Molten Ash Research Articles

Modelling of fly ash viscous deposition and slagging prediction of biomass-fired boiler

In addressing the ash melting-induced slagging on the high-temperature heating surfaces of biomass-fired boiler, a combined experimental and modelling study of biomass ash viscous deposition has been conducted. A comprehensive ash viscous deposition model based on ash fusion and viscosity characteristics and the critical velocity criterion has been proposed. The deposits of the top furnace and high-temperature superheater of a 130 t/h biomass-fired boiler have been sampled for validation, and a modelling study using ANSYS FLUENT with a user defined function (UDF) and dynamic mesh has been performed. The results show that small particles with diameter of 20 μm lead to less viscous adhesion because of faster response to temperature decrease before reaching the heating surfaces. Deposition efficiency and growth rate of deposits are positively correlated with temperature. Slagging in the top furnace area is dominated by the viscous deposition of molten ash. However, the amount of viscous deposition accounts for a low proportion (20.1 %) in the high-temperature superheater, suggesting a change of dominant slagging mechanism from viscous deposition to gaseous condensation and the subsequent capture of fly ash. The model is suitable for predicting ash melting-induced slagging in high-temperature areas of biomass boilers.

Numerical simulation of solidification of boiler molten ash impinging on vertical wall based on Smoothed Particle Hydrodynamics method

In order to study the whole process of the high temperature liquid slag fly ash produced after the combustion of high alkali coal as power coal in the boiler furnace, which moves in the furnace and impinges on the water wall, the heat exchange occurs and then the cooling phase transformation deposits and slags, and the possible factors of slagging particles are studied by changing the environmental variables. In this paper, based on Smoothed Particle Hydrodynamics (SPH), the dynamic behavior of boiler molten ash impacting vertical wall is studied, and the flow spreading and solidification model of molten slag impacting vertical wall is established. Thus, the process of phase change deposition of slag particles against wall is simulated. By observing the phase distribution of slag and the change in spreading process, the complete process of the slag particles hitting the wall, spreading, shrinking and solidifying is analyzed. Based on the cohesive and repulsive surface force method, a temperature-dependent surface tension model is established to simulate the effect of surface tension on flow characteristics when molten ash impinges on low-temperature vertical walls. The influences of slag impact velocity, initial temperature and water wall temperature on deformation and solidification heat transfer are discussed. The results show that the initial velocity and initial temperature of slag particles affect the phase change deposition velocity and the final spreading factor of slag, while the temperature of the water wall has little effect on the deposition deformation and solidification process of slag.

The effect of ash fusion characteristic on the structure characteristics of carbon and the migration of potassium during rice straw high-temperature gasification process

The effect of ash fusion characteristic on the structure characteristics and gasification activity of residual carbon, and the migration of K during biomass (rice straw, BRS) gasification at 1100 °C were investigated. Results showed that the carbon and AAEMs were wrapped by molten ash, and the residual carbons in BRS gasification char were composed of pyrolytic carbon and partial-gasified carbon. The molten ash formed spherical particles adhering to the surface of gasification char, and the particle size gradually enlarged. With the progress of BRS gasification, the BET surface area and microporous surface area decreased monotonously, and the ID1/IG values increased firstly and then decreased, and the total active sites were poorer at the gasification time of 15 and 20 minutes. The gasification activity of residual carbon had no linear relationship with gasification time, and followed the order of DBRS-25> DBRS-10> DBRS-20≈DBRS-15≈DBRS-5. Furthermore, the total content of K increased obviously, and the water-soluble K, ion-exchanged K, and acid-soluble K were changed to insoluble K with the increase of gasification time.

Slagging characteristics in the superheater area and K/Ca distribution of biomass-fired grate boiler

The slagging on heating surfaces severely impedes the biomass combustion in grate boiler. Aiming at this situation, the slagging characteristics of a grate boiler firing straw/woody biomass has been investigated. By field sampling of deposition, the slagging mechanisms including gaseous K/Ca condensation, molten ash adhesion and viscous capture of ash particles have been clarified. The severity of slagging is ordered as: mid-temperature superheater > high-temperature superheater > low-temperature superheater > upper furnace. Ca plays a key role in initial layer formation in high-temperature superheater through condensation of gaseous sulfates. In mid-temperature superheater initial layer formed by gaseous KCl condensation and the viscous capture of ash particles lead to serious slagging. The distribution of K/Ca in the boiler during combustion were obtained based on ash collecting and characterization, counter-balance efficiency calculation and fuel consumption calculation. 44.6% of K and 35.8% of Ca contained in the biomass fuel deposited on the heating surfaces inside the boiler, indicating that more K and Ca tend to deposit in the boiler rather than remain in the discharged ash, which also requires more attention to Ca-related slagging. This study can provide basis for slagging predicting and controlling in grate boiler using high K/Ca biomass fuel.

