An experimental study on bubble formation mechanism during the

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An experimental study on bubble formation mechanism during the sintering of coal and biomass ash blend Hao Zhou, Dan Liu, and Weichen Ma Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02882 • Publication Date (Web): 16 Nov 2018 Downloaded from http://pubs.acs.org on November 16, 2018

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Energy & Fuels

An experimental study on bubble formation mechanism

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during the sintering of coal and biomass ash blend

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Hao Zhou, Dan Liu, Weichen Ma

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State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering,

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Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China

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ABSTRACT: This paper presents an experimental study on bubble formation in the process of

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sintering of Zhundong coal (ZD) and corn stalk ash (CS) blends in a horizontal-chamber furnace.

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The effect of blend ratio of biomass ash was investigated. Bubble parameters, such as number, area,

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and porosity, were measured based on a metalloscope equipped with charge-coupled device (CCD)

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camera and digital image processing technique. After sintering experiments, ashes and condensed

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matters in the bubbles were sampled and analyzed by X-ray diffraction (XRD). In addition, chemical

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equilibrium calculation was conducted to reveal the influences of biomass ash on the formation of

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bubbles. The experimental results show that 50% CS blend has the greatest melting degree and the

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formation time of bubbles is earlier than other cases. While low ratio of CS ash has limited influence

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on bubble formation and mineral composition. The formation mechanism of bubbles is proposed

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based on the results. The condensed matters in the bubbles mainly contain NaCl, CaSO4, and KCl.

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The results indicate that chlorine promotes the transformation of alkali metals to gaseous phase.

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Chemical equilibrium calculation verified the experimental results.

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KEY WORDS: Sintering; Bubble; Pore; Zhundong coal; Biomass ash.

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1. Introduction

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The increasingly grave energy crisis with depletion of fossil fuels and consequent

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environmental problems are threatening human society. Co-firing of coal and biomass in utility 

Corresponding author. Tel: +86-571-87952598. Fax: +86-571-87951616. E-mail: [email protected].

ACS Paragon Plus Environment

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boiler is a promising choice for generating heat and power.1–3 China's biomass power generation

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industry is under rapid development. Biomass power generation projects have been conducted in 28

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provinces around the country. The total power generation capacity reached 12 226.21 MW by the

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end of 2013, including 7 790 MW connected to the grid, and it will reach 30 000 MW in 2020.4 As

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a global concern, biomass is CO2-neutral , and greenhouse gas can be reduced by co-firing biomass

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with coal.5,6 In addition, the low content of sulfur and nitrogen in biomass leads to the reduction in

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emissions of SOx and NOx during the combustion.7,8 Despite the advantages, a limitation of its

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application is the ash-related problems such as fouling, slagging, and corrosion.9,10 Wigley et al.11

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investigated the ash deposit behavior on co-firing of two bituminous coals and five different

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biomasses (miscanthus, short rotation coppice, olive residue, palm kernel expeller and sawmill

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residue), and the addition level of biomass is up to 60 wt.%. They found that higher biomass

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replacement level increased both the deposition efficiency and the degree of sintering of the ash

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deposits. Ash deposition on heat-transfer tubes results in a reduction of plant efficiency and

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availability.12 The forecast reserves of 390 billion tons in Zhundong, Xinjiang have made it the

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largest intact coalfield in China and even in the world, and could supply China for coal consumption

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for the next 100 years.13 Corn is widely planted in China, with a production of 219.55 million tons

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in 2016,14 and corn stalk is commonly used in biomass power plants in China.15 The ash deposition

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and ash fusion behavior of Zhundong coal have been studied by many researchers.16,17 However,

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few investigations reported the effects of biomass ash on the ash sintering behavior of Zhundong

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coal. The process of ash slagging is reported to include three stages and among them the sintering

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stage plays a significant role in heat transfer deterioration. After semi-molten particles depositing

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on the tube and building up the initial layer, the surface temperature of ash deposit rises and sinter


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begins. The sinter degree depends on the melting point of ash particles and rises with the surface

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temperature, resulting in significant effects on heat transfer.18 Lots of investigations reported that

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during the sintering, bubbles were formed and resulted in pore structures. Zhang et al.19 found that

