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VALIDATION AND APPLICATION OF COMPUTATIONAL

MODELLING TO REDUCE EROSION IN A CIRCULATING

FLUIDIZED BED BOILER

Peter Blasera, Giorgio Corinab

a CPFD Software LLC, 10899 Montgomery Blvd. NE, Albuquerque, NM 87111 USA. Email: peter@cpfd-software.com

b Biomasse Italia S.p.A., S.S. 106 Km. 263, 88816, Strongoli (KR), Italy. Email: gcorina@biomasseitalia.it

To Be Presented At: 12th INTERNATIONAL CONFERENCE

MULTIPHASE FLOW IN INDUSTRIAL PLANTS Ischia (Napoli), Italy

September 21-23, 2011

ABSTRACT

The 40 MW Strongoli power plant, located in the Calabria region of Italy, produces power from 100% biomass sources. The combustion of wood biomass, exhausted olive residues and palm kernel shells, occurs in a sand-filled, Circulating Fluidized Bed (CFB) combustor. Operational experience with the unit dates back to 2003.

This paper describes the optimization of the boiler in order to minimize erosion on internal surfaces and structures. Detailed three-dimensional, transient, multiphase gas-solid flow fields were computed and are presented. Details of the complex geometry include the combustion chamber, cyclone, cyclone dipleg, seal pot, fluidized bed heat exchanger and cyclone outlet structures including suspension tubes.

The gas-solid flow was computed using the commercially-available software package Barracuda®, a CFD software based on a unique Eulerian-Lagrangian formulation that was essential to the success of the subject work. Both instantaneous and time-averaged results were obtained. Results were validated against operational erosion experience. The validated model, in turn, was utilized to redesign various components of the boiler, optimizing both erosion characteristics and performance behaviour of the system. The redesigned unit is scheduled for commissioning in late 2011.

1. INTRODUCTION

Circulating fluidized bed combustors have several advantages over other types of combustion technologies used in power plants. The fluidized solids moderate operating temperatures, inhibiting the formation of nitrous oxides (NOx). The fluidization media often incorporates a sulphur-absorbing material, such as limestone, thus significantly reducing sulphur oxide (SOx) emissions. Finally, CFBs permit for the use of a wide range of combustion fuels ranging from fossil fuels to renewable resources or some mixture thereof.

The 40 MW Strongoli power plant, located in the Calabria region of Italy, utilizes multiple CFB boilers to generate electricity from 100% biomass sources. Fuels include wood biomass, exhausted olive residues and palm kernel shells. The plant is the largest of its kind in Europe, and has been in operation since 2003 (Figure 1).

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Figure 1. 40 MW Strongoli power plant (left), CAD model of the primary CFB loop (right)

The primary CFB loop consists of a combustion chamber, cyclone, dipleg and seal pot as shown in Figure 1. Air and biomass fuel react in the combustion chamber and transfer heat to sand particles. The gas-particle mixture then enters a cyclone where the solids are returned to the combustion chamber via a dipleg and seal pot. The gas exits through the cyclone outlet. Some of the recirculating solids are diverted to a Fluidized Bed Heat Exchanger (FBHE) through a Spiess valve. The overall height of the unit is nearly 30 m.

Notwithstanding the advantages noted above, one consequence of the fluidized solids transport in a CFB combustor is erosion on boiler surfaces as a result of particle impingement. Erosion can be mitigated by the use of refractory lining. However, refractory-lined surfaces are not useful for heat transfer.

Figure 2. Eroded surfaces: cyclone inlet (center), suspensions tubes (top), FBHE (bottom)

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Several locations inside the Strongoli CFB loop suffered from erosion levels higher than anticipated. The most severe region was inside the cyclone inlet, as shown on the centre of Figure 2. This region, although refractory-lined, experienced erosion damage severe enough to require replacement of the refractory in as little as 3 to 4 months. Regions of secondary concern include suspension tubes in the cyclone outlet stream, as shown at the top of Figure 2 and heat exchanger tubes inside the FBHE, as shown on the bottom of Figure 2. Some other regions exhibited only minor erosion behaviour after 8 years of operation.

