Combustion behaviors and kinetics of sewage sludge blended with pulverized coal: With and without catalysts

Combustion Characterization and Kinetic Analysis of Mixed Sludge and Lignite Combustion

To investigate the feasibility and reaction mechanism of combusting sewage sludge and brown coal in a mixture. Thermal behavior evaluation of combustion characteristics, interactions, and kinetic analysis of sludge-lignite mixture combustion by thermogravimetry (TG). The results showed that the combustion performance of the mixed samples was all in between that of the lignite and sludge samples. The combined combustion index gradually decreased with the increase in sludge mixing. The addition of sludge favors the ignition of the mixture but is not conducive to overall stable combustion. The synergies between the sludges, as assessed by the mass loss curves, are reflected in the ash removal and coke oxidation stages. When the mixture of sludge and lignite is burned at a ratio of 10 wt %, the calorific value can still reach 20.3 MJ/kg, which is only about 4.2% lower than that of burning lignite alone. Application of the kinetic models of FWO, Starink, KAS, and Friedman, in turn, determined a minimum average activation energy of only 132.50 kJ/mol. In addition, the reaction was judged to be a simple complexation reaction by analyzing the thermodynamic parameters (ΔG, ΔS, ΔH, and A), with the combustion process approaching thermodynamic equilibrium and forming stable products. The nucleation model A4.2 can be used as the best reaction mechanism model for sludge-lignite mixed combustion.

Prediction of CO/NOx emissions and the smoldering characteristic of sewage sludge based on back propagation neural network

Smoldering can achieve effective disposal of sewage sludge (SS) with high moisture content at low energy input, providing social and economic benefits. However, smoldering is accompanied by the emission of high concentrations of CO/NOx, and thus, it requires sufficient attention. This study comprehensively investigates the effects of SS characteristics and experimental parameters on CO/NOx emissions and smoldering characteristics. Results showed that when the moisture content of SS increases from 35% to 50%, CO concentration increases while NOx formation is simultaneously inhibited. After airflow rate exceeds 5 cm/s, the concentrations of CO and NOx begin to decrease. When SS concentration is increased to 20%, the emission concentration of gas pollutants is directly increased. However, high temperatures inhibit the formation of NOx. When the particle size range is 180-270 μm, the formation of CO/NOx is promoted. Finally, a back propagation (BP) neural network model is constructed with SS characteristics and experimental parameters as input conditions, and CO/NOx emission concentration, smoldering velocity, and smoldering temperature as output parameters. The BP neural network model can effectively predict the emission concentration of CO/NOx and smoldering characteristics, providing support for intelligent control scenarios related to SS smoldering, it will help to further explore the great potential of smoldering treatment.

Effect of electromagnetic induction drying on the drying-incineration process of dyeing sludge: focus on migration and conversion of sulfur

Secondary sulfur pollution in dyeing sludge (DS) during drying and incineration is a major environmental problem necessitating in-situ control. To robustly immobilise sulfur during drying-incineration, the authors introduce an electromagnetic induction (EMI) drying method and reveal the corresponding migration and conversion of sulfur in DS. The EMI-drying efficiency reached 10.69%/min, five times that of thermal drying. EMI drying increases the relative sulfoxide ratio from that of thermal drying. In a sludge-sulfur model, the proposed treatment promoted the oxidation and decomposition of organic sulfur without noticeably affecting the inorganic sulfur. The selective oxidation process during EMI drying promotes sulfur stabilisation in dried DS, decreasing the performance and stability of DS combustion. The sulfur-containing pollutants released during the incineration of DS mainly contain H2S, followed by CH3SH and SO2. EMI drying increases the outputs of SO2 and CH3SH but decreases the outputs H2S and total sulfur compared with the outputs of thermal drying. Under the sulfur-model conditions, EMI promoted the conversion of inorganic sulfur to sulfur-containing gases (especially H2S) during incineration. In contrast, the sulfur stabilised by partial oxidation of organic sulfur in the EMI-dried DS was not easily converted to gaseous sulfur during subsequent combustion. Overall, EMI inhibits the release of sulfur during the combined drying-incineration process of DS.

Measurement and control of containing-fluorine particulate matter emission during spent pot lining combustion detoxification process

The particle size distribution (PSD), composition, morphology, and formation mechanism of particulate matter (PM) released from the combustion of spent pot lining with and without CaSiO₃ were investigated. The results showed that NaF and Na₃AlF₆ were found to be the main compositions of PM, and the particle size distribution of PM shows a bimodal distribution. CaSiO₃ substantially inhibited the emission of PM by transforming NaF, Na₃AlF₆, and CaF₂ into stable Ca₄Si₂O₇F₂. Moreover, CaSiO₃ also limited the formation of high hazardous PM_0.2 by providing SiO₂, Al₂O₃, and NaAlSiO₄ with high melting points as the core of promoting the growth of PM in particle size.