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bubbles inside slag reduce the slag thermal conductivity and viscosity, the thickness of slag layer

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decreases with the increase of bubbles inside slag. Rushdi and Gupta20 reported that pore structure

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was formed in the sintering layer of the ash deposit from coal combustion. As the deposit grew and

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slag layer formed, porosity had a decreasing trend. Xu et al.21 investigated the effects of heating rate

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on the porosity of coal char. Zhai et al.22 studied the variation of porosity of rice hull char during

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gasification process. Qiu et al.12 found plenty of pore structures in the ash deposits produced from

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co-combustion of coal and rice husk. They inferred that the formation of HCl was the key reason.

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In our previous work, bubbles forming (left) and bursting to form pores were observed during co-

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combustion of coal and corn stalk , as shown in Figure 1,and the final deposit thickness can reach

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6.1mm during the co-firing of Shen Hua coal and 10% corn stalk, for Shen Hua coal with 5% corn

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stalk, the maximum ash layer thickness is 5.19mm.23 In conclusion, the formation of bubbles is

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normal during ash sintering. Gaseous matter in the enclosed space of molten slag forms bubbles. As

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a result, the slag transforms from single phase to gas-liquid two phase, having significant influences

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on flow properties.24 In addition, the pore structure produced by bubbles and the gaseous matter

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could directly change the heat conductivity coefficient of the ash deposits, which plays an important

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role in ash deposit deteriorating heat transfer of boiler.25,26 The effective thermal conductivity of

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bubbly slag was calculated by the Maxwell-Eucken model, the results show that when gas volume

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fraction of bubbles changes from 0 to 0.1, the effective thermal conductivity is reduced by 20%.19

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Therefore, it is of great value to investigate bubble formation mechanism during ash sintering. Pang


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et al.27 studied the morphological characteristics of coal and biomass ash pellets. They obtained

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porosity from the cross-sectional SEM image of ash pellets. Kweon et al.28 and Wang et al.29 also

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conducted quantitative analysis on pore structures of ash slag by SEM method, and the porosity and

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length of pores were measured. Rushdi and Gupta20 used densitometer and SEM methods to study

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the pore structure of ash deposits. Shen et al.30 applied a high temperature stage microscope on

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studying the formation mechanism of bubbles on molten slag surface. During their experiments, the

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diameter of bubbles was measured. Nevertheless, few investigation analyzed the variation of pore

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structure parameters with time. In addition, the composition of contained matters in pores was rarely

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measured.

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The aim of this work is to investigate the formation of bubbles during the sintering of Zhundong

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coal and corn stalk ashes. The influence of corn stalk blend ratio was studied. A polarizing

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metallomicroscope was used to obtain cross-sectional images of ash blocks after different sintering

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times. The number, area, porosity of pores and the condensed matters in pores were measured. The

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transformation of minerals during the sintering of ash was also measured. Finally, Chemical

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equilibrium calculation was conducted to reveal the influences of corn stalk ash on the sintering

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behavior.

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2. Materials and methods

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2.1. Materials

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The ashes investigated in this study were prepared from their parent fuels at 823 K under air

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atmosphere in a muffle furnace, according to ASTME1755-01. The proximate fuel analysis, and

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ultimate fuel analysis of the Zhundong (ZD) coal ash and corn stalk ash (CS) are shown in Table 1.

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Ash fusion temperature and ash compositions are also presented. It can be seen that different from


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the conventional steam coal, ZD coal ash contains high Na2O (7.6%), chlorine (10.72%) and low

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Al2O3 (14.5%), SiO2 (27.43%). CS ash contains high K2O (8.773%) and MgO (6.17%). Before the

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sintering experiment, the biomass ash and coal ash were mixed and grinded in a mortar. In this

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study, two blend ratios of 10 and 50 wt% were investigated. 2 g ash mixture was pressed at 2 MPa

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with a specially designed mold and a tablet machine to obtain a standard ash block which had a

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cross-section of 15×15 mm and a height of 5 mm. For each blend, three blocks were prepared and

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studied to ensure the repeatability of the experiments.

