Experimental Investigation of Ash Deposit Shedding in a Straw-Fired

512

Energy & Fuels 2006, 20, 512-519

Experimental Investigation of Ash Deposit Shedding in a Straw-Fired Boiler Ana Zˇ bogar, Peter Arendt Jensen,* Flemming J. Frandsen, Jørn Hansen, and Peter Glarborg CHEC research group, Department of Chemical Engineering, Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark ReceiVed February 7, 2005. ReVised Manuscript ReceiVed NoVember 9, 2005

Straw is used as fuel in grate boilers in Denmark to produce heat and electricity. One of the technical problems in these biomass boilers is the formation of an alkali-rich ash deposit, which may seriously affect heat transfer to the plant steam cycle and cause degradation of steam tubes by corrosion. While the processes of boiler ash deposit formation for both coal and biomass have been the scope of many studies, knowledge on the shedding process is limited. A newly developed cooled deposit probe was applied in order to perform shedding measurements in the boiler chamber of the straw-fired Avedøre grate boiler, Unit AVV2, where both qualitative measurements, i.e., video recording, and quantitative measurements, i.e., probe heat uptake and probe deposit mass gain, were performed. The probe was placed in the top of the furnace, close to the superheater. The experimental study showed that in the case of the straw-fired Avedøre Power Station the flue gas temperature around the secondary superheaters is so high (900-1100 °C) and the deposit has such a low melting temperature that the main shedding mechanism is the removal of molten ash deposits due to gravity. It was observed that the flue gas temperature influences the fraction of melt in the deposit and thereby to a high degree controls the ash deposit removal rate. The newly developed deposit probe proved to be a useful tool for the investigation of shedding of ash deposit.

1. Introduction Biomass is considered to be a CO2-neutral and renewable fuel and is as such preferable for the production of heat and electricity. Ash is formed from inorganic species during solid fuel combustion, and fly ash particles and inorganic vapors in the boiler chamber can be deposited on boiler heat transfer surfaces. The deposits act as a thermal resistance to the steam cycle, and especially potassium and chlorine rich deposits may cause boiler metal corrosion. If not removed, deposits can drastically reduce the boiler thermal performance and can in severe cases lead to unscheduled, costly and time-consuming boiler shutdowns. Deposit measurements in boilers1-3 have shown that straw firing leads to the formation of large quantities of ash deposits, especially on superheater tubes, with high contents of potassium, chlorine, and silica. The straw ash deposits melt at a relatively low temperature and may cause high corrosion rates of superheater tubes.3 Because of the above-mentioned reasons, adequate ash deposit removal, i.e., deposit shedding, is important to the boiler operation. Shedding can be caused naturally by erosion, gravity, or thermal shock. To improve the ash deposit removal from the boiler surfaces, artificial shedding techniques, such as sootblowing, are sometimes applied in parallel with natural * To whom correspondence should be addressed. Phone: (+45) 4525 2849. Fax: (+45) 4588 2258. E-mail: [email protected]. (1) Andersen, K. H. Deposit Formation During Coal-Straw Co-Combustion in a Utility PF-Boiler. Ph.D. Thesis, Technical University of Denmark, Department of Chemical Engineering, 1998. (2) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hoerlyck, S.; Karlsson, A. Influence of Deposit Formation on Corrosion at a Straw-Fired Boiler. Fuel Process. Technol. 2000, 64 (1-3), 189-209. (3) Jensen, P. A.; Stenholm, M.; Hald, P. Deposit Investigation in strawFired Boilers. Energy Fuels 1997, 11, 1 (5), 1048-1055.

