Ten residual biomass fuels for circulating fluidized-bed gasification

Biomass and Bioenergy 20 (2001) 45–56

Ten residual biomass fuels for circulating uidized-bedgasi�cation

A. van der Drifta;∗, J. van Doorna, J.W. Vermeulenb

aNetherlands Energy Research Foundation (ECN), P.O. Box 1, NL-1755 ZG Petten, NetherlandsbNV Afvalzorg, P.O. Box 6343, NL-2001 HH Haarlem, Netherlands

Received 21 July 1999; accepted 10 July 2000

Abstract

In co-operation with a Dutch company (NV Afvalzorg) and the Dutch agency for energy and environment (Novem), ECNhas successfully tested 10 di�erent biomass residues in its 500kWth circulating uidized-bed gasi�cation facility. Among thefuels used are demolition wood (both pure and mixed with sewage sludge and paper sludge), verge grass, railroad ties, cacaoshells and di�erent woody fuels. Railroad ties turn out to contain very little (heavy) metals. Initially, fuel feeding problemsoften impeded smooth operation. Contrary to feeding systems, the circulating uidized-bed gasi�cation process itself seemsvery exible concerning the conversion of di�erent kinds of biomass fuels. The fuel moisture content is one of the mostimportant fuel characteristics. More moisture means that more air is needed to maintain the process temperature resulting inbetter carbon conversion and lower tar emission but also lower product gas heating value and lower cold gas e�ciency. So,for a good comparison of the gasi�cation behaviour of di�erent fuels, the moisture content should be similar. However, themoisture content should be de�ned on an ash-free basis rather than on total mass (the usual way). Some of the ashes producedand retained in the second cyclone were analysed both for elemental composition and leaching behaviour. It turned out thatthe leaching rate of Mo and Br, elements only present in small concentrations, are preventing the ash to be considered as inertmaterial according to the Dutch legislation for dumping on land�ll sites. c© 2001 Elsevier Science Ltd. All rights reserved.

Keywords: CFB; Gasi�cation; Biomass; Cacao shells; Sewage sludge; Paper sludge; Ash

1. Introduction

For thousands of years, plants and trees were themost important sources of energy for mankind. In thedeveloped countries this role has been taken over byfossil fuels. Recently, plants and trees, now called

∗ Corresponding author. Tel.: +31-224-564-515; fax: +31-224-563-487.E-mail addresses: [email protected], [email protected] (A. van

der Drift).

biomass, have gained interest again. Worldwide agree-ments have been made to reduce the emission of CO2

being the major greenhouse gas. The Dutch govern-ment has formulated the policy intention that by 202010% (288 PJ) of energy generation in the Netherlandsmust come from sustainable energy sources and 120PJ of that must come from biomass and waste [1].This provides a major impetus for the application ofbiomass as a fuel. Since the Netherlands is a denselypopulated country, the potential of any single type ofbiomass with given physical and chemical properties

0961-9534/01/$ - see front matter c© 2001 Elsevier Science Ltd. All rights reserved.PII: S0961 -9534(00)00045 -3

46 A. van der Drift et al. / Biomass and Bioenergy 20 (2001) 45–56

Table 1Selected biomass fuels with estimated raw fuel price, pre-treatment steps needed for the CFB gasi�cation tests and costs involved. In thelast column, the physical shape is given of the actual materials tested. Fuels #5 and #6 have been tested mixed with fuel #1. Pre-treatmentsteps: a: mix, b: crush=chip, c: sieve=shift, d: compost, e: dry (thermal), f: pelletize, g: (non)ferro removal

No. Biomass fuel Raw fuel Pre-treatment Costs Physical shape ofprice a b c d e f g (Euro=GJ) product(Euro=GJ)

1 Demolition wood (painted wood and −8 to −10 x x x 2 Chips ¡ 35 mmchip board)

2 Park and public garden wood −2 to −4 x x x 4 Chips ¡ 25 mm3 Chip board materials −8 to −10 x x x 2 Chips ¡ 20 mm4 Verge grass 0 to −5 x x x x 6 Pellets5 Paper residue sludge x x 2 Granulates6 Sewage sludge −8 to −16 x x 1 Granulates7 Woody excess sieve fraction from ODWa −12 x x x 15 Chunks ¡ 15 mm

composting plant8 Park and public garden wood (dried by composting) −2 to −4 x x x 8 Chips ¡ 40 mm9 Railroad ties −8 to −12 x x x 2 Chips ¡ 20 mm10 Cacao shells 0 x 0.4 Shells ¡ 30 mm

aODW: organic domestic waste.

is limited. This implies that biomass energy conver-sion facilities in the Netherlands should preferablybe exible concerning the fuel input requirements.Circulating uidized-bed technology is considered tobe one of the most suitable techniques to thermallyconvert di�erent fuels into useful energy. However,practical experience is available for only a limitednumber of fuels and conditions. This paper describesthe results of bench-scale circulating uidized-bed(CFB) gasi�cation experiments with 10 di�erent(mixtures of) biomass materials. The purpose of thetests was to judge the suitability of the selected fuelsfor energy production via CFB gasi�cation concern-ing fuel gas and ash quality and process characteris-tics. Preliminary tests revealed that short-term testsgive su�cient information on the main process- andfuel gas characteristics. In this publication, only thegasi�cation step is considered. In the beginning of2000 the plant has been equipped with gas cleaningcomponents and a gas engine.

