Coal Ash Fusion Temperature Research Articles

Impact of complex reacting atmosphere on ash fusion characteristics and minerals conversion in coal combustion process

ABSTRACTThe surrounding atmosphere of coal particles changes continually during the combustion process in pulverized coal boilers. A part of noncombustible mineral composition in char will be melted under high temperature and then leads to the plugging of pores in char. The O2 molecule used for combustion is hard to diffuse into the inside of char. However, the investigation on ash fusion properties of coal in real combustion atmosphere is still ambiguous. In the present study, the ash fusion temperatures (AFTs) of high Fe coal and low Fe coal were tested under complex atmospheres, which simulated the real condition during coal particles combustion. The ash samples were prepared at 1373 K in the typical conditions and then analyzed by X-ray diffraction to explain the conversion of minerals. In addition, the chemical equilibrium compositions calculations were carried out to obtain the transformation of atmosphere-sensitive Fe species with the atmosphere change. Results indicated that the AFTs reached the lowest point in a weak reducing atmosphere (N2:CO:CO2 = 80%:5%:15%). The Fe2O3 species existed in the ash that prepared in oxidizing atmosphere were reduced due to small quality of CO in weak reducing atmosphere and then formed the Fe2+ compounds with lower melting temperature. If the concentration of CO increases, the Fe2+ compounds would be reduced continually, which would convert into elemental Fe with relatively high melting temperature. Compared with the low iron coal, the AFTs of the high iron coal reduced more obviously, when oxidizing atmosphere turned into weak reducing atmosphere.

Prediction of correlation between ash fusion temperature of ASTM and Thermo-Mechanical Analysis

Ash problems, such as slagging and fouling, are important because of their negative effect of reducing the heat transfer on the furnace wall and tube in pulverized coal boilers and decreasing the boiler safety. However, slagging has positive effect on gasifier. Slagging is protecting furnace wall and reducing heat loss in gasifier. Slagging and fouling are related to ash deposition, which is determined by the coal-ash fusion temperature, chemical composition of coal ash, and boiler operating conditions et al. The coal-ash fusion temperature is a particularly important factor in slagging and fouling. To measure the ash fusion temperature, the ASTM D1857 test is used, not only by thermal power plants and steelmaking companies in South Korea but all over the world. In this study, the fluid temperature (FT) of ASTM D1857 was predicted by Thermo-Mechanical Analysis (TMA). Three bituminous coals (Glencore, Trafigura, Moolarben) and four sub-bituminous coals (KPU, Berau, Jembayan, PRB) were selected for the TMA tests, wherein T25, T50, T75, and T90 were measured. These four temperatures corresponded to shrinkage levels of 25%, 50%, 75%, and 90%, respectively. The T90 of the bituminous coals and T50 of the sub-bituminous coals were similar to the initial deformation temperature (IDT) of ASTM. The T90 of the bituminous coals was greater than 1300°C, and the T90 of the sub-bituminous coals was ∼1250°C. Two FT-prediction equations—for bituminous coal and sub-bituminous coal—were established according to the T25, T50, T75, and T90 relationships of each coal. The FT-prediction equations indicated an average gap of 71°C in the sub-bituminous coals. For the sub-bituminous coals, the average difference was quite low: ∼23°C. The FT-prediction equations matched the FT of ASTM and are useful for determining the ash fusion temperature in a short time.

Investigation of ash fusion characteristics and migration of sodium during co-combustion of Zhundong coal and oil shale

Zhundong coal, which has high sodium content, induces severe fouling and slagging during its combustion in a boiler. Co-combustion is an effective method for mitigating these processes. In this study, oil shale is selected as an additive during combustion in order to study the properties of mixed fuel. The study results reveal that water-soluble sodium is the predominant component of Zhundong coal, which originates from the salt in seawater. With an increase in the oil shale blending ratio, the ash fusion temperatures of the mixed coal and the difference between softening temperature and deformation temperature first decrease and then increase. The maximum sodium retention of 86.01% occurs at a blending ratio of 10% and a final temperature of 815°C. With an increase in temperature, aluminosilicates, such as albite and nepheline, form in the ash. Therefore sodium gradually migrates from the water-soluble phase to the aluminosilicate phase. With an increase in the SiO2/Al2O3 ratio, the ash fusion temperature of mixed coal decreases gradually, which is attributed to the effects of the SiO2 and Al2O3 contents on the melting point. Additionally, a mathematical model for sodium volatilization in the coal is constructed by MATLAB R2014a for fitting and optimization.

