PGU_DryScrubberRecycle

8/10/2019 PGU_DryScrubberRecycle

1/8www.powergenu.com

Dry Scrubber Recycle Ash Systems:

Considerations forRecycling Solids


2/82 www.powergenu.com

Educational ObjectivesOn completion of this course, students will:

The design/sizing criteria for a pneumatic recycle ash conveying system wil l be presented along with the actual layoutof the pneumatic conveying system that utilizes conven-tional commercially available blower equipment. Todaysdry scrubber power plant projects that burn Powder RiverBasin (PRB) coal have substantially increased the quantityof solids in the ue gas stream to be collected in the bag-house or pulse jet fabric lter (PJFF).

In an e ort to fully utilize the un-reacted lime and reduceoperating costs, utilities are recycling a portion of the yash removed in the baghouse/PJFF for re-use in the scrub-bing process. As a result, traditional utility-grade y ashpneumatic conveying systems are approaching their vacuumpipe design limits, compromising the traditional high designallowable margins, and sub-dividing the vacuum conveyinglayout into smaller sub-systems and/or combination vacuum/ pressure pneumatic conveying systems. Higher ash genera-tion production/capacities, di cult material characteristicsand lower operating temperatures provide additional designconstraints directly resulting in larger/more complicatedpneumatic conveying systems (i.e. larger equipment, morepneumatic pipes, ttings and valve stations).

At the same time, the pneumatic conveying systemequipment location and pipe routing must be arranged andoften re-arranged during the design calculation period sothat the total equivalent conveying length (TEL) includingtting losses, etc is determined to be less than the avail-

able TEL calculated length up to and including the vacuumexhauster losses. This paper explains the methodologyused for developing system design criteria, presents designconstraints, lessons learned and the nal system /equipment

selection and layout for a recent project involving a 600MW, Ultra Supercritical PRB- red unit with a Spray Dryer Absorber (SDA) and Pulse Jet Fabric

Introduction and BackgroundIn 2006 American Electric Power (AEP) awarded the bal-ance of plant (BOP) engineering and construction to TheShaw Group for a 600 MW ultra supercritical coal redpower plant. One of many systems included in the BOPscope was the complete ash handling system including bothy ash and bottom ash. This paper discusses the speci cproject approach used in developing and specifying a com-plete y ash/recycle ash (pneumatic) conveying system, andshows the actual site arrangement for the nal system.

The BOP engineering started from the Owners (AEP)preliminary design document and pre-purchased equip-ment that included the dry scrubber system and the pulsejet fabric lter (PJFF).

The speci cs of the plant for the y ash/recycle ash sys-tems are as follows: 600 MW ultra supercritical power plant Steam Condition = approximately 4,400,000 lbs/hr Steam temperature/pressure = ap-

proximately 1114 F / 3800 psig Fuel = Powder River Basin (PRB) coal red Burn Rate = 375-TPH (PRB Coal) at ap-

proximately 8400 Btu/lb

Ash content = 3.5% to 7.3% (by weight) Ash split ratio = 80/20: y ash to bottom ash Dry FGD System consisting of two Spray

Dryer Absorber (SDA) Vessels

Dry Scrubber Recycle Ash Systems:Considerations for Recycling Solids

1. Understand the design/sizing criteria for apneumatic recycle ash conveying system.

2. Understand the limits of traditional yash pneumatic conveying systems.

3. Understand the methodology used fordeveloping system design criteria.

4. Be able to identify speci c lessonslearned based on a case study.


3/8www.powergenu.com 3

Lime slurry injected via rotary atomization Flue gas residence time: Approximately 12 seconds

Pulse Jet Fabric Filter (PJFF) Baghouse 10 compartments 3.7:1 Air-to-Cloth Ratio 13,100 total fabric lter bags.

