Comprehensive Combustion Index Research Articles

Bioenergy and bioexergy analyses with artificial intelligence application on combustion of recycled hardwood and softwood wastes

Novel biomass bioenergy-bioexergy analyses via thermogravimetry analysis and artificial intelligence are employed to evaluate the three biofuels from wood wastes (softwood-SW, hardwood-HW, and woods blend-WB). The chemical characterization of SW has the highest bioenergy (higher heating value – HHV: 18.84 MJ·kg-1) and bioexergy (specific chemical bioexergy – SCB: 19.65 MJ·kg-1) with the SCB/HHV ratio of wood waste as about 1.043-1.046. The high C-element has a significant influence on the HHV-SCB. The SCB/HHV ratio of wood waste is recognized as about 1.043-1.046. The three distinct zones of wood waste combustion are identified: moisture evaporation (Zone I, up to 110 °C), combustion reaction – degradation of three major lignocellulosic components (hemicelluloses, cellulose, and lignin) at Zone II, 110-600 °C, and ash remains (Zone III, 600-800 °C). The ignition (Dig=0.01-0.04) and fuel reactivity (Rfuel=3.82-6.97 %·min-1·°C-1) indexes are evaluated. The comprehensive combustion index (Sn:>5×10-7%2·min-2·°C-3) suggests that wood waste has a better combustion performance than bituminous coal. The statistical evaluation presents that the highest HHV-SCB values are obtained by performing combustion for SW-250 μm at 15 °C·min-1. The S/N ratio and ANOVA results agree that the wood waste type and particle size denote the most influential parameters. The artificial neural network prediction shows an excellent result (R2=1) with 1 hidden layer and 5 neuron configurations.

Production of hydrochar fuel by microwave-hydrothermal carbonisation of olive pomace slurry from olive oil industry for combustion application

This work is the first investigation on microwave-assisted hydrothermal carbonisation (MHTC) of real olive pomace (OP) slurry from the olive oil industry to produce hydrochars with improved fuel properties for combustion applications (e.g., in boilers). Experiments were conducted based on the central composite design of response surface methodology with two main process variables: temperature (180–250 °C) and holding time (2–30 min). Severity factors (log R0) were calculated from the above variables and used to explain the process effect in a simpler way. Increasing the MHTC severity resulted in significant changes in the structure of OP (studied by FTIR and NMR analyses) as well as reductions in yield, bulk density, volatile matter, and ash content in the hydrochars. The high-severity hydrochar was positioned in the lignite zone (van Krevelen diagram). It also exhibited substantial reductions in the alkali index (86.53 %), slagging index (76.89 %), and fouling index (96.07 %) compared to the raw material. Overall, the best conditions for hydrochar production with improved combustion characteristics were found to be 250 °C for 30 min (HHV = 28.45 MJ/kg, energy densification ratio = 1.25, equilibrium moisture content = 31.1 mg/g, comprehensive combustibility index = 2.94 × 10−7%2 min−2 ºC−3). These properties indicate that high-severity hydrochars could be utilised as biofuels for energy applications.

Co-combustion of municipal sewage sludge and food wastes-derived anerobic digestates: Combustion characteristics, HCl/NO/SO2 emission and ash behaviors

Co-combustion of sewage sludge (SS) and food wastes-derived anerobic digestate (AD) is a promising way of waste-to-energy. Therefore, it is crucial to examine their combustion performance, ash behaviors, and gaseous pollutants emissions during the combustion of AD-SS blends using a thermogravimetric analyzer and a tube furnace combustion system. Results show that obvious interactions between AD and SS are observed in their co-combustion process. In terms of comprehensive combustion index, co-combustion can improve their combustion performance with the decrement of burnout temperature and enhanced combustion reactivity. Higher blending ratio of AD favors the increment of their ash fusion temperature, i.e., from 1109 to 1403 °C for deformation temperature due to the formation of high melting point minerals (i.e., Ca9Fe(PO4)7, 2CaO·Al2O3·SiO2). Significant in-situ reduction of NO and HCl as well as self-desulfurization is observed during the combustion of AD-SS blends. Compared to the theoretical values, the emission amount of HCl, NO, and SO2 is reduced by 0.13, 1.39, and 0.35 mg·g−1, respectively at 850 °C and blending ratio of AD to SS = 0.75:0.25. These phenomena are associated with the reducing environment and reducing substances by their high volatiles as well as the abundant minerals such as CaCO3 and Fe2O3 in AD and SS. However, high combustion temperature is unfavorable for chlorine fixation. This work provides a better understanding of their thermal behaviors, ash characteristics, and gaseous pollutants emission characteristics to realize their stable and clean incineration.