Experimental and DFT Research on the Effects of O2/CO2 and O2/H2O Pretreatments on the Combustion Characteristics of Char

The use of a coal-based energy structure generates a large amount of CO₂ and NOx. The numerous emissions from these agents result in acid rain, photochemical smog, and haze. This environmental problem is considered one of the greatest challenges facing humankind in this century. Preheating combustion technology is considered an essential method for lowering the emissions of CO₂ and NO. In this research, the char prepared from O₂/CO₂ and O₂/H₂O atmospheres was employed to reveal the effects of the addition of an oxidizing agent on the combustion characteristics of char. The structural features and combustion characteristics of preheated chars were determined by Raman, temperature-programmed desorption (TPD), and non-isothermal, thermo-gravimetric (TGA) experiments. According to the experimental results, the addition of oxidizing agents promoted the generation of smaller aromatic ring structures and oxygen-containing functional groups. The improvement in the surface physicochemical properties enhanced the reactivity of char and lowered its combustion activation energy. Furthermore, the combustion mechanisms of the char prepared from the O₂/CO₂ and O₂/H₂O atmospheres were investigated using the density functional theory (DFT). The simulation results illustrated that the combustion essence of char could be attributed to the migration of active atoms, the fracture of the benzene ring structure, and the reorganization of new systems. The addition of oxidizing agents weakened the conjugated components of the aromatic ring systems, promoting the successive decomposition of CO and NO. The results of this study can provide a theoretical basis for regulating the reaction atmosphere in the preheating process and promoting the development of clean combustion for high-rank coals.

Gas composition during thermochemical conversion of dry solid fuels and waste-derived slurries

Coal has long remained a promising and widely used energy resource all over the world. Special emphasis is usually put on the research and development of environmentally friendly technologies for the use of coal and coal processing waste. The development of slurry fuels based on coal waste is one of the promising ways to use raw materials with energy potential, recover wastes, and reduce the environmental load. However, no combustion technology has yet been created for heterogeneous wastes as water-based slurries. The physical principles and parameters of the corresponding processes have not been studied adequately. In this research, the environmental combustion indicators (CO₂, CO, H₂, NO_x, and SO₂ concentrations) of slurries based on water and petrochemical, coal, and plant wastes were analyzed for the first time in a wide range of temperatures covering all the typical stages of thermochemical fuel conversion: pyrolysis (400-700 °C), gasification (700-900 °C), and combustion (700-1000 °C). We established the key patterns and aspects of changes in gas concentrations at all the main stages during the thermal decomposition of fuels. The use of water-based fuels at the pyrolysis stage was notable for up to 96% higher concentrations of the key combustible gases (CO, H₂). The temperature extrema were 50-100 °C lower than those of bituminous coal. In terms of the key anthropogenic emissions (CO₂, NO_x, and SO₂), the combustion of slurries also appeared to be 20-77% more environmentally friendly than that of coal depending on the temperature conditions and fuel composition. The maximum positive effect from adding biomass to coal-water slurries was achieved in the temperature range of 850 to 1000 °C. The research findings can be used for developing the technologies for thermal recovery of waste as water slurries, in particular, by intensifying the pyrolysis, gasification, and combustion.

Investigation of interaction mechanisms during co-combustion of sewage sludge and coal slime: Combustion characteristics and NO/SO2 emission behavior

Co-combustion of sewage sludge (SS) and coal slime (CS) is a promising method to achieve resource utilization of both solid wastes. However, the emission characteristics of NO/SO₂ and the interaction mechanisms between SS and CS are unclear. In this paper, the co-combustion characteristics and NO/SO₂ emission behavior of SS and CS were investigated using a thermogravimetric analyzer and a tube furnace combustion system, and the interactions between SS and CS were explored. The results revealed the presence of remarkable interactions between SS and CS during the co-combustion. For the combustion characteristics, non-catalytic factors (interaction between volatiles and heat synergy) and catalytic factors (catalysis of inorganic components) controlled the combustion stage of the heavy volatiles and fixed carbon of the blends, respectively, leading to an earlier combustion process. For NO and SO₂ emission characteristics, SS-CS co-combustion had significant NO in-situ reduction and self-desulphurization characteristics at 800 °C and 900 °C. The best inhibition occurred at 900 °C and 50 % CS ratio, and NO and SO₂ emission amounts were reduced by 0.25 mg/g and 1.37 mg/g, respectively, compared to the theoretical values. At 1000 °C, co-combustion promoted the emissions of both NO and SO₂. The interaction mechanisms suggested that the reducing atmosphere created and the reducing gases released by SS combustion promoted the reduction of CS-NO, while the char formed by CS exhibited a significant reduction of SS-NO. In addition, the effect of CS addition on the mass transfer process enhanced the sulfur fixation of inorganic components in SS, which led to the suppression of SO₂ production. These findings provide a better understanding of the interactions between SS and CS during SS-CS co-combustion.

