FIRST RESULTS OF DEPOSIT AND CORROSION TESTS USING A HIGH TEMPERATURE CORROSION PROBE IN A 30KW BFB DURING COMBUSTION OF WOOD PELLETS DOPED WITH ZN

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1 FIRST RESULTS OF DEPOSIT AND CORROSION TESTS USING A HIGH TEMPERATURE CORROSION PROBE IN A 30KW BFB DURING COMBUSTION OF WOOD PELLETS DOPED WITH ZN DOROTA BANKIEWICZ a,*, ELISA ALONSO-HERRANZ b,, PATRIK YRJAS a, TOR LAURÉN a, HARTMUT SPLIETHOFF b, MIKKO HUPA a ABSTRACT a Process Chemistry Centre, Åbo Akademi, Biskopsgatan 8, Åbo, Finland b Lehrstuhl für Energiesysteme, Technische Universität München, Boltzmannstrasse 15, Garching, Germany * Phone: , Fax: , dbankiew@abo.fi The development and improvement of handling recovered wood waste (RWW) towards higher economical and environmental standards such as use in order to produce heat and electricity has increased in many countries. Waste wood originates mainly from Construction and Demolition (C&D), MSW, and Commercial and Industrial (C&I) sources. It is well known that recovered waste wood may contain substantial concentrations of heavy metals, especially Zn and Pb. In this paper the importance of zinc in the corrosion process was investigated, while lead will be studied later. In order to better understand the fate of Zn and its role in high temperature corrosion of heat exchanger tubes in waste wood fired fluidized bed boilers, high temperature corrosion probe tests have been performed. Combustion tests were carried out in a 30 kw bubbling fluidized bed reactor. A temperature controlled, air cooled probe was placed in the freeboard section where the mean gas temperature was about 780ºC. The probe surface was cooled to 450ºC, 500ºC and 550ºC, respectively. Three types of wood pellets were burnt in the reactor. Two doped with varied amounts of ZnCl 2, to obtain different levels of Zn and one untreated. Two commercially used types of steel were selected for tests: low alloy, ferritic (10CrMo9-10) and stainless, high alloy steel (Sanicro 28). After 8h and 28h of exposure time, the steel rings were analysed using scanning electron microscope and x-ray to determine the oxide layer thickness and the elemental distribution in the oxide scale and in the deposit. The deposit samples from both windward and leeward sides were collected for analysis. The flue gas composition was monitored by means of conventional on-line gas analyzer for O 2, CO, CO 2, NO, NO 2, NO x and SO 2. In addition, Electric Low Pressure Impactor (ELPI) measurements, and online FTIR measurements were carried out in order to determine particle size distribution and gas components (HCl and SO 2 ), respectively. This paper describes preliminary results obtained. Keywords: Recovered Wood Waste, High Temperature Corrosion, Zinc

2 1. INTRODUCTION Lately a lot of effort is being put to different aspects of using Recovered Wood as raw material or energy carrier with the aim, to develop and improve the management of Recovered Wood towards higher common technical, environmental and economic standards in Europe. The main target is to develop strategies to avoid landfilling and waste incineration without energy use of recovery. According to Rector (2006) other factors supporting the use of waste wood as a fuel are: the increased cost of oil and natural gas and the increased regulatory incentives to use renewable energy sources. An estimation of the amount of Recovered Wood in 20 out of 22 EU members countries (Austria, Belgium, Bulgaria, Croatia, Denmark, Finland, Germany, Greece, Hungary, Ireland, Italy, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovenia, Spain, Sweden and UK) shows that: about 30 Million tons of Recovered Wood are produced annually which corresponds to about 13% of the annual round wood consumption of 227 Million tons and about 444 PJ/a or 0.7% of the primary energy consumption of 67,000 PJ/a. Currently, 34% of Recovered Wood is used for power generation, 38% is being recycled and 28% is being composted or put into landfills. Thus, energy recovery of RWW can reduce the amount of waste that goes to landfill and produces heat, thereby establishing an economic outlet for the waste wood resource. Moreover, RWW is considered CO 2 neutral and its combustion emits less SO 2 and NO x than most fossil fuels. Whether the tree is rotting in nature or being burned for heating purposes, the same amount of CO 2 is always released (Fig1) (Krook et al., 2004 and Wood pellets-basic training, 2007). Consequently, already 11 Million tons CO 2 - emissions per year can be avoided through the substitution of fossil fuels and approx. 10 Million tons of fresh wood can be saved. Figure 1. Demolition wood - the renewable source of energy. Wood pellets-basic training (2007). RWW includes all kinds of wooden material that is available at the end of its use as a wooden product. It is expected that the amount of RWW used in biofuels boilers might increase in the coming years, because already in Sweden since 2002 it is illegal to dispose combustible waste in landfills (Krook et al., 2004 and COST Action E31). However, RWW is usually substantially contaminated with heavy metals, causing pollution and downstream operational problems such as slagging, fouling, and corrosion may occur. According to Krook et al. (2002, 2006) the wood-based materials, RWW mainly consists of, can be divided into four main categories: untreated wood, surfacetreated wood, industrial preservative-treated wood and different types of building boards such as plywood and particle boards. Of these materials only surface-treated

