Ash Deposition Behavior Research Articles

Dynamic CFD modeling evaluation of ash deposition behavior and morphology evolution with different tube arrangements

A comprehensive CFD model is presented to investigate the deposition behaviors of ash particles with high AAEM contents. The ash deposition behaviors of deposited particles are described via the melt fraction sticking model, the critical impact velocity model coupled with the elastic–plastic deformation theory, and the critical wall shear velocity model. The morphology evolution of ash deposits is predicted by the dynamic mesh method. The effects of longitudinal tube spacing, transverse tube spacing and tube geometry are studied. The oscillatory turbulent vortices, ash particle kinetic energy, and tube arrangement have a combined effect on the fates of impacting ash particles. A large S1/D has the tendency to enhance the accumulation of ash deposits on the rear rows of tube bundles. The peak of the ash deposits on the windward side is higher at a small S2/D. Elliptical tube bundles are practically promising and feasible applications due to the reduction of vortex shedding and the alleviation of ash deposit formation.

Numerical simulation of ash deposition behavior with a novel erosion model using dynamic mesh

In this paper, a piecewise sticking model that considers both particle viscosity and energy conservation upon collision was presented to predict the development of deposition on a temperature-controlled probe. Furthermore, a novel erosion model which takes into account both the energy consumed during particle impact and the melt fraction of the deposition surface was introduced to improve the accuracy of the simulation. The erosion model in this work is applicable in all stages of deposition growth of common coal ash. The influence of the furnace temperature was also investigated by setting up three cases (1473 K, 1523 K, and 1573 K). The predicted characteristics were compared with experimental data in a 300 kW furnace. The results show that the mass of impact decreases while the deposition efficiency increases with the increase of deposition time. Compared with the simulation without erosion, the relative deviation reduces from 41.61% to 12.88% for thickness and from 3.39% to 2.35% for surface temperature with the erosion model. Additionally, Higher furnace temperatures lead to higher deposition efficiency and higher surface temperature. However, high temperatures may also be a factor impeding the deposit particles to accumulate.

Effects of leaching and additives on the formation of deposits on the heating surface during high-Na/Ca Zhundong coal combustion

Zhundong coal, high-quality coal with large reserves in China, faces great challenges on its large-scale utilization in utility boilers due to the ash-related problems caused by high alkali and alkali earth metals (AAEMs). In this research, to simulate ash deposition behavior on heating surfaces in high-temperature flue gas zone (panel superheater) and that in medium-temperature flue gas zone (superheater or reheater), deposition experiments were carried out during Zhundong coal combustion in a drop tube furnace (DTF) at the deposition temperature of 1200 °C and 900 °C. Meanwhile, water leaching, Na2CO3 additive and kaolin additive were employed to elucidate the formation mechanism of ash deposition and alleviate deposition and slagging. Results showed that leaching reduced the contents of troublesome elements such as AAEMs and S and increased the relative contents of Si and Al, thus alleviating ash deposition. Compared with raw coal, the deposition rate of leached coal decreased from 5.82 mg h−1 cm−2 to 0.88 mg h−1 cm−2 at the deposition temperature of 900 °C. The primary reason for the decreased deposition rate might be that the dominant minerals in leached coal ash were refractory SiO2, CaO and Ca2SiO4. The addition of Na2CO3 would aggravate the slagging and fouling on the deposition surface significantly. The gaseous Na2O decomposed from Na2CO3 reacted with SO2 and SiO2 in the flue gas to form low-melting Na2SO4 and Na2Si3O7, which were easy to cause severe slagging on the deposition surface. The inhibition effect of kaolin addition on the formation of deposition and slagging was similar to that of leaching. After the addition of kaolin, the dominant compounds of fly ash altering to high-melting minerals such as CaSiO3, CaSO4, Ca2MgSi2O7 and NaCaAlSi2O7, resulting in a lower deposition rate than raw coal. Besides, when deposition temperature increased from 900 °C to 1200 °C the slagging degree of deposition surfaces was aggravated, while the deposition rate of four fuels followed different change trends. Specifically, the deposition rate of raw coal and leached coal decreased, whereas that of the case with Na additive and kaolin additive presented an increasing trend. At last, the role of water leaching and additives on the formation of ash deposition during Zhundong coal combustion was clarified.