Evaluation of the reactivity of dense lanthanum‑gadolinium zirconate ceramics with Colima volcanic ashes

The effects of ingestion of airborne particles from pyroclastic events of active volcanoes by aircraft turbines and their subsequent reaction with thermal barrier coatings have attracted the attention of the scientific community in recent years. The reaction products of infiltration experiments of lanthanum‑gadolinium zirconate (LGZO) ceramics with molten ashes from the active Colima volcano at 1250 °C for 10 h are presented and discussed as a function of the Gd3+ content. Five ceramic compositions, varying the Gd3+ content in solid solution were synthesized by the chemical coprecipitation and calcination method of pressed powders. These compositions include pure lanthanum and gadolinium zirconates, LZO, and GZO, respectively. Penetration depth and identification, and in some cases quantification of the reaction products between the molten ash and LGZO ceramics were performed by scanning electron microscopy, chemical composition with energy dispersive X-ray spectroscopy, grazing incident X-ray diffraction as well as micro-Raman spectroscopy. The LZO ceramic exhibited the greatest infiltration resistance with an infiltration depth of approximately 23 μm from the surface. The phase characteristics of the reaction layers were dependent on the gadolinium content. LZO, LGZO25, and LGZO50 (x = 0, 0.25, and 0.5) showed the presence of apatite as well as monoclinic and tetragonal zirconia, while LGZO75 and GZO (x = 0.75 and 1), additionally showed the presence of cubic zirconia and anorthite. As the Gd3+ content increases in the LGZO solid solutions, the wavenumber value corresponding to the stretching vibrational mode of the silicon tetrahedra in apatite shifts from 862 to 877 cm−1, which is associated with a decrease in SiO bond lengths. These findings indicate that the amount and kind of rare earth cations dissolved in the melt plays an important role in the precipitation of the reaction products.

Novel Design for Rotary Burner for Low-Quality Pellets

The burning of low-quality fuels causes several problems in the operation of combustion equipment, which can negatively affect the equipment’s efficiency. The possibilities for the burning of pellets made from low-quality raw materials are limited mainly by the fusibility of the ash, which settles and melts on the surfaces of the burner, gradually causing it to clog. Smelted ash also causes a decrease in heat transfer efficiency, which negatively affects the overall efficiency of the heat source. A possible solution is provided by burners with a rotating combustion chamber, where the contact time of the molten ash with the walls of the burner is shortened, and thus there is no significant melting of the ash in the burner. This manuscript is dedicated to summarizing the current state of development of burners with a rotary chamber, presenting a novel design for such a burner, and providing an analysis of that design. To conclude, the results of experimental measurements on a classic burner and a burner with a rotary chamber are presented, including a comparison and evaluation mainly in terms of emissions. The novel-designed rotary burner achieved a higher heat output than the retort burner, but a similar thermal efficiency. The rotary burner produced 32.5% lower CO emissions, 12.5% higher NOx emissions, 23% lower OGC emissions, and 44.7% higher PM emissions in comparison with a retort burner under the same conditions. This novel rotary burner concept could, after optimization, be a suitable option for efficient combustion of alternative biofuels.

Molten-Volcanic-Ash-Phobic Thermal Barrier Coating based on Biomimetic Structure.