The plant operator, Biomasse Italia S.p.A., desired to take substantive action to mitigate the most significant erosion problems in order to extend the operational life of the unit, reduce maintenance costs, and minimize the risk of unplanned shutdowns. Several potential redesigns were proposed but a means to evaluate the relative benefits of each potential redesign and arrive at an optimal design was required. It was determined that a computational fluid dynamics (CFD) model could be used for this purpose.

This paper discusses the use of a computational model to understand and optimize the performance of the Strongoli CFB power plant with respect to erosion at the cyclone inlet and suspension tubes, while maintaining overall system performance. Additional work has been performed to reduce FBHE erosion while maintaining thermal performance.

2. COMPUTATIONAL METHOD

The CPFD®, or Computational Particle Fluid Dynamics, method was used to simulate the gas-solid flow inside the CFB loop. The CPFD method [1] solves the transient fluid and particle mass, momentum and energy equations in three dimensions. The fluid is described by the Navier-Stokes equation with strong coupling with the discrete particles. The particle momentum has been adapted from the Multi-Phase Particle-In-Cell (MP-PIC) numerical approach [2,3,4] which is a Lagrangian description of particle motion coupled with the continuum fluid. The CPFD method is utilized by the commercially-available Barracuda software package, which has been validated for a wide range of fluid-particle flow problems [5,6,7] including CFB Combustors [8].

This computational method was chosen for the CFB work due to several unique technical capabilities, all critical to the subject work:

The ability to model the full particle size distribution (PSD) for any number of solid species;

The capacity to model any solids loading, from fully dilute up to close-packed, in the same simulation and without prior knowledge of what the loading is likely to be;

Complete Lagrangian formulation for the solids, which is critical for computation of erosion;

A chemistry modelling capability to account for the combustion and heat transfer in the unit; and

The ability to model systems with physical very large particle counts, up to or in excess of 1e16 particles.

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3. COMPUTATIONAL MODEL

A 3D solid model of the overall circulation loop was created as shown on the right of Figure 1. A computational grid was cut from a 580,000 cell Cartesian mesh, as shown on the left of Figure 3. The resulting numerical model included 172,000 computational cells for scalar calculations, 570,000 cell faces for vector calculations and 1.5 million computational particles to resolve the granular field. Note that cell counts in CPFD models are often lower than those used in many CFD models due to the added sub-grid resolution of the particulate phase. A grid resolution study was not performed due to the strict time-constraints of the work. However the validity of the model was confirmed against plant operation, and is discussed in the following section. Suspension tubes in the cyclone outlet stream were included in the model as shown on the right of Figure 3. However, other fine details, such as individual coils in the FBHE, were not included here. These were treated in a separate, localized model. The optimization of FBHE erosion is beyond the scope of this paper.

Air was introduced at multiple locations at rates corresponding to actual plant operating conditions. Air injection locations included primary and secondary combustion chamber inlets, seal pot fluidization, FBHE fluidization and fluidization in the empty transfer chamber between the seal pot and FBHE. Cyclone outlet pressure was maintained at 101 kPa.

Figure 3. Computational grid (left), details of suspension tubes in outlet stream (right)

Three particle species were defined corresponding to sand, fly ash and bottom ash. Sand density is 2650 kg/m3 while the ash is lighter with a density of 1500 kg/m3. The PSD for each specie is shown on the right of Figure 4, and was determined from an analysis of the particles at the plant. The scale of the plot is limited to 1 mm or less to provide a visual comparison of the three unique PSDs. However, some bottom ash as large as 2.2 mm was present in the system.

Seventy tonnes of sand was initialized in the bottom of the combustion chamber and seal pot as shown on the left of Figure 4. In the actual unit, ash is produced during combustion. In the computational model, ash was introduced through the secondary air inlets. This assumption was

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not expected to have any impact on the predicted erosion behaviour in the cyclone inlet or suspension tube regions.

