolysis at 550 °C. The main focus of this study was to investigate the impact of HTC on the pyrolysis products in terms of both energy and mass yield, considering both HTC and pyrolysis for delivering bioavailable and fermentable substances. HTC, in fact, can dissolve most of hemicellulose, providing an hydrochar which produces high (up to 8.7% w/w with 200 °C HTC) yield of levoglucosan. Such yield, which is close to that achieved from cellulose pyrolysis with the same pyrolysis apparatus, suggests that HTC is an effective pre-treatment for enhancing the selectivity of pyrolysis toward anhydrosugars. To highlight the overall potential of products from the hydrothermal carbonization-pyrolysis (HTC-Py) scheme, further characterizations were conducted on the HTC liquids (HTC-L) and aqueous phase liquids of obtained bio-oils (APL) using various techniques, including targeted GC-MS and size exclusion chromatography (SEC-RID). Sankey diagrams with overall yield were built to evaluate the overall selectivity of HTC-Py in producing bioavailable substances and sugars. Results showed that HTC-Py, through hemicellulose fractionation and improved pyrolysis selectivity, allows increasing from 52% to 58% the yield of bioavailable organics and, more importantly, HTC pre-treatment allowed to increase the yield products that can be fermented with biotechnological workhorse (namely sugars, dissolved hemicellulose, and carbon monoxide) from 4% to 16%.

Hydrothermal carbonization coupled with pyrolysis: An innovative approach to digestate management

Anaerobic digestion presents a viable approach for managing biodegradable waste. However, the process generates a significant amount of digestate, which, if not appropriately managed, can contribute to eutrophication and salinization. This research explores the integration of hydrothermal carbonization (HTC) and pyrolysis as a potential solution for digestate management, emphasizing volume reduction and energy recovery. The study specifically focuses on the production of biochar from agricultural waste digestate (AWD) in an energy-efficient manner. Utilizing HTC as a pretreatment was found to enhance the heating value of both the AWD and the subsequent pyrolytic char, and it also improved the carbonization degree of the resulting char. Energy balance analysis revealed that the pyrolysis of AWD shifted from an endothermic to an exothermic reaction upon HTC pretreatment. This transition decreased the reaction's energy absorption from 41.1 to 1557.6 MJ to an energy release of 685.1 to 960.6 MJ per ton of digestate, thereby optimizing energy consumption in digestate management. Additionally, it was noted that using acetone as a solvent in gas chromatography for biooil samples with high ammonia content led to the formation of diacetonamine, an outcome that is deemed undesirable.

Hydrothermal carbonization pretreatment makes a remarkable difference in activation of rice and lettuce in food waste

Cooked rice and the vegetables like lettuce are common kitchen waste, which are carbonaceous materials and have the potential as feedstock for the production of activated carbon. Cooking is similar to hydrothermal treatment (HTC), which might impact the subsequent activation of kitchen waste. In this study, the HTC of lettuce, rice, or their mixture and the activation of the resulting hydrochars were conducted. The results indicated that cross-polymerization between the N-containing organics from lettuce and the sugar derivatives from rice took place in their co-HTC, which significantly increased the hydrochar yield. Activation of the hydrochar from the co-HTC generated the AC with a yield of 2 times that from direct activation of mixed lettuce/rice. However, the co-HTC facilitated aromatization, reducing reactivity with K2C2O4 in activation and producing the AC with main micropores and low specific surface area. Activation of the hydrochar from HTC of rice followed the above trend, while that from lettuce was the opposite. The organics in lettuce were thermally unstable and could not undergo sufficient aromatization. The activation of hydrochar from HTC of lettuce thus generated the AC with the lowest yield, but the highest specific surface area (1684.9 m2/g), abundant mesopores, and superior capability for adsorption of tetracycline. However, the environmental impacts and energy consumption for the production of AC from the hydrochar of lettuce were higher than that from hydrochar of co-HTC.

Chemical looping with oxygen uncoupling of biomass-derived hydrochar with Cu-based oxygen carriers modified by alkaline earth metals

In this paper, the effect of alkaline-earth metals (AEMs, Ca, Ba, and Sr) on the performance of CuO/ZrO2 oxygen carriers (OCs) and the hemical looping with oxygen uncoupling (CLOU) process of wheat-derived hydrochars with these Cu-based OCs were investigated. The results showed that after multiple cycles, the crystal phases of modified Cu-based OCs were the same as that of pure CuO/ZrO2 OC without any interaction between them. Due to the catalytic effects of AEMs, an increase in the decomposition of wheat and hydrochar-235 in modified Cu-based was observed at 500 °C. The combination of Ca and HTC promotes reoxidation, while Ba and Sr inhibit reoxidation. Notably, the Cu-based OCs exhibited moderate agglomeration after 20 redox cycles at 950 °C. The results of the kinetic analysis indicate that the chemical order models and diffusion models can suitably describe the first combustion stage for wheat and hydrochars with Cu-based OCs, respectively. The geometrical contraction model was also found to most suitably describe the second combustion stage of all samples. These results suggest that a combination of hydrothermal carbonization pretreatment with AEMs modification is an efficient approach for CLOU using CuO-based OCs.

