combustion system upgrade on ckpi’s biomass-fired boiler · a biomass day bin with live bottom...

Combustion System Upgrade on CKPI’s Biomass-Fired Boiler J. Stephen Campbell, Jr., P.E. Jansen Combustion and Boiler Technologies, Inc. Kirkland, WA 98034, USA Matthew A. Henderson, P.E. Jansen Combustion and Boiler Technologies, Inc. Kirkland, WA 98034, USA Marcel D. Berz, P.E. Jansen Combustion and Boiler Technologies, Inc. Kirkland, WA 98034, USA Blair Rydberg, P.Eng. Canadian Kraft Paper Industries Ltd. The Pas, Manitoba, Canada MB R9A 1L4 ABSTRACT

Canadian Kraft Paper Industries Ltd (CKPI) operates the No. 2 Power Boiler at its mill in The Pas, Manitoba, Canada. The boiler was designed to generate up to 275,000 lb/hr of steam on Bunker C oil firing or a combination of oil and biomass fuel. The unit was originally equipped with a stationary pinhole grate that sloped downward from front to rear, with a steeper drop over the front half. This design caused most of the biomass combustion to occur at the bottom of the steeply sloped grate portion, resulting in limited steam generation from biomass, frequent oil co-firing, high carbon monoxide (CO) emissions, and high carryover of biomass fuel particles into the upper furnace.

In 2017, the mill contracted Jansen Combustion and Boiler Technologies, Inc. (JANSEN) to design and supply major elements for a combustion system upgrade. The mill’s goals included increasing steam generation from biomass, reducing oil co-firing, and decreasing excess air and char carryover. The project began with a boiler evaluation that characterized the deficiencies of the existing boiler, defined necessary pressure part modifications to support a new continuous ash discharge grate, and determined upgrades to optimize fuel and combustion air delivery.

The boiler upgrades were installed between July and September 2018. JANSEN’s scope of supply included design and supply of modifications to the combustion air system, fuel delivery equipment, and ash handling equipment, as well as boiler pressure part modifications to accommodate a new Detroit® RotoStoker VCG grate supplied by Detroit Stoker Company. Boiler tuning during start-up in September and again in November showed greatly improved combustion conditions without the need for oil co-firing. Improved combustion led to reductions in fly ash collection of 60% to 80%. Average reductions in CO were approximately 50% while operating at higher biomass firing rates, based on spot check field measurements before and after the upgrade.

This paper describes the process that led to a successful upgrade project, including data collection and analyses, Computational Fluid Dynamics (CFD) modeling, equipment design, supply and installation of the modifications, operator training, start-up assistance, and post-start-up tuning.

INTRODUCTION

The No. 2 Power Boiler was originally supplied by Foster Wheeler with an inclined stationary pinhole grate. The unit was designed to generate 275,000 lb/hr of steam at 775 psig and 825°F while burning Bunker C oil either alone or in combination with biomass (roughly 50% heat input from biomass). Since the original installation, two of the four load burners, located on the rear wall, have been removed and the openings sealed with refractory. Prior to the grate retrofit, the unit burned mainly biomass with frequent co-firing of Bunker C and waste oil. Figure 1 shows a side sectional view of the original boiler.

Boiler combustion performance on biomass firing was suboptimal, due largely to the boiler’s steeply sloping stationary pinhole grate. The grate’s slope forced most of the combustion to occur in the rear half (bottom) of the grate that was less sloped. This led to fuel piles on the flatter rear section, which limited the ability of undergrate air (UGA) to penetrate the fuel bed and burn out the fuel. Excess air levels were high due to uneven UGA distribution, tramp air infiltration through the fuel chutes, and high burner air flows. An upgraded overfire air (OFA) system was installed in 1999 to provide more OFA than the original system. Although improvements were achieved, the non-uniformity of combustion in the lower furnace, coupled with limitations in the boiler’s fuel feed system, continued to hinder the boiler’s performance.

In 2017, CKPI decided to upgrade the No. 2 Power Boiler with the following goals:

1. Reliably achieve up to at least 167,000 lb/hr of steam generation from biomass fired on the grate, roughly double the amount from historical averages.