2. Residual biomass fuels

A total of 10 di�erent biomass fuels have beenselected for the tests. The criteria for this selec-tion were availability in combination with presenttreatment=disposal costs. In Table 1, selected fuels are

listed together with the pre-treatment steps as usedfor each of the fuels, the resulting physical shape forthe CFB gasi�cation tests and the costs involved. Thepre-treatment costs are estimated by the supplier of thematerials. The raw fuel costs are estimates from [2].

3. Experimental

3.1. Fuel analysis

Each fuel has been subjected to ultimate and proxi-mate analysis as well as the analysis of the main met-als and heating value according to the standards asrecommended in the so-called best practice list [3].The ash content was determined according to Swedishstandard SS 187171 at 550◦C.

3.2. CFB gasi�cation facility

The circulating uidized-bed (CFB) gasi�er, calledBIVKIN and situated at the Netherlands EnergyResearch Foundation (ECN in Petten, the Nether-lands), is an atmospheric air-blown facility of about500 kWth. Fig. 1 shows a scheme of the facility, seealso [4]. It is equipped with various feeding systemsof which two can be used simultaneously in order tobe able to feed fuel mixtures. The fuel feeding point

A. van der Drift et al. / Biomass and Bioenergy 20 (2001) 45–56 47

Fig. 1. Scheme of CFB gasi�cation facility at ECN.

is 1m above the bottom of the riser. The riser is a20 cm diameter and 6m high refractory lined pipe.The circulation loop contains a bubbling uidized-bedseal operated on pure nitrogen. The total nitrogen ow (purging feeding system and uidising seal) is15m3

n=h in average. Gas ows are controlled by cali-brated mass ow controllers. The accuracy is 3% forthe air and 2% for the nitrogen ows within the owrange used. The produced fuel gas is ared. The bedmaterial used is sand (97% silica) of 0.4–0.6mm indiameter.

3.3. CFB gasi�cation experiment

To be able to compare the di�erent tests, each testwas performed at approximately 850◦C and a lineargas velocity (in top of riser, wet gas, actual temper-ature) of 6m=s. For a given fuel this choice �xes allparameters like fuel feeding rate and fuel=air ratio.In all experiments, except the test using cacao shells,secondary air was introduced (with reduced primaryair ow). Each test consists of approximately 2 h ofstationary operation.

3.4. Analysis during gasi�cation

The fuel feeding rate was measured, dependingon the feeding system used, either gravimetrically

by recording the weight of the feeding bunker orby monitoring the number of revolutions of a dos-ing rotary valve. Second cyclone ash output ow(including unburned carbon) was determined byweighing the ash collected in periods of 1–2 h. Thefraction inert material in the “ash” is determinedby ashing the material at 550◦C. The burnt frac-tion is assumed to have a higher heating valueof 29 MJ=kg and contains 92wt% C, 1wt% H,6.4wt% O and 0.6wt% N. These are average valuesof measurements from four di�erent tests. The con-centrations of H2; CO; CH4 and CO2 in the fuelgas are measured continuously using TCD (H2) andNDIR detectors, respectively. Oxygen is measuredusing a paramagnetic sensor to check sample in-tegrity. The amounts of ethene, acetylene, ethane,benzene, toluene and xylene are measured discon-tinuously (every 10 or 20min) using gas chro-matography. The concentrations of HCl, NH3 andH2S are measured in triple for each experimentby sampling a known volume (approx. 10–20 l)of dust-free fuel gas through two washing bottles�lled with water and 0.05 M HNO3 respectively. Thesolutions are subsequently subjected to standard anal-ysis techniques. Concentrations of dust (particles)and heavy tars are determined by gravimetric meth-ods. By isokinetic sampling, a known volume of fuelgas is passed through a 250◦C (dust collection) and


125◦C quartz �lter (tar collection), respectively. Thefraction inert material of dust is determined by ashingat 550◦C. The burnt part is assumed to have similarhigher heating value and composition as the non-inertfraction of the second cyclone ash. Based on threedi�erent analysis, heavy tar is assumed to contain81wt% C, 6wt% H, 10wt% O and 3wt% N.From this, a higher heating value of 35 MJ=kg tarhas been assumed based on di�erent calculationmethods.

3.5. Ash analysis

Second cyclone ash of three tests has been analysedfor most metals as well as PAHs (polycyclic-aromatichydrocarbons), cyanide, chlorine and uorine. Proce-dures used for determination of metals and halogensare similar to the ones used for fuel analysis. The de-termination of PAHs is done according to standardVPR C 88-11. The ash samples were also subjectedto column leaching tests according to Dutch standardNEN 7343 (L=S=1).

3.6. Calculations following each gasi�cationexperiment

The concentration ofN2 in the fuel gas is calculatedby subtracting all measured gas concentrations from100%. The balances of N and H are subsequentlyused to calculate the fuel gas ow and fuel gas wa-ter content respectively. With these data, balances ofenergy, carbon and inert material are made. The totalheat loss of the facility used in the heat balance is40 kW. The carbon conversion was calculated bysubtracting the measured carbon in second cycloneash and dust from 100%. The cold gas e�ciency(CGE) was calculated by dividing the energy con-tent of the fuel gas (excluding heavy tar) by theenergy content of the incoming fuel (lower heat-ing value). From measured values for hydrocarbons(HCs), an estimate is made for other HCs up tonaphthalene based on earlier measurements. Theair=fuel-ratio is expressed as ER (equivalence ra-tio). ER is de�ned as the ratio of the amount ofoxygen (air) supplied and the amount of oxygen(air) needed for stoichiometric combustion of thefuel.

