Ash Fusion Test Research Articles

Investigating the potential of a waste-derived additive for enhancing coal combustion efficiency and environmental sustainability in a circular economy

This study examines the impact of a waste-derived additive from alumina and shale oil production on the performance of coal combustion. The effects of individual additive components were investigated under oxidant-limited and oxidizing conditions using the isothermal flow reactor (IFR) equipped with gas analysers. The raw materials, as well as fly chars/ashes derived from the IFR, were characterized using standard physicochemical analysis, oxide analysis, oxygen functional group determination, the ash fusion test, thermogravimetry, scanning electron microscopy and energy dispersive X-ray spectroscopy. Results from experiments conducted under oxidant-limited conditions demonstrated that the analysed additive, at a 1% share, increased hydrogen content in char by over 3.5 times (from 600 ppm to 2160 ppm) and enhanced methane conversion by nearly 20%. Under oxidizing conditions, the additive reduced unburned carbon loss by approximately 50%, emissions of NOx from 400-460 ppm to 340–390 ppm and SO2 from 1410-1475 ppm to 1325–1410 ppm. The study emphasized the influence of moisture on thermochemical processes, confirming that a certain amount of water vapour accelerates the conversion of H2, SO2, and NOX. The analysis supported the commercial utilization of the additive from economic, environmental, and operational standpoints.

Insight into the influence of water leaching on ash fusion behavior and alkali metal migration for corn and rice straw

Alkali-induced slagging and silicate melt-induced slagging are the two mechanisms of the ash-related problem for herbaceous plants and agricultural residues. Water leaching changes ash-forming composition and mitigates the risk of alkali-induced slagging, but the fusion behavior and the related melt-induced slagging after water leaching are seldom studied. In this study, a comprehensive assessment strategy, including systematic experiments, optimized ash fusion test and equilibrium calculations, was used to study the influence of water leaching on alkali metal migration and fusion behavior of corn straw (CS) and rice straw (RS). The total potassium released in biomass after water leaching accounted for approximately 15% of the raw materials. Although water leaching increased the fusion temperatures of CS and RS, the residual potassium still induced the same lowest eutectic melting of K-Ca-Mg silicate, causing them to start sintering at around 700 °C. However, the changes in fusion behavior obtained from optimized ash fusion test showed that water leaching effectively mitigated the risks of deposition and slagging of RS. The characteristic of fusion behavior for RS transformed from the “melting-dissolution” mechanism to the “softening-melting” mechanism, which indicated a low risk of deposition after water leaching. The fusion behavior of CS after water leaching was less apparent due to the different mineral compositions and characteristics of the two types of biomass, as Ca and Mg in CS acted as modifiers for the silicate structure. Water-leaching is recommended for pretreating biomass with a predominance of K. For biomass with substantial alkaline earth metals, water leaching combined with other countermeasures is required. These findings can help promote the thermochemical utilization of agricultural residues on an industrial scale.

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.

The influence of mineral addition on the Optimised Advanced Ash Fusion Test (OAAFT) and its thermochemical modelling and prediction

Specific minerals in ash are triggers for ash fusion during combustion. This study analyses, for the first time, the link between individual minerals and the ash fusion of pseudo ash pellets using the Optimised Advanced Ash Fusion Test (OAAFT) and FactSage modelling. The study analysed 20 pseudo ash pellets whose composition spanned a wide range of fuels used in the power generation industry. Varying quantities of the 4 main minerals were used to create the pseudo pellets; CaO (0–40%), Fe2O3 (0–40%), MgO (0–25%), and Silica-Alumina ratio (0.5:1–4:1). The OAAFT produced characteristic ash fusion curves for the pseudo pellets and individual minerals. The study also gained insight into the link between mineral transformations and ash fusion by comparing these profiles to the slag formation predictions in the FactSage modelling. Excellent alignment was obtained between the OAAFT curves and FactSage data. The OAFFT curves can be described as individual fingerprints of the ash fusion behaviour of the sample, which can be broken down into individual components. This data cannot be obtained from the conventional ash fusion test. By combining OAAFT and FactSage data, power generators can replicate slagging and fouling issues and identify the major components which are causing issues. The addition of mineral additives can be tested to analyse how slagging and fouling issues can be tackled for specific fuels. This will be of increasing importance as fuel blending and new complex fuels such as refuse derived fuels enter the market.