2. Reduce excess air and char carryover to improve boiler efficiency.

3. Reduce wear on the superheater and back pass components by minimizing carryover.

4. Eliminate manual grate cleaning to improve safety for the operators.

The upgrade included a new OFA system, new UGA system, modifications to the bottom ash and fly ash handling systems, a biomass day bin with live bottom feed system, and pressure part modifications in the lower furnace, plus a new Detroit® RotoStoker VCG air-cooled, horizontal vibrating grate and new pneumatic fuel distributors from Detroit Stoker Company. In addition to equipment design and supply, JANSEN’s support of the upgrade included a process engineering evaluation to finalize the upgraded boiler’s predicted performance, on-site technical support during equipment installation, operator training, start-up assistance, and post-start-up tuning.

OFA SYSTEM DESIGN APPROACH

The No. 2 Power Boiler air system upgrade was designed to use relatively few OFA nozzles located on the furnace sidewalls sized to provide 35% to 50% of the total biomass combustion air above the grate [1]. The nozzle’s convergent geometry allows operation with low pressure loss while delivering high OFA jet momentum (mass flow times velocity) for deep penetration and turbulent mixing with combustible gases and airborne fuel particles. For the No. 2 Power Boiler, the low pressure loss design of the OFA nozzles allowed the existing forced draft fan to supply the new OFA system, as well as the new UGA system and the oil burners. The OFA system design also reused the existing control dampers and portions of the existing ductwork. Figure 2 shows the original boiler side sectional view with the upgraded grate, combustion air, and fuel delivery systems added.

An illustration of the OFA nozzle (Multi-Range High Energy Combustion Air NozzleTM) is shown in Figure 3. The nozzles are partitioned horizontally into two sections with separate manual or automated dampers for each section. The dampers are located in a low velocity region far from the nozzle tip to minimize pressure loss and to avoid damage or jamming from overheating. At lower OFA flow demands, certain nozzle sections can be taken out of service by closing their nozzle dampers so that the reduced OFA flow is delivered through a smaller flow area. This increases the air velocity exiting the remaining open nozzle sections and improves OFA jet penetration and mixing in the boiler furnace. The nozzle design provides flexibility in achieving optimal system performance over a large load range. Operational flexibility in the OFA system is essential for the No. 2 Power Boiler, as the unit is operated at lower steam generation rates in the summer months compared to the winter months.

INITIAL ENGINEERING EVALUATION

Data Collection and Analysis

The upgrade project began with an engineering evaluation to define current boiler operations and limitations. Engineers visited the CKPI mill in October 2017 for data collection, including measurements with portable instruments, boiler fuel and ash sampling, and downloading data from the data historian. During the on-site data collection period, the boiler’s steam generation averaged 124,000 lb/hr, compared to 117,000 lb/hr over the

previous 12 months. Even at these reduced steaming rates, oil firing in the previous one-year period typically constituted 20% to 25% of the heat input to the furnace, resulting in steam generation from biomass firing of only about 80,000 lb/hr.

Data collected during the site visit demonstrated that the boiler tended to operate with flue gas oxygen levels around 10% on a wet basis at the boiler outlet. Such high levels were due partly to the fact that the majority of biomass combustion occurred on the rear, less inclined portion of the grate. As a result, most of the UGA entering though the sloping part of the grate did not contribute significantly to combustion, and thus only increased flue gas oxygen. Other causes were tramp air leakage into the furnace through the fuel chutes and high burner air flow rates, such that burner excess air levels were often well over 100% and ranged as high as 300%.

Field measurements showed frequent high CO readings. With the boiler co-fired on biomass and oil, CO levels ranged from 580 to 1,930 dry ppm, both corrected to 3% dry oxygen. (Note: All CO values in this paper are stated in dry ppm corrected to 3% dry oxygen.) However, when oil firing was discontinued and the boiler was fired on biomass only, CO levels increased to as much as 3,560 ppm. These readings are indicative of the poor combustion conditions that were present in the boiler.

Boiler Performance Projections and Process Design

Data collected during the October 2017 site visit were used to evaluate the boiler performance, identify limitations, and predict performance at conditions with the new grate and combustion system improvements. Calculations based on standard ASME power boiler test procedures showed the boiler’s maximum biomass firing rate with the new grate to be 26% higher than that of the original boiler design (59,000 lb/hr compared to 47,000 lb/hr). Improved performance resulting from the upgrades and from reducing excess air was predicted to allow an increase in steam generation from the previous annual average of 117,000 lb/hr on biomass plus oil co-firing to 167,000 lb/hr on biomass alone, roughly doubling the amount of steam generation from biomass. In other words, steam generation capability from the upgraded boiler would be greatly increased while oil co-firing would be greatly decreased.

