Ash Deposition Characteristics Research Articles

Numerical simulation of ash deposition behavior in radiant syngas cooler coupled with impact surface temperature

Radiant syngas cooler (RSC) is a crucial heat recovery device for entrained-flow coal gasification technology. However, ash deposition in an RSC can severely affect operating efficiency and safety. An ash deposition model coupled with the impact surface temperature was established based on the ash particle deposition morphology. The effects of different operating conditions on ash deposition characteristics are discussed. The simulated data agreed well with the industrial data. The results indicate that ash particles were primarily deposited in the collision deposition zone. The maximum deposition thickness of radiation screen and cylinder membrane wall is 9.82 mm and 1.88 mm, respectively. The ash deposition hindered heat transfer process and the highest heat flux was observed in the middle part of the RSC. Ash deposition around 5.3 m increases significantly with an increase in the inlet syngas temperature. Increasing the load increased probability of ash particle collision and the area of influence.

Investigation of Ash Deposition Characteristics in Full-Scale Circulating Fluidized Bed Boiler Burning Zhundong Coal

ABSTRACT Due to the high content of alkali and alkaline earth metals (AAEMs) in Zhundong coal, serious ash-related problems such as slagging and fouling occurred in the process of utilization, which has attracted wide attention. In this work, the ash deposition characteristics in a new type circulating fluidized bed boiler burning Zhundong coal was studied. The effect of steam cooled flue on alleviating slagging problems was evaluated, and the influence of flue gas temperature and wall temperature on ash deposition characteristics were studied. The results showed that the mass of ash deposition and the concentration of submicron particles decrease gradually along the direction of flue gas flow in the steam cooled flue. The deposited ash changed from tightly bound to wall to loosely deposited, which indicated that the arrangement of the steam cooled flue greatly alleviated the slagging problems. Compared with flue gas temperature, the wall temperature had a greater impact on ash deposition characteristics. Mineral elements such as Na, S, Fe and Mg were more likely to enrich on the high temperature wall, while the Ca showed the opposite trend. On the low temperature wall, Na existed in the form of Na2SO4 and Na2Si2O5, while on the high temperature wall, the direct condensation of alkali metal vapor was prevented and the thermophoresis disappeared, so Na mainly existed in the form of Na2Si2O5.

Investigating the thermal-hydraulic enhancement and ash deposition characteristics of leeward-cut spiral finned tube heat exchangers

The spiral finned tube (SFT) heat exchanger is a vital component in industrial boilers for energy utilization and recovery. To enhance its efficiency and reduce material usage, a leeward-cut spiral finned tube (LSFT) structure is proposed. In this study, LSFTs with different cutting angles are numerically simulated within the range of Re = 6800–10,850 using a CFD model, which is later validated using a self-developed experimental platform. The discrete phase model (DPM) coupled with the van Beek collision model is utilized to depict the ash deposition characteristics with different particle sizes on LSFTs. The results demonstrate that the LSFT outperforms SFT. The 60° cutting angle LSFT, for example, the Nu improves by 12% and the deposition density decreases 28.3% at Re = 9500 compared to SFT. The light gradient boosting machine (LightGBM) model and multi-objective particle swarm optimization (PSO) algorithm are adopted to optimize the design parameters including longitude and transversal tube pitch (sL, sT), fin high and pitch (Ph, Pf), cutting angle (θ). Compared with the original designed SFT, energy efficiency index JF increase by 15.1%–23.6% at various Re when using the optimized LSFT. Besides, the deposition density of particles in optimized LSFT decreases by 63.6%, while the overall weight lessens by 21%.

