Ash Fusion Process Research Articles

Effects of phosphorous-based additive on flow properties of high silicon-aluminum coal ash

• Effects of Ca 3 (PO 4 ) 2 addition on the fluidity of Huainan coal ash were revealed. • Phosphorus decreased ash fusion temperatures and temperature of critical viscosity. • Phosphorus contributed to the formation of slag during the ash fusion process. • Polymerization of slag structure was enhanced by phosphorus at high temperatures. • Crystallization in the slag during cooling was restricted by phosphorus. Co-gasification with coal is a promising way to dispose the sewage sludge with high phosphorus content, but the lack of data on the flow properties of phosphorus-containing ashes and slags under reducing atmosphere limits the gasification of this fuel. In this study, Ca 3 (PO 4 ) 2 was chosen as the phosphorus model compound of sewage sludge and was added into one high-silicon-aluminum coal (Huainan, HN). Ash fusion temperatures (AFTs) and viscosity-temperature properties of blends were measured and the mechanism of effects of phosphorus were explored based on the results of X-ray diffraction (XRD), thermodynamic software FactSage, Fourier transformation infrared spectroscopy (FTIR) and scanning electron microscope with energy dispersive spectrometer (SEM-EDS). To eliminate the interference of calcium, a phosphorus-free control group was set up with CaCO 3 and the same tests were conducted. The experimental and modelling results showed that phosphorus had a significant fluxing effect on the fusion of coal ash by providing the initial liquid phase and changing the main minerals. The effects of phosphorus on slag viscosity were rather complex. At the temperature above 1530 °C, phosphorus-bearing slags had slightly higher viscosity values than phosphorus-free slags due to the increased polymerization degree of the network structure. However, at the cooling process, phosphorus restricted the crystallization in the slag and decreased the viscosity values and the temperatures of critical viscosity (T CV ). In this paper, a suitable addition ratio of Ca 3 (PO 4 ) 2 for the slagging gasification of HN coal was concluded.

The sintering analysis of biomass waste ash based on the in-situ exploration and thermal chemical calculation in the gasification process

The plentiful biomass wastes are promising fossil energy alternatives to generate indispensable energy. Biomass gasification is an effective way of producing high-value-added productions and reducing CO2 emissions. However, some ash-related issues, especially ash sintering, restrict the wide utilization of biomass energies. To minimize the negative effect of the sintering problem, this work studied the ash sintering process of typical cornstalk waste ash by in-situ experimental methods, including high-temperature stage microscope (HTSM) and thermomechanical analysis (TMA). Then the thermochemical calculation was combined to clarify the sintering mechanism. The result indicated that sintering was the first step during the ash fusion process. The initial sintering temperature (624 °C) was lower than the ash deformation temperature (919 °C), which was the first characteristic temperature of ash fusion tests. The chemical reaction of the inherent ash-related components was the main reason for the appearance of the initial sintering temperature. From the element perspective, the chemical reaction of alkali and alkali earth metals (AAEMs) was mainly involved. The alkali metals tended to react with SiO2 while alkali earth metals were prone to combine with P2O5. Except for ash sintering behavior during the heating process, the TMA trace showed another two stages, swelling and melting. Massive bubbles would be generated from the inside melts at the swelling stage and cause the drastic fluctuation of the ash-melting system until the end of the melting range. Moreover, the high heating rate was beneficial for alleviating the initial sintering temperature while having a tiny influence on changing the total range of sintering and swelling zones.

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 ℃.

The mineral transformation and molten behaviors of biomass waste ashes in gasification-melting process

Understanding the various ash transformation properties is vital for improving gasification behavior and optimizing industrial operation conditions during the gasification-melting process of biomass wastes. In this work, five typical biomass wastes were selected to investigate the slag property. The ash fusion process, mineral transformation and AAEMs solidification behaviors were performed and predicted by mineral transformation analysis and thermochemical calculation. The results indicated that ash fusion process contained sintering, swelling, bubbling and melting stages, which was related to ash chemical compositions. Besides, the ash flow temperature had a linear relationship with the basic-to-acid ratio of biomass waste ashes. High SiO2 content would lead to strong mineral crystallinity at high temperatures. The mineral transformation was also highly related to ash compositions. The complicated reactions of inorganic matters occurred when SiO2 content lied between 50 wt% and 70 wt%. For AAEMs solidification, the alkali was prone to react simultaneously with both SiO2 and Al2O3 to form eutectoid, and the range of 35–65 wt% for SiO2 + Al2O3 contents was beneficial for alkali solidification. All the obtained data was meaningful to provide a modifying reference for the efficient and clean utilization of various biomass wastes during gasification process.