The evaluation included an analysis of the boiler’s natural circulation conditions in order to design and size the required pressure part modifications to accommodate the new grate and allow the boiler to still be capable of producing its original design steaming rate of 275,000 lb/hr. The circulation evaluation used water flow data measured during the October 2017 site visit in the boiler downcomers using ultrasonic flow monitoring (UFM) equipment [2]. The analysis results indicated that the installation of additional sidewall relievers from the upper sidewall headers to the steam drum would be required if the boiler was to operate at steaming rates exceeding 245,000 lb/hr.

CFD Modeling

The base for the boiler CFD modeling was the commercially available CFD software package FLUENT. FLUENT incorporates multiple aspects such as fluid flow (three-dimensional velocities and pressures), general combustion, gas component concentrations, and energy balance calculations. Using this framework, proprietary solid fuel boiler-specific processes, such as fuel drying, release of volatile material, combustion in suspension and on the grate, and carryover of ash and burning particles are added. These models have been used numerous times to successfully improve combustion conditions and diagnose operating problems for boilers firing biomass [3] and refuse derived fuel [4]. For the No. 2 Power Boiler, the model was customized for its geometry with the new grate and to handle the biomass composition and size distribution profiles as determined from fuel sample analyses. The CFD model calculation results included profiles of oxygen, CO, temperature, carryover, and flue gas velocities throughout the furnace. (Nitrogen oxides (NOx) modeling is also available, but was not needed for the No. 2 Power Boiler.) The primary purpose of the modeling was to quantify flue gas flow patterns, oxygen and CO levels, turbulence, temperatures, carryover, etc., in order to illustrate predicted combustion characteristics following a combustion system upgrade.

Modeling included operations at the maximum upgrade steam load of 167,000 lb/hr on biomass firing only. Combustion air was delivered through the new grate as UGA, and through the new OFA nozzles, the new fuel distributors, and the existing burners (cooling air only). OFA flow constituted 32% of the total air flow to the boiler and UGA was 50%, similar to other upgraded biomass-fired boilers.

The CFD results demonstrated good performance from the upgraded combustion system. Figure 4 illustrates contours of velocity in a vertical plane through the furnace. As the figure shows, velocities are relatively high in the region of the OFA nozzles, indicating deep penetration of the OFA jets through the furnace cross section. Velocities are lower below the nozzles and through the upper furnace. As a result, entrainment of ash and unburned fuel was predicted to decrease compared to the old combustion system, leading to reduced ash and char carryover out of the furnace.

Figures 5 and 6 show contours of flue gas oxygen and CO, respectively. The OFA nozzles deliver large quantities of air at high momentum above the grate, creating a turbulent mixing zone that rapidly burns volatiles and fine fuel particles rising from the lower furnace. The modeling results provided confidence that the combustion system modifications would meet the project goals for biomass firing rate while reducing CO emissions and ash and char carryover compared to previous performance.

IMPLEMENTATION OF THE COMBUSTION SYSTEM UPGRADE

Following the boiler performance evaluation and CFD modeling, the design, supply, and installation phase for the recommended combustion system upgrades was initiated. This phase of the project included multiple visits to the mill site to collect equipment information, drawings, and field measurements. The overall boiler upgrade included the following items:

Combustion air system:

o Eight multi-range OFA nozzles (four on each sidewall in an interlaced pattern) and supply ductwork.

o UGA system consisting of six UGA zones and supply ductwork.

o Control dampers to regulate the flow to the OFA and UGA systems.

Biomass supply and combustion:

o Three air-swept biomass fuel distributors and chutes.

o Live bottom fuel bin with six screws (two per distributor), plus chutes connecting to the distributors.

o Ambient fuel distributor air fan with single-speed motor and ductwork to supply air to the distributors.

o Vibrating, horizontal, air-cooled grate.

Ash removal system:

o Twin submerged conveyors to collect ash from the grate discharge and from the siftings hoppers under the grate.

o Fly ash conditioner for ash collected in the tubular air heater (TAH) and mechanical dust collector (MDC) hoppers.