Fly ash characteristics:

Class C 45#/CF High calcium content High a nity for moisture Cementitious under certain conditions

Recycle ash characteristics: 48#/CF (+/-) Contains y ash Contains un-reacted lime Contains calcium sul te (Reacted Lime) 98% Solids

1-2% moisture but generally is less than 1% Contains Halogenated PowderedActi-

vated Carbon, injected into the ue gas streamfor the control of mercury emissions

Much cooler ash compared to non- scrubber plants Approximately 175F exiting the PJFF Very susceptible to plugging in the pipe

Fly ash / recycle ash is collected downstreamof the dry scrubber in the (PJFF)

Initial Studies AEP operates a large eet of pulverized coal- red generatingunits and has considerable experience operating and maintain-ing y ash removal systems. However, the unique demands ofa Dry FGD system with a PJFF and y ash recycle had notbeen encountered. A design criteria document was preparedby AEP based on their y ash removal system experience,including limited criteria for the recycle ash system based ontechnical discussions with vendors, other utilities, etc. Thisdesign criteria document was provided to Shaw to serve as astarting point to develop the nal recycle ash system designand to prepare a system speci cation. Although the designcriteria document required some revisions for speci c PRBrecycle ash handling methods and higher material quantities,the document provided basic system criteria as follows: Vacuum conveying from the PJFF Conveying design capacity of 2-times production 3-days capacity for the waste ash silo 8-hours capacity for the recycle ash silo Truck removal from the waste ash silo De ned the level of redundancy for equipment

Redundancy A key component for the success of the Fly Ash Removal/ Recycle System (FARRS) system is the level of redundancy(LOR) required for the project. The LOR required by

AEP is for 100% operating and 100% sparing capacity. Tomeet this requirement becomes a function of the numberof pneumatic conveying pipes and equipment layout. Fora two pipe vacuum system (one pipe per bank of hoppers)three vacuum producers and three pre-heated air fans arerequired to meet the LOR criteria but the capacity factor isa ected (discussed later).

For a four pipe vacuum system (two pipes per bank ofhoppers) six vacuum producers and six pre-heated air fansare required to meet the LOR criteria, but add unwantedcomplexity, resulting in higher capital costs and additionalmaintenance.

System ConstraintsThe Turk dry scrubber process receives ue gas exiting theair heater at approximately 290 F. The ue gas contains yash, and is combined in the Spray Dryer Absorber (SDA) vessels with the nely atomized lime slurry reagent. Resi-

dence time in the SDA is nearly 12 seconds, as the moisturein the slurry is evaporated, lowering the overall temperatureof the ue gas to approximately 170 oF. The ue gas andsolids ( y ash and FGD waste including un-reacted lime)enter the downstream PJFF where the solids are removedfrom the ue gas stream, captured on the fabric lter bags,and collected in the PJFF hoppers to be removed by theFARRS.

Speci c system constraints encountered for handlingthe recycle ash included developing system layouts thatfall within the vacuum limitations, using commerciallyavailable equipment (staying with the OEM design limits),and dealing with lower recycle ash temperatures, materialmoisture, pipe routing, system complexity (two or four pip-ing con gurations), maintenance considerations and capitalcosts. These constraints have an impact in the nal equip-ment selection and in developing the system design andarrangement as follows:1. For this project, several recycle ash mass ow

quantities were provided by the boiler and SDA/ PJFF supplier. They ranged from high lime usageto high fuel su lfur content. The case studies rangedfrom a low (recycle ash) quantity of 56,312 lbs/Hrto a high (recycle ash) quantity of 191,177 lbs/Hr.

In designing the optimum recycle ash removal sys-tem for this project; the highest ow rate value (191,177lbs/Hr) was used for equipment sizing, pipe sizing/ routing and to identify how to convey the ash from theoutlet of the PJFF hoppers depending on the number ofconveying pipes needed.

2. The PJFF hopper arrangement set the outlet ange ofthe hopper at just 4-feet above the top of concrete.

This distance was already pre-determined and givento Shaw (BOP Engineer) to begin our design. Thisdistance does not allow adequate vertical space for apressure pneumatic conveying system consisting of


4/8

Figure 1 Hopper Flange Clearance

Figure 2 Traditional Vacuum System

Figure 3 Pre-Heated Pneumatic Air

4 www.powergenu.com

airlock chambers, isolation vales, etc. and therefore apressure pneumatic conveying system was eliminatedfrom consideration allowing for only a vacuum pressurepneumatic conveying system to remove the recycle ashas also de ned in the AEPs design criteria document.

3. High calcium recycle ash, with its sticky and/or cementi-tious ash properties, an a nity for moisture, and

lower overall process temperatures provide signi cantchallenges to designing a pneumatic conveying system.

