Ash Melting Research Articles

Investigation of mitigation of nitric oxide emission characteristics and slagging properties from biomass combustion by the additive of coal gangue

The co-combustion of coal gangue (CG) and biomass was expected to increase the utilization rate of solid waste and compensating for individual feedstock disadvantages. However, the emission of nitric oxide (NO) and ash melting characteristics during this process was of concern, but no relevant study had focused on this. In this work, the combustion behavior of Salix limb(SL)/Wheat straw(WS), CG and biomass-CG blends was studied by non-isothermal thermogravimetry and tube oven method. The results indicated that when CG was mixed into biomass fuel as an additive, the burn-out effect of CG was improved. The NO emissions from the experimental value of 80% SL/WS blend with 20% CG were reduced by 8.9%/10.1% compared to the theoretical value. Moreover, the potassium aluminum silicates with high melting temperatures were formed in the ash of the composite fuel, which reduced the serious slagging phenomenon that has been observed in the biomass combustion process. The composite fuel of 80% biomass (SL/WS) + 20% CG could promote combustion and reduce NO emissions, which was a meaningful method to treat CG to the maximum benefit and reduced the phenomenon of biomass slagging.

Effect of Si/Al additives on Cl fate during MSWI fly ash thermal treating process

SiO2 and Al2O3 (Si/Al) are proper additives for municipal solid waste incineration (MSWI) fly ash melting. However, Si/Al additives impact the emission of chlorine (Cl), which is harmful to the ecology and environment. In this study, co-thermal treating experiments of fly ash and SiO2 or Al2O3 were carried out to determine chlorine migration and transformation. Results show that the release of HCl is significantly promoted by the addition of SiO2 or Al2O3. Thermodynamic equilibrium calculation shows that the reaction rate between additives (SiO2 or Al2O3) and CaCl2 is the main factor limiting the release of HCl. Besides, SiO2 has an obvious effect on reducing Cl percentage in residues, as the addition of 30% SiO2 raises the Cl removal rate to 99.72% at 1250 °C. Main component in the residuals is Ca-silicate after addition of SiO2. Compared to SiO2, the addition of Al2O3 is more likely to retain Cl in the residues. Al2O3 reacts with fly ash to generate Cl-containing minerals, such as Ca10(SiO4)3(SO4)3Cl2 and Ca12Al10.6Si3.4O32Cl5.4, leading to a lower removal rate of Cl. Moreover, Cl combined with Na and K is transformed to secondary fly ash by volatilization and condensation. Our findings may assist in pollutant discharge control in developing a new melting furnace.

CFD Modeling of the Melting Behavior of a Fayalitic Slag

The melting behavior of a fayalitic slag was investigated with computational fluid dynamics and validated with experimental data. A heating microscope was employed to perform a typical ash melting behavior (AMB) test, in order to gain information about the melting behavior of the slag. The numerical approach is based on using the volume of fluid (VOF) method, in order to track the interface between the slag and the gas phases, and on the enthalpy–porosity model, in order to simulate the slag as a phase change material (PCM). The combined VOF-PCM model was therefore applied to perform a parameter study on the mushy zone constant A_{rm mushy}, which enters the equations of the enthalpy–porosity sub-model and is an a priori unknown. The goal of the investigation was to determine the correct value for A_{rm mushy}, by comparing the numerical results to the AMB test.