Interaction mechanism and pollutant emission characteristics of sewage sludge and corncob co-combustion

The co-combustion of sewage sludge and biomass has demonstrated significant potential for reducing carbon emissions and realizing the resource utilization of solid waste. In this study, the co-combustion behavior and pollutant emissions of sewage sludge and corncob were systematically investigated using thermogravimetric-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) and a fixed-bed reactor. When the corncob blending ratio reached 30 %, the ignition temperature was 179.01 °C lower than that of sewage sludge, and the comprehensive combustion index increased by 13.92 times. The corncob in the mixed fuel dominated the volatiles' release and combustion process. The interaction strength increased as the corncob blending ratio increased. When the corncob blending ratio reached 70 %, it promoted the thermal weight loss process of sewage sludge. The release rate of CO2 increased as the biomass ratio increased, and a 30 % corncob content could inhibit CO2 emissions. Simultaneously, the interaction between sewage sludge and corncob inhibited the emissions of CO and NO during the co-combustion process, improving the combustion efficiency. This study provides theoretical support for developing the fuel value of sewage sludge, improving the amount of solid waste collaborative disposal, and realizing the leading carbon peak in thermal power industry.

A Lab-Scale Experiment on the Spontaneous Combustion Behavior of Chronic Naturally–Oxidized Coal in Re-Mining Coal Mine

ABSTRACT Spontaneous combustion of coal in re-mining coal mine is a serious threat to the safety recovery of coal resources. In this paper, six coal samples, including two raw bituminous coal samples from the new mining working face and four oxidized bituminous coal samples from the re-mining working face, are selected. The pore structures, thermogravimetric properties, and spontaneous combustion characteristics of raw coal and oxidized coal are studied. The pore structures show that the surface of oxidized coals has more cracks and higher macropore volume and porosity as compared with raw coal. The thermogravimetric experiments demonstrate that the oxidized coal has a higher combustion rate and turns to the combustion reaction zone in advance. Moreover, the oxidized coals perform a better combustion property by showing a comprehensive combustion index of 6.7 × 10−9 min−2·°C−3 and 8.9 × 10−9 min−2·°C−3, higher than the raw coal of 4.0 × 10−9 min−2·°C−3. The spontaneous combustion characteristics show that the oxidized coal performs a greater rate of oxygen consumption and more oxidation products released than raw coal. The CPT of raw coal is higher than 157°C, while the CPT of oxidized coal is between 132°C and 140°C. The research results can provide a risk assessment method for coal spontaneous combustion in the re-mining coal mine, and provide guidance for avoiding the occurrence of coal fires.

Study on thermal behavior and gas pollutant emission control during the co-combustion of rice straw-modified oily sludge and coal

Oily sludge (OS) is a solid waste generated from oil industry activities that can threaten the ecological environment and human health if not properly treated or disposed of. In this paper, the thermal behavior and emission of gas products during the co-combustion of rice straw (R) modified OS (OS5-R5) and coal (C) were investigated using an automatic calorimeter and model analysis. The effect of combustion atmosphere and additives on the gas products emission during the co-combustion of C, R, OS5-R5, and mixed fuels were investigated. The results show that as the content of OS5-R5 increased in mixed fuels, the performance of mixed fuels worsened gradually. When the mass content of OS5-R5 is 30 %, the lower heating value (LHV) was about 20 MJ/kg, which is about 16 % lower than that of C, but the comprehensive combustion index had no significant decrease. The additives could control the emission of pollutant gases. The increase of heating rate from 5 °C/min to 20 °C/min resulted in an increase of the overall combustion performance of the mixed fuel by 5.07-time. Similarly, increasing the oxygen concentration from 10 % to 100 % led to a 3.09-time increase in the combustion index of the mixed fuel. The adsorption of polar molecules by montmorillonite was conducive to reducing the emission of pollutant gases. BaO and CaO do not affect NO and NO2 emission control but reduce H2S and SO2 emissions because BaO and CaO could react with H2S and SO2 to form sulfate resulting in reduced emission. The research provides a theoretical basis for OS's safe disposal and efficient resource utilization.

