characteristics and availability of biomass waste and

Characteristics and availability of biomass waste and residuesin the Netherlands for gasification. Biomass and BioenergyCitation for published version (APA):Faaij, A., van Doorn, J., Curvers, T., Waldheim, L., Wijk, van, A., & Daey Ouwens, C. (1997). Characteristics andavailability of biomass waste and residues in the Netherlands for gasification. Biomass and Bioenergy. Biomassand Bioenergy, 12(4), 225-240. https://doi.org/10.1016/S0961-9534(97)00003-2

DOI:10.1016/S0961-9534(97)00003-2

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https://doi.org/10.1016/S0961-9534(97)00003-2

https://doi.org/10.1016/S0961-9534(97)00003-2

https://research.tue.nl/en/publications/4b3ee013-11d4-40a0-b784-ecc5cb5bc165

~ Pergamon Biomass and Bioenergy Vol. 12, No. 4, pp. 225-240, 1997

(c3 1997 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

P I I : S0961-9534(97)00003-2 0961-9534/97 $17.00 + 0.00

CHARACTERISTICS AND AVAILABILITY OF BIOMASS WASTE A N D RESIDUES IN THE NETHERLANDS FOR

GASIFICATION

ANDR~ FAAIJ,* JOEP VAN DOORN,t TOINE CURVERS,t LARS WALDHEIM,:~ EVA OLSSON,:~ AD ~ VAN WIJK* AND CEES DAEY-OUWENS¶

*Department of Science Technology and Society, Utrecht University, Padualaan 14, NL-3584 CH. Utrecht, The Netherlands

¢Netherlands Energy Research Foundation, P.O. Box l, NL-1755 ZG, Petten, The Netherlands ~Termiska Processer AB, S-61182, Nyk6ping, Sweden

¶Province of Noord-Holland, P.O. Box 3088, 2001 DB, Haarlem, The Netherlands

(Received 16 September 1996; revised 10 January 1997; accepted 20 January 1997)

Abstract Characteristics and availability of biomass waste streams and residues for power production by means of integrated gasification/combined cycle technology (BIG/CC), are evaluated for The Netherlands. Four main categories are investigated: streams from agriculture; organic waste; wood; and sludges. Altogether 18 different streams are distinguished. Gross availability and net availability are inventorized. Various properties (composition, heating value, supply patterns) are analysed and the suitability of these streams for conversion in a BIG/CC unit is studied. The costs at which various streams are likely to be available are assessed. The gross energetic availability amounts annually to approximately 190 PJ (HHV) primary energy. Because of competing useful and higher value applications than fuel of various streams, such as fodder and fertilizer, the net availability is slightly less than 90 PJ (HHV). For a number of streams the costs are negative due to present waste-treatment costs. Costs of waste streams vary from - 10-5 ECU/GJ. For a small fraction the costs are higher than for energy crops (estimated to be approximately 4.5 ECU/GJ). Because there are large variations in properties and contaminants between various streams, the conversion system needs to flexible when a diversity of streams is treated in one installation. Some streams require mixing with cleaner fuels to make them suitable for use in a direct atmospheric biomass integrated gasifier/combined cycle system. Important technical limits for the use of biomass fuels in the system studied, are the moisture content (maximum 70% of wet fuel) and ash content (maximum 20% dry matter content) of the fuel. © 1997 Published by Elsevier Science Ltd.

Keywords--Biomass wastes; biomass residues; availability; characterization; costs; gasification; electricity production.

I. INTRODUCTION

Since The Ne the r l ands is a densely popu la t ed count ry , it p roduces subs tan t ia l quant i t ies o f o rganic wastes and residues in numerous s t reams. The present t r ea tmen t routes for these s t reams are re-use as compos t , fodder or fibre boa rd , landfi l l ing (which is cur rent ly dominan t ) and incinera t ion. As landfi l l ing o f organic mater ia l s will be p roh ib i t ed in the near future, waste t r ea tment capac i ty needs to be increased. The use o f these b iomass waste s t reams fo r energy p roduc t i on in an in tegra ted gasif icat ion/ c o m b i n e d cycle ( B I G / C C ) system, m a y prov ide an env i ronmenta l ly fr iendly and economica l ly a t t rac t ive a l ternat ive , since such systems p r o m - ise high convers ion efficiencies and po ten t ia l ly low capi ta l costs per capac i ty unit instal led. This t echnology can also con t r ibu te to a reduc t ion in CO2 emissions in tha t it util izes the energet ic

po ten t ia l o f b iomass wastes and residues more efficiently than mass b u r n i n g ?

Wi th regard to power p lan ts fired with b iomass and b iomass wastes, the character is t ics o f the fuel are very impor t an t . Because the compos i t i on o f the b iomass mater ia l s and degree o f c o n t a m i n a t i o n vary widely, the p re - t r ea tmen t and convers ion systems need to be flexible. In add i t ion , it should be no ted that the costs o f the fuel and the required p re - t r ea tmen t can be ma jo r factors in overal l electricity p roduc t ion costs.

To al low an assessment o f the possibi l i t ies and cons t ra in ts o f the use o f var ious b iomass wastes and residues in an in tegra ted direct gasif icat ion c o m b i n e d cycle p lant , insight into the avai labi l i ty , costs, compos i t i on and technical l imi ta t ions o f the convers ion technology is required. The object ives o f the pape r are,

225

226 A. FAAIJ et al.

therefore, to identify and quantify these characteristics for organic wastes and residues in The Netherlands.

The feasibility of gasification of various biomass waste streams and residues for electricity production will be discussed in general terms. In this paper the focus will be on direct atmospheric circulating fluidized-bed gasification coupled to a General Electric LM 2500 gas turbine, as described in Elliott and Booth, 7 and Faaij et al?

First, the applied methodology is described. Results are presented regarding the availability of biomass streams in The Netherlands. This is followed by an evaluation of the cost ranges of the biomass streams. The composition and degree of contamination are given for a representative selection of streams. Then the technical limitations of an integrated atmospheric gasifier coupled to a combined cycle are discussed in relation to fuel properties. Uncertainties are discussed and conclusions are drawn about the gross and net energetic potential of biomass streams that are at present available for gasification in The Netherlands.