Effect of purified dust from CaC2 production and bottom ashes/slags on slagging characteristic of Zhundong coal blend

Coal cofiring with carbon-containing solid wastes can achieve resource utilization and promote the circular economy development. This work aims to investigate the effects of C-rich bottom ashes/slags (BAS), fine/coarse BAS separated by mechanical sieving, and the mixture of untreated BAS with purified dust (PD) from CaC2 production acting as additive on slagging behavior and mineral phase transformation of Zhundong (ZD) coal blend. Detail characterizations including chemical composition, micro-morphology, mineral phases and weight loss of BAS were performed. The ash fusion tests for coal blend with varying proportions of additive were conducted. The compositional analyses of deposits on an un-cooled probe at different sampling temperatures (600/800/1000/1200 ℃) in a drop-tube furnace were investigated. The results show that three types of untreated BAS have similar physicochemical properties. The BAS addition has a negative influence on ash fusion temperatures, but they would completely change the types of crystal minerals in the melted deposits dominated by amorphous substance at 1200 ℃. The addition of 5 wt% coarse (>1 mm) or fine (<53 μm) BAS promotes glassy slags at 1200 ℃ converting into deposits with a low melt fraction, but the influence of fine BAS is more pronounced. A group composed of untreated BAS and PD at an appropriate ratio, promotes the formation of more crystal minerals with high melting points in deposits at 1200 ℃.

Using dissipation factor method in testing the ash sintering process of cereal pellet and coal fuels

The aim of the paper is to present the results of alternating current electrical tests made on ashes of selected solid fuels (biomass and coal from Polish mines). This work is meant to supplement the current state of knowledge with an alternating current method for assessing the state of sintering of biomass and coal ash in relation to thermogravimetric and calorimetric tests, statistical (oxide indicators), sintering temperatures i.e. ash fusion test and analyses of multicomponent thermodynamic equilibria. An ultimate and proximate analysis were made for all fuels. Thermogravimetry analysis and differential scanning calorimetry measurements have been carried out for the sintering process of ash in the temperature range up to 900 °C. The thermodynamic analysis has been performed using the FactSage software. The dissipation factor measurements of the sintering process of ash have been carried out. A model of dissipation factor change during sintering has been proposed, which then has been used to calculate the activation energy of the ash sintering process for all fuels. The obtained data has been compared with data available in literature. The good correspondence between them leads to the conclusion that both the assumed model and the adopted measurement method are correct.

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.

Investigation on ash fusion temperature and slagging characteristic of Zhundong coal blends, Part 1: The effect of two solid wastes from calcium carbide production

The potential of purified dust (PD) and removed dust (RD) from calcium carbide production as additive to improve the slagging characteristic of Zhundong (ZD) coal blends was studied. Ash fusion tests with varying proportion of two additives (2.5, 5.0 and 7.5 wt%), and ash sample preparation at 1150/1250 °C were conducted. The deposits at different temperatures (1200/1000/800/600 °C) were collected by using an un-cooled probe which was inserted into a drop-tube furnace. The detail characterizations of ash or slag samples were used for analyzing the transformation of mineral phases, and evaluating the effectiveness of additives. The results show that PD and RD have a feature of Ca and C enrichment. The former has a higher Ca content, while the later owns more abundant C element. The addition of PD or RD improves ash fusion temperatures of coal blends, but the effectiveness of 5.0% PD is the best. The addition of 5.0% PD could convert the glassy slags collected at 1200 °C into deposits with low melting fraction, due to the formation of calcium-rich crystal minerals. Controlling the CaO/SiO2 molar ratio at the range of 2.2– 3 would significantly relieve the high temperature melting behavior of ZD coal blends featured by high Ca and low Si.