Insights into PVC-promoted hydrothermal carbonization of manure: Dechlorination, inorganic metals removal, and combustion behaviors

Hydrothermal carbonization (HTC) can serve as a promising route to effectively co-valorize biomass and plastic wastes for solid fuel production. However, manure-derived hydrochar for fuel application remains a significant challenge due to its high ash content and low energy density. Polyvinyl chloride (PVC), as a chlorine-containing plastic, subjected to HTC commonly results in the generation of massive acidic wastewater. Herein, we proposed a PVC-promoted HTC route for high energy–density hydrochar production. By utilizing PVC as an in situ acidic additive, the HTC condition was optimized at 275 °C, 30 min, with a PVC addition of 10 %. The hydrochar obtained under the optimized condition exhibited a higher heating value (HHV) of 25.89 MJ/kg, comparable to bituminous coals, and achieve a high energy recovery rate of nearly 70 %. Moreover, the excellent removal efficiencies of inorganic metals and chlorine both exceeding 95 % (except for Fe at 86 %) were observed, owing to the strong interactions between dechlorination and inorganic metals removal. Importantly, it was evidenced that acidic reaction environment was demonstrated to enhance the carbonization degree and modify the hydrochar formation mechanism. Furthermore, the high comprehensive combustion index S of 3.22 × 10−7 %2/( °C3·min2) and low average Ea of 134.20 kJ/mol were achieved, indicating the favorable combustion behaviors. Overall, this work systematically and comprehensively investigated the PVC-promoted HTC of manure waste for advanced solid fuel production and examined its combustion characteristics, unveiling the hydrochar formation mechanism.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Characterisation and double parallel random pore model fitting kinetic analysis of faulty coal oxygen-enriched co-combustion

This study used various analytical techniques to explore the structural characteristics of anthracite, lean coal, and bituminous coal. Thermogravimetric analysis assessed the combustion performance of individual coals and their blends. Results revealed distinct structural differences among the three coals. Although lean coal's functional groups distribution resembled bituminous coal, its carbon ordering aligned with anthracite. Increasing the proportion of bituminous coal minimally improved combustion in air. Under O2 + N2, rising oxygen levels lowered ignition and burnout temperatures, enhancing the comprehensive combustion index. At high heating rates, 30 %O2 + 70 %CO2 outperformed 30 %O2 + 70 %N2. The fitting performance of the double parallel random pore model (DRPM) was superior to that of the random pore model (RPM). Kinetic analysis suggested a DRPM for lean coal and bituminous coal co-combustion, unfolding in two stages. Activation energy increased with O2 concentration in O2 + N2 but remained lower than in O2 + CO2.

Thermogravimetric analysis of co-combustion characteristics of sewage sludge and bamboo scraps combined with artificial neural networks

The increasing amount of sludge resources has brought great challenges to the ecological environment and sustainable development. Co-combustion of low calorific value sewage sludge and biosolid waste has become a new and effective way to treat sludge. The co-combustion characteristics of sewage sludge and bamboo scraps in air were studied by thermogravimetric mass spectrometry and artificial neural network. The thermogravimetric analysis of combustion at three different heating rates was performed with five different mixing ratios. Through comprehensive combustion index and cooperative analysis, it is concluded that the co-burning effect and cooperative effect of sludge and bamboo are better. Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose were used to calculate the apparent activation energy. The average activation energy of the sludge mixed with 75% bamboo scraps was the lowest (126.3 kJ mol−1) under the co-combustion condition. The ion current of combustion gas products during combustion was studied by mass spectrometry. Co-combustion can increase NOx and reduce SO2, and 75% bamboo slag mixed sludge has the least gas production. Finally, the prediction model of sludge co-combustion is established by artificial neural network, which provides a prospective guidance for the resource utilization of solid waste.