2. METHODOLOGY

To determine characteristics of biomass waste streams we investigated the four following aspects: availability and supply patterns; costs; composition and degree of contamination; and technical limitations for applying biomass waste for energy production in a BIG/CC unit. Four main categories of biomass wastes and residues are considered: agriculture; organic waste; wood; and sludges. These categories are subdivided into 18 streams. The investigation will be focused on the situation in The Netherlands in 1994.

2.1. Availability and supply

Gross availability of organic waste and residues is investigated by means of production statistics for agriculture, forestry and waste in 1994.

Net availability is determined by correcting the gross availability using data on useful applications per stream, such as utilization as fodder or fertilizer and re-use of waste wood. Volumes of waste are translated into energy potential by means of (both higher and lower) heating values per stream.

Time supply patterns are investigated for each stream by the analysis of seasonal effects, such

as harvest time and maintenance activities of public parks, which are undertaken only during specific periods. Next, the supply during (part of) the year is described. On this aspect detailed data are not available, therefore, it is assumed that the available volume of a stream is released in certain months in equal quantities per month. The time-dependency of supply is an important aspect to take into account when assessing the use of waste in larger BIG/CC units that are expected to function all year round (baseload power production).

2.2. Costs

An inventory of the waste treatment costs per waste stream and of the market value of the organic material is compiled. The costs are determined either by the present costs of waste treatment (such as composting, landfilling, incineration),~. ~0 which results in negative costs of the biomass stream, or by the market prices of the material if used in other applications (such as fertilizer, raw material, etc.). Costs are given in ECUI994 and derived from literature or personal communication. Market prices can vary strongly over time because of varying supply and demand),~7.3° Therefore, the costs of biomass streams are expressed in ranges compiled of the minimum and maximum costs resulting from the inventory. In The Nether- lands the cost of biomass from energy farming is projected to be approximately 4.5-6 ECU/GJ (HHV). 16 When the cost of a "waste" stream appears to be higher than energy crops because of its high value for alternative applications, it is considered unavailable for energy production purposes.

Transport costs from producer to conversion plant are not included in this analysis.

The 1994 situation in The Netherlands will be presented as supply curves that illustrate the minimum and maximum costs per (net available) GJ containect in each biomass waste stream. This gives insight in to what cost band biomass streams are likely to be obtainable. It should, however, be kept in mind that a large demand for biomass waste to produce energy could increase prices.

2.3. Composition and degree of contamination

The chemical composition, degree of contamination, moisture and ash content and calorific value of the waste streams are determined. Moisture and ash content are determining parameters for the heating values

Biomass wastes and residues for gasification 227

of the biomass materials. Higher heating values are derived from both characterization experiments and literature.

Experiments were performed to determine the chemical composition of a selection of biomass streams. These streams are verge grass, waste paper, demolition wood, sewage sludge, cacao shell (being a waste stream from the food and beverage industry), organic domestic waste (ODW: separately collected organic waste from households and services) and for comparison miscanthus and willow. The experimental characterization is described in detail in van Doorn. 6 Carbon, hydrogen, nitrogen and sometimes sulphur are determined in a single analysis step. Chlorine, fluorine and sulphur are measured in a bomb calorimeter. The moisture content and the volatile matter content are determined gravimetrically. Ash is characterized according to standard procedures. Furthermore, initial deformation, softening,

*The lower heating value of a biomass material is calculated by the following formula: LHV = HHV~ry*(I - W) - E~*(W + H*mmo) in which E~ is the energy required for evaporation of water (2.26 MJ/kg), W the moisture content, H the hydrogen content (wt% of wet fuel) and m._,o the weight of water created per unit of hydrogen (8.94 kg/kg).

hemispherical and fluid ash deformation temperatures are measured. To determine metal contents ash samples are dissolved and analysed according to standardized procedures.

The streams considered represent a large diversity of biomass fuels. In some cases, if data are lacking, some properties are estimated by comparison with similar streams.

The higher heating value (HHV) represents the heating value of the dry biomass material including ash. Lower heating values are calculated from the higher heating values and the moisture content*. With respect to contaminants the focus is on nitrogen, sulphur, chlorine and heavy metals, since these are most relevant with respect to emission standards.

2.4. Technical limitations

The physical properties of the organic waste are compared with the demands and limitations of options to utilize this Waste. Here, focus lies on BIG/CC technology. Fuel criteria relevant for this technology involve moisture content, ash content, density (because of handling and feeding), heating value, ash fusion temperature, halogen content (because of corrosion and emissions) and heavy metal content (because of emissions and potential application of residues).

Table 1. Organic waste streams and residues investigated in this analysis. The last column indicates the time of year at which the supply peaks. Main data sources on biomass materials are de Jager et al . , ~ Mocking et al . , ~7 Siemons et al . "-~

and Sikkema? ° Other sources are mentioned per stream

Stream Description Supply pattern

Agriculture In general residues from agriculture are used in their entirety as fodder, fertilizer, etc. A number of residues are therefore not included in this analysis

Straw Residue from cereal production; considerable fluctuations July September in supply and price per year can occur. Composit ion data and prices stem from NOVEM L9 and Siemons 2s

Bulb cultivation Includes several streams such as straw, plant material and Peaking May June peelings from bulbs. Composit ion data of this waste stream lack average moisture and ash contents are given in Stoop ~' the heating value of the organic fraction is assumed to be similar to straw. Almost all waste is composted in either central facilities or at farms

Greenhouse Residues from greenhouse culture. Remains of crops Peaking mixed with some plastics. Composit ion data are obtained September November by personal communication. Almost all waste is composted in central facilities 2 Prunings from fruit trees including uprooted trees and stubs. Compost ion of this woody stream is estimated by assuming a somewhat higher ash content compared with clean wood. Other composition data are similar to thinnings. Residues are either left on the land, partly composted or have a market value as fuel wood t~. ~4, ~s Organic waste from auctions, plant-like material (flowers, vegetables) mixed with packaging materials. Composit ion data with respect to moisture and ash are reported in Mocking? 7 Auction waste is both composted and landfilled at present

