Article pubs.acs.org/EF
Experiences from a Novel Sensor for Fireside Corrosion Monitoring during Grate Combustion of Corn Stover/Wood Chip Blends Timo J. Leino,*,† Martti J. Aho,† S. Juhani Gynther,‡ Tommi A. Ruuskanen,‡ and Matti H. Hak̈ kinen‡ †
VTT Technical Research Centre of Finland, Post Office Box 1603, FI-40101 Jyväskylä, Finland Savcor Forest Oy, Insinöörinkatu 8, FI-50100 Mikkeli, Finland
‡
ABSTRACT: The operation of a pilot-scale online corrosion sensor system was studied at VTT’s 100 kW grate pilot plant. The feedstock composition in tests was varied from 100% wood chips to a blend that also contained 40 en-% d.b. corn stover. The mass flow of alkali chlorides was varied with sulfur-containing additives. The measurements included electrical resistance (ER) of deposit and linear polarization resistance (LPR) for both St45.8 and AISI347H alloys. The number of fine particles (online data), alkali chloride mass flow, chloride content of the deposit, and flue gas composition (online data) were simultaneously measured to study how the furnace conditions correlate with the ER and LPR signals. Information on the risky furnace conditions can be obtained with these methods. The decrease in sensor signals after starting to feed in fuel blended with corn stover indicated increasing corrosion. The ER and LPR values started to increase again after changing the fuel back to 100% wood chips. The deposit composition, alkali chloride flow, and corrosion sensor signals predicted a low corrosion rate on the alloys with the corn stover-containing blend, when the mass flow of additive was sufficient to destroy the corrosive alkali chlorides before meeting the probe.
1. INTRODUCTION Environmental constraints, emission regulations, and market competition have forced power plants to increase the efficiency of electricity production. Firing utility boilers with feedstock containing biomass and waste is a cost-effective method to reduce net CO2 emissions in energy production, but such feedstock can also cause severe availability problems, such as fouling, slagging, bed material agglomeration, and corrosion, which can significantly increase the production costs of heat and electricity. Because superheater fouling and corrosion play an important role in the economy of the plant, knowledge of their state on heat exchangers online ensures maximum availability of the power plant without unexpected breaks in operation and provides information on maintenance requirements before scheduled revisions. Superheater corrosion in biomass boilers is usually caused by deposition of alkaline chlorine salts on metal surfaces followed by reactions between chlorine and metal. The corrosion mechanism consumes elemental Cl in a chain reaction, which forms iron and chromium oxides through their chlorides and is possible at high steam temperatures (Figure 1).1−4 The corrosion rate is dependent upon the superheater metal temperature, gas temperature, alloy composition, construction and placement of the superheaters, and mass flow of alkali chlorides. Aluminum,5 ferric,5 and ammonium sulfates,6−8 elemental sulfur,7 and kaolin8,9 have been found to destroy alkali chlorides in the furnace. The oxidation of sulfur first to SO2 and onward to SO3 can be formulated as7 S + O2 → SO2 (1) SO2 + 1/2O2 → SO3
Figure 1. Possible chemical reactions on a corroding boiler tube.4
X 2(SO4 )3 → X 2O3 + 3SO3
where X is Al or Fe. The simplified reaction of SO3 with alkali chlorides is5−8 2MCl + SO3 + H 2O → K 2SO4 + 2HCl
© 2013 American Chemical Society
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Special Issue: Impacts of Fuel Quality on Power Production and the Environment
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Received: March 20, 2013 Revised: August 27, 2013 Published: August 27, 2013
The decomposition of sulfates also forms SO35−8 (NH4)2 SO4 → 2NH3 + SO3 + H 2O
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where M is Na or K. The alkali capture reaction by kaolin is8,9
have successively been used.5,8,9,25,26 Therefore, new information on both corrosion measurement and phenomena can be obtained with several parallel measurements. In contrast to the earlier studies, the goal in this study has been to keep the main process parameters (temperature distribution, O2 in the flue gas, and residence times) as constant as possible and to measure whether corrosion sensors can detect variations in deposit compositions caused by additives. In addition, a highly corrosive biomass blend has been studied here instead of a good-quality biomass with a low Cl concentration.
