coal feed in thermal power

8/2/2019 Coal Feed in Thermal Power

1/16

OPTIMIZING COAL FEED IN A BRAZILIAN THERMALPOWER PLANT: A CASE STUDY

C. P. Bergmann, S. R. Braganc a, M. C. da Silva,J. J. da Rosa, J. Rubio

QUERY SHEET

Q1 Au: Affiliation line correct?

Q2 Au: Spell out @ 1st mentionQ3 Au: ok?Q4 Au: define @ 1st mentionQ5 Au: list all authors


2/16

Optimizing Coal Feed in a BrazilianThermal Power Plant: A Case Study

C. P. Bergmann, S. R. Braganc a, M. C. da Silva,5J. J. da Rosa, and J. Rubio

LACERLaborato rio de Materiais Cera micos andLTMLaborato rio de Tecnologia Mineral e

AmbientalDepartamento de Engenharia de Minas PPGEM, Universidade Federal do Rio Grande do Sul,

10Porto Alegre, RS, BrazilQ1

The rapid increase in thermal power generating capacity in Brazil hasresulted in the production of huge quantities of brown coal of high ashcontent. Current estimates indicate that coal ash production in Brazil islikely to reach 4 million tons per year by 2005. In view of the need to

15burn this type of fuel to the maximum possible extent, a rational use of coal is vitally important. However, the coal from Candiota (a coal minein southern Brazil) contains calcium-bearing compounds that damageequipment, causing process stoppages; this coal may become cost inef- fective unless the noncombustible material and its content is controlled.

20This article reports on the results of the comminution characteristics of coal combustion feed and of industrial tests, which demonstrate theurgent need for optimizing particle size distribution to preclude the presence of calcareous material. Hard calcium-based minerals display acrushing behavior that differs from coal particles, requiring their

25screening and elimination from the feed. Examples of the overall situa-tion, screening optimization, and the beneficiation process at theCandiota power plant are discussed in terms of fragmentation, particlesize analysis, type of screen, and economic benefits.

3b2 Version Number : 7.51c/W (Jun 11 2001)File path : p:/Santype/Journals/Taylor&Francis/Gcop/24(1-2)/24(1-2)-19194/Gcop24(1-2)-19194.3dDate and Time : 17/5/04 and 13:15

Received 5 January 2004; accepted 8 March 2004.Special thanks to CGTEE, a Federal Brazilian Energy Company, for the financial

support and for permitting this publication.

Address correspondence to J. Rubio, Universidade Federal do Rio Grande do Sul,Av. Osvaldo Aranha 99 CEP: 90035-190, Porto Alegre, RS, Brazil. E-mail: [email protected]

Coal Preparation , 24: 115, 2004Copyright # Taylor & Francis Inc.ISSN: 0734-9343 print DOI: 10.1080/07349340490465561

1


3/16

Keywords Selective crushing; Coal beneciation; Screening; Thermal30power plant

INTRODUCTION

The coal reserves of Candiota, a surface mine, are approximately onebillion tons, with an overburden of up to 50 m. The mining conditions areexcellent, ensuring the products low cost. Since no high-cost bene-

35ficiation steps such as washing are required, the coal goes directly to thecrushing =grinding system. The raw coal has a calorific value of 13.395 MJ =kg and 53% ash content in a dry state. The main consumer of Candiotas coal is the thermoelectric power plant Usina Termoele tricaPresidente Me dici, located only 5 km from the mine, allowing the coal to

40be transported to the power plant by conveyor belt. The power plant,with an installed capacity of 446 MW and a demand of 1.5 to 2.9 milliontons =year, is currently operated by the Companhia de Gerac a o Te rmicade Energia Ele trica (CGTEE), a federally owned company.

Despite the high ash content (about 50%) in Brazilian coal, no45extensive beneficiation is practiced; moreover, much of the coal is used as

ROM inQ2 thermoelectric power plants. The main reason for this is that theinorganic compounds are widely distributed throughout the coal. Thepresence of noncombustible compounds causes many operationalproblems during grinding, classification, and combustion. Candiotas

50coal (in southern Brazil) contains calcium-bearing compounds thatdamage equipment, causing process stoppages. Therefore, unless thesecompounds are controlled, the use of this coal may become economicallytoo expensive.