4. It was AEPs intent in the design criteria to engineerthe system as a traditional vacuum conveying system,consisting of a vacuum/material inlet valve locatedat the outlet of the hopper ange, with sweep air orpick-up air entering the conveying pipe through aspring disc air inlet tting located at the starting endof the pneumatic pipe. Material from the hoppersis conveyed (by vacuum) to the top of a storage silo where the ash is separated from the convey ing air

through a lter separator (also located on top of thesilo) and the air pipe is routed from theseparatorto a vacuum (mechanical) exhauster located at grade near the base of the waste ash storage silo.Understanding how recycle ash di ers from convention-

al y ash, coupled with the capacity requirements and theincreased solids contributed by the Dry FGD system, Shawdetermined that the constraints mentioned above were tooprohibitive of a traditional vacuum conveying system, and worked with AEP to optimize the system design to over-come these constraints.

First, the total equivalent conveying length (TEL) fora vacuum system is limited to approximately 600-700 feetincluding the pipe and ttings. Through numerous piperouting arrangements and calculations, Shaw optimizedthe piping and equipment location to be within the allow-able TEL to a de ned transfer point, at which it was deter-mined that the system would switch over to pressurizedash transfer. Next, to prevent pipe plugging issues in the vacuum conveying lines downstream of the PJFF, heatersand pressurized fans added to the design to continuouslyheat the pneumatic conveying air lines (with and without

material in the pipe) to temperatures ranging from 275Fto 350F, to promote the ow of ash. The pre-heated airsystem was located upstream and adjacent to the PJFF. Additionally, large radius sweeping elbows and/or rubberelbows (designed to handle the expected transport airtemperatures), larger pipes to handle increased air mass,insulated conveying pipes and insulated lter separators

were incorporated. As the design evolved to mitigate the constraints men-

tioned above, more detailed engineering of the systembegan, and additional design constraints related more di-rectly to the operation, performance, and maintenance ofthe system and its integration into the overall plant wereencountered.

Additional Design Constraints1. The waste ash silo storage criteria required 3-days

capacity and truck loadout capabilities. Thetrucking operation alone prevented the waste ashsilo from being located close to the recycle ashsilo further complicating the nal pipe routing.

2. The recycle ash silo is located at the opposite endof the PJFF, is approximately 135 ft tall, and isonly 24 ft in diameter. The height of the siloadds to the overall conveying length, and the


5/8

Figure 4 Partial Site Plan Figure 5 2-Pipe Vacuum System

Figure 6 4-Pipe Vacuum System

www.powergenu.com 5

diameter limits the size of equipment that canbe housed and easily accessed on the silo roof

3. Determining how many of the ten (10) PJFF hoppers wil l discharge into a single vacuum convey ing pipeis critical for laying out a pneumatic conveyingsystem so that the material to air ratio is satis edbased on the available (vacuum) pressure. Note; the goal is for fewer conveying pipes rather than morepipes to reduce the number of pre-heated air fans,air heaters, cross-over valve stations, lter separatorsand vacuum (mechanical exhausters) blowers. Thedesired number of conveying lines was two.

4. Equipment redundancy is necessary, but fewerconveying pipes will reduce the actual number of sparefans, heaters, blowers and exhausters to minimizesystem complexity and overall system costs.

5. Conveying design rate vs. actual production rate hastraditionally followed an accepted guideline of 2-timesas was required by the original design basis document. Although the 2-times criteria has traditionally beenused/speci ed to provide for catch-up periods whensome of the PJFF hoppers are removed from operation/ service or maintenance, Shaw determined a 2-timesdesign rate was not possible with only two conveyingpipes beneath the PJFF because the size of the pipe to

meet the lb material to lb air ratio (based on pneumaticpiping calculations) exceeded commercially availableabrasion resistant pipe sizes. Example: (191,700 lbs/Hr) (2 production rate) = 96-TPH 2 conveying pipes pipe

Project Specic Design ApproachOvercoming the additional design constraints proved tobe a challenge. Several piping arrangements were devel-oped and numerous pneumatic piping calculations wereperformed using published pneumatic equations for the2-times production ratio. The conclusion was that the 96-TPH pneumatic conveying rate discussed earlier was notpractical or possible given the FARRS OEM pipe size limi-tations and mechanical exhauster size limitations, withoutadding two (2) additional pneumatic pipelines beneath thePJFF hoppers, two (2) additional lter separators, and 3to 4 additional mechanical exhausters (See Figure 6). Theadded complexity, equipment cost, and future maintenanceconsiderations to meet the 96 TPH rate was determined tobe impractical. It was determined that by using commer-cially available abrasion resistant pipe and the largest rotarylobe blowers, and re-locating the lter/separators to grade,thereby reducing the TEL of the vacuum transport lines,the system could achieve a design ratio of 1.7 times the pro-duction rate (81-TPH). This lower ratio was acceptable to AEP while providing su cient margin to catch up produc-tion if one of the PJFF hoppers were down for service. Thepublished pneumatic equations used for this project are asfollows:

The TEL calculations coupled with the limited area onthe recycle ash silo roof, and AEPs requirement for suf-cient make-up capacity meant that a traditional vacuum-only system simply would not work. A combination vacuum/ pressure pneumatic systemwould be required, allowingthe pressurized portion of the system to convey the ash toeither the recycle ash silo or the waste ash silo due to the

long conveying distances and silo heights. By introducinga pressurized pneumatic conveying system, the complexityof the system increased slightly because of the additionalpressure blowers, silencers, lters etc. However, the pressur-


6/8

Published Pneumatic Equations Figure 7 Final Site Plan

6 www.powergenu.com

ized portion also o ered some bene t to the system sincematerial can be conveyed greater distances with pressurizedtransport, and heat of compression from the pressure blow-ers heats the transport lines to promote the ow of ash andreduce the need for additional heaters, heat tracing, etc.

A description for a vacuum / pressure pneumatic conveying system is as fol lows: A traditional vacuum / pressurepneumatic conveying system has all of the same basic com-ponents as a traditional vacuum-only system including vac-uum/material in let valve located at the outlet of the hopperange, preheated sweep air or pick-up air at the entrance ofthe vacuum pipe except that the lter separators are locatedat grade and designed for pressure piping downstream.

The next challenge was to locate the vacuum / pressure lterseparator station (one lter separator is required per vacuumpipe) very close to the PJFF because the physical distance ofthe pipe combined with the equivalent system losses (equivalentpipe distance) had to be under 600-700 feet. A feasible locationthat met this criteria for the grade-mounted lter separators was adjacent to the PJFF, and the Filter/Separator station waslocated so that the ID fans and ductwork maintenance areasestablished for the plant were kept clear and un-obscured.

Lastly, the lter separators had no level of redundancy.To ensure that the plant would continue to operate in theevent one of the lter separators had to be removed fromnormal operation, a vacuum pneumatic cross-over pipe ar-rangement was provided at the PJFF.

Summary and ConclusionsThe nal arrangement for this project is two vacuum pneu-matic pipes (individual piping systems); one pipe per bank

of ve PJFF hoppers. Pipe sizes and selected blowers stayed within the design limits of commercially available pipe andblowers. The nal arrangement also reduced the quantity ofauxiliary equipment (i.e. heaters, fans, pipes, lter separa-tors and vacuum exhausters) to one operating componentand one spare per system to meet the redundancy require-ments of the design criteria document. The knowledge gained by AEP, Shaw, and the ash handling system OEM asa part of this design evolution, provided tremendous insightin an area that has not been widely addressed in the industryand was not included in AEPs eet operating experience.The original design criteria document has been revised by AEP and now contains speci c criteria for PRB- red unitsutilizing dry FGD and PJFF equipment. Some of the majordesign criteria changes that resulted from this project areas follows: A complete pressure, or combination vacuum/

pressure pneumatic conveying system utilizingpressure to convey the recycle ash to the top ofboth the waste ash silo and recycle silo

1.7-times capacity factor Pre-heated conveying air (vacuum transport lines)

Rotary lobe vacuum exhaust-ers (for combination systems) Rotary lobe pressure blowers Cross-over piping stations to allow for maintenance


7/8

Figure 8 Final Flow Diagram

www.powergenu.com 7

Insulated conveying pipe from the PJFFto the recycle and waste ash silos.

Insulated lter-separators All of the above considerations (and even more when

needed depending on actual site conditions) are key to thesuccess of a pneumatic handling recycle ash solids convey-ing system for a dry scrubber power plant burning PRB coal.

The nal ow diagram for the recycle ash solid conveyingsystem and complete material balance is presented on thenext sheet.

References1. Chemical Industries A series of Reference Books and Text

Books Volume 13 Pneumatic and Hydraulic Conveying ofSolids, O. A. Williams (1983).

