frity iliwr "* i , M | \ ^

W j

Frans.K. van Dijen, Loesche/EIectrabel,prim:. • prim*:

Germany, reports orTtheeffectiveness of

grinding coalrwith)various biomass and its

.potential for reducinq CO, emissions.

Loesche has performed tests on behalf ofElectrabel on the grinding of coal indepen-dently and the co-grinding of coal withwood pellets, wood chips and oliveresidue. The tests were within the scope ofElectrabel technology watch and develop-ment activities. Loesche used a smal! millthat produces results comparable withthose of full-scale coal mills. Co-grinding ofcoal and biomass proved to be rather com-plex and the mill had to be adapted accord-ingly, particularly the classifier. Using a lowair flow through the mill to each fuel mix-ture also proved crucial for obtaining excel-lent results from co-grinding coal and bio-mass. Both companies' interest in co-grind-ing coal and biomass originated from adesire to reduce fossil CO2 emissions,which can potentially be achieved throughco-grinding. By using biomass instead ofcoal sustainable energy is used.Consequently emissions of fossil CO2 arereduced and less fossil fuel is used. The testresults, therefore, can be used as a means bywhich to assess the viability of co-grindingas a method for reducing CO2 emissions.

IntroductionElectrabel operates pulverised coal-firedpower plants. Within these power plants,coal pulverisation is particularly importantfrom a financial point of view because thecosts of maintaining and operating coalmills are relatively high. The company isalso interested in the co-combustion of suit-able biomass in pulverised coal-fired powerplants, which produces sustainable energythat, depending on the country, can be ahighly valued product. In this way fossilCOT emissions and the use of fossil fuels are

1.0Q.

E

o

: pow

ei

15 - .

14 -

13 -

12 -

11 -

10 -

9 -

8 -

7 -

6 -

5 •——

20

•51.5

•

•

•

3^-7 53.5

1

30•

17.8

high vitriniie coals

1 1 130 40 50

• bituminous• subbituminous

vitiinile contents (% mineral mailer free basis)st iown adjacent lo data point

•JO.6•

• 406x • 71.344.4 ••

I I 1 ! 1 1 160 70 80 90 100 110 130

HarOgrave Grindabiiity Index

Figure 1. Power consumption of coal milling in kWh Jt.

reduced as well. If technically possible, co-milling of coal and biomass is one of themost economically sound methods of co-combustion. Interest in coal grinding with-in the company has increased and as aresult studies have been performed withinthe scope of the the company's technologywatch and development activities.1

High quality coal is usually milled toparticles 100% < 0.1 to 0.2 mm. A particlesize of 70% < 0.074 or 92% < 0.090 mm forcoal with a HGI of 50 is suggested.

Secondary fuels, like biomass, are usuallymore reactive than coal. This is indicated bytheir fuel ratio. For co-combustion in a pul-verised coal-fired power plant, the biomassis preferably milled down to approximately100% < 1 to 2 mm.

Studies revealed limited literature oncoal milling and hardly any literature onthe co-milling of coal and biomass. Millingis often combined with classificationbecause it is wise to avoid reducing the par-ticle size more than is absolutely necessary.

Reprinted from World Coal * December 2004

co-grinding coal and wood pellets

100.00

10.00 •—

10 100

microns

1000

• 10wt.%, test 10 . 20wL%, test 15

Figure 2. Particle size distribution of the products after co-grinding.

Fuel analysis

Two standard tests of coal related tomilling are available, these are theHardgrove Grindability Index (HGI) andthe Abrasion Index (AI). HGI should pre-dict the ease of milling the coal, especiallythe power consumption, and AI shouldpredict mill wear. Unfortunately currentlaboratory tests for the determination ofHGI and AI are inadequate and inaccurateand their ability to predict full-scale per-formance of coal mills can be unreliable.2

There are many discrepancies between thedetermination of HGI and the reality ofcommercial coal mills. Wear of coal millsis caused by the presence of large particlesof hard minerals in the coal, such as pyriteand quartz. Recently interesting literatureon the subject appeared,3 however, thedetermination of AI does not simulatecommercial coal mills.

