Behavior of slagging deposits during coal and biomass co-combustion

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Behavior of slagging deposits during coal and biomass co-combustion in a 300KW down-fired furnace Weichen Ma, Hao Zhou, Jiakai Zhang, Kun Zhang, Dan Liu, Chenying Zhou, and Kefa Cen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03050 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Energy & Fuels

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Behavior of slagging deposits during coal and

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biomass co-combustion in a 300KW down-fired

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furnace

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Weichen Ma, Hao Zhou*, Jiakai Zhang, Kun Zhang, Dan Liu,

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Chenying Zhou, Kefa Cen

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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

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Coal and biomass co-combustion in existing utility boilers is a promising option of mitigating

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the fossil energy crisis and reducing the gaseous emissions of NOx, SOx, and CO2. However,

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ash-related problems including fouling, slagging, and corrosion cause damage to heat

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exchange tube and reduce boiler efficiency. In an attempt to give better insights into slagging

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behavior during coal/biomass combustion, an experimental investigation was conducted to

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study the growth of slag when coal was co-fired with wood and corn stalk in a 300 kW

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pilot-scale furnace. For comparison, combustion of pure coal was also conducted. During the

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experiments, biomass proportions of 5% and 10% by weight were examined. Slags formed on

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an oil-cooled deposition probe were collected, sampled and analyzed using scanning electron

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microscopy (SEM) and X-ray diffraction (XRD). Change in slag thickness with time was

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obtained by a CCD monitoring system. With two thermocouples in the probe, the heat flux

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through the slag could be measured. The slag from pure coal combustion showed a layered

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structure with different levels of compactness and hardness. The heat flux decreased by 31.7%

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as the slag grew to 5.19 mm. The results showed that co-firing wood significantly inhibited

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the slagging behavior. Especially in 10% wood case, hardly any slag was collected from the

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probe. Nevertheless, co-firing corn stalk resulted in severe slagging with slag thickness of 5.5

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and 6.1 mm for two blend ratios. The formation of bubbles in the deposits together with

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greater deposit thickness caused heat transfer deterioration. XRD results revealed the

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influence of co-firing biomass and corn stalk caused quite different changes to mineral

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species from wood. It was observed that fly ash under different biomass co-firing conditions

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differed little on mineral compositions.

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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. Demand for alternative and

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renewable energy resources makes co-firing coal with biomass in existing coal-fired boilers a

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suitable option [1-3]. Co-firing biomass fuels could effectively reduce net CO2 and NOx

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emissions other than the coal consumption [4, 5]. Despite the advantages, a limitation of its

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application is the ash-related problems such as fouling, slagging, and corrosion [6, 7]. Ash

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deposition on heat-transfer tubes results in a reduction of plant efficiency and availability [8].

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Properties of biomass fuels account for the increased ash deposition propensity. The contents

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of alkali metals and chlorine in biomass are generally higher than those in coal. Chlorine has


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been reported to promote the transformation of alkali metals from solid to gaseous matters,

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such as potassium chloride [9]. Gaseous alkali facilitates the ash deposition, especially the

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formation of the initial layer of deposits [10, 11].

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Notably, biomass is in various forms and can result in quite different influences on ash

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deposition during co-firing process. In this study, two common biomass fuels, wood and corn

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stalk, were studied due to their great yield and widely cultivation. Until now, many

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researchers have reported investigations on co-firing coal with biomass fuels on pilot-scale

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facilities or real boilers and co-firing wood had positive influence on deposition behavior.

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Firing wood chips was reported to show not problematic for superheater fouling in a 28 MW

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BFB boiler for about ten years [12]. Robinson et al. [13] presented results from a pilot-scale

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combustor which showed that coal-wood blends result in lower deposition rate than

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unblended coal, and there was hardly any ash deposits produced in Red Oak wood case.