Co-combustion of high alkali coal with municipal sludge: Thermal behaviour, kinetic analysis, and micro characteristic

Blending sludge rich in protein and aliphatic hydrocarbons into the high alkali coal (HAC) has been demonstrated to reduce the ash melting temperature of the HAC/sludge mixture, thereby increasing the effectiveness and efficiency of liquid slagging. However, whether the incorporation of sludge can affect the combustion stability of the original coal-fired boiler is still debatable. As the combustion stability of the fuel can directly affect the operational safety of the boiler, it is of great practical value for exploring the effect of sludge incorporation on the combustion performance of HAC. In this work, the thermal behaviour and microscopic properties of individual HAC, municipal sludge (MS) and HAC/MS mixtures were tested using a Thermogravimetric analyser (TGA) and a Fourier transform infrared (FTIR) spectrometer, respectively. The exothermic, thermodynamic and functional group evolution patterns during the combustion of these samples were also evaluated. Ignition temperatures (T_i) of the HAC/MS mixtures were relatively lower than that of individual HAC, and decreased with the increase in sludge mass ratio (SMR). The synergistic effect of the co-combustion of HAC and MS resulted in a slightly higher total heat release during the combustion of MS10HAC90 (i.e., the mass percentage of MS and HAC is 1:9) than HAC alone, however, the total heat release of the blend decreased progressively with increasing SMR. The experimental values of the average E_α for all four mixtures were lower than the theoretical values, indicating that the addition of MS lowered the reaction energy barriers of the mixtures. Consumption rates of the principal groups in samples during the oxidation and combustion all tended to increase progressively with increasing SMR. There are three major synergistic effects existing during co-combustion of HAC and MS: (1) the reaction of free radicals with benzene molecules; (2) the interaction of free radicals; and (3) the catalytic effect of alkali and alkaline earth metals. These findings can provide theoretical guidance for the determination of key parameters (mixing ratio) for the blending of HAC and MS, and can fill the research gap in terms of microscopic reactivity and synergistic effects during the co-combustion of HAC and MS.

Co-Combustion Behavior of Paper Sludge Hydrochar and Pulverized Coal: Low Rank Coal and Its Product by Hydrothermal Carbonization

In this paper, the combustion behavior of low rank coal and its product after hydrothermal carbonization with paper sludge hydrochar were studied. The Raman technique was used to compare the structural differences between raw coal and the product. Thermogravimetric analysis was employed to conduct experiments of single sample and their mixtures with different proportions at a heating rate of 20 °C/min, the activation energy of chemical reactions was calculated. The results showed that upgraded product had higher carbon ordering degree than raw coal and the ignition temperature and burnout temperature of the product were advanced. Compared with raw coal, the combustion characteristic parameters C and S of the product were higher, indicating that its combustibility was better. As for the mixture, when the paper sludge hydrochar ratio was not more than 10%, the mixed fuel combustion curve was still similar to coal curve. After the paper sludge hydrochar ratio exceeded 10%, the activation energy of the mixed combustion reaction of paper sludge hydrochar and upgraded coal was lower than that of raw coal and paper sludge hydrochar. These results indicated that the mixture of upgraded coal and paper sludge hydrochar as mixed fuel was a better option.

Upgrading of banana leaf waste to produce solid biofuel by torrefaction: physicochemical properties, combustion behaviors, and potential emissions

This study is the first report that focuses on investigating the effects of torrefaction on the bioenergy-related properties, combustion behavior, and potential emissions of banana leaf waste (BLW). Experiments were first conducted in a bench-scale fixed-bed reactor operating at light (220 °C), mild (250 °C), and severe (280 °C) torrefaction conditions to torrefy the raw BLW. Torrefaction pretreatments reduced the weight of the raw BLW by about 60%, but the resulting solid biofuel can preserve up to 77% of the energy content of the raw biomass. It was found that torrefied BLW contains more concentrated fixed carbon than the raw BLW, volatile matter content of up to 59.8 wt.%, and a higher HHV of up to 20.7 MJ kg^-1 with higher concentrations of carbon, nitrogen, and ash. Bulk density increased 13.0% over the raw BLW, and the torrefied BLW became a solid biofuel with 51.5% greater energy density under the severe torrefaction condition. The upgrading of BLW by torrefaction enhanced its combustion performance in terms of comprehensive combustion, ignition, burnout, and flammability indices. Compared with commercial hard coal, BLW torrefied at the mild condition (250 °C) had lower potential emissions per unit of energy, 25.3% less CO₂ emission, 3.1% less CO emission, 96.4% less SO₂ emission, and 18.4% less dust emission, except for NO_X emission. This study conclusively indicates that BLW after torrefaction has enhanced bioenergy-related properties, improved combustion performance, and reduced emissions potential, proving to be a promising method for its valorization.