This article summarizes the existing alternatives to diminish the overall55problem, focusing on the significance of the presence of calcium-bearing

compounds and their selective removal. Our initial step in this work wasto assess the problems in order to evaluate the validity of the applicationof possible solutions. Possible ways of separating the inorganic impuritiesfrom the coal, which involved studying the preparation cycle in terms of

60granulometry, equipment efficiency, ideal cutting diameter to separateimpurities, points in the cycle for this separation, etc., were examined anddiscussed.

The main extrinsic impurities deriving from coal extraction arecalcareous rock and argillite. Owing to its nature, the extrinsic material

65appears in the form of incrustations and concretions, filling cracks andfissures in the coal layers. Technical surveys indicate that the removal of these impurities during mining is unviable.

In the presence of moisture, argillite can form a highly plastic paste,impairing the efficiency of crushers and mills. The same problem may

2 C. P. Bergmann et al.


4/16

70occur on the dosage tables that feed the mills, sometimes requiringequipment maintenance stoppages.

Low cleavage calcareous rock that is harder than coal sometimes

displays a larger particle size than that of coal. The presence of this rockleads to a series of problems. Owing to its greater hardness, the rock75causes substantial wear of crushers and mills and is therefore directly

responsible for increasing production costs. Large rocks (which areoccasionally present) may lock the chain of the dosage table, causinglinks to break and the table to rise, making it necessary to stop the millingprocess. Moreover, the periodic accumulation of stones in the mills

80reduces their efficiency.The second section of this article analyzes the results obtained from a

pilot test carried out in the coal preparation cycle of the President Me dici

thermoelectric power plant in Candiota, state of Rio Grande do Sul,Brazil with the purpose of separating the impurities contained in coal

85from the mine runoff. Samples from the pilot tests were collected andcharacterized in terms of particle size distribution, calcareous rockcontent, ash content, total sulfur, and superior calorific power.

TECHNOLOGICAL ALTERNATIVES FOR OPTIMIZINGOF CLEANING CANDIOTA COAL

90The alternatives were initially analyzed to decide whether or not theconcentration of calcium-bearing compounds was beneficial, andsecondly whether these compounds should be removed or ground.Decisions regarding their removal are determined based on an analysis of the potential advantages of desulfurizing the gases emitted during com-

95bustion versus the disadvantages in relation to the coal preparation cycleand heat loss in the combustion stage. A bibliographic research [15] wasmade to analyze the experimental and theoretical proposals of severalauthors. This research indicated that the greatest problem when injecting

calcareous rock together with coal is the passage of the calcareous 100material through the combustion flame zone (high temperatures of approximately 1400 C).

According to the literature, the relation between temperature anddesulfurization is as follows:

The optimal temperature of the sulfatization reaction is 800900 C.105Excessively high temperatures lead to calcination, which transforms

the calcareous material (calcination to death), resulting in the lossof reactivity.Temperatures of over 1000 C cause a high reaction rate, and theproduct of the reaction (sulfate) with the largest molar volume blocks

110the particle surface, which prevents the progress of the reaction.

Optimizing Coal Feed in a Brazilian Thermal Power Plant 3


5/16

In addition to this discontinuation in the feed, one must also consider thelow amount of calcareous material that is fed. Values reported in theliterature demonstrate desulfurization efficiency of 60% for a molar ratio

of 2 to 3 for Ca =S (ratio of calcium in calcareous matter versus sulfur in 115coal). In the Candiota coal, a Ca =S close to 0.42 is usually sent to theboiler: therefore, this maximum amount of limestone present in coal isalways much lower than required for a significant desulphurization.

Under the power plants combustion conditions, the calcareousmaterial showed low desulfurization efficiency, leading to the conclusion,

120in terms of sulfurous oxide removal, that any benefit obtained byinjecting the calcareous material with the coal would be minimal.

It is also worth pointing out that the presence of calcareous materialin the coal feed is not constant because this material is present only in

one layer of the mine. In other words, possible beneficiation by125desulfurization would be a discontinuous and low efficiency process.

The alternative of separating the calcareous material offers theadvantage of increasing the coals calorific value and reducing opera-tional problems. However, this separation must be selective so that itdoes not imply significant loss of combustible matter. Therefore, the

130choice of the best way to implement a removal system should be based onan in-depth study of the various possible options.

The options examined for separation of the calcareous materialinclude:

removal of calcareous rock in the coal extraction stage via selective135mining,

alteration of the mines crushing system, andintroduction of a classification system by size (screening).