Methods for the determination of composition, mineral phases, and process-relevant behavior of ashes and its modeling: A case study for an alkali-rich ash

The mineral matter contained in feedstocks has a generally limiting impact on the process design of high-temperature conversion processes. In the present study, the composition, mineral phases, and process-relevant properties of ashes are investigated by different experimental and modeling methods, which were reviewed in the literature regarding the frequently applied methods. Various analyses are exemplarily performed for the ashes of a high-sodium coal from China, generated at temperatures of 150–950 °C. X-ray fluorescence (XRF) analysis, microwave-assisted inductively-coupled plasma optical emission spectrometry (MW-ICP-OES), and the same technique with electrothermal vaporization (ETV-ICP-OES) are applied to analyze the chemical composition of the bulk material. The chemical composition of the near-surface region is studied by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). Mineral phases are analyzed by X-ray diffraction (XRD) and thermochemical calculations. The process-relevant ash fusion behavior is studied by a common ash fusion test (AFT) and thermomechanical analysis (TMA) and supported by thermochemical calculations. The different ashing temperatures have a recognizable impact on the composition, formation and transformation of mineral phases, and resulting ash fusion behavior, while each property is monitored by at least two different methods. For this purpose, a detailed analysis of the results achieved by the individual methods is performed. Finally, the results obtained by different methods for the same ash property are compared for monitoring the validity of the results and, for example, extracting additional information about the gas phase transfer of selected ash components.

Effect of different additives on ash fusion characteristic and mineral phase transformation of iron-rich Zhundong coal

Fundamental research on the mineral transformation of iron-rich coal is very necessary for extending its utilization at elevated temperature. In this work twelve runs with six types of additives (halloysite, alumina-rich ash, dolomite, SiO2, CaO and MgO) at different mixing proportions were sorted, to adjust the relative content of Si, Al, Ca or Mg element in coal ashes targeting to improve the slagging characteristic. Ash fusion tests and the preparation of coal ashes at elevated temperatures (1100, 1150 and 1250℃) were performed, respectively. The effects of additive type and blending ratio on the fusion characteristic and the mineral phase transformation were explored. Elemental composition, crystalline mineral phases and micro-morphologies of coal ashes were characterized via various technical approaches. The results showed that 3% alumina-rich ash or dolomite significantly improve the deformation temperature of iron-rich coal. While the addition of halloysite or SiO2 with a same proportion sharply decreases the softening and flow temperatures of coal ash, except for slightly increasing the deformation temperature. The composite additive consisting of alumina-rich ash, MgO and dolomite, could promote the formation of MgFe2O4, FeAl2O4, Mg(Fe,Al)2O4 or MgAl2O4 which inhibit the melting of coal ash. However, a low proportion of Si-rich additive into iron-rich coal is not suggested.

An overview of slagging and fouling indicators and their applicability to biomass fuels

Slagging and fouling are common problems associated with biomass firing. The different nature of the mineral and phase composition of biomass ash makes the vast experience with coal firing insufficient for its translation to biomass fuels, especially when it comes to slagging and fouling behavior. Biomass tends to have lower ash content than coals; however, it is often rich in volatile alkalis. The mineral deposits found on boiler walls and superheater tubes are often comprised of alkali compounds. Numerous studies on ash melting and particle sticking behavior have been conducted. Laboratory observed ash fusion temperatures are commonly used to evaluate the slagging and fouling propensity of fuels. The tests are often time consuming, therefore several predictive indices have been developed to estimate the propensity based on the ash composition alone. Thermodynamic models as well as neural networks have also been applied to this end. However, for practical in the field purposes, the ash fusion tests and predictive indices are preferred because of their convenience. An overview of these indices is presented in this work. A sizeable dataset has been collected in order to statistically evaluate the applicability of the indices and of several AFT prediction formulas. General trends in ash composition on this extensive dataset have also been illustrated. Finally, a more convenient graphical solution is presented for preliminary slagging and fouling predictions.