Experiment study on entrained flow gasification technology with dry slag by second-stage water supply

Experimental system of a new entrained flow gasifier with second-stage water supply (20kg/h) was set up in this paper, and effect of molar ratio of O/C on temperature distribution, syngas composition, carbon conversion and effective syngas were analyzed with a typical high AFT coal (Guizhou LJ coal). The results of non-second-stage water and second-stage water with changing molar ratio of O/C indicate that, when the molar ratio of O/C is larger than 0.96, gasification efficiency with dry slag by second-stage water would become better than non-second-stage water. Specifically, when the molar ratio of O/C was 1.1, the effective syngas fraction and carbon conversion of second-stage water gasification were 80.2% and 93.24% respectively. Correspondingly, the effective syngas fraction and carbon conversion of non-second-stage water gasification were 78.5% and 91.67% respectively. Furthermore, the gasification temperature was lower than the ash fusion temperature of test sample, the slag was non-melt during all the gasification experiments. The optimal gasification was conducted with the process temperature under 1450°C, which was lower than the ash fusion temperature of LJ coal. These experimental results prove the applicability of entrained flow gasification technology with dry slag by second-stage water supply on using high AFT coals, which is helpful to generalize the large-scale gasification technology using abundant high AFT coals in China.

Adjustment behaviors of blending coal on the ash fusion characteristics of coal with a high ash fusion temperature

To explore the effects of blending coal on the ash fusion temperature (AFT) of coal with a high AFT, Yima coal (YM) and Shenmu coal (SM) were added to Jincheng anthracite (JC) in different mass ratios. The variation in ash fusion characteristics was investigated on ash fusion analyzer. X-ray fluorescence spectrometer, X-ray diffracto-meter combined with Rietveld-based SIROQUANT software package and scanning electron microscopy were used to explore the adjustment mechanism of blending coal on the AFTs. Both blending coal can make the JC AFTs decrease to some extent. For YM, the AFT of the mixture decreased smoothly; however, for SM, the mixture AFT initially decreased smoothly (SM: 0–20 %), then sharply (SM: 20–30 %) and gradually again. The increase of low MP matter (anorthite, hercynite) content results in the AFT decrease of JC mixed ashes. Gehlenite formation due to high content of calcium oxide in SM makes the AFT variation in JC mixed ashes for two blending coals is difference. Compared to YM, the gehlenite could react with hercynite and quartz to generate glassy material, which made mixed JC decrease markedly when SM mass ratio increased from 20 to 30 %.

Modeling and Prediction of Coal Ash Fusion Temperature based on BP Neural Network

Coal ash is the residual generated from combustion of coal. The ash fusion temperature (AFT) of coal gives detail information on the suitability of a coal source for gasification procedures, and specifically to which extent ash agglomeration or clinkering is likely to occur within the gasifier. To investigate the contribution of oxides in coal ash to AFT, data of coal ash chemical compositions and Softening Temperature (ST) in different regions of China were collected in this work and a BP neural network model was established by XD-APC PLATFORM. In the BP model, the inputs were the ash compositions and the output was the ST. In addition, the ash fusion temperature prediction model was obtained by industrial data and the model was generalized by different industrial data. Compared to empirical formulas, the BP neural network obtained better results. By different tests, the best result and the best configurations for the model were obtained: hidden layer nodes of the BP network was setted as three, the component contents (SiO2, Al2O3, Fe2O3, CaO, MgO) were used as inputs and ST was used as output of the model.

Influence of biomass addition on Jincheng coal ash fusion temperatures

The ash fusion temperatures (AFTs) of Jincheng (JC) coal, two kinds of biomass (cotton stalk (CS) and sargassum natans (SA)) and various coal–biomass mixtures were investigated in this study. Fluid temperatures (FT) were found 1531°C, 1534°C and 1428°C for JC, CS and SA ash respectively, which were higher than the normal operating temperature of entrained bed gasifier. However, the FTs of JC and CS mixture ash were lower than that of either single JC or CS ash, which showed a minimum value of 1292°C with 80% CS, and the FTs of JC and SA mixture ash showed the similar trend with a minimum value of 1258°C when SA proportion was 40%. At high temperature, refractory minerals were presented in three respective ashes, which were mullite of JC ash, lime and periclase of CS ash, merwinite and akermanite of SA ash. And they could support the ash skeleton structure. Interaction between coal and biomass ash occurred during blending ash melting process, which formed co-eutectic and decreased the AFTs. The relationship between the ash fusion temperatures and the ash chemical composition was also discussed.