Environmental impacts of cement kiln co-incineration sewage sludge biodried products in a scale-up trial

The biodrying technology as a pretreatment technology can overcome the limitations of cement kilns co-incineration sewage sludge (SS) on energy consumption. But the impact of SS biodried products on cement kilns and the route carbon reduction potential of biodrying + cement kilns have not been studied. In this study, SS biodrying and cement kiln co-incineration biodried product trials were conducted to highlight the matrix combustion characteristics, and the impact of biodried products on cement kilns (clinker capacity, coal consumption, and pollutant discharge). The carbon emissions of the four scenarios were assessed based on these results. The results showed that water removal rate reached 65.5 % after 11-day biodrying, and the wet-based lower heating value of the biodried product increased by 76.0 % compared with the initial matrix. Comprehensive combustibility index of the biodried product (0.745 × 10−7 %2℃−3min−2) was better than that of SS (0.433 × 10−7 %2℃−3min−2) although a portion of the organic matter was degraded. Cement kiln co-incineration of biodried products (150 t/d) resulted in per tonne of clinker saved 5.61 kg of coal due to the heat utilization efficiency of biodried products reached to 93.7 %. However, it led to an increase in the emission concentrations of NOX and SO2. Assessment results indicated that the biodrying + cement kiln pathway reduced CO2 emissions by 385.7 kg/t SS. Biodried products have greater potential to reduce emissions as alternative fuels than as fertilizers. This study indicated the advantages of SS biodrying + cement kiln co-incineration route.

Investigation on combustion characteristics and ash-related issues of Calliandra calothyrsus and Gliricidia sepium using thermogravimetric analysis and drop tube furnace

Calliandra calothyrsus and Gliricidia sepium are developed as wood energy forest plantations in degraded land. They have good survivability, are fast to be harvested, and have good calorific value to be utilized as biomass fuel. This study aims to investigate their combustion characteristics by thermogravimetric analysis and their ash-related issues by combustion experiments in drop tube furnace and ash observation. The results show that G. sepium has a better combustion performance with higher values of ignition index, comprehensive combustion index, and flammability index, while C. calothyrsus shows better ash-related issues with less risk in theoretical slagging prediction, clearer metal surface in the fouling area, and less ash deposit formation. The domination of K, Ca, and S elements in the ash deposits of both biomasses results in several sticky aggregate particles and low-melting minerals. This study provides an in-depth understanding of C. calothyrsus and G. sepium combustion for broader utilization.

Analysis of pyrolysis and combustion characteristics of several coals with different coal properties

Studying the pyrolysis and combustion properties of coal is advantageous for promoting the environmentally friendly and effective exploitation of this resource. To investigate the pyrolysis and combustion characteristics of coal with varying types and properties, a thermal gravimetric experiment was conducted. This experiment aimed to determine the characteristic index and comprehensive combustion index. The pyrolysis characteristics of pulverized coal in a nitrogen atmosphere and the rapid combustion characteristics in an oxygen atmosphere were examined separately. The obtained results provide insights into the governing principles of the carbon content effect on the pyrolysis and combustion processes of pulverized coal particles. The findings indicate a positive correlation between the carbon content of coal and the efficiency of pyrolysis and combustion processes for pulverized coal particles. Moreover, a notable enhancement in the performance of pyrolysis and combustion is seen with increased carbon content.

A thermogravimetric assessment of eco-friendly biochar from oxidative torrefaction of spent coffee grounds: Combustion behavior, kinetic parameters, and potential emissions

This study investigated how temperature and residence time variations impact the combustion behavior, kinetic, and potential emissions of biochar produced through oxidative torrefaction of spent coffee grounds (SCG). The study examined the characteristics of combustion and kinetics by conducting thermogravimetry analysis under heating rates of 10 °C·min−1. The Coats-Redfern model was utilized to calculate kinetic parameters. While the emission indices were approximated using the data obtained from elemental analysis. The results indicated that biochar's comprehensive combustion index (Cci) from oxidative torrefaction was lower than that of raw SCG, suggesting stable combustion behavior. Moreover, with the escalation of torrefaction intensity, the activation energy (Ea) values exhibited an upward trend for the char combustion stage, ranging from 22.08 kJ mol−1 to 38.46 kJ mol−1. Concurrently, the Ea values pertaining to the oxidative pyrolysis stage decreased from 64.26 kJ mol−1 to 52.65 kJ mol−1. Besides, this study emphasized that the ash content of the biochar was lower than that of coal and remained consistent with the ash content of raw SCG (p > 0.05). Moreover, the study revealed that biochar from oxidative torrefaction emitted less CO2 (67.35 g MJ−1) than lignite coal (76.55 g MJ−1). Additionally, biochar exhibited up to 27 times lower dust emissions than bituminous coal, emphasizing its eco-friendly fuel potential.

