harvesting of cotton residue for energy production

Harvesting of cotton residue for energy production

T.A. Gemtosa, *, Th. Tsiricogloub

aLaboratory of Farm Mechanisation, University of Thessaly, Pedio Areos, 38334 Volos, GreecebTEI of Larissa, 41110 Larissa, Greece

Abstract

The possibility of collecting cotton stalks in Greece and using them for energy production was investigated. The

production and properties of cotton stalks were studied and a system for collection of the aerial part is proposed asa feasible solution to avoid wet conditions under the local climate. A successful method for collection andpackaging of the residue was applied, using conventional but highly advantageous equipment, o�ering reduced

investment cost and use of existing machinery. The energy required to harvest cotton stalks was measured by aninstrumented tractor. The tractor was able to measure the developed forces between tractor and implement, thepower absorbed through the PTO, as well as tractor velocity and fuel consumption. The energy consumed for theoperation was calculated and when compared to the energy of the biomass collected gave a positive balance. The

work proved the feasibility of harvesting cotton stalks using conventional machinery giving the possibility to collectenergy material with a total energy content of 500,000 tons of oil equivalent at national level. # 1999 ElsevierScience Ltd. All rights reserved.

Keywords: Biomass; Cotton stalks; Residue harvesting; Energy

1. Introduction

During recent decades, biomass use for energy

production has been more and more proposed as

a substitute for fossil fuels. Biomass, as a zero

CO2 emission fuel, can o�er an immediate sol-

ution in the reduction of CO2 atmosphere con-

tent. Although energy crops can o�er a basis for

larger energy producing plants, the use of crop

residues can o�er a more immediate source of

biomass for energy production in small installa-

tions. The economics of a system which produces

energy from crop residue highly depends on the

cost of collection, transportation and storage of

the raw material. It is probably true that

specially constructed machinery could give, in the

long run, the best results. However, at the initial

stages of the residue use the purchase of new

equipment would increase the cost of operation

with an unknown acceptance by the end user.

Therefore the best solution to promote biomass

utilisation is to employ, when possible, existing

equipment for the collection of the raw material.

Cotton is cultivated in Greece in more than

400,000 hectares [14]. It is harvested by cotton

pickers between the end of September and the

beginning of December leaving stalks in the ®eld.

Biomass and Bioenergy 16 (1999) 51±59

0961-9534/99/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved.

PII: S0961-9534(98 )00065-8

PERGAMON

* Corresponding author. Tel. 30-4216-9781; Fax: 30-4216-

3383; E-mail: [email protected].

Climatic data in the cotton growing plains ofGreece show that there is considerable rainfall atthe end of the harvesting period. Greek farmersusually chop the stalks by cotton shredders andthe residue is then incorporated into the soil byploughing. Only a few farmers leave the stalksuncut and drill winter cereals without anytillage [1]. The latter is mostly applied duringvery wet years. Cotton stalks are a residue, whichis left unused for the time being, but has the po-tential to be used as biomass for energy pro-duction or as raw material for other industries.

Ebeling and Jenkings [2] studied the physicaland chemical properties of di�erent crop resi-dues. For cotton stalks the following values wereobtained: higher heating value 15.83 MJ/kg; vol-atile 65.40%; ash 17.30%; carbon fraction17.30%. The stoichiometric analysis of the stalksgave C 39.47%, H 5.07%, O 39.14%, N 1.20%,S 0.02%, residue 15.10%.

Sumner et al. [3] have reported, the gross heatof combustion to be 18.1 MJ/kg and 18.4 MJ/kgfor dry cotton stalks and roots respectively.Sumner et al. [4] have done a study of the par-ameters needed for the design of a cotton stalkpuller. They quoted from Demian an averagepulling force to uproot cotton stalks of 903 Nwith maximum 1188 N and from Colwick anaverage uprooting force of 489 N. They reportedthat the cotton stalk mean diameter was 13 mm(12±14 mm). The average pulling force wasfound [4] to be 317 N (256±373 N). Sumner etal. [5] measured the moisture content of thestalks (average 42.7% wb, range 23±62.3% ) andof the roots (average 60.8% range 53±64.4%).Dry matter yield was 4.42 t/ha with range of 3±7.04 t/ha. Roots were 23.2% of the whole plantin average with the measured values rangingbetween 14.3 and 29.1%. In the same experimentthe moisture content was found to drop from50% to under 20% when the stalks were left inthe ®eld, after uprooting, for three weeks.Sumner et al. [4±6] have suggested a method toharvest cotton stalks by uprooting them. Theyconstructed a machine with two rubber wheelsturning in opposite directions. The wheelstrapped the stalks and, as they turned, uprootedthem. The stalks were then left on the ground