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Wang et al. [14] also conducted experiments in a large-scale furnace and the results showed

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that co-firing pine branches and peach stones reduce the deposition rate of deposits as

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compared with that of the pure coal firing, while the addition of wheat straw and olive stones

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result in an increase of about 50% in the deposition rates. Kupka et al. [15] used a 50 kW

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slagging reactor to carry out related study. They observed that the addition of the saw-dust to

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the coal with low and high ratios decreases ash deposition rate a lot. Savolainen [16] reported

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that slagging and fouling were at normal levels during the wood co-firing tests at 9-25% ratio


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in a 315 MW pulverized coal boiler. Smajevic et al. [17] observed no significant increase in

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ash deposition during low wood ratio co-firing tests compared to unblended coal on both

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lab-scale furnace and 110 MW power station. Molcan et al. [18] carried out co-firing tests on

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a 3 MWth combustion test facility and they found the co-firing of woody biomass and coal

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would help to reduce unburned carbon in the fly ash but result in severe fouling, and the

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increased contents of alkali metals under co-firing conditions should be the reason. Many

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studies on the ash characteristics of corn stalk have been done [19-21], and only a few

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experiences were reported to investigate ash deposition during co-firing process using corn

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stalk in large-scale furnaces. Lupiáñez et al. conducted an experimental characterization of

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anthracite and corn stover co-firing under air and oxy-combustion in a 100 kW fluidized bed

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reactor, reporting significant influences of chlorine content in corn on ash deposition [22].

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They reported that high desulphurization efficiency by supplying limestone diminishes the

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deposition rates [23]. Xiong et al. [24] reported that the addition of kaolin or calcite to the

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corn stover considerably reduces the severe slagging tendency. Yang et al. [25] demonstrated

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that the deposition amounts of corn straw follows a linear pattern as a function of

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temperature.

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These studies, however, usually collected deposits after experiment and then analysed, and

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lacked the detailed information during the formation of ash deposits. In other word, few

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studies described how the ash deposits grew and how biomass fuel influenced the growth


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process. It is imperative to illuminate the growth law of deposits to reveal the deposition

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mechanisms. Li et al. [26] exhibited a deposition trend of a “fast− slow−fast−slow” process

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for corn stalk, and the deposition mechanisms were distinct in four periods. However, the

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deposition sampling method in their work was ineffective and discontinuous, limited by

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sampling time (only four times in 240 minutes). In this study, the focus of biomass co-firing

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analysis was on the slag thickness and heat flux versus time at different biomass blend ratios.

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A measurement based on image processing technique was applied to continuously measure

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the thickness of deposits. Heat flux through the deposit with time was also measured to give a

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better understanding of how biomass co-firing influenced heat transfer. Mineral compositions

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of ash deposits and fly ash were compared to evaluate the changes induced by adding

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biomass.

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2. EXPERIMENTAL

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2.1. Test Facility, fuel and experimental methods

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The growth of ash deposits during co-combustion of coal and biomass fuels were studied in

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a 0.3 MW down-fired furnace. Figure 1a shows the test rig which comprised feeding and

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burner system, furnace, measurement system, and flue gas treatment system. The furnace can

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be divided into four zones, namely, the first to the fourth stage from the top to bottom. During

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the one-hour preheat period, oil was firstly fed and combusted to increase the furnace

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temperature fast. Then coal was combusted instead of oil. Adjustable feeding capacity


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enabled stable thermal output under co-combustion conditions. When the temperature of the

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third stage of the furnace achieved 1300 °C, the ash deposition probe was inserted into the

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middle of the third stage which was about 1875 mm from the top (Fig. 1a). Ash deposited on

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the probe head, shown as the deposition area in Fig. 1b. The whole probe head was placed in

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the furnace with the two thermal-couples arranged in the vertical direction, as shown in Fig.

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1b.The temperature of the cooling oil circulated in the probe was kept at 230 °C, so that the

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surface temperature of the probe was similar to that of water wall tube in actual condition.

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Meanwhile, a CCD camera unit was placed opposite the probe. As shown in Figure 1c, the

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CCD monitoring device was cooled by water. Compressed air prevented fly ash from

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depositing on the lens. Fly ash was collected from the bottom of the cyclone separator after

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the experiments.