There are probably no standards relatedto the milling of biomass. Special hammermills from the wood processing industry(Pallmann) proved suitable for the pulveri-sation of dry wood.4

Modelling of millingThe following factors may influence thepower consumption of coal milling:

• Particle size of coal before milling.

• Particle size of coal after milling.

• Type of coal or value of the HGI.

• Type of coal or moisture content.

• Type of mill.

• Mill operations.

• Others.Equation 1 presents the operational

costs of coal milling^

Pm - (Pp x fcl / HGI03) / LHV + (Pth +Pec) x fc2 / HGI2

Where:Pm = costs of milling in €/GJ;LHV - Lower Heating Value in GJ/t;HGI - Hardgrove Grindability Index

value;Pp - the costs of power in €/MWh,;Pth + Pec = the costs of coal at the loca-

tion in €/GJ;The factor fcl = 0.075 and the factor fc2 =

20.The operational costs are related to

power consumption and combustion effi-ciency. Equation 1 implicates Equation 2 forpower consumption:

W/M = fc3 / HGI05

Where:W/M = work in kWhe/t;fc3 = 75.Equation 3 implicates the influence of

the particle size before and after coalmilling on the power consumption:''

W/M = C x ((DPin/DPout)05/ DPout)05

Where:W/M = work in kWhe/t;DPin - the average particle size of the

coal before milling in cm;DPout - the average particle size of the

coal after milling in cm;C - a materials constant. (C is approxi-

mately 0.2 for coal, 10 for dry wood and 4for residue of olives, with kernels).

Equation 4 presents the costs of themaintenance of coal milling:5

Mmw = Cmw x (245 x AI/HGI2 + 1.8) x(U-9xBxlO- 4 )

Where:Mmw - the costs of maintenance in

€/MWhe;Cmw - 0.083;B = the Brinell hardness number of the

wear parts of the mill;HGI - the Hardgrove Grindability Index

value;AI - the Abrasion Index value.The value of B was 600.For the remodelling of the power con-

sumption of coal milling a more complicat-ed model was made. This is represented byEquation 5 (Van Dijen's equation):

W/M = (Cex/HGP) x ((DPin/DPout)05

/DPout)05 x(l-W)111

Where:W/M - work in kWh /t;

Reprinted from World Coal • December 2004

Cex, n and m are factors;HGI = Hardgrove Grindability Index

value;DPin = average particle size of the coal

before milling in cm;DPout = average particle size of the coal

after milling, in cm;W = the water fraction in the coal.The values of n and m have to be deter-

mined. So far, the first indication for thevalue of n is 1.25 and for the value of m is0.11 The value of Cex is approximately 40.n

We anticipate a value of 1 for m.The work for milling is without power

for the primary air fans.

Data fittingData of full-scale mills was available fromliterature 7-8 and in-house. The powerplants of interest were Langerlo 1/2,Gelderland 13, Ruien 5, Hemweg 8 andMaasvlakte 1. Fitting this data with equa-tions 2, 3 and 5, where n = 1.25 and m = 0,resulted in values of fc3, C and Cex as pre-sented in Table 1. From these data it wasconcluded that coal with a HGI value of 50can be milled with a power consumption ofapproximately 11 kWhe/t. However, muchhigher power consumption may beencountered in practise.2'7 Further researchis needed to find out the causes of this highpower consumption. Possible causes maybe:

• Measurements errors.

• Type of coal mill.

• Poor quality coal, other than a lowvalue of the HGI.

The large spread in power consumptionby about a factor 2 for coal with a low HGIvalue, as indicated by Figure 102 andshown in Figure 1, may be related to:

• Differences in particles sizes of thecoal before and after milling.

• The type of coal mill.

• Operations of the coal mill.

• Power consumption with or withoutprimary air fans.

• Others.

The results obtained by Loesche alsoindicate a large variation of power con-sumption for coal with a low HGI value.

