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Fouling characteristics of coal biomass cocombustion and the influence of deposition surface Hao Zhou, Jiakai Zhang, Weichen Ma, Yong Xu, and Menghao Zhao Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on June 1, 2017
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Energy & Fuels
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Fouling characteristics of coal biomass co-combustion
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and the influence of deposition surface
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Hao Zhou*, Jiakai Zhang, Weichen Ma, Yong Xu, Menghao Zhao
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Zhejiang University, Institute for Thermal Power Engineering,
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State Key Laboratory of Clean Energy Utilization, Hangzhou, 310027, P. R. China
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Abstract:
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Fouling deposition is a severe problem in boilers, especially co-firing coal with
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biomass. In this study, the influence of the deposition surface on the fouling
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mitigation and heat flux through surface of the heat transfer tube are investigated.
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Chromium (Cr) was coated on the surface of the heat transfer tube; an online
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measurement is applied to monitor the growth of the fouling deposit. Meanwhile,
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mineralogy and microstructure of the deposits are obtained by the X-ray diffraction
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(XRD) and scanning electron microscopy (SEM). The results showed that average
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thickness of the fouling deposits decreased from 11.09 mm to 4.32 mm and the
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average heat flux increased from 238.93 to 261.11 kW/m2 as the surface of the heat
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transfer tube coated with Cr. The XRD results showed that the content of the sodium
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which has a great effect on the adhesion force is reduced as the surface of the heat
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transfer tube coated with Cr. Consequently, the Cr coating showed good performance
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in reducing the fouling deposition.
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Keyword: surface modification; fouling mitigation; co-firing.
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1. Introduction:
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In order to reduce the CO2 emissions, biomass is used in the power industry and
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the co-firing coal with biomass is an economic way to use the biomass [1,2]. However,
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the fouling of heat transfer surfaces increase due to the high content of alkali and
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chlorine in biomass [3-7], causes severe operation problems in boilers, such as
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corrosion of the heat transfer surfaces, reduction of heat flux through the boiler tubes
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and the decreasing of system reliability [8-11]. It is essential to understand the fouling
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deposition mechanism and characteristics. In recent years, much work has been
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conducted to study the fouling propensity of the high-phosphorus solid fuels [12], the
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possibility with the alkali index [13]. Khan et al. reported that silica has significant
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influence on fouling [14]. The concentration of the chlorine has a great effect on the
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growth of the ash deposition [15]. The sulphur can react with Ca and alkalis to form
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the low melting temperature compounds which have a great effect on the fouling [16].
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Meanwhile, some studies focused on the mitigation mechanism of the fouling
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deposition in boiler. Afgan et al. developed an expert system to reduce the fouling
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deposition on the boiler heat transfer surfaces [17]. Spent bleaching earth (SBE) as an
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additive are applied in straw combustion to reduce the ash deposition by converting
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the low melting temperature compounds into high temperature melting substances
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[18]. A promising technique to reduce ash deposition is surface modification.
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Naganuma and Naruse investigated the adhesive forces between the ash deposit and
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several modified surfaces to control the deposition [19-22]. The nano-modified
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surfaces showed a good performance in reducing the fouling [23]. Al-Janabi et al.
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used the electroless Ni-P coatings to decrease the fouling rate [24]. However, limited
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studies monitor the growth of fouling online. In most of the studies, fouling deposits
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collected and analyzed after the combustion, the mechanism is not well investigated.
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In this paper, the reduction mechanism of fouling from co-firing coal with wood
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has been investigated with an online measurement. The influence of the ash
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deposition surfaces on fouling mitigation conducted in a 300kW furnace, the Cr is
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applied in this paper to modify the heat transfer surface to control the ash fouling. The
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chemical and physical characteristics of ash deposits and fly ash are analyzed using
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X-ray fluorescence spectrometer, scanning electron microscopy and X-ray diffraction
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(XRD) spectrometer.