Highly porous carbon derived from hydrothermal-pyrolysis synergistic carbonization of biomass for enhanced CO2 capture

Highly porous carbon adsorbent is much desired to diminish greenhouse effect caused by CO2 emission and benefit for subsequent carbon cycle. Herein, a low-cost and environment-friendly carbonization strategy was reported for preparation of highly porous carbon derived from biomass. Such carbonization is a synergistic process with hydrothermal treatment coupled with a subsequent pyrolysis using sugarcane as biomass. The obtained hydrothermal-pyrolysis derived porous carbon (HPC) has a BET surface area of 938 m2/g, which is much higher than that of carbon produced through direct pyrolysis (PC, 528 m2/g). During the hydrothermal carbonization pretreatment, highly thermostable aromatic skeletons are formed and converted to carbon skeletons in the post-pyrolysis process, leading to formation of highly stable micropores and higher surface area. Further, HPC shows remarkable adsorption ability of CO2 (2.8 mmol/g, 25 °C, 1 bar), higher than that of PC (2.1 mmol/g). In addition, the HPC exhibits high CO2/N2 adsorption selectivity and recyclability, demonstrating its potential application in CO2 capture. This work not only gives new light for tailoring carbon materials with highly porous structure but also provides novel adsorbent for CO2 adsorption.

Hydrothermal carbonization of sewage sludge: Optimization of operating conditions using design of experiment approach and evaluation of resource recovery potential

Scientific disposal of sewage sludge (SS) is a tedious task worldwide. Landfilling of the SS after centrifugation may have serious adverse impacts on the environment. The present study investigated efficacy of hydrothermal carbonization (HTC) as pretreatment for the centrifuged sewage sludge with an aim to improve resource recovery. In this study, both solid and liquid fractions recovered after the HTC were subjected to the detailed analysis for examining the opportunities for recovery of value-added chemicals and biogas. The HTC conditions were optimized using design of experiments (DOE) approach. The runs were performed in a high-pressure reactor at 160–240 °C temperatures and total pressures of 6.1–34.2 kg/cm2 for 1–3 h duration without supply of air. The optimum HTC temperature and duration were found to be 200 °C and 1 h, respectively. Under these conditions, the dried hydrochar (HC) had higher heating value of ~13 MJ/kg. Its properties were found similar to that of the sub-bituminous coal based on the atomic ratio of elements. The HTC pretreatment resulted in substantial reduction in carbon and oxygen due to decarboxylation, dehydration and oxidation (due to oxygen present in reactor headspace) reactions. The process wastewater (PWW) recovered after HTC at the optimum conditions had high concentration of the recoverable value-added compounds such as humic acid (HA) and PO43--P (concentration = 20 g/l and ~0.5 g/l, respectively). Moreover, significant quantity of biogas could be generated from the PWW. Hence, hydrothermal pretreatment can be considered a potential option for SS valorization.

Improving Kraft Pulp Mill Energy Efficiency through Low-Temperature Hydrothermal Carbonization of Biological Sludge

Of the various waste and side streams created in a kraft pulp mill, the biological sludges from the wastewater treatment plant are some of the most problematic to handle. Incineration is becoming a common solution as landfilling is no longer permitted by legislation in many countries, but this is also problematic due to the high moisture content, poor drying characteristics, and high ash content in the solids. This study evaluates the technical potential of mild hydrothermal carbonization (HTC) at 160 °C for 3 h to improve the energy efficiency of on-site incineration as a biosludge handling method. HTC treatment transforms wet organic substrates into a hydrophobic carbonaceous material (hydrochar). The heating value and elemental composition of both the sludge and the hydrochar product were analyzed. Based on this, a hydrothermal carbonization model developed earlier was adjusted for the feedstock, and process integration modelling performed to evaluate the performance impact on the power and heat generation at the mill. The results indicate that if the alternative is combustion in the power boiler, HTC pre-treatment could allow a significant increase in power generation. If the sludge is combusted in the recovery boiler, a practice often avoided in order to not introduce non-process elements to the chemical recovery cycle but sometimes necessary due to, e.g., absence of a power boiler, a much smaller increase is obtained. The increase is smallest if the freed evaporator plant capacity cannot be utilized for increasing the firing liquor dry solids content.