Furnace water wall modifications to accommodate the new, horizontal grate and provide new openings for the fuel distributors and OFA nozzles. Pressure part modifications also included new lower headers for the front and rear furnace walls, changes to the lower sidewall headers, and furnace downcomer alterations.

The equipment was installed on the boiler during the summer 2018 outage. A photo of the new OFA nozzle arrangement on the boiler left sidewall is shown in Photo 1, while Photo 2 shows the new biomass fuel distributors installed on the boiler front wall.

OPERATING EXPERIENCE

The upgraded boiler was started up in September 2018 with assistance from start-up engineers. Operator training sessions had been conducted the previous month, and on-the-job training continued during start-up. Start-up activities included adjusting fuel delivery settings to achieve a more uniform fuel distribution on the grate, setting the OFA and UGA systems to promote good fuel burnout with minimal carryover, and working through modifications to the control system to ensure proper functionality. Because load demand for the No. 2 Power Boiler was fairly low during the September start-up, additional boiler tuning was conducted in November 2018 at higher loads.

Table 1 shows results for two post-upgrade operating conditions, one following tuning in November 2018 when the boiler was fired on biomass only, and one in February 2019 when the boiler was fired at a higher steaming rate on biomass plus oil co-firing. Also included are data for the one-year pre-upgrade data period referenced earlier (September 2016 – September 2017). As discussed earlier, pre-upgrade conditions were characterized by low steam generation from biomass due to the grate design and low OFA pressure due to the greater number of OFA nozzles in service with the old air system. Following the upgrade, the boiler’s biomass combustion capacity was substantially increased. Steam generation from biomass during tuning in November 2018 reached 93% of the mill’s goal of 167,000 lb/hr and exceeded the goal during operations in February 2019. Peak performance for the upgraded boiler is reported to be as high as 180,000 lb/hr steam flow on biomass only. Flue gas oxygen levels were less than 6% on a wet basis, compared to over 9% before the upgrade, due largely to reduction of tramp air infiltration through the fuel chutes and better control of UGA and OFA splits.

The upgraded OFA system showed good performance. Prior to the upgrade, measured CO levels varied widely and at times exceeded 3,500 ppm due to weak OFA penetration and poor grate combustion conditions. For the upgraded boiler, following tuning of the combustion air and fuel delivery systems, CO levels ranged from 500 ppm to 1,130 ppm even though the amount of biomass firing was approximately doubled. Although the measurement period was short in both cases, the reduction in CO demonstrated the expected improvements in biomass combustion and emissions performance.

In addition to reduced CO, another measure of improved combustion performance with the upgraded boiler was in fly ash generation. Carryover of unburned fuel and ash was a major issue prior to the upgrade. With the boiler upgrade, the substantial improvement in grate combustion conditions and the greater air jet penetration and mixing from the OFA system resulted in much reduced carryover levels. The November 2018 tuning efforts resulted in a drop in ash collection from the TAH and MDC hoppers from approximately 12 dumpsters per 12-hour shift to three or four dumpsters per shift (each dumpster holds around 3,500 lb of ash). Again, this is particularly impressive considering the large increase in biomass firing following the upgrade.

The boiler’s new ash handling system also improved safety for the operators. Ash removal from the original grate was performed by manually raking the material off the grate with the boiler on-line, a potentially dangerous task. With the upgrade, grate ash removal is performed automatically by the periodic grate vibration and the submerged ash conveyors. The continuous discharge of ash from the grate also increases biomass burning availability compared to the previous disruptions from grate cleaning efforts.

CONCLUSIONS

Upgrade and performance optimization of a biomass-fired boiler requires attention to all aspects of boiler design and operation, including the furnace, air and fuel supply systems, ash collection equipment, control systems, and auxiliary equipment. Collection of reliable and critical boiler operating information, followed by thorough analysis, is required to identify current design and operational issues. The project goals and possible future operating scenarios can then be viewed in light of the findings from the evaluation to determine the necessity of equipment upgrades and modifications.

