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April 2006, Vol. 24 (1)

(a) Sketch of pressurized vertical spindle mill. (b) Flow patterns in upper half ofmill with four outlet pipes.

(a) (b)

BALANCING COALFLOW DISTRIBUTION FROM PULVERIZERS

Being able to achieve balancedcoal and air flows to individual burnersin a pulverized coal boiler is importantfor obtaining efficient combustion andlow emissions. Up to now, control ofthe pipe-to-pipe distribution of coalhas been difficult to achieve inpulverizers having multiple coal pipesconnected directly to the dischargeturret at the top of the pulverizer.However, research carried out by theEnergy Research Center has resultedin a new technology for balancingcoal flow among the outlet pipes ofpulverizers of this type.

Supported by government andindustry, the research involvedevaluation of possible coal flowb a l a n c i n g a p p r o a c h e s u s i n gcomputer simulations, followed bytesting of the most promisingapproaches in a laboratory-scalepulverizer model. The pulverizerteam, consisted of Harun Bilirgen,Aly Elshabasy and Edward Levy andwas led by Dr Bilirgen.

Bilirgen explains, “From ourprevious research on control of coalflow distributions from two and three-way splitter junctions in coal pipesystems, we knew that there can bes ubstantial benefits from achievingbalanced coal pipe flows. Coal flowbalancing tests which we performedat one unit showed that coal flowmaldistributions resulted in higher flyash unburned carbon and unstablecombustion with large excursions inc a r b o n m o n o x i d e(CO) emissions. Eliminating the coalflow imbalances in that boiler resultedin higher boiler efficiency, improved

heat rate, lower NOx and COemissions, and reduced ammoniainjection rate for the SCR system.

Based on that successfuloutcome, we initiated a researchproject on coal flow balancing frompressurized vertical spindle mills.With funding from DOE, the Babcockand Wilcox Company and theP e n n s y l v a n i a I n f r a s t r u c t u r eTechnology Alliance, we performedcomputer simulations of the flowpatterns in the top half of a pulverizerwith four outlet pipes . The computerresults revealed the regions of h ighand low coal concentration. Thisinformation was used to devel o p anew approach to coal flow controlwhich uses flow control elementsstrategically posit ioned in the

pulverizer to change the distribution ofcoal flow in the outlet pipes.

We then performed lab scaletests in a one-seventh scale modelpulverizer. These tests provided dataon the effectiveness of flow controlelements located at various places inthe pulverizer in adjusting the coalflow distribution.”

Elshabasy continues, “Thelaboratory tests were carried out inthe Center’s Coal Flow Laboratory,using a scale model of a pressurizedvertical spindle mill. I modified themill internals by adding flow controlelements at strategic locations.These flow control elements wereeasily adjusted from outside themodel while coal was flowing throughthe system. Each of the four outlet

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Laboratory model (in right side of picture) of top half of a pulverizer withfour outlet pipes connected to cyclones for particle capture.

pipes connected to the top of thepulverizer model was connected to acyclone which separated the coalfrom the conveying air. By accuratelymeasuring the weight of coal whichaccumulated in containers at thebottoms of the cyclones, I was ableto determine the coal flow rate ineach coal pipe. With this approach,I was able to adjust a flow controlelement and immediately determ inethe effects on the coal flow rates inthe four outlet pipes.

The original pipe-to-pipe coal flowimbalance was 16 percent. Mylaboratory test program showed thatby adjusting the coal flow controlelements, it was possible to reducethe imbalance to +/- 2 percent. Thetes ts showed there was nomeasurable effect of coal flowbalancing on the outlet distribution ofthe primary air flows. This is quiteimportant because being able toindependently control coal andprimary air flows greatly simplifies theproces s of balancing air to fuel ratio.

A third finding from my tests isthat the coal flow distribution wasinsensitive to the coal feed ratethrough the mill. This indicates thecoal flow balance will not change asmill coal loading varies.

Finally, the coal flow controlmechanisms were designed toproduce negligible pressure drop. Mymeasurements showed the pressuredrop through the mill increased byless than one inch H2O due to the

presence of the f low controlelements.”

Bilirgen adds, “We feel we have atechnique for coal flow control whichwill achieve very good results inpressurized vertical spindle mills.The next step in this process is todemonstrate the technology in a fullsize mill at a power plant. We havehad discussions with Babcock andWilcox and a utility company about afield demonstration of the technologyand we expect to be able to performthe field tests in the 2006-2007 timeframe.

The commercial availability ofcoal flow balancing technology willcreate interesting opportunities toimprove combustion. Used incombination with flame sensortechnology or on-line coal f lowsensors, it will be possible to achieveon-line closed loop control of burners t o i c h i o m e t r y , w i t h r e s u l t i n gimprovemen ts i n combus t i one f f i c iency and reduc t ions inemissions.”

-12

-8

-4

0

4

8

12

16

20

Trial Number

Co

al Im

bal

ance

( %

)

Pipe 1

Pipe 3

Pipe 4

Pipe 2

0 1 2 3 4 5 6

Trial 0 – Without Flow Control Elements (FCE)Trials 1 to 6 – FCE's Adjusted to Various Settings

Coal flow imbalance during balancing process: initial imbalance was +16percent. After installation of FCE’s, coal flow balance to within +4 percent wasachieved on the third trial. After upset of balance (Trial 4), balance to within+2 percent was achieved within two additional trials.

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RESEARCHERS’ PROFILES

• Dr. Nenad Sarunac has a Ph.D. in Mechanical Engineering and is anAssociate Director of the Energy Research Center. His research focuseson power plant heat rate improvement, emissions control and processoptimization.

• Dr. Harun Bilirgen has a Ph.D. in Mechanical Engineering and is aResearch Scientist in the Energy Research Center. One of his researchspecialities is the fluid mechanics of gas-particle flow systems.

• Dr. Carlos Romero is an Associate Director of the Energy ResearchCenter with a Ph.D. in Mechanical Engineering. He is a specialist incombustion kinetics and emissions control.

• John Sale has BS and MS degrees in Mechanical Engineering. He isDirector of the Center’s Energy Liaison Program.

• Aly Elshabasy has BS and MS degrees in Mechanical Engineering. Heis expected to receive his Ph.D. degree in Summer, 2006 and hasfocused his research on control of coal flow in pulverizers.

• Dr. Xiaodong Bian has a Ph.D. degree from Lehigh University. HisPh.D. research dealt with application of artificial intelligence and adaptivemodeling techniques to optimization of power plant processes.

• Zhang Wei is pursuing a Ph.D. degree in Mechanical Engineering. HisPh.D. research focuses on computer applications and modeling of