Biochar washing to improve the fuel quality of agro-industrial waste biomass

Low energy density and low ash melting temperatures of agro-industrial waste biomass are bottlenecks for their sustainable use in combustion utilities. Torrefaction at low temperature (350 °C for 30 min) for energy density enhancement followed by very simple aqua-washing technique to remove soluble mineral matters from biochar (BC) samples have been employed in this research work to investigate possible improvement of combustion performance of tobacco stalk (TS), sugarcane bagasse (SB), and poultry litter (PL) biomasses (BM). Though the energy density of the BC was significantly higher than their respective BM, the propensity for slagging was also higher, particularly in TS-BC and PL-BC. On char washing, ash content as well as the slagging and fouling tendencies were estimated to be decreased. Energy densification ratio (as compared to biomass) as well as GCV were further increased in washed biochar (WBC). As per the DSC-TGA investigations, combustion reactivity of BC was improved by washing. The significant advantages of BC washing were observed in TS-BC and PL-BC due to the removal of a large amount of soluble materials from respective BCs. As the water extracted material from SB-BC is insignificant, unwashed SB-BC having sufficiently high GCV (20.6 MJ/kg), energy densification ratio (1.5), carbon content (66.7%) can be directly used without washing. Water washing of BC is highly beneficial for TS and PL in respect of combustion performance and a valuable potassium-rich fertilizer material was obtained on evaporation of the leachate depicting unique value-addition of entire process.

Experimental evaluation on NOx formation and burnout characteristics of oxy-fuel co-combustion of ultra-low volatile carbon-based solid fuels and bituminous coal

Pyrolyzed semi-coke (SC) powder and gasification residual carbon are usually regarded as solid wastes, which feature high NOx formation amount and inferior burnout performance during burning process. Oxy-fuel co-firing of these ultra-low volatile carbon-based solid fuels (LVFs) with bituminous coal (BC) could reduce the NOx generation and realize the carbon capture to a large extent. However, the NOx formation and burnout features of oxy-fuel co-combustion of BC and LVFs are still unclear. Here, a two-staged drop-tube co-firing experimental system was employed to probe the oxy-fuel co-combustion characteristics of various blends. The experimental results show that the low ash melting temperature of coal-water-slurry gasification residual carbon yields an increase in Cfh with the temperature of burnout zone. The Cfh of BC/SC blend is decreased by 64.3% with the oxygen flow rate of primary air raised from 0.25 to 0.45 L min−1, which is the largest among all blends. The rise of difference between tube lengths in furnace of BC and LVF generates a decrease in NOx formation and an increase in Cfh. The present research could offer guidance for clean and efficient utilization of carbon-based solid wastes using oxy-fuel co-combustion technique, together with the encouraged development of carbon capture technology.

Oxy-fuel co-combustion dynamics of phytoremediation biomass and textile dyeing sludge: Gas-to-ash pollution abatement

The environmental pressures of major wastes in the circular economies can be abated leveraging the complementarity and optimal conditions of their co-combustion. The oxy-fuel co-combustion of phytoremediation biomass of Sedum alfredii Hance (SAH) and textile dyeing sludge (TDS) may be a promising choice for sustainable CO2 capture and a waste-to-energy conversion. This study characterized and quantified their co-combustion performances, kinetics, and interactions as a function of blend ratio, atmosphere type, and temperature. With a focus on the characteristic elements of SAH (Ca, K, Zn, and Cd) and TDS (Al and S), changes in the mineral phases and ash melting and slagging trends of K2O-Al2O3-SiO2 and CaO-Al2O3-SiO2 systems were quantified. The Zn and Cd residual rates of the co-combustion of 75% SAH and 25% TDS rose by 58.52% and 5.93%, respectively, in the oxy-fuel atmosphere at the 30% oxygen concentration, relative to the mono-combustion of SAH in the air atmosphere. The co-combustion in the oxy-fuel atmosphere at the 20% oxygen concentration delayed the release peaks of SO2, C2S, and H2S, while the Ca-rich SAH captured S in TDS through the formation of CaSO4. Our findings provide new and practical insights into the oxy-fuel co-combustion toward the enhanced co-circularity.