The option to remove the calcareous rock as the coal was extracted wasadopted before 1989. With the change in mining region, calcareous rock

140began to emerge with such a level of impurities that it became very mixedwith coal during the extraction, making it impossible to separate themduring the process (a fact attested to by the mines technicians). Todaythere are no technical conditions, such as appropriate equipment, thatwould allow for this operation; moreover, this would lead to problems in

145the breaking up (by explosives) of the mineral rock.An alteration in the current crushing system requires a change in the

current layout, which is shown in Figure 1.Altering the layout would require changing the secondary crusher for

another type of crusher to allow separation of the coal from the calcar-150eous material by screening. In that case, the double roll crusher could be

shifted to the third crushing stage since, if the calcareous material wereremoved beforehand, the double roll would be a very attractive coalcrushing =milling system.



6/16

Because it would require major modifications of the current layout,155this alternative would mean a prolonged stoppage of the entire coal

preparation cycle.The introduction of a system of classification by size through the

installation of a vibratory screen depends on an analysis of the particlesize distribution curves of the coal and calcareous rock to determine the

160screens classification mesh. This information can also serve as the basisfor defining the installation site. The basis of the installation of thisseparationclassification system is to exploit significant size differencesthat allow the removal of calcareous matter accompanied by a minimumof coal.

165The characteristics of coal and calcareous rock differ considerably interms of natural cleavage lines, yielding significant differences inresistance to impact or shearing stress. Hence, in the case of double rollcrushers, breaking by compression should not, from the granulometricstandpoint, provide good differentiation between coal and calcareous

170rock, since the crushing force produced by this type of crusher is muchgreater than the compressive strength of these two materials. In otherwords, either the material is crushed or it does not pass through therollers.

However, particle size reduction in hammer crushers occurs by impact.175Thus, in this case, the absence of cleavage, greater hardness, and higher

impact strength of calcareous rock enables its granule size to remainmuch larger than that of coal, requiring simply the proper adjustment of

FIGURE 1. Crushing system at Candiota mine, 2 km from the power plant.



7/16

the hammer crusher (as well as the determination of the internal mesh).Figure 2 illustrates the difference between the shearing =impact strength

180of coal and calcareous rock.In order to determine the size distribution difference between the

materials after crushing, samples were collected at several points of thecycle, and the distribution was analyzed by simple screening.

The analysis of the size distribution revealed the following advantages185obtained by installing the screen after the hammer crushers (HCs)

more differentiated particle size distribution curves,less loss of coarse coal, andless quantity of material separated and lower transportation andplacement costs.

190EXPERIMENTAL

Pilot Study

A pilot test was found to be necessary because of the large amount of material processed at the power plantapproximately 400 t =h, whichmeant that results on a laboratory scale would not ensure reliable data.

195Minor design errors could lead to significant deviations on a real scale,which would mean a large amount of material would be wasted.

Based on an analysis of the results obtained for the choice of the bestmethod to remove the inorganic matter in the coal, the pilot test wasstarted in the plants coal preparation cycle, aiming to separate the

200impurities contained in the runoff mine coal. A screen (32 mm mesh) was

FIGURE 2. Fragmentation differences between coal and calcareous rock.



8/16

installed at the exit of HC-1, and the design of the screen, such as cuttingdiameter and installation site, was defined based on the aforementionedresults.

The duration of the test was and 150 min,Q3 during which period 120.1 t 205of ROM coal were processed. Of this total, 52.1 t (43.3%) were retainedby the bypass of the HC-1, while 0.9 t (0.7%) were retained in the screenand 67.1 t (55.9%) passed through it.

Samples representative of all the fractions (coal impurities) werecollected and characterized by particle size distribution, calcareous rock

210content, ash content, total sulfur content, and upper calorific value.

Methods

A suspended vibratory screen (Briterpa) belonging to CompanhiaRiograndense de Minerac a o, (CRM) was used in the test. The infor-mation collected during the project indicated that the best site for the

215temporary installation of this screen was right after the exit of the HCs.Figure 3 shows a flowchart of this installation.

An independent structure was set up to install the screen. The feedflow was bypassed to the screen by means of a conveyor belt startingfrom the exit channel of HC-1, as shown in Figure 4. The fluxes of the

FIGURE 3. Flow chart of the installation of a screen at the HC-1 exit.



9/16

220passing material (coal) and retained material (reject) were collected,removed by truck (Figure 5), and weighed. The material for sampling wasplaced in the conveyor belt feeder, and representative samples wereremoved with the help of a template.

