Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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In Situ Diagnostics on the Dynamic Processes of Ash Deposit Formation, Shedding, and Heat Transfer in a Self-Sustained DownFired Furnace Qian Huang,†,‡ Yang Xu,† Qiang Yao,† and Shuiqing Li*,† †
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering and ‡Zhou Pei-Yuan Center for Applied Mathematics, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: Ash deposition is the most acute problem that hinders safe and efficient operations of coal-fired boilers. In this paper, we investigate the ash deposition process by incorporating in situ observations with standard procedures of ash deposit sampling. A specially calibrated digital camera is used. A sodium-rich Zhundong (ZD) lignite, a Si/Al-dominated bituminous coal, and their blend were burned in a 25 kW down-fired furnace. The deposits were collected at a position with flue gas temperatures of 850 °C. The newly developed RGB pyrometry quantitatively measures the deposit surface temperature in a range of 800− 1150 K. During the dynamic deposition process, the deposit surface temperature approaches the local flue gas temperature within 1 h of elapsed time. The heterogeneity of the deposit surface is caused by either the deposition of burning coarse particles or the shedding event of the deposit layer with a low cohesive strength. The deposit structure is characterized by its packing ratio, which positively correlates with the deposit mass load. Moreover, the effective thermal conductivity of the deposit can be derived from a “two-resistance” model and ranges from 0.14 to 0.32 W m−1 K−1. After ZD lignite is blended with the bituminous coal, the shedding period of the ash deposit is greatly reduced. The effective conductivity of the deposit is also enhanced in a nonlinear manner.
1. INTRODUCTION The utilization of solid fuel, including coal and biomass, still plays an important role in worldwide power generation. As a result of the existence of non-combustible materials in the fuel, the ash-related problem has been a major issue in coal-fired power plants. The residual ash particle may deposit onto heat transfer tube surfaces, causing undesired slagging and/or fouling detrimental to safe and efficient boiler operations. The deposited layer could inhibit heat transfer, corrode tube surfaces, and even plug flow passages.1−3 Numerous studies have investigated the ash deposition problems for the mechanisms and countermeasures.1−9 The experiments thus far have mainly relied on off-line analyses of the ash deposit samples.1−8 The main mechanisms of ash deposition include inertial impaction, thermophoresis, condensation, surface reaction, and eddy deposition.2−4 Furthermore, the ubiquitous existence of the layered structure in the ash deposit is essential to the fouling initiation, especially during the combustion of low-rank coal samples.5−9 In particular, the Chinese Zhundong lignite (abbreviated here as ZD lignite) has attracted widespread attention in recent years, as a result of its huge reserve and extreme fouling propensity.6,9 The fly ash formation6,10 and deposition6,9,11−14 problems have been extensively investigated in the literature for better remedial measures. However, the off-line analysis suffers from some disadvantages. First, only limited information on the fly ash particle can be obtained from the deposit. To address this issue, some recent studies have incorporated “on-line” measurements of fly ash aerosols with the conventional deposit sampling.6,15 Second, the off-line analysis provides no clue to the deposit © XXXX American Chemical Society
surface temperature, which has a prominent effect on the sticking probability of impacting particles, the intensity of thermophoretic deposition, and the extent of deposit sintering.1,3,9 Moreover, the off-line analysis is not enough for a thorough investigation of the deposit shedding, an important process of ash deposit removal.16−19 Therefore, in situ diagnostic methods are needed to monitor the growth and shedding of the deposit layer and to conduct non-contact measurements of the deposit surface temperature. The in situ measurement of the deposit surface temperature can be achieved by various methods. Robinson et al. applied optical pyrometers to measure the surface temperature of a rotating deposit layer outside a down-fired furnace.20,21 Then, the two- or three-color pyrometry had been developed to reduce the temperature uncertainty brought by the emissivity.22,23 Recently, Marshall and co-workers proposed a novel RGB pyrometry with a single and relatively inexpensive RGB digital camera.24 Our co-workers have successfully developed this technique to measure the surface temperature of burning pulverized coal particles.25,26 By taking advantage of the broad visual spectrum, this RGB pyrometry is more appropriate in contrast to those using two or three typical wavelengths for a temperature deduction. In addition, this simple optical setup is featured with a potential for multi-point temperature measureSpecial Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 19, 2017 Revised: January 21, 2018 Published: January 22, 2018 A
DOI: 10.1021/acs.energyfuels.7b03208 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels ments. Therefore, it is desirable to incorporate this novel and convenient method with conventional off-line characterizations to further investigate the ash deposition process of representative coal samples. In this paper, the development of camera-based RGB pyrometry facilitates a direct observation of the ash deposition process and the in situ measurements of the mean temperature of the deposit surface. The sodium-rich ZD lignite, a common Si/Al-dominated bituminous coal, and their blend were used. The combustion experiments were conducted inside a 25 kW down-fired coal combustor. The ash deposits were sampled at a location with the flue gas temperature around 850 °C, under which the fouling is most severe in practical boilers.6,27 In combination with the conventional deposit sampling in parallel tests, we characterized the global packing ratio, the cohesive strength, and the effective thermal conductivity of the deposit layer. The method has demonstrated its potential for practical use for the deposit monitor and characterization in complex environments.
Figure 1. Cumulative distribution of the particle volumes of the pulverized coal samples.
2. EXPERIMENTAL SECTION 2.1. Coal Properties. Two kinds of coal samples, the ZD lignite and a high-ash-fusion bituminous coal (denoted HAF-2), were used in this work. Table 1 lists the coal properties. ZD lignite is low in ash
Table 1. Coal Properties ZD lignite
HAF-2 bituminous
Proximate Analysis (wt %, Dry Basis) fixed carbon 63.54 55.17 volatile 30.58 22.51 ash 5.88 22.32 HHV (MJ/kg, dry basis) 28.83 27.19 Ultimate Analysis (wt %, Dry and Ash-Free Basis) C 71.60 85.44 H 3.16 5.18 N 0.78 1.42 Stotal 0.52 0.35 Cl 0.06 0.004 O (by difference) 23.88 7.61 Ash Composition (wt %) SiO2 5.31 69.08 Al2O3 5.25 22.04 Fe2O3 2.30 1.66 CaO 36.00 0.58 MgO 6.90 0.53 SO3 34.75 0.39 K2O 0.34 2.17 Na2O 5.76 0.49
Figure 2. Schematic of the experimental setup. Left is the front view of the 25 kW down-fired self-sustained furnace. Right is the top view of the configuration for ash deposit collection and in situ diagnostics. The RGB calibration curves of the digital camera are also presented. Here, bg denotes blue/green, gr denotes green/red, and br denotes blue/red.
Table 2. Operating Conditions of the Self-Sustained Furnace ZD coal feed rate (kg/h) air (Nm3/h) gas temperature at P4 (°C) flow rate of the probe cooling air (L/min)
HAF-2 ZD/HAF-2 = 2:1
2.8 26.4 835 200
3.2 24.2 847 190
3.0 26.0 856 196
conditions. Besides, a gas analyzer (ABB EL3020) was applied to monitor the flue gas compositions at the exit. The normal combustion condition was achieved with CO concentrations of