Volcanic ash is a major threat to aviation safety. The softening/melting temperatures of volcanic ash lie far below typical aero-engine operating temperatures. Thus, molten ash can accelerate the failure of thermal barrier coatings (TBCs). Here, inspired by natural superhydrophobic surfaces (e.g., the lotus leaf), a molten-volcanic-ash-phobic TBC, which provides a large possibility to eliminate molten ash issues of TBCs, is developed. A hierarchically structured surface is first prepared on a (Gd0.9 Yb0.1 )2 Zr2 O7 (GYbZ) pellet by ultrafast laser direct writing technology, aiming to confirm the feasibility of the biomimetic microstructure to repel molten volcanic ash wetting. Then biomimetic-structured GYbZ TBCs are successfully fabricated using plasma spray physical vapor deposition, which reveals "silicate" phobicity at high temperatures. The exciting molten-volcanic-ash-phobic attribute of the designed surfaces is attributed to the lotus-leaf-like dual-scale microstructure, emulating in particular the existence of nanoparticles. These findings may be an important step toward the development of next-generation aviation engines with greatly reduced vulnerability to environmental siliceous debris.

High temperature interactions between K-rich biomass ash and MgO-based refractories

MgO-based refractories are used in lime kilns to withstand the high temperature and chemical environment. Efforts to reduce CO2 emissions have led to an increased interest to use bio-based fuels as alternatives to traditional fossil sources. The potential for refractory corrosion from a potassium-rich biomass ash was investigated by studying the infiltration of olive pomace ash into magnesia/spinel refractories. Refractory samples were exposed to the ash at up to 1400 °C for 15–60 min in a CO2–rich atmosphere. Molten ash infiltrated the refractories through pores and grain boundaries to a depth of up to 9.6 mm, which was quantified with a new systematic procedure. The phase KAlO2 was identified inside the refractories after exposure, indicating an attack of spinel components by potassium. Phases found in the ash residues also indicated the migration of refractory constituents. Thermochemical equilibrium calculations were also used to investigate the ash/refractory chemistry.

Comparative insights into flue gas-to-ash characteristics on co-combustion of walnut shell and bio-oil distillation sludge under atmospheric and oxy-fuel condition

The clean recovery of bio-oil distillation sludge (DS) is a pivotal sector to perfect the biomass refinery system in responses to effective waste management and environmentally friendly consensus. This work aimed to comparatively characterize atmospheric and oxy-fuel co-combustion of walnut shell (WS) and DS in terms of (co-)combustion performance, distributed kinetics, flue gas evolution and ash characteristics. The results indicated co-combustion of WS and DS significantly increased the allocated proportion of char combustion stage and improved co-combustion performance, which was reflected in the enhanced four quantitative combustion indices. The fourth pseudo component, corresponded to the char combustion stage, continuously dominated the whole combustion process by adding more fraction of DS to co-combust with WS, whose activation energy was diminished, thereby optimizing the initial reaction energy barrier. In addition, oxy-fuel combustion promoted the recovery and sequestration of CO2, facilitated the formation of CO, and further endowed the conversion of polycyclic aromatic hydrocarbons into phenols rather than aromatics. Although the composition for mixed molten ash presented the decreased contents for easy-slagging minerals, the co-combustion process was still with potential ash fouling risk owing to basically unchanged ash configuration consistent with WS ash. However, the addition of DS to some extent strengthened anti-slagging performance, consequently optimizing the empirical indices of slagging and fouling for mixed ashes. Also, a higher ash content was yielded in oxy-fuel combustion than that in atmospheric combustion owing to the reducibility of CO2, in which the presence of CO2 contributed to the formations of carbonates and low melting-temperature potassium-containing silicates.

Mathematical modeling for molten ash slagging moving bed coal gasifier considering the impact of particle behavior characteristics

Molten ash slagging moving bed gasification technology is significant for lump coal utilization. Mathematical modeling is essential for a comprehensive understanding of transport and reaction phenomena in a gasifier. In this work, a model based on the impact of particle behavior characteristics was proposed, encompassing: (i) particle fragmentation and agglomeration vary the particle size and bed voidage, (ii) gas channeling weakens the gas–solid heat transfer efficiency and variation of the gas phase temperature, and (iii) molten ash covering coal surface attenuates the gas–solid reaction rate. A good agreement was found between the simulation results of the proposed model and industrial data. Finally, the effects of the operating and model parameters were studied. The results indicated that the steam flow rate had a significant influence on the product gas composition, and the temperatures of gas and solid phases were sensitive to the gas channeling factor.