Impact of blending ratio and injection position on slagging behavior in a 1000 MWe wall-fired furnace with a modified viscosity model

A commercial CFD package coupled with user-defined functions was used to observe coal particle combustion and tracking, particle capturing and deposition, and other related phenomena. A modified Watt-Fereday model for deposition employing experimental coefficients derived from the ash fusion test was used to predict ash viscosity, which showed that the samples had a nonadditive behavior due to the eutectic phenomenon. The furnace results showed that when the Bayan coal, containing a large amount of volatile matters, was injected at the top burner, the slagging propensity increased compared to when it was injected in the bottom burner. In addition, the specific blending ratio of 25 % Flame and 75 % Bayan coal, which had a lower melting temperature compared to the individual coals. This study suggests that sub-bituminous coal should be injected in the bottom burner, and the specific blending ratio that yields a low-temperature eutectic mixture should be avoided.

Low-cost additives to improve the fusion behaviour of hydrochar ash

Hydrochars produced from agro-industrial wastes contain significant amounts of alkali metals and other inorganic elements that lead to different ash-related issues in energy systems. This work aims to modify the composition of hydrochar ash by adding rice hulls and/or mineral additives (i.e., lime, bentonite and kaolin) to improve their fusion behavior. Two series of samples were prepared to analyze the effect that the use of additives before or after the HTC processing of olive mill waste at 225 °C for 2 h had on the hydrochar properties. Hydrochars were then ashed, in agreement with UNE-EN 18122:2016, and the corresponding ashes subsequently characterized through XRF and ash fusion tests to determine their ash composition and ash characteristic fusion temperatures (AFTs), respectively. Among the studied additives, rice hulls led to the best results, as their use increased the initial ash deformation temperature to >1500 °C and resulted in the lowest hydrochar ash content. Using rice hulls before or after the HTC process did not affect the ash behavior, but it did change the hydrochar properties as a fuel. The joint HTC of rice hulls and feedstock led to a less carbonized solid with 13% higher moisture content and 15% lower energy density. Accordingly, the addition of rice hulls to ready-produced hydrochars is recommended to maintain the energy benefits of HTC as possible. The use of rice hulls as a low-cost additive to improve the fusion behavior of hydrochar ash opens new avenues for hydrochars in energy conversion processes with limited ash-related operational problems.

New Experimental Evaluation Strategies Regarding Slag Prediction of Solid Biofuels in Pellet Boilers

Pellet boilers and pellet stoves are widely used for heat production. But in most cases, only specific wood pellets with a low ash content are approved due to the increased risk of slagging and limited deashing capacity. The ash fusion test (AFT), according to prCEN/TS 15370-1, is currently the only standard method for the prediction of slagging. This method is not feasible for all biomass fuel types, since sometimes the characteristic temperatures cannot be determined or the characteristic shapes do not occur for temperature determination. Furthermore, the method is costly and requires complex instrumental infrastructure. Hence, a demand for more expressive or more rapid methods to characterize slag formation potential of fuels is often claimed. Based on a literature study, four such laboratory test methods were chosen, partly adapted, and then experimentally investigated. These methods included thermal treatment of the fuel itself or the ashes of the fuel and were the rapid slag test, CIEMAT, the slag analyzer, and the newly developed pellet ash and slag sieving assessment (PASSA) method. Method performance was practically assessed using 14 different biomass fuel pellets, which were mainly from different assortments of wood, but also herbaceous or other nonwoody fuels. The results from the tests with these four alternative methods were evaluated by comparing to both results from standard AFT and results from full-scale combustion tests performed over a maximum of 24 h. Seven different pellet boilers were assessed, of which one boiler was used to apply all 14 test fuels. According to the granulometric ash analysis (i.e., the ratio of >1 mm-fraction toward total ash formed), the sensitivity of the new test methods to depict slagging phenomena at a suitable level of differentiation was assessed. Satisfactory conformity of the boiler ash assessment (reference) was found for both, the slag analyzer and the PASSA method. The latter may, in particular, be seen as a promising and relatively simple low-input procedure, which can provide more real-life oriented test results for fixed-bed combustion. The standardized AFT could, however, not sufficiently predict the degree of slag actually formed in the reference boiler, particularly when only wood fuels are regarded.

