Different plant parts as raw material for fuel and pulp
production
K. Pahkala *, M. Pihala
Agricultural Research Centre of Finland,FIN-31600Jokioinen,Finland
Accepted 8 October 1999
Abstract
Reed canary grass (Phalaris arundinacea L.), meadow fescue (Festuca pratensis Hudson), tall fescue (Festuca arundinaceaSchreber) and goat’s rue (Galega orientalisL.) were harvested at seed ripening stage and in the following spring when the plants were totally dry. The amounts of different plant parts (grasses: stem, leaf sheaths, leaf blades and panicles; goat’s rue: stem, leaf blades and pods) were measured and the composition of ash, silica (SiO2), iron
(Fe), manganese (Mn), copper (Cu) and potassium (K) was analysed for each plant fraction. Plant species, plant part and harvesting time affected the mineral composition; grasses contained more SiO2and K, but less Cu than goat’s
rue. The mineral concentrations were highest in leaf blades. In each species, stem fractions had the lowest ash, SiO2,
K, Fe, and Mn contents. The proportion of stem was highest in reed canary grass and goat’s rue when harvested in spring. The K concentration was clearly lower in plants harvested in spring than at seed ripening stage in autumn. However, the concentrations of SiO2, Fe, Cu and Mn were highest at spring harvesting. Spring harvest of reed canary
grass yielded clearly higher fibre contents for each plant fraction than harvesting in autumn. Of the species studied, reed canary grass suits best for raw material, if the leaf blades are removed and harvesting is done in spring at senescence stage of plants. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Grass species; Goat’s rue; Plant fractions; Mineral composition; Fibre
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1. Introduction
Non-wood plant species have been found to be potential sources for fibre production (Nieschlag et al., 1960; Nelson et al., 1966; van Dam and Shannon, 1994) and raw material for energy pro-duction (Burvall, 1993). In northern Europe,
espe-cially reed canary grass (Phalaris arundinacea L.) has aroused interest as an energy crop (Burvall, 1993; Leinonen et al., 1998) and in recent years as raw material for paper (Berggren, 1989; Paavi-lainen and Torgilsson, 1994). Also other grasses,
such as tall fescue (Festuca arundinacea Schr.)
(Janson et al., 1994), and cereal straw (Atchison, 1988; Lo¨nnberg et al., 1996) can be used for paper production. In central Europe, elephant grass (Miscanthus sinensisAnderss.) has been studied as a raw material for paper and energy (Walsh, 1998).
* Corresponding author. Tel.:+358-3-41881; fax: + 358-3-41882437.
E-mail address:katri.pahkala@mtt.fi (K. Pahkala)
When herbaceous species are pulped for paper-making or used for bioenergy, most frequently, the whole plant with all plant parts is used as raw material. Except for fibrous material from cell walls, this biomass contains inorganic elements, which are essential or useful for plant growth and development. However, these mineral substances have a negative effect on the pulping and combus-tion processes, so their quantities should be as low as possible (Keitaanniemi and Virkola, 1982; Il-vessalo-Pfa¨ffli, 1995; Obernberger et al., 1997). Silicon (Si), potassium, manganese, copper and iron are harmful for the pulping process (Keitaan-niemi and Virkola, 1982) and undesirable in fuel use (Obernberger et al., 1997). In papermaking, silicon wears out the installations of a factory (Watson and Gartside, 1976), lowers the paper quality (Jeyasingam, 1988) and complicates the recovery of chemicals and energy (Ranua, 1977; Keitaanniemi and Virkola, 1982; Ulmgren et al., 1990). In combustion, high alkali metal concen-trations decrease the melting point of ash and cause deposits and damage in boilers (Burvall, 1993; Obernberger et al., 1997).
The concentration of each particular mineral substance varies depending on the plant species and the plant part (Rexen and Munck, 1984; Petersen, 1988; Theander, 1991). The plant age or stage of development when harvested and the concentration of other minerals have also a sig-nificant influence (Tyler, 1971; Gill et al., 1989; Marschner, 1995; Landstro¨m et al., 1996). The main botanical components in a grass plant are stem (nodes, internodes), leaf sheaths, leaf blades and panicles, in legumes stem, leaves and pods or flowers, respectively. The weight distribution of these components varies within and between plant species (Salo et al., 1975; Petersen, 1988). Harvest-ing at different stages of development also affects the stem to leaf ratio in the biomass. There are differences in the chemical composition of stem and leaves (Muller, 1960; Salo et al., 1975; Buxton and Hornstein, 1986; Albrecht et al., 1987; Pe-tersen, 1988) which also cause variations in the mineral content of the harvested biomass.