Exploration in ash-deposition (AD) behavior modification of low-rank coal by manure addition

High-alkali low-rank coal aggravates ash deposition (AD) during conversion. To mitigate AD formation during its gasification, the AD characteristics of Xiaolongtan lignite (XLT) and Yima coal (YM), their variations from chicken or pig manure addition under a reducing atmosphere, and their mechanisms were investigated by using ash fusion temperature tester, self-made AD analyzer, and X-ray diffractometer. The results indicated that the mass fraction of AD (MAD) of XLT was higher than that of YM because of the XLT with a high Ca, Fe, Na, and K content. At a low temperature (<950 °C), MAD increased with an increasing content of Na, K, and Fe due to their low melting-point eutectic formations; the MAD decreased with an increase in ash fusion temperature at a high temperature. The MAD of the XLT or YM decreased rapidly (1%–10%) and then decreased slowly (10%–15%) with manure ash addition. The XLT MAD decreased 50.32% for 4.93% chicken manure (CM) addition at 1000 °C. High-P and -Ca CM addition caused the formations of high melting-point Ca3P2O8 and KCaPO4, which inhibited alkali silicate generation and made the MAD decrease. Manure addition can mitigate AD formation effectively during low-rank coal gasification.

Ash deposition during pressurized oxy-fuel combustion of Zhundong coal in a lab-scale fluidized bed

The paper presents the ash deposition behavior during pressurized oxy-fuel fluidized bed combustion (POFBC) of Zhundong coal at 0.3 MPa, which has never been reported before. Ash deposits collected from different positions (windward, leeward and vertical sides) of the probe under different atmospheres were investigated in detail. The results showed that the inside deposits, i.e. the initial deposit layers, on the different positions shared the similar composition and morphology. However, the atmosphere had a significant impact on the ash deposition behavior. The inside deposits have more regular spheres and larger size under 30% O2/70% CO2 condition than those from 21% O2/79% N2 and 21% O2/79% CO2 combustion, probably due to the intensive submicron particle coagulation. Furthermore, the particles contained in the inside deposits had higher Fe and Cl in 30% O2/70% CO2 combustion compared with that in the other two combustion modes. However, the content of S in the deposits was the highest under 21% O2/79% CO2 condition, which may be attributed to the fact that high CO2 partial pressure strengthens self-desulfurization ability of ash. In addition, S and Si contributed significantly to the compositions of the inside deposits.

Effects of oxy-fuel condition on morphology and mineral composition of ash deposit during combustion of Zhundong high-alkali coal

Zhundong coalfield is a super-large coal reserve, with high-alkali feature exacerbating ash deposition. Oxy-fuel combustion technology could propel the clean utilization of Zhundong high-alkali coal. While the ash deposition behavior of high-alkali coal under oxy-fuel condition has yet to be sufficiently investigated. The present study compared the differences of ash deposits between oxy-fuel and air combustion, and also examined the effects of oxygen content on ash deposition mechanism, employing a drop-tube furnace equipped with a specially designed sampling probe and some analysis methods, such as X—ray diffraction equipment, simultaneous thermal analyzer, etc. Experimental results indicated that ash deposition was weaker, with fewer contents of sodium chloride, calcium sulphate and less agglomeration ash in oxy-fuel atmosphere compared to the air case with same oxygen content. The content of the ash particle distributed in the range of 0–40 μm was up to 60% under oxy-fuel condition. The first weight loss of ash deposits, around 850 °C, was put down to the decomposition of carbonate and the second one, about 1150 °C, was ascribed to the decomposition of the sulphate minerals in the thermal process. Ash deposition worsened with more large particles (>120 μm), as the oxygen content rose. Sodium chloride content reached 9.7% with 50% oxygen content. The present study not only focuses on the morphology and chemical components, but also probes into the thermal volatility of ash deposits, which benefits the further understanding of the ash deposition mechanism and utilization of Zhundong high-alkali coal during oxy-fuel combustion.

An experimental investigation into the effect of flue gas recirculation on ash deposition and Na migration behaviour in circulating fluidized bed during combustion of high sodium Zhundong lignite

The effect of flue gas recirculation (FGR) on ash deposition and Na migration in a laboratory-scale circulating fluidized bed (CFB) burning Zhundong lignite was investigated. The CFB dense phase was maintained at temperature 950 °C, whereas that of the furnace exit was adjusted to 730 and 680 °C respectively by manipulating the FGR flow rate. Two ash deposition probes maintained at 550 °C were installed in the furnace chamber (P1) and cyclone outlet (P2). The deposited ashes along with the bottom and fly ash were collected and analysed by using XRD, SEM-EDX and ICP-OES. Results showed that the P1 deposit consisted of sintered Ca/Mg/Na sulphates and aluminosilicates at 730 °C, but became granular Na2SO4/CaSO4 at 680 °C. The P2 deposit and fly ash were sintered and rich in CaSO4 and Na2SO4 at 730 °C, but did not show signs of sintering at 680 °C. The bottom ash was enriched in CaSiO3 and NaAlSi3O8 at 730 °C, but were dominated by Na6(AlSiO4)6 at 680 °C. Na in the bottom ash increased from 31.7 to 35.2 mg g−1 as FGR flow rate increased, leading to severe bed particle agglomeration consequently. The FGR thus exerted a trade-off effect on the sintering and deposition propensity of fly ash and agglomeration of bed materials.