Effect of phosphorus-based additives on ash fusion characteristics of high-sodium coal under gasification condition

Ash fusion characteristics and melting mechanism of two high-sodium coals including Luan (LA) and Hongshaquan (HSQ) with addition of ammonium dihydrogen phosphate and tricalcium phosphate under gasification condition were investigated to reveal reactions between phosphorus-based additives and high-sodium coal ash in detail. Methods such as ash fusion test, X-ray diffraction, and high temperature heating stage microscope were used for that purpose. High-sodium coal ashes have low ash fusion temperatures (AFTs) due to their high SiO2/Al2O3 ratio and alkaline and alkaline-earth contents. Low-temperature eutectics are formed between nepheline and akermanite, as well as anorthite and akermanite at 1210 °C and 1274 °C, respectively. Phosphorus has a higher affinity to alkaline and alkaline-earth metals for the higher ionic potential of acidic components than basic components. As a result, high melting point phosphates like Ca9MgNa(PO4)7, Ca9Fe(PO4)7, and Na3Ca6(PO4)5 are generated in coal ash with phosphorus-based additives. Additionally, when PO43-/Na molar ratio increases, contents of quartz and phosphates are also increased, which enhance the formation of skeleton structure with high AFTs. However, the formation of low-temperature eutectic mixtures of Fe-Na silicates accelerates the melting process of coal ash, which weak the enhanced AFTs of high-sodium coal by phosphorus-based additives. Furthermore, the melting mechanism of high-sodium coal ash with and without phosphorous-based additives follows the “melting-dissolution” mechanism.

Mechanical and energy properties of pellets formed from walnut shells blended with spruce sawdust

Various waste materials have energy potential. It is important to make use of this potential and prepare the product for further use by treating the waste. Treatments such as compressing waste into pellets leads to increasing the energy density of this fuel, which benefits transport and storage costs. However, low bulk density, high ash content, low-ash melting temperatures, and low calorific values of non-woody pellets can cause problems during their combustion. This article deals with the energy usage of walnut shells, which were blended with spruce sawdust in various amounts and compressed into pellets. The mechanical and energy properties of these were measured and compared with recommended or standardized values. The formed pellets met the quality limit for bulk density, ash content, moisture content, the content of nitrogen and sulfur, and net calorific value according to ISO 17225. However, low ash melting temperatures were noticed for pellets from pure walnut shells, and also lower mechanical durability for produced pellets with walnut shells contents higher than 10% were detected.

Comprehensive adhesion model based on the surface characteristics of the coal particles

A comprehensive model for the adhesion of coal ash or char particle was established in this study, which includes the particles’ properties and slag layer. The physical properties of the particles include their kinetic parameters and surface characteristics. The particle surface characteristics, such as particle ash ratio and melting fraction, play an important role in particle adhesion. A sub-model of adhesion probability, constructed based on particle surface properties, was also introduced as a part of this comprehensive model. The particle adhesion propensity was determined by considering the kinetic energy criterion of the particles. The model was validated using two sets of particle deposition experiments at different temperatures. Discussion of the model shows that, when the particle kinetic energy is within the medium range, more significant than the critical velocity and less than the critical kinetic energy required to overcome viscosity or surface tension, the particle adhesion is determined by the probability corresponding to the surface characteristics. Furthermore, it is found that the adhesion probability increases with ash-carbon ratio and carbon conversion, and exhibits different trends above and below the flow temperature.

Characterization of Ash Melting of Reed and Wheat Straw Blend

Ash melting could cause severe problems in boiler operation, such as agglomeration of the fluidized bed. Our previous work has shown that the ash melting behavior of the blend of reed and wheat straw is complex and needs further investigation. The ash melting behavior was studied using different laboratory methods such as simultaneous thermal analysis, heating microscope, scanning electron microscopy with energy dispersive X-ray (SEM/EDX) analysis, and X-ray diffraction (XRD). In the thermodynamic modeling, we used FactSage software, which supplements well the results obtained by other methods and vice versa. The results indicated that melting started at 660–680 °C when Na and K salts were melted and molten K2SO4 covered the ash material; the content of liquid mass fraction was 13.8%, revealing that the studied ash blend could already be sticky at 680 °C. Intensive melting took place in the temperature range of 800–980 °C. The rapid melting between 950 and 980 °C was caused by the melting of SiO2 and K2MgSi5O12; the ash material became glassy and amorphous. We propose an alternative distribution of ash melting stages for the studied blend.