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Bituminous coal, wood chips, and corn stalk are the fuels used for the experiments. The

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coal, called Shenhua, was produced from north China. Wood chips and corn stalk represented

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wastes from forestry and agriculture with great production, respectively. Their properties are

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presented in Table 1. It can be seen from the proximate and ultimate analysis that, in

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comparison to coal, biomass contained much less fixed carbon and more oxygen. It should be

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noted that wood had much less ash content than corn stalk, even less than coal. Coal and corn

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stalk had similar ash fusion temperatures, much lower than those of wood chips. Ash

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compositions analyzed by X-ray fluorescence spectrometer (PANalytical Axios) are also


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presented in Table 1. Coal ash contained high iron of 9.84%. The potassium contents in

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biomass ash were much higher than that in coal ash, 4.84% and 8.77% in wood and corn stalk,

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respectively. Wood ash had a very high calcium content of 62.9%. Before experiments, coal

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was pulverized with a mean diameter of 12.9 µm. The size distribution of pulverized coal was

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measured by a laser particle size analyzer (Coulter LS230), as shown in Figure 2. Biomass

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fuels were ground and sieved to fine particles less than 1mm. Pulverized coal was then mixed

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with wood or corn stalk and fed into the furnace by a screw feeder. The experimental

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conditions are shown in Table 2. The biomass blend ratios of 5% and 10% were selected to

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use in this study. Repeated trials were conducted for each test. If the deviation of the deposit

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thickness between the tests is less than 15%, and then the results are considered to be

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repeatable.

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To study the mineral transformation mechanism in ash deposit and fly ash, samples

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collected after tests were analysed by XRD analysis (PANalytical X’Pert PRO). The

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microstructure and element distributions of ash samples were observed and identified using a

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Hitachi S-3700N scanning electron microscopy (SEM) equipped with energy dispersive

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spectrometer (EDS).

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2.2. Measurement principle of the deposit thickness and the heat flux

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The frame rate of the CCD camera was set at 3 frames per second to capture the images of

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ash deposit during its growth process. A series image processing operations were done to the


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image (Figure 3). In this study, the highest thickness in the vertical direction where the

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thermal-couples were arranged was used to evaluate the ash deposition. Firstly, target area

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was cropped from the original image and then converted to a binary image. Secondly, the

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edges of the probe and ash deposit were extracted from the background, and the center of the

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probe was located. The pixel numbers of the probe radius and the distance from the center to

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the peak of ash deposit were calculated (denoted as PR and P). The deposit thickness was

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obtained as follows:

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T =(

P − 1) × R PR

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(1)

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where T is the thickness; R is the radius of the probe, 20 mm.

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The diagram of the probe head is shown in Figure 1b. The heat conduction model of

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cylinder was applied to calculate the heat flux through the ash deposit. The equation is listed

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below.

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q=

λ (t2 − t1 ) R ln(

(2)

R2 ) R1

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where q is the heat flux; λ is the thermal conductivity of the deposition probe material, 16

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W/(m·K); t2 and t1 are the outer and inner temperature of the probe; R2 and R1 are the distance

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from the outer hole and inner hole to the center of the probe, respectively.

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3. RESULTS AND DISCUSSION


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3.1. Growth of deposits

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Images of ash deposits collected from the probe are shown in Figure 4, and the

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cross-sections of deposits are shown in Figure 5. The surface of slag from pure coal

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combustion showed a semi-molten structure (Figure 4a). The slag had a hard texture and its

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length was about 50 mm. The cross-section of coal slag showed a three-layered structure,

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namely, layer1, layer2, and layer 3 from bottom to top (Figure 5a). Layer 3 contained some

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little pores and had melting features. The color of layer 3 was darker than the other two layers.

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Layer 1 was hard to visually distinguish from layer 2 because they had the same color and

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layer 1 was pretty thin. However, layer 1 had quite different textures from layer 2. Layer 1

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was studded with relatively large ash particles and its texture was so loose that it could be

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removed from the deposit easily. Layer 2 had, by contrast, much harder and more compact

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textures.

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During the co-combustion of coal and 5% wood, regular shedding of deposit occurred. We

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successfully collected the shedding deposit after the second shedding process. The ash

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deposit (Figure 4b) showed a smaller size than pure coal slag, with a length of approximately

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20mm. The deposit was quite crispy and had a deep color. The surface of the deposit was

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granular. The cross-section was two-layered: layer 1 was loose and easily removed by touch;

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layer 2 was harder and more compact. When the wood ratio rose to 10%, hardly any ash

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deposit was collected. Observing the probe taken from the furnace after the test, an


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accumulation of large ash particles was found (Figure 4c). The ash particles did not stick to

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the probe surface tightly and were easy to blow off. The result indicates that the deposit from

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co-firing wood had high fusion temperature and low capacity to adhere to the surfaces.