Furnace size and geometry must be taken into consideration, as a furnace with short height or small cross section can limit increases in steam generation from biomass as a stand-alone fuel or as a replacement for auxiliary fuel. The air delivery system evaluation should cover key aspects of design and performance, including combustion air distribution and penetration, the presence of tramp air infiltration, and fan capacities. The fuel delivery system evaluation should examine whether the existing equipment has adequate fuel delivery capacity and the ability to provide optimal fuel distribution on the grate. This includes the bin, feed screws, distributors, and the grate itself. Through complete evaluation of the air and fuel systems, the shortcomings of all equipment items can be defined and remedies identified.

In optimizing performance of biomass-fired boilers, two areas in which upgrades are often necessary are fuel delivery and OFA supply. Achieving more uniform fuel distribution and combustion conditions on the grate is of paramount importance in biomass combustion. The OFA system should be designed with adequate flow capacity and good jet penetration to generate an intense and turbulent mixing zone above the grate. The application of CFD modeling is a valuable tool in comparing performance of an upgraded air system to the existing system. Once equipment upgrades are defined, the boiler control systems for combustion air and fuel delivery must also be modified to allow optimal performance of the new and existing equipment. In particular, maintaining effective biomass combustion conditions despite changes in load demand and fuel quality is possible when combining modern control systems with effective combustion equipment upgrades.

This process for boiler evaluation and upgrade design contributed to a successful upgrade for the CKPI No. 2 Power Boiler. The upgraded boiler has exceeded the mill’s objectives, and the new air and fuel delivery systems have performed well. A major factor in achieving this success was close communication between all parties during each phase of the project: process evaluation; equipment design, supply and installation; and start-up and tuning.

ACKNOWLEDGEMENTS

We would like to thank Daniel Castonguay, Hal Lagimodiere, Steve Reimer, and Kim Dixon of CKPI, the CKPI boiler operators, Paul Wielgosz of PW Controls & Consulting, ProcessBarron, Can Ecosse, and CIMS for helping to make this project successful.

REFERENCES

1. La Fond, J. F., Keathley, D.C., Lantz, D., and Ritter, L., Combustion of Clarifier Underflow Solids in a Hog Fuel Boiler with a New High Energy Air System, TAPPI Engineering Conference, San Francisco, California, 1994.

2. Walsh, A. R., Ultrasonic Flow Monitoring for Boiler Circulation Analyses, Fall BLRBAC Meeting, Atlanta, Georgia, 1998.

3. Pethe, S. J., Dayton, C. R., Bastianelli, R. R., and Schwartz, L. J., Biomass Replaces Coal at the Hibbard Power Generation Facility, CIBO Fluid Bed and Stoker Fired Boiler Operations and Performance Conference, Louisville, Kentucky, 2013.

4. Pethe, S. J., Britt, M. B., and Morrison, S. A., Carbon Monoxide Emission Improvements from Combustion System Upgrades at the Wheelabrator Portsmouth Refuse Derived Fuel Plant, 20th Annual North American Waste-to-Energy Conference, Portland, Maine, 2012.

FIGURE 1. SIDE VIEW OF THE NO. 2 POWER BOILER (ORIGINAL CONFIGURATION)

FIGURE 2. BOILER SIDE VIEW WITH UPGRADED AIR AND FUEL SYSTEMS

FIGURE 3. ILLUSTRATION OF MULTI-RANGE HIGH ENERGY COMBUSTION AIR NOZZLE

FIGURE 4. PREDICTED FURNACE VELOCITY PROFILES FROM CFD MODEL

Velocity (m/sec)

FIGURE 5. PREDICTED OXYGEN DISTRIBUTION IN THE FURNACE FROM CFD MODEL

Oxygen (vol. fraction)

FIGURE 6. PREDICTED CARBON MONOXIDE DISTRIBUTION IN THE FURNACE FROM CFD MODEL

CO (vol. fraction)

TABLE 1. COMPARISON OF PRE-UPRGRADE AND POST-UPGRADE OPERATING CONDITIONS

Units

Pre-Upgrade One-Year Data Period (Sept 2016 – Sept.

