Visual investigation of slagging characteristics in a pilot-scale facility:Influence of deposition surface☆

Hao Zhou,Jiakai Zhang,Weichen Ma,Kun Zhang,Chenying Zhou

Zhejiang University,Institute for Thermal Power Engineering,State Key Laboratory of Clean Energy Utilization,Hangzhou 310027,China

ABSTRACT Slagging is a major problem in boilers,especially the low-rank coal applied in boilers.In this study,the influence of heat transfer surface on the slagging characteristics of a pilot-scale furnace was investigated.Ni coatings were applied in modifying the deposition surface to control slagging.The growth characteristics of the slag were studied using an online digital image technique.Scanning electron microscopy linked with energy-dispersive X-ray analysis and X-ray diffraction(XRD)were applied to investigate the microstructure,semi-quantitative chemical composition,and mineralogy of slag samples.Ni coating demonstrated a positive effect on the mitigation of slagging.Results revealed that the thicknesses of the slag initially increased with experimental time and then inclined toward stable values for both cases(Case 1:substrate material;Case 2:modified surface).The stable thicknesses for Cases 1 and 2 were 4.91 mm and 3.95 mm,respectively.The heat transfer efficiency was improved by approximately 18.2%with the application of Ni coating for probe surface modification.The mechanism of the mitigation of slagging was investigated in this study.XRD results revealed that the content of alkali reduced when the surface was coated with Ni.The alkali significantly affected the adhesion behavior of the deposition.Hence,Ni coating showed potential in reducing slagging and increasing the efficiency of boilers.The overall study makes a contribution to a deep understanding of the effect of Ni coating on the growth characteristics of the slag.

Keywords:Slagging Deposition surface Heat flux Digital image technique

1.Introduction

Slagging is a serious problem that decreases heat flux through a heat transfer tube.It causes a few significant damages to boilers,including reduction of efficiency [1],high-temperature corrosion[2,3],boiler outfire,and safety accidents [4,5].Numerous studies have been recently conducted to investigate the characteristics of slagging.Several works have focused on the physical-chemical properties of slags [6-10],such as fusibility,density,viscosity,and expansiveness.Some researchers have investigated the effect of reaction atmosphere,mineral composition,and chemical composition on slagging [11-14].Several characteristic values (e.g.,FeO-CaO mass ratio and ash fusion temperature) have been proposed to represent the characteristics of slagging[11,13,14].Computational fluid dynamics(CFD)has been applied to investigate the characteristics of slagging [15,16].

Researchers have proposed several methods,such as coal blending,chemical pretreatment,additive,and coating,to mitigate slagging.Anti-sintering additives,kaolin and dolomite,were applied to mitigate ash deposition [17,18].Raclavska et al.[19]reduced ashrelated problems in biomass combustion by co-firing biomass with coal.Mechanical dewatering and leaching were applied to remove the inorganic constituents of biomass,which resulted in severe slagging[20].Meanwhile,some researchers have focused on modifying surface to control ash deposition.Naganuma et al.[21,22],Naruse et al.[23,24],and Chen et al.[25]controlled ash deposition using modified surfaces.They found that Ni alloy played an effective role in reducing deposition and that it could control the diffusion of Fe compounds from the probe to the ash deposition.Ni coating could reduce the adhesion force of ash particles by controlling the content of alkalis.However,few studies have investigated the growth characteristics of slagging when the deposition surface is changed;in addition,how surface properties influence heat fluxes through a heat transfer tube has been investigated incompletely.

In the current work,an online digital image technique was applied to study the effect of Ni coating on the growth characteristics of slags in a pilot-scaled furnace,only our previous study had used this technique[26].The heat flux through a heat transfer tube was measured to study the effect of surface properties on heat fluxes.The experimental data in this study could be used for simulations where few studies have simulated the effect of coating on slagging.In order to clarify the mechanism of the mitigation of slagging,X-ray diffraction(XRD)and energy-dispersive X-ray spectrometry(EDX)were applied to analyze the mineralogy and chemical composition of slag.

2.Materials and Methods

2.1.Experimental facility and the fuel properties

Fig.1 shows the pilot-scale furnace,which consisted of coal combustion,temperature measurement,image sampling,and deposition systems.The detail size of the experimental facility has been previously reported [26].The experimental conditions are listed in Table 1.When the experimental conditions reached a stable state,the ash deposition probe was inserted into the furnace.A charge-coupled device (CCD) camera was placed opposite to the deposition probe to monitor the growth of slagging.The temperature of the probe was controlled using an oil circulating system.Type-S thermocouples were applied to measure the temperature of the furnace.Each test was repeated twice.If the deviation between the two tests is less than 15%,then the average of the two tests is taken as the result.If the deviation between the two tests is greater than 15%,then a third test commences.If the third experiment and the previous two tests in any one of the deviation is greater than 15%,then the work is void and the test is restarted.A similar experimental procedure has been earlier reported [26].Shenhua coal,which is widely used in power plants in China,was utilized in this study.The coal particle size distribution is shown in Fig.2.The ultimate analysis,proximate analysis,ash fusion temperature,and ash compositions are shown in Table 2.