The impact of ash pellet characteristics and pellet processing parameters on ash fusion behaviour

The Ash Fusion Test (AFT) is considered to be the most popular method of characterising the melt characteristics of solid fuel ash. This study shows how pellet preparation can make significant improvements to repeatability. Pelleting pressure, pellet particle size, pellet shape, and furnace ramp rate were investigated to establish the most repeatable representation of ash melting relevant to pulverised fuel combustion in a furnace in an oxidizing atmosphere up to 1600 °C. A 5 mm machine pressed pellet was found to produce the best results as it identified the earliest initial deformation temperature (IDT), gave the least error, and displayed the greatest visible change in pellet height to enable easy identification. Reducing maximum ash particle size to <72 µm and increasing the pressure of the pelleting process was also shown to produce a 120 °C reduction in the IDT when compared with other methods. Reducing the ashing temperature and retaining volatiles lost during high temperature ashing were shown to have a negligible impact on IDT. The characteristic AFT curve was also used to quantify the extent of shrinkage and swelling during the test.

Ash Deposition Characteristics of Industrial Biomass Waste and Agricultural Residues

Abstract Resolution of industrial waste as thermal source is an effective way to transform the unwanted scheduled waste into renewable energy. Plant residues also contributed to enhance its utilization and considered a reliable resource for energy uses. Both sectors of feedstock are aimed to produce significant amount of thermal energy through combustion. Ash deposition behaviors of biomass fuel from industrial sludge and agricultural waste have been analyzed. Sludge and effluent were carefully taken from petroleum crude processing plant (PGB) and palm oil mill (POME), respectively. Rice husk and kenaf were obtained from local neighborhoods farm. The effective utilization of potential biofuel is evaluated based on its ash content, chemical composition and its ash slagging and fouling tendency. PGB and POME were evaluated to have energy content about 21.35 MJ/kg and 15.238 MJ/kg respectively. While kenaf and rice husk each has 16.14 MJ/kg and 15.19 MJ/kg, respectively. Ash content of PGB sludge was around 30.0% where POME sludge was about 38.0%. Kenaf showed a better biomass type as the ash content was only 2% ash, whereas rice husk ash content was 11%. To illustrate the realistic combustion condition under different temperature, the effect of ashing temperatures that are 525°C, 575°C and 625°C to the ash deposition characteristics is also examined. This variation of ashing temperature will also deflect the chemical composition especially the heavy metal element in the sludge ash. Furthermore, the physical ash deposition and deformation characteristics are observed by ash fusion test. Conclusively, PGB, POME, rice husk and kenaf biomass are feasible to produce sufficient thermal energy as well as stable at high temperature with their distinct chemical composition, but their individual range ashing temperature may deficient the thermal process differently.

Sintering and collapse of synthetic coal ash and slag cones as observed through constant heating rate optical dilatometry