Ash melting behaviour of reed and woody fuels blends

Ash melting behaviour is an important issue regarding operation of boilers, for instance, to avoid the agglomeration of the fluidized bed. Previous studies have shown that ash melting behaviour of the blends could differ from that of single parent fuels. This study focuses on the melting behaviour of fuel ash blends of reed, pine wood pellets and Douglas fir wood chips. For that purpose, simultaneous thermal analysis, heating microscope and scanning electron microscopy with energy dispersive X-ray analysis, X-ray diffraction, and FactSage modelling were used. The ash sample was heated up to the final temperatures stepwise, and the morphology and composition of the ash samples were examined at different temperatures. The results revealed that the melting onset for reed and woody fuels blend ashes was related to KCl-K2SO4 eutectic. The melting started at 700 °C and chlorine was released. The decomposition of CaCO3 was observed for the ashes containing wood pellets. In the case of reed and wood pellet ash, both processes performed simultaneously. The second mass loss and sulphur release were observed at 1100 °C, which is related to decomposition of K2SO4. In reed and woody fuels blend ashes, the gas release was accompanied with the melting of other substances (e.g. Si-Ca or Si-Ca-K system) and the gases were trapped into melt. The feldspars abundant in Douglas fir wood chips, could cause the sluggish melting behaviour of ash material. The FactSage modelling indicated the melting onset at lower temperatures compared to the results obtained by the other methods applied.

Ash fusion characteristics of biomass pellets during combustion

Combustion of biomass pellets is one of the essential approaches to biomass energy utilization. However, the ash produced from the combustion of pellets may melt and affect combustion efficiency. Therefore, this study evaluated the ash formation and melting evolution characteristics, as well as their causes, of several typical biomass fuels, such as peanut shell pellets, wheat straw pellets, and corn straw pellets, during high-temperature combustion. The ash fusion temperatures of the biomass ash ranged from 1100 °C to 1300 °C. Combined with scanning electron microscopy–energy-dispersive spectrometry and X-ray diffraction analysis, the irregular aggregation of KCl, K2SO4, and other fusible matters was identified as the main factor affecting the ash fusion of wheat straw and corn straw pellets. Ash melting inhibited the combustion reaction of the wheat straw and corn straw pellets, which have the highest amount of silicon content in their molten ash at 1100 °C. However, the ash completely melted at 1300 °C, and the Si, MgO, and CaO contents increased and formed low-melting-point eutectics that were widely distributed in the molten ash. Higher temperatures that exceeded the ash flow temperature promoted the formation of various structures and eutectic materials of molten ash and accelerated carbon conversion. At 1300 °C, the ash melting process improved the combustion reactivity of the three biomass pellets, among which the combustion reaction of the corn straw pellets was the most obvious. At 1100 °C, the ash melting process only improved the combustion reactivity of peanut shell pellets and inhibited the combustion reactivity of wheat straw pellets and corn straw pellets. This study has provided a basis for solving the problem of fuel ash melting affecting combustion in the combustion equipment of biomass pellets.

Co-Combustion Behavior, Kinetic and Ash Melting Characteristics Analysis of Clean Coal and Biomass Pellet

Co-Combustion Behavior, Kinetic and Ash Melting Characteristics Analysis of Clean Coal and Biomass Pellet

Effect of Atmospheres on Transformation of Heavy Metals during Thermal Treatment of MSWI Fly Ash: By Thermodynamic Equilibrium Calculation.