Fruit farming

Auction

Peaking June August

Irregular, increase in summer

c o n t i n u e d

228 A. FAA1J et al.

Table" 1 C o n t i n u e d

Stream Description Supply pattern

Organic wastes Households

Services

Swill

Verge grass

Waste paper

Food and beverage industry

Wood Thinnings

Prunings

Industrial waste wood

Demolition wood

Wood products Sludges

Waste water treatment

Maintenance of waterways

Separately collected organic fraction of domestic waste. Contains remains of vegetables, fruit, plants and garden waste. Composition data are reported in a background report, ~70DW is largely composted and partly digested. Separately collected organic fraction. Mainly contains remains of food processing. This stream is, like ODW, largely composted and digested. The composition is assumed to be equal to ODW since they are treated together in the same facilities" Food remains released at restaurants, hospitals, etc. Very wet material with a moisture content of approximately 80%. Part of this waste is discharged by the sewer system, other fraction is composted or digested 39 Mowed grass released during maintenance of road sides. Information about supply, use and compostion is taken from Siemons38 Verge grass either has a useful application as fodder or has to be removed in order to be landfilled or composted 24 Surpluses of separately collected old paper which are not used for paper production. The volume strongly 3s varies with market price and demand for raw material. An estimate of the supply is given by van Doorn. 5 Composition data are taken from van Doorn. 6 Residues from food processing industries. Highly variable composition per industry. Streams cover sludges, remains of food crops, processing waste (e.g. sugar, tobacco and beer production, meat and fish). Several streams are 100% re-used as fodder or in other applications. An overview of this waste stream category is given in de Jager et al. 9 For this overview only waste streams are included with a moisture content below 80%. Those streams are often digested or have a useful application that does not require additional treatment 9

All year with increase in spring and autumn

Constant

All year

Peaking in May-June and September-October

All year

All year. Some streams are released during seasonal production

By-product of commercial forestry. Compostion data are taken from van Doorn 6 and Sipkens. 33 Prices vary according to demand for pulp wood and other applications. A cost range is obtained from Sipkens) TM

Woody stream released during maintenance of public parks, belts, etc. Mainly consists of wood, partly leaves and some other waste. Composition is assumed to be equal to forest thinnings but with a slightly higher ash content. Most cleaner wood is shredded and left in the parks or has some value as fuel w o o d ]7'29'31

Wood released during wood processing. Generally clean and dry material. Prices, avialability and utilisation of various waste wood categories are discussed in Knoef,12. i~ Okken et al . , 2) Renia and Sikkema 27 and Sikkema? Average moisture content for waste wood is 15% and very low in ash. We use a broad price range for all waste-wood categories determined by waste treatment costs (incineration and landfilling). High quality waste wood is excluded from this overview since it has a high value and considered unavailable for energy production. Wood released in the building sector. Demolition wood is partly re-used, but largely landfilled or incinerated at presenP Discarded wood products, like pellets, furniture, etc. Manure and sludges from industrial processes are not included in the analysis. Sludge produced at waste water treatment facilities. Initially very wet. De-watering and/or drying is applied. High ash content and contaminated with heavy metals. Average composition data are taken from 5'6. Sludge is at present incinerated, dried, landfilled and composted. Cost data are taken from 3s. Mowing waste released during maintenance of waterways. Consists of reed, grass, plant material with high fractions of sand and mud which are sometimes contaminated. Information about moisture and ash content is taken from The material is either left on the side of waterways or in some cases removed and compostedfl 2'23

Delivery all year; storage in woods

Peaking April-June and September-November

All year

All year

All year

Constant

Peaking in summer


Gasification is assumed to take place in an atmospheric circulating fluidized-bed with a tar cracker as the first gas cleaning step (similar to TPS technology). 7 Lab-scale fuel reactivity experiments are performed with a selection of biomass streams and resulting gas compositions of the fuels are derived with the help of model calculations. These experiments are reported in detail in Lassing et al. ~5 In this paper technical limitations concerning ash and moisture content and various contaminants will be evaluated in general terms only.

3. RESULTS

3.1. Availability

The biomass residues and waste streams that are taken into account for this analysis are described in Table 1. Streams are divided into four categories: agriculture; organic wastes; wood streams; and sludges. In general, biomass residues from agriculture on arable land are all re-used as fodder or fertilizer. 22, 23,29 Only some straw remains available.

The category "organic waste" covers streams that are collected separately, such as organic domestic waste (ODW). Organic waste from the food and beverage industry is also included in this category. Sludges cover sewage sludge and organic material released during the maintenance of waterways. Manure, which is a substantial stream in The Netherlands, is not considered here because its dry matter content is too low for thermochemical conversion. Furthermore, the surplus depends on a number of factors that are due to change in the near future (such as reduction of stock and different treatment technology). 35

The results for all the streams considered regarding the gross and net availability, lower and higher heating values and energy potential are given in Table 2. Moisture and ash content are given per stream; the values mentioned represent average values but in some cases they can cover a wide range. 6' ~7,20

Estimates of ash and/or moisture content are made for the waste streams from bulb cultivation and the greenhouse sector, swill, fruit farming and sludge from the maintenance .of waterways. For simplicity values for the food and beverage industry are averaged to avoid great complexity, since this category includes a large number of specific streams. The total annual gross energy supply amounts to

approximately 190 PJ (HHV). The net supply is slightly less than 90 PJ (HHV).

The main periods (months) in which streams are released are indicated in Table 1. Figures 1 and 2 present the energy supply (expressed in higher heating value) of the above-mentioned biomass residues and wastes in The Nether- lands. Figure l shows the total energy supply of available biomass streams per month. Figure 2 shows the energy supply for those streams that are especially bound to seasons.

Overall the energy supply peaks between spring and autumn. The minimum and the maximum supply varies from approximately 6-9 PJ per month. A large fraction of this supply comes from streams of which the supply remains more or less constant during the year, such as from the food and beverage industry and organic domestic wastes. When it is considered desirable to use streams from agriculture, verge grass, prunings, etc. for energy production, one has to take into account that the supply of those streams during winter months will be negligible, as illustrated by Fig. 2.