2. Ash Handl ing and Disposal Seminar, University of WisconsinExtension Center, October 28-29, 1985.

3. Gardner-Denver, Cyclo-Blower Selection Data for Vacuumand Pressure Blowers.

Acknowledgements:This course is based on the presentation Dry Scrubber Recycle

Ash Systems: Considerations for Recycle Solids by Dan Jennings, Material Handl ing Manager Technical Support,Shaw Power Group; and Matt Usher PE, MechanicalEngineer-New Generation Design & Engineering, AmericanElectric Power, at POWER-GEN International 2008. Thepresenters acknowledged the technical contributions made byUnited Conveyor Corp. to the approach method sections ofthis paper.


8/88 www.powergenu.com

Questions

Online CompletionUse this page to review the questions and choose your answers. Return to www.powergenu.com and sign in. If you have not previously purchased the programselect it from the Online Courses listing and complete the online purchase. Once purchased the exam will be added to your User History page where a TakeExam link will be provided. Click on the Take Exam link, complete all the program questions and submit your answers. An immediate grade report will beprovided and upon receiving a passing grade (70%) your Certicate of Completion will be provided immediately for viewing and/or printing. Certicates ofCompletion can be viewed and/or printed anytime in the future by returning to www.powergenu.com, sign in and return to your User History Page.

1. What type of coal is

combusted in this unit?a. Low sulfur eastern bituminousb. High sulfur Illinois #6c. Low sulfur PRB

2. What is the range of percentageash, by weight, in this fuel?

a. 3.5% to 7.3%b. 4.5% to 8.3%c. 5.5% to 9.3%

3. What is the approximate residencetime of the ue gas in the scrubber?

a. 60 secondsb. 30 secondsc. 12 secondsd. 2 seconds

4. Fly ash and scrubber particulatesare removed by a pulse-jet fabric

lter (PJFF). PJFFs consist of sev-eral compartments in which the gasows through many frame-support-ed bags that are regularly pulsed with air to dislodge ash that fallsto hoppers below. How many bagsare utilized for this 600 MW unit?

a. 7,500b. 13,100c. 15,000

5. One of the de ning criteria for afabric lter device is the air-to-cloth ratio. This is amount oflter area provided for the designue gas ow, and is measuredin units of acfm (actual cubicfeet per minute) of air ow persquare foot of cloth. What is the

air-to-cloth ratio of this PJFF?a. 3.7 to 1b. 4.2 to 1

c. 5.7 to 1

6. The list of y ash properties men-

tions that the ash can be cementi-tious under certain conditions.This is quite true, and many boilersthat have been switched frombituminous coals to PRB have en-countered concrete-like ash forma-tion in backpass locations. The listalso includes the chemical elementthat can in uence the cement-likenature of the deposits. What is it?

a. Calciumb. Magnesiumc. Aluminum

7. The paper also includes a listof recycle ash characteristics.What chemical is in the ash thatprovides the incentive to reusethe material in the scrubber?

a. Aluminum oxideb. Anhydrous ammonia c. Un-reacted lime

8. What is injected into the ue gasstream for mercury removal?

a. Non-halogenated activated carbonb. Halogenated activated carbonc. Limestoned. None of the above

9. What type of transport system was util ized for moving the ashfrom the PJFF hoppers to therecycle and waste silos?

a. Water sluicing b. Pneumaticc. Belt conveyor

10. To what temperature is the gas cooled in the scrubber?

a. 140 Fb. 150 Fc. 160 F

d. 170 F

11. The recycle system size was

based upon what criteria?a. The maximum calculated ow rateb. The minimum calculated ow ratec. The average calculated ow rate

12. The design of the conveying linefrom the PJFF hoppers to thesilos includes heaters to raisethe temperature such that pipeplugging of the calcium-bearing,

hygroscopic material. What isthe range to which these heatersraise the air temperature in thepneumatic conveying system?

a. 250 to 275 Fb. 275 to 300 Fc. 275 to 350 F

13. How much storage capacitydoes the waste ash silo have?

a. One day b. Two daysc. Three daysd. Four days

14. Economic evaluations dictated thata two-pipe pneumatic conveyingsystem from the PJFF hoppers tothe silos was best. However, pipe

sizing for a 2-times productionratio proved to be impractical.What ratio was selected?

a. 1.7 to 1b. 1.5 to 1c. 1.3 to 1

15. What was the maximum distancecalculated to be allowable forthe ash conveyor run from

the PJFF to the silos?a. 700 feetb. 1000 feetc. 1500 feet

PGU_DryScrubberRecycle

Documents

Transcript of PGU_DryScrubberRecycle