2MCl + Al 2O3 · 2SiO2 + H 2O → M 2O·Al 2O3 ·2SiO2 + 2HCl
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When sprayed on the superheater area, sulfates can be more effective than elemental sulfur mixed with fuel because SO3 is formed directly from the additive at optimal furnace zones, enabling effective sulfation of alkali chlorides and minimizing the consumption of SO3 in other reactions, such as calcium sulfate formation or reduction to SO2.10 The disadvantage of the addition of sulfur-containing chemicals to the furnace is the possible increase in SO2 emissions. Superheater corrosion monitoring at power plants is usually performed during revisions by visual examination and ultrasound tube thickness measurements. In some cases, corrosion probe measurements have been performed by manufacturers or research institutes.2,3,11 The measurements in the corrosive environment are typically based on the mass loss of material sleeves installed in temperature-controlled probes. The method can be used in various environments, and the results are accurate. The major disadvantage is the lack of information on the instantaneous corrosion rate, such as the influence of temporary fluctuations to the corrosion rate because of variations in the fuel composition, flue gas temperature, load, etc. The possibility to measure the corrosion rate online is becoming increasingly important because of increased corrosion as a result of lower (and more fluctuating) fuel quality and increased final steam temperature and pressure at power plants. The information also helps to identify the relationship between process conditions and corrosion. Online corrosion sensors using various measurement technologies are available,12 but few applications can be found for a high-temperature furnace environment.13−24 Only a restricted amount of literature studying the performance of the online corrosion sensors at power plants exists.13−20 Changing process conditions in real time (fuel composition and boiler load) and complex corrosion mechanisms in fullscale boilers make the data analysis difficult. The study of sensor performance under controlled conditions by varying each process parameter in turn helps to understand better the relationship between furnace conditions and corrosion. Sensors have been studied in laboratory-scale ovens at high temperatures by covering them with ash and using a synthetic combustion gas environment.21−23 These studies provide interesting information on sensor performance, but continuous flow of real flue gas and continuous material deposition onto the metal surface cannot be arranged. Pilot-scale combustors provide a compromise between laboratory- and full-scale combustors. Process parameters can be varied as in laboratory scale, but fresh flue gas is in contact with the sensor and continuous deposition occurs as in full scale. Simultaneous measurements of corrosive gas and particle flows from the furnace assist in corrosion sensor signal analysis. Only a few studies combining corrosion sensor measurements with such furnace measurements have been performed.20,23,24 To expand the research to cover the correlation between the alkali chloride reactions in furnace and metal corrosion, an online corrosion probe applied to a pilot-scale combustor was designed and constructed in cooperation with VTT and Savcor Forest Oy. The pilot-scale combustors at VTT enable flexible variation of experimental conditions affecting corrosion. In addition, several methods for the alkali chloride measurements
2. EXPERIMENTAL SECTION 2.1. Test Description. The 100 kW grate combustor is shown in Figure 2, and the set of experiments is shown in Table 1. The diameter of the rotating grate and the furnace is 0.40 m, and the height of the reactor (from grate to flue gas cooler) is 5.0 m. The rotating grate (3 rpm), sufficiently high furnace, well-controlled gas velocities, and gas mixing provide enough residence time for effective combustion with low CO emissions. The ratio between primary and secondary air was set to 50:50. More details of the reactor have been presented earlier.25,26 Wood chips and a blend containing wood chips and corn stover [60:40 en-% d.b.)] were burnt in the performed tests (Table 1). In earlier studies, 40% share of corn stover mixed to wood chips was found to be the maximum to reach a stable combustion process.25 A 100% corn stover combustion is not possible in the reactor because of instability problems (fuel feeding and ash melting on the grate). The fuel blend was prepared on the basis of analyzed heating values. The corn stover was the main source of chlorine and ash, whereas the wood served more as a dilutant. The wood chip dimensions were a length of 30 ± 10 mm, width of 15 ± 5 mm, and thickness of 3 ± 1 mm. The straw-like corn stover was crushed. Particles were needle-shaped fibers with 10−50 mm in length and 0.5−5 mm in diameter. The bulk density of the wood chips was 250 ± 20 kg/m3, and the bulk density of the corn stover was 90 ± 5 kg/m3. CO and SO2 were measured with an infrared (IR) technique (Servomex 4900); O2 was measured with a paramagnetic principle (Servomex 4900); and HCl was measured with Fourier transform infrared (FTIR) spectroscopy (Gasmet DX series). Flue gas was sampled to traditional online analysers before stack and to the FTIR analyzer through port Y7 (see Figure 2). The sampling lines for gas analysers included a particle filter and a heated polytetrafluoroethylene (PTFE) line. Flue gas temperatures in the furnace