Online corrosion measurements in combination with deposit and

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Online corrosion measurements in combination with deposit and aerosol analysis during the co-firing of straw with coal in electrically heated, small-scale pulverized fuel and circulating fluidized bed systems Christian Wolf, Timo J. Leino, Andreas R. Stephan, Martti J. Aho, and Hartmut Spliethoff Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03976 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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Online corrosion measurements in combination with deposit and aerosol analysis during the co-firing of straw with coal in electrically heated, small-scale pulverized fuel and circulating fluidized bed systems Christian Wolf *, †, Timo J. Leino ‡, Andreas R. Stephan †, Martti J. Aho ‡, Hartmut Spliethoff †, § †

‡

Institute for Energy Systems, Technical University of Munich, D-85747 Garching, Germany

VTT Technical Research Centre of Finland, Post Office Box 1603, FI-40101 Jyväskylä, Finland §

Bavarian Centre for Applied Energy Research, D-85748 Garching, Germany

Keywords: Online corrosion monitoring, Linear polarization resistance, Co-combustion, Straw, Coal, Aerosols, Emissions, Alkali mitigation strategies, Pulverized fuel, Circulating fluidized bed

ACS Paragon Plus Environment

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ABSTRACT

A measurement campaign has been conducted both in a pilot-scale pulverized fuel and in a pilotscale circulating fluidized bed test rig to evaluate the behavior of two different online corrosion sensors during the co-combustion of straw with bituminous coal. The online corrosion sensors based on the linear polarization method were equipped with material rings of the alloy10CrMo910 and air-cooled to a material temperature of 530 °C (PF) and 560 °C (CFB). They were implemented at a flue gas temperature of approx. 750-800 °C in both test rigs to simulate superheater tubes. The derived signals were compared with flue gas measurements (O2, CO2, SO2 and HCl) as well as selected fine particle measurements and deposit sampling during cofiring tests of 0, 10, 25, 40, 60 and 100 % straw with coal on an energy basis. Slight deviations between the fuels tested in the different test rigs were observed. Main differences were measured in the coal ash composition and chlorine content of the straw. Online corrosion sensors reacted quickly to changes in the blend composition. While no enhanced corrosion was detected during the co-combustion of 10 % and 25 % straw, both sensors identified possible corrosive processes on the metal surface during the 60 % straw case. The detected signal change could be correlated to an increased share of chlorine in the fine particles (in the PF and the CFB test rig) and deposits (only in the CFB tests). Interestingly a smaller signal change was detected during the 40 % straw case in the PF combustion, in contrast to a larger signal gradient during the 40 % case in the CFB tests. Two reasons could be identified for this behavior: On the one hand, the sensor used in the PF tests showed a lower sensitivity due to a different design of the sensor head. On the other hand, a significant amount of chlorine was detected in the aerosolic particles in the CFB tests in contrast to no chlorine in the PF experiments during this case.


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The known interaction mechanisms of alkali mitigation during combustion of difficult fuels (sulfation and embedding in alumino-silicates), which lead to a chlorine reduction in the fine particles, were investigated thoroughly. It was found that sulfation might be more pronounced under conditions typical of CFB systems.


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INTRODUCTION The direct co-combustion of biomass materials with coal in existing large-scale power stations is known to be among the most cost-effective possibilities to use biomass for energy generation.1 In times of an urgent need to reduce global warming, the concept can help to decrease CO2, NOx and SO2 emissions worldwide and replace coal in the energy system. While co-combustion activities in Europe are currently decreasing and a trend towards mono-combustion of biomass in large-scale power plants can be observed, China recently announced in its 13th Five-Year-Plan (2016-2020) the intention to extend co-firing in order to enhance biomass utilization in the country.2,3 Co-firing of different biomass types (e.g. wood, agricultural residues like straw, sewage sludge and animal waste) with coals of different ranks has been investigated thoroughly in pulverized fuel (PF) and fluidized bed (FB) boilers all over the world.4–6 In particular, a lot of attention has been paid to the mobilization of residual fuels like straw for co-combustion in order to save fuel costs and utilize unused material streams. For both systems an increased share of biomass materials while maintaining operability and high efficiencies is still desired and has been investigated in recent studies.7,8,9 Straw is considered a difficult fuel due to a usually high amount of K and Cl. Chemical fractionation studies reveal that a high share of water-soluble Cl (up to 1 wt-% as received (ar)) can be present in straw.10 Water-soluble Cl is usually fully released as KCl and HCl at temperatures and flue gas compositions with sufficient water contents present in co-firing combustion systems.11,12,13 Several studies have shown that the presence of vaporized KCl is responsible for a variety of problems occurring inside the boiler during biomass combustion and co-combustion.14,15 The main issues are: an increased formation of fine particles16, an enhanced