In the early 1990s, the Agricultural Research Centre and the University of Helsinki together with the Finnish Pulp and Paper Institute set out
to search for the most promising crop species for raw material of papermaking (Pahkala et al., 1995). In those studies, reed canary grass, tall fescue, meadow fescue and goat’s rue were chosen for further studies, including field experiments to determine the proper harvesting system and fertil-isation level for biomass production. The fibre and mineral compositions of the total yields have been reported in an earlier study (Pahkala et al., 1994). In the present study, we wanted to find out whether the quality of the raw material could be improved by screening for the plant fraction, which is most appropriate for the pulping and combustion processes.
2. Material and methods
2.1. Field experiments
In field experiments, three grass and one legu-minous species, reed canary grass (P.arundinacea
L.), meadow fescue (Festuca pratensis Hudson),
tall fescue (F. arundinacea Schreber) and goat’s rue (Galega orientalisL.), were studied. The data presented in this paper were collected from previ-ous field experiments designed to determine the proper harvesting system and fertilisation level for biomass production. The field experiment for each grass species comprised two fertilisation levels combined with four harvest times in split-plot design with 3 – 5 replicates. The fertilisation treat-ments were completely randomised into blocks. For goat’s rue, four harvest times and one fertili-sation level were used. Because Pahkala et al. (1994) showed earlier that harvesting to produce biomass for industrial purposes is best at seed ripening stage and in the following spring as senescent dry crop, we focused only on these two harvest times in this study. The results presented here are from the lower fertilisation level (100 kg N ha−1
), because the higher fertilisation level
(200 kg N ha−1) was shown to be too high for
economic reasons.
The experiments were performed on farm-scale fields of sandy loam with pH values between 5.8 and 6.3. The plot size was 15 m2. More details of
2.2. Har6esting and sampling
Harvest at seed ripening stage felt usually be-tween the last week of July and the second week of August, depending on the year (Table 1). Har-vest in spring was at the end of April or in the beginning of May. Delayed harvest was done in early spring after the snow and ice had melted and the soil had dried enough to carry the harvest machine (Haldrup forage plot harvester). For analysing the DM content in harvested biomass, two samples of 200 g were dried first for 2 h at 105°C and then 17 h at 60°C. For plant part
analyses, a sample of 25×50 cm (consisting
about 100 – 120 plants) was taken from each plot, cutting the plants near the soil surface. The dried grass samples were separated into stems, leaf blades, leaf sheaths and panicles. The goat’s rue samples were separated into stems, leaf blades and pods. The weight distribution of the plant parts (%) was determined after drying the samples for 17 h at 60°C.
2.3. Mineral composition and crude fibre
For grass species, chemical analyses were per-formed on samples of stem, leaf sheaths and leaf blades separately, for goat’s rue on samples of stem and leaf blades. After drying, the samples
were milled to less than 1 mm. The concentrations of K, Fe, Mn and Cu were measured by flame AAS (flame atomic absorption spectrophotome-ter), the concentration of silica (SiO2) and ash by
gravimetry, in both cases after dry ashing at 500°C. Crude fibre is the fibre fraction, which is not soluble in the acid – alkali treatment. Crude fiber was determined by using the Fibertec system M, which consists of hot and cold extraction units. The sample was boiled first in dilute acid
(H2SO4) and then in dilute alkali (KOH). The
residue, which contains cellulose, some hemicellu-lose and lignin, was measured gravimetrically af-ter ashing it at 500°C. Analyses were performed at the Chemistry Laboratory of the Agricultural Re-search Centre of Finland.