In Situ Analysis and Mechanism Study of Char-Ash/Slag Transition in Pulverized Coal Gasification

The behaviors of char-ash/slag transition of three different rank coals during char-CO2 gasification were studied by the combination of thermogravimetric analyzer and an in situ heating stage microscope. The effects of coal ranks and gasification temperatures on the interactions of char-ash/slag were analyzed. The results show two reaction forms, which relate to the reaction conditions and coal properties in the char-ash/slag transition processes. (1) When the temperature was lower than the ash fusion temperature of coal, the char particles exhibited the shrinking particle mode in the initial and midterm reaction stages, and then showed the shrinking core mode at the late stage. The accumulated ash might block some pore structure of semicoke, which was unfavorable for the char-CO2 reaction proceeding at high carbon conversions. (2) When the gasification reaction was conducted above the ash fusion temperature of coal, the liquid film formed because the ash melted with the shrinkage of char particles. Moreover, the molten slag enclosed residual carbon at high carbon conversions. These two forms of the char-ash/slag transition can well explain the variation of the reactivity of coal char at different reaction temperatures. A threshold value of gasification conversion (x = 0.9) exists during the char-ash/slag transition processes of both Yunnan lignite coal and Shenfu bituminous coal, while the threshold did not appear in the gasification processes of Zunyi anthracitic coal for its high ash fusion temperature. The present work shows that the coal ranks, coal ash properties, and reaction conditions have great effects on the whole char-ash/slag transition processes.

Impact of alkaline oxide on coal ash fusion temperature

Ash fusion temperature (AFT) is an important index for coal gasification. A blend of fluxing agent with coal can decrease the AFT to meet the requirements of specific gasification process. Effect of alkaline oxide and silica alumina ratio on decreasing AFT is focused in this study. Simulation ash was used to investigate the effect of alkaline oxide on AFT. The experimental results showed that AFT of simulation ash significantly decreased by adding single or composite fluxing agent of Fe2O3, CaO, Na2O into it and the silica alumina ratio has specific effect on the AFT. The single or composite fluxing agent was also added to three coals with high AFT, consistent results were obtained. Biomass was blended to adjust the silica alumina ratio of high AFT coal and the AFT was evidently decreased.

Effect of Stone Coal Chemical Composition on Sintering Behavior during Roasting

During the roasting of stone coal, a sintering phenomenon usually occurs, which is related to the chemical composition of the stone coal. In this study, the sintering beginning temperature (SBT) and ash fusion temperatures (AFTs) of stone coal with different CaO, Fe2O3, SiO2 and Al2O3 content were investigated. FactSage has been used to calculate the phase distribution of roasted stone coal, to analyze how these components influence the sintering behavior during roasting. For stone coal with low CaO content, CaO reacted with albite, leucite, and quartz to generate an amorphous phase, which would easily cause sintering, while for stone coal with high CaO content, CaO took part in the formation of andradite and Na2Ca3Si6O16. Most Fe2O3 in roasted stone coal stably existed as hematite in the system and did not react with others, which had almost no effect on sintering. SiO2 and Al2O3 in the stone coal contributed to hinder the formation of amorphous phases, especially for high Al2O3 stone coal, in which mull...

Improvement of ash flow properties of low-rank coal for entrained flow gasifier

In order to meet the demand of slagging for entrained flow gasifier, addition of refractory agent kaolinite and coal with high SiO2 and Al2O3 contents was applied to improve ash flow properties of low-rank coal including fusion temperature and viscosity. The FACTsage software package was used to predict proportion of solid phase and minerals as a function of temperature. X-ray diffraction (XRD) analysis was carried out to identify the major crystalline phases in ash. The results show that the variation of ash fusion temperatures (AFTs) with addition of kaolinite is similar to that with addition of coal with high SiO2 and Al2O3 contents. It first decreases with increasing amount of kaolinite addition and coal with high SiO2 and Al2O3 contents, and then decreases and reaches a minimum value before increasing again. The location of ash composition in pseudoternary phase diagram and mineral changes in ash can be used to explain the variation of ash fusion temperatures of low-rank coal with addition of kaolinite or coal with high SiO2 and Al2O3 contents. Viscosity of slag also increases with increasing additions of kaolinite and coal with high SiO2 and Al2O3 contents. The sensitivity of slag viscosity to temperature excursion decreases with increasing of kaolinite addition and coal with high SiO2 and Al2O3 contents, because the sensitivity of the SiO2–Al2O3–‘FeO’–CaO–MgO system to temperature excursions decreases with increasing kaolinite addition and coal with high SiO2 and Al2O3 contents. Meanwhile, coal blending method has a larger tapping temperature range and shows a better effect on improving ash flow properties for entrained flow gasifier.