3.2. Costs o f biomass residues and wastes

The results of the cost inventory of biomass waste streams and residues are given in Table 3. Negative costs represent the current waste treatment costs, positive values represent the present market value of the residue. For comparison the projected biomass production costs of energy crops (willow and miscanthus) in The Netherlands are also given/* In these figures transport costs to the conversion plant are not included. Figure 3 shows supply curves of net available biomass streams to illustrate the distribution of costs relating to energy available in biomass streams. The upper curve represents the maximum costs at which waste streams could currently be obtained, the lower curve the minimum costs. In reality the costs of biomass wastes will be somewhere in between the two curves and show a more gradual progressing curve moving from one type of biomass to the other. However, such detailed data were not available.

Negative costs may serve as a source of income to the conversion plant because it serves as a waste treatment facility when it converts such biomass materials into energy.

The costs mentioned generally do not involve pre-treatment (size reduction and drying), which is required for the gasifier coupled to a

Tab

le 2

. P

oten

tial

, av

aila

bili

ty,

ener

gy c

onte

nt a

nd h

eati

ng v

alue

s of

bio

mas

s w

aste

str

eam

s an

d re

sidu

es

Moi

stur

e A

sh

HH

V~r

y(G

J/

LH

V~e

,(G.1

/ K

ton/

ycar

d (%

wet

mat

eria

l)

(% d

ry m

ater

ial)

to

n)

ton)

PJ

Luv

/yr

Gro

ss

Net

G

ross

N

et

PJ.

Hv/

yr

Gro

ss

Net

Agr

icul

ture

Str

aw

800

400

15

10

18

14

! 1.1

5.

5 12

.2

6. l

Bul

b cu

ltiv

atio

n 26

0 26

0 60

10

18

5

1.4

1.4

1.9

1.9

Gre

enho

use

100

100

80

1 20

2

0.2

0.2

0.4

0.4

Fru

it f

anni

ng

200

200

50

5 19

8

1.5

1.5

1.9

1.9

Auc

tion

14

0 14

0 60

10

18

5

0.7

0.7

1.0

1.0

Hou

seho

lds

Org

anic

was

tes

1000

10

00

60

20

IL6

4 4.

4 4.

4 6.

4 6.

4

Serv

ices

20

0 20

0 60

20

16

4

0.9

0.9

1.3

1.3

Swil

l 10

0 10

0 80

1

19

2 0.

2 0.

2 0.

4 0.

4 V

erge

gra

ss

500

400

60

10

18

5 2.

6 2.

1 3.

6 2.

9 f&

b in

dust

ry b

3000

55

0 10

15

19

16

47

.1

8.6

51.3

9.

4

Was

te p

aper

a 80

70

3700

60

1

19

6 45

.5

20.9

61

.3

28.1

Woo

d

Thi

nnin

gs

1600

11

00

50

1 20

8

13.0

9.

0 16

.0

11.0

P

runi

ngs

330

230

50

5 19

8

2.5

1.8

3.1

2.2

Indu

stri

al w

aste

woo

d 25

0 50

15

1

19

15

3.7

0.7

4.0

0.8

Dem

olit

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woo

d 42

5 20

0 15

1

19

15

6.2

2.9

6.9

3.2

Wo

od

pro

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s 37

5 30

0 15

2

18

14

5.2

4.2

5.7

4.6

Slu

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Was

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tre

atm

ent ¢

30

0 30

0 0

40

14

13

3.8

3.8

4.2

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2600

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3 1.

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ilab

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ar.

Tot

als

152

69

189

87

~Ave

rage

val

ues

are

give

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r m

oist

ure,

ash

and

hea

ting

val

ues.

C

Exp

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be

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dry

for

m.

Act

ual

moi

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e co

nten

t de

pend

s on

the

deg

ree

of d

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g, d

ryin

g, e

tc.

dFig

ures

wit

h re

spec

t to

ava

ilab

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y, p

oten

tial

moi

stur

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var

ious

str

eam

s st

em f

rom

nu

mer

ou

s so

urce

s gi

ven

in T

able

1

per

stre

am.

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heat

ing

valu

es a

re d

eriv

ed f

rom

lit

erat

ure.

In

case

s w

here

no

valu

es w

ere

avai

labl

e an

est

imat

e w

as m

ade

by o

n th

e ba

sis

of

com

pari

son

wit

h a

sim

ilar

bio

mas

s m

ater

ial.

Dif

fere

nces

, ho

wev

er,

in h

ighe

r he

atin

g va

lues

of

dry

biom

ass

are

larg

ely

caus

ed b

y di

ffer

ence

s in

min

eral

fra

ctio

n (a

sh c

onte

nt)

and

not

by d

iffe

renc

es i

n ch

emic

al c

ompo

siti

on (

oxyg

en,

carb

on a

nd h

ydro

gen

cont

ent)

sin

ce t

hese

are

com

para

ble

for

alm

ost

all

biom

ass

mat

eria

ls.

Exc

epti

ons

to t

his

are

pape

r an

d sl

udge

. rT

he l

ower

hea

ting

val

ues

are

calc

ulat

ed b

y th

e fo

llow

ing

form

ula:

LH

Vw

e, =

HH

Vd~

y*(1

-

W)

- E

w*(

W +

H

*mm

o) i

n w

hich

Ew

is

the

ener

gy r

equi

red

for

evap

orat

ion

of w

ater

(2.

26 M

J/kg

),

W t

he m

oist

ure

cont

ent,

H

the

hydr

ogen

con

tent

(w

t%

of w

et f

uel)

and

m,2

o th

e w

eigh

t o

f w

ater

cre

ated

per

uni

t o

f hy

drog

en (

8.94

kg/

kg).


10

9

8

7

6

~ s 4

3

2

1

0 Jan Mar

I,I May Sep Nov

I I

ll, il Jul

Feb Apr Jun Aug Oct Dec

Months

Fig. 1. Summated potential of energy supply from net available biomass wastes and residues in The Netherlands

(PJHHv) during the year.

combined cycle and considered part of the conversion.