surface to dry and were collected after remaining

there for two weeks. The soil which remained on

the roots when uprooted was dried and mostly

removed due to the movement of the balling ma-

chinery. This drying period permitted the collec-

tion of the residue quite free of soil. However,

there was no exact estimation of the amount of

the soil collected with the residue. Similar pro-

cesses were also used by Kemp and Matthews [7]

and the NIAE in Sudan and by others [4].

Coates [8] reported results of a research of cotton

stalks harvesting systems in Arizona, USA. He

used two machines that undercut the plants

before they pulled them o� the ground, leaving

most of the root in the soil. One system chopped

the material immediately after pulling while the

other left it on the ground before chopping or

packaging it. Packaging was carried out by

round balers or seed cotton module builders.

Coates found the e�ective cotton stalks yields to

range from 2.34 (chopped by ¯ail mower) to 5.7

t/ha (for hand harvested). Soil contamination

ranged from 0.9 to 7.7% while total energy

required for harvesting ranged from 47.9±52.1

kWh/ha or 8.6±10.9 kWh/t dry matter.

In the literature cited cotton stalks were har-

vested by uprooting them. In most cases, the

local climate allowed su�cient time for the

uprooted plants and the soil stacked on the roots

to dry. As a result of these conditions, the cotton

residue could be collected dry enough for storage

and without signi®cant soil impurities. In Greece,

however, it is unusual to experience continuous

periods of dry weather as required by the pre-

viously described procedure. In anticipation of

the wet period of the year, farmers plough their

®elds for the next crop as soon as cotton is

picked, so that their ®elds are not too wet for

ploughing. It is well known that ploughing under

wet conditions causes compaction that deterio-

rates the physical properties of soil and adversely

a�ects crops [9].

In order to investigate the possibility of using

cotton stalks for energy production under Greek

conditions, a research programme was underta-

ken. The objectives of the programme were:

T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±5952

(a) to quantify the yield of cotton stalks and itspotential in Greece;

(b) to study the physical properties of cottonresidue;

(c) to identify and propose a procedure for har-vesting cotton stalks;

(d) to investigate the possibilities for storing thematerial in a simple and economical way;

(e) to study the energy budget and the economicsof harvesting cotton stalks; and

(f) to investigate possible utilisation methods ofenergy production by burning.

The present paper reports some of the resultsobtained during the project, which was fundedby the Greek Ministry of Education.

2. Material and methods

2.1. Theoretical and preliminary tests

The size of the stalks, their distribution, theirproperties in the ®eld and the yield were studiedafter seed cotton picking. Rows of 2±10 m length(cotton is cultivated in rows of 0.95 to 1.00 mapart) were studied in di�erent ®elds in centralGreece. The diameters of the stalks at soil leveland at a height of 0.05 m from the soil level (theheight a mower cuts the stalks) and their heightwere measured. The plants in a row were thenuprooted and without any disturbance of the soilstacked on the roots were placed in bags. Thebags were weighed in a laboratory. Then thestalks were removed from the bags and sampleswere taken from di�erent plant parts for moist-ure content determination. They were placed inan oven to dry at 728C for 48 h. The soil wasthen removed from the roots by washing it withwater and its percentage in the total weight wasestimated. The Higher Heating Value was esti-mated from samples taken from freshly uprootedplants. The measurements were made using anIKA 400 adiabatic bomb calorimeter. The ma-terial was dried in an oven and divided intoroots, stalks and branches and bolls. Then a millground it and 1 g samples were formed by aspecial small, hand operated press. The material

was burnt in the oxygen enriched atmosphere ofthe bomb of the calorimeter.

Additionally, uprooting of the stalks was car-ried out to study the uprooting forces. Uprootingwas performed using a small vice and thehydraulic system of a tractor. Each stalk wasclamped in the vice and pulled out of the soil bythe upward movement of the three-point linkageof the tractor. A mechanical balance did theforce measurement and it was recorded by avideo camera.