4. Results

4.1. Fuel analysis

Table 2 shows the composition of the fuels usedin the experiments. Metal concentrations are given ing=GJ lower heating value (dry base). As a reference,average values are given for willow from an energyplantation in the Netherlands [5,6].As can be seen from Table 2, all fuels contain

less than 17% water. Fuel moisture content is avery important parameter for cycle e�ciency in abiomass-based gasi�cation combined cycle [7]. Asfar as the heating value is concerned, most fuels havehigher heating values (HHV) very close to 20 MJ=kg,based on ash-free and dry basis. The exceptionsare paper-residue sludge containing relatively highamounts of cellulosic material and sewage sludgecontaining more fats and other components low inoxygen. Generally speaking, sewage sludge containshigh amounts of almost any element (not consideringC, H and O) compared to the other fuels whereasrailroad ties (sleepers) surprisingly are “cleaner”than the other materials. The woody excess fractionfrom composting organic domestic waste containsrelatively high amounts of heavy metals (chromium,manganese, and nickel).Potassium, considered as one of the major elements

responsible for agglomeration during thermal conver-sion, is present in relatively high amounts in vergegrass, cacao shell and the woody excess fraction fromcomposting organic domestic waste.

4.2. Fuel mixtures

Both sludge fuels (fuel #5: paper residue sludgeand fuel #6: sewage sludge) have only be tested asa mixture with demolition wood in test #5 and #6,respectively. Table 3 shows the fraction of both fuelsin the mixture.

4.3. Gasi�cation tests

Although feeding all the di�erent biomass materi-als turned out to be not straightforward, satisfactoryfeeding could be achieved for all fuels after modifyingand renewing the existing feeding systems. Althoughsome general aspects could be discovered, feeding


Table2

FuelcompositionoffuelsusedinCFB

gasi�cationtests.Valuesprintedinitalicsarenotmeasured,insteadaveragevaluesfrom

thePhyllisdatabasearegiven[6].

Abbreviations:ar:asreceived(wetbasis),daf:dryandashfree,na:notanalysed

Fuelno.

01

23

45

67

89

10Willow

Demolition

Park&Public

ChipboardVergegrassPaperresidueSewage

Woodyexcess

ParkwoodRailroadCacaoshells

wood

gardenwood

materials

sludge

sludge

fractionof

(bio-dried)ties

ODW

Testno.

01,5,6

23

4a,4b

56

78

910

Water

wt%ar

17.0

6.1(1)

3.5

10.8

7.3

10.7

10.6

12.8

14.8

10.6

13.4

10.1(5,6)