However, in case of some woody streams (prunings, fruit farming, waste wood) chipping is applied as part of the harvest and collection process of the material. When delivered in chipped form the price of the material will be in the upper part of the cost range presented in Table 3. 30

A large part of the sewage sludge produced at waste-water treatment plants in The Nether- lands is and will be dried with conventional rotary driers (to approximately 10% moisture content). Part of the total sludge supply is, therefore, available in dry form. Other options available for water removal are mechanical dewatering (by means of a sieve-band press or filter press) which can reduce the moisture content down to 50%. 35 Dried sludge still needs to be landfilled or combusted, thus, although the volume is strongly reduced by water

3.5

3.0

2.5

2.0 O..

1.5

1.0

0.5

0

Straw m Fruit farming E3 Bulb cult ivation

Verge grass m Greenhouse

sector E3 Prunings

, , ,[-7,[1, , Jan Mar May

Feb Apr Jun Jul Sep Nov

Aug Oct Dec

Months

Fig. 2. Energy supply during the year of net available biomass wastes and residues of which the supply is bound

to seasons.

removal, the negative value per ton remains. in Table 1 figures of the available dry sludge are presented.

3.3. Composition and degree o f contamination

The composition of the biomass material has consequences for the capacity and nature of the gas cleaning, the composition of the ashes, the composition of the waste water stream from the scrubber and for the emissions from the entire system. The nitrogen, chlorine and sulphur content and contamination with heavy metals of a representative selection of streams are shown in Figs 4 - 8 . Those data are largely derived from characterization experiments and partly from literature. A more detailed description of the composition and characterization experiments of various biomass wastes and residues is given in van Doorm 6 and Mocking et al. ~7 Such extensive sets of data are not available for all the streams mentioned in Table 1. However, the selected streams clearly show the range of various components for a wide range of fuels from sludge to clean wood. The most important components with regard to the legislation and standards on emissions to air are heavy metals, nitrogen (NOx), chlorine and sulphur.

More attention is given to the technical and economic consequences of these variations in the evaluation of the conversion system with various fuels in van Ree et al. 26

Figure 4 shows a BIG/CC-system based on atmospheric direct gasification. Fuel enters the system in the pre-treatment section where the incoming fuel is reduced to the required particle size and dried to a moisture content of 15%, which is considered acceptable for gasification. In the system the fuel gas is cleaned by various gas cleaning steps: tar cracking; cooling; baghouse filter for removal of particles and alkalis; and a wet scrubber for the removal of mainly ammonia. The latter also removes the water present in the gas since it condenses during the scrubbing. In some cases additional gas cleaning might be necessary in order to meet emission standards.

The degree of contamination differs widely per stream and between streams. This has consequences for the required capacity of the gas cleaning system, especially when a conversion unit is to be fuelled with various fuels.

3.3.1. Nitrogen. A high nitrogen contents is found in verge grass and sludge (up to 7 wt% of dry matter, see Fig. 5). Miscanthus and willow

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Biomass v, astes and residues for gasification

nitrogen content

233

-10

1 s'T' 0 10 20 30 40 50 60 70 80 90

PJ (HHV~

Fig. 3. Cost supply curves of the net available energetic potential of biomass wastes and residues in The Netherlands in P J/year. The upper curve represents the maximum costs at which various streams could be available, the lower curve

the minimum costs.

Fig. 5. Nitrogen content of a selection of biomass sources (presented in ranges). Tick marks present average values

when available.

have the lowest nitrogen content (up to 2% for miscanthus). Nitrogen in the biomass is mainly converted to ammonia which is removed in the scrubber. A higher nitrogen content will directly result in a higher ammonia concentration in the waste-water stream. This can have some effect on the waste-water treatment costs and also on the required capacity or" the scrubber. It might be possible to add an acid to

the scrubber water to improve ammonia removal.

3.3.2. Chlorine. Chlorine content is especially high for verge grass and to a lesser extent in organic domestic waste (see Fig. 6). The chlorine concentration observed vary widely between samples. For verge grass the use of salt on roads to abate icing can be a reason for the high chlorine contents. A higher chlorine

BIOMASS + Solids .......... Water/steam ~ gas.

Sizing& I~ll cleanm 9 . . . . . . Gas screening ~ - ~ - ~ . . . . . .

Ash Gas cooling

Generator ~ ~ C~!t r ~ - - - ~ C ° n e ~ ° ~ - - I

. . . . . FLel;;s co~mpressor <" Fig. 4. Scheme of an integrated direct atmospheric gasification combined cycle system based on TPS technology. After pre-treatment biomass is gasified. Tars are cracked in a secondary reactor. Further fuel-gas cleaning involves cooling in order to condense alkalis, which are removed together with dust by a baghouse filter. A wet scrubber removes mainly ammonia and some remaining contaminants. The gas is then compressed and combusted in a gas turbine. The hot flue gases generate steam in a heat recovery

steam generator in order to drive a steam turbine.

234 A. FAAIJ et al.

concentration can also be found close to the sea.

Chlorine can cause corrosion in the BIG/CC system because of the formation of HCI during gasification. In gasification experiments it is observed that the dolomite which is applied in the tar cracker will react with chlorine to form CaC12 and will lead to a higher dolomite consumption. The adsorption of HC1 is considered to be comparable with the level of adsorption in waste incineration facilities and is generally high; adsorption levels of more than 900 can be expected. HC1 will also react with particulates which are removed later by the baghouse filter. Nearly all of the chlorine can be removed from the gas as a result of reaction with lime. 15 Any HC1 that remains will dissolve in the scrubber water.

3.3.3. Sulphur. Generally speaking the sulphur content in biomass is (very) low. However, some streams, especially sludge but also organic domestic waste and verge grass, show a higher sulphur content than woody streams and energy crops (see Fig. 7).