Based on observations of the amount of thesoil stacked on the roots during ®eld work, itwas decided to investigate the possibility of col-lecting only the aerial part of the residue, leavingthe roots in the ®eld. It was anticipated that thecollected material would be free of soil and withless moisture content. These factors would makeits storage easier and its use for energy pro-duction by burning more attractive.

Based on the analysis and the measurements ofthe strength of cotton stalks and the frictionproperties between cotton stalks and mowerknife [10] the feasibility of using existing farmmachinery was investigated. The cutting resist-ance encountered by the mower during hay cut-ting (grasses or legumes) is estimated fromKepner et al. [11]. They quoted from Elfes anaverage PTO power for cutting mixed hay of 1.9kW at a speed of 7.9 km/h. Part of this power,0.89 kW, was due to cutting resistance of theplants. From measurements of cotton stalksstrength, carried out during the presentinvestigation [10], the following equation givingthe energy required for cutting the stalks as afunction of their diameter was found:

WORK=4.71+0.78*DCUT with r2=0.69where: WORK is the energy required to cut

the stalk in J,DCUT stalk diameter at a height 0.05 cm from

the ground in mm.According to this equation a stalk with base

diameter of 10 mm requires 12.51 J to be cut.Based on the plant base diameter distributionfound from the ®eld measurements and assuminga cotton plant population of 100,000 per hectareor 10 plants per meter on the row, the totalwork which is needed for cutting 2 m of row is

T.A. Gemtos, T. Tsiricoglou / Biomass and Bioenergy 16 (1999) 51±59 53

approximately 750 J. Given that only two rowscan be cut down by a 1.70 m working widthmower, the total power requirements for a work-ing speed of 2 m/s, are 1500 J/s or 1.5 kW. Thismeans that the mower will encounter resistanceof the same order cutting two rows of cotton ascutting grass at a width of 1.70 m and the samespeed. The di�erence will be in the concentrationof the load, which in the case of the cotton stalksharvesting, will be only applied on two knives. Itwas concluded that a mower with reciprocatingknives could be used without the risk of anydamage, apart from a minor possibility ofdamage to the knives due to the concentration ofthe load. To illustrate this, a reciprocating knifehay mower was used to cut cotton stalks of tworows at a speed of about 2 m/s. The machineworked well, with a slight deterioration of theknives. That means that the mower should beused with stronger knives, possibly with teeth.The stalks cut by the mower were left on the soilsurface. A rotating head rake was used to collectmore material in one row. The rake worked welland collected four rows in one, in one run.Actually only two rows were moved on top ofthe other two.

From the beginning of the project, the use of ahay baler for small square bales which waswidely used in the area, was rejected for packa-ging the cotton stalks. It appeared rather di�cultfor stalks with high moisture content to be fedinto the compression chamber and to be cut bythe ram knife. Additionally the material had highdensity, which would cause heating and destruc-tion of the material. Some preliminary testsshowed the di�culties in using it. Two additionalpossibilities were also investigated. The ®rst wasa special machine which gathered vine pruningsand packed them into round bales of 0.50 m indiameter and length. In the ®rst experimentalyear, the stalks used for the analysis of theirproperties were fed into the machine.

The machine worked well and formed bales ofabout 10 kg. The bales were loose enough andthe moisture content of the stalks dropped fromabout 40% to under 20% w.b. in less than 20days without any warming of the material. Balesleft outdoors proved that they absorbed water

easily after a rainfall but under Greek climaticconditions they dried again within 10 days. Thesebales could be handled easily by hand and fedinto a bunch shape biomass boiler. In the secondyear, the machine was used to collect materialimmediately after cutting. It proved that therewere a lot of blockages due to the high moisturecontent of the stalks which blocked the pick-upand made baling impractical.

The second machine used successfully was aClaas Rollant 44 round baler with a ®xedchamber and metallic rollers. The wrapping ma-terial was a plastic net. The machine worked wellduring the second year just after the cutting ofthe stalks, producing bales with dry matter ofabout 190 Kg. The bales were 1.20 m in diameterand in length and could be handled by a fronttractor end fork lifter and transportable by aplatform.