Ash

wt%dry

2.13

2.08

3.23

2.9

17.6

47.3

3642.8

18.3

2.6

10.5

Volatileswt%daf

83.5

80.0

80.2

79.2

79.0

82.2

86.2

75.2

74.5

81.3

75.8

HHV

MJ=kgdaf

20.1

20.3

20.2

20.0

20.0

15.1

23.1

19.4

19.9

21.7

20.5

LHV

MJ=kgdaf

18.8

19.1

18.9

18.7

18.7

13.9

21.5

18.2

18.6

20.4

19.3

Cwt%daf

48.7

51.0

51.2

50.3

49.9

43.5

52.2

50.7

51.5

54.4

53.0

Hwt%daf

5.91

5.55

5.88

5.87

5.68

5.58

7.27

5.70

5.70

5.78

5.85

O(di�.)wt%daf

44.5

42.9

42.5

42.3

40.9

48.9

30.7

41.7

41.3

39.4

37.6

Nwt%daf

0.88

0.46

0.26

1.44

2.47

1.37

7.06

1.56

1.16

0.27

3.39

Swt%daf

0.045

0.043

0.049

0.082

0.18

0.65

2.61

0.26

0.25

0.10

0.22

Cl

wt%daf

0.016

0.073

0.064

0.090

0.86

0.046

0.15

0.068

0.027

0.016

0.016

Fwt%daf

0.003

0.002

0.002

0.001

0.003

0.036

0.044

0.010

0.013

0.002

0.001

Al

g=GJ

3.3

1833

58na

9040

1470

820

370

11na

As

g=GJ

0.07

0.36

0.10

0.11

0.06

0.10

0.41

0.34

0.17

0.10

0.06

Bg=GJ

0.54

0.36

0.27

0.40

na3.6

4.5

8.4

2.4

0.15

naBa

g=GJ

0.22

6.4

0.8

37na

7.5

2913

8.5

4.2

naCa

g=GJ

310

170

200

270

840

13100

2800

2200

990

96130

Cd

g=GJ

0.13

0.03

0.01

0.03

0.06

0.08

0.10

0.06

0.01

0.02

0.06

Co

g=GJ

0.033

0.1

0.04

0.1

0.1

0.3

0.5

1.0

0.3

0.03

0.1

Cr

g=GJ

0.76

2.2

1.1

1.0

0.7

2.7

4.6

30.7

8.5

0.5

0.2

Cu

g=GJ

0.71

1.3

0.3

0.4

0.6

7.1

332.8

2.7

0.4

2.0

Feg=GJ

3.7

2233

41na

400

3700

500

240

76na

Hg

g=GJ

0.003

0.047

0.003

0.033

0.003

0.007

0.17

0.007

0.007

0.005

0.003

Kg=GJ

140

38120

461400

500

160

980

400

131600

Mg

g=GJ

2818

2653

120

340

270

240

130

10280

Mn

g=GJ

0.5

3.6

1.1

4.5

9.7

1029

149.2

1.9

4.9

Mo

g=GJ

0.03

0.03

0.03

nana

0.23

0.46

0.20

0.13

0.03

naNa

g=GJ

1029

2763

9752

120

380

240

121.7

Ni

g=GJ

1.4

0.3

0.4

0.4

0.1

1.0

12144

350.3

0.6

Pg=GJ

393.7

146.6

160

210

1600

210

462.6

190

Pbg=GJ

139.1

5.5

9.4

0.9

2.2

1320

8.5

2.4

1.9

Sbg=GJ

0.14

0.03

0.03

0.10

0.06

0.09

0.02

0.15

0.07

0.05

0.06

Seg=GJ

0.05

0.01

0.01

na0.06

0.11

0.03

0.07

0.05

na0.06

Sig=GJ

34160

320

210

2500

8700

2400

3400

1300

65440

Sng=GJ

0.092

0.032

0.027

0.085

0.65

0.82

1.7

0.19

0.11

0.03

0.29

Srg=GJ

0.76

0.64

0.93

nana

1625

7.1

3.2

nana

Te

g=GJ

0.05

na1.1

na0.06

nana

nana

na0.06

Ti

g=GJ

0.22

8.6

nana

na178

3019

14na

naV

g=GJ

0.015

0.037

0.060

0.14

0.4

1.8

1.0

1.2

1.2

0.043

0.12

Zn

g=GJ

5.3

131.8

152.9

8.5

6717

7.2

135.2


Table 3Fraction of fuels used in tests #5 and #6

Test #5 Test #6Fuel A Fuel #1: demolition woo Fuel #1: demolition woodFuel B Fuel #5: paper residue sludge Fuel #6: sewage sludge

Fraction of sludge, based on dry weight 22.4% 19.1%Fraction of sludge, based on higher heating value 10.4% 15.1%

systems turned out to be very fuel-speci�c. From sev-eral experiments using chipped railroad ties it can beconcluded that feeding appeared to be crucial for op-erating the plant. Feeding problems caused fast chang-ing air=fuel ratios which in turn caused measured tem-perature excursions up to 1060◦C. Bed agglomerationwas the result of this and tests had to be aborted, feed-ing problems being the culprit.In Table 4 the main numerical results, including

some calculated values, are given for each test per-formed with the CFB gasi�cation facility at ECN.

4.4. Ash analysis

The measured composition of second cyclone ashfrom tests 1, 2 and 7 are used to calculate the fractionof each element in the fuel that ends up in the secondcyclone ash. Table 5 shows the result. Table 6 showsthe concentration of total PAHs and cyanides in thesecond cyclone ash of tests 1, 2 and 7. These compo-nents, together with many elements mentioned inTable 5, determine whether a material is considered as“dangerous waste” or not, according to the Dutch lawBAGA [8]. All together, the second cyclone ashes oftests 1 and 7 are indicated as dangerous according tothe BAGA law only because of its high concentrationof arsenic (84mg=kg, 50 mg=kg being the limit) andchlorine (8300mg=kg, 5000 mg=kg being the limit),respectively. The ash of test 2 contains no componentsin concentrations exceeding the limits mentioned inthe BAGA law and therefore is considered to be nota dangerous waste. Note that the judgement of theBAGA law or any concentration-based legislation canbe in uenced by the performance of the gasi�cationunit to a large extent. For example, the carbon con-version and bed material abrasion can have signi�cante�ects on the amounts of carbon and bed materialin the ash, in this way diluting the concentrations ofother components.

The same ashes were used for leaching tests. Table 7shows the measured values together with the limits U0and U1 used in the Netherlands mentioned in the reg-ulation for dumping on land�ll sites. If all elementsare below U0, the material is considered as inert anddumping is relatively cheap. All three ashes testedcannot be considered as inert and exceed limit U0 forsome elements. Especially bromine and molybdenumturn out to be leached too easily. Neither of the ashesshow leaching behaviour exceeding the U1-limit. Ifvalues exceed U1, the material has to be dumped inspecial areas at relatively high costs. However, thisalso means that treatment to change the propertiesmight be an economic alternative.

5. Discussion

Not surprising, but nevertheless interesting is thefact that the water content of the fuel highly in uencesthe heating value of the fuel gas produced during gasi-�cation. Fig. 2 shows the e�ect of both moisture con-tent (line 1) and ash content (lines 2 and 3) on fuel gasheating value calculated using a simple model presentat ECN. Contrary to the moisture content, the ash con-tent of a fuel hardly determines the fuel gas heatingvalue. In Fig. 2 the ash content is varied while keepingthe water content constant, both de�ned on a wet basis(line 2) and on ash-free basis (line 3). The di�erencebetween these lines shows that when predicting theheating value of the fuel gas it is important to relatethe amount of water to the amount of organic material(ash-free) rather than the whole lot including the inert(ash) fraction. This is illustrated by the di�erence be-tween tests 7 and 0. In test 7 the fuel contains almost13% water (on a wet basis). However, because the ashcontent is as high as 43% on a dry basis (that is 37%on wet basis) only 50% is left to generate the heatto evaporate the water. The resulting product gas


Table4

Numericalresultsofthegasi�cationtestsusingdi�erentbiomassfuels.Abbreviations:ar:asreceived,HHV:higherheatingvalue,CC:carbonconversion,CGE:coldgas

e�ciency,na:notanalysed,HC:hydrocarbonsuptonaphthalene

Testno.