Sulphur can cause corrosion problems when sulphuric acid is formed in the heat-recovery steam generator, although this can be solved by keeping the gas temperature sufficiently high. Sulphur (which is largely converted to H2S during gasification) will react to some extent with dolomite to form CaS. In Lassing et al. ~5 it is concluded that in the case of various fuels (sludge, verge grass, organic domestic waste) the H2S concentration can reach an equilibrium concentration in the fuel gas. This concentration is too high to meet emission standards for waste incineration? 6 Additional sulphur removal may, therefore, be necessary in order to meet SO2

sulphur content

Fig. 7. Sulphur content of a selection of biomass sources (presented in ranges). In the case of organic domestic waste one value only is available and no range can be presented.

standards for flue-gas emissions, especially for sludge. This can be done by adding a basic to the scrubber water (possibly with a two stage scrubber).

3.3.4. Heavy metals. Very wide ranges of heavy metal concentrations are found per biomass material and between biomass streams. Generally speaking sludge contains very high heavy metal concentrations, although this depends largely on the source of the sludge. Levels are especially high for zinc, copper, chromium and nickel. Waste wood shows, again depending on the source and sample, relatively high concentrations of heavy metals too (e.g. for copper, lead and zinc, see Fig. 8).

Some of the heavy metals will remain in the gasifier ash, although they will partly evaporate in the gasifier, particularly those with a low melting point (cadmium, mercury, lead)(Fig. 9). The extent to which heavy metals will end up in

U

chlorine content

Fig. 6. Chlorine content of a selection of biomass sources (presented in ranges).

v 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 . 1 2 3 4 5 6 7 8

Fig. 8. Cu, Pb and Zn concentration ranges of a selection of biomass streams

Cu, Pb, Zn concentrations


As, Cd, Cr, Ni concentrations 120

100

80

E

E 40

20

0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

Fig. 9. As, Cd, Cr and Ni concentration ranges in a selection of biomass streams.

the fuel gas during gasification differs from combustion process conditions. The temperature in the fluidized-bed of the gasifier is relatively high compared with combustion temperatures. Furthermore, the atmosphere is reducing in contrast to the oxidizing conditions that prevail during combustion. This reducing atmosphere promotes the evaporation of heavy metals in metallic form. 4°

However, metals present in the gas stream, pass the tar cracker and are cooled to 140°C in the gas cooler. Generally, the metals present will condense (like the alkali metals) on particulates and be removed by the baghouse filter. The distribution of heavy metals over the two ash fractions is related to the behaviour of the metals in the hot reducing atmosphere in the gasifier and has not been studied in detail. Emissions will, however, be very low due to the cooling and filter followed by wet scrubbing of the fuel gas.

3.3.5. Ash. The ash production in the gasifier and the baghouse filter differs per fuel. When the ash fraction of a fuel contains a great deal of sand (heavy particles) most of the ash is removed from the gasifier. When the ash content is low (such as for clean wood) more fly ash is produced. Also the dolomite consumption (e.g. by reaction with chloride and sulphur) varies per stream influencing the ash production.

The distribution of the total ash production can be 30% from the baghouse filter and 70% from the gasifier. In the case of materials with a low ash content and little or no sand content, the proportion of the total ash in the fly-ash stream will be relatively large. It is assumed that

the gas cleaning system proposed, which includes cooling to near ambient temperatures is sufficient to remove alkalis. Therefore, limited attention has been given to alkalis in this paper. In van Doorn 6 alkali contents (Na + K + Li) are reported ranging between approximately 1000 mg/kg dry matter for clean wood up to 5000 mg/kg dry matter for certain sludge samples.

3.4. Suitability of biomass residues and waste streams .['or gasification

Gasification is possible with a broad range of fuels with varying properties. Generally speaking, fuels with a higher moisture and ash content will give a fuel gas with a lower heating value. With the selected gasification technology the relations between the heating value of the fuel gas and moisture and ash contents are given in Figs 10 and 11. The reason why a decrease in heating value occurs with a higher moisture content is that part of the heat released during partial combustion in the gasifier is needed to evaporate the water. This also implies that more air is fed to the gasifier whenever more water evaporates because more energy is needed for evaporation and, consequently, more fuel needs to be (partially) combusted. Both the evaporated water and additional air (nitrogen) dilute the gas which causes a decrease in the heating value of the gas.

Decreasing heating values are also observed when the biomass fuel has a high ash content. The ash of the incoming fuel will be heated to gasification temperature (800-900°C). More ash implies that more fuel needs to be combusted to

LHV Irv~m3(n)]

v , q I L ~ , I

10 20 30 40 50

moisture in fuel [%]

Fig. 10. LHV of fuel gas produced by a directly heated ACFB gasifier as a function of the moisture content (using data of poplar wood with an ash content of 1.3%). Wet gas represents the gas composition including water present in the biomass and evaporated in the gasifier and water formated in the gasification process itself by partial oxydation. The graph is compiled with help of a gasifier

simulation model (courtesy of TPS).

236 A. FAAIJ et al.

L~/ 7

[M,J/n'~(n)l 6 dry gas

; I t t 1

10 20 30 40 ash in fuel [% on dry substance]

Fig. 11. LHV of the fuel gas produced by a directly heated ACFB gasifier as a function of the ash content (using data of poplar wood with a moisture content of 15%). Wet gas represents the gas composition including water present in the biomass and evaporated in the gasifier and water formated in the gasification process itself by partial oxydation. The graph is compiled with help of a gasifier

simulation model (courtesy of TPS).

heat the inert material, thus lowering the heating value of the fuel gas.

Figures 10 and 11 give the relation between the LHV of the fuel gas and moisture and ash content of the fuel, respectively. They are composed by using a gasifier model. 15 Curves for both wet and dry gas (in which the water is condensed out) are given. For the system described, the wet gas represents the gas stream leaving the tar cracker, the dry gas is the same gas on a dry basis. The moisture content of the gas entering the gas turbine will lie between these values. The exact value will depend on the degree of cooling and resulting condensation of water vapour, which in turn will depend on the temperature in the scrubber.