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2. Experiment
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2.1 Description of the experimental system
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In this experiment, a vertical pulverized coal combustion system is shown in
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Figure 1[25], is applied. It is a pilot-scale furnace (height: 3950 mm and inner
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diameter: 350 mm) and the power of the facility is 300 kW. The main components of
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the system: feeding system, temperature measurement system, sampling system. The
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experiment setup is listed in Table 1. In this study, 10 wt% of wood is mixed with coal.
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The wood and coal are pulverized and mixed before feeding into the furnace. The
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feeding rate of the fuel is 40-45 kg/h, determined by a screw feeder. After about 3 h
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pre-heating, the temperature of the furnace reaches the set point and stabilizes, and
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then the heat transfer tube is installed into the furnace. The temperature of the heat
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transfer tube is controlled by the oil-cooling system to simulate the heat transfer tube
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in boiler. A charge-coupled device (CCD) camera is placed opposite the heat transfer
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tube is applied to monitor the growth of the fouling; the water-cooling system is
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applied to protect the CCD camera.
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2.2 Measurement method of the deposit thickness and the heat flux
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The measuring system is shown in Figure 2, the detailed information already
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reported [25]. The video of the growth of fouling deposition is obtained through the
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CCD camera. Images are extracted from the video every minute and Figure 3 shows
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an example, these images are processed by the MATLAB. As shown in Figure 4, the
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original image is converted to binary image. The edges of the heat transfer tube and
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the fouling deposition are extracted. The pixels corresponding respectively to the heat
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transfer tube radius ( ) and the maximum thickness ( ) of the fouling deposition are
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calculated. The maximum thickness of fouling is used to represent the thickness of the
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fouling.
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The thickness (ℎ) of the fouling deposition is defined by
ℎ = ×
(1)
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Where ℎ : the thickness of the fouling deposition; : the pixel of the heat transfer
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tube radius; : the pixel of the thickness of the fouling deposition; : the radius
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of the heat transfer tube and its value is 20 mm.
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In order to get a better evaluation for the performance of the Cr coating in
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reducing the fouling deposition, the average thickness is introduced, the average
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thickness ( ) of the fouling deposition is defined by
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=
(2)
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Where : the average thickness of the fouling deposition. ℎ : the thickness
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of the fouling deposition described in section 2.2; : the experiment time.
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The usual shape of the fouling is shown in Figure 2b. In order to simplify the
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calculation of the heat flux through the deposition surface, assume that the heat
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transfer is one-dimensional, the heat conduction model of cylinder is applied to
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calculate the heat flux.
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The heat flux through the deposition surface is defined by =
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(
)
"#$( )
(3)
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Where : the heat flux through the deposition surface; % :the thermal conductivity of
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the substrate material (22.5 W m-1 K-1); &'( and &)$$ : the temperatures of the
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outer and inner surfaces of the heat transfer tube respectively; *'( : the distance
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between the outer hole and the center of the heat transfer tube; *)$$ : the distance
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between the inner hole and the center of the heat transfer tube (see Figure 2b).
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The average heat flux is defined by
+ =
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,
(4)
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Where + : the average heat flux; : the heat flux through the deposition
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surface described in section 2.2; : the experiment time.
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The temperatures of inner and outer surfaces are measured by two type K
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thermocouples inserted into the small holes in the heat transfer tube (see Figure 2b).
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2.3 Surface treatment
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Cr coating was applied to modify the surface of the heat transfer tube in this
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study. Cr was coated onto the heat transfer tube by automatic plating equipment to
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approximately 44µm in thickness. The properties of the coating and the substrate
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material is listed Table 2.
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2.4 Fuel properties and analytical methods
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Table 3 shows the properties of the fuel in including ultimate analysis, proximate
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analysis, ash fusion temperature and ash compositions. The fuel particle size
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distribution is shown in Figure 5. The ashes of coal and wood are produced according
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to Standards Australia 2000. The Hitachi S-3700N scanning electron microscopy
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(SEM) is applied to obtain the microstructure of the fouling deposition. The
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mineralogy of fouling deposition is analyzed by the X-ray diffraction (XRD)
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spectrometer.