01

23

4a4b

56

78

910

FuelA

Willow

Demolition

Park

ChipboardVerge

Verge

Demolition

Demolition

Woody

ParkwoodRailroad

Cacao

wood

wood

materials

grass

grass

wood+paperwood+

excess

(biodried)

ties

shells

residue

sewage

fraction

sludge

sludge

ofODW

Processconditions

Fuel owkg=har

6961

56.2

60.7

87.8

102.6

82.6

7776.3

8056

59.8

Watercontentoffuelwt%ar

17.5

6.1

3.5

10.8

7.3

7.3

10.2

10.2

12.8

14.8

10.6

13.4

Energyinput(HHV)kW

323

324

305

303

386

450

367

375

219

316

304

275

Temperature

◦ C827

847

861

843

803

815

829

827

832

805

855

822

Fuelgas

COvol%dry

9.40

11.35

11.65

9.64

9.89

10.64

9.24

10.53

5.34

8.31

10.57

8.00

H2vol%dry

7.20

7.05

6.77

6.42

7.28

8.45

6.08

8.02

1.80

5.38

5.85

9.02

CO2vol%dry

17.10

15.75

15.51

15.63

15.61

15.18

16.11

15.02

16.09

16.04

13.94

16.02

CH4vol%dry

3.30

3.25

3.17

2.77

2.58

2.79

2.81

3.19

1.24

1.72

2.88

2.34

C2H4vol%dry

1.10

0.85

0.99

0.87

1.15

1.04

1.02

1.12

0.40

0.60

0.95

1.13

C2H6vol%dry

0.10

0.03

0.04

0.03

0.05

0.04

0.04

0.04

0.02

0.02

0.03

0.05

Benzenevol%dry

0.21

0.21

0.25

0.22

0.12

0.13

0.19

0.23

0.12

0.14

0.28

0.16

Toluenevol%dry

0.065

0.031

0.043

0.040

0.035

0.041

0.054

0.058

0.019

0.04

0.11

0.06

Xylenevol%dry

0.013

0.005

0.009

0.007

0.004

0.005

0.070

0.008

0.008

0.007

0.11

0.03

EstimatedremainingHCs

0.26

0.19

0.22

0.20

0.25

0.23

0.24

0.26

0.09

0.14

0.26

0.26

vol%dry

H2Svol%dry

0.005

nana

nana

nana

nana

0.023

0.014

0.002

HClvol%dry

na0.02

0.0002

0.0002

na0.0002

0.0003

0.0029

0.0019

0.0002

0.0003

0.0001

NH3vol%dry

0.180

0.21

0.13

0.56

1.25

1.25

0.28

0.42

0.39

0.31

0.04

0.81

Ar(calculated)vol%dry

0.58

0.61

0.62

0.63

0.64

0.60

0.63

0.60

0.76

0.66

0.61

0.58

N2(calculated)vol%dry

60.47

60.42

60.60

62.92

61.01

59.47

63.22

60.46

73.68

66.58

64.35

61.45

H2O(calculated)vol%wet

14.26

9.90

8.14

12.34

10.37

8.36

13.54

12.58

17.74

16.79

12.30

10.32

Heavytars

mg=

m3 n

510

383

245

658

60273

241

537

2050

253

78Dustm

g=m3 n

1700

5000

6600

6400

6100

6000

8000

8500

3800

10400

8200

13100

HHVfuelgas(calc.)

4.92

4.77

4.93

4.41

4.64

4.90

4.51

5.13

2.05

3.28

4.97

4.61

MJ=

m3 ndry

Dryfuelgas ow(calc.)

151

139

142

141

171

206

160

150

133

160

128

131

m3 n=hdry

Wetfuelgas ow(calc.)

172

153

154

159

189

224

182

169

156

187

144

145

m3 n=hwet


Table4

Continued

Testno.

01

23

4a4b

56

78

910

FuelA

Willow

Demolition

Park

Chipboard

Verge

Verge

Demolition

Demolition

Woodyexcess

Parkwood

Railroad

Cacao

wood

wood

materials

grass

grass

wood+paper

wood+

fraction

(biodried)

ties

shells

residue

sewage

ofODW

sludge

sludge

Balancesandothervalues(calculated)