*The reasons for selecting this turbine are: (1) it is under development for low calorific gas firing in connection with the World Bank GEF project in Brazil; 7 (2) the size of a BIG/CC unit based on the LM 2500 is approximately 30 MWe, which is still a reasonable capacity from the point of view of the availability of fuel and transport distances; and (3) this turbine, being an aeroderivative, combines a high efficiency with a high turbine outlet temperature, which is required to achieve a high overall combined cycle efficiency.

tMaximum allowable concentrations in the flue-gas flow to the expander of the turbine are 4 ppbw (parts per billion weight) for alkalis (Na + K + Li), 600 ppbw for particulates < 10/~m, 0.6 ppbw for 10-13 #m particulates and 0.6 ppbw for > 13 Fm particulates? 6

:~Assuming that energy from the flue gas is used for drying and that a conventional rotary dryer is involved (requiring approximately 3 GJ/tonmo evaporated). Fuel-gas temperature is approximately 250°C, the temperature of the gas leaving the stack is just over 100°C. Overall electrical efficiency is 40% on a LHV basis. More efficient drying systems such as fluid-bed dryers and (indirect) steam dryers, can lower the heat demand and can cope with somewhat wetter fuels.

Figures l0 and I I are composed with composition data of poplar wood. The limits given are approximate and are valid for similar fuels.

The fuel gas quality has to meet strict demands to be suited for gas-turbine firing. The most important restriction is the heating value of the fuel gas which is required for stable combustion. A General Electric LM 2500 is the selected gas turbine (described in more detail in van Ree et al?6) * in this research project.

The GE LM 2500 gas turbine which uses low calorific gas has to satisfy the following conditions: 25

1. The heating value of the gas for which stable operation of the turbine is guaran- teed must be at least 5.6 MJ/Nm 3

2. Acceptable variations in heating value of the fuel gas amount to _+ 5% of the Wobbe index: (5.5-5.7 MJ/Nm 3)

3. The flue gas must be largely free of particles and alkalis to prevent excessive wear and corrosion of the gas turbine blades.t

Figure l l shows that biomass streams with an ash content higher than 15-20% (moisture content of incoming fuel 15%) are not able to fulfil the first requirement. For direct gasification the moisture content of a given fuel needs to be lower than 20%. A higher moisture contents will result in heating values below gas turbine limits, as is shown in Fig. 10.

Several waste streams have a moisture content of 70% and above. When biomass fuels are dried with waste heat from the stack, there is sufficient heat to process fuels with a moisture content below 70- 80%.:~ 8 Waste streams with a higher moisture content require a different waste treatment process, such as anaerobic digestion or need t o be mixed with drier material. This would be the case for several waste streams from the food and beverage industry, sludges, swill and wastes from the greenhouse sector and bulb cultivation. A fuel that combines a high moisture content with a high ash content is not suited for gasification unless the material is dried beforehand. Such fuel properties can be found in sludges.

Besides the moisture and ash content, the elemental composition is a factor that affects the heating, value, although the differences between various kinds of biomass are small.

Pre-treatment of streams such as grinding,


densification and drying can ensure that the fuel has the required moisture content and physical properties, but this will increase the costs. Mixing of fuels can also be seen as a pre-treatment option to achieve the required fuel properties.

4. DISCUSSION

4.1. Availability and supply

Many types of biomass waste and residues have been considered in this paper. Some of them are excluded for energy production. One criterion for exclusion is the market price of the waste stream. When the price is significantly higher than the projected costs for energy crops such streams are not considered to be available for energy production. However, it should be noted that market prices are sometimes subject to strong fluctuations. When there is a surplus of biomass streams with a relatively high value, fractions of such streams can become of interest for energy production. This can occur with some categories of wood and straw.

The wastes produced by the food and beverage industry form a complex category. To deal with this complexity, difference was made between very wet and dryer streams. Very wet streams, with a moisture content over 70-80% are not counted in the streams potentially available for thermochemical conversion. How- ever, the diversity in materials is not reflected in the data presented here. Therefore, before application all streams should be studied and analysed separately, but this is beyond the scope of this study.

There is uncertainty about the availability of biomass materials that are derived from natural reserves, such as turf from heath and reeds which are harvested for several reasons. All this material is presently used for other purposes such as fertilizer. However, they could represent a future potential. 34

The periods mentioned in this study during which a number of biomass residues and wastes are released are not as strict as stated. In practice supply spreads over longer periods and also variations in harvest time occur from year to year. At the same time, however, those variations make a more exact presentation of supply not very useful.

The time dependence of the supply of biomass streams differs strongly per material. The supply of a number of types of biomass, such as straw,

remains of bulb production and the greenhouse sector, fruit farming, thinnings, verge grass is coupled to the seasonal production in agriculture or to maintenance activities. Most of this material is released from May to September. When such streams are used for conversion in a power plant in base-load operation, substi- tutes must be found from other sources part of the year. Such back-up fuel is desired anyway because the supply will differ in time and in quantity from year to year because of weather conditions. Storage is mainly suited to woody material, preferably dry, because otherwise decomposition will lead to significant losses of organic matter when stored over longer periods. The main conclusion is that for a number of streams year-round operation of a conversion plant requires input of other fuels for at least part of the year. The low supply in winter can be compensated by using energy crops such as willow, which is harvested in winter or by using waste-wood streams, which are available all the year round. Consequently the system should be capable of dealing with the varying properties of a diversity of other biomass material(s).

4.2. Costs

The costs of biomass streams are hard to determine since they partly depend on market conditions, and an increase in demand for a given material can cause in increase in prices. The costs given in this paper represent the present situation in The Netherlands. The given ranges, which are often wide, illustrate the large variations that can occur in biomass costs. More insight is needed in to the way these costs are influenced by changing and fluctuating demand, especially when these streams are to be converted into energy on a large scale.

The lower values of the costs as presented in Fig. 6 are the absolute minimum at which at present organic waste streams and residues are available in The Netherlands. These costs are based on the present costs for waste treatment and reflect the current situation in waste treatment in The Netherlands. Waste treatment costs may change substantially in the future? Both lower and higher costs are possible. Important in this context is the ban on landfilling organic materials which will come into force in the near future in The Netherlands. In fact, this can push up the costs for waste treatment further (and thus lead to more negative biomass costs) because the present

238 A. FAAIJ et al.

alternative is incineration, which is considerably more expensive than landfilling.