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Co-firing corn stalk caused more serious slagging than pure coal combustion. As shown in

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Figure 4d and c, slags from co-combustion of coal and corn stalk had lengths larger than

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70mm. Both the two slags had highly molten surfaces and three-layered structures. It

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indicates that co-firing corn stalk yielded deposits with low fusion temperature. The textures

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of layers became harder from the bottom up. The outer layers of both the two slags contained

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many large pores. The gas bubbles inside the deposit (believed to be formed by gaseous alkali

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species and chlorine species)usually combine with the differential shrinkages of the mineral

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components to form the pores [8, 19]. The most significant difference between the two cases

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was that the thickness ratio of layer 3 in 10% case was larger than that in 5% one.

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Figure 6 shows the comparison of the growth of deposits for the four cases. The growth

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curve for pure coal was divided into 3 stages: the first (0-30 min), second (30-110 min), and

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third stage (110-140 min), corresponding to the rapidly increasing, slowly increasing, and

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stable stage, respectively. The slopes of the first and second stage were 0.08 and 0.034

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mm/min. The final deposit thickness was 5.19 mm. It is inferred that the three stages of

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growth curve were bound up with the three-layered structure of the ash deposit. The corn

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stalk co-firing cases had similar growth curves. For the 5% corn stalk case, the first stage


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lasted 25 min and the second stage lasted from 25 to 115 min. Their corresponding slopes

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were 0.07 and 0.043 mm/min. The first stage was similar to that for pure coal. However,

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deposit increased faster during the second stage than pure coal case, resulting in a larger final

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thickness of 5.5 mm. As the blend ratio of corn stalk increased to 10%, deposit grew more

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rapidly. The three stages corresponded to 0-35, 35-105, and 105-140 min. The slopes of the

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first and second stages were 0.103 and 0.041 mm/min, respectively. The final deposit

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thickness was 6.1 mm. The lasting time of first two stages was shorter than 5% case. This

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may relate to the result that the thickness ratio of layer 3 in 10% case was larger than that in 5%

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one.

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The 5% wood curve underwent two sharp drops, corresponding to two shedding processes

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of ash deposit. The two growing stages were quite similar and the deposit thicknesses before

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shedding were almost the same. Therefore, it is inferred that the shedding was regular and

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would continue to occur if the probe stayed in the furnace for a longer time. The thickness of

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collected deposit was 2.09 mm.

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It is concluded from the above results that co-firing corn stalk increased the slagging

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propensity. With corn stalk added to coal, the slag grew faster and the melting degree also

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increased. In addition, the effect of adding corn stalk increased as the blend ratio rose. By

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contrast, adding wood to coal remarkably reduced the ash deposition. Co-firing wood made

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the texture of deposits loose and easy to remove, thus shedding occurred. Hardly any ash


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deposit was formed with a higher wood ratio. The melting behavior of the deposit should be

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the reason for the different performances of the two biomasses. Co-firing wood yielded

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deposits with high fusion temperature. On the contrary, deposits had low fusion point in corn

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stalk co-firing cases. The melted, sticky surface of the deposit contributes to the capture of

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coarse ash particles [6]. Abreu et al. [27] reported a similar result that co-firing sawdust with

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coal significantly decreased the deposition rate due to both the sawdust low ash content and

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its low alkalis content. They concluded that deposits from co-firing wood and coal had high

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levels of silicon and aluminium, thus, with high fusion temperature. They also reported that

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existence of potassium contributed to the reduction of the fusion temperature of the deposits,

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which could account for the melting behavior for corn stalk cases in this study.

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3.2. Heat transfer characteristics of deposits

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In this paper, the ratio of current heat flux to initial heat flux (q/q0) was adapted to describe

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the heat transfer deterioration. The inner surface and outer surface temperatures versus time is

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shown in Figure 7a. The variation of the heat flux ratio with time is also shown in Figure 7.

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Similar to the growth curves, the heat flux curves for pure coal and corn stalk co-firing cases

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were also divided into three stages: rapidly decreasing, slowly decreasing, and stable stages.

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The separation nodes were consistent with those of deposit thickness curves. This result

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indicates that heat flux was mostly influenced by deposit thickness. The sharp rise in the heat

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flux curve for 5% wood indicates the occurrence of deposit shedding.