Effect of interactions during co-combustion of organic hazardous wastes on thermal characteristics, kinetics, and pollutant emissions

Thermal disposal of organic hazardous wastes (OHWs) in a rotary kiln is an effective method to destroy organic pollutants and reduce the volume, but the complex interactions between various OHWs may result in sharp degradation on combustion performance and the increase in gaseous pollutants emission. Herein, three typical types of OHWs (i.e., pesticide waste, dyeing waste, and organic resin waste, labeled as HW1, HW2, and HW3, respectively) were chosen and thermally co-treated, and the co-combustion characteristics, kinetics, and gas evolutions were systematically studied. A strong positive interaction between HW1 and HW2 was found between 440 and 680 °C possibly due to the catalytic effect of Fe (III) and alkali metals in HW1. The experimental DTG peaks of mixtures at 480 °C were advanced by 60 °C compared with the calculated ones, resulting from the volatiles combustion of HW2 and the catalytic effect from Fe₂O₃ formed during the combustion. The decrease of ignition temperature of mixtures was found helpful for stable combustion, while the decrease of burnout temperature during co-combustion of HW2 and HW3 exhibited the potential to reduce the clinker ignition loss.

Comparative Analyses between Raw and Preoxidized Pulverized Coals: Combustion Behaviors and Thermokinetic and Microcharacteristics

Investigating the difference in the combustion performance and microcharacteristics of oxidized and raw pulverized coal (PC) can contribute to effectively prevent and control the spontaneous combustion of deposited coal dust in high-temperature environment and further help guarantee the safe operation of coal-fired boiler. In this study, the combustion performance and thermokinetic and microcharacteristics of three raw coal samples and their preoxidized forms were explored by a thermogravimetric analyzer (TGA) and Fourier transform infrared spectroscopy (FTIR). According to the characteristic temperatures and variations of the mass loss rate during heating, the entire combustion process of PC was divided into four periods. For each type of coal, the preoxidized PC had relatively lower characteristic temperatures than the corresponding raw PC. The preoxidized samples had larger values of ignition index (C _ig) and comprehensive combustibility index (S), but lower values of burnout index (C _b) than raw samples. The values of apparent activation energy (E) for the preoxidized PC were below that of the corresponding raw PC at the same conversion rate (α), which suggested the preoxidized PC required relatively less energy to react and was more prone to spontaneous combustion. In addition, although parts of -OH, C=O, and aliphatic hydrocarbon groups were consumed after the preoxidation treatment, the increase in C-O and -COO- bonds compensated for the adverse effect of the reduction of the aforementioned groups on coal combustion.

Ash-to-emission pollution controls on co-combustion of textile dyeing sludge and waste tea

Given the globally increased waste stream of textile dyeing sludge (TDS), its co-combustion with agricultural residues appears as an environmentally and economically viable solution in a circular economy. This study aimed to quantify the migrations and chemical speciations of heavy metals in the bottom ashes and gas emissions of the co-combustion of TDS and waste tea (WT). The addition of WT increased the fixation rate of As from 66.70 to 83.33% and promoted the chemical speciation of As and Cd from the acid extractable state to the residue one. With the temperature rise to 1000 °C, the fixation rates of As, Cd, and Pb in the bottom ashes fell to 27.73, 8.38, and 15.40%, respectively. The chemical speciation perniciousness of Zn, Cu, Ni, Mn, Cr, Cd, and Pb declined with the increased temperature. The ash composition changed with the new appearances of NaAlSi₃O₈, CaFe₂O₄, NaFe(SO₄)₂, and MgCrO₄ at 1000 °C. The addition of WT increased CO₂ and NO_x but decreased SO₂ emissions in the range of 680-1000 °C. ANN-based joint optimization indicated that the co-combustion emitted SO₂ slightly less than did the TDS combustion. These results contribute to a better understanding of ash-to-emission pollution control for the co-combustion of TDS and WT.

Response surface optimization, combustion characteristics and kinetic analysis of mixed fuels of Fenton/CaO conditioned municipal sewage sludge and rice husk

The co-combustion characteristics and kinetics of Fenton/CaO conditioned MSS, and biomass rice husk (RH) are studied by thermogravimetry, and the condition optimization was carried out by response surface methodology (RSM). The results show that the mixed fuel with RH is helpful to decrease T_i and T_b values and increase combustion characteristic index (CCI). The CCI of MSS after conditioning is 0.59-0.88 times lower than that of the pure MSS. In addition, the total E_m of S2, MSS/RH mixed combustion after Fenton/CaO conditioning is lower, the combustion reactivity is stronger. According to RSM, the optimum conditions are considered to be: RH mixing ratio 56%, Fenton/CaO conditioner dosage 147 mg g^-1 dry solids, heating rate 30 K min^-1, the maximum CCI 25.3305 × 10^-7%² °C^-3 min^-2, and the minimum E_m 10.6403 kJ min^-1. This study supplies new insights into combustion technology of sludge.