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deposit buildup, especially on superheater tubes17, corrosion in the superheater area of the boiler18 and deactivation of the catalyst for NOx reduction19. In fluidized bed systems bed agglomeration can additionally occur.20 Fast deposit buildup can especially reduce the availability of the boiler. Unplanned shutdowns to remove the deposited material need to be avoided. Furthermore, the deposit layer causes deteriorated heat transfer, which in turn reduces the steam production efficiency of power stations. The main mechanisms leading to the formation of deposit layers on superheater tubes are inertial impaction, thermophoresis of small particles due to a temperature gradient, diffusion of aerosols, heterogeneous condensation of inorganic vapors and chemical reactions of such vapors with the deposit.21 Due to the lower content of vaporized inorganics during coal combustion, deposit build-up takes usually longer when compared to the combustion of straw or other high-alkali biomass materials.22 In a pilot-scale study, Nielsen et. al.17 showed that the initial deposits formed during pure straw combustion are mainly composed of condensed KCl and K2SO4. Potassium-containing compounds have low melting points (e.g. KCl at 770 °C and K2SO4 at 1070 °C) which can contribute to melt formation in the deposits on heat exchanger surfaces. Cl in the deposit is known to cause severe corrosion problems at steam temperatures above 500 °C, especially when a molten phase is present on the metal surface.18,23 The chlorine-induced corrosion process (also called active oxidation) caused by Cl in the deposits is described in detail in Nielsen et. al.18. HCl or Cl2 is released close to the metal surface via sulphation of deposited KCl to form metal chlorides. These metal chlorides usually have low vapor pressures and can transport metal atoms away from the metal surface. Direct gas phase corrosion attack by HCl or Cl2 is usually regarded as of lower importance in biomass combustion systems.18 The deposit-driven corrosion process is still under investigation and highly dependent of several factors, e.g. the present flue gas


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atmosphere, the combustion system itself, the deposit composition and the alloy used in the superheater section. The formation of the cycle process by regenerating the chlorine for corrosive attack on the metal surface and the role of sulfation inside the deposit is still not fully understood.24 Additionally, the influence of stress corrosion cracking due to higher load flexibility of solid fuel power stations is a topic of current investigations. The main cause for the above-mentioned problems is the formation of vaporized KCl during the combustion of fuels with a high K and Cl content. Several mitigation strategies to reduce the share of vaporized KCl and other alkali chlorides in the flue gas have been developed. Most of them are based on two reactions known to transform the chlorine to less problematic gaseous HCl. The first one is the gaseous sulfation of KCl according to reaction (1) and (2): 2 KCl (g) + SO2 (g) + 0.5 O2 (g) + H2O (g) → K2SO4 (g) + 2 HCl (g),

(1)

2 KCl (g) + SO3 (g) + H2O (g) → K2SO4 (g) + 2 HCl (g).

(2)

Reaction (2) is reported to be by a magnitude faster than reaction (1).25 The gaseous oxidation of SO2 to SO3 is often mentioned to be a required step in order to achieve a satisfying sulfation rate.26–28 Wu et. al.29 point out the temperature window enabling the formation of SO3: 900 °C < T < 1100 °C via homogeneous oxidation (kinetically limited at lower temperatures and thermodynamically restricted at higher temperatures). The sulfation has to take place in a sufficient distance to metal surfaces in the superheater in order to assure that the formed HCl does not interact with the deposit.30 The generation of a sufficient amount of SO3 in the flue gas can be achieved by adding additives (e.g. ammonium sulfate, ferric sulfate, etc.) into the boiler.31 Another option is the co-firing of a sulfur containing fuel like coal or peat.32–34 The second mechanism is the embedding of K in reactive alumino-silicates as described in reaction (3):


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2 KCl (g) + H2O (g) + Al2O3·xSiO2 (s) → K2O·Al2O3·xSiO2 (s) + 2 HCl (g)

(3)

Gaseous HCl is released simultaneously. Several minerals (kaolinite, bauxite, halloysite, montmorillonite etc.) capable to capture potassium are described in the literature. Wang et.al.35 investigated the K-capture by kaolin in an entrained flow reactor. No temperature influence on the capture rate could be observed between 1100 °C to 1500 °C. During co-firing of coal with biomass, the alumino-silicates are usually provided by the coal fly ash. Depending on the coal type different minerals are present.36 Both of the described alkali mitigation reaction mechanisms can occur simultaneously during co-firing of biomass with coal. The co-combustion of straw with coal has been examined in detail in Denmark during a special research program.37 Zheng et.al.38 investigated straw shares of up to 30 % in blends with different coals in an entrained flow reactor. In blends with bituminous coals, no Cl was observed in the deposits even at the highest straw shares. Due to the low share of water-soluble K in fly ash, it was assumed that the embedding mechanism described in reaction (3) is the pre-dominant one during co-firing of straw with coal in PF systems. Long-term corrosion investigations during co-firing of straw and coal showed only slightly enhanced corrosion for straw shares up to 20 %.39,40 In CFB systems higher co-firing ratios with a variety of different fuels have been investigated.41,42 In a study conducted by Aho et.al.7, the fine particle formation and their composition during the co-combustion of two straws and sawdust with two Chinese coals were examined. The co-combustion of a high-sulfur coal showed a better alkali mitigation capability by the sulfation mechanism compared to the other coal with a six times lower S-content. Such high S content can, however lead to very high SO2 emissions and cannot be used without a


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desulfurization unit. A critical assessment of the coal composition is required to estimate the possible co-firing ratios. In order to maximize the share of difficult fuels during co-firing it is crucial to have detailed knowledge of the behavior of the corrosive processes in the superheater. The possibility to measure corrosion online can help monitoring the state of the metal in the superheater, the share of difficult fuels in the fuel blend can be optimized. A commonly used approach is the Linear Polarization Resistance (LPR) method, which indicates the state of the oxide layer on the studied metal surface based on electrochemical measurement.43,44 The former German company Corrmoran developed one OCS (now owned by Babcock Borsig Steinmüller GmbH). This sensor has been used in several experiments in waste incineration plants and during pure biomass combustion.45–51 Retschitzegger et. al.47 investigated the influence of the flue gas temperature and the material temperature in long-term experiments in a wood-fired grate combustor. Generally, low corrosion rates were measured; both increased flue gas and material temperatures caused higher corrosion rates of the metal. No Cl-induced corrosion was observed. A different online sensor made by the Finnish company Savcor Tempo Oy was tested by Leino et. al.52 in a pilot-scale grate combustor firing pure wood and a blend of wood and corn stover. Corrosive processes were detected during the combustion of a blend of 60 % wood and 40 % corn stover on a thermal basis. The derived corrosion signals could be correlated to results of separate deposit measurements indicating a share of up to 30 wt-% chlorine in deposits on the lee and side position of the material rings. In Stephan et. al.53 the behavior of a similar sensor as used in Retschitzegger et. al. is investigated in a co-combustion environment of straw with different bituminous coals in a small-scale PF test rig. Less corrosion