2.2. Field trials

A ®eld application of the method was carriedout in 1994 in order to prove the feasibility ofharvesting cotton stalks by conventional machin-ery, assess their performance and measure theenergy required. Harvesting was applied in a 1.4ha ®eld using the machinery shown in Table 1.The particular ®eld is typically representative ofthe area.

During ®eld work the performance of the ma-chinery used was monitored and their e�ciencywas determined. An instrumented tractordescribed by Gemtos and Tsiricoglou [12] andTsiricoglou and Gemtos [13] was used to measurethe forces developed during the work, the power

Table 1

Farm machinery used for harvesting cotton stalks

Machinery Speci®cation

Tractor Two wheel drive, 50 kW

Mower Reciprocating knives, 1.70 m width

Raker 1.70 m width

Round bales baler Bales 1.2 m in diameter and

height, with plastic net wrapping

Fork lifter Mounted on tractor

Platform Carrying up to 8 bales


requirements and the fuel consumption. The trac-tor had six loading cells (three measuring hori-zontal forces, two vertical and one side forces)measuring the forces in the space. Additionally atorque and rotation frequency meter wasinstalled on the PTO shaft of the tractor tomeasure the power absorbed through PTO.Analogue signals after ampli®cation were con-verted to digital by an A/D converter andrecorded by a portable PC. Samples were takenat 1000 s/s for each transducer. A fuel dischargemeter and a radar type linear velocity meter wereinstalled. Mean values of fuel consumption andspeed were given on a liquid crystal display as amean for each run. Based on the recordedmeasurements the energy budget of cotton stalksharvesting was determined.

3. Results and discussion

3.1. Theoretical and preliminary results

The results of cotton stalks size, water contentand yield, as well as the soil stacked on the rootsduring uprooting, are summarised in Tables 2±4,for two years of measurements. It is clear thatcotton stalks were quite di�erent during the twostudied years. It is also clear that large amountsof soil were stacked on the roots when pulled byhand. If collected immediately the movement willremove a part of the soil caused by the harvest-ing equipment. Even if most of the soil isremoved a considerable amount will remain andbe collected with the plants, which would even-

tually cause problems in the energy conversionplant. This e�ect is indicated by the reportedresearch, where the stalks remained in the ®eld

for long periods not only for the stalks to drybut also the soil, which then would be easilyremoved. The results of the higher heating valuemeasurements are shown in Table 5. In Tables 5and 6 the yield of the stalks and the roots arepresented. Total yield averaged 3,144 kg/ha, ofwhich 2,547 kg/ha were the aerial part. Meanwater content at the end of the harvesting period

Table 2

Size of cotton plants and their distribution in the ®eld (in mm)

Year Diameter Diameter Diameter Diameter Distance Distance Height of Height

at soil at at 0.5m at between between plants of

level soil height 0.5m plants plants mm plants

mm level mm height mm mm mm

mm mm

mean st dev mean st dev mean st dev mean st dev

1st 11.7 1.9 9.5 1.6 NR NR NR NR

2nd 14 3.3 11.9 3 14.1 5.3 934 134

Table 3

Distribution of the stalk diameter at a height of 0.05 m

Diameter range Frequencies % Frequencies %

in mm 1st year 2nd year

5±6 6.2 0

6±7 7.5 0

7±8 7.5 13.3

8±9 18.8 6.7

9±10 28.8 13.3

10±11 10 14.7

11±12 17.6 34.7

12UP 3.7 17.4

Table 4

Amount of Soil stack on the cotton roots when uprooted

a/a Soil weight on Soil Percent of the soil

the root Moisture on the weight of

content the whole plant-soil

g %

Mean 1377 23.6 69.3

Range 200±2419 22.9±24.1 58.0±79.1


was 41.5% for the aerial part and 54.8% for theroot. The reported dry matter yields of cottonstalks varied from 2.3 to 5.7 t/ha with water con-tent mean 34.9% and range 23.3±41.4% [8].Sumner et al. [6] reported yields ranged from 3to 7 t/ha (including roots) with water contentsranging from 23.2% to 62.3% for stalks andfrom 53% to 64.4% for roots in the periodbetween mid November till end of February.The Greek results are similar to the results ofSumner et al. [6], showing that a considerableamount of energy could be produced by cottonstalks, which can give about 1250 kg of dieselequivalent per ha or a total of 500,000 tons of oilequivalent for the country. Total Greek energyconsumption in 1991 was 22,214,000 tons of oilequivalent [14].

