Co-combustion Of Sludge Research Articles

Effect of Sewage Sludge Addition on the Co-Combustion Characteristics of Municipal Solid Waste Incineration

This study employs a numerical computation model based on a municipal solid waste (MSW) incinerator in Nanning to investigate the impact of different sewage sludge (SS) co-combustion ratios and MSW incinerator temperatures on combustion efficiency. Using the FLUENT simulation method, this study systematically analyzes the distribution characteristics of the temperature field, velocity field, and pollutant concentration field within the furnace under various SS mixing ratios (5%, 7%, 10%, and 15%) and MSW incinerator temperatures (800 K, 1000 K, and 1200 K). The simulation results indicate that the combustion efficiency was optimal at an MSW incinerator temperature of 800 K, where the co-combustion of SS with MSW mixed effectively, leading to a stable and efficient combustion process. Furthermore, an SS co-combustion ratio of 7% was identified as the most effective in maintaining high combustion efficiency. These findings contribute to the optimization of co-combustion strategies for MSW and SS, enhancing both operational efficiency and environmental compliance.

Study of Characteristics and Heavy Metals Migration During Co-Combustion of Coal and Sludge

Study of Characteristics and Heavy Metals Migration During Co-Combustion of Coal and Sludge

Investigation on ash fusion characteristics and mineral transformation mechanism of sorghum straw by paper-making sludge addition

The co-combustion of sorghum straw and paper-making sludge (PMS) is beneficial to alleviate the ash-related problem in sorghum straw combustion process. In this paper, the mineral migration and transformation behavior in sorghum straw ashes with the addition of PMS ashes under an oxidizing atmosphere were investigated using an X-ray diffractometer, thermogravimetric-derivative thermogravimetric, scanning electron microscopy-energy dispersive X-ray detector, and FactSage software. The results demonstrated that the increase in the PMS ash mass ratio led to an increase in ash fusion temperature (AFT) of sorghum straw mixtures. The generations of high melting-point (MP) calcium feldspar (e.g., Ca2MgSi2O7, Ca2Al2SiO7, Ca3MgSi2O8, and Ca7Mg (SiO4)4) and phosphates (e.g., Ca3P2O8, Ca5P2SiO12, and Ca7P2Si2O16) caused the increases in the AFT. The maximum weight loss of sorghum straw ashes decreased with the increasing PMS ash mass ratio, and the surface of the ashes underwent the transition from smooth to a whole loose and rough, which alleviated the ash-related problems of sorghum straw. The addition of PMS ashes decreasing the liquid-phase content calculated by FactSage at the same temperature also explained the sorghum straw AFT increase. The changes of AFT and its corresponding mechanisms in the sorghum straw with PMS ash addition under an oxidizing atmosphere provided the support for large-scale combustion of biomass.

The effect of additives on the co-pelletization of sewage sludge and extraction residue on combustion behavior and off-gas emission reduction

Co-combustion of biomass and sludge is a promising method for waste treatment and power generation. In this study, additives such as ammonium dihydrogen phosphate (ADP), calcium oxide (CaO), attapulgite (AT), and their combinations were added into the co-pelletization of semi-dry sludge (with approximately 50% water content) and extraction residue from vegetable oil processing. The objective of the studies was to examine the pelletization behavior, reduce off-gas emissions during the combustion, and investigate the mechanisms influenced by these additives through the thermogravimetric analysis and the tube furnace combustion experiment. The results of the analysis of variance (ANOVA) indicated an increase in pellet density due to the addition of additives. During the combustion, the addition of AT and compound additives generated more complex compounds with high melting temperatures, thereby mitigating the high slagging tendency. Meanwhile, additives facilitated the conversion of ultrafine particles (PM0.1) to fine particles, leading to a reduction in PM0.1 emissions by 15.60–42.40%. The additives exhibited efficacy in reducing SO2 emissions, with a reduction range of 5.00–23.20%. However, the addition of ADP additives resulted in a 12.00% increase in NO emissions. Consequently, adding additives to pellets could address the slagging issue and reduce the emission of air pollutants during combustion.

High-Temperature Ash Melting and Fluidity Behavior upon the Cocombustion of Sewage Sludge and Coal.