shedding. Erosion4,5 is the process of removal of solidified deposit, in which case the deposit is hit by hard irregularly shaped fly ash particles, mainly quartz. Particles with sharp edges and a high velocity upon impact can lead to deformation and cutting of the deposit. When the gravity force on a piece of deposit is above a certain critical value gravity shedding occurs.6,7 Breakdown may happen (1) inside the deposit, when the deposit strength is exceeded by the tension induced by the gravity force, or (2) between deposit and tube metal. Melting8 can cause a type of gravity shedding where the outer layer of the deposit flows down. Thermal stress9-11 is caused by changes in the flue gas or the steam temperature, which leads to a thermal expansion/contraction of the deposit-tube system. A fracture due to a thermal shock can occur in the deposit itself, or if the thermal expansion coefficients of the deposit and the tube differ significantly, uneven expansions can lead to deposit detachment from the tube surface. It depends on the ash characteristics and (4) Raask, E. Mineral impurities in coal combustion; Hemisphere Publishing Company: New York, 1985. (5) Raask, E. Erosion wear in coal utility boilers; Hemisphere Publishing Company: Washington, 1988. (6) Kaliazine, A.; Cormack, D. E.; Piroozmand, F.; Tran, H. Sootblower optimization II: Deposit and sootblower interaction. Tappi J. 1997, 80 (11). (7) Kaliazine, A.; Tran, H.; Cormack, D. E. The Mechanisms of Deposit Removal in Kraft Recovery Boilers. J. Pulp Paper Sci. 1999, 25. (8) Benson, S. A. Ash Formation and Behavior in Utility Boilers. www.microbeam.com. (9) Wain, S. E.; Livinston, W. R.; Sanyal, A.; Williamson, J. Thermal and Mechanical Properties of Boiler Slags of Relevance to Sootlowing. Inorganic Transformations and Ash Deposition During Combustion; Benson, S. A.: Florida, 1991. (10) Hasselman, D. P. H. Micromechanical thermal stresses and thermal stress resistance of porous brittle ceramics. J. Am. Ceram. Soc. 1969, 52 (4), 215-216. (11) Hasselman, D. P. H. Griffith criterion and thermal shock resistance of single-phase versus multiphase brittle ceramic. J. Am. Ceram. Soc. 1969, 52 (5), 288-289.

10.1021/ef050037a CCC: $33.50 © 2006 American Chemical Society Published on Web 12/14/2005

Ash Deposit Shedding in a Straw-Fired Boiler

the conditions in the boiler chamber which shedding mechanism will be the dominant one. The deposit ash composition will directly influence the deposit strength, the melting temperature, and the thermal expansion coefficient. The flue gas temperature will affect the deposit surface temperature and thereby the temperature distribution inside the deposit. That will determine the state of the deposit, i.e., if it is partly molten or solid. Several studies have been conducted on deposit formation2,3,12 in straw-fired power plant boilers, while no experimental investigations on shedding in a straw-fired boiler have previously been performed. Jensen et al.3 presented experimental results from two Danish straw-fired power stations (Haslev, Slagelse), where the deposition fluxes in two boiler regions (the furnace chamber and the convective pass) were measured with air-cooled probes with a metal surface temperature of 510 °C. The average flue gas temperatures were around 850 and 650 °C in the furnace chamber and the superheater area, respectively. The Haslev boiler used a so-called cigar burner to feed the straw to the boiler, while the Slagelse boiler was grate fired. Power plant straw boilers commissioned during the past 10 years are grate fired, but the flue gas temperature near the superheaters is much higher than 650 °C in the new boilers. The probes in the Haslev and Slagelse experiments were placed in the boilers for 8 h, while a well-characterized fuel was applied. The deposition flux was calculated according to the deposited mass that was collected from the probe when it was removed from the boiler chamber. The deposit flux in the Slagelse boiler chamber was measured to be in the range of 15-160 g/m2 h and around the superheaters to be in the range of 2-25 g/m2 h. The largest deposit flux was measured for straw with a potassium content of 2.5 wt %, while the lowest deposit flux was observed for a straw with 0.5 wt % potassium. The deposits were dominated by high contents of potassium and chlorine, which worked as glue thereby making it possible for the silicon- and calciumrich ash particles to stick on the probe surface. In two studies, Hansen et al.2 and Jensen et al.,12 mainly the morphology and chemical composition of deposits from two other grate-fired straw boilers (Masnedø and Ensted) were investigated by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. The flue gas temperatures were from 900 to 1020 °C, and the superheater steam temperatures were from 380 to 520 °C at the positions where deposit samples were removed from the boiler superheaters. Deposits collected from air-cooled probes had a layered structure, with an inner layer of KCl and an outer layer of sintered fly ash. The mature deposits formed on the boiler superheater surfaces were quite thick (up to 10 cm) and all had a characteristic layered structure, with a dense inner layer of K2SO4 or KCl present adjacent to the metal surface. In cases with moderate flue gas temperatures (around 900 °C) or in case of the probe deposits the outer part of the deposits was dominated by calcium and silica rich particles fixed in a solid matrix of KCl. For high-temperature boiler deposits (flue gas temperature above 900 °C), the outer layer was depleted of chlorine and an outer porous deposit layer rich in potassium, calcium, and silica appeared. While no shedding experiments have been conducted in strawfired boilers, a few shedding studies have been conducted on boilers using high alkali fuels. Kaliazine et al.6 and Sabet13 investigated the mechanisms of removal of ash deposits by (12) Jensen, P. A.; Frandsen, F. J.; Hansen, J.; Dam-Johansen, K.; Henriksen, N.; Ho¨rlyck, S. SEM Investigation of Superheater Deposits from Biomass-Fired Boilers. Energy Fuels 2004, 18, 378-384. (13) Ebrahimi-Sabet, S. A. A Laboratory study of deposit removal by debonding and its application to fireside deposits in kraft recovery boiler. Ph.D. Thesis, University of Toronto, 2001.