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was detected during the co-combustion of coals with a higher share of Al in the ash, possibly showing better alkali capture ability. This paper, investigated the co-firing of one bituminous coal with increased shares of a highchlorine straw both in PF and CFB combustion in small-scale test rigs to compare flue gas compositions, fine particle emissions and deposit compositions. Additionally, it examined if differences in the occurring alkali mitigation reactions exist in both firing systems. Online corrosion measurements were performed with two sensors in defined conditions. Online corrosion signals were compared to the other measurements to explain the derived signals.

EXPERIMENTAL SETUP Two test rigs were used for the research. An overview of the conducted tests is shown in Table 1. Six blends with increasing straw shares of 0, 10, 25, 40, 60 and 100 % were investigated in both firing systems. Fuel mixtures were blended on an energy basis (e.b.) based on their lower heating value (dry). Pulverized fuel experiments were conducted in an atmospheric entrained flow reactor (EFR) (see Figure 1) at the Technical University of Munich (TUM). The reactor consists of a 2000 mm long, externally heated ceramic tube, called radiation zone, and an approx. 620 mm long so-called convective zone composed of an insulated steel pipe. Two different pulverized fuels can be fed simultaneously into the reactor by two separate vibration channels. The fuel input in the PF test rig was kept constant at 8 kWfuel for all blends. Fuel was premixed with primary air and secondary air inserted tangentially in order to generate a swirling flow to increase residence time and enhance mixing in the upper region of the radiative part. Airflows were adjusted to achieve an excess oxygen content of 2 – 5 vol-% dry at the end of the radiative section. During the


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experiments, the radiation zone was heated to 1200 °C by five external heating elements. In the convective zone flue gas temperatures drop to approx. 700 °C – 900 °C. Figure 2 (left) shows the temperature-flue gas residence time profile of four straw-coal blends during combustion in the PF test rig derived by computational fluid dynamic (CFD) simulations. Modeling was conducted similarly to the methods described in Kleinhans et. al.54. One temperature was measured by a suction pyrometer to validate the derived results. After the convective duct, a cyclone removed the coarse fly ash.

Figure 1: Schematic pictures of both test rigs with measurement locations; PF test rig (left), CFB test rig (right). VTT’s CFB pilot plant (see Figure 1) was used for fluidized bed experiments. The height of the riser is 8 m and the inner diameter 170 mm. Fuel can be fed into the combustor through two separate fuel feeding lines. The fuel input was kept constant at 50 kWfuel for all combusted blends, also blended on an energy basis. The combustion air can be divided into primary, secondary and tertiary air. In the presented tests, air staging was performed with a ratio of 60:40 % of primary to secondary air. The overall air-to-fuel-ratio was set to 1.26 ± 0.04. The


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combustor is equipped with several separately electrically heated and water/air-cooled zones in order to control the process conditions almost independently. The bed temperature was held constant at approx. 850 °C. Figure 2 on the right shows the temperature-flue gas residence time profiles of the combusted blends in the CFB test rig measured by thermocouples. 4

4 100% Colombian

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Before cooler: ELP I/DLP I

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Between cyclones: FTIR Corrosion probe 560°C

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Figure 2: Temperature-flue gas residence time profiles of selected blends; in the PF test rig (left), profiles derived by CFD simulations, one measurement point by a suction pyrometer during 100% straw combustion; the CFB test rig (right) profiles derived by thermocouple measurements. A brief overview of all conducted measurements is shown in Table 2. Further information is given in the following text. Flue gas measurements in the pulverized fuel test rig were conducted at port level 3, approx. 1.75 m below the burner at the end of the radiative section. Flue gas was sucked out of the reactor via an air-cooled probe through a heated particle filter (filter degree: 0.1 µm) maintaining the flue gas temperature > 180 °C. Measurements were performed at constant conditions for a time span of at least 10 minutes. Two different measurement devices were used to analyze the flue gas: a Siemens Ultramat 23 using infrared spectroscopy for CO2, SO2; and O2 by electrochemical method and a MCS300P system made by SICK that measured