The use of kaolin and dolomite bed additives as an agglomeration mitigation method for wheat straw and miscanthus biomass fuels in a pilot-scale fluidized bed combustor

Renewable biomass fuels are frequently used for power generation. Biomass ash causes bed agglomeration in fluidized bed boilers due to the formation of alkali silicate melts. Very few prior studies have tested dolomite and kaolin bed additives for agglomeration mitigation with agricultural biomasses. In this work, pelletized miscanthus and wheat straw were tested in a pilot-scale 65kWth fluidized bed combustor with varying dosages of dolomite and kaolin on a silica sand bed. Neither additive improved defluidization time with wheat straw, whereas additive use at all dosages prevented bed defluidization with miscanthus. Agglomerates were studied through a novel, detailed SEM/EDX analysis across structural features. SEM/EDX analysis presented evidence of chemical reaction between both additives and fuels. Potassium in ash migrated into kaolin particle at depths of up to 60 μm. With dolomite, calcium and magnesium raised melt temperatures. Thermochemical modelling of the ash and additive combinations predicted that additive use would substantially reduce ash melt formation. It is proposed that the wheat straw pellet acted as a “ready-made” agglomerate structure due to release of molten ash to the pellet surface which bed material then sticks to, hence the lack of change to defluidization time regardless of additive use. Future studies into this behaviour would improve additive use.

Effect of filler on adhesion characteristics of precursor ceramic coating against high-temperature molten ash

Abstract In view of the widespread high-temperature molten ash adhesion problem of heat exchanger surface in the chemical, power, metallurgy and other industries, precursor ceramic coating is proposed to reduce ash adhesion strength. Three ceramic coatings were obtained by adding hexagonal boron nitride (h-BN), zirconia (ZrO2) and aluminium oxide (Al2O3) inert fillers on TP347 steel sheets through the Czochralski method and high-temperature sintering. Results of the static contact angle test show that at 860°C, the contact angles of the molten ash on the coating surfaces of h-BN, ZrO2 and Al2O3 are 58°, 54° and 53°, respectively, which are greater than that (39°) of the steel sheet sample. The spreading radiuses of the three coatings are significantly less than that of the bare sample, indicating a strong anti-adhesion performance, in which the h-BN coating is the best. Spreading dynamic analysis of the bare and h-BN coating samples reveals that the spreading dynamics of both sample surfaces is driven by a combination of reaction, dissolution and diffusion. The inert filler in the coating makes the reaction and dissolution of molten ash on the coating surface weaker than those of the steel sheet. The micromorphology of the melt ash-sample sheet cross-section shows that the melt ash binds closely to the steel sheet sample oxide layer, whereas the boundary with the coating is clear and obvious. This result indicates that the coating can effectively weaken the wetting and permeability of the molten ash. The research results provide new ideas for precursor ceramic coating to solve the high-temperature melting ash adhesion problem of the heat exchanger surface.

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.

Ash-Decorated and Ash-Painted Soot from Residual and Distillate-Fuel Combustion in Four Marine Engines and One Aviation Engine.

Soot is typically the dominant component of the nonvolatile particles emitted from internal combustion engines. Although soot is primarily composed of carbon, its chemistry, toxicity, and oxidation rates may be strongly influenced by internally mixed inorganic metal compounds (ash). Here, we describe the detailed microstructure of ash internally mixed with soot from four marine engines and one aviation engine. The engines were operated on different fuels and lubrication oils; the fuels included four residual fuels and five distillate fuels such as diesel, natural gas, and Jet A-1. Using annular-dark-field scanning transmission electron microscopy (ADF-STEM), we observed that ash may occur either as distinct nodules on the soot particle (decorated) or as continuous streaks (painted). Both structures may exist within a single particle. Decorated soot was observed for both distillate and residual fuels and contained elements associated with either the fuel (V, Ni, Fe, S) or with the lubrication oil (Zn, Ca, P). Painted soot was observed only for residual-fuel soot, and only contained elements associated with the fuel. Additional composition measurements by inductively coupled plasma mass spectrometry (ICP-MS) of filter samples indicated that the internal mixing trends of ash with soot were consistent with the overall ash-to-carbon ratio of the sampled combustion aerosols. Painted soot may form when molten ash coagulates with or condenses onto soot within engines.