Energy & Fuels, Vol. 20, No. 2, 2006 513

Figure 1. Scheme of the Avedøre boiler. Table 1. Content of Ash and Main Inorganic Elements in Straw Used at the Avedøre Boiler Plant Compared with Typical Danish Straw14 wt % (As Received)

typical Danish straw Avedøre straw

ash

Cl

S

K

Si

Ca

3.9 5.5

0.34 0.32

0.13 0.05

0.86 0.69

0.69

0.34

sootblowing in recovery boilers. A general conclusion was that the brittle deposits were removed by internal breaking up after being hit by a sootblower jet, while harder deposits will be debonded from the tube surface, since they are difficult to break. The aim of this work was to gain more information about ash deposit shedding in a straw-fired boiler to provide information for strategies for effective deposit removal and to provide data that can be applied to verify a model of the shedding process. This study was conducted by constructing a shedding probe that could measure the heat uptake and deposit mass changes and by using the probe to shedding measurements in a straw-fired boiler. 2. Experimental Section The probe measurements were performed in the straw-fired boiler at the Avedøre Power Station (Copenhagen, Denmark), Unit AVV2, in the region of the secondary superheaters. The boiler is gratefired, with a maximum thermal load of 105 MWth. A simplified scheme of the boiler showing the probe position within the boiler is seen in Figure 1. The content of K, Cl, S, and ash in the applied straw fuel is provided in Table 1. It is seen that the applied straw does have a composition reasonably similar to typical Danish straw used in power plant boilers. The newly developed shedding probe was kept inside the straw-fired boiler for a total of 18 days. During this period the boiler was shut down twice, after the 13th and the 17th days of the experiment, respectively. The shedding probe was designed for this project in order to be able to conduct measurements of (1) the temperatures and the flows of the cooling air and water (which will give the heat uptake by the probe) and (2) the deposit mass changes. The scheme of the experimental setup is shown in Figure 2. In the experiments: a color charged-coupled device (CCD) camera was used to take pictures of the deposit, and a thermocouple was applied to measure the flue gas temperature. The probe is constructed as a double annular tube, made of stainless steel, which is cooled with the counter-current flows of water and compressed air. This cooling arrangement ensures a reasonable uniform surface temperature along the whole length of the probe. The probe surface temperature was set to 500 °C in order to simulate typical superheater tubes in a biomass-fired boiler. The constant probe surface temperature was maintained by regulating the flow of the cooling air. Twelve thermocouples were placed inside the outer probe metal tube, i.e., four thermocouples placed circularly at 90° in three horizontal positions (middle position and at the two ends of the measurement zone), as shown in Figure 2 to ensure a reliable registration of the metal surface temperatures. To calculate the heat flux, the system is assumed to be in a steady state. Thus, the heat transferred from the flue gas to the probe is

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514 Energy & Fuels, Vol. 20, No. 2, 2006

Figure 2. Scheme of the applied deposit probe.