Ash agglomeration and slagging are responsible for costly maintenance problems in coal-fired boilers. Boiler design and operation rely on critical ash fusion temperatures defined by geometric criteria that explicitly ignore shrinkage or warping. Reproducibility of ash fusion test (AFT) results is known to be a major problem, given the subjective nature of some criteria used in critical temperature reporting. Decades of faulty predictions based on AFT data underscore the need for an objective measure of the likelihood for agglomeration and slagging as a function of coal ash composition and temperature. Heating microscopes and ash fusibility determinators are AFT instruments that are widely available and readily adapted for optical dilatometry of ash cones through digital image analysis. Optical dilatometry enables the continuous measurement of cone height and face area, and the calculation of changes in cone volume and density as a function of temperature during constant heating. A strong correlation was found between the sintering onset temperatures of two synthetic ash compositions measured through optical dilatometry and their corresponding solidus temperatures predicted through thermophysical modeling. The sintering onset temperatures of synthetic slag cones appears consistent with glass transition temperatures estimated using the Kauzmann-Beaman rule. Cone collapse is highly correlated with softening, hemispherical, and fluid temperatures for ash cones. Rapid changes in solid, liquid, and gas volume fractions near the collapse temperature suggest that a transition from capillary to slurry state may play an important role in triggering collapse. Slag cone collapse is better explained by a decrease in viscosity as a function of temperature in the glass working range. The sintering onset temperature and cone collapse temperature are readily understood through thermophysical modeling based on ash composition and provide two additional critical temperatures in the modeling of coal ash agglomeration and slagging.

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.

Scavenging of refractory elements (Ca, Mg, Fe) by kaolin during low rank coal combustion

In this paper, special attention is paid to the scavenging of refractory elements (Ca, Mg, Fe) by kaolin during combustion of a low rank coal. Combustion experiments were carried out on an electrically-heated drop tube furnace at 1300 °C. The raw coal and its blends with kaolin additive (1–10 wt% of coal mass) were respectively burned in simulated air atmosphere. Combustion-generated ashes were collected and analyzed by Computer-Controlled Scanning Electron Microscopy (CCSEM). The data were used to elucidate the scavenging of Na/Ca/Mg/Fe by kaolin and the transformation of inorganic species after kaolin addition. Quantitative X-ray diffraction (XRD) analysis and ash fusion tests were carried out to characterize the amount of amorphous species and the fusion temperatures of bulk ashes after kaolin addition, respectively. Backscattered electron images (BSE) were collected on the raw coal and coal/kaolin mixtures to demonstrate the role of the dispersion state of kaolin in metal scavenging. It is found that apart from Na, the added kaolin can also strongly scavenge the refractory elements (Ca and Mg, except Fe) during low rank coal combustion. The scavenging of Na/Ca/Mg by kaolin promotes the formation of amorphous and molten alumino-silicate particles. The high dispersion of fine kaolin particles in coal is a key factor affecting the scavenging process, which provides sufficient opportunities for the contact and reaction between Ca/Mg species and kaolin. Two possible routes are proposed for the scavenging of Ca/Mg by kaolin. One is that kaolin firstly reacts with sodium vapors, forming melting sodium alumino-silicates that favor the capture of Ca/Mg containing species. Another possible route is that, kaolin directly reacts with active CaO/MgO fumes produced from coal combustion, forming Ca/Mg containing alumino-silicates. Kaolin dosage has significant impacts on the ash fusion temperatures. Which firstly decreased with the increase of kaolin dosage, reaching to the lowest at 4 wt%. Then it sharply increased with the increase of kaolin dosage.

Ash Fusion Characteristics and Transformation Behaviors during Bamboo Combustion in Comparison with Straw and Poplar

In this work, the bamboo ash fusion and sintering characteristics were studied to evaluate its potential application in combustion for the production of heat and power. Poplar and wheat straw were used in the experimental test as the reference fuels for comparison. Standard ash fusion tests and ash sintering tests were carried out at elevated temperatures. The results indicate that bamboo has a low ash melting temperature of 862 °C, much lower than that of poplar. In spite of the high K content in bamboo ash, no severe melting and sintering was observed under the temperature lower than 1000 °C. The ashes after the tests were analyzed using SEM/EDX, XRF, and XRD techniques to illustrate the ash transformation behavior. Standard ash fusion tests indicated that the melting temperatures of bamboo, wheat straw, and poplar ashes are 862 °C, 770 °C, and 1088 °C, respectively. No severe sintering can be observed for poplar due to the large existence of refractory compounds. Ash sintering occurred when the tempera...