Effect of hydrothermal carbonization on woody biomass: From structure to reactivity

Hydrothermal carbonization (HTC) pretreatment is an effective method to upgrade the low quality biomass properties. In this work, the pinewood (PIW) and poplar wood (POW) were selected to study the effect of HTC (180–240 °C) on woody biomass structure and reactivity. The results showed that the slight and moderate HTC can make the cellulose structure more ordered, while the severe HTC leaded to the decomposition of cellulose structure. The mesoporous specific surface area and pore volume increased with the HTC temperature, and the pore structure of PIW was more developed than that of POW. The slight HTC had little effect on cellulose structure and the diffusion effect played the dominant role. The severe HTC increased the relative content of lignin and the pyrolysis followed the high order reaction model. The model-free method was used to determine the relationship between T-β-X, and the specific temperature THTC/Traw was calculated. The THTC/Traw value of PIW and POW located in the range of 0.938–1.340 and 0.924–1.422, indicating the HTC pretreatment had great effect on POW pyrolysis reactivity.

Impacts of cooking, pyrolysis and HTC pretreatments of noodles waste on evolution of products in activation

Cooking and other thermal treatments like hydrothermal carbonization (HTC) and pyrolysis might impact the evolution of varied products in the activation of noodles. In this study, raw noodles were pretreated via cooking at 100 °C, pyrolysis at 500 °C and HTC at 260 °C, and subsequently activated with K2C2O4 at 800 °C. Correlation of the varied pretreatments with nature of the resulting activated carbon (AC), bio-oil and gases was paid particular attention. The results showed that cooking treatment enhanced the formation of mesopores in AC, but decreased overall yields of bio-oil and gases from 82.0 to 80.3 wt% during the activation. HTC pretreatment decreased the production of both AC and gases, but remarkably enhanced bio-oil formation. Pyrolysis increased aromatization degree of biochar and suppressed cracking reactions in activation, producing the highest overall yields of AC. However, pyrolysis and HTC destroyed sugars in noodles and reduced the occurrence of reactions for transformation of the functionalities like -OH, -C-H, C=O and C-O-C, which hindered development of pores. The highest specific surface area of AC from direct activation of raw noodles thus showed superior capability for the adsorption of tetracycline (TC) from aqueous solution. Life cycle analysis (LCA) indicated that HTC plus activation showed the least environmental impact, although the lowest yields and specific surface area of AC was obtained via this route. The results of this study will be interested to anyone working in the general field of bioenergy science and engineering.

Optimizing the effect of hydrochar on anaerobic digestion of organic fraction municipal solid waste for biogas and methane production.

Anaerobic digestion (AD) is the biological waste treatment method for the organic fraction of municipal solid waste (OFMSW). AD is notable for its ability to reduce volume and produce biogas from waste. However, the conventional AD of OFMSW has a low degradation rate. In recent years, some treatment method has been used to promote the biogas and methane production of AD. One of these methods is hydrothermal carbonization (HTC). This study aimed to evaluate the effect of hydrothermal carbonization (HTC) temperature and hydrochar: OFMSW ratio as factors on biogas production, methane production, and methane content of anaerobic digestion (AD) as responses was investigated. This study determined the biomethane potential of raw and pretreated OFMSW (hydrochars) in 118 ml serum glass bottles. Based on the Hansen method, all tests were conducted at mesophilic temperature (37 ± 1 °C) in an incubator for 45 days. The response surface method and central composite model were used for designing experimental conditions. Quadratic models were used to estimate the correlation between factors and responses. Also, the optimal conditions for maximizing responses were determined. Biogas production of mixing hydrochar and OFMSW was 41% more than control groups which contained OFMSW and inoculum. The optimal operating conditions to maximize all responses were applied in HTC temperature and hydrochar: OFMSW ratio of 179.366 °C and 2.406, respectively. In this condition, the maximum biogas production, methane production, and methane content were 394 mL/g VS, 284.351 mL/g VS, and 73.176%, respectively. As an OFMSW HTC pretreatment for AD, hydrochar additive has a significantly positive and negative effect on biogas production, methane production, and methane content of biogas depending on operating conditions. Therefore. It is necessary to consider the individual and interaction effects of the temperature and hydrochar: OFMSW ratio, obtain the optimal conditions and determine responses.