carried away by the cooling water and air. The probe heat uptake Q [W] was calculated by eq 1 where m [kg/s] is the mass flow, Cp [J/kgK] is the heat capacity, Tin and Tout [K] are the appropriate inlet and outlet temperatures of the cooling fluids (w ) water, a ) air), and T h is the mean air temperature between the inlet and the outlet Q ) mwCp,w(Tw-out - Tw-in) + maCp,a(T h )(Ta-out - Ta-in) (1) The mass change due to deposition was measured using a load cell on which the probe hangs. The probe was fixed in position by a hinge placed adjacent to the boiler wall and the load cell as seen in Figure 2. The load cell detects changes in mass due to deposition by detecting the force needed to keep the probe in a horizontal position. The resulting momentum balance on the load cell is a summation of the momentum induced by the deposit mass, the probe weight, and the drag force from the flue gas flow. Since the probe mass is constant and the drag force on a long time scale can be assumed to be constant, the collected deposit mass on the probe can be calculated by the momentum balance shown in eq 2 (mLC,0 - mLC)ga ) mDgb

(2)

mLC,0 is the measured mass signal from the load cell at t ) 0, mLC is the signal from the load cell at a given time t, mD is the mass of the formed deposit at time t, g is the gravity acceleration, a is the distance from the load cell to the hinge point (probe mounting), and b is the distance from the deposit center of mass to the hinge point. The calculation of the deposits mass is based on the assumption that the deposit is approximately equally distributed a long the length of the probe. By visual inspection of the probe when it was withdrawn from the boiler this was confirmed to be an adequate assumption. It can be expected that the drag force induced by the flue gas flow to some extent affects the deposit mass measurements. Calculation shows that a flue gas flow of 10 m/s at 1000 °C will correspond to a change of the deposit mass of approximately 150 g. Fluctuations of the flue gas velocity, and thus drag force on the probe, do induce some fluctuations in the deposit mass measurements as seen in Figure 11. However, the deposit mass is on a much higher level than the drag force induced fluctuations. Additionally, the flue gas temperature near the probe was continuously measured, using a simple thermocouple in a protective shell. An IFRF15 suction pyrometer was used to confirm the accuracy of the flue gas temperature measurements obtained by (14) Sander B. Properties of Danish biofuels and the requirements for power production. Biomass Bioenergy 1997, 12 (3), 177-183. (15) Information document: IFRF Suction pyrometer, International Flame Research Foundation.

Figure 3. Image of the shedding probe 55 min after insertion into the boiler (Tfg ) 980 °C).

the thermocouple. A typical difference of approximately 15 °C between the flue gas measurements with the thermocouple and the suction pyrometer was measured. The thermocouple measured a lower temperature due to radiation to the cooled furnace walls. The probe was continuously monitored using a high-resolution CCD video camera placed below the probe, in front of a glass window on the furnace wall. The camera was pointed toward the probe and not the furnace walls most of the time to make it possible to make comparisons of deposit mass gain, heat uptake and video images. Because of constant fouling of the glass window in front of the camera, the window needed to be periodically cleaned with pressurized air.

3. Results and Discussion This section initially discusses the capacity of the developed probe, then the deposit formation and shedding process based on the video recordings are discussed and later the results of the probe mass and heat uptake measurements are presented. Applied Experimental Technique. The surface probe metal temperature was set to 500 °C, and it was possible to keep it within a range of (50 °C along the whole length of the probe. Temperatures at the downstream side of the probe fluctuated



Figure 6. Image of the shedding probe after 9 days (Tfg ) 930 °C), showing deposit dominated by a solid-phase surface.

Figure 4. Image of the shedding probe after 21 h (Tfg ) 1080 °C) showing ash droplets.

Figure 7. Image of the probe after it was removed from boiler chamber and the outer deposit layer was removed (see Figure 8). The total thickness of the screeched layer is approximately 5 mm.

Figure 5. Image of the shedding probe after 22 h (Tfg ) 1000 °C), showing the attachment of an unburned straw particle.

more compared to temperatures at the upstream side, most probably due to less deposition (less deposit formation by inertial impaction) and thus increased heat transfer on the down stream side. The shedding probe could provide steady and stable measurements of heat uptake and accumulated deposit mass. It was possible to detect large mass loss changes but not the shedding of pieces with small masses, as is the detachment of individual ash droplets. Video Camera Observations. The temperature measurements showed that the flue gas temperature at the probe position fluctuated strongly, in the range between 800 and 1100 °C, with an average temperature around 1000 °C. For these conditions

Figure 8. Deposit outer sintered layer.

