INVESTIGATION ON ASH FOULING FORMATION OF INDUCED FAN BLADE AND HEAT EXCHANGER SURFACE IN A 1000 MW COAL-FIRED POWER PLANT He-xin LIU a, Jia-fan XIAO a, Hou-zhang TAN a*, Yi-bin WANG a, Fu-xin YANG a a MOE Key Laboratory of Thermo-Fluid Science and Engineering, Xi’an Jiaotong University, Xi’an, Shannxi, 710049, China *Corresponding author: [email protected] The control of fouling deposition on the main equipment has always been an important issue concerned by scientific research and industrial application. However, severe fouling deposits on the induced fan blade and the low- temperature economiser were found in a 1000MW coal-fired power plant with ultra-low emission. The deposit samples were collected and analyzed through X-ray diffraction spectrometer, X-ray fluorescence, elemental analyser and scanning electron microscope with energy dispersive spectrometers. The result shows that the deposits are mainly composed of tschermigite ((NH4)Al(SO4)2•12H2O), letovicite ((NH4)3H(SO4)2), calcium sulphate (CaSO4) and quartz (SiO2). The ammonium sulfate is the main component of the fouling deposits. It acts as an adhesive and makes an important contribution to the deposition. The analysis shows that the ammonia slip from denitrification system and the unreasonable temperature setting are the main reasons for fouling deposition. It is suggested that the high concentration of ammoniam slip at denitrification system and the rapid condensation of the sulphuric acid mist at heat exchanger should be paid more attention in coal-fire power plants. Key words: fouling deposition, ammonium sulfate, coal-fired power plant, induced fan blade, low-temperature economiser 1. Introduction With the improvement of ecological environment awareness, the impacts of pollutant emission from coal-fired power plants on frequent haze weather have gradually attracted attention [1, 2]. Currently, China has fully carried out strict ultra-low emission standard for coal-fired power plants, which specifies particulate matter (PM), sulfur dioxide (SO2) and nitrogen oxides (NOx) shall not exceed 5, 35 and 50 mg/Nm3 [3]. Correspondingly, the technologies that achieve the ultra-low emission target through a series of processes or devices are called ultra-low emission technologies. In terms of PM removal, there are many ways such as the low-low temperature electrostatic precipitator (LLT-ESP) [4], wet electrostatic precipitator (WESP) [5] and high-frequency power supply (HFPS) [6]. For the control of SO2, the more effective technologies include the single tower integrated desulphurization and dust removal purification (SPC-3D) [7], double absorption tower and two circulations per tower [8]. Besides, the main emission reduction technologies for NOx consist of low NOx combustion [9], selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) [10]. Although the ultra-low emission technologies are effective ways to control the main air pollutants, there are still many new problems existing in coal-fired units during technical reconstruction. For 1 example, the gaseous reductants, ammonia or urea solution, which are injected in SCR, react chemically with the NOx in the catalyst layers, but the vanadium-containing catalyst can convert about 1% of SO2 into SO3 in this process [11]. Then, some of the gaseous reductants will directly pass through the SCR system, and react with sulphuric acid downstream to produce ammonium sulphate or bisulphate (ABS) [12]. Compared with ammonium sulphate that exists as dry solid powders within the operating temperature range of the downstream air preheater, ABS presents sticky and easily induces serious acidic deposit on the metal elements of the air preheater. Jimenez [13] found that ABS begins to deposit at approximately 157 oC. Menasha et al. [14] showed that the formation temperature of ABS is 227-247 oC for the typical flue gas with ammonia of 2-10 ppm and sulphuric acid of 5-45 ppm. Chetan et al. [15] oberved that ammonium sulfate becomes the main component in the flue gas when the molar ratio of ammonia to SO3 is greater than 2. It can be summarized that the formation of ABS in the flue gas is affected by three main factors consisting of temperature, concentration and molar ratio of ammonia to SO3. Furthermore, the concentration product of ammonia and surphur trioxide can also reflect the formation rate of ABS, which means the low ammonia slip can also easily form ABS deposits when the concentration of SO3 is high enough. Thus, high conversion rate of SO2 to SO3 and ammonia slip from SCR are one of important causes of serious blockage of ABS. In addition, in order to remove PM and sulphur more efficiently, LLT-ESP technology has been widely used in the Chinese power plants with