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3. Results and discussions
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3.1 The Growth of the Fouling Deposition
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Figure 6 shows the photos of the fouling deposits for the two cases (Case A:
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substrate material; Case B: modified surface). These deposits are collected after
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experiment, because it is difficult to collect the shedding deposits during the
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experiment, the size and the shape of fouling deposits for the two cases have the
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significant difference. Apparently, the color and the shape of the two cases are
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different. The color of the case B is darker than cases A. The growth curves of the
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deposit thickness for the two cases are shown in Figure 7. The growth curves for the
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two cases have similar variation tendency, the shedding phenomenon of the fouling
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deposition occurs in both two cases. The shedding frequency of the fouling deposition
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in case B is greater than that in case A, the thickness of the fouling deposit in case A is
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greater than that in case B in most time. An interesting finding, fouling deposit in case
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B is almost removed after shedding (see the data symbols in the ellipses in Figure 7).
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Meanwhile, it never happens in case A, there is still some fouling on the heat transfer
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tube after every shedding in case A. It indicates that the adhesion force between
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deposit and the surface of the heat transfer tube in case A is greater than that in case B,
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it is difficult to remove all the fouling after shedding in case A because of the great
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adhesion force between the deposit and the surface of the heat transfer tube.
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The average thicknesses are calculated through equation (2). The average
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thicknesses of the fouling deposits are 11.09mm and 4.32mm for case A and case B,
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respectively. The average thickness for case A is much greater than that for case B.
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Naganuma et al. controlled the ash deposition with coating. They found that coating
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could influence the adhesion force [19,20]. In this study, it indicates that the adhesion
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force between deposit and the surface of the heat transfer tube in case A is greater than
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that in case B [19, 20]. The Cr coating had a good performance in reducing the
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fouling deposition.
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3.2 The heat flux through the heat transfer tube
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The temperature data of the inner and outer surface of the heat transfer tube is
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shown in Figure 8. Figure 9 shows the heat flux through the tube versus deposit
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thickness. Case A and Case B show a similar tendency. It can be observed that the
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tendency of the scatterplot can be divided into two stages: stage 1 (stageO-M), stage 2
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(stageM-N). In stage 1, the heat flux decreases rapidly. In stage 2, the heat flux
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decreases slowly. Figure 10 shows the heat flux through the tube versus time. The
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heat flux through the heat transfer tube is an important index to evaluate the
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performance of the Cr coating in increasing the thermal efficiency. However, in figure
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10, it can be observed that there is shedding (in the ellipses) of fouling deposition
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existing in both two cases and heat flux waved. It is difficult to make a comparison
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between these two cases. The average heat flux is applied to evaluate the effect of Cr
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coating on the heat flux.
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The average heat fluxes through the surface of the heat transfer tube are 238.93,
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261.11 kW/m2 for case A and B, respectively. This indicates that Cr coating had a
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good performance in increasing the thermal efficiency.
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3.3 The mineralogy of the fouling deposition
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The XRD patterns of the fouling deposits for the two cases are shown in Figure
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11. In order to investigate the influence of the Cr coating on the fouling
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characteristics, samples collected from the bottom part of the fouling deposition
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(scraped from the surface of the heat transfer tube) are analyzed by XRD. The main
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mineral compositions in case A are anorthite (sodian), mullite, and quartz. Meanwhile,
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the anorthite, mullite, and quartz are the main compositions in case B, it is consistent
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with analysis result of the fuel that the aluminum, silicium, and calcium are rich in
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the fuel, the Rietveld-based technique applied to calculate the relative proportions of
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the minerals (shown in table 4). The proportions of the minerals in the two cases are
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similar. The major mineral is anorthite. However, an interesting finding is that the
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anorthite in case A contained sodium, while sodium did not detected in case B. This
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result suggested that the Cr coating could influence the content of the sodium.