Emission-to-ash detoxification mechanisms of co-combustion of spent pot lining and pulverized coal

In response to the global initiative for greenhouse gas emission reduction, the co-combustion of coal and spent pot lining (SPL) may cost-effectively minimize waste streams and environmental risks. This study aimed to quantify the emission-to-ash detoxification mechanisms of the co-combustion of SPL and pulverized coal (PC) and their kinetics, gas emission, fluorine-leaching toxicity, mineral phases, and migrations. The main reaction covered the ranges of 335-540 °C and 540-870 °C while the interactions occurred at 360-780 °C. The apparent activation energy minimized (66.99 kJ/mol) with 90% PC addition. The rising PC fraction weakened the peak intensity of NaF and strengthened that of Ca₂F, NaAlSiO₄, and NaAlSi₂O₆. The addition of PC enhanced the combustion efficiency of SPL and raised the melting temperature by capturing Na. PC exhibited a positive effect on solidifying water-soluble fluorine and stabilizing alkali and alkaline earth metals. The leaching fluorine concentrations of the co-combustion ashes were lower than did SPL mono-combustion. The main gases emitted were HF, NH₃, NO_x, CO, and CO₂. HF was largely released at above 800 °C. Multivariate Gaussian process model-based optimization of the operational conditions also verified the gas emissions results. Our study synchronizes the utilization and detoxification of SPL though co-combustion and provides insights into an eco-friendlier life-cycle control on the waste-to-energy conversion.

Optimizing environmental pollution controls in response to textile dyeing sludge, incineration temperature, CaO conditioner, and ash minerals

The dynamics of heavy metal speciation and flue gas emissions during the incineration of textile dyeing sludge (TDS) were quantified as a function of four addition levels of CaO, incineration temperature, and ash minerals using thermogravimetric analysis and experimental tube furnace. The TDS incineration was most improved with the addition of 10% CaO. The increased fractions of CaO coupled with the ash minerals changed the retention behaviors of eight heavy metals. The CaO addition increased the Cu, Zn, As, and Pb retentions, did not significantly change Cr, Mn, and Cd, but decreased the Ni retention. The CaO addition enhanced the speciation stability of Cu and transferred the Cr, Cd, and As speciations to the mobile fractions. The increased temperature weakened the Zn and Pb retentions and the speciation stabilities of As and Pb and turned the Cr, Mn, Ni, Cu, Zn, and Cd speciations into the stable fractions. The CaO addition inhibited HCN, NO, NO₂, COS, SO₂, CS₂, and SO₃ emissions from the TDS incineration. Neural network-based multi-response optimization was implemented to determine the optimal operational temperature for the TDS incineration and the reduction of the 12 gas emissions. The range of 640-755 °C with(out) 5% CaO appeared to be most beneficial in terms of both environmental quality and economic efficiency.

Thermochemical and Toxic Element Behavior during Co-Combustion of Coal and Municipal Sludge

The thermochemical and kinetic behavior of co-combustion of coal, municipal sludge (MS) and their blends at different ratios were investigated by thermogravimetric analysis. Simulation experiments were performed in a vacuum tube furnace to determine the conversion behavior of toxic elements. The results show that the combustion processes of the blends of coal and municipal sludge are divided into three stages and the combustion curves of the blends are located between those of individual coal and municipal sludge samples. The DTG_max of the sample with 10% sludge addition reaches a maximum at the heating rate of 20 °C/min, indicating that the combustion characteristics of coal can be improved during co-combustion. Strong interactions were observed between coal and municipal sludge during the co-combustion. The volatilization rates of toxic elements decrease with an increasing proportion of sludge in the blends during co-combustion, which indicates that the co-combustion of coal and sludge can effectively reduce the volatilization rate of toxic elements. The study reflects the potential of municipal sludge as a blended fuel and the environmental effects of co-combustion of coal and municipal sludge.

Do FeCl3 and FeCl3/CaO conditioners change pyrolysis and incineration performances, emissions, and elemental fates of textile dyeing sludge?

The pyrolysis and incineration performances of sulfur-rich textile dyeing sludge (TDSS) were determined in response to the additions of FeCl₃ or FeCl₃ + CaO. The emissions of eight air pollutants from the incineration and pyrolysis were systematically identified. The 3-to-8% FeCl₃ additions increased the comprehensive combustibility index by 2.14 and 1.62 times, respectively, as opposed to the 5-to-10% FeCl₃ + 8-to-15% CaO additions. The CaO addition inhibited the TDSS incineration, while the FeCl₃ addition increased HCl emission. NO_x, SO₂, and H₂S emissions decreased initially and increased between 600 and 950 °C. SO₂ and NO_x emissions rose with FeCl₃ but FeCl₃ + CaO. FeCl₃ catalyzed NO_x, while CaO retained SO₂. The main pyrolysis gas/liquid products were alkane, alkenes, nitrile, heterocyclic compounds, benzene, and its derivatives. Benzene and its derivatives accounted for 55.33% of the control group and 42.25-57.23% of the treatment groups. The FeCl₃ and FeCl₃ + CaO additions did not significantly influence the pyrolysis products. The measured versus thermodynamically simulated SO_x and HCl emissions were consistent. Neural network-based simultaneous optimizations of the non-linear dynamics of eight kinds of gases pointed to 50% and 14.4% reductions in the emissions and the pyrolytic temperature, respectively, with the 3% FeCl₃, relative to the control.