One criterion selected to evaluate the potential of biomass wastes and residues, was that streams more expensive than the projected costs for energy farming are not available for energy purposes. When developments in energy farming result in a significant decrease in the costs per unit of energy, this can also influence (decrease) the amounts of waste to be utilized for energy production. On the other hand, if energy crops compete economically with streams such as wood from thinnings the price of these residues might drop if the supply of energy crops is sufficiently large. A more detailed analysis of this subject is required.

4.3. Composition and contamination

The best way to present our data on the composition and degree of contamination of biomass wastes and residues is to express them in ranges and not as average values, since the composition also depends on the time of year and location where the stream is produced (e.g. chlorine is likely to be present in higher concentrations in verge grass near the sea6). The combination of both literature sources and experiments gives insight in to observed ranges of the composition of various biomass streams. It should, however, be kept in mind that it is not always clear from some sources what characterization procedures have been applied. Furthermore, the origin of the biomass samples for characterization may have a strong influence on the compositions found due to the hetero- geneity of specific streams. The ranges found in composition data should, therefore, be considered merely as indications of the possible variations instead of absolute values.

A given conversion plant will have to meet environmental standards irrespective of whether it uses clean or contaminated sources of biomass. The characterization results, therefore, provide important input for an environmental assessment of energy production from biomass and for the design and selection of the gas cleaning technology t h a t is required to meet specific standards. Chlorine does not seem to be a problem in the gasification process considered. High sulphur and nitrogen contents might mean that there is a need for additional gas cleaning steps. Some wastes when utilized will produce ash streams that are substantially contaminated with heavy metals (sludge).

The ash melting temperature of biomass is

not discussed in this paper, although it can be an important restriction on its use in gasifiers. However, initial ash deformation temperatures for a wide range of possible biomass streams as inventorized in van Doorn 6 and Lassing et al/5 indicate that most fuels have higher ash melt temperatures than the gasification temperature of 800-900~'C. More detailed testing may, however, be required to assess the behaviour of fuels with high ash contents at high gasification temperatures.

4.4. Gasification o f waste streams

This paper focuses on direct (atmospheric) gasification in a BIG/CC unit. The limits obtained here for the moisture and ash content of the biomass in order to obtain fuel gas with a high enough heating value for the gas turbine do not apply to indirect or oxygen blown gasification processes, because of the medium heating value fuel gas that is obtained from such processes.

The ash and the moisture content are the most important parameters that determine whether gasification and application of the produced fuel gas in a gas turbine is possible. The boundaries for acceptable moisture and ash content of the fuel entering the gasification system depend, on one hand, on the demands of the gas turbine and, on the other hand, on drying technology and heat recovery from the ash. The properties of the fuel gas have to satisfy fixed criteria to enable gas-turbine firing. The drying has to be such that the fuel entering the gasifier does not exceed 15-20%, because otherwise the heating value of the gas becomes too low to satisfy gas turbine requirements. Another factor lowering the heating value is the ash content. This may (partly) be prevented by recovery of heat from the ash, which can be achieved by a counter-current fuel-feeding system, but this optimization option is not investigated further here.

Adaptations to the gas turbine, especially the combustion chamber, can be made to achieve a higher tolerance to fuel gas with even lower heating values. Such modifications will, in turn, enable gasification of fuels with a higher ash content in BIG/CC systems.

In general the data presented on biomass waste streams and residues represent the current situation in The Netherlands. However, the technical potential, availability a n d costs of various streams are n o t static. Future developments can affect the production and availability


of various streams. It is beyond the scope of this paper to make projections for the future availability of all the above-mentioned streams, since this will depend on the economic activity in various sectors and changes in land use (e.g. the total land used for cereal production and forestry).

5. CONCLUSIONS

The current gross energetic potential of organic waste and residues in The Netherlands is substantial, namely about 190 PJ (HHV) per year. In practice this potential can be used only partially for energy purposes due to alternative applications, such as raw material, fodder and fertilizer. However, even if these factors are taken into account, the energetic potential remains significant, namely about 90 PJ (HHV) per year.

Some streams are not available in winter months. Application of such streams in a base-load power plant requires compensation in winter. Peaks in the supply of specific types of biomass can be levelled either by storage (mainly suited for drier woody material) or by using other fuels during part of the year. Energy crops, usually harvested at the end of the year, could prove necessary in a situation where a large part of the potential biomass waste and residues is used for energy production, in order to compensate for decreasing supply of biomass wastes and residues in winter months.

An important criterion for selecting a fuel is its cost. The inventory for 1994 showed large differences in the costs per GJ of various organic wastes and residues. These varied from - 1 0 ECU/GJ to above the projected cost levels of energy crops in The Netherlands (5 ECU/GJ).

The conversion system, particularly in relation to the gas cleaning, has to be flexible when a variety of fuels is used in view of the variations found in compositions. In this respect the removal of nitrogen and sulphur is important. Chlorine seems to cause less problems because it is removed along with the ash streams.

When used in a BIG/CC system for power production, fuels with a high ash content ( > 20%) have to be mixed with cleaner material. This is the case for sludges and possibly organic domestic waste. Very wet streams ( > 70% m.c.), such as organic streams from the greenhouse and bulb cultivation

sector, swill and food and beverage industry need to be mixed with drier materials. Drying is always required since no biomass stream has a moisture content lower than 15% from origin.

The results can serve as input for assessments of the technical, economic and environmental aspects of gasification of organic waste in The Netherlands. The presented methodology could be useful in specific regional studies designed to evaluate the feasibility of new biomass conversion units to be fired (partly) with various types of waste and residue.

Acknowledgements--The authors are grateful to CEC DG XII for the sponsoring this project within the framework of the EC JOULE II + programme. Co-sponsoring was provided by the Noord-Holland gasification project and NUTEK. They thank Prof Dr W. C. Turkenburg for critical comments and suggestions. The authors are grateful to Sheila McNab for linguistic assistance.

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characteristics and availability of biomass waste and

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