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Naganuma et al. also found the similar phenomenon [19]. Alkalis condensed on the
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heat transfer surface at the beginning of the formation of the ash deposition. They
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found that the coating could effect on the condensation of the alkalis [19]. The
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condensation liquid phase strongly affects in the adhesion behavior. Accordingly it is
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thought that the Cr coating influences the adhesion behavior by control the
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condensation of the alkalis.
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3.4 The microstructure of the fouling deposition
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The microstructures of the bottom part and the upper part of the fouling
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deposition are obtained by the scanning electron microscopy (SEM). Figure 12
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showed the SEM images for the two cases. For case A, the bottom part of the
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deposition appeared to have a porous and loose structure, which the black spots are
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the tiny holes in the deposition. This structure results in the relatively low heat
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conductivity [25]. However, a continuous phase is presented in the upper part and its
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heat conductivity is higher than that of bottom part. This result is consistent with the
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variation of the heat flux versus thickness in section 3.2. These two structures
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correspongding to stageO-M and stageM-N. For case B, it appears similar results with
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case A. There are many spherical particles (in the ellipses) adhering to the matrix in
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the bottom part of the fouling in both cases. There is no significant agglomeration of
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the particles in case B compared with case A. This may result in the deposit easier
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removed from the tube surface.
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4 Conclusions
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In this study, an online measurement is applied to investigate the influence of the
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deposition surface on the fouling characteristics for the co-firing coal with wood
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biomass. In addition, the XRD and SEM are applied to analyze the fouling deposition,
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in order to study the reduction mechanism of fouling. The result of the deposition
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growth is that the average thicknesses of the fouling deposits are 11.09 and 4.32 mm
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for case A and case B, respectively. The Cr coating shows a good performance in
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reducing the fouling deposit. Meanwhile, the average heat fluxes through the surface
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of the heat transfer tube are 238.93 and 261.11 kW/m2 for case A and B, respectively.
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The results of XRD and SEM show that the Cr coating influences the adhesion
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behavior by control the condensation of the alkalis. The bottom part of the deposit
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has more porous and losser structures than upper part in both cases. This result is
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consistent with the variation of the heat flux versus thickness. There is no significant
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agglomeration of the particles in case B compared with case A. It is main reason, the
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fouling deposit in case B is easier removed from the heat transfer tube. Therefore, it
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is analyzed that Cr coating has effective control on the fouling deposition.
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Author information Corresponding
Author:
Hao
Zhou,
*Tel:
+86-571-87952598;
Fax:
+86-571-87951616. E-mail address:
[email protected] 218 219 220 221
ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (51476137).
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Figures
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Figure 1. Schematic diagram of the combustion system [25]
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Figure 2. Schematic diagram of the measuring system: (a) schematic diagram of the
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heat transfer tube, (b) detail of the cross section of the heat transfer tube, (c)
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schematic diagram of image sampling system.
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Figure 3. The Images of fouling deposit extracted from the video for case A: substrate
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material
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Figure 4. Processing of the image: (a) original image, (b) edge image.
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Figure 5. The size distribution of the fuel
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Figure 6. The photos of the fouling deposits: (a) Case A: substrate material; (b) Case
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B: modified surface
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Figure 7. Variation of the thickness with time: (a) case A: substrate material (b) case B:
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modified surface.
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Figure 8. The temperature data of the inner and outer surface of the heat transfer tube:
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(a) case A: substrate material (b) case B: modified surface
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Figure 9. The change of heat flux versus thickness for two cases: (a) case A: substrate
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material (b) case B: modified surface
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Figure 10. The heat flux through the tube versus time: (a) case A: substrate material (b)
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case B: modified surface
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Figure 11. The XRD patterns of the fouling deposits for the two cases: (a) case A:
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substrate material (b) case B: modified surface.
331
Figure 12. The SEM images for the two cases: (a) case A: substrate material (b) case
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B: modified surface
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Figure 1. Schematic diagram of the combustion system [25]
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(a)
368 369
(b)
370 371
(c)
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Figure 2. Schematic diagram of the measuring system: (a) schematic diagram of the
373
heat transfer tube, (b) detail of the cross section of the heat transfer tube, (c)
374
schematic diagram of image sampling system.