Residual and ecological risk assessment of heavy metals in fly ash from co-combustion of excess sludge and coal

Co-combustion of municipal excess sludge (ES) and coal provides an alternative method for disposing ES. The present study aims to investigate the residual and ecological risk of heavy metals in fly ash from co-combustion of ES and coal. The total concentration and speciation distribution of heavy metals, characterization of SEM, EDX, XRD and leaching test were carried out to assess the fly ash in this study. The results showed that the total concentrations of Cu, Zn and Mn were higher than others in fly ash, and most heavy metals were concentrated in fine particles. For Cd, Cr and Pb, the percentages of speciation of F4 and F5 were all over 90%, suggesting the relatively lower leaching toxicity. The leaching percent of all heavy metals was lower than 5% by two diluted HNO₃ solutions for fly ash. The potential ecological risks increased with the decrease of particle size of fly ash, and Cd accounted for the main fraction for ecological risk despite of lower concentration in comparison to other measured heavy metals.

Catalytic combustion performances, kinetics, reaction mechanisms and gas emissions of Lentinus edodes

This study aimed to quantify the catalytic effects of CaO, Fe₂O₃, and their blend on the Lentinus edodes stipe (LES) and pileus (LEP) combustion performances, kinetics and emissions in bioenergy generation. Apparent activation energy (E_a) of LES and LEP increased with CaO, decreased with Fe₂O₃ and differed with their blend. The catalysts mainly affected the maximum intensity of volatiles combustion and partly the fixed carbon combustion. CaO, Fe₂O₃, and their blend decreased the release intensity of NO_x from the LES combustion. Fe₂O₃ increased SO₂ emission, while CaO, and the blend narrowed the emission temperature to the range of 200 to 450 °C. Kinetic triplets were estimated via the integral master-plots methods, and the best-fit reaction for the three sub-stages were obtained coupled with the model-free models. Our study provides a reference for the catalyzed biomass combustion in terms of pollution control, bioenergy generation, optimal design of incinerator, and industrial-scale application.

Combustion of single particles from sewage sludge/pine sawdust and sewage sludge/bituminous coal under oxy-fuel conditions with steam addition

Co-pelletization of sewage sludge (SS) and conventional fuels for combustion is considered to be a feasible SS disposal method. Oxy-fuel combustion is recognized as a promising technology to reduce the emission of CO₂. In practical applications, the combustion atmosphere in oxy-fuel boiler is O₂/CO₂/H₂O, which is different from that in the conventional boiler (O₂/N₂). Therefore, the effects of gas composition on the combustion characteristics of boiler fuels should be both taken into consideration. In this work, the SS/pine sawdust (PS) and SS/bituminous coal (BC) blended fuel particles were prepared, and the single particle combustion experiments were conducted in O₂/N₂, O₂/CO₂ and O₂/CO₂/H₂O atmospheres. The influences of SS blending proportion and gas composition on combustion characteristics of fuel particles were analyzed. The results reveal that increasing the blending proportion of SS from 20 to 40 wt% decreases the ignition delay time, burnout time and combustion temperature. The substitution of N₂ by CO₂ increases the ignition delay time and burnout time, while decreases the combustion temperature. Replacing CO₂ by 10 vol%, 20 vol% and 30 vol% H₂O decreases the ignition delay time and burnout time, while increases the combustion temperature.

Co-combustion of industrial coal slurry and sewage sludge: Thermochemical and emission behavior of heavy metals

A combination of thermogravimetric analysis and lab-scale fixed bed combustion experiments was carried out to study the thermochemical, kinetic and heavy metals emission behavior during co-combustion of industrial coal slurry (CS) and sewage sludge (SS). The results found that the blends had integrative combustion profiles which reflected both coal slurry and sewage sludge. During co-combustion, the ignition performance of CS could be significantly improved with the addition of SS. Synergetic effects of the co-combustion were observed at lower temperature, while the high-temperature char combustion of the blends was inhibited because of high ash components of SS or formation of inactive alkali metal aluminosilicates. Kinetic analysis confirmed the improve iginition behavior of blends. Both the comprehensive combustibility index S and the activation energy suggested that the blends with 20% SS may have the best promoting effects. Compared with CS, the higher concentration of Cl in SS increased the volatilization ratios of Cu, Zn, As, and Pb. When added CS into SS, the volatilization ratios of arsenic decreased during combustion. The inhibition effects for arsenic during co-combustion might be associated with the capture of arsenic vapors by the new-formed Ca/Al from CS thermal decomposition.