Totalmassbalance%

101.0

99.0

100.6

99.3

94.9

96.2

96.7

97.5

88.9

97.4

98.1

99.3

C-balance%

104.3

98.1

107.0

100.0

93.5

99.5

99.0

98.8

92.6

90.0

94.5

103.3

O-balance%

101.6

98.0

100.4

97.7

94.0

93.9

95.7

95.3

96.9

101.5

96.3

98.6

Inert-balance%

8868

4694

5866

4966

2164

8677

E-balance(HHV)%

102.2

96.0

104.9

99.5

91.8

97.5

97.1

99.3

87.0

88.4

98.5

109.5

CC%

97.0

91.6

91.7

92.2

94.7

92.7

88.2

85.2

98.4

96.5

92.4

85.9

CGE%

6658

6659

5864

5658

3747

6062

ER

0.37

0.36

0.38

0.38

0.37

0.36

0.37

0.32

0.60

0.43

0.34

0.34

Lineargasvelocitym=s

6.1

5.55

5.7

5.7

6.6

7.9

6.5

6.0

5.6

6.5

5.3

5.2

NH3=fuel-N%

7372

8265

8183

7346

5448

2742

H2S=fuel-S%

4939

504

Table 5Fraction of elements from fuel appearing in second cyclone ash(transfer coe�cients), na: not analysed

Test 1 Test 2 Test 7

Al 1.05 0.65 0.33As 0.59 0.41 0.43B 0.54 0.62 0.10Ba 0.66 0.85 0.50C 0.06 0.05 0.01Ca 0.53 0.65 0.52Cd ¡0:02 ¡0:13 0.15Cl 0.21 0.17 1.04Co 0.64 0.47 0.11Cr 0.49 0.23 0.06Cu 0.56 0.50 0.51F 0.10 0.13 naFe 0.81 0.68 0.48Hg na na ¡0:02K 0.91 0.53 0.32Mg 0.54 0.51 0.40Mn 0.49 0.64 0.61Mo ¡0:32 ¡0:28 0.52N 0.05 0.06 0.01Na 0.30 0.45 0.30Ni 0.55 0.34 0.01P 1.18 0.67 0.55Pb 0.17 0.02 0.06S 0.22 0.13 0.12Sb 1.66 0.34 0.59Si 0.98 0.25 0.34Sn 0.96 0.47 1.70Sr 0.66 0.58 0.48Ti 0.52 na 0.69V 0.74 0.71 0.37Zn 0.16 0.24 0.52

Table 6Concentration of PAHs (polycyclic aromatic hydrocarbons) andcyanides in the second cyclone ash of given tests

Test 1 Test 2 Test 7

PAHs 14.4 8.7 0.14Cyanides 5.8 3.4 6

therefore has a very low heating value, even lowerthan in test 0 with willow having a water content ofover 17% (but only 2% of ash).The measurement of ash fraction is extremely dif-

�cult for fuels where the ash content is high and theresult of the presence of sand, stones and other in-ert material likely to be present not homogeneouslyin the fuel. Because of the large inaccuracy in the ash


Table 7Leaching properties (mg=kg) of three di�erent second cyclone ashes determined according to Dutch standard NEN 7343. U0 and U1 arelimit values used in the Netherlands for judging the land�ll category. Marked grey are the concentrations exceeding limit U0

Element Limit U0 Limit U1 Test 1 Test 2 Test 7

As 9 9 0.09 0.15 0.16Ba 20 60 16.3 1.25 8.5Br 0.5 6 2.0 2.0 2.5Cd 0.05 0.2 ¡0:0002 ¡0:0003 ¡0:0008Cl 50000 50000 1070 1360 13600Co 2 6 0.0008 0.002 ¡0:001Cr 30 30 0.012 0.013 0.01Cu 7 10 ¡0:01 ¡0:01 ¡0:01F 25 280 0.35 1.0 0.11K — — 1360 4500 7200Mo 0.04 3 0.14 0.41 0.62Ni 8 10 ¡0:003 0.001 ¡0:005Pb 4 25 ¡0:006 0.012 0.01Sb 0.1 0.7 0.24 0.098 ¡0:004Se 0.02 0.3 0.003 0.017 0.008Sn 0.1 6 0.004 0.014 ¡0:002V 0.4 20 ¡0:02 0.17 ¡0:0005Zn 10 40 0.005 0.027 0.015Sulphates 80000 80000 31 31.5 310

Fig. 2. Calculated e�ect of moisture content (line 1) and ashcontent (line 2 and 3) of fuel on higher heating value (HHV) offuel gas. Line 3 simulates the addition of dry inert material (e.g.sand, stones, etc.) to the fuel while line 2 simulates the additionof wet inert material to fuel.

content measured in this kind of fuels, a predictionof gasi�cation behaviour and heating value of prod-uct gas is di�cult. This is mainly caused by the ef-fect mentioned above, i.e. the water content should bebased on the amount of organic material rather thantotal weight including ash.Fig. 3 shows the relation between the air=fuel ratio

(expressed as ER : equivalence ratio) and the higherheating value of the product gas. From the �gure it

Fig. 3. Higher heating value (HHV) of product gas from CFBfacility as a function of equivalence ratio (ER). The line representsa linear �t of the points.

becomes clear that the HHV of the product gas ismainly determined by the ER. This can be explainedby the fact that ER is not an independent parameterbut is the result of some important variables like thewater content, composition and heating value of thefuel, the total heat loss of the gasi�cation unit andto a lesser extent the ash content, the carbon con-version and the amount of inert purge gas used dur-ing gasi�cation. The concentration of hydrocarbons


Fig. 4. Relation between equivalence ratio (ER) and concentrationof some hydrocarbons in product gas. The lines represent linear�ts of the points.