3 wood and preservative-treated wood contain heavy metals, thus being potential sources of contamination with harmful components. Zinc and lead occur at the highest concentration in surface-treated wood present in RWW, and originate mainly from white pigments. Before RWW is used as a biofuel, the material is processed. Common processes include: crushing to wood chips, treatment with magnetic separators, eventually screening, and removing the finest fraction of the processed material. However, even preprocessing treatment neither removes all metallic e.g. non-magnetic parts nor contaminated surfaces. The aim of this study is to investigate the effect and importance of problematic RWW s ash elements such as Zn and later on Pb regarding high temperature corrosion processes. This study presents preliminary results of lab-scale corrosion tests carried out at the Technical University in Munich, Germany. 2. EXPERIMENTAL This study describes preliminary experiments carried out in a small-scale fluidized bed reactor (30 kw th ). Wood pellets doped with ZnCl 2 were used as a fuel Lab-scale facility The setup outline is shown in Fig. 2. The reactor had a cylindrical shape with an inner diameter of 190 mm and a height of 550 mm. The freeboard section had diameter of 310 mm and a height of 450 mm. At the bottom of the fluidization column a Figure 2. Lab-scale setup with 30 kw th fluidized bed reactor (Klein WS)

4 perforated plate (91 holes; diameter = 1.8 mm) was used for the distribution of fluidizing air. Fuel was fed by means of a screw feeder and pellets were falling on the top of the bed with the secondary air flow. Electrical resistances (heater) placed around the reactor heated up the bed up to 570 C during the start up. The bed temperature was regulated by the preheating of the fluidizing air and it was controlled by means of 4 thermocouples. Quartz sand ( mm) was used as bed material in a total quantity of 10 l for each experiment. Bed material was changed before every test with a new fuel. Combustion air was preheated up to 600 C by an electric heater and an additional electrical resistance. Gas velocity in the reactor was about 0.5 m/s. Particulate matter was separated from the flue gases in a cyclone. Reactor temperatures, pressure drop in the bed, and, air flow were monitored and recorded continuously during the experiments. Flue gas was sampled from the freeboard for online HCl and SO 2 analysis (FTIR). The flue gas composition was also monitored by means of a conventional online gas analyzer for O 2, CO, CO 2, NO, NO 2, NO x and SO 2. After the cyclone, flue gas was filtered before being emitted into the atmosphere Experimental procedure Fuel feeding rate and fluidization air were kept approximately constant and were equal to 3 kg/h and 15 Nm3/h respectively. The aim was to obtain a constant lambda value of about 1.2. When the bed temperature reached 570 C, fuel feeding was started. Once the bed temperature reached a stationary value and freeboard temperatures became stable, the corrosion probe was introduced into the freeboard. The freeboard temperature varied from 740ºC to 770ºC during different tests. Combustion conditions were kept constant for 7 hours. At the end of the test the corrosion probe was removed from the reactor. After each test, samples of bed material, and ashes from cyclone and flue gas duct after the freeboard were collected for SEM/EDX analysis Corrosion probe A temperature controlled corrosion probe was used in the experiments. Temperature controlled probe techniques and examples of results that can be achieved with this kind of instrument were described by Laurén (2007). The working principle is shown in Fig. 3. The temperature of one of the test rings on the probe is controlled with a PIDcontroller. The temperature of the other test ring is monitored and logged during Figure 3. Outline of corrosion probe