Laboratory Study on Adhesive Ash Deposition Characteristics of Ammonium Bisulfate in Conditions Simulating an Air Preheater for Hard Coal Combustion

The ash blockage of the rotary air preheater is a serious problem of the coal-fired boiler that urgently needs to be solved, which is caused by the adhesive deposition of ammonium bisulfate (ABS) and the fly ash. A comprehensive experimental study was performed to investigate the adhesive ash deposition characteristics based on an experimental platform established. The influences of the gas temperature, the gas velocity, the mass ratio of the ABS to the fly ash (R), and the ash particle size on the ash deposition characteristics were mainly analyzed and discussed under different conditions. The experimental results indicate that the liquid ABS is the root cause of the ash particles adhering to the heat transfer elements of the air preheater. The experimental results indicate that when the gas temperature is in the range of 420–493 K, the ABS ash deposition intensity and the ABS adhesion rate both increase with the increase in the gas temperature. When it is 493 K, the ABS adhesion rates of the corrugated plate and the positioning plate both reach maximum values, which are 31.7% and 27.9%, respectively. With the decrease in gas velocity, the total ash deposition intensity, the ABS ash deposition intensity, the ABS adhesion rate, and the growth rate of the ABS adhesion all increase. The content of ABS in the fly ash is also an important factor. When R rises, the ash deposition intensity and the ABS adhesion rate increase significantly. The particle size of the fly ash has little influence on the total ash deposition intensity, but has a great influence on the ABS ash deposition intensity and the ABS adhesion rate. With the increase in the particle size in the range of 30.8–100 μm, the ABS ash deposition intensity decreases by nearly 50%, and the ABS adhesion rates of plates A and B decrease by about 43.9% and 49.6%, respectively. According to the study results, some effective measures can be taken to solve the ash blocking problem of the rotary air preheater, including using the steam air heater, optimizing the operation parameters of the soot blower, and inhibiting ABS formation.

A Numerical Study of Ash Deposition Characteristics in a 660MW Supercritical Tangential Boiler

AbstractWhen the boiler is operated at different loads, the characteristics of ash deposition in the boiler will change accordingly. In this paper, a 660MW tangentially fired power boiler is taken as the research object. The combustion characteristics and ash deposition on the boiler walls are studied by numerical simulation. The results show that: 1) In the middle and lower part of the furnace wall, more particles are deposited, the temperature is higher and the CO concentration is higher, so slagging is easy to occur. 2) The ash accumulation in the bottom hopper mainly comes from the bottom two layers of burners for >98%. 3) The deposition ratio is negatively correlated with the height of the burner. 4) With the particle diameter increasing, the particle deposition from the lower burner increases significantly. Selecting pulverized coal with particle size of 65 µm can reduce the risk of ash deposition.

Correction to “Ash deposition and heat transfer characteristics on the convective heating surface of calcium‐rich Zhundong coal combustion in a 0.4‐MWth facility”

Correction to “Ash deposition and heat transfer characteristics on the convective heating surface of calcium‐rich Zhundong coal combustion in a 0.4‐MWth facility”

Effect of the Coal Fly Ash Blending Ratio on Biomass Slagging Structure Modification and Alkali Metal Migration.

The analysis of ash deposition samples by electron microscopy (EM), scanning electron microscopy (SEM)/energy dispersive scanning (EDS), and X-ray diffraction (XRD) was carried out at three sampling points of a one-dimensional furnace (sampling temperatures were TL5 = 960 °C, TL6 = 855 °C, and TL7 = 820 °C, respectively). The improvement of ash deposition characteristics of biomass combustion with the different addition ratios of coal fly ash (CFA) (Mr = 1:1, 1.5:1 or 2:1) and the heterogeneous removal of gas phase alkali metal heterogeneous were studied. The results showed that under the condition of Mr = 1:1, the ash sample presented a three-dimensional porous loose structure like "coral reef" with the rice husk ash as the skeleton and the combustion products of CFA as the bonding point. The spherical and filamentous crystals mainly composed of SiO2, Al2O3, and silicates were distributed in the slag, and the surfaces of these crystals were accompanied by the eutectic composite salt of K-Al-Si with a high melting point, reflecting the heterogeneous curing effect of CFA on the gas phase migration of the alkali metal (K). With the decreasing addition ratio of CFA (Mr = 1.5:1 or 2:1), the spherical and filamentous crystals were increased in the slag. MgO, Fe2O3, and Al2O3 may be involved in the formation of high-temperature refractory Mg-Fe-aluminate, Ca- and Mg-silicate, which were the precursors of the rigid slag. The alkali metals mostly presented in the form of K2O and KCl in the gas-solid conversion under the temperature effect, serving as binders for bridging, agglomeration, and slagging of the above precursors. In addition, the kalium enrichment was accompanied by Al and S in the CFA, indicating that the Al- and S-active sites have obvious adsorption to kalium.