Effect of the ash melting behavior of a corn straw pellet on its heat and mass transfer characteristics and combustion rate

The effect of ash melting behaviors on the characteristics of heat and mass transfer and combustion rate for the corn straw pellet were investigated by experiment and simulation methods. Considering that the fusion temperature range of ash is from 1418 K to 1558 K, four temperatures (1193 K, 1326 K, 1465 K, and 1618 K) were chosen as the operating temperatures. Results show that the molten ash exists as the spherical particles. At operating temperature between the initial deformation temperature and flow temperature, the spherical particles sharply increase, which contributes to the increase in oxygen concentration at the pellet surface, but limits that inside the pellet. Molten ash delays the char reaction. The tendency reaches the maximum at this temperature range. When the operating temperature is higher than flow temperature, the molten ash peels off from the pellet surface, which exposes the part of the pellet covered by molten ash to the environment directly, thereby accelerating the increase in oxygen concentration inside the pellet.

High-temperature pyrolysis of biomass pellets: The effect of ash melting on the structure of the char residue

The formation of ash during pyrolysis of biomass pellets (corn straw and rice husk) was studied, at high temperature conditions where the formation of tars is minimized. In particular, the influences of the melting ash on the physical and chemical properties of the biomass chars were examined. Cylindrical pellets (9 by 19 mm) were pyrolyzed at three temperatures (1200, 1300, and 1400 °C) and three resident times (10, 20, and 30 min), at moderately high heating rates. The biomass behavior during pyrolysis and the properties of the formed chars were assessed by a variety of experimental techniques. Results show that the char yields decreased with increasing pyrolysis temperature and increasing residence time. The char yields from rice husk were significantly higher than those from corn straw. The ash on the surface of corn straw pellets melted and polymerized, and balls of ash formed and agglomerated, especially at the highest temperature. Molten ash of corn straw contained Ca, O, Al, Mg and Si, forming mainly CaAl2Si2O8, quartz, and cristobalite. In contrast, the surface morphology of rice husk pellets remained intact and the molten ash on the surface of chars consisted mainly of quartz and cristobalite. In the case of corn straw, the melting ash blocked the pore structure and decreased the accessible surface area. To the contrary, in the case of rice husk, the effect of ash melting on its pore structure was negligible and the accessible surface area increased upon pyrolysis. As the pyrolysis temperature increased from 1200 °C to 1400 °C, the carbon structure of the biomass chars became more orderly. Overall, increasing the pyrolysis temperature and duration increased the amount of pyrolysis gas, decreased the char yields and decreased the extent of ash melting and polymerization on the surface of the chars.

Chemical fractionation of inorganic constituents in entrained flow gasification of slurry from straw pyrolysis

Pressurized entrained-flow gasification (PEFG) of straw biomass is currently being studied as a potentially sustainable and economically viable process to produce fuels and other vital chemicals. In the process chain the gasification is integrated and straw is converted via pyrolysis into a bioslurry consisting of a liquid, tar-rich phase and char. Afterwards, the slurry is gasified into a tar-free, low-methane syngas which is a basic reactant for the synthesis of biofuels. At the high temperatures over 1200 °C the ash constituents of the char in the bioslurry melt and flow down the inner wall as slag. The slag viscosity has to be in a certain range to form a protective layer at the reactor wall and to guarantee a continuous removing. For this reason, the composition of the molten ash at the reactor wall has to be well known. Due to several fractionation processes in the gasifier the composition of the slag at the reactor wall does not correspond directly with the slurry ash. Therefore, experiments were conducted to identify depletion and enrichment processes in the gasifier. Finally, the composition of the slag at the reactor wall is obtained and can be used for the adjustment of the viscosity.

Alleviation of thermal corrosion caused by molten ash on heat-exchange tubes in MSW incinerators: Effects of Ni-, Co-, Fe-based HVOF coatings