Study on the combustion characteristics and kinetics of water hyacinth co-combustion with anthracite

Water hyacinth(WH) is a promising resource produced as a by-product of treating pollution in watersheds, but little study has been done on its high-value applications. In this work, the combustion characteristics of WH, anthracite (AN), and their blends under different ratios, and heating rates were investigated by the thermogravimetric (TG) analyzer. The Kissen-Akahira-Sunose (KAS) and Flynn-WallOzawa (FWO) methods are used to calculate the dynamics, and the interaction between the mixed materials is discussed. The results show that the combustion characteristics of WH are better than those of AN because of its higher volatile components. With the increase of WH, the comprehensive combustion index increased from 0.15(pure AN) to 2.22(WH: AN=5:5). With the increase in heating rate, there is no significant difference in residual. The difference in thermogravimetric curves of mixed fuel in the volatile combustion stage and fixed carbon combustion stage shows that there is an interaction between WH and AN in the process of co-combustion, indicating that the mixed fuel is suitable for co-combustion. This study provides a theoretical basis for improving the efficiency of clean energy and reducing the consumption of fossil energy.

Co-combustion of binary and ternary blends of industrial sludge, lignite and pine sawdust via thermogravimetric analysis: Thermal behaviors, interaction effects, kinetics evaluation, and artificial neural network modeling

Co-combustion behaviors, performances, and kinetic parameters of binary and ternary blends of industrial sludge (IS), lignite (HL), and pine sawdust (SD) were investigated by thermogravimetric analysis, as well as interaction effects and artificial neural network (ANN) modeling. The measure of blending HL and SD into the IS would boost the ignition temperature, while decreased burnout temperature. For the binary blends of IS and HL, the combustion performance would be best with 30% HL ratio, and the combustible index (Ci), combustion stability index (G) and comprehensive combustibility index (CCI) were separately 7.57 × 10−5 %/min·°C2, 3.22 × 10−5 %/min·°C2, 1.39 × 10−7 %2/min2·°C3. With the SD ratio increasing, the combustion performance of the IS and SD blends, and ternary blends improved. It was observed that the interaction effects did exist during the co-combustion of the binary and ternary blends, which were beneficial for the IS combustion disposal. The combination of Coats-Redfern and Malek methods was utilized to the combustion kinetics analysis of blends. The combustion process of binary and ternary blends of IS, HL, and SD could be effectively predicted by the ANN model, and the regression coefficients of the training, validation, testing, and all of the ANN model were all 0.99996.

Synergistic retention of heavy metals and in-situ reduction of NO and SO2 by co-combustion of sewage sludge and coal gangue: A promising approach for contaminant management and emission reduction

This study aims to evaluate the influence of co-firing municipal sewage sludge with coal gangue at 800–1000 °C on the emissions of pollutants such as NO, SO2 and heavy metals (HMs). The results revealed that co-combustion resulted in smoother combustion (e.g., lower ignition and burnout temperatures, and a higher comprehensive combustion index) as well as a reduction in activation energy by approximately 30 kJ/mol when the sewage sludge ratio was above 60%. Moreover, the retention of HMs in the ashes during co-combustion was significantly improved through reactions between the dominant minerals (especially quartz (SiO2), berlinite (AlPO4), hedenbergite (CaFeSi2O6), magnetite (Fe3O4) and gehlenite (Ca2Al2SiO7) with HMs species presented in the wastes, leading to an increase in HMs content by 1 to 3 times. Additionally, in-situ desulfurization and denitration were observed during co-combustion at above 900 °C and below 850 °C, respectively. It is likely that berlinite has a direct impact on NO emissions, while gehlenite, whitlockite (Ca2.86Mg0.14(PO4)2) and diopside (CaMgSi2O8) present in the ashes are associated with the evolution of SO2 during co-combustion. These findings suggest that co-combustion of multiple wastes is a feasible approach for their resource utilization while reducing the environmental impacts associated with solid waste disposal.