the main shedding mechanism in the straw-fired boiler was observed to be melting of the outer surface of the deposited ash and detachment of ash droplets. Observations of the shedding probe by the video camera showed that the deposit build-up consists of several distinct stages. The time duration of these stages most probably depends on the flue gas temperature, fly ash composition and ash flux. The formation of a white salt-rich inner layer has been identified as the initial innermost deposit layer in straw-fired boilers.2,12 In the course of this work, an initial phase with formation of this layer could not be observed directly on the video (it took some time to mount the video equipment after the probe was inserted; because of the construction of the


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Figure 9. The heat uptake by the probe and the flue gas temperature as a function of residence time in the boiler (both averaged over 10 000 seconds).

Figure 10. The deposit mass build-up on the probe and the flue gas temperature as a function of residence time in the boiler (both averaged over 10 000 seconds).

equipment, the video can first be mounted after the probe have been installed), but its existence was detected when the probe was removed from the boiler. The second stage of deposit formation can be described as the solid deposit formation phase. Partially combusted straw and fly ash particles form a porous particle-rich deposit on the probe. The video indicates that some of the large attaching particles do burn prior to or just after the attachment to the tube, although this is difficult to distinguish clearly due to a very bright environment inside the boiler. The formation of a porous solid deposit starts rather early after the probe has been inserted into the boiler (it was noticed approximately 25 min after the probe was fully inserted), which indicates that some deposit formation by alkali salt condensation and particle inertial impaction occurs simultaneously. Figure 3 shows the shedding probe 55 min after it was fully inserted into the boiler. In Figure 3, an approximately 40 mm large straw particle can be observed,

which was the largest particle that was observed to impact on the probe. Also two ash droplets can be seen, and this was the first time that a visual observable melt appeared. As the time passes and the thickness of the deposit increases, the formation of drops in the deposit becomes visible, which can be noted as the third stage of the deposit formation process. Figure 4 shows the shedding probe after 21 h in the boiler, when the flue gas temperature was around 1080 °C. At this temperature, the outer layer of the deposit is completely molten, a slag flows down the probe, and ash droplets often detach. The deposit surface melting is not constantly present but depends on the flue gas temperature. During the experiment the outer deposit layer was completely molten at some periods; huge droplets ran from the upstream side of the tube downward. This took place when the flue gas temperature was high, around 1100 °C. At approximately 1000 °C, droplet formation was less pronounced as seen in Figure 5, some parts of the deposit were



Figure 11. Measured deposit mass and flue gas temperature on days 9-11 (raw measuring data).

molten but droplet detachment happened slower. Solid particles, which are then present in the melt, keep the deposit together, acting as a sort of reinforcement. When the flue gas temperature approaches 900 °C, the solid structure becomes dominant, as shown in Figure 6. Video observation of the boiler walls and the superheaters near the probe position confirmed that the slag flows down the boiler superheater tubes in the same manner as it flows down the side of the shedding probe. This was further proven by the fact that the window used for the camera was periodically “plugged” by melt flowing down the furnace wall, so that occasionally the video recording was interrupted. After the probe was removed from the boiler, it was found that the deposit consisted of two major mutually weakly attached layers. Figure 7 shows the shedding probe after it was taken out of the boiler and the outer layer was removed. A white powdery layer can be seen which was probably mainly formed by condensation. The layer was approximately 5 mm thick, was porous, powdery, and sintered, contained small white and transparent crystals, and was quite difficult to remove from the probe. Figure 8 shows the cross sectional structure of the outer deposit layer which was attached to the probe (tube curvature is still visible). The outer deposit layer was brown, 5-8 cm thick, strongly sintered, and highly porous. Gas voids in the solid were potatolike in shape and had a typical size of 1-5 mm. The observed deposit morphology was in agreement with previous straw deposit studyes.3,12 Measurements of Heat Uptake and Deposit Mass. The heat uptake and the weight measurements for the 18 days during which the probe was exposed, averaged over 10 000 s, are presented together with the flue gas temperature in Figures 9 and 10. In Figure 11, an example of the raw mass and temperature measurements (not averaged over 10 000 s) from days 9-11 is shown. The data presented in Figures 9 and 10 was averaged over 10 000 s to make the general trends observable while removing the short time fluctuations. Between days 5 and 7, the flue gas temperature was not measured, while on day 6, the data acquisition system was not started. On day 13, the boiler was shut down for the first time, and the probe stayed in the boiler chamber. On day 15, the boiler was restarted again, and finally on day 17, it was shut down for the second time and the probe was withdrawn.