Fusibility Characteristics of Slag from Fluidized-Bed Gasification of Jincheng Anthracite

To investigate fusibility characteristics of slag from Jincheng Anthracite (JC) pressurized ash agglomerate fluidized bed (AFB) gasification of Jincheng Anthracite (JC), the slag samples were examined by an ash fusion temperature (AFT) analyzer, scanning electron microscope (SEM), and X-ray diffraction (XRD). The results show that the AFTs of three kinds of slag are lower than that of JC greatly, and the AFTs of three kinds of slag decrease from the slag formed that on the central tube (SC) to distribution plate (SD) to inner face (SI), as a result of the increase of total base content and the differences in their mineral compositions accordingly. Not all slag during AFB gasification are composed of molten mineral matters. This can be explained that the phenomena that the slag formation during fluidized-bed gasification when the operating temperature is below the AFTs of coal.

Study of Effect of Ternary-Component Blended Coal on Coal Gasification Reaction at High Temperature

Three typical Chinese individual coals which existed remarkable difference on coal ash chemical composition and ash fusion temperature were selected to carry out coal blending experiments to study the coal gasification reaction at high temperature by means of using ternary-component blended coal technique and TGA-DTA method. According to ternary-component blended coal with a certain proportion, ash chemical composition and coal-char/CO2 gasification reactivity were analyzed by X-ray fluorescence (XRF) and thermogravimetric analysis-derivative thermogravimetric analysis (TGA-DTG), respectively. The results show that the ash chemical components change because ternary-component blended coals change the mineral composition, and hence, the gasification reactivity can be affected as well. Moreover, in accordance with reactivity index R, it indicates that the order of gasification reactivity of three individual coals and four blended coal options is coal x > option B > option A > option D > option C > coal z >coal y. Meanwhile, a new mathematical model called per unit ash alkali index B* was established by using the ash chemical component dates, which has a good corresponding relationship with R for four blending coal options. Utilizing ternary-component blended coal technique could improve the high-temperature coal ash gasification reaction.

The influence of phosphorus on ash fusion temperature of sludge and coal

Sewage sludge gasification can result in significant environmental benefits. However, the influence of sewage sludge on the process is not recognized in an adequate degree. The purpose of the present study was to investigate the ash fusion temperature (AFT) of coal-sewage sludge (SS) and the influence of phosphorus on it. Ash fusion characteristics of six types of coals, one type of sewage sludge, and the corresponding coal-sewage sludge blends were measured. Chemical and mineralogical compositions of blended ashes at different temperatures were determined. Ash fusion temperatures of most coals are lowered by adding sewage sludge, because Ca and Na in sewage sludge ash can react with refractory minerals from coal to form low temperature eutectic. The influence of phosphorus on mixture AFT depends greatly on the Al2O3/(CaO+Na2O+K2O) (A/CNK) molar ratio of bulk ash. Phosphorus tends to react with calcium to form refractory minerals in subaluminous systems (A/CNK<1), but mainly exists in amorphous materials and beneficial for decreasing ash fusion temperature in peraluminous systems (A/CNK>1).

Physical property behaviour of North Dakota lignite in an oxygen/steam blown moving bed gasifier

In this study lignite originating from North Dakota (USA) was thermally treated in an oxygen/steam blown commercial-scale moving bed gasifier operating on lump coal at the Dakota Gasification Company (DGC) in order to identify the physical property changes that occur during heating. After reaction, the solid particulate remnants were extracted from the reactor and characterised using standard techniques. Thermal fragmentation was found to be severe with the coal tested, i.e. the lump coal was found to decrease in size to about 90% <6.3mm particles in the drying and de-volatilisation zones of the reactor. The particle size (mean Sauter diameter) however increased slightly in the reduction and combustion zones due to ash melting and subsequent agglomeration. The particle as well as the bulk density residual profiles of the fuel bed samples in the reactor was found to be in alignment with the residual ash content profile. The ash fusion temperature (AFT) of the feed coal to the commercial-scale moving-bed gasifier was found to be higher as compared to the ash bed. Fluxing elements in the dominating glass phase determined in the reduction and combustion zones are most probably the reason behind this phenomenon.

Relationship between coal ash composition and ash fusion temperatures

Coal ash fusion characterization and the ash fusion temperatures (AFTs) are important parameters for coal industry. Previous research has proved that the AFTs are correlated with the chemical composition of coal ash samples. In this paper, a five-component SiO2–Al2O3–CaO–Fe2O3–K2O system is set. The AFTs of 34 synthetic ashes were measured in a carbon atmosphere, and the trends of the AFTs analyzed. Different from other studies, X-ray diffraction (XRD) and scanning electron microscope (SEM) were combined to explore the relationships between the measured AFTs and the mineral composition, morphology and microstructure of ash samples. Furthermore, the effect of the element valence state is also taken into account and X-ray photoelectron spectrometer (XPS) is used to analyze it firstly. The results show that the AFTs decrease with the increasing Fe2O3 content and silica-to-alumina (S/A) ratio. However, for the effect of CaO content, the AFTs reach a minimum value and then increase once again. Furthermore, the AFTs show no significant change as the K2O content varies. At last, all the conclusions found from the synthetic samples are also applicable for the seventeen bituminous coal ashes collected locally and abroad.