Dynamic simulation on ash deposition and heat transfer behavior on a staggered tube bundle under high-temperature conditions

In this paper, a predictive tool was developed to simulate the formation of ash deposit on tube bundles under high-temperature conditions based on ANSYS FLUENT platform. The effects of the factors on ash deposition and heat transfer performance were investigated, including the morphology variation, wall temperature, transverse pitch, and longitude pitch of the tube bundle. It was found that impact mass flux on the tube bundle was significantly reduced by the changed morphology of the tubes during the growth of ash deposit. The final thermal efficiency of the tube bundle increased with the increase of the wall temperatures of the tubes. Furthermore, the thermal efficiency of the tube bundle decreased from 0.74 to 0.65 when the ratio of the transverse pitch and tube diameter increased from 1.58 to 2.63. However, it was slightly affected by the change of the longitude pitch with the same change as the transverse pitch. The results indicate that it will be an effective way to reduce ash deposition by changing the shape of the tubes with considering the temperature-dependent thermal conductivity of ash deposit. Additionally, a small transverse pitch for the tube bundle with a proper longitude pitch is recommended for this simulation condition.

Comparison Study of Ash Partitioning and Deposition Behavior between Two Rice Husk Fuels under a 100 kW Combustor

Utilization of biomass when combined with carbon capture and sequestration (CCS) is one feasible solution for “bioenergy with carbon capture and storage” (BECCS), which can reduce the CO2 emissions in the atmosphere. Rice husk is a unique biomass resource because its ash content is typically higher than 15% and more than 88% of this mineral matter is composed of silica. Ash partitioning and deposition mechanisms in rice husk combustion are still under investigation and will be the focus of this paper. Two different rice husk resources were tested in this work: one is from China, and another is from the United States, which are abbreviated as RH_CN and RH_US, respectively. The experimental work was performed in a 100 kW down-fired oxy-fuel combustor (OFC), and both fuels were co-fired with natural gas to achieve combustion conditions that are comparable to coal combustion. Two conditions were tested: (1) air combustion and (2) oxy-combustion with 70% O2 and 30% CO2 in oxidant gas (denoted as OXY70). Result...

Simulating growth of ash deposit in boiler heat exchanger tube based on CFD dynamic mesh technique

Ash deposition on heat transfer surfaces is still a significant problem in existing conventional utility boilers. Understanding the formation of ash deposit is thereby very important, which is helpful to optimize the design and operation procedure of boilers. In this study, a dynamic numerical method based on the commercial software ANSYS FLUENT was developed to predict the growth of ash deposit on the deposition probe in a 300 kW slagging test furnace fired with Zhundong coal. Based on the CFD dynamic mesh technique, both shape variations and surface temperature variations of ash deposit with time were predicted. Time-dependent deposition behaviors on the deposition probe with different initial metal wall temperatures were presented. The simulation results (e.g. thickness, heat flux, surface temperature, and shape of ash deposit) are in good agreement with the experimental data. It shows that increased surface temperature of ash deposit can significantly increase ash deposition mass. It was also determined that the shape variation of ash deposit can obviously reduce the impact mass and deposition mass during ash deposition by the comparative studies. The developed simulation method is proved to be a useful tool to predict ash deposition behavior with more detailed information.

Influence of Fine Fuel Particles on Ash Deposition in Industrial-Scale Biomass Combustion: Experiments and Computational Fluid Dynamics Modeling

This work focuses on the contribution of fuel particles with diameters smaller than 1 mm on the ash deposition behavior of different kinds of biomass feedstock. Special emphasis lies on their impac...