5 the test run. The cooling temperature of the probe was adjusted by the pressurized air flow inside the tube. The probe was provided with two removable rings (Fig. 4). Tests were performed with two commercially available superheater materials: one low alloy steel 10CrMo9-10 and one austenitic steel Sanicro 28. Tab. 1 presents the detailed steel compositions of the tested steels. Figure 4. Corrosion probe tip with two removable rings after 7h exposure time (combustion of wood pellet doped with 0.5 wt. % of ZnCl 2 ). Before the experiments all steel rings were cleaned in ethanol using the ultrasound bath. Hereafter, the material rings (2 at a time) were fixed on the probe and introduced into the freeboard for 7 or 28 hours when the freeboard temperature exceeded 700ºC. Different cooling probe temperatures were set for each test (550 º C, 500 º C and 450 º C). After the corrosion test, the test rings were cooled down to room temperature outside the furnace. The rings were then placed in resin thinned with 99.8% ethanol. The aim of thinning was in order to let resin to penetrate the very thin layer of collected deposit, and preserve. The rings were then fully cast in resin and cut off in the middle. Similar samples treatment prior to SEM analysis was described by Westén-Karlsson (2008). The samples were then ready to be analyzed with SEM/EDX in order to indentify elemental composition of the deposit and to estimate the oxide layer thickness. Table 1. Detailed compositions of the steels used in the corrosion tests. Steel element Composition, wt. % 10CrMo910 Sanicro28 Fe Cr Mo Mn Si Ni C P S

6 2.4. Fuel Three types of wood pellets were burnt in the reactor. Two doped with varied amounts of ZnCl 2, to obtain different levels of Zn and one untreated as reference. Pellets were doped with 0.1 and 0.5 wt. % of ZnCl 2 what corresponds to 480 and 2400 mg of Zn per kg of fuel respectively. Solid zinc chloride was dissolved in ion-exchanged water and the solution was then evenly sprayed over a batch of wood pellets. After doping, pellets were left for drying and then packed and stored in tight barrels. The aim of fuel doping with ZnCl 2 was to imitate demolition wood contaminated with Zn. The amount of Zn in doped pellets corresponds to a mean and high content of Zn in demolition wood found in the literature (Krook et. al 2006). 3. RESULTS AND DISCUSSION The performed experiments are summarized in Table 2. Results of experiments with untreated wood pellet are still under evaluation. Table 2. Summary of the performed tests Run Fuel Exposure time [7 h] 1 wood pellet+0.1 ZnCl 2 wt. % 28 2 wood pellet+0.5 ZnCl 2 wt. % 7 3 wood pellet+0.5 ZnCl 2 wt. % 7 4 wood pellet+0.5 ZnCl 2 wt. % 7 5 wood pellet+0.5 ZnCl 2 wt. % 7 6 wood pellet 7 7 wood pellet 7 8 wood pellet 7 Steel sort Probe temp. [ºC] Oxide layer thickness, composition and structure The thickest oxide layer growth was noticed on the 10CrMo9-10 low alloy steel when combusting wood pellet doped with 0.5 wt. % of ZnCl 2. The measurements taken on a 45º angle of windward side showed growth of oxide layer over 70 µm thick after 7 hours exposure time (0 º - windward side, 180 º - leeward side). The rest of the measurements taken in a one representative point of 0º, 45º and 180º angle on each sample are presented in Table 3. (where wp means wood pellet doped with 0.1 and 0.5

7 wt. % of ZnCl 2 respectively). The typical composition of the oxide scale was mainly Fe 2 O 3 with significant amounts of KCl on the top of the scale and Zn possibly as ZnCl 2 (as not much oxygen was found) in between of the mentioned compounds (Fig. 5, area 1). In all cases the thickest oxide layer growth was notice on 0º-45º section of the windward side. austenitic steel showed good resistance, with only negligible iron and chromium oxides growth, difficult to measure and possible to be distinguished only on the x-ray maps (Fig. 6). The visible deposit consisted mainly of KCl and significant amounts of Zn most likely in a form of ZnCl 2 rather than ZnO what is noticeable on the x-ray maps. Also, in the impactor samples ZnCl 2 was clearly present and more Cl was found than K would be able to bind. This implies that not all ZnCl 2 decomposed in the combustion process. ZnCl 2, ZnO, Zn KCl 1 Fe 2 O 3 Steel ring Figure 5. SEM image of the exposed 10CrMo9-10 steel rings. Wood pellets + 0.5wt. % of ZnCl 2, 500⁰C-probe temperature and the analysis spectrum of the marked square in the image. Table 3. Measurements of the oxide layer thickness (10CrMo9-10 steel) Run Fuel Exp. time[h] Probe temp. [ºC] Oxide layer thickness [µm] 0º 45º 180º 1 wp wp wp wp wp Note: wp = wood pellet 3.2. The deposit composition The rate of deposit build-up during 7 h tests was negligible in all cases. Figure 7 presents the elements found in the wind- and leeward side of deposits after 28 h test. Windward sides deposit analysis shows enrichment in ZnO as a main compound,