The vaporization behaviors of eight heavy metals (Pb, Zn, Cu, Cd, Cr, Co, Mn, and Ni) in municipal solid wastes incineration (MSWI) fly ash during thermal treatment under air atmosphere (21% O2/79% N2), an inert atmosphere (100% N2), and a reducing atmosphere (50% CO/50% N2) were evaluated based on a thermodynamic equilibrium calculation by FactSage 8.1. The results show that the reducing atmosphere promotes the melting of MSWI fly ash, resulting in a more liquid phase than in air or an inert atmosphere. Except for Cd, the formation of liquids can dissolve heavy metals and reduce their vaporization ratio. In the air and inert atmospheres, Pb, Zn, Cu, Co, Mn, and Ni vaporize mainly in the form of metallic chlorides, while Cd volatilizes in the form of metallic Cd (g) and CdO (g). In the reducing atmosphere, Co, Mn, and Ni still vaporize as chlorides. Zn and Cd mainly vaporize in the form of Zn (g) and Cd (g), respectively. In terms of Pb, in addition to its chlorides, the volatiles of Pb contain some Pb (g) and PbS (g). Cr has a low vaporization ratio, accounting for 2.4% of the air atmosphere. Cr, on the other hand, readily reacts with Ca to form water-soluble CrCaO4, potentially increasing Cr leaching. Except for Cd, the results of this study suggest that the reducing atmosphere is used for the thermal treatment of MSWI fly ash because it promotes the melting of fly ash and thus prevents heavy metal vaporization.

Hydrogen production with an auto-thermal MSW steam gasification and direct melting system: A process modeling

Municipal solid waste steam gasification and direct melting system is proposed in this study for H2 production and ash melting simultaneously. Part of the H2 generated in gasification is extracted for combustion with pure oxygen in the melting zone to provide the energy necessary for auto-thermal operation. A simulation model is developed with Aspen Plus to investigate the performance and optimum conditions of the system. For the feedstock with a lower heating value of 18.91 MJ/kg used in this study, 39.8% of the generated H2 needs to be extracted to maintain the heat balance of the system at the gasification temperature of 900 °C, melting temperature of 1400 °C, and S/M of 1. The net H2 yield is ∼77.3 kg/t-MSW with a net cold gas efficiency of 49.1% under the same operating condition. An optimum operation condition for T (850–1000 °C) and S/M (0.6–1.0) is determined considering the balance between H2 production ability and the auto-thermal energy balance.

Ash Melting Behavior of Rice Straw and Calcium Additives

Rice straw is potentially an appropriate feedstock material for biofuel production, since a huge amount of this postharvest residue is generated every year. The transformation of such agricultural biomass into densified products with a higher energy value and their subsequent combustion is associated with several questions. One of them is that rice straw exhibits a large formation of ash during combustion; thus, it is essential to know the nature of its ash melting behavior. Generally, during the combustion of straw biomass, ash sintering occurs in relatively low temperatures, resulting in the damaging of heating equipment. This negative aspect can be overcome by the addition of calcium-based additives. This paper aimed to study the ash melting behavior at a laboratory scale and to determine the ash melting points of rice straw mixed with calcium carbonate (CaCO3) and calcium hydroxide (Ca(OH)2) in different proportional ratios. The standardly produced ash samples from the rice straw obtained from Cambodia were constantly heated up in a muffle furnace, and characteristic temperatures of ash melting, i.e., shrinkage, deformation, hemisphere, and flow temperature, were recorded. The results showed that increasing the additive ratio did not bring linear growth of the melting temperatures. The addition of 1% CaCO3 showed an optimal positive impact of higher ash melting temperatures, and thus a better ability to abate the sintering of the rice straw ash.

Effect of CaO-SiO2-FeO slag system on coal gasification reaction in CO2-Ar atmosphere and kinetic analysis

A new method of coal gasification with steelmaking slag as heat carrier and catalyst is put forward. At the same time, from the carbon cycle perspective, CO2 is used as a gasification agent instead of water vapor. The effects of coal type, slag ratio, and experimental temperature on coal gasification reaction were discussed through constant temperature thermogravimetry, heating thermogravimetry experiments, and the kinetic analysis of thermogravimetry data carried out. The results show that steelmaking slag has a catalytic effect on coal gasification reaction, which is beneficial to coal gasification reaction, but the additional amount of slag has a suitable value. Because low-rank coal contains more oxygen-containing functional groups, the catalytic effect of slag on low-rank coal is stronger. The gasification reaction temperature should be controlled near the ash melting point temperature of coal. At the same time, the kinetic parameters of coal gasification reaction under different conditions were obtained, and the kinetic mechanism model was constructed. This study is of great significance to the recovery and utilization of waste heat of steelmaking slag and the emission reduction of greenhouse gases.