Experimental study on the ash deposition and heat transfer characteristics of a heat pipe air preheater

In this paper, a scaled-down air preheater for a heat pipe is prepared and a visualization experiment system for ash deposition is established. The local and global characteristics of ash deposition and the heat transfer performance of the air preheater are investigated, and the effects of parameters including the gas velocity and the ash concentration are discussed. Experimental results show that a loose ash deposition is formed on the fin of the heat pipe with test time. The ash deposition mass has a maximum at Row #2 and a secondary peak at Row #6 due to the enhanced flow disturbance. With increasing gas velocity, the mass of ash deposition on the windward side gradually decreases, but there is a mass peak on the leeward side. The effect of gas velocity on the mass of ash deposition in the heat pipe fins is severe on the windward side. When the gas velocity reaches or exceeds 10 m/s, the ash deposition mass on the fin is mainly concentrated on the leeward side (>80.1%). Furthermore, as the gas velocity increases or the ash concentration decreases, the total ash deposition mass in the preheater decreases, and the overall heat transfer coefficient increases. The descending range of the heat transfer coefficient over the test time decreases from 10.0% to 4.3% as the gas velocity increases from 6 m/s to 12 m/s and increases from 3.2% to 7.8% as the ash concentration increases from 23 g/m3 to 38 g/m3.

Investigation of the combustion and ash deposition characteristics of oil palm waste biomasses

AbstractBiomass serves as an alternative energy solution for decarbonizing coal-fired power plants, which have been reactivated in several countries due to the global energy crisis. Oil palm waste, owing to its abundant availability, holds significant potential as a biomass fuel. This study aimed to investigate the combustion performance of various oil palm wastes in comparison to coal. Biomass combustion is associated with ash-related problems such as slagging, fouling, and corrosion, which may accelerate ash deposit acceleration, reduce heat transfer, and damage refractory equipment in boilers. Ash-related problems were evaluated using the method commonly adopted for solid fuel, including experimental drop tube furnace combustion and ash observation. The results indicate that each oil palm waste has different combustion characteristics. Palm leaves, empty fruit bunch, and palm fronds with clean probe observation have a relatively low tendency of slagging and fouling and can be recommended as biomass fuel for co-firing. However, their high alkali and iron contents need to be considered. Palm fiber has similar combustion characteristics to coal, but it has a high slagging and fouling tendencies. The palm stems with high chlorine content have a high corrosion tendency confirmed by probe observation, scanning electron microscopy, and X-ray diffraction analyses.

Experimental investigation of combustion and ash deposition characteristics of coal-NH3 cofiring in a 200 kW furnace

The coal-NH3 co-firing experiment was conducted in a 200 kW one-dimensional furnace to study the flue gas emission characteristics and ash deposition characteristics under different ammonia co-firing ratios. As ammonia is a carbon-free fuel, CO2 shows a decreasing trend, while H2O gradually increases with the increase of ammonia co-firing ratio. With the increase of ammonia co-firing ratio, the proportion of unburned carbon increases slightly. Compared with the ammonia in flue gas, the ammonia attached to ash is the main reason for ammonia slip. The total ammonia slip rate of the three co-firing ratio conditions is 0.26% (5% NH3), 0.29% (10% NH3) and 0.67% (20% NH3), respectively. The NOx emission reaches the maximum of 349.05 ppm when the ammonia co-firing ratio is 10%, then decreases as the ratio increase to 20%. Ammonia co-firing does not change the composition of ash particles but has a certain deterioration of heat transfer of heat exchange surface in high temperature zone of furnace. This is related to the adhesion of ammonia on the surface of fly ash and the reduction of the size of fly ash.