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2.2. Experimental Facility and Procedure

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The tests were conducted in an electric heating horizontal-chamber furnace, as shown in Figure

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2a. According to the temperature during real combustion of boiler, 1473K is selected as the furnace

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temperature. When the furnace temperature was heated to 1473K, the ash samples were sent into

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the furnace. Each ash block was kept in the high temperature environment for the scheduled

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sintering times. The sintering times were selected according to the sintering degree. Ashes were

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sintered one minute apart at the beginning, then 2-3 minutes apart in the middle and later stages.

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Then the samples were taken out and quenched by liquid nitrogen to maintain the pore structure and

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the distributions of the mineral components. For each test, three ash blocks were placed in a

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porcelain boat, as shown in Figure 2b.

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A vernier caliper was used to measure the height of each ash block. The area of ash block was

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measured by image processing technique which combined calibration method and edge extraction.

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In order to measure parameters of pores, ash block was firstly coated with epoxy resin and then

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sliced up. A metalloscope (Zeiss Axioskop) equipped with Axiocam MRc5 CCD camera was used

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to obtain the images of pore structure. For each ash block, over 10 images were recorded and cover


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the pore areas. Then the images are processed by software AxioVision SE64. The metalloscope and

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software interface are shown in Figure 3.

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Figure 4 shows the digital image processing method based on AxioVision SE64 software.

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Figure 4a and b are the image of pore structure and microscopic photo. Then pores were labelled

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using boundary extraction function of the software. Finally, the background noise was eliminated

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and parameters of pore structure were measured, such as number, area, and porosity of pores. For

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each ash block, average value was obtained to reduce the errors. After experiments, ash block was

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sampled and analyzed by XRD method. The condensed matter in the pore was also sampled and

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analyzed, as shown in Figure 5. Finally, chemical equilibrium calculations was conducted using

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FactSage software (version 5.2) which provides “Equilib” and “Phase Diagram” modules to

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investigate the chemical and physical properties of the minerals.

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3. Results and discussion

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3.1 Morphologic change

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The images of ash blocks after different sintering times are shown in Figure 6. It can be found

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that as the time increases, the color of the ash block continues to deepen. The shapes of the ash

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blocks first soften, and then the top of the blocks raise, indicating the formation of bubbles. After

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that, the blocks collapse and their appearances show obvious sintering characteristics. In the final

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stage, ash blocks are badly melted. Comparing the images of ash blocks after different sintering

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times for coal and biomass blends, it can be seen that the morphologic characteristics of the pure

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coal ash and 10% CS blocks are similar, and the time points at which the ash block changed

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remarkably in shape are also similar. The sinter process of 50% CS block is significantly faster than

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other two cases, and the degree of sintering and melting is also deeper. After 4 minutes, 50% CS


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block completely collapses. The surface of 50% CS block shows obvious sunken features (4-8

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minutes) which indicate the collapse of the bubbles. After 10 minutes, the ash block is totally melted.

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The comparison shows that blending 50% corn stalk ash promotes the sintering and melting

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behavior of Zhundong coal ash. Figure 7 shows the morphological changes of pure corn stalk ash

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blocks. It is found that the sintering process is very slow and the sintering degree is pretty low when

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compared with other cases. The ash block begins to collapse at 40 minutes and the feature of bubbles

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is not obvious. Figures 8 and 9 present the variation of height and area of ash blocks, respectively.

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The height curve corresponds to the formation of bubbles, and the area curve presents the sintering

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and melting behavior of ash block. It can be seen from Figure 8 that the height curves of ZD and

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10% CS are similar. Both of them can be divided into three stages: 1) descending stage,

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corresponding to the shrinkage of ash during sintering process; 2) rapid increasing stage, indicating

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the formation of large numbers of bubbles; 3) decreasing stage, presenting much fluctuation. On

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one hand, bubbles continue to form. On the other hand, small bubbles merge to large ones. The slow

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shrinkage of the ash block is mainly due to the sintering and melting behavior of the ash block.31

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The sudden rapid fall of the height curve indicates bubbles breaking up. The maximum height of 10%

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CS is larger than that of ZD, indicating a larger bubble volume. The height curve of 50% CS is

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divided into two stages, rapid increasing stage and descending stage. The reason for the lack of

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descending stage in the initial stage is that the formation of bubbles is advanced and the degree of