Wastewater treatment produces a large amount of sludge, where the minimizing of the disposed sludge is essential for environmental protection. The co-combustion of sludge with coal is a preferable method for sewage sludge disposal from the economic and environmental perspective. The co-combustion of sludge has been widely used in the industry with the advantages of large processing capacity. The melting characteristics of ash are an important criterion for the selection of the co-combustion methods and furnace types. In this study, two types of sludge and four types of coal with different ash melting points were selected, where the ash melting behavior upon co-combustion is investigated by experimental and thermodynamical approaches. Especially, the slag fluidity upon co-combustion is explored via a modified inclined plane method. It has been found that the presence of SiO2 and CaO in sludge substantially enhances its fusion temperature owing to the high content of CaO, while SiO2 acts as a solvent, facilitating the co-melting of other oxides and raising the sludge fusion temperature. Fe2O3 exhibits a specific mass fraction within the range of 10-20%. Furthermore, the presence of CaO and SiO2 prohibits the flow ability of the slag at high temperatures, and Fe2O3 promotes the flow ability for sludge at high temperatures. With increasing base/acid ratio, the sludge flow velocity increases remarkably and peaks at 1.6. The interaction between Fe-Ca and Si-AI significantly affects the fluidity significantly. The findings are expected to optimize the condition of co-combustion and desirable furnace design for the incineration of sludge.

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.

Heavy metals behavior of co-combustion ash from sewage sludge and coal slime: Temperature and mixing ratio dependence, interactions and speciation

Co-combustion of municipal sludge (SS) and coal slime (CS) is an effective route to achieve the co-disposal of solid wastes. This work addresses the problem of heavy metal pollution in co-combustion ash, aiming to reveal the effect of interactions in SS-CS co-combustion on the behavior of heavy metals and the control mechanism of minerals using the inductively coupled plasma mass spectrometry (ICP-MS) technique and the Community Bureau of Reference (BCR) method. The residual rate of heavy metals in CS ash monotonically decreased with increasing combustion temperature, while the residual rate of heavy metals in SS tended to increase when it was burned at high temperatures. Increased temperature enhanced the effect of SS-CS interaction on heavy metals. The interaction significantly inhibited the volatilization of As, and the As residue of the blend containing 50% CS (5SS5CS) at 1000 °C was 14.9% higher than the theoretical value. Typical minerals in SS had significant effects on heavy metal migration in CS, and the effects of CaO and Fe2O3 on heavy metals were mainly due to chemical interactions, in which CaO fixed As in the form of Ca3(AsO4). Density functional theory calculations show that the optimal adsorption sites for both As and As2O3 on the CaO(001) surface were O-top sites, and the presence of O defect on the surface led to enhanced interaction of As2O3 with the surface. Co-combustion led to an increase in the residual fractions of heavy metals, but the exchangeable and acid soluble fraction of As in 5SS5CS was still as high as 19.3%, so the ecological impact of As should be considered in further utilization of the co-combustion ash.

Co-combustion of bituminous coal and industrious sludge under non-isothermal and isothermal conditions: Thermal behaviors, kinetic analyses, and heavy metals migration

Considering the environmental pollution caused by the direct stacking or landfilling of various sludge, the combustion characteristics of bituminous coal, Fe-rich industrious sludge, and their blend were studied using thermogravimetric analysis at non-isothermal and isothermal conditions. It was found that the volatile matter in sludge with low C/H ratios will be released and burned in the low-temperature stage, then the generated heat will promote extra heat for the combustion of fixed carbon in coal when the addition of sludge was greater than 60%. The used Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods were more accurate for coal, sludge, and their blend with low sludge addition. After comparing the activation energy, a suitable addition of sludge was found to improve the combustion and kinetic characteristics of fixed carbon but do less help to volatile matter in coal. As to the heavy metals emission, the synthetic effect of the blend combustion enhanced the Pb emission when sludge addition was more than 60% but almost did not affect Cr, Ni, Zn, and Cu. By sampling dynamically during the co-combustion, it was found that the leaching toxicity of heavy metals was greatly related to the combustion degree. The stabilization of Al, Ca, Cr, and Zn was finished as their leaching toxicity decreased to a constant value before the burnout of fixed carbon (100∼120s) while that of Ni was slightly delayed, especially when the coal was added. Besides, the leaching concentration of Cu gradually increased with combustion time and its stabilization cost more time. Such a time-controlled combustion strategy may also be worthy of further research to help dispose of other solid wastes with similar composition.

Techno-economic analysis of municipal sludge co-combustion power generation with different fuels

Municipal sludge production has been growing due to urbanization, and the sludge co-combustion process has been proven to be a promising alternative. Bituminous coal, straw, and general industrial solid waste (GISW) were used as fuel with sludge co-combustion power generation (CPG). The life cycle cost (LCC) theory and life cycle assessment (LCA) were applied to study the techno-economic and environmental impact of three CPG processes. The results show that the sludge CPG process demonstrates significant economic benefits. Using 1 ton of wet sludge as the functional unit. The net gains for using GISW, rice straw, and bituminous coal as fuel are ¥947.38, ¥261.31, and ¥135.78, respectively. However, the sludge CPG process imposes a burden on the environment, and incineration is the main stage leading to environmental degradation. Therefore, it is essential to focus on the emission of pollutants during this stage. This study can promote the economic sustainable development of sludge CPG.