were measured with suction pyrometers to avoid heat radiation errors. Ferric sulfate was sprayed into the furnace via port Y3 (see Figure 2) because sulfates in spray form can be found to be effective when sprayed just above the flame.26 The 0.4 mm (inner diameter) spray nozzle mounted on an air-cooled probe was placed at the center of the furnace and directed upward. The chemical was pumped to the nozzle and sprayed with air to the furnace. The additive flow rate depended upon target S/Cl dosage. More details of the spraying procedure are found in ref 5. Elemental sulfur was fed with a separate feeder directly on the fuel-feeding screw. The additive container was mounted on the top of a scale, which enables the determination of mass flow rates as a weight loss against time. The additive dosages were determined as molar Sreagent/Clfuel. 2.2. Online Corrosion-Monitoring System. A short response time is a benefit of the electrochemical methods, which enable rapid changes in corrosion rates to be identified. The electrical resistance (ER) and the linear polarization resistance (LPR), which is the most common electrochemical method, are applied in the corrosionmonitoring system. ER is inversely proportional to the electrical conductivity of the deposit. In ER, the electrical conductivity of the deposit between the sensors is measured with an electrical resistivity measurement. LPR indicates the state of the oxide layer on the studied metal surface. A low LPR signal indicates a weak oxide layer and an increased metal corrosion risk. The LPR signal is based on the measurement of polarization resistance ΔE/ΔI between electrodes, which have been made of reference and tested material. A small 5654
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Figure 2. Scheme of the furnace of the pilot-scale grate combustor at VTT. The reactor has a rotating grate and several sampling ports through which flue gas composition and ash deposition can be measured.
Table 1. Test Description experiment number 0 1 2 3
tests, different types of alloys were selected. St45.8 is an alloy with poor high-temperature corrosion resistance, whereas AISI347H is an alloy with a much better high-temperature corrosion resistance. The signal analysis is performed in a separate data-logging computer, which sends the measurement values to the operator display via an automation system. Because of ER and both LPR measurement sequences and signal analysis, new values are obtained once in 4 min. The online corrosion probe was tested at sampling port Y4 (Figure 2), which is located 3.5 m above the grate. Port 4 was selected to the online corrosion probe because the flue gas temperature there is typical to a superheater zone in the grate boiler and there should be enough residence time between ports 3 and 4 for chemical reactions during ferric sulfate spraying. The air-cooled probe was set to 470 °C in the tests to simulate a superheater in a grate-fired power plant. This metal temperature was assumed to exist somewhere at the second superheater in a power plant producing 80 bar/500 °C steam. 2.3. Deposit Collection and Analysis. One way to give a longer lifetime to the superheater with the highest steam temperature is to place it after the secondary superheater, so that the latter superheater meets the hot flue gas flow first. Then, the gas temperature at the third
dosage: molar Sadded/Cl
blend
additive
wood corn stover/wood (40:60 en-% d.b.) corn stover/wood (40:60 en-% d.b.) corn stover/wood (40:60 en-% d.b.)
no additive no additive
0 0
Fe3(SO4)2
0.8
sulfur
2.6
external potential shift ΔE given between the sensors produces a small measurable current ΔI at the corroding electrode. ΔI is directly proportional to the corrosion current Icorr caused by the electrochemical mechanism between the corroding material and deposit. More information on the used methods can be found in refs 12−15. The measurement section of the probe with a diameter of 24 mm (Figure 3) consists of four sensors insulated from each other. To find out whether differences in LPR signals can be obtained in a few hour 5655
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Figure 3. Schematic picture of the online corrosion measurement probe. axis in the reactor as the corrosion probe in Y4. Interactions between the probes were assumed negligible. Deposits were collected on removable sleeves with outer diameters of 16 mm and lengths of 30 mm simultaneously to the corrosivity measurements. Sleeves were components of the probe on which the surface temperature can be adjusted to the desired level. Deposits on the corrosion probe were collected similarly to removable sleeves (outer diameters of 24 mm and lengths of 25 mm). Composition of deposits on the sleeves were analyzed in three locations (Figure 4) with scanning electron microscopy (SEM) type LEO Gemini FEG equipped with an energy-dispersive X-ray microanalysis system (EDXA) type Thermo Noran. The device was operated by Åbo Academi University, Finland. Bulk analysis was conducted on a few square millimeters of the deposit layer. 2.3. Sampling and Analysis of Aerosols. Condensed aerosols, in which the concentration is dependent upon the concentration of gaseous alkali chlorides at the sampling point, were determined through port Y5 in two ways: A electric low-pressure impactor (ELPI) measures the number of aerosolic particles and gives an estimate on the optimal sampling time to a low-pressure impactor (so-called mass impactor), which shares the fine fly ash (