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dilation is greater than the degree of shrinkage induced by sintering behavior. The ash fusion

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temperature of co-combustion of corn straw with coal shows a “V” shape trend, when the content

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of corn straw is 50% ,the fusion temperature is lowest.32 The slope of increasing stage and maximum

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height for 50% CS are larger than those of the other cases, indicating that for 50% CS, bubbles are


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formed more rapidly and have larger volume. The curve does not fluctuate at the descending stage,

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firstly decreases rapidly and then becomes stable. It indicates that the addition of 50% CS ash

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obviously promotes the melting behavior of ZD ash, and despite the dilation by bubbles, ash blocks

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melts down. In addition, it can be seen from Figure 6 that large numbers of bubbles break up on the

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surface of ash blocks. This also decreases the height of ash block. The variation of height for pure

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CS is smaller and slower than the other cases.

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Observing Figure 9, it can be found that the area curves for all the cases decrease firstly and

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then increase. The decreasing stage corresponds to the shrinkage induced by sintering behavior, and

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the increasing stage indicates that ash block collapses. By comparison, the area curves for ZD and

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10% CS are similar and fluctuate in the increasing stage. The fluctuation is related with formation,

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coalescence, and breakage of bubbles. 50% CS has the maximum increase in area and greatest

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increasing slope, indicating a higher melting degree. The ash fusion temperature of co-firing of corn

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straw with coal is lower than each single sample.32 The area curve of pure CS has the smallest and

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slowest change, indicating a lowest melting degree. The result is consistent with the observation in

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Figures 6 and 7.

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3.2 Pore structure

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This section conducts quantitative analysis on the process of formation, coalescence, and

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breakage of bubbles. Gas could be trapped in deposits during ash deposition, especially when

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surface began to melt and flow. KCl and K2SO4 are the main K-containing substance.33 The relative

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content of sodium, K and chlorine will significantly increase when adding CS ash into ZD coal ash.

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Figure 10 shows the variation of number of bubbles with time. The formation time of bubbles differs

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among the cases. For pure ZD and 10% CS, bubbles begin to form after 3 minutes. The formation


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time for 50% CS is earlier than the two cases, about 2 minutes. This observation is consistent with

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the height curve trend in Figure 7. In the first two minutes, pure ZD and 10% CS are sintered and

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shrink, no bubbles generate. 50% CS promote the formation of gaseous matters, large numbers of

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bubbles begin to form in the second minute. It can be found that the number curves for 10% CS and

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50% CS follow decrease-increase-decrease trend. Based on the analysis on bubble number

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combined with area data (Figure 11), some inferences could be made. (1) In the initial stage, bubble

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number decreases but bubble area increases for 10% and 50% CS. It indicates that small bubbles

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coalesce into large ones. (2) Though bubble number for ZD increases in the beginning, ZD case has

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the lowest bubble area. It indicates that bubble coalescence for ZD is less intense than CS blends.

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(3) Bubble number for 50% CS decreases the fastest among the cases in the middle stage (5-8

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minutes). Meanwhile, bubble area increases the fastest and decreases a lot in the end. It indicates

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that bubble coalescence for 50% CS is the most intense, and breakage of bubbles occurs frequently.

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The high melting degree observed in Figure 6 for 50% CS is the key reason. After 4 minutes, ash

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block begins to soften and the top part shows some feature of melting behavior. Melting behavior

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makes the ash block which contains bubbles a gas-liquid two-phase mixture. This is conducive to

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bubble coalescence. The small pores could connect with each other to form large pores when the

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deposits melted and began to flow.12 The height of ash block continues to decrease due to the intense

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melting behavior, and it is easier for large bubbles to move to the surface and break up and form the

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pits on the surface, as shown in Figure 6. Sintering behavior is dominant for ZD and 10% CS.

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Compared with 50% CS, it is harder for bubbles to coalesce and break up in ZD and 10% CS blocks.