Ash deposition characteristics during oxy-fuel combustion of biomass in a drop tube furnace

Ash deposition characteristics during oxy-fuel combustion of biomass in a drop tube furnace

An Experimental Study on SO2 Emission and Ash Deposition Characteristics of High Alkali Red Mud under Large Proportional Co-Combustion Conditions in Fluidized Bed

As an industrial solid waste, the discharge of a large amount of red mud (RM) causes serious environmental problems; thus, a large proportion of RM co-combustion has been proposed to solve the consumption problem. In this paper, an experiment with various proportions of RM co-combustion was conducted on a 0.2 t/h circulating fluidized bed (CFB) boiler. Desulfurization performance, combustion characteristics, and ash deposition characteristics were analyzed, especially under the large proportional co-combustion conditions. As the study results showed, the desulfurization efficiency was positively correlated with the RM co-combustion proportion. When the RM co-combustion proportion reached 50%, the desulfurization efficiency was over 94%. After a period of cyclic combustion, the highest desulfurization efficiency exceeded 99.5%. The smaller size of RM was beneficial to improve the combustion efficiency and the combustion stability. However, a large area of sintering formed on the top of the heating surface in the furnace, which was lighter than the sintering of high alkali fuels such as Zhundong coal. Meanwhile, the content of sulfates, such as Na2SO4 and CaSO4, in the ash increased, which clearly proves that RM has the desulfurization effect. Therefore, a large proportion of co-combustion could meet the requirements of in-situ desulfurization and realize the resource utilization of RM.

Ash deposition characteristics of flue gas across a tube using the Eulerian multiphase model with dynamic mesh technique

The ash deposition problem majorly affects the stability, safety, and economy of industrial production processes. To investigate the transient ash deposition characteristics of the flue gas flowing across a tube, the Eulerian multiphase model combined with a dynamic mesh technique is first used in this work. The deposition and the removal processes are both taken into account. Four deposition mechanisms are considered in the deposition process, including Brownian and eddy diffusion, gravity, thermophoretic, and turbophoretic forces. Based on the experimental verification results, it is found that the Eulerian multiphase model combined with a dynamic mesh technique can more accurately predict the ash deposition behavior. The overall fouling characteristics are analyzed by the changes in net deposition mass and deposition morphology under seven different working conditions. The results showed that the ash deposits are mainly formed in two regions: β = 165° ∼ 195° and β = 305° ∼ 55°. In addition, one peak is observed in the deposition region on the windward side, and three peaks exist in the deposition region on the leeward side. The periodic shedding regularity of vortices, the separation process of the boundary layer, and the flow patterns of each feature point on the tube surface are studied to investigate the influence of the flow field characteristics on the deposition characteristics. In addition, the causes of the ash deposition in various locations are discussed in detail from the perspective of the deposition mechanism. The findings of this study are beneficial for understanding the ash deposition process and provide an improved approach for ash deposition simulation.

Research on the effects of the fly ash reburning on element migration and ash deposition characteristics of high-alkali coal in a full-scale slag-tapping boiler

Burning high-alkali coals in boilers can cause severe slagging problems and affect boiler safety, which restricts the utilization of high-alkali coals. Some studies have shown that high-alkali coals can be burned well in the laboratory or medium-scale slag-tapping boilers, but few studies have been conducted in a full-scale boiler. The effects of the fly ash reburning on element migration and ash deposition characteristics of high-alkali coal in a full-scale slag-tapping boiler were investigated in this work. The results showed that the primary components in different samples were SiO2, Al2O3, Fe2O3 and CaO. The liquid slag had high Si and Fe contents and low Na content, while the ash deposition showed an opposite trend. Moreover, element Cl was only found in the ash deposition. After the fly ash reburning system was put in, the element content changed greatly. The Si and Al contents increased, and the Fe and Ca contents decreased in the liquid slag and fly ash samples. At the same time, the Na content in #1 liquid slag (LS) and #2 LS increased by 10.6% and 8.86% and decreased by 14.76%, 8.05%, 2.27% and 21.49% at the fifth superheater, the secondary reheater, the primary reheater and the economizer, respectively. Fly ash reburning can enhance the absorption of alkali metals by liquid slag and reduce alkali metals in the flue gas. Microscopic morphology analysis showed that the ash particles had a smooth surface after fly ash reburning. NaCl acted as a binder between ash particles, causing particle aggregation in the ash deposition. Mineral composition analysis indicated that fly ash reburning could affect the mineral composition contents in the ash samples. The sodium in the flue gas mainly existed in the form of NaCl. This work can guide industrial boilers to burn high-alkali coal directly and is beneficial for improving boiler design and optimizing combustion operation.