Effect of hydrothermal carbonization pretreatment on the pyrolysis behavior of the digestate of agricultural waste: A view on kinetics and thermodynamics

Anaerobic digestion is the most promising disposal methods to treat organic waste. Also, a feasible management is necessary for the resulted digestate. Hydrothermal carbonization (HTC) combination with pyrolysis could be a proper solution to use for the treatment of digestate. In this study, the effect of an HTC on the pyrolysis of the digestate of agricultural waste (AWD) was investigated, focusing on the kinetic and thermodynamic aspects. Three model-free methods, including Friedman, KAS, and OFW methods, were used to evaluate the kinetic performance of the total and pseudo pyrolytic reactions of AWD and its hydrochar. Furthermore, kinetic predictions were made to provide more information for further studies. It was found that the HTC treatment decreased the activation energy ranges of the pyrolysis of AWD from 182.9–274.43 kJ/mol to 144.59–205.20 kJ/mol by using the Friedman method. For a more thorough understanding of the effect of HTC treatment on the pyrolysis of AWD, the pyrolysis reactions of AWD and its hydrochar were divided into two pseudoreactions using the Fraser-Suzuki deconvolution method. The mean activation energy of the deduced pseudo 2 pyrolytic reaction of hydrochar was 175.64 kJ/mol, which was 28.11 kJ/mol less than that of AWD. In addition, the ΔH‡ values of the pseudo 2 reactions of AWD and its hydrochar were 197.97 and 169.68 kJ/mol, respectively. The results of kinetic isothermal predictions suggested that the peak temperature for the further research and application of the pyrolysis of AWD and its hydrochar should not be lower than 450 °C.

Influence of sequential HTC pre-treatment and pyrolysis on wet food-industry wastes: Optimisation toward nitrogen-rich hierarchical carbonaceous materials intended for use in energy storage solutions

Due to elevated protein content, the food-industry bio-wastes are promising feedstock to produce hierarchical (micro-mesoporous) carbonaceous materials with the intended use as electrodes in the energy storage solutions. However, the high initial water content, makes their direct activation through high-temperature processes costineffective due to significant heat requirements. In this study, the influence of pretreatment with hydrothermal carbonization (HTC) on wet food-industry bio-wastes, further pyrolysed, was investigated. Selected wastes (brewer's spent grains, spent coffee grains and spent sugar beets) were pre-treated by HTC at 180 °C or 240 °C, and then pyrolysed at 500 °C or 700 °C. Obtained materials were examined using elemental analysis, gas adsorption (N2 and CO2) and FT-IR. Besides minor differences caused by the bio-composition of wastes, the general trends were similar for feedstock. The pre-treatment had a beneficial influence on the properties of all wastes. The HTC at 180 °C and pyrolysis at 700 °C for all wastes show the most promising total specific surface area 560 ± 10 m2/g and accessible specific surface area 96 m2/g. Those conditions simultaneously did not reduce the total solid yield in comparison to the one-step process. The pre-treatment at 240 °C led to elevated nitrogen incorporation in the carbonaceous structure compared to HTC at 180 °C. However, it formed a hierarchical structure that was not stable for the thermal treatment. Study proves the HTC pre-treatment at 180 °C is beneficial for the conversion of food-industry bio-wastes into hierarchical carbonaceous material for their use in the energy storage systems application.

Bifunctional biomass-based catalyst for biodiesel production via hydrothermal carbonization (HTC) pretreatment – Synthesis, characterization and optimization

The hydrothermal carbonization (HTC) technique is known for its advantages in producing hydrochar from biomass samples with high water content compared to conventional pyrolysis techniques. This study utilized HTC to produce an activated carbon catalyst from renewable mesocarp fiber derived from palm oil processing. The introduction of K2CO3 and Cu(NO3)2 produced a bifunctional catalyst suitable for conversion of used cooking oil to biodiesel. The catalyst possessed a mesoporous structure with a BET surface area of 3909.33 m2/g. An optimum treatment ratio of 4:1 (K2CO3: Cu(NO3)2) provided elevated basic (5.52 mmol/g) and acidic (1.68 mmol/g) concentrations on the catalytic surface, which promoted esterification and transesterification reactions. Maximum yield (96.4%) of biodiesel was obtained at 70 °C for 2 h with 5 wt% catalyst and a 12:1 molar ratio of methanol to oil. The catalyst endured up to 5 reaction cycles while maintaining biodiesel yields of more than 80%. These findings indicated that HTC pretreatment yielded a high-quality bifunctional catalyst for conversion of low-quality used cooking oil for production of biodiesel.