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The relationships between deposit thickness and heat flux for different conditions were

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established to give a better insight into how biomass co-combustion influenced the heat

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transfer deterioration. The comparison of heat flux ratio versus thickness for different

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conditions is shown in Figure 8. For pure coal, the curve was nearly linear before

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stabilization. The slope was about 0.061 mm-1. It indicates that the structure and physical

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characteristics, especially the coefficient of thermal conductivity, did not change much during

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the growth of ash deposit. The descending curve changed to two-staged pattern with 5% corn

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stalk added. The first descending stage with a slope of 0.142 mm-1 indicates that adding

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biomass had a great influence on the formation of the initial layer of ash deposit, by physical

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and chemical means. As the corn stalk ratio rises to 10%, the variation of curve became much

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more complex. The first stage had a pretty large slope, approximately 0.217 mm-1, indicating

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greater effects were made by adding a higher ratio of the corn stalk. The formation of the

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initial layer involves two mechanism, vaporized alkalis condensing on the tube surface and

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submicron ash particles moving to the tube by thermophoresis force [28, 29]. Co-firing

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biomass with coal formed more alkali metals and chlorine containing species in the gaseous

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phase, such as NaCl, KCl, Na2SO4, and K2SO4 [30]. It is inferred that co-firing corn stalk

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would produce more small ash particles, combining with the condensation of alkali vapors to

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promote the formation of the initial layer. It is inferred that little bubbles, which could

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remarkably reduce the effective thermal conductivity, formed during the formation of the first


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two layers of deposit. The drop after 3.5 mm was inferred to result from the large pores

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observed in Figure 5e. The observation of the formation of bubbles on the head face of the

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probe under 10% corn condition verified this inference (Figure 9).

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For 5% wood case, the shedding of deposit caused a sudden rise in heat flux curve. Notably,

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during the second growing and shedding process, heat flux was smaller than that of the first

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process. It is because not all the deposit shed from the probe and part of the initial layer still

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stuck to the surface of the probe.

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At the end of the test, heat flux for pure coal was reduced by 31.7%. The values were

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36.8%, 58.7%, and 24.2% for 5%, 10% corn stalk, and 5% wood, respectively. The results

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indicate that co-firing corn stalk caused exacerbation of heat transfer by producing thicker

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slags. The influence became greater with a higher blend ratio. Nevertheless, by

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co-combusting wood, heat transfer was improved. The results are consistent with the

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conclusion drawn in section 3.1.

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3.3. Mineral composition of deposits

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Figure 10 gives the XRD results for all the cases. Each layer of ash deposit was sampled

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and then analyzed. The main mineral was quartz for all the deposits. In addition, hematite

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was identified in all the deposits due to the high iron content in coal ash. In layer 1 of the

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pure coal deposit, anhydrite was detected. However, in the upper layers, the peak of anhydrite

266

disappeared and anorthite formed.


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In comparison to pure coal, more anhydrite peaks appeared in layer 1 with 5% wood added.

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When the wood ratio rose to 10%, the intensity of anhydrite peaks notably increased. This

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result revealed the influence of co-firing wood on mineral composition of deposits. Priyanto

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[31] reported that a significant amount of anhydrite is usually produced by the combustion of

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woody biomass.However, co-firing wood introduced little change to mineral species.

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Peaks of augite and albite were identified in XRD pattern for 5% corn stalk, and the peaks

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disappeared in layer 3 where the temperature was pretty high. This is because augite and

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albite have low fusion temperature.

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The detection of sylvine in the layer 1 of the deposit for 10% case was due to the high

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potassium content in corn stalk. In addition, another two new minerals, magnesioferrite

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(MgFe2O4) and diopside (CaMgSi2O6), were formed with 10% corn stalk added. Among the

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fuels listed in Table 1, corn stalk ash had the highest content of magnesium. Adding high

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ratio of corn stalk promoted the formation of Mg-based minerals. Tiainen et al. [32] reported

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that the presence of amorphous material in a sample creates a broad hump or halo in the

281

diffraction pattern. Both the two cases had humps between 2ɵ=20° and 35° in layer 3,

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indicating the existence of amorphous matters. The amorphous matters are usually formed by

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melting behavior [31]. In addition, diopside is formed from melt crystallisation at 1100–

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1300 °C [33], thus the high intensity indicates the occurrence of melting. This result is


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consistent with the observation of the molten surfaces of deposits in Figure 4d and e for corn

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stalk cases.