The Effect of Flux on the Melting Characteristics of Coal Ash from the Liu-qiao No. 2 Coal Mine in North Anhui Province

The effect of flux on the melting characteristics of coal ash from the Liu-qiao No. 2 Coal Mine in north Anhui province (AQ) was investigated. The change of the compositions of mineral under various temperatures with the atmosphere of N2 and H2 mixture before and after adding flux into AQ was analyzed by infrared ray. Results show that mullite formed over 1000°C has an effect on increasing the ash fusion temperature of coal, and this is the major reason that leads to a high ash fusion temperature of AQ. The coal ash fusion temperature was obviously reduced by adding calcium-based flux, magnesium-based flux, and ferrum-based flux, respectively. While the added mass amount of calcium-based flux is 6.2% (coal as reference), or the added mass amount of magnesium-based flux is 2.8%, the ash fusion temperature of AQ could be reduced to below 1350°C to meet the requirements of the shell gasifier slag-tapping process. The low temperature eutectic mixtures are easily formed between flux with aluminium silicate minerals under high temperatures, which will remarkably decrease the coal ash fusion temperature.

Prediction of Coal Ash Fusion Temperature by Least-Squares Support Vector Machine Model

Coal ash fusion temperature (AFT) is one important parameter for coal-fired boiler design and evaluation in a power plant. The relationship between coal AFT and the chemical composition of coal ash is rather complex in nature and makes the modeling of AFT difficult. In this work, a least-squares support vector machine (LS-SVM) model, which was based on a dynamically optimized search technique with cross-validation, is developed to predict the coal ash softening temperature (ST). The accuracy of this LS-SVM model was verified by comparison with the experimental AFT data of different types of coal. Further, the comparison of the present LS-SVM model and the traditional models, for example, multilinear regressions (MLR) and multi-nonlinear models (MNR) as well as the artificial neural network (ANN) models, showed that the LS-SVM model was much better to provide the highest generalized accuracy with the mean squared error of 0.0128 and correlation coefficient of 0.9272. Furthermore, based on the LS-SVM model,...

Study on the ash fusion temperatures of coal and sewage sludge mixtures

The coal, sewage sludge, water and chemical additives are milled to produce coal–sludge slurry as a substitute for coal–water slurry in entrained-flow gasification, co-gasification of coal and sewages sludge can be achieved. The ash fusion temperature is an important factor on the entrained-flow gasifier operation. In this study, the ash fusion temperatures (DT, ST, HT and FT) of three kinds of coals (A, B and C), two kinds of sewage sludges (W1 and W2) and series of coal–sewage blends were determined, and the mineral composition during the ash melting process was analyzed by X-ray diffraction (XRD). The results showed that the ash fusion temperatures of most coal–sewage blends are lower than those of the coals and sewage sludges. The ashes have different mineral composition at different temperature during the heating process. It was found that the mineral composition of AW1 blend ash is located in the low-temperature eutectic region of the ternary phase diagram of SiO 2–Al 2O 3–CaO. The minerals found in BW1 blend ash are almost the same as those in B coal ash. Kyanite is detected in CW1 blend ash, which results in the ash fusion temperatures of CW1 blend ash higher than those of C coal. We found that sodium mineral matters are formed because of NaOH added to W2, which can reduce the ash fusion temperature of coal–sewage blends.

Effect of ferrum-based flux on the melting characteristics of coal ash from coal blends using the Liu-qiao No.2 Coal Mine in Wan-bei

The effect of ferrum-based flux on the melting characteristics of coal ash in coal blends from the Liu-qiao No.2 Coal Mine in Wan-bei (AQ) was investigated. The change of the compositions of mineral under various temperatures before and after adding ferrum-based flux into AQ was studied by XRD and FT-IR. Results show that the presence of mullite formed over 1000°C increases the melting point of coal-ash, leading to high melting temperature of AQ coal-ash. The coal ash fusion temperature will be decreased by adding ferrum-based flux. The eutectic mixtures such as fayalite and hercynite are easily formed between ferrum compounds under high temperature, which remarkably decreases the coal ash fusion temperature.