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Thus the curves in Figure 11 are flatter than that for 50% CS. For pure CS case, the formation of

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bubbles is very slow and bubbles begin to appear after 20 minutes. Bubble number is small and


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decreases with time. It can be seen form Figure 11 that the area of bubbles for pure CS is small and

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does not change much during the sintering. This result indicates that during the sintering of pure CS

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ash, bubbles are formed mainly in small size and bubble coalescence is not intense. Figure 12 shows

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the variation of porosity of the ash blocks with time. The curves for ZD and CS blends can be

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divided into three stages, named increasing, fluctuating, and descending stage. The increase of the

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porosity is related with the formation of bubbles. As the sintering degree rises, small bubbles

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coalesce into large ones and break up on the surface, resulting the fluctuation of porosity. In the

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final stage, melting degree rises and the thickness of block is quite small. Large bubbles are easier

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to move to the surface and break up. In addition, the formation of gaseous matters gradually stops.

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Thus the porosity decreases in the end. By comparison, the porosity of 50% CS is smaller than ZD

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and 10% CS. The higher melting degree is the reason. For 50% CS, bubbles coalesce and break up

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more frequently. The melted ash fills the void immediately, resulting in a low porosity. The porosity

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of pure CS ash block is the smallest among all the cases, and changes little in a long sintering time.

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This result is consistent with the bubble number and area discussed above.

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3.3 Mineral composition of ash blocks

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Figure 13 shows XRD results of the ash blocks in the process of sintering. It can be found from

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panel (a) that in the initial stage, ZD ash mainly contains anhydrite, halite, quartz, diopside, anorthite,

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and gehlenite. As the sintering time rises, the peaks of halite and anhydrite decrease, this may be

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caused by these two minerals transform to gaseous phase. In the final stage, anorthite is the dominant

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mineral and the peak of anhydrite disappears. The formation of anorthite is related with the high

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contents of silicon, aluminum, and calcium. Vassileva et al.34 reported that anorthite is formed by

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two means: 1) Formed by solid-phase reactions (800–1100℃). Such as reactions of quartz with CaO,


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Al2O3, or with gehlenite. This may explain the disappearance of gehlenite. 2) Formed by melt

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crystallization (1100-1300℃). The solid-phase reaction can be expressed as follows.

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CaO  Al2O3  2SiO2  CaAl2 Si2O8 ( Anorthite)

(1)

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By comparing Figure 13b with 13a, it can be found that low ratio of CS ash has limited

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influence on mineral composition. Peaks of sylvine increase with 10% CS ash added due to the high

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potassium content in CS ash. As the sintering degree rises, peaks of sylvine gradually disappear. It

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is inferred that sylvine is released as gaseous phase. Figure 13c shows the XRD results for 50% CS.

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There are significant changes in mineral composition in the process of sintering when compared to

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ZD results. The intensity of quartz increases remarkably and peaks of anhydrite decrease. This result

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corresponds to the high silicon content and low calcium content in CS ash. In the initial stage, a

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large amount of albite and sylvine are formed, while gehlenite is not formed. Albite is formed by

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reactions of Al-Si oxide with NaCl or Na contained oxide.35,36 The reactions are as follows. After

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10 minutes, peaks of crystalline mineral disappear and plenty of amorphous matter is found, which

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is consistent with the melting characteristics in Figure 6.

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5SiO2  Al2O3 gSiO2 +2 NaCl  H 2O  2 NaAlSi3O8 ( Albite)  2 HCl

(2)

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Na 2O  Al2O3 g2SiO2 +4SiO2  2 NaAlSi3O8 ( Albite)

(3)

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After the process of sintering, the dominant mineral in pure CS ash is diopside. The formation

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of diopside can be expressed as the following reaction.37 Leucite is detected after 5 minutes and then

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disappears. Compared to other cases, pure CS ash lacks peaks of halite and anhydrite. The intensity

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of sylvine is weak. This result indicates that pure CS ash has less component that can transform to

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gaseous phase, accounting for the low number of bubbles in Figure 10.