ultra-low emission modification, which maintains the flue gas temperature of about 85-95 oC before ESP through a heat exchanger [16]. The SO3 in the flue gas can react with water vapor to form the sulphuric acid mist. These mist then stick to the surface of PM and improve the removal and collection of SO3 by ESP [17]. Although the sulphuric acid mist can be removed under design conditions, it may fail under actual conditions and then severe corrosion and deposition will occur at the downstream of the LLT- ESP because of low temperature. Therefore, in order to address the problems in the ultra-low emission modification, it is of scientific and practical significance to conduct study on the ash fouling formation of coal-fired unit equipment to provide more reasonable suggestions. In this work, serious fouling deposits were investigated on the blade of the induced draft fan and the surface of the two-stage low-temperature economiser (LTE) in a 1000MW coal-fired power plant. The deposition and fouling were firstly sampled when the coal-fired unit was shutdown. The chemical compositions and micromorphologies of these samples were characterized through X-ray diffraction (XRD), X-ray fluorescence (XRF) and scanning electron microscope with energy dispersive spectrometers (SEM-EDS). Then the performance of upstream equipment and the formation of the fouling were then analyzed and evaluated. Finally, based on the above analysis, a formation mechanism of ash fouling was proposed for further understanding ash fouling problems in coal-fired power plants. 2. Field parameters and sampling location The essential information of the coal-fired power plants will be presented, including system composition, operating temperature, coal properties and ash compositions. Then, the thermodynamic model of the acid dew point for coal-fired flue gas will be provided, which can be used to evaluate the change of the acid dew point temperature with coal properties and further analyze the formation mechanism of fouling deposition. 2 2.1. Coal-fired power plant The research object was the #1 unit of a 1000MW ultra-supercritical coal-fired power plant with ultra-low emission, which was located in Shouguang City, Shandong Province. As shown in Fig.1, a three-layer SCR with V2O5-TiO2 catalyst was equipped and the design efficiency of denitration reached 87%. The first two layers of the catalyst were enable while the last one was on standby. A two-stage LTE was used for recoving latent heat of the flue gas and improving the removal efficiency of ESP and WESP. The first stage LTE was installed at the inlet of the ESP and the second LTE was located at the inlet of the WFGD. The induced draft fan was installed at the outlet of the ESP to supply the driving force for flue gas flow and balance the system resistance. The removal efficiency of PM for the ESP was higher than 99%. The distributed control system (DCS) showed that the concentration of PM at the 3 outlet of ESP was about 5 mg/m with small fluctuations, and the concentration of SO2 was about 1200 mg/m3 at full load. WFGD SCR Boiler Sampling equipment WESP LLT-ESP Air First-stage Induced Second- Stack preheater LTE draft fan stage LTE Figure 1. Schematic diagrams of the coal-fired power plant and the sampling equipment According to the data of DCS, the boiler load was distributed between 50-100%. When the boiler was at full capacity, the flue gas temperature of the outlet for the first-stage LTE and the second-stage LTE was ranged within 103-121 oC and 98-101 oC, respectively. While the boiler was at 50% capacity, the temperature ranges were 100-111 oC and 95-100 oC for the first-stage and the second-stage LTE, respectively. Meanwhile, it could be found that the temperature drop of the second-stage LTE would experience 15-20 oC when the temperature of the first-stage LTE decreased 2-5 oC. 2.2. Coal properties and ash compositions The typical properties and the ash compositions of the coal are presented in Tab.1 and Tab.2. It can be seen that the sulphur contents in design coal and checked coal are both below than 1%, but severe fouling deposits are found at the downstream equipment of ESP, which indicates that ammonia escape in SCR systems may have been existing. Table 1. Typical properties of design coal and checked coal Proximate analysis/% Element analysis/% LHV/MJ∙kg-1 Samples Mt Mad Aar Vdaf Car Har Oar Nar St,ar Qar,net Design coal 18.50 10.66 10.24 35.61 57.33 3.26 9.34 0.61 0.63 21.23 Checked coal 17.90 9.33 15.10 34.68 53.25 3.00 9.70 0.58 0.47 20.02 Note: The subscripts t and ad represent the total and the air dried basis, respectively; the subscript ar and daf represent the air received basis and the dry ash-free basis, respectively.