Co-milling of biomassWith the co-milling of biomass, fuels of inter-est are dried sewage sludge, wood chips,wood pellets, kernels and nuts shells. It isgenerally perceived that coal mills are lesssuitable for only milling biomass, with theexception of dried sewage sludge. During

the co-milling of coal and biomass, (jhe bio-mass is mainly milled by the coal. With woodpellets there is hardly any need for milling,only for desagglomeration. Questions to beanswered regarding the co-combuspon ofcoal and biomass are as follows:

• How much power is consumed forcoal milling?

• How much power is consumed! by thebiomass milling?

• What is the particle size distributionof the coal after milling?

• What is the particle size distributionof the biomass after milling?

• What are the influences of coal |>n bio-mass and of the biomass on co^l dur-ing milling?

• What is the moisture content |)f bio-mass and coal respectively! aftermilling?

• How well does the classifier operate?

• Is co-milling of the biomass successful?

The co-milling experiments per;at individual pulverised coal-fired

ormedpower

plants are often limited from a scientificpoint of view. The results only concludewhether 'it works' or 'it does notThese results are presented withoutwith limited explanation and the

work.'any orexperi-

ments are not well documented. Problemsencountered with co-milling of coil andbiomass are:

• Mill vibrations.

• Reduced throughput.

• No good particle size reduction],

• High energy consumption.

• Excessive wear.

An example of the cause of excessivewear may be the sand in purified wastewood chips. Mill vibrations may be ;ausedby accumulation in the mill of hard andcoarse minerals or wood chips. Differentpower plants claim different results withthe co-milling of coal and woodSome of these claims are that woocare not ground at all.

chips,chips

ExperimentsIn order to improve our knowledge bf coalgrinding and co-grinding of biomass andcoal, tests have been performed by Lbescheand Electrabel, the latter operating tpe for-mer's mills for coal pulverisation. B omassmaterials tested were: olive residue (with-out kernels), wood pellets and dry woodchips. Some characteristics of the fuels test-ed are presented in Table 2. The tests arelisted in Table 3.

A coal mill with a table 360 mm in diam-eter was used. The roller grinding mill wasnamed LM 3.6. Tests were conducted onlyusing coal, in which air was blown throughthe mills with a capacity of 1220 Nm3/s andat 90 C after the mill. The grinding pres-sure was 70 bar. Tests 3 to 18 were per-formed under conditions relevant to the co-grinding of biomass. The airflow wasreduced to 980 Nm3/h and at 90 °C after themill. The grinding pressure was 70 bar. Theclassifier was not adapted. Biomass was co-milled with Consol coal.

After grinding the products wereanalysed. The following aspects were mea-sured:

• The particle size distribution of thecoal and the mixtures, using screens.

• The particle size distribution of thecomponents after the mixtures wereseparated into their components,using screens if possible.

• The moisture contents of the compo-nents of the mixtures, if possible.

• Microscopic analysis.

ResultsThe average particle sizes of the fuels aftertesting are presented in Table 4. The calcu-lated results of measurements during test-ing are presented in Table 5. For the calcu-lations an average primary particle size ofthe wood in the wood pellets of 1.5 mm wasused. It became clear that primary woodparticles of the wood pellets were reducedin size in the co-grinding tests. Equation 5was used, where Cex = 21.4, n = 1.25 and m= 0, to calculate the power consumption forcoal grinding.

Microscopic analyses showed that theparticle size distribution of the residue ofolives, without kernels, was similar to thatof the coal. Microscopic analysis alsoshowed that the particle size of the woodafter co-grinding of both wood pellets andwood chips was coarse, whereas that of thecoal was fine.

Figure 2 shows two typical particle sizedistributions of wood pellets and coal mix-tures after milling as in tests 10 and 15. Thecoal is approximately 100% < 0.150 mm andthe wood approximately 100 % > 0.150 mm.

DiscussionBased on the coal analysis, it appears thatthe grinding behaviour of coal is not easy topredict. For this reason grinding tests arestill necessary when purchasing anunknown coal, purchasing a new mill or co-grinding coal with another fuel.