On-line analysis of the correlation between gasification characteristics and microstructure of woody biowaste after hydrothermal carbonization

Woody biowaste is a component which is difficult to be converted among multiple solid waste (MSW) during the hydrothermal carbonization (HTC). In this paper, poplar sawdust was pretreated by HTC to study the correlation between microstructure and gasification characteristics. The results showed the fixed carbon and higher calorific value increased from 13.44 % and 19.41 MJ/kg to 41.47 % and 25.85 MJ/kg after HTC, respectively. The cold gas efficiency of hydrochars prepared at 220 °C was the highest of 93.57 % compared with that of raw material of 76.65 %. It was found that carbon structure had a greater influence on hydrochars gasification characteristics than pore structure. The crystallinity of hydrochars had a good correlation with the total yield and H2/CO of syngas, which can provide guidance for HTC pretreatment of woody biowaste and MSW.

Coupling hydrothermal carbonization with anaerobic digestion: an evaluation based on energy recovery and hydrochar utilization

This work evaluates the effect of hydrothermal carbonization (HTC) as a pretreatment and post-treatment technique to anaerobic digestion (AD) of dairy sludge. HTC's effect on AD was evaluated based on energy recovery, nutrient transformation, and hydrochar utilization. The first approach was executed by performing HTC under a range of temperatures before mesophilic AD. HTC optimal pretreatment temperature was 210 °C for 30 min residence time. HTC pretreatment significantly increased the methane yield potential by 192%, the chemical oxygen demand removal by 18%, and the sludge biodegradability during AD by 30%. On the other hand, the application of HTC after AD (post-treatment) increased the total energy production, i.e., in addition to methane, a hydrochar with a caloric value of 10.2 MJ/kg was also obtained. Moreover, HTC post-treatment improved the steam gasification performance of the AD digestate. From the fertilizer quality point of view, HTC implementation generally boosted the concentrations of macro, micro, and secondary nutrients, suggesting its suitability for use as a liquid fertilizer. Overall, the findings of the present study indicate that if bioenergy production were the main target, HTC post-treatment following AD would lead to the most promising outcomes.

Thermochemical conversion of sewage sludge-derived hydrochars: Volatiles release and char gasification kinetics

Hydrothermal carbonization (HTC) combined with subsequent thermochemical conversion is an effective approach for sewage sludge (SS) to energy. In this work, HTC pretreatment of SS was performed. Then the properties of released volatiles and the remaining char-CO2 gasification after pyrolysis were evaluated via the multiple kinetic model and combined instrumental analyses. The gasification behavior of these samples exhibited slightly different due to high ash content and limited fixed carbon, although the aromatic structure in the hydrochars developed as HTC proceeded. Kinetic analysis indicated that volume reaction model (VRM) with chemical reaction order of 1.5 showed high correlation coefficients (R2>0.99) for all carbonized samples. There was an obvious kinetic compensation effect in the char gasification step, and activation energy (E) and pre-exponential factor (A) of these samples followed a positive correlation relationship as lnA = 0.1037 × E + 0.3579. The formation of fixed carbon in the hydrochar samples was conserved during pyrolysis, which lowered the average gasification reactivity of char. Py-GC/MS demonstrated that the chemical composition of pyrolytic volatiles was found to be mainly determinant by HTC conditions. The share of aromatics in the volatiles was in a positive correlation with the evolution of carbon functionality in the hydrochar samples. Above in all, HTC temperature exerted a more significant effect on the fuel properties of hydrochar samples than HTC duration did, which also indicated by their subsequent thermochemical conversion. These findings can provide essential knowledge on the thermochemical conversion of hydrochar samples and favor the designing, optimization, and scaling up of the process.

The volatilization of heavy metals during combustion of polyvinyl chloride after hydrothermal carbonization

This paper experimentally investigated the volatilization characteristic of five heavy metals (cadmium, chromium, nickel, lead and zinc) during combustion of hydrochars of polyvinyl chloride with hydrothermal carbonization pretreatment. The differences of volatilization rate of cadmium and lead between polyvinyl chloride and hydrochars were within 5%, indicating that hydrothermal carbonization had no significant effects on their volatilization behavior. Higher hydrothermal carbonization temperature and lower combustion temperature resulted in lower volatilization rate of zinc. For chromium and nickel, although their volatilization rates were lowered, the absolute volatilization rates increased remarkably because of the increase of original concentrations. Nevertheless, the total emissions of heavy metals studied reduced. Hydrothermal carbonization performed at 230 °C showed the most significant absolute volatilization rates reductions ranged from 79.2% to 86.8% depending on combustion temperature. The highest concentrations of chromium and nickel in bottom ash reached up to 20.1 g/kg and 7.9 g/kg, respectively.