2.4. Statistical analyses
An analysis of variance was done for each
measured variable. Each plant species was
analysed separately. Statistical analysis was done for testing the harvest time effect. When testing the differences between plant parts, the plant part was handled as a repeated factor. For reed canary grass, the year was analysed as a repeated factor when analysing the total DM yield and stem proportion. The repeated measurements were cor-related. This correlation was taken into account
Table 1
Cultivation practices, varieties, localities, sowing years and harvesting dates of the experiments Sown
Variety Locality Harvested
Spring Autumn
DM yield and plant parts
Reed canary grass Venture Jokioinen 1990 24th July 92a 27th April 93a Venture Jokioinen
Reed canary grass 1990 29th July 93 25th April 94
Reed canary grass Venture Jokioinen 1990 3rd August 94 11th May 95 8th May 96 Jokioinen 1990
Reed canary grass Venture 25th July 95
12th May 97 Venture
Reed canary grass Jokioinen 1990 16th August 96
11th May 98 Venture
Reed canary grass Jokioinen 1990 16th August 97
21st April 93a 28th July 92a
1988
Tall fescue Retu Helsinki
21st April 93a Helsinki 1988
Meadow fescue Kalevi 28th July 92a
Gale Jokioinen 1990
Goat’s rue 5th August 92a 6th May 93a
Fibre
Venture Jokioinen 1993 19th August 96 16th May 97 Reed canary grass
Fig. 1. Dry matter yield (kg ha−1) of reed canary grass in autumn 1992 – spring 1998. Relative proportion of different plant parts in total DM (%) are given in columns.
on a selected model (1). The covariance structure of the repeated measurements was chosen by com-paring potential structures using Akaike’s infor-mation criterion (Wolfinger, 1996).
Yij=m+Ti+Bj+oij+Pk+(T×P)jk+(B×P)ik
+dijk (1)
where m, Ti, Bj and oij are equivalent in the
analysis of variance for the classical randomised complete-block design.Pkand (T×P)jkrepresent
the fixed effect of plant part (or year in reed
canary grass) and plant part (or year)×harvest
time interaction. (B×P)ikrepresents the random
effect of plant part (or year)×block interaction.
dijk are the experimental error terms.
Assumptions of the model were checked by graphical methods; box-plot for normality of er-rors and plots of residuals for constancy of error variance (Neter et al., 1996). The parameters of the models were estimated by the restricted maxi-mum likelihood (REML) estimation method us-ing the SAS system for Windows 6.12.
3. Results
3.1. Total yields and weight distributions of different plant parts
3.1.1. Reed canary grass
When harvested in spring, the total DM yield of reed canary grass was on average 2025 kg ha−1
higher than at autumn harvest (P0.0012) (Fig. 1). The age of the crop stand did not affect the harvested DM yield significantly in spring, and the yield level remained constant throughout the experiment period. In spring, the DM yield varied
between 6408 (1993) and 7716 kg ha−1
(1997). Variation in yield was greater when harvested in autumn; 3298 (1992) to 6357 kg ha−1
The stem proportion of reed canary grass was significantly (P 0.0133) higher in spring (61.3%) than at autumn harvest (51.6%) (Fig. 1). The proportion of stem varied greatly depending on
the year (P 0.0001). Variation in stem fraction
ranged between 42.4 (1993) and 61.8% (1996) in autumn and in spring between 46.6 (1993) and
73.9% (1997). The stem yield (kg ha−1) varied
yearly more than stem proportion. The stem yield
was lowest in 1992 (2269 kg ha−1) and it
in-creased later, varying from 2966 to 4311 kg ha−1
in older stands. The proportion of leaf blades averaged 28.2% in autumn and 20.1% in spring, leaf sheaths 17.4 and 18.5% of the DM yield, respectively. Panicles were found only at seed stage in autumn when their proportion was 3.3% of the DM yield.
3.1.2. Fescues
The total DM yields for both tall fescue and meadow fescue over a period of several years have been presented earlier (Pahkala et al., 1994). In both fescues, the total biomass yield was lower
and variation was greater in spring than in au-tumn (Fig. 2). The harvest effect was significant in tall fescue (P0.0450). In fescues, the major part of the harvested biomass consisted of leaf blades (Fig. 2), the stem fraction being a minor part and the stem proportion even decreased during winter. In autumn, the proportion of stem varied from 24.1 to 33.1% in tall fescue and from 29.9 to 34.6% in meadow fescue and in spring from 11.1 to 39.9% and from 7.1 to 27.9%, respectively. The proportion of panicles in tall fescue was 8.2%, and in meadow fescue 11.6% of the DM yield at seed ripening stage.
3.1.3. Goat’s rue
The spring yield of goat’s rue was significantly
lower than that harvested in autumn (P 0.0299)
(Fig. 2). The proportion of stem was as high as 73.1% in spring, which was significantly higher (P
0.0250) than in autumn (48.6%). However, the
stem yield (kg ha−1) was comparable at both
harvest times.