The results of the uprooting force from twoyears of measurements are shown in Table 7.According to the results obtained, when cottonresidue is harvested by uprooting the plants, a

large amount of soil will be collected with itunless a rather long drying period in the ®eld isavailable. It appears to present a problem withGreek farmers who cannot delay soil tillage forthe next crop until the soil stacked on the rootsis dry enough to be easily removed. Althoughduring collection of the stalks some of the soilwill be removed at any case, a considerableamount will remain stacked. This amount of soilwill eventually cause problems in the work ofany burner and of any system for biomass con-version.

3.2. Field trial results

During ®eld trials the experiment ®nished inone day. Fifteen bales were formed weighing4923 kg of fresh material (2880 kg of dry matter)with a water content at 41.5%. During the workthe performance of the equipment was monitoredas well as their power consumption. The resultson the performance of the machinery as well astheir energy consumption during the work are

Table 5

Higher heating value of cotton residue (mean values for

measurements of several years)

plant part Dry matter Higher heating Moisture

yield value content

kg/ha MJ/Kg %

Bowls 440 17.5 25.4

Stalk and branches 2107 18.1 44.80

Total aerial part 2547 18.0 41.5

Roots 597 18.3 58.40

Total aerial part 3144 18.05 44.71

Table 7

Uprooting force for cotton stalks

Year a/a Mean diameter Mean diameter Pulling

soil level 05 m height Mean

mm mm N

1st Mean 11.7 9.5 333

Range 11.5±11.7 9±10 278±389

2nd Mean 14.2 11.8 385

Range 13.8±14.8 11.4±12.3 376±392

Table 6

Cotton residue production. Samples collected by hand

Variable Mean Range

Dry matter yield of stalks kg/ha 2578 1168±3391

Dry matter yield of roots kg/ha 566 301±690

Stalks moisture content % mid November 50.29 39.7±64

Root moisture content % mid November 60.6 52.9±69.5

Stalks moisture content % mid December 41.5 36.9±52.0

Root moisture content % mid December 54.8 47.8±62.5

Energy content of dry aerial part MJ/ha 45,836.7 21,024±61,038

Energy content of dry root MJ/ha 10,925.1 5,508±12,627

Energy content of total dry material MJ/ha 56,762


given in Tables 8 and 9. Table 8 presents the

e�ective speed of work, as measured in the ®eld.

Pulling power was calculated from the mean

horizontal force developed between tractor and

implement and the mean linear velocity of the

tractor during the run was measured by the

radar. Power from the PTO was calculated from

the torque and the rotating frequency of the

PTO shaft of the tractor. Fuel consumption was

the mean for each run. Speci®c fuel consumption

was calculated from the fuel consumption and

the total useful power produced by the tractor. It

is clear from Table 8 that a smaller tractor

should have been used for part of the work. This

underused power has caused a high speci®c fuel

consumption. This indicates the di�culty of

obtaining optimum results during ®eld work

because there is rarely a close ®t between power

requirements of the equipment and the power of

the tractor. In Table 9 the performance of the

machinery is given. Field e�ciency coe�cients

are generally small. For the mower and the raker

Coates [8] found higher e�ciencies for the ma-

chinery used. Hunt [15] gave the e�ciency coe�-

cient range for mowers as 75±89%, for rakers

62±89%, for ¯ail mowers 50±76% and for balers

65±85%. The values of the present work are at

the lower end of the range of the literature which

is reasonable given the small harvested area. In

larger areas, operator experience gained during

the work would increase e�ciency. The e�ciency

of the round baler given by Coates is rather high

for conventional machines unless a non-stop

model was used which was not made clear in the

Table 8

Energy consumed during cotton stalks harvesting

Machinery Speed Pulling Power in Fuel Speci®c

km/h power PTO consumption consumption

kW kW l/h g/kWh

Hay mower 8.2 3.21 2.35 7.7 1100

Raker 8.25 2.90 2.60 6.3 930

Baler beginning 6.5 1.13 2.71 5.2 1083

Baler end of cycle 6.5 6.88 2.81 6.5 542

Baler mean 6.5 4.01 2.76 5.85 812

Transportation 10.1 10.4 Ð 4.8 370

Table 9

Performance of machinery used for cotton stalk harvesting in a ®eld of 1.4 ha

Machine working power theoretical e�ective e�ciency energy consumption energy consumption energy consumption