287

3.4. Microstructure and chemical composition of deposits

288

After the experiments, SEM-EDS was used to observe the microstructure and

289

quantitatively analyze the chemical composition of the deposits. SEM images are shown in

290

Figure 11. It can be seen that the outer layers have compact and smooth structures, whereas

291

the inner layers have loose and porous structures. The smooth surface indicates that the layer

292

was molten during the experiments. The microstructures show that adding corn stalks

293

promoted the occurrence of melting behavior. The structures of layer 2 and 3 of the 10% corn

294

case are most compact and smooth, indicating the slag was highly molten. In contrast, wood

295

case was less melted. The SEM results are consistent with the discussion on the melting

296

behavior in Section 3.1.

297

EDS results in form of element distribution are shown in Table 3 and normalized to 100%.

298

The main elements in each deposit were Al, Si, Ca, and Fe. The contents of other elements,

299

by contrast, were quite lower. The contents of alkali metals generally decreased from inner to

300

outer layer. The release of potassium increases with temperature [34]. The higher temperature

301

in outer layer was inferred to promote the transformation of alkali metals to gaseous phase.

302

As far as the influence of co-firing biomass is concerned, the element distributions showed

303

different changes with different biomass. Adding wood remarkably increased the calcium


Page 16 of 40

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Energy & Fuels

304

content. Co-firing corn stalk resulted in increases in contents of alkali metals, especially the

305

potassium content. The biomass fuel properties accounted for the results.

306

3.5. Fly ash

307 308

After each test, fly ash was collected from the bottom of the cyclone separator. Then the physical and chemical properties were analyzed.

309

XRD patterns of fly ash for all the conditions are shown in Figure 12. The main minerals

310

were quartz and mullite. Mullite related to the formation of anorthite shown in Figure 10a.

311

Pintana et al. [35] reported that the reaction between the mullite and calcium oxide will

312

produce anorthite according to the equation below.

313

950℃ Al6Si2O13 (Mullite) + CaO  →CaAl2Si2O8 ( Anorthite)

(3)

314

Portlandite was detected in all the samples. Notably, there is little difference among the

315

conditions, indicating that co-combustion brought hardly any change to the mineral

316

composition of fly ash. The relatively low blend ratio may account for this.

317

The size distributions of the fly ash for the difference cases were obtained by Coulter

318

LS230 laser particle size analyzer (Figure 13). The mean diameters of fly ash particles with

319

different blend ratios are shown in Figure 14. Three runs of analysis were done for each

320

sample to gain an average value. Particle size for corn stalk cases had a descending trend with

321

the blend ratio. The result verifies the inference in Section 3.2. Lopes [36] investigated the

322

particulate emissions from coal co-firing with biofuels, reporting that the presence of aerosol


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

323

forming elements such as K, Na, and Cl in biofuels had close relationships with the formation

324

of very fine particles. Niu [6] and Wang [37] reported that alkali metal aerosols increases

325

stickiness of fly ash particles and KCl reduces the melting temperature of the fly ash, and

326

thus slagging is increased. The overall trend for wood cases increased and the particle size for

327

10% wood was far larger than that for pure coal. Gani [38] and Al-Naiema [39] also reported

328

that co-firing sawdust shifts the particle size distribution of the ash particles from fine

329

particles to coarse particles. Large particles usually have high impact energy and may

330

rebound back into the flow after hitting the probe instead of depositing on the surface [40, 41].

331

This result may account for the observation that no deposit formed during co-combustion of

332

10% wood and coal. It can be inferred that change in particle size of fly ash should be an

333

important reason for the very different deposition behavior in wood co-firing case.

334

4. CONCLUSION

335

The growth of ash deposits for co-firing coal with wood and corn stalk was investigated

336

by image processing technique, respectively. Deposit thickness and heat flux versus time

337

were measured. The influence of co-firing biomass was revealed by XRD and SEM-EDS

338

analysis. The mineral composition and size of fly ash was also analyzed. The main

339

conclusions are as following:

340

(1) Co-firing corn stalk increased the slagging propensity significantly. The slag grew faster

341

and the melting degree also increased. In addition, the effect of adding corn stalk


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Energy & Fuels

342

increased as the blend ratio rose. The deposit thickness increased from 5.19 (coal) to 5.5

343

(5%) and 6.1 (10%) mm. By contrast, adding wood to coal remarkably reduced the ash

344

deposition with a deposit thickness of 2.09 mm for 5% ratio. Hardly any ash deposit was

345

formed with a higher wood ratio.