Table 2
Ash, silica (SiO2) and potassium (K) contents in different plant parts of reed canary grass, tall fescue, meadow fescue and goat’s rue in autumn 1992 and in spring 1993a
SiO2(% of DM) K (g kg-1of DM) Ash (% of DM)
Harvest time Harvest time Harvest time
Spring Pvalue Autumn Spring Pvalue
Autumn Autumn Spring Pvalue
Reed canary grass
5.04 ab 0.4824 1.67 ab 4.04 ab
Stem 4.71 ab 0.0005 14.47 ab 2.77 ab 0.0019
9.00 b 0.2572 4.27 b 7.40 b 0.0002
8.44 b 19.73 b
Leaf sheath 3.57 a 0.0004
11.53 c
Leaf blade 13.00 c 0.0263 5.73 b 10.67 c 0.0022 21.13 b 4.30 a 0.0001
Tall fescue
4.15 a 0.0132 0.67 a 1.94 a 0.0039
6.03 a 25.93 a
Stem 8.83 a 0.0114
9.03 b
Leaf sheath 8.53 b 0.5180 3.24 b 5.06 b 0.0029 28.30 ab 13.07 b 0.0336 10.07 b 0.1931 3.54 b 7.33 c
Leaf blade 10.90 c 0.0014 30.77 b 6.33 a 0.0007
Meadow fescue
Stem 6.12 a 3.78 a 0.0077 0.53 a 2.54 a 0.0266 24.87 a 3.00 a 0.0001 7.76 b 0.1168 2.86 b 5.83 b 0.0071
Leaf sheath 8.71 b 23.90 a 4.30 a 0.0001
10.89 c 0.3735 2.63 c 8.51 c 0.0006
11.37 c 30.87 b
Leaf blade 4.47 a 0.0001
Goat’s rue
3.62 a 0.0012 0.08 a 0.48 a
Stem 4.89 a 0.0766 12.57 a 2.50 a 0.0014
11.10 b 0.0166 0.34 a 2.03 b 0.0243 14.57 a 3.37 a
10.77 b 0.0012
Leaf blade
aThePvalue are given for the harvest time effect.
bMeans for each species, which are written in columns, are not significantly different at the 0.05 probability level if they are followed by the same letter.
3.2. Mineral and fibre contents of different plant parts
3.2.1. Ash
The ash content was lowest in the stem fraction for all species and at both harvest times, in leaf sheaths and especially in leaf blades the ash content was clearly higher (Table 2). Harvest time had not a very clear effect on the ash content. Leaf blades of reed canary grass and goat’s rue had a higher ash content in spring than in autumn, while the ash content in the stem of fescues and goat’s rue decreased in spring.
3.2.2. Silica (SiO2)
The grass species had a higher silica content than goat’s rue. In spring, the silica content was higher than in autumn in all plant parts. Silica was lowest in the stem fraction for all species at both harvest times (Table 2). The silica content in leaf sheaths and blades was almost the same in autumn, but in spring the silica content was
clearly higher in leaf blades than in leaf sheaths.
3.2.3. Potassium (K)
Harvest time had a considerable effect on the potassium content in all plant species and plant parts. The potassium content in all plant species and in all plant parts was clearly lower at spring harvest than in autumn (Table 2). Between plant parts the differences were small. In autumn, the potassium content was lower in stem than in leaves.
3.2.4. Copper (Cu), iron (Fe) and manganese
(Mn)
The copper content of leaf sheaths and blades in all plant species was higher in spring than in autumn, but the difference was not so clear in stem (Table 3). Except for tall fescue, in spring the copper content was significantly lower in stem than in leaf blades. The copper content of goat’s rue was higher than that of grasses.