Experimental research on the features of spontaneous combustion during secondary oxidation of coal with various particle sizes

The secondary oxidation of remaining coal in the process of secondary mining of coal seams poses a major threat to the safety of personnel and property, as well as the effortless re-mining of relict coal. The grain size composition of the relict coal in the mining area is complex, and it is an essential component determining the coal's spontaneous combustion characteristics, particularly the more complex secondary oxidation reaction. The secondary oxidation characteristics of coal treated with various pre-oxidation temperatures at different grain sizes were investigated using TG-DSC coupled experiments and electron paramagnetic resonance (EPR) experiments. The results showed that as the particle size of the coal samples decreased, the ignition point temperature T6 and the maximum burning rate temperature T7 decreased gradually; the burnout index and the comprehensive combustibility index increased gradually; the endothermic ‘ladder’ on the DSC curves are more pronounced, with the peaks shifting to lower temperatures; the activation energy of coal samples gradually declines from the ignition point temperature T6 to the maximum burning rate temperature T7 during the combustion process, as determined by Coats-Redfern and Achar-Brindley-Sharp-Wendworth respectively; the g-factor of coal samples tends to decrease, while the concentration of free radicals declines first and subsequently increases. With the same particle size, the g-factor of the coal samples remained virtually constant as the pre-oxidation temperature increased, whereas the free radical concentration gradually increased.

Experimental investigation on the characteristics of XG/GG/HPAM gel foam and prevention of coal spontaneous combustion

Polymer gel foam shows excellent development potential and application prospects in the prevention of coal spontaneous combustion. To reveal the relationship between the characteristics of gel foam and the prevention of coal spontaneous combustion, XG/GG/HPAM gel foam consisting of polymer xanthan gum (XG), guar gum (GG), and polyacrylamide (HPAM) is prepared. The characteristics of gel foam are investigated in terms of microstructure, viscosity, thermal stability, and film-forming. The effects of gel foam on gas products, characteristic temperature, combustion characteristics parameters, and functional groups during coal oxidation are studied. The results show that polymer delays the foam coarsening and improves the foam stability. XG/GG/HPAM gel foam exhibits a slow foam coarsening process, concentrated bubble diameter distribution, shear-thinning behavior, excellent thermal stability, and film-forming properties. Moreover, the inhibition rate of XG/GG/HPAM gel foam on coal is 43.63% higher than that of ordinary foam. XG/GG/HPAM gel foam effectively improves the apparent activation energy of coal, the maximum weight loss rate of the coal is decreased by 7.55%, and the combustibility, combustion stability, and comprehensive combustion index are diminished by 8.50%, 7.64%, and 10.17%, respectively. Simultaneously, XG/GG/HPAM gel foam reduces the oxidation activity of aliphatic hydrocarbons and oxygen-containing functional groups in coal, blocks the chain reaction, and thus inhibits coal spontaneous combustion. The results provide support for achieving efficient fire prevention performance of gel foam.

Phosphorus extracted municipal sludge-derived hydrochar as a potential solid fuel: Effects of acidic leaching and combustion mechanism

Hydrochar from hydrothermal liquefaction (HTL) of municipal sludge is often seen as waste. Initially, acidic leaching has been performed on hydrochar to recover phosphorus. This study evaluated the resulting hydrochar as solid fuel, thus achieving zero waste for integrating HTL into wastewater treatment plants. The effects of various leaching conditions on acid-modified hydrochar properties were examined. Compared to raw hydrochar, acid-modified hydrochar had a significant ash reduction (up to 44%). The leaching conditions (10 mL of 0.6 N HNO3/g for 2 h) for near-complete phosphorus extraction also achieved the following most desirable properties of acid-modified hydrochar: Maximized fuel ratio (1.3), higher heating value (20.5 MJ/kg), carbon content (48%), and minimized ash content (34%) on a dry basis. By removing most alkali and alkaline earth metals, acid-modified hydrochar could carry a low slagging and fouling risk compared to a high/severe risk for raw hydrochar. Acidic leaching also enhanced ignition temperature from 317 to 351 °C for safer storage and transportation of hydrochar, with a higher comprehensive combustion index (up to 9.9 × 10−8 min−2 °C−3). Overall, hydrochar fuel property was improved by acid modification, comparable to bituminous coal. For the first time, it was identified that hydrochar combustion was controlled by a two-step nth-order reaction mechanism f(α) = (1 − α)n with the reaction order changing from 3 to 1 as progressed. The acid modification did not affect combustion mechanisms but reduced activation energy in the first stage. The results demonstrated a sustainable approach for closing the waste loop and provided a pathway for transforming hydrochar into biofuel.