346

(2) Co-firing corn stalk resulted in sever heat-transfer-deterioration. The deposit thickness

347

increased on one hand, on the other hand, bubbles formed and decreased the effective

348

thermal conductivity. Nevertheless co-firing wood would improve heat transfer by

349

inhibiting ash deposition.

350

(3) The comparison of the mineral composition of deposits between pure coal and biomass

351

co-firing conditions demonstrated that co-firing corn stalk produced more alkali and

352

Mg-based minerals. Co-firing wood brought little change to mineral species but increased

353

the intensity of anhydrite.

354

(4) Co-firing wood increased the particle size of fly ash, which had a significant influence

355

on deposition behaviour. Nevertheless co-firing corn stalk decreased the particle size.

356

AUTHOR INFORMATION

357

Corresponding Author

358

*Telephone: +86-571-87952598. Fax: +86-571-87951616.

359

E-mail: [email protected].

360

Notes


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

361

The authors declare no competing financial interest.

362

ACKNOWLEDGEMENTS

363

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

364

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365

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366

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[3] Bartolome, C.; Gil, A. Ash deposition and fouling tendency of two energy crops (cynara

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technique. Appl. Therm. Eng. 2014, 70, 77−89.

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chlorine, and sulphur in ash, particles, deposits, and corrosion during wood combustion

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[10] Li, L.; Yu, C.; Huang, F.; Bai, J.; Fang, M.; Luo, Z. Study on the deposits derived from a biomass circulating fluidized-bed boiler. Energy Fuels 2012, 26, 6008−6014.

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[12] Stam, A.; Haasnoot, K.; Brem, G. Superheater fouling in a BFB boiler firing wood-based fuel blends. Fuel 2014, 135, 322−331. [13] Robinson, A.; Junker, H.; Baxter, L. Pilot-scale investigation of the influence of coal– biomass cofiring on ash deposition. Energy Fuels 2002, 16, 343−355.

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biomass: Experimental studies on a laboratory-scale furnace and 110MWe power unit.


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temperatures. Fuel 2013, 103, 454−466.

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Manuel Andres, J. Effect of co-firing on emissions and deposition during fluidized bed

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modeling of particle impaction and sticking. Fuel 2016, 165, 41−49.

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transformation by co-firing of coal with high ratios of woody biomass and effect on

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agglomeration problems in FB boilers. A powder XRD and SEM-EDS study. Energy

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pellet combustion of soft wood and wheat straw. Fuel Process. Technol. 2016, 143,

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from combustion of pulverized lignite with high calcium content in utility boiler. Energ.

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Page 25 of 40 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

Energy & Fuels

Table 1. Fuel properties

477

Fuel

Coal

Wood

Corn

Moisture, (wt. %, ar)

2.77

14.8

12.1

Proximate (wt. %, db)

analysis, Volatile matter

31.1

65.4

52.5

Fixed carbon Ash analysis, Carbon

55.3 10.8 67.8

18.2 1.62

10.7 24.7 31.4

Ultimate (wt. %, daf)

Hydrogen Nitrogen

3.86 0.810

56.9 5.23 1.34

Sulfur Oxygen

0.690 13.3 27.4 1180

0.120 36.5 17.6 1483

0.240 26.8 12.4 1156

1198 1208 1219

>1500 >1500 >1500

1212 1224 1257

15.7 20.2

1.83 62.9

10.4 9.29

9.84

1.19

3.84

0.510 3.07

4.84 4.20

8.77 6.17

0.174

0.181

0.082

TiO2

0.890

0.223

0.509

Na2O

0.580

0.384

0.774

P2O5

0.09

1.08

1.66

SiO2

34.2

4.04

52.7

SO3

14.6

1.70

2.16

HHV (MJ/kg) Ash fusion temperature, IT

3.48 1.27

(℃) ST HT FT Ash analysis(wt% ash) Al2O3 CaO Fe2O3 K2O MgO MnO2

478 479 480 481


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

Page 26 of 40

Table 2. Experimental Conditions

482

Fuel

Coal and biomass

Excess air ratio

1.2

Biomass blend ratio (wt%)

Case A Case B

0 5

Case C the second stage

10 ~2.8 ~1623

the third stage Oxygen concentration at the furnace outlet (%)