Visualization research on ash deposition characteristics of Zhundong coal in a vertical liquid slagging cyclone furnace

A 300 kW vertical liquid slagging cyclone furnace is built for the combustion experiment of Hami coal(HM), a kind of Zhundong coal. The sampling probe and CCD camera are used to monitor the growth process of deposition in the furnace online. In addition, the slags at various positions of the furnace are collected and analyzed to study the metal migration law and slagging characteristics of HM during combustion in the furnace. The results show that compared with the traditional furnace, deposition thickness on the probe of the liquid slagging cyclone furnace is much smaller. The stable thickness of the deposition on probes 1 and 2 is 0.85 mm and 3.30 mm respectively. Fe2O3 shows enrichment in liquid slag. The alkali metals in the gas phase condensed on the low-temperature surface of the sampling probe and existed in the initial layer of the deposition. The SO3 reacts with alkaline oxides at low temperature to form viscous compounds such as Na2SO4 and CaSO4. Due to the fast cooling rate, almost no diffraction peaks can be observed in the XRD curves of the liquid slag and glass filaments, which indicates that the material is a completely amorphous solid.

Ash deposition characteristics on heating surface in biomass circulating fluidized bed boiler

ABSTRACT A mathematical model of biomass ash deposition based on inertial impaction, thermophoresis deposition, and condensation was established to predict the ash deposition on the heating surface in a biomass boiler. Compared with the conventional numerical simulation methods, the mathematical models established are more focused on important design parameters in industry and can quickly calculate changes in parameters such as ash and heat flux density of heating area, and the simulated values were in good agreement with the measured results of the actual boiler. The growth of ash deposition on the heating surface of the boiler under different operating conditions was calculated by MATLAB program. The results show that the deposition caused by inertial impaction is dominant among the three deposition mechanisms. While the deposition caused by condensation is the least, accounting for only about 1%. The heat flux is only 1/2 of the initial stage when the ash deposition reaches a steady state. The ash deposition degree increases with the increasing of flue gas velocity, flue gas temperature, and particle size. When the flue gas temperature increases from 923 K to 1123 K, the ash deposition thickness increases from 7.6 mm to 14.2 mm. When the particle size increases from 12 μm to 20 μm, the thickness of ash deposition increases from 5.3 mm to 10.7 mm; when the gas velocity increased from 6 m/s to 15 m/s, the thickness of ash deposition increased from 6.5 mm to 10.6 mm. The ash depositions in two corn stalk-fired pure-combustion circulating fluidized bed (CFB) boilers were calculated, and the predicted results of the model were consistent with the actual ash deposition level.

Numerical study on dynamic ash deposition and heat transfer characteristics of radiant syngas cooler

Ash deposition greatly influences the performance of Radiant Syngas Cooler (RSC) which plays a pivotal role in the heat recovering process in coal gasification technology. Therefore, a numerical model of ash deposition coupled with heat transfer is established in present study to predict the ash deposition of RSC, and the effects of inlet temperature and division wall structure on deposition characteristics are further discussed. The simulation results indicate that the particle impact mainly occurs in the transition region of the jet flow to plug flow, while the particle deposit mainly occurs in the recirculation region. The deposition of inner wall and division wall shows a malignant growth pattern successively with the width increase of division wall. The division wall of 400 mm width is more conducive to the safe and stable operation of the RSC than those of 250 mm and 650 mm. The deposition becomes heavy with the increase of inlet temperature, and the deposition of division wall belongs to the slow growth, the large-scale detaching and the malignant growth pattern at the inlet temperature of 1573 K, 1673 K and 1773 K, respectively. Present result may provide a reference for the structural design and operational adjustment of RSC.