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HCl by hot non-dispersive photometry. The sampling line for the MCS300P was heated to 180 °C. More information on HCl measurements are summarized in Wolf et. al.8. Flue gas compositions in the CFB test rig were analyzed after the cooler and between the cyclones. A Servomex 4900 was used after the cooler for standard gas analysis and a Gasmet DX-series FTIR device for the measurement of HCl emissions. A similar approach for the measurement of mass flows of aerosolic particles was taken for in both systems. In the PF test rig, an electric low-pressure impactor (ELPI) made by Dekati was used. A specially designed particle-sampling probe was inserted into the reactor from below. Due to the limited space in the reactor, the particle sampling tests were performed separately from the longer deposit and corrosion experiments. Fine particles were extracted in such a way that they are sampled at the same position where the online corrosion sensor is placed in the long-term experiments. The whole measurement system is described in detail in Balan55. A Dekati Fine Particle Sampling (FPS) system was used to extract a defined flue gas flow of 2.7 Nl/min from the convective part. The sampling flow was diluted with 6 Nl/min pre-heated N2 in the particle sampling probe. Particles > 10 µm were separated by a cyclone heated to 300 °C. Downstream, the sample flow was divided iso-kinetically, one stream analyzed for its CO2concentration to control the N2-dilution, and the other one entered the FPS system for further dilution by pre-heated air (180 °C) before entering the ELPI impactor cascade. Overall dilution ratios were set to approx. 33 for each measurement. The system was used to both measure the particle number distribution by sintered metal targets in the ELPI impactor cascade and to sample fine particles for subsequent composition analysis on aluminum targets. The mass distribution was calculated from data derived during the measurement with sintered metal targets by assuming an average particle density of 1 g/cm³ according to the ELPI instructions. Sampled


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particles on aluminum targets were investigated subsequently in a JEOL JSM 5900 LV electron microscope by energy-dispersive X-ray (EDX) spectroscopy. Particles were sputtered with gold before the analysis. The sampling system used in the CFB test rig is described e.g. in Aho et. al.7. Electric and low pressure impactors (LPI) were used to separate corrosive vapors from the (solid) fly ash for concentration and mass flow analysis. ELPI measuring particle number flow as function of particle size in range < 10 µm in real time was used to determine optimal sampling moment and time to LPI. Probing to the impactors (ELPI and LPI) was conducted between the cyclones. Impactor samples were extracted with 30 mL of Milli-Q-water 5. Sample tubes were held two hours in ultrasonic bath and two hours on a vertical-shaft mixer during extraction. Concentrations of sodium and potassium were measured from the solutions by inductively coupled plasma mass spectrometry (ICP-MS). Concentrations of chloride (Cl-) and sulfate (SO42), were measured by ion chromatography (IC). The detection limits were about 0.02 mg/L, for K and Na and about 0.1 mg/L for Cl- and SO42-. The deposit probing system used in the PF test rig is described in detail in Balan et. al.56. The deposit probe was inserted perpendicularly to the online corrosion sensor. Material rings were mounted on the probe head and cooled to 530 °C on the windward side. The rings were built of the low-alloyed steel 10CrMo9-10 with an outer diameter of 13 mm. Deposits were sampled for 2 h for the presented blends (see Table 1). Material was scraped off at the windward, ~ 45° to windward and lee side of the rings and subsequently analyzed by EDX for its composition (an approach also described in Leino et. al.52). Deposits in the CFB test rig were directly taken from the online corrosion sensor, described in the following text.


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Two different online corrosion sensors (OCS) that apply the linear polarization technique were used in the experiments. The probe heads have a modular construction and consist of measuring electrodes 1, 2, 3 (see Figure 3) to apply the LPR method. The material of the measuring electrodes (1) and (3) can be chosen freely according to the material of the superheater, the reference electrode (2) is usually made out of Inconel.

Figure 3: Picture and schematic drawing of the sensor head of the online corrosion sensors in order to apply the linear polarization resistance technique (adapted from Leino et.al.52). The OCS of former company Corrmoran was investigated in the PF combustion test, later also called “PF sensor”. One sensor was used at port 4 approx. 2.3 m from the burner at a flue gas temperature of approx. 750 °C for the whole measurement duration of 48 h. The sensor head was cooled to 530 °C, the alloy 10CrMo9-10 was mounted on the sensor head for the experiments. The outer diameter of the sensor was 48 mm, the distance between the electrode rings filled by ceramic was 3 mm. Before the start of the long-term test, the sensor was exposed to the flue gas and fly ash stream of pure coal combustion for 16 h to form a deposit on the sensor head. A stable signal formed during those preparation tests. A different sensor of made by the Savcor Oy company, was used in the CFB test rig, later also called “CFB sensor”. A new sensor with a fresh sensor head was implemented between the cyclones at a flue gas temperature of approx. 800 °C


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for each blending ratio for approx. 14 h. The outer diameter of this sensor was 24 mm and the distance between the electrode rings 0.4 mm. The measurement system measured both LPR of two materials and the electrical resistivity of the deposit on the probe head consisting of four sensors. Electrode rings were composed of 10CrMo9-10 and 310S, the materials were cooled to 560 °C. In this paper, the values of the LPR signal of both sensors for the material 10CrMo9-10 are given in Ohm*cm² as described in Andrade et. al.44. A decrease of the signal indicates higher corrosive activities on the metal surface. In order to test the sensor under harsh conditions, the co-firing of a high-chlorine straw from Italy with a bituminous coal from Colombia was investigated. Straw pellets and larger coal particles were milled and sieved to fulfill the requirements of the PF test rig. In the CFB tests the pellets were used directly; the coal was crushed to particles smaller than 4 mm. Both fuels were analyzed for their composition at TUM and VTT, respectively, according to relevant standards. Additionally, chemical fractionation analysis of both fuels was conducted according to the procedure described in Zevenhoven et.al. 57.