in uences the heating value of the product gas becausethese components have relatively high heating valuesand as such “concentrate” the energy in a smaller vol-ume. The concentration of hydrocarbons depends verymuch on the amount of oxygen available and this inturn is related to the ER. In Fig. 4 the relation is shownbetween some hydrocarbons and the ER. This relationadds up to the e�ect that ER has on heating value ofthe product gas explained above since hydrocarbonshave relatively high heating values.The concentration of heavy tars in the product gas

depends, amongst others, on: temperature, equiva-lence ratio, H=C-ratio (mainly determined by watercontent of fuel) and fuel size. Besides these parame-ters, elements like K, Ca and C (char) may function ascatalysts for tar cracking reactions. From the resultsit appears that no straightforward relation can be de-duced between tar concentration and one of the afore-mentioned parameters. This means that either parame-ters like ER, fuel size and H=C-ratio more or less com-pensate each other or catalytic cracking of tars playsa major role. Furthermore, the tar concentration ofthe product gas from railroad ties is remarkably lowgiven the fact that the fuel itself contains as much as1.5wt% PAHs as a wood preservative. Apparently,these components do not survive the gasi�cationprocess. Measured tars probably are made duringgasi�cation. This also means that the concentrationof PAHs in the fuel should not be of interest whenassessing biomass as fuel for gasi�cation. Since rail-road ties contain very little (heavy) metals, even lessthan willow for almost all metals, railroad ties can

even be considered as clean wood when used as afuel for gasi�cation.The concentration of NH3 in the product gas is de-

termined by the amount of nitrogen present in thefuel. For the tests reported above about 60% (in aver-age) of the fuel-bound nitrogen is converted to NH3.Probably, a small part is converted to HCN and otherN-containing components. The remaining part is re-duced to N2. Others report similar fuel-N to NH3 con-versions [9,10].The CFB-gasi�cation facility used for the tests is a

small-scale unit compared to what usually is consid-ered as full scale for this kind of processes. The maindi�erences with full-scale installations are (1) the rel-atively high heat loss and (2) the relatively high to-tal ow of inert purge gases. Although heat losses arereduced as much as possible by good insulation andelectrically heated walls, still 40 kW (12% of the av-erage energy input) is lost in the small-scale unit. The ow of inert purge gases is about 12 m3

n=h, whichis 12% of the average amount of air used for thesmall-scale process. A reduction of the relative heatloss results in a signi�cant increase in heating valueof the product gas. This is caused by a direct in-crease of the energy content of the product gas. How-ever, most importantly the amount of air needed toreach the desired temperature is reduced: the ER re-duces. This results in an increase of the heating valueexpressed per m3 of product gas (the product gasyield decreases). Besides this, an extra increase canbe added because of the e�ect the decrease of ER hason the concentration of hydrocarbons. In total a re-duction of heat losses from 12 to 3% will result inan 35–40% increase of the heating value of the prod-uct gas. Simultaneously reducing the purge gas owfrom 12 to 5% results in an extra increase of 10%of the product gas heating value. In conclusion it canbe said that scaling-up will have a signi�cant e�ecton the heating value of the product gas. For exam-ple test #1, the higher heating value of the productgas might increase from 4:8 MJ=m3

n in the small-scaleCFB gasi�er to 7.0–7:2 MJ=m3

n for a full-scale unitwith 3% heat loss and 5% inert purge gas as a frac-tion of air ow. The minimum value for use in gasengines and gas turbines according to [11] is approx-imately 4.3 and 6.0, respectively. These however areestimates and depend on the composition. Large-scalesystems exist with gas turbines operating on gas with


Table 8Mean values (and range) of some important parameters for thetests reported

Parameter Mean Range

CC (Carbon conversion) (wt%) 92 85–97C-balance (wt%) 98 90–107Cold gas e�ciency (%) 61 56–66Fuel-N to NH3 conversion (%) 62 27–83Heavy tars (mg=m3

n) 270 20–660

lower heating values as low as 3 MJ=m3n [12,13].

This is not impossible when realised that gas turbinesusually operate with 1150◦C or lower turbine inlettemperature and the adiabatic ame temperature forstoichiometric combustion of the cold gas produced intest #1 already is 1360◦C. Even the cold fuel gas pro-duced in test #8 (HHV=3:3 MJ=m3

n) has an adiabatic ame temperature of 1050◦C.Metals considered as relatively volatile, like cad-

mium and lead, turn out to be present in only smallquantities in the ash from the second cyclone. Sincethe second cyclone is operated at around 800◦C thisis what might be expected. Potassium however, alsoconsidered to be volatile, turns out to be retained inthe ash for a considerable part. Also chlorine is re-tained in the ash for several tens of percents to even100% for test number 7. The measured concentrationof HCl in the gas is extremely low for the tests de-scribed. For all tests, except test #1, less than 15% ofthe fuel-bound chlorine is converted to HCl. The dis-crepancy between what is found in the gas and ashand what is coming in with the fuel can only be ex-plained by time dependent e�ects like accumulationin the bed or refractory lining, probably being the re-sult of the relatively short test period. This e�ect willbe subject to further research.The second cyclone ashes produced in three tests all

show relatively high leaching rates of Br, Mo and Sb.For this reason, Dutch legislation prohibits these ma-terials to be categorised as inert. Dumping on land�llsites only is allowed on special areas. It is interestingto observe that above mentioned elements are not theones having a high concentration and indicating theash as dangerous.For the reported tests Table 8 summarises the

mean values of the main parameters characterisingthe CFB-facility.