Thermal corrosion on heat-exchange tubes had an adverse effect on the economical and safe operation of municipal solid waste (MSW) incinerators. In order to alleviate the corrosion caused by molten ash, high velocity oxygen fuel (HVOF) thermal spraying technology was considered for its ability of enhancing corrosion resistance. In this study, 10 typical Ni-, Co-, Fe-based coatings were employed to find the relationship between material composition and high temperature corrosion resistance. The MSW incineration ash adhesion tests at 873 K were performed to specially simulate the corrosion caused by molten ash on tubes. Besides, mass gain measurements under the exposure of Na/K chlorides and sulfates were also performed to assess the failure of tubes. From experimental results, the strong positive correlation between MSW incineration ash adhesion force and mass gain rate proved the feasibility of using the former as an in-furnace corrosion indicator. Fe-based coatings with Fe accounting for up to 80 wt.%, rather than common coatings with Fe content of ca.50 wt.%, had better performance against thermal corrosion. For Ni/Co-based coatings, the optimal content of ternary components, including body metal (Ni/Co), eutectic metal (Cr) and the other strengthening elements (Mo+W+Si+B, et al.), seemed to be 60–20–20 wt.%. The mutual allocation of three kinds of elements, rather than the content of one specific element, affected its MSW incineration ash adhesion tendency. The microstructure characterization indicated the prevented permeation of corrosive species containing Na, K, S, Cl by the presence of strengthening elements such as Mo. Further comparison of NiCr and NiCrMo coatings explored the effect of Mo reinforcing the coating microstructure free from cracking, besides the widely known effect of forming protective oxidation film. This investigation provided insight for the corrosion resistance in MSW incinerators of harsh operating conditions.

Influence of biomass ash additive on fusion characteristics of high-silicon-aluminum coal ash

The influence of cow dung ash (CDA) additive on the ash fusion characteristics of high-silicon-aluminum coal ash (SHA) was investigated using ash fusion temperature analyzer (AFTA), X-ray diffraction (XRD), thermogravimetric analyzer (TG-DSC) and FactSage thermodynamics software. In order to clarify the shrinkage/expansion and flow behavior of ash, high-temperature heating stage coupled with optical microscope (HSOM) was used to record the morphological change of ash in the heating process. The results show that the ash fusion temperatures (AFTs) decreased firstly and then increased with the addition of CDA. There was a minimum value of AFTs when the addition amount of CDA was 60%. Due to the high content of silicon and aluminum in SHA, the mullite with skeleton structure became the main crystalline phase in molten ash, which led to the high AFTs of SHA. The addition of CDA resulted in the increase of calcium and phosphorus in coal ash. Consequently, low-melting-temperature minerals such as anorthite and chlorapatite, which were responsible for the decrease in AFTs of blended ash, would be formed. With the continuous addition of CDA, the decrease of amorphous content led to the increase of AFTs. According to the results of ash fusion characteristics, SHA melting process only underwent the sintering stage due to the high AFTs, and its ash fusion behavior followed “softening-melting” mechanism. For CDA and blended ash (60% CDA), the melting process went through the sintering and melting stages, and the ash fusion behavior followed the “melting-dissolution” mechanism.

A critical review of ash slagging mechanisms and viscosity measurement for low-rank coal and bio-slags

Gasification or combustion of coal and biomass is the most important form of power generation today. However, the use of coal/biomass at high temperatures has an inherent problem related to the ash generated. The formation of ash leads to a problematic phenomenon called slagging. Slagging is the accumulation of molten ash on the walls of the furnace, gasifier, or boiler and is detrimental as it reduces the heat transfer rate, and the combustion/gasification rate of unburnt carbon, causes mechanical failure, high-temperature corrosion and on occasions, superheater explosions. To improve the gasifier/combustor facility, it is very important to understand the key ash properties, slag characteristics, viscosity and critical viscosity temperature. This paper reviews the content, compositions, and melting characteristics of ashes in differently ranked coal and biomass, and discusses the formation mechanism, characteristics, and structure of slag. In particular, this paper focuses on low-rank coal and biomass that have been receiving increased attention recently. Besides, it reviews the available methodologies and formulae for slag viscosity measurement/prediction and summarizes the current limitations and potential applications. Moreover, it discusses the slagging behavior of different ranks of coal and biomass by examining the applicability of the current viscosity measurement methods to these fuels, and the viscosity prediction models and factors that affect the slag viscosity. This review shows that the existing viscosity models and slagging indices can only satisfactorily predict the viscosity and slagging propensity of high-rank coals but cannot predict the slagging propensity and slag viscosity of low-rank coal, and especially biomass ashes, even if they are limited to a particular composition only. Thus, there is a critical need for the development of an index, or a model or even a measurement method, which can predict/measure the slagging propensity and slag viscosity correctly for all low-rank coal and biomass ashes.