Figure 4. Initial sand location (left), particle size distribution (right) Additional simplifying assumptions were made to enable the numerous calculations to be performed in a practical time frame. While chemical reactions and heat transfer are important aspects of a CFB combustor, it was assumed that non-reacting and isothermal simplifications would not affect the erosion prediction in the regions of interest, provided that the velocities in the impact regions were accurately represented. To ensure correct impact velocities, the isothermal temperature of 850°C was taken from the operational temperature measurements at the top of the combustion chamber. Further, the gases from combustion products were also included in the model. Since combustion was not computed, the additional gas flow was added to the primary and secondary air inlets in the combustion chamber.

An erosion index was defined to tabulate particle impacts on surfaces. The erosion index is dependent upon the following functional form:

Cαm1.5v3.5 (1)

where Cα is a coefficient as a function of impact angle, m is the particle mass and v is the particle velocity. The angular coefficient, Cα, was set with a maximum value for impacts normal to the surface, to simulate erosion characteristics of a brittle material [9, 10]. The mass and velocity coefficients were left at their default values in the software, some of which can be found in the literature [10, 11]. The functional form is summed for all particles impacting a given wall surface and is normalized by the area of the surface patch and time. Note that this model is dependent upon the angle of impact between the particles and the wall, and thus only may be used for an Eulerian-Lagrangian formulation of the problem: That is, discrete particles must be modelled in order to use this impact-based erosion calculation.

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4. MODEL VALIDATION

Before using the model to optimize the overall CFB erosion performance, the model was validated based on past operational experience at the plant. Two different cyclone inlet geometries were used in the past. The original inlet resulted in very severe erosion in the cyclone inlet region requiring repair every 3 to 4 months. The cyclone inlet was subsequently widened, imcreasing the inlet area in an effort to reduce the velocities in that region. That approach was successful and local erosion was reduced enabling nearly one year of operation before replacement of the refractory was required.

The computational model was used to predict the relative erosion characteristics of the two prior inlet designs. Two separate baseline calculations were run, representing the original and modified inlet geometries. All other model parameters were unchanged.

Figure 5. Calculation start-up. Particle speed shown at 1, 2, 3, 4, 5 and 20 seconds.

Figure 5 shows the start-up behaviour of the system. Particles are coloured by speed. Results are shown at 1, 2, 3, 4, 5 and 20 seconds, from left to right across Figure 5. A quasi-steady operating mode is obtained by 20 seconds in terms of gas and solids circulation behaviour. Time-averaging and erosion tabulation was activated after 20 seconds. The calculations were run to 80 seconds of real time in about 5 weeks of CPU time on a single-processor, multi-core workstation computer. Subsequent analysis shows that the same engineering conclusions could have been made with a shorter run of about 50 seconds, which could have reduced the runtime to about 3 weeks.

Figure 6 shows a comparison of the computed erosion index for the two baseline calculations. The top row shows results for the original inlet configuration, while results for the modified inlet are shown in the bottom row. Only regions where the erosion index exceeds a given tolerance are shown. Increasing tolerance levels are applied from left to right across Figure 6. Erosion regions are coloured by severity, with red representing the highest erosion.

In all cases, the extent and severity of the erosion are lower for the modified inlet compared with the original inlet. This agrees well with operational experience at the plant. After modifying the cyclone inlet geometry, the CFB combustor was able to operate approximately three times longer before requiring maintenance.

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Figure 6. Model prediction of high-erosion regions for the baseline calculations. Original inlet is shown on the top row. Modified inlet is shown on the bottom row.

It should be noted that although the results of the erosion model are quantitative, additional data is required to determine meaningful erosion index values. The model is entirely based upon the mass, velocity and impact angles of the particles against the wall. In actuality, erosion is based on a myriad of additional parameters such as surface material, particle material, particle shape, etc. Thus by comparing the model results with operational experience, it was possible to discern what threshold levels of the erosion index were cause for concern. Erosion index values below the identified threshold were considered acceptable.