Experimental study on ash deposition of Zhundong coal in oxy-fuel combustion

To facilitate the large-scale utilization of high-alkali and -alkaline earth metals (AAEMs) coals in power generation, the ash deposition behaviors of a typical Zhundong coal in oxy-fuel combustion were experimentally investigated using a drop tube furnace. A wall-temperature-controlled ash deposition probe by which the bulk gas temperature could be measured simultaneously was designed and employed in the experiments. The deposition tendencies, ash morphologies, chemical compositions of deposited ash particles were studied respectively under various oxygen concentrations, bulk gas temperatures, probe surface temperatures and probe exposure times. The experimental results revealed that the oxygen concentration had a significant influence on the deposition behavior during oxy-fuel combustion of high-alkali coal. Compared with air case, more fine ash particles were generated during the combustion of Zhundong coal in 21% O2/79% CO2 atmosphere but the deposition tendency was weaker. However, a higher oxygen concentration could aggravate the tendency of ash deposition. The high contents of iron (Fe), calcium (Ca), sulfur (S), and sodium (Na) in Zhundong coal could result in the generations of low-melting point compounds. Calcium in flue gas existed as CaO and was captured prior to SO3 by the probe surface during the ash deposition process. At the initial 30 min of the ash deposition process, the dark spherical fine ash particles rich in Fe, Na, oxygen (O), and S were largely produced, while in the range of 60–90 min the light spherical fine ash particles with high contents of Ca, barium (Ba), O, and S were generated on the other hand. The deposition mechanisms at different stages were different and the melted CaO (BaO)/CaSO4 (BaSO4) would give rise to a fast growth rate of ash deposit.

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.

Ash deposition behavior in co-combusting high-alkali coal and bituminous coal in a circulating fluidized bed

High contents of alkali and alkali earth metals (AAEMs) induce severe ash slagging and fouling during coal combustion. In this regard, co-combustion is an effective method to mitigate ash deposition. The ash deposition behavior in co-combusting high-alkali Zhundong (ZD) coal and a bituminous coal, i.e. Shenhua (SH) coal was tested in a 30 kW circulating fluidized bed test system. The results revealed that the condensation of Na2SO4 triggered the slag formation of ZD coal in the furnace. The Na-Ca-Fe eutectics were responsible for the melting or partial melting of slags at high temperatures. After blending SH coal with ZD coal, the volatilization of Na was effectively suppressed, and compacted slags practically disappeared at 20% blending ratio of SH coal. In the convective backpass, fine particles (<10 µm) tended to be deposited on the leeward side of the heating surface because of the eddy impaction. As the blending ratio of SH coal increased, the size distribution of fly ash shifted to a large size range, because of the dilution effect and capture of AAEMs by silicates or aluminosilicates. The minimum blending ratio of SH coal was recommended as 20% to mitigate the ash deposition tendency of ZD coal.

Ash deposition behaviors during combustion of raw and water washed biomass fuels

Biomass-fired boilers have the tendency to suffer from severe problems of fouling and slagging due to the high potassium content of biomass fuel. The troublesome potassium, however, can be removed efficiently by water washing pretreatment. In this study, the ash deposition behaviors during combustion of raw and water washed biomass fuels were investigated by a one-dimensional furnace and a deposition probe. Two biomass fuels (corn stalk and wheat straw) were used, and deposition mass, deposition efficiency, composition and morphology of the deposit were studied. The ash deposition while firing raw biomass exhibits a “fast−slow−fast−slow” trend with the sampling time. After water washing, the deposition mass decreases dramatically, and the deposition efficiency reduces gradually as the sampling time increases. The analyses of elemental composition, morphology and chemical composition on the deposit from raw biomass imply that the condensation/thermophoresis is quite significant in the earlier deposition stage, whereas the chemical reaction is remarkable in the later stage. After water washing, the potassium content of the deposit decreases significantly. Morphology and chemical composition analyses indicate that the deposit from water washed biomass ascribes to the physical accumulation of non-viscous fly ash particles. The deposition mass can easily approach a maximum value. The ash fusion temperatures of deposits increase remarkably after water washing. In addition, ash deposition mechanisms during biomass combustion are discussed.

Ash Deposition in Air-Blown Gasification of Peat and Woody Biomass in a Fluidized-Bed Gasifier

The ash deposition behaviors in air-blown gasification of a woody biomass and a Canadian peat were studied using a pilot-scale bubbling fluidized bed gasifier operating at an equivalence ratio (ER) of 0.2 to 0.35. An air-cooled probe was installed in the furnace of the gasifier to collect ash deposits on its surface. Four different bed materials including olivine, dolomite, limestone, and iron ore were used in this study to compare their effects on ash deposition rate during the gasification. The experimental results demonstrated that among the four bed materials, the use of olivine resulted in the lowest ash deposition rate <1.0 g–2 h–1, compared with ∼16 g–2 h–1 for limestone in the gasification of pine sawdust at an ER of 0.35. The superb performance of olivine in retarding ash deposition could be accounted for by its outstanding thermal stability and mechanical strength. The other three bed materials, in particular, limestone, were fragile during the fluidized bed gasification, and the fractured fines...