Two-in-One Fuel Synthetic Bioethanol-Lignin from Lignocellulose with Sewage Sludge and Its Air Pollutants Reduction Effects

Developing effective, economical, and environmentally sound approaches for sewage sludge management remains an important global issue. In this paper, we propose a bioethanol-lignin (nonfood biomass)-based sewage sludge upgrading process for enhancing the heating value and reducing air pollutants of hybrid sewage sludge fuel (HSF) for the effective management of sewage sludge. Sewage sludge paste with the lignin-CaO solution implies drying at 105 °C accompanied by torrefaction at 250 °C. During torrefaction, moisture and partly volatile matter begin to evaporate, and are almost vaporized out to the surface. In this study, the proposed process enhances the net caloric value (NCV) to 37%. The lignin-embedded HSF shows a two-in-one combustion peak regardless of the mixing ratio, resulting in a 70% reduction of unburned carbon (UBC) emissions, which is one of the particular matter (PM) sources of combustion flue gas. Other air pollutants, such as CO, hydrocarbon, NOx, and SOx, were also reduced by the proposed process. In particular, SOx emission remained at ~1 ppm (average value) regardless of the sulfur content of the fuel.

Co-biodrying of sewage sludge and organic fraction of municipal solid waste: A thermogravimetric assessment of the blends

This study investigated the thermogravimetric properties of sewage sludge and organic fraction of municipal solid waste (OFMSW) during their co-biodrying at different fractions. Sewage sludge and OFMSW were co-biodried at the mass proportion of 0%, 42.5% and 85% (of the total wet weight), respectively, with 15% cornstalk as the bulking agent. Results show that of these three raw materials, OFMSW exhibited the lowest ignition temperature and the highest burnout temperature. Moreover, OFMSW had a better comprehensive combustion performance (S) than sewage sludge. Blending OFMSW, sewage sludge and cornstalk showed the highest S value (4.0 × 10^-7%² min^-2 °C^-3). In addition, there existed certain interactions between the co-combustion process, especially at high temperature stage. The burning characteristics, including ignition performance, burnout efficiency, DTG_max and S increased with fluctuations in the first 6-9 days of co-biodrying process, and then declined in all treatments. Hence, 15-day of biodrying made the product with poor burning behavior (S value of 1.0 × 10^-7-1.4 × 10^-7%² min^-2 °C^-3). More importantly, the optimal combustion performance was observed when co-biodrying the same amount (42.5%) of sewage sludge and OFMSW with the peak of 8.3 × 10^-7%² min^-2 °C^-3 achieved on day 9. In addition, the blends were easier to burn after the biodrying process.

Kinetics, thermodynamics, gas evolution and empirical optimization of (co-)combustion performances of spent mushroom substrate and textile dyeing sludge

Spent mushroom substrate (SMS) and textile dyeing sludge (TDS) were (co-)combusted in changing heating rates, blend ratios and temperature. The increased blend ratio improved the ignition, burnout and comprehensive combustion indices. A comparison of theoretical and experimental thermogravimetric curves pointed to significant interactions between 350 and 600 °C. High content of Fe₂O₃ in TDS ash may act as catalysis at a high temperature. Ignition activation energy was lower for TDS than SMS due to its low thermal stability. 40% SMS appeared to be the optimal blend ratio that significantly decreased the activation energy, as was verified by the response surface methodology. D3 model best described the (co-)combustions. SMS led to more NO and NO₂ emissions at about 300 °C and less HCN emission than did TDS. The addition of 40% SMS to TDS lowered SO₂ emission. The co-combustion of TDS and SMS appeared to enhance energy generation and emission reduction.

Thermogravimetric and mass-spectrometric analyses of combustion of spent potlining under N2/O2 and CO2/O2 atmospheres

Thermal decomposition and gaseous evolution of the spent potlining (SPL) combustion were quantified using thermogravimetric and mass-spectrometric analyses in CO₂/O₂ and N₂/O₂ atmospheres using three heating rates (15, 20 and 25 °C/min). The thermal decomposition of SPL occurred mainly between 450 and 800 °C. Based on the four kinetic methods of Friedman, Starink, Kissinger-Akahira-Sunose and Flynn-Wall-Ozawa under the various conversion degrees (α) from 0.1 to 0.7, the lowest apparent activation energy was estimated at 149.81 kJ/mol in the 70% CO₂/30% O₂ atmosphere. The pre-exponential factor, and changes in entropy, enthalpy and free Gibbs energy were also estimated. The reaction model did not suggest a single reaction of the SPL combustion. With the α value of 0.25-0.7, the following function best described the reaction based on the Malek method: f(α) = 1/2α and G(α) = lnα². The gases released during the combustion process included CO₂, CO, NO_x, HCN, and HF.