RESULTS and DISCUSSION Results of proximate and ultimate fuel analysis of the two fuels in both test rigs are listed in Table 3. Additionally, measured values of the lower heating value and particle sizes in the PF test rig are shown. Despite the effort to test identical fuels in both test rigs, a number of important differences were observed in the fuel data. The straw used in the PF test rig contained more Cl and slightly more K, indicating a higher corrosivity compared to the straw in the CFB test rig. The Al and Si content of the straw in the PF tests is increased compared to the straw batch used in the CFB test rig. This might lead to a small reduction of released KCl, but the


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expected influence is low. More importantly, the coal batch used in the PF test rig contains clearly more Al and Si compared to the coal used in the CFB tests. Especially the higher Al content (increased by approx. 45 %) can lead to enhanced alkali capture according to reaction (3). Furthermore, the S content of the coal used in the PF test rig is approx. 35 % higher compared to the coal used in the CFB test rig. Those differences may arise from the heterogeneity of solid fuels. Quite large batches were used for the presented investigations, making it difficult to achieve perfect comparability. Moreover, the particle size fraction in both test rigs differed by three orders of magnitude. Heavy elements and minerals may accumulate in fine particle fractions, leading to higher shares in small particles. Results of the chemical fractionation analysis of both fuels are shown in Figure 4. The high ratio of leachable K and Cl in the straw (98 % and100 %) indicate that a large amount of those components will be volatilized and are able to form gaseous KCl during combustion. Additionally, the high ratio of not soluble Al in the coal (approx. 94 %) indicates the presence of an alumino-silicate. 30000

30000 Leached in H2O Leached in Acetate Leached in HCl Rest fraction, analysed Untreated Fuel

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Figure 4: Chemical fractionation results for Colombian coal (left) and Italian Straw (right).


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Due to the S content of approx. 1 wt-% dry in the coal and the presence of a sufficient amount of reactive mineral, it is expected that both interaction mechanisms described in reaction (1) – (3) may occur during the combustion of blends of straw with the Colombian coal. The temperature-residence time profiles of both test rigs shown in Figure 2 reveal a major difference between them. While maximum flue gas temperatures of up to approx. 1500 °C are expected in the PF test rig, temperatures in the CFB test rig do not exceed approx. 900 °C. These temperature differences can greatly influence the ongoing chemical processes. Overall residence times are comparable for all blends in both test rigs. In the PF experiments, the combusting particles are in a zone of temperatures equal to or higher than 1200 °C for approx. 1.5 s. Overall conversion of more than 96 %, calculated by the remaining carbon content in ash, was achieved for all blends in the PF test rig and more than 99.5% in the CFB test rig. Figure 5 shows the trends of gaseous emissions in both test rigs over the share of combusted straw in the fuel blend. Dashed lines indicate the concentrations of SO2 and HCl measured in the PF test rig at port 3 in mg/Nm³ reduced to 6 % O2 dry. The SO2 concentration decrease for an increasing share of straw in the fuel blend due to the much lower S content in straw. The measured HCl concentration shows a maximum at 40 % e.b. straw in the fuel blend, subsequently decreasing slightly for a share of 60 % e.b. and more pronounced for pure straw combustion. Filled lines show concentration trends of SO2 and HCl measured in the CFB test rig. The overall trends are comparable to the ones of the PF test rig, however, the following differences can be observed: SO2 concentrations are slightly lower in the CFB test rig for all straw shares. HCl concentrations are constantly increasing in the CFB test rig, by contrast to the PF system, where a drop for pure straw combustion is visible.


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SO₂PF

SO₂CFB

HCl PF

HCl CFB

2000

350

1800 300 250

1400 1200

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HCl [mg/Nm³ red. 6% O2 dry]

1600

SO 2 [mg/Nm³ red. 6% O2 dry]

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400 50 200 0

0 0

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40

60

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Straw share on energy basis

Figure 5: Measured HCl (right y-axis) and SO2 (left y-axis) emissions for different straw shares in the blend in the PF test rig (dashed lines) and the CFB test rig (full lines). The drop for pure straw combustion might be explained by the higher temperatures in the PF test rig. Dayton et. al.58 observed a slightly increased release of Cl as KCl during the combustion of switchgrass at 1100 °C when compared to the release at 800 °C, consequently lowering HCl concentration. Deviations in the SO2 concentrations might be caused by different S contents in the combusted coals (see Table 3). Additionally, inherent sulfur capture in the bed material in the fluidized bed might decrease the measured concentrations. Due to temperatures between 800 – 900 °C in CFB systems, the oxidation of SO2 to SO3 and consequently the sulfation process of KCl according to reaction (2) might be more pronounced in this firing system leading to a consumption of SO2 and production of K2SO4. Figure 6 shows the measured mass flows and composition of the aerosolic particles. Those particles are formed from condensed alkali chlorides and sulfates during cooling in the sampling line. Therefore, the mass flows of S, Na, K and Cl are displayed.


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mg/Nm³, dry red. 6% O2

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220 200 180 160 140 120 100 80 60 40 20 0

Cl K Na S

51 12

in particles < 0.8 µm

in particles < 0.62 µm 113

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Figure 6: Fine particle mass flows of S, Na, K and Cl for different straw shares in the PF test rig in the CFB test rig. In the PF test rig, a very low overall fine particle mass flow was detected for 10 % e.b., 25 % e.b. and 40 % e.b. straw in the blend. The overall mass flow increased to approx. 77 mg/Nm³ red. 6 % O2 dry for the 60 % e.b. straw blend. Results are similar for 10 % e.b. and 25 % e.b straw in the blend in the CFB test rig with almost no fine particles in the flue gas. The 40 % e.b. straw blend shows slightly higher concentrations of 22 mg/Nm³ red. 6 % O2 dry, while for the 60 % e.b straw blend the mass flow is 211 mg/Nm³ red. 6 % O2 dry, thus approx. 3 times higher compared to the PF test rig. Due to the higher mass flow of fine particles containing K and Cl, it may be concluded that alkali mitigation in the CFB rig is not happening at the same rate as in the PF test rig. Higher content of Al in the coal used in the PF test rig enables better alkali capture leading to a reduced amount of KCl in the flue gas. Additionally, the higher temperatures present in the PF test rig, are influencing the alkali behavior in the system. Sulfation reactions are not as likely to occur at the temperatures present in the PF test rig as in the temperature range of the CFB test