The reason for the reduced erosion in the case with the modified inlet is the lower particle velocities upon impact. Figure 7 shows the gas speed as shown on a plane through the cyclone inlet regions. Note the lower gas velocity in the case with the larger inlet area on the right of Figure 7. The lower gas velocity results in lower particle velocities and thus reduced erosion. The erosion index is a function of velocity to the 3.5 power, so reducing impact velocity is the most effective means of reducing likely erosion.

While this result is expected, the impact of this change on other components in the system is not immediately obvious. The model calculations revealed that the larger inlet reduced the cyclone efficiency for sand separation from 99.994% to 99.961%. While both of these numbers may seem similar, it is important to note that the amount of sand exiting through the cyclone vortex tube is drastically different in these two cases. The lower efficiency for the modified inlet case results in about 6 times more sand passing through the suspension tube region in the cyclone outlet stream. Thus the simulation predicted an increase in the suspension tube erosion as a result of the cyclone inlet change.

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Figure 7. Gas speed in cyclone inlet region (original inlet on left, modified inlet on right)

After the model predicted the increase in suspension tube erosion, it was also confirmed via plant operational experience. The modified inlet, though good for reducing cyclone erosion, had in fact increased erosion in the suspension tubes. It should be noted that the modeller was not informed of this information prior to reaching this conclusion. On the basis of this validation work, it was concluded that the model could predict not only the effect of geometry changes on cyclone inlet erosion, but also how those changes affect other parts of the system. Thus, the model was validated for use in optimizing the overall erosion performance of the system.

5. MODEL APPLICATION

From the model validation effort, it became clear that while a change may have a positive impact on one aspect of the unit, the same change could have a detrimental effect on another region. For this reason, it was decided to optimize the erosion performance of the overall CFB unit in two phases. In the first phase, several design alternatives were studied, including their effect on system performance. These design alternatives were compared and contrasted to arrive at a final design that was a combination of those design alternatives that appeared most effective at reducing erosion in the subject region while having the least undesirable impact elsewhere in the unit. In the second modelling phase, this new, final redesign was modelled in detail, fully assessed and compared with the baseline models.

Many alternative designs were proposed for analysis. The ideas for alternate designs came from a variety of sources including both plant operational experience and insights obtained by a detailed analysis of the baseline models. The list of alternative geometries was then down-selected to include only ideas which were reasonable and feasible to implement, considering both the cost of modifications and the risk of failure. After running an initial set of alternate designs, several more designs were suggested based on the results of the first cases.

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Figure 8. Comparison of cyclone erosion characteristics

In the end, five different designs were studied and compared using the validated computational model. Alternative 2 had the same inlet area as the current design. Alternatives 1, 3 and 5 utilized different inlets which were 9% larger than the current design, while the inlet for Alternative 4 was 18% larger than the current design. The performance of these designs was then compared based on three objectives. The primary objective was to reduce the erosion at the cyclone inlet. Minimizing erosion in the suspension tube region and minimizing pressure drop across the cyclone were secondary objectives.

Figure 8 shows the extent and severity of erosion index for the five alternative designs compared with the two baseline cases. Alternative 3 shows a high level of erosion, comparable to that present when using the original inlet design. Thus Alternative 3 was eliminated from further consideration. Alternative 2 is expected to have similar erosion characteristics to the current design (i.e. modified inlet), while alternatives 1, 4 and 5 were expected to further reduce erosion compared with the current design.

Figure 9 compares the mass flow of the various solids through the suspension tube regions in the cyclone outlet streams for the various designs studied. The cyclone is extremely efficient at separating the bottom ash from the flue gas stream; very little passes through the cyclone vortex tube. Similarly, all the design changes have a minimal impact on the separation of the fly ash. However, it is apparent in Figure 9 that the amount of sand passing through the cyclone is the primary factor influencing the erosion on the suspension tube surfaces. This is not immediately

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obvious, since the sand separation efficiency is so high. However, due to the high circulation rate present in CFB systems, these small differences can add up to significant additional sand flow, as shown in Figure 9.