~1573 4-5

Exposure time of the ash deposit probe (min)

140

Primary air velocity (m/s) Furnace temperature (K)

483 484 485 486 487 488 489 490

Table 3. Elements distributions by EDS wt. %

Layer

Na

Mg

Al

Si

K

Ca

Ti

Fe

Coal

1

0.55

1.64

12.86

42.56

1.97

14.01

0.42

25.99

2 3

0.52 0.52

1.69 1.47

12.21 14.45

48.53 51.8

1.34 1.23

19.27 15.55

0.63 0.85

15.81 15.03

5%wood

1 2

0.54 0.00

1.98 1.32

12.61 8.52

45.46 63.04

0.95 1.34

24.86 19.39

0.80 0.00

12.80 6.39

5%corn

1 2 3

1.16 0.87 -

2.67 2.04 0.69

11.37 13.67 4.44

54.02 49.89 78.82

4.37 2.77 2.56

11.66 20.44 7.93

0.68 0.74 -

14.07 9.58 5.55

10%corn

1 2 3

1.32 1.19 0.73

1.5 1.88 1.94

9.14 7.89 8.37

45.91 59.74 46.79

5.43 6.27 2.47

12.44 11.11 15.85

0.64 0.51 0.53

23.62 11.4 22.54

491


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Energy & Fuels

492

493 494

(a)

495 496

(b)

497 498

(c)

499

Figure 1. (a)Schematic of the test rig, (b) ash deposition probe, and (c) CCD monitoring unit

500

[8].

501


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

502 503

Figure 2. Size distribution of pulverized coal.

504 505 506 507 508 509 510


Page 28 of 40

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Energy & Fuels

511 512

Figure 3. Digital image processing, (left) original image and (right) edge image.

513 514 515 516 517 518 519 520 521 522 523 524 525 526


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

Page 30 of 40

527 528

(a) coal

529 530

(b) 5%wood

(c) 10%wood

531 532

(d) 5%corn

533

Figure 4. Image of deposits.

(e) 10%corn

534 535


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Energy & Fuels

3 2 2 1 1

536 537

(a) coal

(b) 5%wood

3

3

2

2

1

1

538 539

(c) 5%corn

(d) 10%corn

540

Figure 5. Cross-sections of collected deposits.

541 542 543 544 545 546 547 548


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

549 550

Figure 6. Deposit thickness versus time.

551 552 553 554 555 556 557 558


Page 32 of 40

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Energy & Fuels

559 560

(a)

561 562

(b)

563

Figure 7. (a) Probe surface temperatures versus time, (b) q/q0 versus time.

564 565 566 567


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

Page 34 of 40

568 569

(a) coal

(b) 5%wood

570 571

(c) 5%corn

572

Figure 8. q/q0 versus thickness.

(d) 10%corn

573 574 575 576 577 578 579 580


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Energy & Fuels

581 582

Figure 9. Bubbles forming (left) and bursting to form pores (right).

583 584 585 586 587 588 589 590 591


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

Page 36 of 40

592 593

(a) coal

594

(b) 5%wood

(c) 10%wood

597

(d) 5%corn

(e) 10%cornCoal

598

Figure 10. Mineralogy of ash deposits.

599

Q: Quartz SiO2; Ah: Anhydrite CaSO4; He: Hematite Fe2O3; Ao: Anorthite CaAl2Si2O8;

600

M: Magnetite Fe3O4; Ma: Magnesioferrite MgFe2O4; D: Diopside - CaMgSi2O6;

601

Au: Augite (Ca, Na)(Mg, Fe, Al, Ti)(Si, Al)2O6; Ab: Albite NaAlSi3O8; S: Sylvine-KCl.

595

596

602 603


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Energy & Fuels

Layer 1

Layer 2

Coal

5%wood

5%corn

10%corn

604

Figure 11. SEM micrographs of each layer of deposits.


Layer 3

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

605 606

Figure 12. Mineralogy of fly ash.

607

M: Mullite Al6Si2O13; Q: Quartz SiO2; P: Portlandite: Ca(OH)2.

608 609 610 611 612 613 614 615


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Energy & Fuels

616 617

Figure 13. Size distributions of the fly ash.

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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

632 633

Figure 14. Mean diameters of fly ash with different blend ratios.

634


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Behavior of slagging deposits during coal and biomass co-combustion

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