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CaO  MgO +2SiO2  CaMgSi2O6 ( Diopside)


(4)

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For each test, three ash blocks were placed in a porcelain boat. After the test, breaking one of

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the simples and taking out the condense matters from pores for analysis. The mineral composition

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is shown in Figure 14. Due to the limitations of experimental methods and analytical techniques,

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we fail to collect the gas which does not condense in the bubbles. No condensed matter for pure CS

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is collected due to the pretty low number and small size of bubbles. It can be found from XRD

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patterns that the main components in the bubbles are NaCl, CaSO4, and KCl. Consistent with the

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results that discussed above: the peaks of NaCl, CaSO4, and KCl are weakened as the degree of

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sintering is deepened. It is confirmed that these three minerals could transform to gaseous phase to

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form bubbles during sintering. Besides, reaction 2 shows that NaCI could react with H2O, SiO2 and

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AI-Si oxide to release HCI, HCI could be trapped inside the deposits to form pores.12 As the blend

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ratio of CS ash increases, the intensity of NaCl and CaSO4 decreases remarkably, while peaks of

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KCl increase. No sodium sulfate or potassium sulfate is detected, indicating that sulfur tends to react

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with calcium to form calcium sulfate. Wang et al.13 investigated the ash evaporating to condensing

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of ZD coal. They proposed an ash deposition mechanism when burning ZD coal, in which CaSO4

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plays an important role. This paper gives the direct evidence that CaSO4 is formed in the ash and

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transforms to gaseous phase.

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3.4 Bubble formation mechanism

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Combining bubble parameters, sintering and melting degree, and mineral composition analysis

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of ash blocks. The general law of formation of bubbles can be obtained. In the initial stage of

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sintering, the alkali metal chloride and calcium sulfate in the ash block transfer into gaseous state to

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form small bubbles. As the number of bubbles increases, small bubbles coalesce into large ones. In

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the middle term, deepening of the melting behavior causes the ash block to soften and collapse, and


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thickness is thinned. In addition, melted ash and bubbles are in gas-liquid two-phase condition, and

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large bubbles are easy to moves to the surface of the ash block and break up, in which the gaseous

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substance escapes and the number of bubbles decreases. In the final stage, the low content of alkali

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metal chloride and calcium sulfate makes the formation of bubbles gradually stops, while the high

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degree of sintering and melting causes the bubbles to continually rupture on the surface of the ash

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block. These two reasons reduce the number and area of bubbles to a low level.

271

Figure 15 is the molten/gaseous phase fraction calculated for the cases. It can be found that

272

the molten phase fraction increases with the blend ratio of CS ash. However, pure CS has the lowest

273

value. For gaseous phase fraction, 50% CS has the highest value and pure CS has the lowest one.

274

The results of chemical equilibrium calculation are consistent with experimental results.

275

4. Conclusions

276

Bubble formation mechanism in the process of sintering of ZD coal and CS ash blends is

277

investigated. Ash blocks are sampled after different sintering times. Variations of bubble parameters

278

with time, such as number, area, and porosity, are obtained. Transformation of mineral composition

279

during the sintering is measured by XRD analysis. The composition of condensed matters in the

280

bubbles is also measured. Chemical equilibrium calculation is conducted to reveal the influence of

281

biomass ash. The main conclusions are drawn as follows.

282

(1)In the initial stage of sintering, small bubbles are formed. As the number of bubbles

283

increases, small bubbles coalesce into large ones. In the middle term, the thickness of ash block

284

decreases due to higher melting degree. In addition, melted ash and bubbles are in gas-liquid two-

285

phase condition, which makes it easy for large bubbles to move to the surface of the ash block and

286

break up and release the gaseous matters. Therefore, the number of bubbles decreases. The higher


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

287

melting degree is, the more intense bubble coalescence and breakage are. In the final stage, the

288

formation of bubbles gradually stops and bubble number and area decrease to a low level.

289

(2)Blending 10% CS has limited influence on bubble parameters and mineral composition of

290

ash blocks. Blending 50% CS remarkably changed the mineral composition. Intensity of KCl is

291

increased, which promotes the formation of gaseous matters and advances the formation time of

292

bubbles. In addition, high ratio of CS ash promotes the melting behavior of ZD ash. Chemical

293

equilibrium calculation verifies the experimental results.

294

(3)The condensed matters in the bubbles mainly contain NaCl, CaSO4, and KCl. The results

295

indicate that chlorine promotes the transformation of alkali metals to gaseous phase. Sulfur tends

296

react with calcium to from CaSO4.