8 containing elevated amounts of KCl and most likely also K 2 SO 4. Although, no sulphates were detected on the oxide scale/deposit interface suggesting occurrence of solid K 2 SO 4 further from the tube surface what was indicated in the studies done by Åmand et al. (2006). Cl K Zn Ca KCl Cr Fe O S Figure 6. a) BSE-SEM image and b) x-ray maps of steel ring when combusting wood pellet wt. % ZnCl 2, probe temperature 550 o C, exposure time 7 h. Sulfur released during combustion binds alkali metals or replaces chlorine in alkali chlorides forming alkali sulfates, simultaneously preventing alkali chlorides formation. Then, chlorine preferably tends to form HCl leaving the system with the flue gases. Figure 7. SEM/EDX analyses of wind- and leeside deposits, probe temp. 450ºC, 28h test, wood pellets wt. % of ZnCl 2 However, in this study addition of Cl (in a form of ZnCl 2 ) enhanced formation of KCl which appeared as an enrichment of KCl in the oxide scale/deposit interface (Fig. 4). This is in agreement with Åmand et al. (2006). 4. CONCLUSIONS Preliminary Test results showed that ZnCl 2 has a profound impact on high temperature corrosion

9 Higher ZnCl 2 concentrations in the fuel resulted in higher corrosion rates of low alloy steel Zn was mainly found as ZnO on the windward side of the corrosion probe but there are also indications for ZnCl 2, which implies that ZnCl 2 has not totally decomposed during the combustion process. Corrosion rate on the high grade steel seems to be negligible under the tested conditions and exposure times. ACKNOWLEDGMENTS The tests presented in this paper were done at the Institute of Energy Systems at the Technische Universität München as a part of project under INECSE Marie Curie Early Stage Programme activities. This research was mainly financed by INECSE Marie Curie Early Stage Training Programme. The work was also partly financed by Chemcom, a mainly Tekes financed project at the Process Chemistry Centre at Åbo Akademi University. Other funders were Andritz Oy, Foster Wheeler Energia Oy, International Paper Inc., Metso Power Oy, Oy Metsä-Botnia Ab, Clyde Bergemann GmbH, amd UPM-Kymmene Oyj. SEM analyses and help given to this work was made by Mr. Linus Silvander and is greatly appreciated. I would also like to express my gratefulness to all TUM co-workers for their help and assistance during tests especially to Mr. Matthias Abst. Special thanks are also given to Mr. Albert Daschner and Mr. Martin Haindl for continuous help and fast problems solving. Mrs. Maria Zevenhoven is also acknowledged for her help and valuable comments. REFERENCES CHIRONE, R., MICCIO, F., SCALA, F. (2006). Mechanism and prediction of bed agglomeration during fluidized bed combustion of a biomass fuel: Effect of the reactor scale. Chemical Engineering Journal vol.123, pp COST Action E31 Management of Recovered Wood KROOK, J., MÅRTENSSON, A., EKLUND, M. (2004). Metal contamination in recovered waste wood used as energy source in Sweden. Resources, Conservation and Recycling vol. 41, pp KROOK, J., MÅRTENSSON, A., EKLUND, M. (2006). Sources of heavy metal contamination in Swedish wood waste used for combustion. Waste management vol. 26, pp LAURÉN, TOR. (2007). Methods and Instruments for Characterizing Deposit Buildup on Heat Exchangers in Combustion Plants. Report 07-05, ISBN Primary Energy Consumption, GermanY

10 RECTOR, L. (2006). Emissions from Burning Wood Fuels Derived from Construction and Demolition Debris. NESCAUM. Wood pellets-basic training (2007). Maine Energy Systems. Training documentation. WESTÉN-KARLSSON, M. Assessment of a Laboratory Method for Studying High Temperature Corrosion Caused by Alkali Salts. (2008). Lic. Thesis. Åbo Akademi University, Åbo, Finland, ISBN ÅMAND, L-E., LECKNER, B., ESKILSSON, D., TULLIN, C. Deposit on heat transfer tubes during cocombustion of biofuels and sewage sludge. (2006) Fuel vol. 85, pp