Effect of blending sewage sludge with coal on combustion and ash slagging behavior

Blending sewage sludge (SS) with Zhundong coal (ZDC) for combustion in coal-fired power plants is a recent approach that can alleviate the shortage of high-quality coal resources and achieve the harmless treatment of SS, while also having a significant influence on combustion and ash slagging. Due to the high content of alkali and alkaline earth metals (AAEMs) in ZDC, its combustion ash has a strong likelihood of slagging. This study aims to investigate the effect of blending SS with ZDC on combustion and ash slagging. Thermogravimetry (TG) results indicate that blending with SS could lower the ignition and burnout temperatures of ZDC. With an increase in the ratio of sludge, the comprehensive combustion index (S) first increases and then decreases, showing that blending SS with ZDC in an appropriate proportion could improve the overall combustion. Through the analysis of the interaction, it is confirmed that SS and ZDC could complement each other during co-combustion due to their different components. X-ray fluorescence (XRF) was used to test the ash components of different blending ratios (10–30%) and combustion temperatures (800–1100 °C). Slagging indices including alkali acid ratio (B/A), silicon ratio (G), and silica–alumina ratio (SiO₂/Al₂O₃) were also calculated. The results suggest that the slagging behavior of ZDC is greatly reduced even if the blending ratio is only 10%. However, with an increase in the blending ratio, the effect on slagging gradually weakens. Considering the dual influence of SS blending on combustion and slagging, this study assumes the optimal blending ratio of 20%. Influenced by the components of the combustion ash, B/A and SiO₂/Al₂O₃ are more suitable for evaluating the slagging tendency of ash; however, there is great deviation in the results for G. This research is beneficial to coal-fired power plants for the selection of operation parameters during co-combustion with SS.

Blending sewage sludge (SS) with Zhundong coal (ZDC) for combustion in coal-fired power plants is a recent approach that can alleviate the shortage of high-quality coal resources and achieve the harmless treatment of SS, while also having a significant influence on combustion and ash slagging.

A Review of Sludge-to-Energy Recovery Methods

The increasing volume of sewage sludge from wastewater treatment facilities is becoming a prominent concern globally. The disposal of this sludge is particularly challenging and poses severe environmental hazards due to the high content of organic, toxic and heavy metal pollutants among its constituents. This study presents a simple review of four sewage to energy recovery routes (anaerobic digestion, combustion, pyrolysis and gasification) with emphasis on recent developments in research, as well as benefits and limitations of the technology for ensuring cost and environmentally viable sewage to energy pathway. This study focusses on the review of various commercially viable sludge conversion processes and technologies used for energy recovery from sewage sludge. This was done via in-depth process descriptions gathered from literatures and simplified schematic depiction of such energy recovery processes when utilised for sludge. Specifically, the impact of fuel properties and its effect on the recovery process were discussed to indicate the current challenges and recent scientific research undertaken to resolve these challenges and improve the operational, environmental and cost competitiveness of these technologies.

Activation of the Fuels with Low Reactivity Using the High-Power Laser Pulses

In this paper we have proposed the simple and effective approach to activation of the low reactivity industrial fuel which can be used immediately inside the furnace. The high-power laser pulses initiates partial gasification of the fuel together with its ultra-fine atomization. The gas-aerosol cloud surrounding the initial coal-water slurry droplet can consist of approximately 10% (after absorption of hundred pulses) of the initial droplet weight. The ratio of the syngas and aerosol weights is like 1:2 when pulse intensity is higher than 8 J/cm 2 . The size and velocity distributions of the ultra-fine aerosol particles were analysed using the original realization of the particle tracking velocimetry technique.

Combustion behaviors of spent mushroom substrate using TG-MS and TG-FTIR: Thermal conversion, kinetic, thermodynamic and emission analyses

The present study systematically investigated the combustion characteristics of spent mushroom substrate (SMS) using TG-MS (thermogravimetric/mass spectrometry) and TG-FTIR (thermogravimetric/Fourier transform infrared spectrometry) under five heating rates. The physicochemical characteristics and combustion index pointed to SMS as a promising biofuel for power generation. The high correlation coefficient of the fitting plots and similar activation energy calculated by various methods indicated that four suitable iso-conversional methods were used. The activation energy varied from 130.06 to 192.95 kJ/mol with a mean value of 171.49 kJ/mol using Flynn-Wall-Ozawa and decreased with the increased conversion degree. The most common emissions peaked at the range of 200-400 °C corresponding to volatile combustion stage, except for CO₂, NO₂ and NO. The peak CO₂ emission occurred at 439.11 °C mainly due to the combustion of fixed carbon.