297 298

Acknowledgements

299

This work was supported by National Natural Science Foundation of China (51476137) and the

300

Innovative Research Groups of the National Natural Science Foundation of China （No.51621005）

301


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397 398 399 400

Table 1 Fuel properties. Fuel Proximate analysis, (wt. %, ad) M A V FC Ultimate analysis, (wt. %, ad) C H N S O Ash fusion temperature, (℃) IT ST HT FT Ash compositions (wt %) Na2O MgO Al2O3 SiO2 P2O5 SO3 Cl K2O CaO TiO2 MnO Fe2O3

401

ZD Coal

CS

15.6 12.3 32.79 52.91

12.13 24.78 52.46 10.63

64.07 3.58 0.65 0.18 19.22

31.35 3.48 1.27 0.24 26.75

1101 1163 1172 1178

1156 1212 1224 1257

7.6 2.9 14.5 27.43 0.03 3.82 10.72 0.33 27.46 0.87 0.07 4.26

0.77 6.17 10.39 52.66 1.66 2.16 3.52 8.77 9.29 0.51 0.08 3.84

Note. M = Moisture; V = Volatile matter; FC = Fixed Carbon; A = Ash; ad= air dry

402 403


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Figure 1. Bubbles forming (left) and bursting to form pores (right) [19].

406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425


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426

a

427

b

428

Figure 2. (a) Schematic of the test rig and (b) placement of ash block.

429 430 431 432 433 434 435 436 437 438 439 440


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441

a

b

442

Figure 3. (a) Metalloscope equipped with CCD camera and (b) interface of AxioVision SE64.

443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

467

a

b

468

c

d

469

Figure 4. Digital image processing.(a) Image of pore structure; (b) microscopic picture; (c) extracting edge; (d) eliminating noise and counting parameters.

470 471 472 473 474 475 476 477 478 479 480


Page 24 of 36

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481 482

Energy & Fuels

Figure 5. The condensed matter in the pores.

483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Time (min)

ZD

10% CS

1

2

3

4

5

6

7


Page 26 of 36

50% CS

Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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8

10

12

15

512

Figure 6. Images of ash blocks after different sintering times for coal and biomass blends.

513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531


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Time (min)

1

5

10

40

60

90

Page 28 of 36

20

CS

Time (min) CS

532

Figure 7. Images of ash blocks after different sintering times for pure CS ash block.

533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553


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555 556

Figure 8. Variation of height of ash blocks with time (1473K).

557 558 559 560 561 562 563 564 565 566 567 568 569


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

570

571 572

Figure 9. Variation of area of ash blocks with time (1473K).

573 574 575 576 577 578 579 580 581 582 583 584 585


Page 30 of 36

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586

587 588

Figure 10. Variation of number of bubbles (1473K).

589 590 591 592 593 594 595 596 597 598 599 600 601


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

602

603 604

Figure 11. Variation of area of bubbles (1473K).

605 606 607 608 609 610 611 612 613 614 615 616 617


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618

619 620

Figure 12. Variation of porosity (1473K).

621 622 623 624 625 626 627 628 629 630 631 632 633


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634 635

a ZD

636 637

b 10% CS

638 639

c 50% CS

640 641

d pure CS

642

H: halite– NaCl; S: Sylvine–KCl; Q: Quartz– SiO2; G: Gehlenite– Ca2Al2SiO7;

643

A: Anhydrite– CaSO4; D: Diopside– CaMgSi2O6; An: Anorthite– CaAl2Si2O8;

644

Au: Augite– (Mg,Fe,Ti,Al)(Ca,Na,Fe,Mg)(Si,Al)2O6; L: Leucite– KAlSi2O6;

645

Al：Albite– NaAlSi3O8.

646

Figure 13. XRD patterns of the ashes in the process of sintering.


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647 648

a ZD

649 650

b 10% CS

651 652

c 50% CS

653

H: halite– NaCl; A: Anhydrite– CaSO4; S: Sylvine–KCl.

654

Figure 14. Mineral composition of condensed matters in the pores.

655 656 657 658 659 660


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661 662 663 664 665 666 667 668 669 670 671

672 673

Figure 15. Molten/Gaseous phase fraction of the blends.

674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689


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An experimental study on bubble formation mechanism during the

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