Article pubs.acs.org/EF
Effects of Coal Type on Growth and Shedding of Ash Deposit in Pulverized Coal Combustors Naoki Sato,*,† Shunichiro Ueno,† Dedy E. Priyanto,† Emi Ohno,‡ Yoshiaki Matsuzawa,§ Yasuaki Ueki,∥ Ryo Yoshiie,⊥ and Ichiro Naruse∥ †
Chemical Engineering Department, IHI Corporation, Yokohama 235-8501, Japan Combustion Engineering Department, IHI Corporation, Tokyo 196-8686, Japan § New Products Incubation Center, IHI Corporation, Yokohama 235-8501, Japan ∥ Division of Systems Research, Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8603, Japan ⊥ Department of Mechanical Science and Engineering, Nagoya University, Nagoya 464-8603, Japan ‡
ABSTRACT: Removal of ash deposits formed on boiler tubes or furnace walls is required to inhibit the slagging and fouling problems. The effect of coal type on the growth and gravity shedding of ash deposits was evaluated by ash deposition experiments and scanning electron microscope (SEM) analysis of the cross-sectional structures of the ash deposits. Three types of pulverized coalbituminous, subbituminous, and lignite coalswere tested in the experiments. The behavior of ash deposition on a water-cooled tube inserted at an inner furnace temperature of 1673 K was evaluated by video camera observation, temporal variation of the tube surface temperature, and chemical equilibrium calculations. For the subbituminous and lignite coals, the cycle periods of ash shedding were longer than that of the bituminous coal because of the formation of a wider ash deposit. The distributions of particle packing fraction (PPF), the particle sizes, and the chemical compositions of the ash particles within the deposits were analyzed by an SEM and a computer-controlled SEM. A powder layer was formed near the tube, and the PPF in the deposit gradually increased with the growth of the deposit because of ash coalescence. The effects of the PPF on the ash shedding were larger than that of the chemical compositions. Therefore, the deposit detachment occurred within the powder layer that became low strength due to the low PPF. Additionally, the thickness of the residual deposit that remained on the tube after ash shedding depended on the thickness of the powder layer in the deposit. The subbituminous and lignite coals have formed a deposit with a thin powder layer because ash coalescence progressed near the tube surface. Therefore, their residual deposits have been thin compared with the bituminous coal. The findings of this study can be put to practical use in the design of pulverized coal-fired boilers.
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al.,16−18 and others have measured the bond strengths of ash deposits by thermo-mechanical analysis. Kaliazine et al. calculated the mechanical stresses at the interface between tubes and ash deposits by sootblower jet impacts in kraft boilers.19 Pophai et al. have predicted the jet impact force by using computational fluid dynamics.20 Bashir et al. discussed the effects of tube surface temperature and exposure time at the maximum impact pressure of the jet on removing the ash deposits in a straw and wood pellet cofired boiler.21 Furthermore, the erosion of ash deposits has been studied by Raask,2 Oka,22 and others. Zboger et al. summarized several ideas to evaluate the shedding of ash deposits.23 In a previous study by the present authors, it was confirmed by the SEM observation of the cross section of ash deposits on a tube that the reductions of the particle packing fraction (PPF) and Fe content in the powder layer of ash deposit contributed to the shedding of the ash deposits.24 However, the effects of coal type on the ash shedding have not been elucidated yet. Therefore, boiler designs have generally depended on empirical indicators. However, the empirical indicators cannot predict the precise
INTRODUCTION Ash deposition on boiler tubes or furnace walls inhibits heat transfer in coal-fired boilers because of the low heat transfer coefficient of the ash deposits. The boiler tubes may incur damage by falling ash deposits and blocking gas lines due to the growth of ash deposits between the superheater tubes. Therefore, the removal of the deposits is important for stable boiler operation. In particular, if low-rank coals are used as fuel, ash deposition in the boiler is generally more severe owing to the low ash fusion temperature. Therefore, it is necessary for boiler design to elucidate the mechanisms of the growth and shedding of ash deposits during combustion of different types of coals. The phenomena of ash adhering to the surfaces of boiler tubes have been studied by many researchers. Bryer,1 Raask,2 and Couch3 summarized the studies on ash formation and ash deposition in furnaces. The behavior of ash deposition in combustion of low-rank coals, biomass, and coal blends has also been studied.4−12 The predictions of ash deposition have been studied by Baxter et al.,13 Rushdi et al.,14 and others. However, there have been a few studies on the shedding of ash deposits. Raask predicted that the bond strength of an ash deposit is influenced by the galvanic reaction at the interface between the tube and the deposit.2 Piroozmand et al.,15 Naganuma et © XXXX American Chemical Society
Received: December 3, 2015 Revised: June 16, 2016
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DOI: 10.1021/acs.energyfuels.5b02836 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Properties of Tested Coals Fuel heating valuea (kJ/kg) moistureb (wt % as received) proximate analysisb (wt %, dry)
ultimate analysisc (wt %, dry)
ash fusion temperatured (K)
HHV LHV volatile matter fixed carbon ash carbon hydrogen oxygen nitrogen total sulfur combustible sulfur noncombustible sulfur oxidizing
softening hemispherical fluid softening hemispherical fluid
reducing
ash compositione (mol %)
Si Al Fe Ca Mg Ti Na K P S
Coal A
Coal B
Coal C
30 100 29 044 4.0 33.6 54.7 11.7 72.9 4.25 9.27 1.41 0.49 0.47 0.02 >1773 >1773 >1773 >1773 >1773 >1773 63.6 27.3 3.0 0.16 0.93 0.88 0.96 2.0 0.13 1.0
28 250 26 824 7.8 44.5 41.0 14.5 65.0 5.47 14.33 0.61 0.22 0.07 0.15 1503 1543 1613 1513 1553 1663 41.5 30.5 4.4 10.5 3.5 0.90 2.6 0.50 0.90 4.7
23 150 21 897 9.0 46.9 46.4 6.7 67.91 4.57 20.14 0.68 0.18