Numerical Study on Coal–Sludge Cocombustion Characteristics and NOx Emission Behaviors in a 330-MW Wall-Fired Boiler

Numerical Study on Coal–Sludge Cocombustion Characteristics and NOx Emission Behaviors in a 330-MW Wall-Fired Boiler

Introducing hydrothermal carbonization to sewage sludge treatment systems—a way of improving energy recovery and economic performance?

Hydrothermal carbonization (HTC) can mitigate the disposal costs of sewage sludge in a wastewater treatment plant. This study analyzes the impact of integrating HTC with anaerobic digestion (AD) and combustion from a combined energy and economic performance perspective. Net energy balance and investment opportunity are investigated for a number of technical scenarios considering i) different combinations of the technologies: AD + HTC, AD + thermal dryer + combustion, and AD + HTC + combustion, ii) different options for HTC process water treatment: wet oxidation (WO) + AD, and direct return to AD, and iii) different products: heat-only, heat and electricity, hydrochar, and phosphorus.The results show trade-offs between investment cost, self-supplement of heat, and output electricity when WO is used. In AD + HTC, net heat output decreases compared to the reference plant, but avoided disposal costs and hydrochar revenue result in profitable investment when the process water is directly returned to the AD. Although HTC has a lower heat demand than the thermal dryer, replacing the thermal dryer with HTC is only possible when AD, HTC, and combustion are connected, or when WO covers HTC’s heat demand. HTC may impair the electricity production because of the necessity for a high-temperature heat source, whereas the thermal dryer can utilize a low-temperature heat source. In conclusion, energy advantages of HTC in AD + HTC + combustion are insufficient to provide a promising investment opportunity due to high investment costs of HTC. The investment opportunity improves by co-combustion of hydrochar and external sludge.

Thermal Behavior Prediction of Sludge Co-Combustion with Coal: Curve Extraction and Artificial Neural Networks

Previous studies on the co-combustion of sludge and coal have not effectively utilized the characteristics of the combustion process to predict thermal behavior. Therefore, focusing on these combustion process characteristics is essential to understanding and predicting thermal behavior during the co-combustion of sludge and coal. In this paper, we use thermogravimetric analysis to study the co-combustion of coal and sludge at different temperatures (300–460 °C, 460–530 °C, and 530–600 °C). Our findings reveal that the ignition improves, but the combustion worsens with more sludge. Then, we further employ curve extraction based on temperature and image segmentation to extract the DTG (weight loss rate) curves. We successfully predicted the DTG curves for different blends using nonlinear regression and curve extraction, achieving an excellent R2 of 99.7%. Moreover, the curve extraction method predicts DTG better than artificial neural networks for two samples in terms of R2 (99.7% vs. 99.1% and 99.7% vs. 94.9%), which guides the application of co-combusting coal and sludge.

Co-combustion characteristics of paper sludge and combustible construction and demolition waste

Combustion characteristics and hazardous gas emissions were investigated by using TG-FTIR under individual and co-combustion of paper sludge (PPS), combustible construction and demolition wastes (CCDW) and its components. The interaction between PPS and CCDW during co-combustion at different PPS ratios was predicted by mass-weighted method. Kinetic parameters under individual and co-combustion of PPS and CCDW were calculated by using three model-free methods involving Flynn-Wall-Ozawa (FWO), Kissinger-Akah-Sunose (KAS) and Starink. The obtained results indicated that the residue of co-combustion with PPS and CCDW gradually increased when PPS ratio increased from 30% to 70%. Co-combustion of PPS and CCDW improves combustion characteristics and mass-weighted method can be used to predict their TG/DTG curves. A significant interaction between PPS and CCDW was clarified, especially at the high temperature stage greater than 400 °C. The activation energy calculated by FWO method has higher accuracy for co-combustion of PPS and CCDW. Co-combustion of PPS and CCDW can effectively reduce the emission of hazardous gases. A suggested PPS ratio of 50% was proposed for improving combustion characteristics and reducing the hazardous gas emissions during co-combustion of PPS and CCDW.