width required ®eld ®eld coe�cient based on measured based on measured based on measured

measured capacity capacity power requirements fuel consumption fuel consumption

by tractor

m kW ha/h ha/h MJ/1,4ha MJ/ha MJ/1,4ha

mower 2 5.56 1.64 1.1 67.7 25.47 352.80 493.92

raker 4 5.5 3.3 2.3 69.70 12.05 215.71 301.99

baler mean 4 6.77 2.6 1.3 50 26.25 342.26 479.16

transport 10.4 20.3 94.68 132.55

full

transport N/A*

empty

Total 84.07 1407.62

Energy of cotton stalks collected: 15 bales *192 kg of dry matter by 18 MJ/kg gives 51840 MJ.

*Measurements during return were not taken due to the high speed. Values of loaded transportation are taken for the return trip.


paper. In columns seven and nine of Table 9 theenergy consumed for harvesting 1.4 ha of cottonstalks is given. In column seven the estimation isbased on the net energy consumed by the ma-chinery while the values in column nine includethe energy consumed by the tractor and all itsparts. It is clear that much more energy is con-sumed to do a ®eld work than it is actuallyrequired by the machinery. This di�erence ismuch higher as tractor power is not well matchedto the equipment size. This mismatch is alsoapparent from the high speci®c fuel consumption(Table 8). In this higher fuel consumption is alsoincluded any low e�ciencies of the tractor enginedue to bad maintenance or settings. However,that energy is a more realistic estimation of theenergy consumed for any ®eld work and shouldbe the base for any assessment of the energy bud-get of cotton stalk collection. It should be notedthat transportation energy is only measured forthe loaded platform as the high speed in thereturn trip could damage the instrumentation.So, a slight overestimation was introduced.Additionally, energy consumption was not esti-mated during the pre-compression of the baler bythe hydraulic system of the tractor and duringfork loading and unloading of the bales. Totaldirect energy consumed for the ®eld operation is1407 MJ. In these ®gures, the indirect energyconsumed for the machinery as well as the energyfor lubricants, repair and maintenance etc shouldbe added to have an exact energy balance. Ananalysis of the indirect energy consumed for themachinery used was given by Gemtos andTsiricoglou [16]. In this work the energysequested for the construction, maintenance etc,of the machinery used ranged from 521 to 695MJ for the 1.4 ha giving a total energy consump-tion of 1929±2103 MJ. The energy content of thecollected stalks was 15 bales of 192 kg dry mat-ter/bale and heating value of 18 MJ/kg, yield51,840 MJ, giving a net energy gain of about49,800 MJ or 35,571 MJ/ha.

The material collected by the baler was 2,880kg dry matter (15 bales of 192 kg each). Withtheoretical yield 3565.8 kg the e�ciency of collec-tion is 80.8% which is quite satisfactory. In orderto investigate the problems encountered during

storage, the bales were stored outside and insidea barn. In both cases, the bales were dried to awater content of less than 20% w.b. in 20 daysof outdoor storage without any heating of thematerial. Cotton stalks are covered at the baseby a cork layer which prevents moisture loss. Anuprooting of the plants without the disturbanceof this layer could decrease the moisture lossspeed. In our case, the cutting of the stalks andtheir bending during baling would cause damageto that layer and thus increase moisture loss.Detailed results of storing experiments weregiven by Gemtos and Tsiricoglou [17].

4. Conclusions

From the present work it can be concludedthat under Greek conditions:

. uprooting of cotton stalks is not a feasiblemethod of harvesting due to the soil stackedon the roots and the limited period availablefor the plants to be left in the ®eld for drying;

. cutting of stalks is feasible with the existinghay mowers with reciprocating knives;

. the whole work of collection, packaging andtransportation of the residue can be ful®lled bythe existing conventional hay making equip-ment which gives a great advantage to themethod;

. the best results were given by the use of a largeround baler;

. the harvesting operation is energy e�ective,giving a net energy of 35,571 MJ/ha;

. the bales can be stored safely in or out ofdoors without heating problems for the ma-terial and, equally importantly, with naturaldrying during the initial storage period.

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