Reprinted from World Coal • December i004

Table 1 . Calculated values of fc3, C and Cex and power consumption for milling only

fc3

C

Cex

kWhe /t

Langerlo 1 /2

64

0.13

17

9.1

Gelderland 13

65

0.145

25

8.2

Ruien S

69

0.145

23

9.0

Hemweg 8

103

0.23

3 1

14.6

Maasvlakte 1

76

0.17

28

9.8

Table 2. Some properties of solid fuels tested

Property

Name

APS bm*

APS bm**

Moisture bm

Ash

HGI

LHV

Unity

mm

mmwt.%

wt.%

GJ/t

Coal

USA CONSOL

5

6.75

8.34

54

28.8

Dry wood chips

C0WI

5

7.6

2

18

Wood pelletswithout kernels

6

1.5

5.8

2

18

Residue of olives

3

20.4

8

14

* Average particle size before milling. ** Primary particles.

Table 3a. The tests with CONSOL coal only

Test

1

2

3

4

Throughput in tph

0.391

0.360

0.439

0.367

* Of the mill.

Table 3b. The tests with biomass and CONSOL coa

Test

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Type of biomass

Residue of olives0

Residue of olives"

Residue of olives0

Wood pellets

Wood pellets

Wood pellets

Wood chips

Residue of olives0

Residue of olives0

Residue of olives"

Wood pellets

Wood pellets

Wood chips

Wood chips

Power consumption in kWhe/t*

8.9

9.3

8.3

9.4

Biomass in wt.%

10

10

10

10

10

10

10

20

20

20

20

20

5

5

Throughput in tphin kWhe/t*

0.293

0.342

0.432

0.211

0.252

0.348

0.232

0.246

0.325

0.358

0.298

0.339

0.404

0.512

Power consumption

11.8

10.3

8.5

16.4

13.6

10.

17.2

13.2

10.5

9.8

12.4

11.0

9.9

8.0

* Of the mill. ° Without kernels.

Knowledge of co-grinding in particular isvery limited indeed.

Our understanding of coal milling needsto be improved. Better models of coal pul-verisation and verification of these modelsare needed. Van Dijen's equation may beused as an overall starting point. Literatureon co-milling is scarce. The best way tolearn about coal grinding and co-grindingshould be in the context of large coal mills.Laboratory experiments and standards areoften inadequate in representing full-scalecoal mills.

In the tests that were conducted by thecompany co-grinding olive residue with-out kernels proved easy. However, powerconsumption, expressed in kWhe / t ofresidue, was high. This was mainly relat-ed to the fact that the residue of oliveswithout kernels was ground much finerthan anticipated. Co-grinding wood pel-lets was also relatively easy. However,again power consumption, expressed inkWhe/t of pellets, was high. In turn, thiswas also related to the fact that the woodpellets were ground finer than necessary

and that wood is hard to grind. This isshown by Table 5 and the high value of C.For wood pellets, C varies strongly andcan hardly be considered a constant. Co-grinding of wood chips is hard, but possi-ble. This is shown by the high values forC of wood chips and the high-energy con-sumption, expressed in kWhe/t of chips(Table 5).

In the futureThe coal-fired power plants of the futuremay be either USC CFB or GCC.

Reprinted from World Coal • December 2004

Table 4. Average particle size in mm of solid fuels after testing

Test

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

Coal

0.035

0.030

0.0350.032

0.032

0.040

0.055

0.025

0.035

0.050

0.035

0.040

0.060

0.065

0.045

0.050

0.045

0.054

Dry wood chips

0.200

0.350

0.600

Wood pellets

0.100

0.200

0.400

0.270

0.400

Residue ofolives without

kernels

0.032

, 0.040

0.055

0.040

0.060

0.065

Table 5. Calculated results of the tests

Test

123

4

5

6

7

89

10

11

12

13

14

15

16

17

18

Power consumption(kWhe/t biomass)

35.933.530.265.059.147.295.135.129.727.533.728.863.642.7

Power consumption(kWhe/t coal)

8.99.3

8.3

9.4

9.1

7.7

6.1

11.08.5

6.5

8.5

7.7

5.7

5.4

7.1

6.5

7.1

6.2

fc3

65.468.461.069.1

C

0.1520.1420.1420.1500.65*0.72*0.82*3.30*5.05*6.78*6.01*0.75*0.87*0.85*3.61*4.14*6.12*6.16*

Cex

22.320.820.822.0

* For the biomass.