A total of four samples, one from each flow, were taken at the end of 225the test, grouped together, and taken to the laboratory for particle size

analysis. The samples were subjected to a classification test through astandard series of screens, and the fractions were stored and identifiedseparately in plastic bags, after which they were sent for quantitativestudies. These fractions were weighed and the calcareous rock and other

230impurities sorted manually (feasible only for particle sizes larger than6.3 mm). After sorting, the samples were again weighed to determine howmuch calcareous material and impurities they contained.

RESULTS AND DISCUSSION

Figures 6 and 7 show that the bypass of the HC-1 removed a large235amount of fine material (99.7% < 25 mm) that did not need to pass

through the crusher, thus preventing blockage and loss of crushingcapacity. The curve of the material removed in the bypass was similar tothe material that exits the HC-1 (Screen Feeder). It was also found thatthe coal underwent a considerable reduction of grain size in the

FIGURE 4. Bypass of the HC-1 exit flow.



10/16

FIGURE 5. Trucks collecting material retained and material passing throughscreen.

FIGURE 6. Particle size distribution of the ROM coal of different fractions.



11/16

240HC-1, passing from a material with 56.7% < 25 mm to one with99.1% < 25 mm. These results demonstrate the importance of optimizingthis bypass, which will allow for the improved performance of the crusherand of the impurity separation system to be installed.

Figures 8 and 9 show a significant reduction of the particle size of 245calcareous rock and impurities (calcareous rock silt pyrite) after

exiting from the HC-1, passing from 17% < 25 mm to 7579% < 25 mm.This reduction in size is undesirable, leading to the conclusion thatcomplementary studies are needed for the operation of the HCs. The aimof these studies was to optimize mainly the mesh size and shape in order

250to lessen the reduction of impurity sizes, maintaining the coals sizeanalysis.

Figures 10 and 11 compare the curves of the coal and those of thecalcareous rock and impurities, before and after passing through the HC-1.Once again it is found that separating the calcareous rock is viable only

255after the HC crusher. If the screen 32 mm mesh were placed before theremoval, not only would 70.5% of the calcareous rock be removed butalso 32.8% of the coal. The screen placed after the HC would remove11.9% of calcareous material and 0.2% of coal. These values would bevalid for a 100% separation efficiency, which is not the case in practice,

260since there is always a percentage of removal of particles smaller than theseparating diameter size.

FIGURE 7. Particle size distribution of the coal (ROMimpurities) for thedifferent fractions.



12/16

FIGURE 8. Particle size distribution curves of the calcareous rock in the variousfractions.

FIGURE 9. Particle size distribution curves of the impurities (calcareous material silt pyrite) in the various fluxes.



13/16

FIGURE 10. Particle size distribution curves of the coal and the calcareous rockbefore and after the HC-1.

FIGURE 11. Particle size distribution curves of the coal and the impurities beforeand after the HC-1.



14/16

Table 1 summarizes the particle size analysis data in terms of contentand removal yield of the impurities. The content of calcareous matter inthe total feed (ROM coal) was 1.84%. After going through the bypass,

265this content increased to 2.74% in the HC feed, dropping to 2.47% aftercrushing due to the particle size reduction of the calcareous rock below6.3 mm, (smallest feasible size for manual sorting). Therefore, thisreduction occurred in relation to the calcareous material present in thecoarse fraction ( > 6.3 mm) and not with the total contained in the HC

270feed, which remained unaltered. The material retained on the screenduring the pilot test showed that the content of calcareous matter contentwas 33.7% and 35.8% of impurities (calcareous rock silt pyrite). Interms of calcareous matter removal, 18% was achieved in relation to the

TABLE 1 Particle Size Distribution and Removal of Impurities

Content (%)

Coal Calcareous Impurities

Total feed 97.29 1.84 2.71HC bypass 98.86 0.67 1.14HC feed 96.08 2.74 3.92Screen feed 96.78 2.47 3.22Material passed through

screen97.22 2.05 2.78

Material retained by screen,32.0 mm

34.51 62.89 65.49


50.90 46.45 49.10


57.99 39.68 42.01

Total material retained byscreen

64.23 33.71 35.77

Removal (%) yield, %

Coal Calcareous Impurities

Screen feed, 32.0 mm 100.00 100.00 100.00Screen feed, 25.0 mm 56.13 69.15 66.25Screen feed, 12.5 mm 8.93 26.30 22.57Screen feed 0.88 18.05 14.71