Since the deposit acts as a thermal resistance, the heat uptake by the probe decreases as the amount of deposit increases. In Figure 9, a general decrease of the measured heat uptake from 13 to 6 kW in the first 6 days can be observed, while as seen in Figure 10 a continuous build-up of probe deposit took place. In the relatively stable period from days 7 to 13, a heat uptake above 7 kW was only observed when the flue gas temperature exceeded 1050 °C. The nonaveraged data for the flue gas temperature, as that presented in Figure 11, show that the flue gas temperatures vary significantly within 1 day (approximately (120° for day 9). Figure 12 illustrates how the heat uptake is influenced by both the flue gas temperature and the amount of probe deposit. In Figure 12, the probe heat uptake as a function of the flue gas temperature for, respectively, days 2 and 10 is shown. The probe had accumulated approximately 2 kg deposit on day 2 and 4.7 kg on day 10. A large amount of deposit acts as an isolation layer, so that at a similar flue gas temperature the heat uptake on day 10 is only 60% of the level on day 2. Two periods can be distinguished in Figure 10, i.e., the initial steep deposit mass increase (days 1-6) and the later (days 7-17) somewhat slower fluctuating deposit mass increase. The deposit mass level in the second period did not reach a completely steady state. In three cases, after large shedding events (days 10.5, 11.8, and 14.6), a steep increase in deposit mass was observed. After the boiler had been restarted (at day 14.6), following the first shut down of the boiler, the initial heat uptake had increased due to a partial detachment of deposit from the probe tube, as can be seen in Figure 10. Only some parts of the outer deposit layer fell off the tube during the shut down period. A gradual increase of the deposit mass appeared after the boiler was restarted. The net deposit mass flux gain on days 15-17 was much higher compared to the initial net mass flux gain (days 0-6). At present, it is not clear why such a fast deposit build-up took place from days 15 to 17, but it may have been caused by a change in the applied fuel composition. An overview of the experiment in terms of the average flue gas temperature, the heat uptake by the probe, the mean net deposit flux, and the deposited mass is shown in Table 2. Results presented in this paper can be compared to previous work3, where deposition fluxes were measured in straw-fired boilers

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Figure 12. The heat uptake by the probe as a function of the flue gas temperature on experimental days 2 and 10. Table 2. Overview of the Mean Heat Uptakes and the Deposition Fluxes Measured during the Experiments time [days]

average gas temperature [°C]

average heat uptake [kW/m2]

deposition flux [g/m2 h]

0-1 1-4.8 7-10 10-13 14.5-17

940 1022 998 1003 996

46.9 46.3 27.3 29.6 30.6

113 108

average mass [kg/m2]

18.4 20.5 281

as has previously been described. Those conditions of the previous measurements, which mostly resemble the present investigation, concerned boiler chamber probe measurements in the Slagelse boiler3. In that case a local flue gas temperature of 850 °C was measured, and the probe surface temperature was fixed at 510 °C. The measured deposition fluxes varied between 15 and 160 g/m2 h depending on the applied straw type. For a straw with a similar content of potassium as in the present study a deposit flux of 45 g/m2 h was measured at Slagelse. As seen in Table 2 a deposit flux of approximately 110 g/m2 h was measured during the first days in this investigation. We believe the differences in the measured fluxes regarding the two experimental studies can be attributed to the differences in the local flue gas temperatures of typically 150 °C. Shedding Observations. It was generally observed that the shedding behavior of the deposit was unambiguously related to the flue gas temperature when the probe was placed in the boiler for more than 1 day, and more than 1 kg of deposit was collected. To illustrate the influence of temperature on the shedding process the images in Figures 6, 13, and 14 can be compared. The images are of mature deposits from days 8 and 9 where a total amount of deposit of approximately 4.5 kg was contained on the probe. It was observed that below a flue gas temperature of approximately 1100 °C the deposit melt contained some solid material, whereas above that temperature the video recording indicated that the deposit surface was nearly completely melted. At temperatures somewhat below 1100 °C, the melt will flow slower, since solid particles increase the melt viscosity. At a temperature of approximately 900 °C, melted particles and shedding were rarely observed. This difference in melt flow can also be detected as a sudden decrease in mass on the probe when the gas temperature increased to above 1100 °C, as shown in Figure 11. It was found that the change in deposit mass, caused by an increase in the flue gas temperature,