Influence of fuel distribution on co-combustion of sludge and coal in a 660 MW tangentially fired boiler

The distribution method of coal and sludge is an important factor affecting co-combustion, but few studies have been carried out in most papers. The influence of different fuel distribution methods for municipal sewage sludge and coal on co-combustion in a furnace was studied using numerical computational fluid dynamics simulations in this paper. In the considered cases, the burners that inject sludge were artificially altered, while the masses and contents of sludge and coal remained unchanged. The combustion temperature, combustion characteristics, and the heat of the boiler were analysed. The concentrations of CO, CO2, and volatile matter were used as characteristic parameters. The conclusion shows that the co-combustion temperature in the main combustion region inevitably decreases compared to coal mono-combustion regardless of the fuel distributions being used. When coal and sludge are premixed, the furnace temperature distribution is more uniform. However, premixing has a greater influence on the boiler heat flux. Feeding sludge into the furnace from the top layer of burners individually can effectively reduce the adverse impact of premixed combustion, and the total heat flux of the furnace is reduced by 4.8%, the heat flux decreased by about 6% for other distribution methods. Considering the broad prospects of sludge co-disposal in boilers, these findings provide guidance and suggestions for the co-combustion of sludge and coal, and they provide theoretical support for the efficient operation of boilers.

Co-Combustion Characteristics of Municipal Sewage Sludge and Coal in a Lab-Scale Fluidized Bed Furnace

The co-combustion of dry coal and sludge was carried out in a self-built laboratory scale fluidized bed furnace. We used FTIR spectroscopy, high-resolution gas chromatography, high-resolution mass spectrometry, and XRF to capture and dynamically observe the flue gas fly ash and bottom ash. The sludge sample was taken from a municipal sewage treatment plant and its mixing ratio was below 20%. The temperature distribution of the furnace was observed and the emissions of the main gaseous pollutants such as SO2, NOx, and dioxins were measured in the combustion process. The heavy metal contents of fly ash and bottom ash were later detected to analyze the migration characteristics. The results showed that all of the gaseous pollutants measured increased linearly with the sludge mixing ratio. However, it could also meet the 1 metal content of bottom ash and fly ash did not change much. Through the elaboration of the above points, this study can provide guidance for future research on the co-combustion of coal boiler sludge and suggest favorable conditions for the co-combustion of coal boiler sludge.

Characterisation of ash particles from co-combustion of bark and sludges from pulp and paper industry

Recycling phosphorus from waste streams for fertilization purposes could contribute to a sustainable society. The production in the pulp and paper industry results in several waste streams, among others nutrient-rich sludges in different forms. This study presents a detailed chemical and 3D characterization of ash from co-combustion of bark and two types of sludges from a paper mill; mixed sludge and biosludge. The combustion performance was investigated for these experiments and advanced analysis methods were used to characterise the ashes to correlate chemical and physical properties relevant for nutrient recycling. The elemental composition was determined by energy-dispersive X-ray spectroscopy; dominating crystalline phases by X-ray diffraction; and morphology, porosity, pore size distribution and active surface area of the slag were analysed with synchrotron-based X-ray micro-tomography and image analysis. Slag was formed in all combustion experiments to a large extent with increasing amounts with a higher proportion of sludge. Nutrient amounts indicate that slag particles from co-combustion of both biosludge and mixed sludge can be useful either as a soil improvement directly or for recovery processes. Slag from combustion of 30 wt% biosludge and 70 wt% bark contained the highest amount of phosphorus, 9 at% on a C and O free basis. Evaluation of tomography data showed that discrete and open pores could be distinguished on a micrometre scale. The porosity of the slag varied between the replicates and fuel mixtures, on average between 17 and 23 vol% for the bark and sludge mixtures. Open pore volume displayed large variations, on average 39–56 vol% of the pores were open pores connected to the surrounding volume. For all samples, 90 % of the pores were small, with an equivalent diameter under 30 μm, but the largest pore volume (80–90 %) consists of pores with an equivalent diameter over 75 μm. In soils, pores with a minimum equivalent diameter over 30 μm generally transmit water and the smaller pores store water. The slag particles have relatively thick walls, with few pore openings to the surroundings, indicating that the slag needs to be pre-treated by milling or crushing before application in the soil.