Morphological changes and ash fusibility of coal, rice straw and their mixture during CO2 gasification

The gasification behavior of single particle of coal, rice straw, and coal-rice straw mixture were investigated by in-situ heating stage microscope, X-ray diffraction, and scanning electron microscopy. The gasification process of coal particle can be divided into three stages consisting of swelling stage, shrinkage stage, and ash fusion stage, while the swelling stage is not observed for the gasification of rice straw particle. The presence of rice straw particle facilitates the shrinkage of coal particle during the co-gasification process due to the migration and catalytic effect of alkaline components which are abundant in biomass. Further, it was found that the average melting temperature of particles is close to the deformation temperature (DT) of ash sample, implying that the DT may reflect the melting properties of particles in a real gasifier. Finally, the ash fusion mechanisms were explored. It was identified that the formation of high-melting points mullite, quartz and anorthite are responsible for the high ash fusion temperatures of coal (with flow temperature over 1500 °C). The addition of 20 wt% rice straw (with flow temperature of 1133 °C) to coal improves the formation of lower-melting points minerals containing alkaline and alkaline-earth elements, and sulphur in the mixture that facilitates the ash fusion process and induces the slag formation. Thus, the flow temperature of the mixture decreases to 1369 °C. The ash fusion behavior of coal follows the “softening-melting” mechanism, while this behavior of rice straw shows a typical “melting-dissolution” mechanism. The ash fusion behavior of coal-rice straw mixture reveals both mechanisms; however, the “melting-dissolution” mechanism seems to be more characteristic for this mixture.

The ash fusion characteristics of bamboo and coal co-firing process

The ash fusion characteristics of bamboo and coal co-firing process were determined by a thermogravimetric analyzer, X-ray fluorescence and X-ray diffraction. The results showed that the ash of bamboo and coal co-firing process had similar chemical compositions with coal ash. Ash components occurred to synergistic interaction during co-firing process of bamboo and coal. Bamboo increased ash fusion temperatures of blends. The ash fusion process included moisture evaporation, oxidation of organic matters, removal and reaction of inorganic matters. The transformation of chemical compositions resulted in mass loss of ash. Fusion mechanism included G–B model, Mampel power model, second-order reaction and the Avrami–Erofeev model. The optimum blend ratio of bamboo and coal was suggested as 30%:70%. The results will be helpful to provide a scientific guidance to use bamboo residues for energy products.

Effect of K2O/Na2O on fusion behavior of coal ash with high silicon and aluminum level

Ash fusion temperatures (AFTs) are widely used for evaluation of fusibility of coal ash for combustion and gasification. K2O and Na2O is regarded as the effective component to enhance the ash fusibility for slag tapping. The combination of K2O and Na2O may enhance the flux efficiency. In this work, the effect of K2O/Na2O mass ratio on fusion behavior of coal ash with high silicon and alumina level was evaluated by the height variation of ash cone during heating. The results showed that AFTs and theoretical fusion range (Tliq-Tini) decreased quickly and then increased slowly as the K2O/Na2O mass ratio in the coal ash was reduced. The sub-liquidus phase changed from mullite into feldspar, which was responsible for change of AFTs. Meanwhile, most of K element occurred in leucite (KAlSi2O6) and the rest existed in nepheline (KAlSiO4), but Na almost existed in nepheline (NaAlSiO4). The competition effect between K2O and Na2O on AFTs was reflected by KAlSiO4/NaAlSiO4 in nepheline, so the SiO2-KAlSi2O6-NaAlSi2O6 ternary phase diagram was constructed, which well explained the variation of AFTs caused by K2O/Na2O mass ratio. Meanwhile, the fusion process was divided into two stages based on variation rate of ash cone height. At first stage, the amount of liquidus phase is limited and the viscosity is high, so only a low shrinkage rate of ash cone exhibits. However, at the second stage, high shrinking rate is caused by not only the rapid increase of slag content but also the sharp decrease of viscosity besides, the addition of K2O/Na2O change ash fusion process from “melting-dissolving” into “softening-melting”.

The role of residual carbon on fusibility and flow properties of rice straw ash

Residual carbon is a non-negligible component in biomass gasification slag and is seen as a factor related to the flowability of slag. One purpose of this study was to explore the relationship between residual carbon and rice straw ash fusibility via an ash fusibility temperatures (AFT) test in an argon (Ar) atmosphere. A visual measurement method for slag flow velocity was also established to further reveal slag flow properties. Furthermore, to illustrate the reaction mechanism of residual carbon with mineral components in ash, the surface morphology and chemical components of ash prepared at high temperatures were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and the effects of residual carbon on rice straw ash fusibility characteristics were quantitatively calculated by thermodynamic equilibrium calculations. The experimental results showed that the increase in residual carbon mass fraction in ash increased ash fusibility temperatures and reduced slag flow velocity. The characterization and thermodynamic calculations indicated that in the ash fusion process, the rod-like structure containing carbon in ash disappeared, and the surface became compact due to the carbothermic reaction with SiO2 with continuously rising temperatures. Then, the unreacted residual carbon and SiC could be completely covered by molten slag due to the further increase in the liquid phase caused by the melting CaMgSiO4 and CaMgSi2O6. The residual carbon in ash inhibited the formation of K2Si4O9, which further resulted in reduction of SiO2 migration to low melting silicate.