HC feed 0.88 16.28 12.08Total feed 0.49 13.72 9.88



15/16

downstream screen feed (fraction of the test) and 13.7% in the total feed275(total material from the mine). The removal of impurities from the

retained material reached values of 69% in material coarser than 25 mm,

indicating a high removal rate of the coarse fractions, which causethe greatest problems in the milling cycle. In regard to coal losses,considering the total material retained, the loss was of 0.9% of the screen

280feed, which is equivalent to 0.5% of the total feed. A few operationalproblems involving the screen occurred during the test, which causedtrapping of fine material ( < 25 mm), although under normal operatingconditions this would be much less. For this reason, an evaluation wasmade of the calcareous content considering only the larger fractions,

285showing that the retained material > 25 mm would contain of 46.5%calcareous rock.

Table 2 gives the results of the analyses of ash, total sulfur, andsuperior calorific power of the different samples collected during the pilottest, using the screen installed at the exit of the HC-1.

290The most significant result that is shown in Table 2 is the low uppercalorific value (CV) in the fraction retained in the screen, which justifiesits removal from the system. The lowQ4 CP value resulted from the accu-mulation of calcareous rock, which reached approximately 34%. Thefraction that passed through was close to the total feed and bypass, in

295terms of ash, albeit with a discrepancy in regard to the CV, although anenergy enrichment was expected from this fraction. This variation invalues consistently occurs in coal analysis data, requiring a larger numberof samples considering the large volume of sampled material passingthrough.

300Table 2 also lists the results of the screen feed simulated from thevalues of material retained and passing through, for which the expectedash and CV contents of 54.18% and 2786, respectively, were obtained forthis flux. The low CV of the retained material, together with the results of the granulometric analysis, clearly confirm what had been expected right

TABLE 2 Characterization of the Samples: Analyses of Ash, Total Sulfur(STOTAL ), and Upper Calorific Value (CV)

Sample Ash (%) S TOTAL (%) CV (kcal =kg)

Total feed 51.90 2.37 3295HC bypass 51.09 1.96 3175Screen feed (simulated) 54.18 1.93 2786Material passed through

screen

54.03 1.93 2810

Material retained by screen 60.18 1.63 1025



16/16

305from the beginning of this study, i.e., the possibility =necessity of removing a fraction of combustion inert material by the dry route.

An explanation should be given for the low total S content in the

fraction of material retained in the screen. This fraction containeda greater concentration of pyrite (FeS), which led to a higher total S310content. However, large quantities of calcareous rock lead to the reduc-

tion of this value since, in this case, there is naturally a smaller amount of coal.

CONCLUSIONS

This case study showed that most of the problems occurring in mills

315and auxiliaries were caused by gangue and calcium-bearing compoundsassociated with the coal. Characterization, bench, and pilot plant studiesled to the minimization of an important coal beneficiation drawback.Optimizing the comminution stage by adjustments of hammer and rollcrushers and installing a screening system was found to prevent particles

320larger than 25 mm from reaching the mill. This prevention was due to thedifferences in the particle size distribution. Furthermore, because of the separation of gangue (pyrite-rich particles) this screening practice hadthe added environmental benefit of reducing the emission of sulfurousgases.

325REFERENCES

[1] J. Ada nez, et al.,Q5 Methods for Characterization of Sorbents Used in FBB, Journal of FUEL , Vol. 73, pp. 355362, (1994).

[2] H. Ataku l, G. O ner, and M. F. Yardim, FBC Research in Turkey, Journal of EnergySources , Vol. 15, pp. 115, (1993).

330[3] P. F. B. Hansen, H. Dam-Johansen, and K. Ostergarrd, High Temperature ReactionBetween Sulphur Dioxide and Limestone , Journal of Chemical Engineering Science ,Vol. 48, pp. 13251341, (1993).

[4] W. Z. Khan and B. M. Gibbs, The Inuence in Air Staging in the Reduction of SO2 byLimestone in a FBC, Congress of World Energy Engineering , Washington, Vol. 90,

335pp. 473478, (1990).[5] A. Lyngfelt and B. Leckener, Model of Sulphur Capture in Fluidized-Bed-Boilers

Under Conditions Changing Between Oxidizing and Reduction, Journal of Chemical Engineering Science , Vol. 48, pp. 11311141, (1993).


coal feed in thermal power

Documents