6. Conclusions

Circulating uidized-bed technology seems to bevery suitable for the gasi�cation of all sorts of dif-ferent biomass materials. The feeding system how-ever turned out not to be as exible as the gasi�cationprocess itself. Many practical problems were directlyor indirectly caused by disturbances in the fuel sup-ply. All feeding problems however were solved duringthe experimental programme by using new systemsor adjusting existing ones. Presently, ECN has thefacilities to test almost any fuel (or mixture) in the500 kWth circulating uidized-bed gasi�cation sys-tem. In the beginning of the year 2000, gas cleaningand a gas engine has been connected to the gasi�cationfacility.Gasi�cation of di�erent fuels resulted in di�erent

fuel gas properties. However, water content is one ofthe most dominant fuel characteristics when outputparameters as carbon conversion, cold gas e�ciencyand heating value of fuel gas are concerned. Forgood comparisons it turned out that the water con-tent can better be de�ned on an ash-free basis ratherthan the usual way: based on total mass includingash. However, parameters not strongly relating tothe energy balance, like for example tar concentra-tion in the fuel gas and conversion of fuel-nitrogento NH3, do seem to be very dependent on processconditions. Agglomeration is also strongly relatedto the fuel fed to the installation. For this phe-nomenon to investigate however, short-term tests arenot suitable since accumulation (long-term) e�ectsare dominant. Tar concentration in the fuel gas doesnot seem to be dependent on the amount of PAHsintroduced in the gasi�cation reactor. This might beconcluded from the relatively low (heavy) tar con-centrations in the fuel gas when using railroad ties asa fuel.The ashes produced and retained in the second

cyclone show considerable concentrations of car-bon. This strongly reduces the concentrations of forexample metals which are used to indicate whetherits dangerous waste or not, according to the Dutch“BAGA”-law. Leaching behaviour seems to be a bet-ter characteristic when judging an ash for further use.This is supported by the fact that the elements havinga high leaching rate, and as such are “available”, arenot the ones present in high concentrations.


Acknowledgements

The Dutch agency for energy and environment(Novem) and the Dutch company NV Afvalzorg aregreatly acknowledged for their (�nancial) contribu-tion to both the erection of the gasi�cation facilityand the following research. We also thank Stork En-gineers and Contractors for their contribution to thedesign and construction of the CFB facility.

References

[1] Dutch Ministry of Economic A�airs. Third EnergyNote; Programme Renewable Energy (Derde energie nota,actieprogramma duurzame energie in opmars), the Hague,Netherlands, 1997.

[2] Arts PAM, Van Beek A, Benner JHB. Bio-massterclass;An overview of materials and a concept for priceindexation (Een overzicht van de stromen en een aanzettot prijsindexering), Novem-EWAB report no. 9916, Novem,Utrecht, Netherlands, 1999.

[3] Heemskerk GCAM. Best practice list for biomass fuel and ashanalysis. Novem-EWAB report no. 9820, Novem, Utrecht,Netherlands, 1998.

[4] Description of the CFB gasi�cation plant at ECN on theinternet: http:==www.ecn.nl=biomass=bivkin=index.html.

[5] Gigler JK, Bosma AH. Energy plantation; harvestingfast growing wood for energy (Energiegewassen;oogst van snelgroeiend hout voor energiedoeleinden),Landbouwmechanisatie 1995;12:34–5.

[6] Biomass and waste composition database on the internet:http:==www.ecn.nl=Phyllis.

[7] Hughes WEM, Larson ED. E�ect of fuel moisture content onbiomass-IGCC performance. Journal of Engineering for GasTurbines and Power 1998;120(3):455–9.

[8] Besluit Aanwijzing Gevaarlijk Afval (BAGA). StaatscourantStb. 663, 8 December 1997.

[9] Kurkela E, St�ahlberg P. Air gasi�cation of peat, woodand brown coal in a pressurized uidized-bed reactor.II. Formation of nitrogen compounds. Fuel ProcessingTechnology 1992;31(1):23–32.

[10] De Jong W, Andries J, Hein KRG. Pressurised uidised-bed co-gasi�cation of coal and biomass. In: Biomassfor energy and industry. In: Kopetz H. et al. editors.Proceedings of the 10th European Conference and TechnologyExhibition, C.A.R.M.E.N. Rimpar, Germany, 1998.p. 1781–4.

[11] Van Ree R, Oudhuis ABJ, Faaij A. Curvers APWM.Modelling of a biomass-integrated-gasi�er=combined-cycle(BIG=CC) system with the owsheet simulation programmeASPENplus. Study performed within the framework ofthe extended Joule-II+ programme of EC DGXII contractJOU2-CT93-0397 “Energy from biomass: An assessment oftwo promising systems for energy production”. ECN-Report:ECN-C–95-041, ECN, Petten, Netherlands, 1995.

[12] Mukherjee DK. Experience with unconventional gas turbinefuels, In: Sipilae K, Korhonen M, editors. VTT Symposium164: Power production from biomass II with special emphasison gasi�cation and pyrolysis R and DD, VTT Energy, Espoo,Finland, 1996, p. 93–107.

[13] Mulder J, Havenaar P, Santen D. Blast furnace gascuts costs at IJmond 1. Modern Power Systems, 1996,September.

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