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34min
35min
36min
378 379
37min
38min
39min
380 381
40min
41min
42min
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Figure 3. The Images of fouling deposit extracted from the video for case A: substrate
383
material
384 385 386
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387 388 389
(a)
(b)
Figure 4. Processing of the image: (a) original image, (b) edge image.
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Figure 5. The size distribution of the fuel
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414 415 416 417
(a)
(b)
Figure 6. The photos of the fouling deposits: (a) Case A: substrate material; (b) Case B: modified surface
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433 434 435
Figure 7. The variation of the thickness with time: (a) Case A: substrate material; (b) Case B: modified surface.
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446 447 448
449 450 451 452
(a)
(b)
Figure 8. The temperature data of the inner and outer surface of the heat transfer tube: (a) Case A: substrate material; (b) Case B: modified surface
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467 468
Figure 9. The change of heat flux versus thickness for two cases: (a) Case A: substrate
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material; (b) Case B: modified surface
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482 483 484
Figure 10. The heat flux through the tube versus time: (a) Case A: substrate material; (b) Case B: modified surface
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495 496
a Quartz: SiO2; b Anorthite: CaAl2Si2O8; c Mullite: Al(Al0.69Si1.22O4.85); d Anorthite
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(Sodian): Na0.25Ca0.71(Al2Si2O8)
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Figure 11. The XRD patterns of the fouling deposits for the two cases: (a) Case A:
499
substrate material; (b) Case B: modified surface.
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511 512
513 514
bottom part
upper part (a)
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516 517
bottom part
upper part
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(b)
519
Figure 12. The SEM images for the two cases: (a) Case A: substrate material; (b) Case
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B: modified surface
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Tables
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Table 1. Experimental conditions Fuel
Coal and Wood
Excess air ratio Deposition surface
1.2 case A
Uncoated
case B
Cr
Standard deviation 0.017
Velocity of the gas (m/s)
2.8
0.122
Average furnace temperature around the heat exchange tube (℃)
1100
17.364
Oxygen concentration at the furnace outlet (%)
4.0-5.0
Experimental time (min)
100
527 528 529 530 531 532 533 534 535 536 537 538 539 540 541
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542 543
Table 2. Properties of coating and the substrate material Material
Cr coating
Substrate material
Mainly chemical composition (wt%)
Cr (>99%)
Mn(≤ 2.0), Cr(18.0-20.0), Ni(8.0-10.5), C(≤ 0.08), Fe(Balance)
Thermal conductivity (W/m.K) Thickness (μm)
93.7 44
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22.5 -
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Table 3. The properties of the fuel Fuel Moisture (wt %, ad) Proximate analysis (wt %, ad)
Ultimate analysis (wt %, ad)
Coal
Wood
8.30
14.75
fixed carbon
42.02
18.22
ash
24.99
1.62
volatile matter
24.69
65.41
C
53.62
47.59
H
3.64
4.37
N
0.83
1.12
S
0.64
0.1
O
7.98
30.55
Qb (MJ/kg, ad)
20.6
17.6
IT
1450
1483
ST
＞1500
＞1500
HT
＞1500
＞1500
FT
＞1500
＞1500
Al2O3
31.484
1.827
CaO
2.997
62.924
Fe2O3
4.041
1.19
K2O
0.451
4.839
MgO
0.34
4.198
MnO2
0.026
0.181
TiO2
0.985
0.223
Na2O
0.231
0.384
P2O5
0.273
1.076
SiO2
42.746
4.038
SO3
0.756
1.704
Cl
0.109
0.294
Ash fusion temperature (℃)
Ash analysis (wt% ash)
561 562 563 564 565 566
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Table 4. The relative proportions of the minerals in the deposits. Proportion (wt%)
Case A: Substrate material
Case B: Cr coating
Quartz
4.8
1.7
Anorthite
-
91.6
Mullite
12.6
6.7
Anorthite (Sodian)
82.6
-
568 569 570 571 572 573 574 575 576 577
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