2017) Post-Upgrade

(Tuning 11/28/18) Post-Upgrade

(2/8/19)

Steam and Feedwater

Steam flow 1,000 lb/hr 117 154 204

Steam flow from biomass 1,000 lb/hr 76.3 154 173

Final steam pressure psig 694 664 678

Final steam temperature °F 803 768 789

Feedwater temperature to unit °F 347 348 346

Fuel

Biomass firing rate 1,000 lb/hr 26.6 48.0 56.4

Oil firing rate liter/min 30.4 0 20.2

Combustion Air

UGA flow 1,000 lb/hr 36.5 75.0 86.8

OFA flow 1,000 lb/hr 105 107 126

Burner air flow 1,000 lb/hr 126 38.5 140

TAH outlet air temperature °F 525 496 550

Flue Gas

Generating bank outlet oxygen vol. %, wet 9.6 5.8 5.5

TAH outlet temperature °F 452 425 483

PHOTO 1. NEW OFA NOZZLES ON THE BOILER LEFT SIDEWALL

PHOTO 2. NEW FUEL DISTRIBUTORS ON THE BOILER FRONT WALL

Gateway to the Future

Combustion System Upgrade on CKPI’s Biomass‐Fired BoilerSteve Campbell, Matt Henderson, Marcel BerzJansen Combustion and Boiler Technologies, Inc.

Blair RydbergCanadian Kraft Pulp Ltd.

Presenter: Steve Campbell

1


2

Presentation Outline• Introduction• Project overview• OFA system design approach• Initial engineering evaluation• Upgrade scope• Operating experience• Conclusions


3

Introduction• Location: CKPI, The Pas, Manitoba• No. 2 Power Boiler• 1975 Foster Wheeler• MCR = 275 kpph• Oil or oil + biomass


4

No. 2 Power Boiler(original configuration)


5

Project Overview – Boiler Issues• Suboptimal combustion• Limited steam generation from biomass• High excess air, tramp air• High unburned carbon• Safety concern with grate raking


6

Project Overview – Upgrade Goals• Increase biomass steam capability from around 80 kpph to 165 kpph

• Cut excess air and tramp air• Reduce wear on superheater• Improve fuel burnout• Improve operator safety


7

Engineering Evaluation – Boiler “As Is”• October 2017 site visit• Typical load = 117 kpph• Heat input from oil = 20‐25%• Boiler outlet flue gas O2 = 10% wet• Flue gas CO as much as 3,600 ppm on biomass


8

Engineering Evaluation – Projections• Max. steam from biomass = 165 kpph• Roughly 2x increase in steam from biomass• Max. steam with oil + biomass = 245 kpph• Additional sidewall relievers needed for 275 kpph


9

OFA System Design Approach• Relatively few, large sidewall nozzles• Large nozzle for deep jet penetration• Split nozzle design• Reuse existing fan, ducts


10

Multi‐Range High EnergyCombustion Air Nozzle


11

CFD Modeling• High OFA nozzle velocity• Lower velocities in upper

furnace• Reduced carryover

Velocity (m/sec)


12

CFD Modeling• Intense combustion in lower

furnace• Low CO in upper furnace• Effective CO burnout at OFA

elevation

CO (vol. fraction)


13

Upgrade Scope• Combustion air system• Eight sidewall OFA nozzles• Six UGA zones• Ductwork, control dampers, instrumentation

• Fuel feed• Live bottom bin• Three pneumatic fuel distributors


14

Fuel distributors OFA nozzles


15

Upgrade Scope• Detroit Stoker VCG horizontal vibrating grate• Ash handling• Submerged drag chain conveyor for grate ash• Ash conditioner for TAH/MDC fly ash

• Pressure part modifications to fit new grate• Updated combustion controls


16

Rear wall pressure parts UGA + bottom ash


17

Pre‐Upgrade Post‐Upgrade


18

Operating Experience• September 2018 startup, November 2018 tuning• Biomass steam met 165 kpph goal after tuning• Flue gas O2 from 10% to <6%• Cut tramp air, burner air• Flue gas CO from >3,000 ppm to around 1,000 ppm


19

Operating Experience – Biomass Steam

Pre‐upgrade Post‐upgrade


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Conclusions• All upgrade goals met or exceeded• Grate, combustion air, ash systems performing as designed

• Mill/vendor cooperation was key• Room for improvement: burner air• Mill: upgrade was “second to none”


21

Thank you!

[email protected]

Direct: 425.952.2840Main office: 425.825.0500

[email protected]

204.623.8585

combustion system upgrade on ckpi’s biomass-fired boiler · a biomass day bin with live bottom...

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