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rig. Sulphation of K leads to the formation of K2SO4, which is capable of forming fine particles as well.59 To assess this in more detail the composition of the fine particles was analyzed. In both test rigs, the main component of the very fine particle fraction in the K, S, Na, Cl domain is K. The ratios of the S, Na, K and Cl concentrations in the fine particles between the CFB and the PF test rig during the 60 % e.b. straw case are: SCFB/SPF = 51/8 = 6.4; NaCFB/NaPF = 12/3 = 4.0; KCFB/KPF = 113/49 = 2.3; ClCFB/ClPF = 35/12 =2.9; the ratio of the overall fine particle mass flow calculates to mfine particles, CFB/mfine particles, PF = 211/77=2.7. The higher amounts of K, Cl and Na in the CFB test rig in the 60 % e.b. straw blend is approximately proportional to the increase of the overall mass flow. However, the higher ratio of S in the fine particles in the CFB test rig compared to the amount of S in the fine particles in the PF test rig does not correlate to the ratio of the overall mass flow. Thus, more S containing fine particles were formed during the CFB experiments, possibly indicating a higher intensity of the sulfation processes according to reaction (1) and (2) in the CFB test rig. Figure 7 and Figure 8 compare the analyzed composition (on an oxide basis) of deposits collected during the combustion of 40 % and 60 % e.b. straw in the fuel blend. Much higher shares of SO3 and K2O were detected in the deposits of the 40 % e.b. straw blend collected in the CFB system. The main elements detected on an oxide basis in the PF test rig deposits were SiO2, Al2O3 and K2O. While in the CFB system the condensation of K2SO4 might lead to the high amounts of K and S in the deposits, especially at the 45° and the lee side; this condensation is overlapped in the PF system by the deposition of particles containing SiO2 and Al2O3. On the one hand, this might be caused by the higher share of Al and Si in the coal ash in the PF system. On the other hand, due to the vertical design of the EFR, the entire ash flow passes the deposition probe in the PF system (see Figure 1), including the bottom ash. This might lead to an enhanced


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deposition of larger ash particles on the probe surface. Additionally, the flue gas velocity at the position of the deposit collection was much higher in the CFB tests (approx. calculated to 11.3 m/s) compared to the PF tests (approx. 0.5 m/s). A higher flue gas velocity leads on the one hand to a higher share of particles depositing due to inertial impaction, on the other hand, the probability for particle rebound is increased.60 Furthermore, a higher flue gas velocity might increase the erosion of already deposited material. Influences of particle and flue gas temperature should be comparable in both test rigs. The results indicate a higher share of S containing species depositing in the CFB test rig. The abundant elements in the deposit of the PF test rig imply a higher share of K-Al-Silicates in the depositing particles.

60% Coal / 40% Straw

wt-% in deposit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 90 80 70 60 50 40 30 20 10 0

P₂O₂ Fe₂O₂ CaO Al₂O₂ SiO₂ MgO Na₂O K₂O SO₂

Wind

45° PF

Lee

Wind

45°

Lee

Cl

CFB

Figure 7: Deposit composition at different positions over the circumferences of material rings sampled during combustion of 60 % e.b. coal and 40 % e.b. straw blend. Material temperature was 530 °C in PF and 560 °C in CFB.


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The deposits of the 60 % e.b. straw blend of both test rigs looked more similar. Comparable shares of SiO2 and Al2O3 were found both in the CFB and PF system. Due to the higher amount of Ca in the straw ash, slightly higher concentrations of CaO were detected. 40% Coal / 60% Straw

wt-% in deposit

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 90 80 70 60 50 40 30 20 10 0

P₂O₂ Fe₂O₂ CaO Al₂O₂ SiO₂ MgO Na₂O K₂O SO₂

Wind

45° PF

Lee

Wind

45°

Lee

Cl

CFB

Figure 8: Deposit composition at different positions over the circumferences of material rings sampled during combustion of 40 % e.b. coal and 60 % e.b. straw blend. Material temperature was 530 °C in PF and 560 °C in CFB. The concentrations of SO3 and K2O decreased in the CFB test rig compared to the 40 % e.b. blend. Possibly, the decreasing S content in the blend caused a lower content of depositing SO3. At the same time, the available amount of Si from straw increases. Contrary to the Si present in the coal, a comparable high amount of Si in the straw is soluble in acetate and HCl thereby indicating a more reactive form (see Figure 4). This may form more sticky deposits containing K-Silicates, leading to an increase of Si in the deposit. A small fraction (approx. 5 wt-%) of Cl could be detected in the deposit of the 60 % e.b. straw blend in the CFB test rig, whereas only traces of Cl were found in the PF system. The chlorine in the deposit might lead to corrosive processes on the metal surface.