2.2.Measurements of slag thicknesses

Fig.3 presents the schematic and image of the measuring system [26].The deposition probe has two parts,namely,deposition sampling and oil cooling sections.The temperature of the sampling section was controlled using an oil circulating system to simulatethe heat transfer tube.A CCD camera recorded the growth of slagging on the sampling section.The images were extracted from the video every 2 min (see Fig.4).The maximum thickness of the slag was chosen to represent the slag thickness.These images were analyzed using MATLAB to obtain the pixel of the probe radius and the slag thickness in the image (see Fig.5).The thickness(Hthickness) of the slag is defined by

Table 1 Experimental condition

where Hthicknessdenotes the slag thickness;Prand Phrepresent the number of pixels for the probe radius and the slag thickness in the image,respectively;and R denotes the probe radius.

2.3.Measurements of heat fluxes

Fig.1.The schematic diagram of the furnace [26].

The heat conduction model of the cylinder was applied to calculate the heat flux[26].The temperatures of the inner and outer surfaces were obtained with a pair of thermocouples [see Fig.3(c)].The heat flux (q) through a heat transfer tube is calculated as follows:where q is the heat flux through a heat transfer tube;λ is the thermal conductivity of the substrate material;Touterand Tinnerare the temperatures of the outer and inner surfaces of the probe,respectively;routeris the distance between the outer hole and the center of the probe;and rinneris the distance between the inner hole and the center of the probe.

2.4.Properties of coating and substrate material

The Ni coating was applied on the probe surface using automatic plating equipment.The properties of the Ni coating and the substrate material are listed in Table 3.The substrate is Q235.

3.Results

3.1.Slag samples

Fig.6 shows the slag samples collected on the probe.Fig.7 illustrates the cross-section of the slag samples (Case 1:substrate material;Case 2:with Ni coating).In both cases,a layered structure exists,and the color of the upper part of the slag is darker.The differences between the two cases are as follows:(1) Case 1 is thicker than Case 2.It revealed that the Ni coating has a negative effect on the thickness of the slag.(2) Three layers exist in Case 1,whereas only two layers exist in Case 2.(3)There are more pores in Case 1 than that in Case 2,and the slag in Case 2 only contains some little pores.(4) A significantly higher amount of white powder is present at the bottom of Case 1 than that of Case 2;the white powder was collected and measured using an electronic balance.The mass of the white powder in Case 1 is nearly two times greater than that in Case 2.This white powder is the initial layer of the slag related with alkalis [26],which strongly affects the adhesion behavior.It is inferred that the Ni coating may affect the adhesion behavior by influencing the content of alkalis [21].

Fig.2.The coal particle size distribution.

Fig.3.The schematic diagram and image of the measuring system:(a) the schematic diagram of the probe;(b)the image of the probe;(c)the cross-section of the probe;(d)the schematic diagram of the CCD monitoring system;(e)the image of the CCD monitoring system [26].

Fig.4.The Images of slagging extracted from the video in Case 1.

Fig.5.The progress of the image processing:(a) original image;(b) processed image.

Table 3 Properties of coating and the substrate material

Fig.6.The image of the slags in two Cases:Case 1:Substrate material;Case 2:Ni coating.

3.2.Slag growth

Slag thickness versus time is shown in Fig.8.There are some similar tendencies in both cases.The slags grow gradually in the fist a few minutes and then grow quickly.Finally,the slags grow up to a stable thickness in both cases.The stable thickness of the slag for Case 1(4.91 mm)is greater than that of Case 2(3.95 mm).The stable stage is caused by the balance of the surface force,viscosity,and gravity [26].The different stable thicknesses can be caused by the different surface forces between the slag and the deposition surface.Meanwhile,the Ni coating can reduce adhesion force[22].Thus the Ni coating may influence the stable thicknesses of the slag through reducing adhesion force.The slagging in Case 2 is 4 min late compared with that in Case 1(see the ellipse in Fig.8).It is inferred that the Ni coating may have negatively affected the initial stage of slagging.This result is consistent with the observation in Section 3.1,in which a significantly higher amount of white powder exists at the bottom of Case 1 than that of Case 2.

3.3.Heat flux through a heat transfer tube

Fig.9 displays the heat flux through a heat transfer tube versus time.The heat flux initially decreases rapidly in the first few minutes and then decreases gradually with time.Finally,the heat flux achieves a stable stage.The stable heat fluxes for Cases 1 and 2 are approximately 275 kW·m-2and 325 kW·m-2,respectively.Meanwhile,the initial and final temperatures measured on the outer surface were 437°C and 396°C in Case A.The initial and final temperatures measured on the outer surface were 436°C and 420°C in Case B.The heat flux in Case 2 has on significant decreasing in the first 6 min (see the ellipse in Fig.9) and is consistent with the result in Section 3.2 that no slag exists in the first 6 min in Case 2.The heat flux decreases rapidly in the first 10 min in Case 1 because the initial layer of the slag significantly affects the heat flux [27].The heat transfer efficiency is improved by approximately 18.2% with the application of Ni coating for the probe surface modification.Thus,the Ni coating performs well in improving heat transfer efficiency.