Figure 13. Image of the shedding probe after 8 days (Tfg ) 1000 °C).

is very fast, i.e., the response to the increased flue gas temperature is almost immediate. To summarize, the deposit behavior could be divided into three distinct regimes dependent on the temperature: (1) When the flue gas temperature is approximately 1100 °C or above, the deposit surface layer melts completely, and it behaves as a running liquid slag (Figures 4 and 14). (2) Around 1000 °C the deposit contains large amounts of melt, but solid particles are still visible, as shown in Figures 5 and 13. These solid particles act as reinforcement making the melt less fluid, but some deposit is still shed by droplet detachments. (3) At 900 °C and below, a solid porous particle rich deposit is visible on the probe (Figure 6). It was considered that for a long-time probe exposure the deposit mass could reach a constant value, due to the equilibrium between ash deposition formation and shedding. As can be seen from Figure 10, this absolute steady-state condition was not reached. This is probably due to changes in fuel ash content, ash composition, and the large fluctuations of the flue gas temperature.


Figure 14. Image of the shedding probe after 9 days (Tfg ) 1102 °C).

4. Conclusion The aim of this study has been to investigate the process of natural ash deposit shedding in the superheater area of a strawfired boiler. The investigation was conducted by performing heat uptake and mass change measurements and video recording of the ash deposit build-up process on a cooled deposit probe, with a metal surface temperature of 500 °C. The probe was placed 18 days in the Avedøre straw-fired boiler near the superheaters, where a flue gas temperature of 800-1100 °C causes melting to be the main deposit shedding mechanism. The probe deposit mass increased steadily the first 6 days, while after day 7, the mass fluctuated up and down with a mean value of 4.5 kg (equal to 20 kg/m2). The flue gas temperature governs the state of the deposit, i.e., the fraction of the deposit that is melted, and consequently governs the shedding. The melting and partial solidification of the deposit happen periodically, following the changes in the flue gas temperature. The deposit behavior after day 7 can be divided into different regimes,


depending on the flue gas temperature: (1) when the temperature is approximately 1100 °C or above, the deposit surface layer is completely melted and the melt run down the probe and a fast reduction in the amount of deposit on the probe is observed. (2) Around 1000 °C, the deposit contains both melt and solid particles and the deposit formation rate and the deposit shedding rates are approximately on a similar level. (3) At 900 °C and below, a solid porous particle-rich deposit is present on the probe and the deposit formation rate exceeds the deposit shedding. Both from a practical and scientific point of view are the obtained measuring data valuable. It is the plan to use the data to validate a model of the deposit formation and shedding process in the superheater area of a straw fired boiler. The present investigation showed that if superheaters in straw fired boilers are constructed so that a thick layer of deposit can be accepted a stable deposit removal process by ash surface melting can be established. With the knowledge that at flue gas temperatures exceeding 900 °C shedding by ash surface melting takes place, it is clear that sootblowing is not needed at a flue gas temperature above 900 °C. There are still a lot of open questions regarding the shedding in biomass-fired boilers. It will be advantageous to supplement probe investigations with local measurements of gas velocity, ash particle flux, ash particle size, and ash particle composition to improve the fundament for using the data to model verification. Another area of interest is to conduct probe measurements further downstream in the boiler where the deposit is solid and sootblowing often is used. At present it is not known which sootblowing steam pressure is needed to remove a given deposit. Parameters such as deposit chemical composition, surface and flue gas temperature, and deposit sintering conditions may have a large influence. Acknowledgment. This work is part of the CHEC (Combustion and Harmful Emission Control) research center funded by the Technical University of Denmark, Nordic Energy Research, Elsam A/S, Energi E2 A/S, PSO funds from Eltra A/S and Elkraft A/S, and the Danish Energy Research program. The deposit probe study was funded by PSO Eltra A/S. The company Energi E2 A/S is acknowledged for letting us use the straw-fired boiler. EF050037A

Experimental Investigation of Ash Deposit Shedding in a Straw-Fired

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