Ash deposition and heat transfer characteristics on the convective heating surface of calcium‐rich Zhundong coal combustion in a 0.4‐MWth facility

AbstractThermal power plants in Zhundong area of Xinjiang equip Π‐typed boiler widely in recent years. With the increasing ratio of Zhundong coal in blended fuels, the slagging is strengthened. And the boiler shutdown was caused by ash deposition on the convective heating surface frequently. In order to study the mechanism of this problem, a 0.4‐MWth pilot‐scale facility was built and expected to study ash deposition and heat transfer characteristics on the convective heating surface of 100% burning calcium‐rich Zhundong coal. Thermodynamic model calculation was implemented on the heat transfer characteristics of two groups of tube panels, on which ash samples were collected after experiments. Then, simulation calculation was applied by software for verification. The ash samples were characterized by X‐ray diffraction spectroscopy (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive X‐ray detector (EDX). FactSage 7.3 thermochemical software package and a high‐temperature viscometer were also used to figure out the viscosity‐temperature characteristics. Results showed that a “step change” of heat transfer characteristics occurred when the flue gas temperature of inlet tube panel #2 approached to 1000°C. After the “step change,” the heat transfer characteristics became stable. The ash deposition mainly consisted of silicon‐calcium compounds of tube panel #1, while in contrast, that of tube panel #2 consisted of gehlenite, CaSO4, and Na2SO4. The ratio of liquid phase in raw coal ash samples increased sharply when the flue gas temperature exceeded 1030°C. With the increase of surface temperature of fouling tube panel, the CaSO4 would be decomposed to release CaO, which reacted with gehlenite or other compounds to form low temperature eutectics. The fouling of ash would transfer to slagging and lead to the deterioration of ash deposition, which induced the formation of “step change” during the experiment.

Strategic valorization of bio-oil distillation sludge via gasification: A comparative study for reactivities, kinetics, prediction and ash deposition

Biomass refinery system is gradually maturing as an emerging technology for waste management and carbon neutral, but bio-oil distillation sludge (DS) as an unexpected product significantly hampers the continuous operation of the system and hazards human health. Herein, we conducted a comparative investigation on the gasification characteristics of bio-oil distillation sludge char (DSc) and walnut shell char (WSc) to explore the clean and efficient strategy for disposal of DS. The biochar was prepared on a fixed bed reactor under two different temperatures (900 °C and 1100 °C), followed by being gasified on a thermogravimetric analyzer with the purposes of evaluating the gasification behaviors and kinetic evolution. Meanwhile, machine learning approach (back-propagation neural network as an example) and thermodynamic equilibrium simulations were innovatively adopted to predict the gasification performances and ash deposition characteristics, respectively. The results indicated the gasification reactivities for WSc varied with gasification conditions and preparation temperatures, whereas the DSc reactivity was far lower than that of WSc. Gasifying WSc accompanied with the development of porous structures and thus was kinetically fitted by random pore model well. However, the gasification process for DSc was more representative by volume model and grain model. The back-propagation neural network performed an excellent prediction effect on gasifying DSc and WSc under whatever (non–)isothermal conditions, with the correlation coefficients beyond 0.9983. In addition, mineral compositions and ash deposition prediction indicated the presence of potassium and calcium in WSc induced the decreased melting temperatures of ash, and thus severe ash slagging and fouling, while DSc ash, composed of iron and silicon, exhibited flocculent morphology and was difficult to deposit.

Ash Deposition Characteristics of Flue Gas Across a Tube Using the Eulerian Multiphase Model with Dynamic Mesh Technique

Ash Deposition Characteristics of Flue Gas Across a Tube Using the Eulerian Multiphase Model with Dynamic Mesh Technique