From Figure 9 it is apparent that alternatives 1, 2 and 4 will result in similar, and even higher, erosion in the suspension tube region compared with the current design. Alternatives 3 and 5 will reduce the erosion in this region. However, alternative 3 proved to have unacceptably high erosion in the cyclone inlet. Thus, alternative 5 was judged to have the best overall erosion characteristics.

Figure 9. Comparison of particle flow rates through suspension tube regions in cyclone outlet stream

The pressure drop across the cyclone was also compared for the baseline and alternative design cases. The highest pressure drops were computed for the baseline case with the original inlet and alternative 3. Note that these two cases also had the highest erosion in the cyclone inlet region and lowest erosion in the suspension tube regions. Alternative 1, 4 and 5 all resulted in lower pressure drop than the current design.

6. DISCUSSION

Upon consideration of the inlet erosion, suspension tube erosion and pressure drop performance predicted by the validated computational model, it became clear that alternative 5 was the best candidate for a redesign. The changes outlined in this paper were combined with other modifications suggested from a similar study of the FBHE erosion. These changes defined a final redesigned CFB combustor. The final redesigned case was then rerun and compared with the two baseline cases. This redesign assessment phase verified the findings reported above were valid even after implementing additional modifications to the lower region of the combustion chamber.

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While the erosion results presented herein enable directional decision-making, a quantification of the improvement proved to be a challenging undertaking. The same difficulty presented itself when analyzing erosion observations at the plant. Although typical refractory life-spans were reported earlier, it was observed that during different maintenance outages, the same refractory installed in the same unit and run under identical conditions resulted in noticeable differences in amount of material eroded.

In order to quantify the improvement in erosion performance in the models, the surface area which exceeded a given erosion index value was computed and contrasted for the baseline and redesigned geometries. An example is shown in Figure 10. In this example, the eroded surface areas for the original inlet, modified inlet and redesigned inlet cases were 12.75 m2, 3.13 m2 and 0.25 m2, respectively. Thus, if a 75% reduction in erosion was observed when changing from the original inlet to the modified inlet, then an additional 92% reduction is expected for the redesigned case for the same refractory material and operating conditions. The comparison of eroded surface areas was repeated for four different erosion index tolerance levels, with the results shown in Figure 11.

Figure 10. Comparison of surface areas exceeding a given erosion index tolerance level.

Overall, the redesigned case is expected to result in a 50% reduction in erosion in the cyclone inlet region, a 47% reduction in erosion in the suspension tube region and a pressure drop 12% lower than the current design.

7. CONCLUSIONS

Erosion on internal surfaces remains a primary concern in CFB combustion systems, especially those fuelled with biomass. This work has presented a validated computational model for erosion prediction in a full CFB loop including combustion chamber, cyclone, dipleg, seal pot, and additional components such as FBHE and suspension tubes in the cyclone outlet stream. The validated model was then used to optimize the design to minimize erosion and pressure drop in the cyclone region. The redesigned case is expected to have a 50% reduction in erosion in the

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cyclone inlet region, a 47% reduction in erosion in the suspension tube region and a pressure drop 12% lower than the current design.

Figure 11. Quantification of expected erosion reduction as a function of previously observed erosion reduction

This work has highlighted the importance of overall optimization. Earlier modifications, although successful at reducing erosion in the cyclone inlet region, resulted in increased erosion on other surfaces. This computational study of the design space enabled optimization considering multiple objectives for the overall system performance.

Finally, this work represents a synergistic solution to the erosion problem. The final redesigned case was computed to have better overall performance than any alternative suggested by the modeller or plant operator alone, and consisted of recommendations that are relatively simple to implement, maintain and remove if necessary. Installation of the redesigned components is scheduled for late 2011.

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