Effect of chemical composition on the fusion behaviour of synthetic high-iron coal ash

High-iron coal is widely distributed throughout China and could be used as the feedstock for slagging gasifiers. To enable cleaner, efficient, large-scale use of high-iron coal by the slagging gasifier, a solid understanding of coal ash fusion behaviour is required. In this study, we investigated the fusion behaviour of synthetic high-iron coal ash (SiO2-Al2O3-Fe2O3-CaO) using an ash fusion temperature (AFT) analyser, X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and thermodynamic modelling. We found that AFTs decreased with increasing α values (CaO/(SiO2 + Al2O3), mass ratio) due to the formation of low-melting-point minerals. AFTs also decreased with increasing S/A (SiO2/Al2O3, mass ratio), attributed to massive generation of the amorphous phase. The ash fusion process is described with three stages, namely, the mineral transformation, formation of the initial liquid phase, and melting of the residual solid phase. In the first stage, the mineral species was considered as the key parameter, and the increase in α and S/A favoured the formation of mineral with a low melting point. S/A also significantly influenced the second stage, with greater S/A ratios correlating with lower temperature and higher content of the initial liquid phase. In contrast, the third stage was accelerated by intense mass transfer, as the viscosity of the liquid phase dropped dramatically with increasing α value. The fusion heat corresponding to the melting of minerals at high temperature showed a negatively linear correlation with the flow temperature of synthetic coal ash, which may provide an alternative method of predicting the flow temperature of real coal ash.

Effect of biomass ash addition on coal ash fusion process under CO2 atmosphere

The ash fusion process of Jincheng anthracite ash, rice straw ash and coal-biomass ashes with different rice straw ash addition ratios during co-gasification under CO2 atmosphere was elucidated by the ash fusion test, chemical analyses, optical heating stage microscopy (OHSM), X-ray diffraction and scanning electron microscopy equipped with energy-dispersive X-ray analyzer. Results showed that the initial sintering and initial melting temperatures of coal-biomass ashes observed by OHSM decreased with the addition of rice straw ash. The deformation temperatures determined by the ash fusion test were close to the initial melting temperatures. The biomass ash addition to coal ash introduced excessive amounts of alkaline compounds in the ash mixtures that led to lower initial temperatures of liquid phase formation. The ash fusion behavior of coal-biomass ashes with a rice straw ash addition ≤3 wt% was similar to that of coal ash and fits to the “softening-melting” ash fusion mechanism. In contrast, the ash fusion behavior of coal-biomass ashes with a greater addition of rice straw ash (>3 wt%) experienced dramatic changes at lower temperature and fits to the ‘‘melting-dissolution’’ ash fusion mechanism.

Fusion characteristics of blended ash from Changzhi coal and biomass

The blended ash fusion characteristics and its fusion mechanism of biomasses (peanut husk (PH) and rice husk (RH)) and Changzhi (CZ) coal with high melting point (MP) were investigated by ash fusion temperature (AFT) detector, X-ray fluorescence, X-ray diffraction, and FactSage software. The results show that the AFTs of CZ ash mixtures decrease with addition of biomass ash, and the fluxing effect of PH ash is better than that of RH ash, which mainly depends on their ash chemical compositions and elements existing form. The formations of low MP minerals anorthite, albite, and leucite are responsible for the decrease in AFT of PH and CZ mixtures. With RH ash addition, feldspar and eutectics are generated, resulting in a decrease in AFT of their mixtures. In the presence of Na2O, CaO, or K2O, SiO2 and Al2O3 would firstly react with them to form low MP aluminosilicate from the perspective of thermodynamic calculation, inhibiting formation of high MP mullite. The mixed ash fusion process could be divided into two major stages: fusion of K- and Ca-bearing minerals, respectively.

Effect of Silica and Alumina on Petroleum Coke Ash Fusibility

Silica (Si) and alumina (Al) elements are always considered to be the important inducements for ash sintering and slagging in the boilers. The effects of SiO2/Al2O3 (S/A) and (SiO2 + Al2O3) ratios on the synthetic petroleum coke (petcoke) ash fusibility are investigated in this work. Experimental methods including ash fusion temperatures (AFTs) tests, X-ray diffraction, and scanning electronic microscopy are applied to investigate the AFTs, the mineral composition, and surface morphologies of high-temperature ashes. Moreover, thermodynamic equilibrium calculations are also used to predict the ash fusion process. The results show that the AFTs of petcoke ash samples are closely related to the addition of S/A and (SiO2 + Al2O3). The dominant crystalline phases formed in high-temperature ashes with different S/A and (SiO2 + Al2O3) are gehlenite (Ca2Al2SiO7), quartz (SiO2), nickel orthosilicate (Ni2SiO4), and mullite (Al6Si2O13). The AFTs drop sharply with increasing S/A until it reaches 1.5, which may be asc...