Ash deposition behavior of a high-alkali coal in circulating fluidized bed combustion at different bed temperatures and the effect of kaolin.

High alkali and alkali earth metals (AAEMs) content in coal causes severe slagging and fouling during combustion in a boiler. In this study, the ash deposition behavior of a high-alkali coal at different bed temperatures and the effect of kaolin were investigated in a 30 kW circulating fluidized bed (CFB) test system using an ash slagging probe and deposition probe. The results show that the ash deposition tendency increases with the bed temperature. The condensation of Na2SO4 is an important inducement for slag formation in the furnace. The melting or partial melting of slags is attributed to Na–Fe–Ca eutectics. At 920 °C, Na2SO4 will react with CaSO4 to form the low-melting compound of Na2SO4–CaSO4. The deposited ash on the convection-heating surface consists of granular particles. On the windward side, the layered-structure ash deposits, i.e. the inner and outer layers, are formed at the bed temperature of 920 °C but are absent at lower temperatures (820 °C and 870 °C). The formation of the inner layer consists of fine particles (<2 μm) and is closely related to Na2SO4. The size of the deposited ash in the outer layer is larger than 10 μm, while that on the leeward side is less than 10 μm. By adding kaolin in the coal, the slags are replaced by loose particles due to the absorption reactions between kaolin and alkali metals. The ash deposition tendency is improved and the optimal result is achieved when kaolin is added at an addition ratio of 3%.

Multilateral approaches for investigation of particle stickiness of coal ash at low temperature fouling conditions

Particle stickiness is a key parameter for increasing ash deposition in gasification process. We conducted multilateral investigations to evaluate particle stickiness of coal ash at low temperature fouling conditions through Watt and Fereday’s viscosity model, dilatometry (DIL) and laser flash apparatus (LFA) technique. Seventeen coals were employed for ash deposition experiments under gasification condition through drop tube furnace (DTF). The low viscosity not only led to increasing ash deposition behavior, but also increasing the particle size of deposited ash. From DIL analysis, the ash sintering behavior increased with increasing temperature due to increase of particle stickiness. The high amount of Fe2O3, CaO and MgO components resulted in low sintering temperature and high reduction of physical length. Through LFA analysis, the thermal conductivity increased with increasing temperature, because of increasing particle stickiness. In addition, its value was correlated with the propensity of common fouling indices.

Experimental investigation of ash deposition behaviour modification of straws by lignite addition

The ash-deposition (AD) characteristics of straws (corn stalk [CS], wheat straw [WS], cotton stems [CCS], and soybean stalk [SS]), and the effects of lignite (Husheng [HS], Huolinhe [HLH]) on the characteristics were investigated. Under the same conditions, the AD mass ratio (Mad) decreases as WS>CS>CCS>SS, and the Mad of the four straws decreases more obviously with HS addition than that for HLH addition. The Mad of SS and CSS increases slowly before 1000°C and then increases rapidly; whereas for CS and WS, the change occurs at 950°C. The differences result mostly from the higher potassium-oxide contents in CS (26.34%) and WS (30.04%) over those in CCS (11.63%) and SS (10.25%). Ca2+ replaces K+ in semi-molten alumino-silicate (orthoclase and leucite) and results in the formation of a high melting-point anorthite. This, together with the generation of high melting-point mullite, leads to a decrease in Mad of the mixed ashes with an increase in lignite mass ratio. The position changes of the ash composition in the ternary phase diagrams and their variation in liquid-phase content with increasing temperature based on the FactSage software calculation may explain the variation in Mad.

Ash deposition behavior under coal and wood co-firing conditions in a 300 kW downfired furnace

Growth of ash deposits when wood was co-fired with coal was visually investigated in a 300 kW pilot-scale furnace. For comparison, combustion of pure coal was also conducted. A total of 10% and 20% wood were mixed with coal. The thickness and heat flux were obtained. The collected ash deposits and fly ash were characterized by a series of analysis methods to determine the physical and mineral properties. Their relationships were also revealed. Results showed that co-firing of coal with wood dramatically increased the ash deposition propensity. During the coal combustion, shedding of ash deposit occurred and the maximum deposit thickness was 15.33 mm. A deposit thickness of up to 27.02 mm was achieved for 10% wood, and the thickness increased to 34.20 mm for 20% wood. The variation in heat flux with deposit thickness substantially changed when wood was co-fired. A significant change was also observed in the mineral composition of ash deposit with the increase in wood ratio. The proportion of anorthite increased because that of lime in fly ash increased with wood ratio. In addition, the mean diameter of fly ash particles increased as wood ratio increased.