Experimental and ReaxFF molecular dynamic study of NO emission during municipal sludge/coal co-combustion

In this work, the emission behavior of NO during the co-combustion of municipal sludge (MS) and Meihuajing bituminous coal (MHJ) was studied using experiments and ReaxFF molecular dynamics methods. Both experimental and ReaxFF MD results indicated that there has negative synergistic effect on NO emission during MS/MHJ co-combustion. In addition, there has positive synergistic effect on N2 and NO2 emission during MS/MHJ co-combustion. Using the atomic labeling method, it was observed that the yield of MS-NO decreased, while the yield of MHJ-NO was almost unchanged during co-combustion. Moreover, the yields of MS-N2 and MS-NO2 during co-combustion were increased. It indicated that MHJ has a significant effect on the MS-NO emission during co-combustion, while MS has little effect on the MHJ-NO emission. It attributed to the fact that MHJ consumed more oxygen, which resulted in more OH radicals generated during co-combustion, accelerating the cleavage of pyrrolic and pyridinic structures in MS to form more nitrogen-containing compounds. As combustion proceeds, the nitrogen-containing compounds from MS were gradually converted to HCN. Subsequently, HCN was oxidized to NCO, which increased the yield of MS-NCO during co-combustion. Then, the increase in MS-NCO during co-combustion promoted the reduction reaction between MS-NCO and MS-NO, increasing MS-N2 yield. In addition, the increased yield of O radicals also promoted the oxidation of MS-NO to MS-NO2 during co-combustion.

Three-dimensional full-loop numerical simulation of coal and sludge co-combustion in a circulating fluidized bed

The Dense Discrete Phase Model (DDPM) method is used to simulate the co-combustion process of coal and sludge over the full-loop circulating fluidized bed in a three-dimensional (3D) Eulerian-Lagrangian framework. Both heterogeneous (fuel conversion through pyrolysis and char combustion) and homogeneous reactions (e.g. volatile combustion) are considered. Comparison of the predicted pressure profile, flue gas composition and bed temperature with measurements show good agreement and validate the DDPM methodology for accurate description of the gas–solid flow and combustion process over the full-loop circulating fluidized bed. The results of coal and sludge co-combustion are analysed qualitatively and quantitatively in terms of flow characteristics, gas composition, reaction rate profile, and particle combustion characteristics. Due to the lower density and higher volatile content of the sludge, as the coal-to-sludge feeding ratio decreases, the combustion reaction rate is gradually skewed upwards in the furnace, shifting up also the corresponding gas concentration profiles. For the given fuel sizes, the faster devolatilization rate of coal yields a shorter burnout time than that of the sludge particles. Our work provides an intuitive perspective to the fluidized bed community to boost comprehension on the coal and sludge co-combustion in a full-loop circulating fluidized bed.

Migration characteristics and ecological risk assessment of heavy metals in ash from sewage sludge co-combustion in coal-fired power plants

Co-combustion in coal-fired power plants is one of the main methods to dispose of municipal sewage sludge (MSS). The high heavy metals content of MSS increases the ecological risk of ash from the coal and MSS co-combustion. In response to this problem, the three types of fly ash or bottom ash from MSS co-combustion in the three coal-fired power plants were sampled. The properties of fly ash and bottom ash were analyzed in terms of ash composition, particle size distribution, heavy metals content and leaching characteristics. The potential ecological risks were evaluated using the ecological risk index assessment method. The results showed that the co-combustion of MSS had no significant effect on the ash composition order and heavy metals content order. However, the total content of Cu, Mn, Pb and Zn, Hg in the co-combustion ash was higher than that in the coal-fired ash without sludge blending but did not exceed the risk screening value for soil heavy metal contamination of residential land. Heavy metals showed significant enrichment in fly ash and dilution in bottom ash. The leaching concentrations of Cr and Ni were several times the limit value. The potential ecological risk levels of fly ash and bottom ash were moderate risk and high risk respectively. Cr and Hg were the main contributors to potential ecological risks. The contents of Cr, Hg and Ni need to be paid attention to in the subsequent utilization of coal ash mixed with co-combustion of sewage sludge.

The synergistic effect during co-combustion of municipal sludge and coal: Experimental and ReaxFF molecular dynamic study

In this work, the behaviors of municipal sludge/coal co-combustion were investigated by experiments and ReaxFF molecular dynamic simulations. The molecular models of coal and sludge were constructed based on experimental characterization. The characteristics of the separately combustion of sludge and coal from ReaxFF MD simulations were consistent with that from thermostatic thermogravimetric experiments. The synergistic effect on the co-combustion characteristics were investigated via performing sludge/coal co-combustion simulations under various conditions. The results showed that the sludge/coal co-combustion has positive synergistic effects on the co-combustion characteristics such as combustion rate, consumption of O2, and yield of CO2. By using the atomic labeling method, it was found that the combustion of sludge was inhibited while the combustion of coal was promoted during co-combustion process. The carbonaceous gas was found to be an important intermediate product in the generation of CO2. The synergistic effect promoted coal combustion because some ·OH radicals produced by the sludge attacked the aromatic ring structure in coal, which accelerated the formation of carbonaceous gas during sludge/coal co-combustion. This work provided new insight into the behaviors of municipal sludge/coal co-combustion.