Investigating a high vanadium petroleum coke ash fusibility and its modification

Petroleum coke (petcoke), especially high vanadium (V) petcoke, has been produced increasingly because of the growing consumption of petroleum oil. Petcoke ash fusibility is closely related to ash slagging, which has an important impact on the clean and efficient utilization of petcoke. This study investigates a high V petcoke ash fusibility and its modification by coal blending and flux addition at reducing atmosphere. X-ray diffraction (XRD) and scanning electronic microscopy (SEM) were used to determine the crystalline minerals and surface morphology of high-temperature ash slag, respectively. The ash fusion process was predicted by thermodynamic equilibrium calculations. The results show that Shengli Oilfield petcoke (SL) has a high ash fusion temperature (AFT), whereas coal blending and flux addition can effectively decrease the AFT of SL, which may be ascribed to the mineral transformations during ash fusion process. The synergistic effects of high-melting vanadium trioxide (V2O3) and coulsonite (FeV2O4) result in the high AFT of SL. When SL blends with Shenmu (SM) coal and Qi coal (QC), the formation of low-melting andradite (Ca3Fe2Si3O12) and the decrease of V2O3 contribute to the progressive decline of AFT. When the addition of flux calcium oxide (CaO) reaches 10%, the low-melting calcium orthovanadate (Ca3V2O8) formed under reducing atmosphere decreases the AFT. With increasing CaO content, the content of V2O3 and FeV2O4 decreases while that of Ca3V2O8 and Ca3Fe2Si3O12 increases gradually, which further leads to a decrease in the AFT.

Size effects on the thermal behavior of superfine pulverized coal ash

The coal ash fusibility is an important parameter that affects the boiler design and operation significantly. Superfine pulverized coal is beneficial to the alleviation of fouling and slagging problems due to its distinctive properties in the aspects of multiphase flow, heat transfer and combustion atmosphere. In this work, the size related ash fusion propensity were emphasized from the point of view of the chemical and mineralogical morphology of coal ash. A comprehensive study on the chemistry and thermal behavior of superfine pulverized coal ash was conducted combining multiple analytical methods such as XRF (X-ray fluorescence), XRD (X-ray diffraction), TGA (thermo-gravimetric analysis), slagging indices, AFT (ash fusion temperatures) tests and phase diagrams. Besides, a quantitative analysis method was proposed to eliminate the subjective judgments in AFT tests through a numerical image-processing platform, which could depict a complete deformation profile of the whole ash fusion process with high precision and reproducibility. The results from this work are helpful for understanding the size dependence of ash thermal behavior, and predicting the ash fusion propensity in the future application of superfine pulverized coal.

Thermomechanical analysis of coal ash fusion behavior

Ash fusion behavior relates to several key issues for designing and running boilers or gasifiers, including fouling, sintering or slagging. In order to illustrate the mechanism of ash fusion behavior, the ash fusion temperature test, thermomechanical analysis (TMA), the high temperature processing microscope (HTPM) and differential scanning calorimetry (DSC) were applied to describe the ash fusion process and the characteristics of fusion temperatures. In addition, the thermodynamic software FactSage™ was used to reveal the relation between liquid phase formation and ash fusion behaviors. The shrinkage trace by TMA not only indicated the ash fusion temperatures, but also described the fusion process quantitatively. The ash fusion process can be divided into sintering, primary fusion and the free liquid stage based on the shrinkage trace, which was supported by HTPM and DSC results. Stage I (sintering stage), in which sintering was initialized by the formation of an initial eutectic, was characterized as the liquid phase sintering stage. The initial temperature of sintering stage is essential to prevent ash sintering or fouling. In stage II (primary fusion stage), most of the solid minerals melted and the formation rate of the liquid phase reached its maximum. The range of the plateau in this stage is determined by the end points of sintering and newly formed eutectics. During stage III (free liquid stage), the remaining solids dissolved in the liquid slag and influenced the flow behavior of slag at high temperatures. Overall, the ash fusion mechanism “Sintering-fusion-dissolving” was proposed according to the fusion behavior. The end points of the sintering and primary stage, denoted as Ts1 and Ts2, agreed well with deformation temperature and flow temperature, respectively. Ts1 or deformation temperature should be used as key criteria to prevent sintering in boilers and gasifiers.