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Energy & Fuels

Figure 9 shows the measured online corrosion signals in both test rigs for blends with 10 %, 25 %, 40 % and 60 % e.b. straw. The signal is given in Ohm*cm² on a logarithmic scale over the respective measurement duration on the x-axis. Signals derived in the PF test rig are shown per blending ratio by splitting the overall signal of the entire measurement duration in the respective intervals. A lower signal value indicates higher corrosive activities ongoing on the metal surface. Stable signals are detected in both test rigs for 10 % e.b. and 25 % e.b. straw in the fuel blend. A small decrease of the signal is visible in both firing systems for 40 % e.b. straw in the blend. The sensor signal in the CFB test rig shows a clearer drop when compared to the signal derived in the PF system. A similar behavior can be seen for the 60 % straw blend, while the signal in PF test rig decreases to approx. 3000000 Ohm*cm², the signal in the CFB test rig goes down to a value of approx. 100 Ohm*cm². The signals react quickly to the blend changes, with reaction times between 0.5 – 3 hours. Differences between the signals of the sensor used in the CFB test rig and in the PF experiments, especially the large difference in the signal magnitude, may occur due to different reasons. Firstly, a new sensor was used for each blend in the CFB test rig. Thus, the metal surface was directly exposed to the flue gas and particles per blend. The buildup deposit consisted solely of the ash and fine particles of the respective combusted blend. By contrast, the deposit on the sensor in the PF test rig was formed during the whole measurement duration of 48 h and consisted of particles from all blends, starting with pure coal combustion. On the one hand, the already present deposit may act as a barrier for corrosive species diffusing to the metal surface. On the other hand, the mixed deposit might cause different intra-deposit reactions when compared to deposits originating from just one fuel blend. This may limit the sensitivity of the signal to the ongoing corrosive processes.


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90% Coal / 10% Straw 19

20

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22

75% Coal / 25% Straw 00

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LPR 10 CrMo [Ohm*cm²]

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60% Coal / 40% Straw LPR 10 CrMo [Ohm*cm²]

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02

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100000000 10000000 1000000 100000 10000 1000 100 10 1 09

t in [h]

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t in [h]

Figure 9: 10CrMo LPR signals derived in the PF and CFB test rigs for different straw shares. Secondly, the diameter of the CFB sensor is only 24 mm, compared to 48 mm of the PF sensor; also, the thickness of the ceramic rings insulating the electrode rings from each other is 0.4 mm compared to 3 mm on the PF sensor. Therefore, the larger PF sensor might be not as sensitive as the smaller CFB sensor. Thirdly, the deposit composition on the sensor has a large influence on the signal. A higher amount of Cl was measured in the fine particles of the 40 % e.b. and 60 % e.b. blend combusted in the CFB test rig compared to the PF experiments. This might cause more enhanced corrosive processes on the metal surface due to corrosion induced by active oxidation. This assumption is strengthened by the Cl found in the 60% straw deposits of the CFB tests leading to the quickest and largest drop of the online corrosion signal. Only traces of Cl were found in the deposits of the 60 % e.b. straw blend combusted in the PF test rig supporting the small deviations of the PF sensor. Finally, different material temperatures (530 °C


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Energy & Fuels

in PF and 560 °C in CFB) can influence the signal as well. Since corrosive processes show an exponential behavior with a temperature increase, the higher material temperature in the CFB test rig may lead to more enhanced corrosion on the metal surface, in turn leading to a more pronounced decrease of the derived signals. Overall, the derived online corrosion signals show a different behavior which can be partly explained by the sensor geometry and supplementary measurements of the flue gas, fine particles and deposit sampling.

SUMMARY and CONCLUSION Combustion of coal-straw blends was investigated in PF and CFB pilot reactors. The examinations comprised flue gas analysis, fine particle measurements, deposit sampling and innovative online corrosion measurements using the linear polarization resistance method. Two different online corrosion sensors were used in test rigs. The online signals showed a quick response time during the combustion of higher straw shares in the blend. Especially a high ratio of Cl in the fine particles and deposits might lead to an increase of corrosive activities on the metal surface of the sensors. Very low corrosion was detected in both test rigs for 10 % e.b. and 25 % e.b. straw in the fuel blend. A slightly increased corrosion was detected for 40 % e.b. straw in the blend in the PF test rig, a more distinct increase at a co-firing ratio of 60 % e.b. straw. The corrosive activity detected was higher for both straw shares in the CFB rig. This might be caused on the one hand by a different sensor behavior due to other sensor head dimensions, i.e. a smaller sensor diameter and a reduced thickness of the ceramic insulation between the measurement electrodes. On the other hand, an increased sensor temperature of 560 °C in the CFB system compared to 530 °C in the PF system might enhance the corrosive activities due to the typically exponential behavior of corrosive processes during a temperature increase.


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Additionally, different alkali mitigation reactions might occur at the lower temperatures present in CFB applications. Results of the fine particle composition indicate a different interaction behavior of the inorganics in the PF and CFB systems. A higher amount of sulfur in the fine particles in the CFB test rig as well as a lower concentration of SO2 in the flue gas compared to the values in the PF test rig gives the impression that sulfation reactions, as mentioned in equation (1) and (2), are more important in the conditions prevalent in the CFB system. By contrast, in the PF system the embedding of alkali in alumino-silicate, according to equation (3), might be the more dominant reaction mechanism due to the reduced share of sulfur in the fine particles. Further research in the field of combustibility of low-grade fuels and their corrosion behavior needs to be conducted in order to meet the de-carbonization goals in Europe, e.g. similar investigations as the presented ones on the co-combustion of pre-treated fuels (e.g. by torrefaction or steam explosion) both with coal and with other biomass materials like wood.24 Additionally, the investigation of co-combustion in an oxy-fuel environment for carbon capture applications is currently intensified.61