Fig.10 shows the heat flux through a heat transfer tube versus the slag thickness.The heat flux decreases with the increase in thickness.The heat flux decreases rapidly in the first 0.59 mm in Case 1 mainly because of the formation of the initial layer of the slag and the significant thermal resistance.However,the heat flux decreases rapidly only in the first 0.34 mm in Case 2.This result is consistent with the observation in Section 3.1 that a significantly higher amount of white powder exists at the bottom of Case 1 greater than that of Case 2.

3.4.Result of XRD pattern

Fig.7.The cross-sections of the slag in two cases:(a) Case 1:Substrate material;(b) Case 2:Ni coating.

Fig.8.The thickness of the slag versus time.

The initial layer of the slag for the two cases is analyzed with XRD to investigate the effect of Ni coating on slagging.The results of the XRD patterns for both cases are shown in Fig.11.The main mineral compositions in Case 1 are quartz,anorthite(sodium),and diopside.Meanwhile,the quartz,anorthite,and iron magnesium oxide are the main compositions in Case 2.The major mineral is quartz in both cases according to the intensity of the XRD peak.However,sodium is only detected in Case 1.Thus,the Ni coating may negatively affect the content of sodium,which contributes to the formation of the initial layer.A similar phenomenon has been found in several studies;Ni coating can control the content of alkalis [21,22]because the adsorption energy between Na atom and NiO is significantly higher than that between Na atom and Fe2O3and the system of Na-F2O3is more stable[28].The quantum chemistry calculation results effectively explain the results of XRD,that is,the Ni coating negatively affects the content of sodium.This result is consistent with the observation in Section 3.1 that a significantly higher amount of white powder exists at the bottom of Case 1 than that of Case 2.The XRD peak of the iron-bearing mineral in Case 1 is higher than that in Case 2.Thus,the Ni coating can control the diffusion of Fe from the deposition probe to the slag [21,24].The iron-bearing mineral significantly influences the thickness of the deposition[29].This result is consistent with the result in Section 3.2 that the stable thickness of the slag for Case 1 is greater than that for Case 2.

It can be concluded that the Ni coating has an effect on the slagging by affecting the content of sodium and Fe compounds,which strongly influences the slagging behavior.

Fig.9.The heat flux through a heat transfer tube versus time.

Fig.10.The heat flux through a heat transfer tube versus the slag thickness.

Fig.11.The XRD patterns of the slags:(a)Case 1:substrate material;(b)Case 2:Ni coating.a—Quartz:SiO2;b—Anorthite (Sodium):Na0.25 Ca0.71(Al2 Si2 O8);c—Diopside:Ca1.007(Mg0.805 Fe0.214)((Si1.75 Fe0.214)O6);b′—Anorthite:Ca Al2 Si2 O8;c′—Iron Magnesium Oxide:(Fe0.95 Mg0.052)2 Mg0.052 O3.

Table 4 The chemical composition of the slag

3.5.Result of EDX

Since the XRD results show that the effect of Ni coating on the slagging mainly focuses on the initial layer,EDX is applied to analyze the chemical composition of the initial deposition.The result of the EDX is shown in Table 4,and the elemental composition has been converted into the composition of oxides.It can be concluded that the content of alkalis in Case 1 (3.03%) is significantly higher than that in Case 2 (0.59%).This result is consistent with that of the XRD pattern,which suggests that Ni coating can reduce the content of alkalis.Meanwhile,the content of Fe2O3is 7.57% in the bottom part of the slag in Case 1,which is considerably higher than that of its central part and that of the slag in Case 2.This result can be attributed to the capability of the Ni coating to control the diffusing of Fe from the deposition probe to the slag [21,24].The result is consistent with the observation in Section 3.4 that the XRD peak of the iron-bearing mineral in Case 1 is higher than that in Case 2.

4.Conclusions

In this study,the effect of Ni coating on slagging is investigated in a pilot-scale furnace.An online digital image technique is utilized to study the growth characteristic of the slag,and XRD and EDX are used to investigate the reduction mechanism of slagging.

(1) The stable thicknesses of the slags are 4.91 mm and 3.95 mm for Cases 1 and 2,respectively.The Ni coating performs well in the mitigation of slagging.

(2) The heat fluxes are approximately 275 kW·m-2and 325 kW·m-2for Cases 1 and 2,respectively.The heat transfer efficiency is improved by approximately 18.2% with the application of the Ni coating for probe surface modification.The Ni coating performs well in improving heat transfer efficiency.

(3) The results of the XRD and EDX reveal that the Ni coating negatively affects the content of alkalis,which significantly influence the formation of the initial layer;moreover,the Ni coating can control the diffusion of Fe from the deposition probe to the slag.In addition,the relationship between the adhesion strength and the content of alkalis will be studied in our future work.

The overall study makes a contribution to a deep understanding of the effect of Ni coating on the growth characteristics of the slag.