Co‐combustion characteristics of black liquor and waste oil sludge

In this study, the co-combustion of black liquor and oil sludge is investigated. The properties of the fuels are analyzed, and the co-combustion characteristics are explored by thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TG-FTIR). The synergistic effect, activation energy as well as gas yield are also studied from TG-FTIR results. The results show that the addition of oil sludge can promote the reaction in the early stage, as indicated by the combustion characteristic index and the flammability index. The TG-FTIR also shows that the swelling behavior of black liquor appears to block the release of volatile matter. The weaker SO2 emission and XRF analysis for the black liquor indicate that most of the sulfur remained in the ash. A positive synergy effect existed in the early stage, and the activation energy decreased with the addition of oil sludge. Distinct stages could be observed in the single pellet combustion experiments. Adding oil sludge to the black liquor promoted surface bubbles and ash melting. Surface bubbles indicate more volatile breaks through the plastic state layer formed on the black liquor and increase the reaction. However, adding oil sludge should be controlled to reduce the hindrance to the reaction due to ash melting.

Pellet Production from Residual Biomass of Greenery Maintenance in a Small-Scale Company to Improve Sustainability

Replacing fossil energy sources with renewable energy sources is a key strategic action to limit environmental issues. To achieve this goal, substitution with biomass is beneficial due to its versatility in various fields. In terms of circular economy and sustainability, the possibility of energy exploitation of residual biomass is particularly desirable in small-medium enterprises. The use of supply chain by-products can improve sustainability and create opportunities for companies. The purpose of this study is to evaluate the suitability of residual biomass of conifers and broad-leaved trees to produce quality pellets using an agri-pellet machine activated by the power take-off of a tractor. This system can be employed at the farm level. Wood biomass of four species was tested; poplar, stone pine, black locust, and oak. Wood chips samples were analyzed to determine their qualitative characteristics following the technical standard ISO 17225-4. Based on the results, different wood blends were created to produce pellets, subsequently characterized according to ISO 17225-2. The analyses carried out on wood chips and pellets were bulk density, moisture, ash content, calorific value, elemental composition, chlorine, sulfur, and heavy metals. In addition, particles size was measured only for wood chips, while the length, diameter, mechanical durability, and ash melting behaviors were determined only for pellets. Some of the analyzed mixtures show acceptable values according to the current ISO technical standards. The values related to the apparent pellet bulk density and the durability test highlight that not all the mixtures are suitable to produce quality pellets. Results also represent a good starting point for future studies.

Ash formation characteristics in co-combusting coagulation sludge and Zhundong coal

The co-combustion of coagulation sludge (CS) and Zhundong coal (ZDc) can simultaneously convert waste to energy and mitigate fouling and slagging problems during circulating fluidized bed (CFB) combustion of ZDc. In order to explore the ash forming characteristics after blending CS with ZDc, the ash melting curve method quantified the real-time ash fusion characteristics, and alkali metal sodium was analyzed. The results revealed that the ash fusion process of ZDc was divided into two stages. During the first stage, shrinkage was caused by the release of Na and the low temperature eutectic of Na2SO4 and CaSO4. The aluminosilicates in CS effectively decreased the release of alkali metal sodium and inhibited the low temperature eutectic, so the shrinkage rate of ZDc ash was effectively decreased by blending CS during the first stage. During the second stage of ZDc ash shrinkage, the ash shrinkage temperature decreased and the shrinkage rate increased by blending CS. It was attributed that the iron salts in the blends generated low melting point substances such as SiO2-Al2O3-Fe2O3 (MP: 1073 ℃), which caused the blends ash to begin to shrink at around 1000 ℃. As the blending ratio increased, iron-bearing minerals formed low temperature eutectic with the minerals previously produced, resulting in the rapid melting of the blends ash at around 1200 °C. In connection with the sodium retention and the ash melting curve, ash formation characteristics were optimized by blending CS with ZDc. The co-combustion of CS and ZDc is suitable for the boiler that operation temperature under 1000 ℃.