Gasification Combined Cycle (GCC)allows for low production costs for heatand power from coal without emissions ofCO2. The CC itself is fired with hydrogen.Advantages of Ultra Super CriticalCirculating Fluidised Bed combustion(USC CFB) over pulverised coal combus-tion may be lower costs and higher fuelflexibility. Higher fuel flexibility is similarto reduced costs of co-combustion of suit-able biomass. The USC CFB power plantmay operate on 100% coal, 100% woodchips or wood pellets and mixtures of

wood chips or wood pellets and coal. Alarge USC CFB power plant fired by coal isnow under construction in Poland.9 A largeCFB power plant fired by wood operates inFinland.10

ConclusionsThe tests showed that co-grinding of bio-mass and coal is possible and may beoptimised further. Different types of bio-mass behave differently in co-grindingwith coal. The particle size distributionof the mixture of coal and residue of

olives without kernels should be similar tothe particle size distribution for millingonly coal. The particle size distribution ofthe coal and wood pellets or wood chipsmixtures should be orientated towards theweight fraction biomass > 0.2 or > 0.1 mm.For example, a mixture with 10 wt.% woodchips or wood pellets should be ground to10 wt.% > 0.1 or 0.2 mm.

Table 5 suggests that co-grinding20 wt.% wood pellets is better than co-grinding 10 wt.% wood pellets because lesspower is consumed for both wood pelletsand coal. This is not the conclusion that thecompany expected to draw. Adjustment ofthe coal mill, especially of airflow throughthe mill and air classifier, proved crucial forsuccessful co-grinding. •

References1. F. van Dijen, Laborelec, Milling of coal and sec-

ondary fuels, 2003.

2. A.M. Carpenter, IEA Clean Coal Centre, Coalquality assessment - The validity of empiricaltests, 2002.

3. J.J. Wells, F. Wigley, D.J. Foster, W.H. Gibb, J.Williamson, The relationship between exclud-ed mineral matter and the abrasion index ofcoal, Fuel 83, 2004, 359-364.

4. F. van Dijen, F. Penninks, The development ofthe technology for the pulverisation of woodchips at Electrabel, VDI Wissensforum,Ersatzbrennstoffe fur Industrieanlagen,Osnabruck, 2001.

5. F. van Dijen, Laborelec, Impact of coal qualityand costs on the costs of power - a complexmodel, 2002.

6. W. Hemming, Verfahrenstechnik, VogelBuchverlag, Wiirzburg, 1993.

7. A. Hoogendoorn, KEMA, Kolenmolenproevenbij Centrale Hemweg 8 ten behoeve van con-ditiebewaking en procesoptimalisatie, 2001.

8. F.J.J.M. van Aart, D. van der Vlist, KEMA,Kolenmolenproeven bij Centrale Maasvlakte1 ten behoeve van conditiebewaking en pro-cesoptimalisatie, 2000.

9. R.G. Lundqvist, Designing large-scale circulat-ing fluidised bed boilers, VGB PowerTech10/2003,41-47.

10. Alholmens: the world's largest biofuelledplant, Modern Power Systems, January 2002,19-28.

11. CM. Roozendaal, KEMA, Maalgedrag vankolen en mengsels. Resultaten experimenter!TU Clausthal, 1999.

12. ACARP, Effects of coal moisture on verticalspindle mill performance, National energyresearch development and demonstrationprogramme C1215,1993.

13. T. van Dijsseldonk, A. Korthout, T. Krause, T.Pistorius, Mitverbrennung von unbehandelterBiomassa in einem 600 MWe Kohle gefeuertenDampferzeuger, in VGB-conferenceKraftwerke im Wettbewerb, Koln, March2003.

Reprinted from World Coal • December 2004