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Energy & Fuels

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +49 89 289 16263. Fax: +49 89 289 16271. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. ACKNOWLEDGMENT The research leading to these results has received funding from the European Union‘s Research Fund for Coal and Steel (RFCS) research programme under grant agreement No. RFCR-CT-2014-00010 OnCord. We thank the team at ABO Akademi for conducting the chemical fractionation analysis. The help of Barbara Waldmann with the evaluation of the OCS results in the PF test rig is gratefully acknowledged. We furthermore want to thank the CFB and PF measurement crews. ABBREVIATIONS ar, as received; CFD, computational fluid dynamics; CFB, circulating fluidized bed; e.b., energy basis; EDX, energy-dispersive X-ray; EFR, entrained flow reactor; ELPI, electrical low pressure impactor; FB, fluidized bed; IC, ion chromatography; ICP-MS, inductively coupled plasma mass spectrometry; LPI, low pressure impactor; LPR, linear polarization resistance; OCS, online corrosion sensor; PF, pulverized fuel; TUM, Technical University of Munich; VTT, Technical Research Centre Finland.


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elektrochemischer Online-Messungen. Dissertation, Universität Augsburg, Augsburg, 2011. (47) Retschitzegger, S.; Brunner, T.; Obernberger, I.; Waldmann, B. Assessment of Online Corrosion Measurements in Combination with Fuel Analyses and Aerosol and Deposit Measurements in a Biomass Combined Heat and Power Plant. Energy Fuels 2013, 27, 5670– 5683. (48) Retschitzegger, S.; Gruber, T.; Brunner, T.; Obernberger, I. Short term online corrosion measurements in biomass fired boilers. Part 1: Application of a newly developed mass loss probe. Fuel Processing Technology 2015, 137, 148–156.


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(49) Retschitzegger, S.; Gruber, T.; Brunner, T.; Obernberger, I. Short term online corrosion measurements in biomass fired boilers. Part 2: Investigation of the corrosion behavior of three selected superheater steels for two biomass fuels. Fuel Processing Technology 2016, 142, 59–70. (50)

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Energy & Fuels

Table 1: Test matrix PF and CFB experiments. Combusted blend

PF test rig Flue gas

CFB test rig

Fine particle

Deposit (2h)

OCS (48h)

Flue gas

Fine particle

Deposit (14h)

OCS (14h)

100 % Colombian (Col)

X

90 % Col /

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

10% Straw 75 % Col / 25 % Straw 60 % Col / 40 % Straw 40 % Col / 60% Straw 100 % Straw

X

X


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Page 38 of 39

Table 2: Information on the measurement equipment utilized in the PF and CFB test rig. Measurement PF test rig

Location & info

CFB test rig Location & info

Flue gas analysis

Siemens Ultramat 23: O2, SO2

Port 3 at 1200 °C, IR method, O2 electrochemical

Servomex 4900: O2, SO2

After the cooler at 180 °C, IR-method, O2 paramagnetic

SICK MCS300P: HCl

Port 3 at 1200 °C, non-dispersive photometry

Gasmet DX FTIR: HCl

Between cyclones at 800 °C, FTIR method

Fine particle sampling

Dekati ELPI system

Port 4 at ~ 750 °C, measurement of particle concentration and composition

Dekati ELPI and LPI

After the cyclones at 730 °C, measurement of particle concentration and composition

Deposition sampling

Cooled deposit probes to temperatures 530 °C

Port 4, 10CrMo as used material, analysis of deposit composition

Deposit analyzed sensors.

Online corrosion measurement

Babcock Borsig Steinmüller sensor (formerly made by Corrmoran), cooled to 530°C.

Port 4, 10CrMo as used material, one sensor for whole measurement, sensor diameter: 48 mm, distance between metal rings: 3 mm

Savcor Tempo Oy sensor, cooled to 560 °C

composition from online

directly corrosion

Between cyclones, 10CrMo as used material, new sensor per blend, sensor diameter: 24 mm, distance between metal rings: 0.4 mm.


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Energy & Fuels

Table 3: Fuel analysis results for fuels in the PF and the CFB test rig.

Properties

PF test rig

CFB test rig

(measured by TUM)

(measured by VTT)

Colombian Coal

Italian Straw

Colombian Coal

Italian Straw

Unit

Lower heating value

MJ/kg, dry

29.7

16.6

29.3

16.8

Total moisture

wt-%

4.6

8.6

7.3

8.2

Volatile matter

wt-% dry

37.1

73.3

38.4

73.2

Ash content (815/550 °C)

wt-% dry

11.3

9.0

8.9

8.5

Particle size d50

µm

24

280

Crushed < 4 mm

Pellets < 6 mm

C

wt-% dry

72.4

46.0

74.1

44.7

H

wt-% dry

5.6

5.8

4.9

5.4

N

wt-% dry

1.5

0.6

1.7

0.66

S

wt-% dry

1.2

< 0.1

0.88

0.08

O (by difference)

wt-% dry

8.0

38.3

9.5

40.4

Major components

Important minor components in the fuels Si

mg/kg, dry

31400

28500

24300

23800

Al

mg/kg, dry

13840

1990

9580

860

K

mg/kg, dry

2660

11090

1550

10700

Cl

mg/kg, dry

430

3570

80

2700

Ca

mg/kg, dry

960

4540

920

5100

S/Cl

mol/mol

30.5

< 0.3

121

0.33

Ca/S

mol/mol

0.06

~3

0.08

5.10

Fuel indices


39

Online corrosion measurements in combination with deposit and

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