Table 3
Copper (Cu), iron (Fe) and manganese (Mn) contents in different plant parts of reed canary grass, tall fescue, meadow fescue and goat’s rue in autumn 1992 and in spring 1993a
Fe (mg kg-1of DM) Mn (mg kg-1of DM) Cu (mg kg-1 of DM)
Harvest time Harvest time Harvest time
Autumn Spring Pvalue Autumn Spring Pvalue Autumn Spring Pvalue
Reed canary grass
Stem 5.90 ab 6.34 ab 0.4946 18.70 ab 61.37 ab 0.1738 20.00 ab 48.00 ab 0.3998 4.11 b
Leaf sheath 7.33 ab 0.0055 66.67 b 267.00 b 0.0015 52.83 ab 140.33 b 0.0423 8.22 b 0.0193 110.33 b 491.00 c 0.0001
5.99 a 80.53 b
Leaf blade 213.67 c 0.0110
Tall fescue
4.65 a 0.0447 15.73 a 68.10 a
Stem 2.47 a 0.0090 35.73 a 62.67 a 0.0851
4.91 a 0.0228 48.00 a 148.33 b
2.19 a 0.0796
Leaf sheath 97.37 b 175.67 b 0.0027
3.63 a
Leaf blade 6.72 a 0.0149 91.97 a 477.33 c 0.0044 105.10 b 186.00 b 0.0024
Meadow fescue
5.73 a 0.2692 25.30 a 157.33 a
4.23 a 0.0195
Stem 42.53 a 63.43 a 0.2313
3.31 a
Leaf sheath 8.54 b 0.0110 55.50 a 391.67 b 0.0019 84.70 b 117.70 b 0.2366 6.81 b
Leaf blade 12.07 c 0.0109 131.00 a 1176.33 c 0.0007 83.53 b 149.67 c 0.1155
Goat’s rue
12.40 a 0.0096
Stem 4.09 a 44.83 a 332.00 a 0.0507 18.33 a 48.57 a 0.0009
26.07 b 0.0512 125.00 a 1192.00 b 0.0215 85.80 b 194.00 b
10.67 a 0.0062
Leaf blade
aThePvalue are given for the harvest time effect.
bMeans for each species, which are written in columns, are not significantly different at the 0.05 probability level if they are followed by the same letter.
between plant parts were significant in spring. The lowest contents were found in stem, the highest in leaf blades. In autumn, the iron content varied greatly which is the reason why there were no significant differences between the plant parts in iron content.
The manganese content varied considerably be-tween different plant parts. It was higher in spring than in autumn (Table 3). The contents were lowest in the stem of all species, the highest in leaf blades.
3.2.5. Fibre
The fibre content in reed canary grass was always clearly higher in spring than at autumn harvest (Table 4). Even the differences between plant parts were significant. The highest fibre content was observed in stem where the harvest time effect was strongest. The amount of fibre was lowest in leaf blades.
4. Discussion
Reed canary grass, meadow fescue, tall fescue and goat’s rue had shown a high yielding capacity in earlier studies, and especially the grass species had been found to be potential fibre crops
(Jan-Table 4
Crude fibre contents (% of DM) in different plant parts of reed canary grass in autumn 1996 and in spring 1997a
Harvest time
Autumn Spring Pvalue Reed canary grass
52.13 ab 0.0001 39.70 ab
Stem
Leaf sheath 36.70 b 39.83 b 0.0032
26.97 c 0.0293
Leaf blade 30.13 c
son et al., 1994; Pahkala et al., 1995). In this study, the highest DM yield was obtained from reed canary grass when the crop was harvested in spring as a dry senescent crop, and the yield level remained constant from the second year through-out the experimental period of 6 years. In Swedish studies (Landstro¨m et al., 1996), the DM yield of reed canary grass increased at delayed harvest with increasing age of the crop stand during the first three ley years.
The stem proportion of plant species studied varied greatly because of different growth habit of the species. On average, more than half of the biomass of reed canary grass and goat’s rue con-sisted of stem and the proportion of stem was higher in spring than in autumn. In fescues, the major part of the harvested biomass consisted of leaf blades, and the proportion of stem even decreased during winter. Reed canary grass is a strawy rhizomatous species and its growth habit is vigorous and erect, while fescues form leafy tus-socks, which also lodge more easily during winter. The total and stem yield losses during the winter are more common in fescues than in reed canary grass or goat’s rue due to the lodging of the canopy.
The chemical composition of a plant part varies depending on the stage of development of the plant when the mobile elements are moving from organ to organ as growth proceeds (Jeffrey, 1988). The concentrations of silicon, iron, manganese and copper have proved to increase at the late stage of development (Tyler, 1971), and also in this study the highest concentrations were found in dead plants in spring. The reason for the great variation in the manganese and iron contents may be connected to the lodging of the canopy and for possible soil contamination at harvesting. The potassium content was clearly lower in spring than in autumn because of the leaching during winter. An increase of fibre fraction in spring can be explained by ageing of the plant, when the relative amount of plant cell walls increases with increasing amount of cellulose and lignin in the secondary wall, as has been described in several forage crops (Buxton and Hornstein, 1986; Bux-ton and Russel, 1988; Albrecht et al., 1987; Gill et al. 1989).
Grasses seemed to accumulate more silica than goat’s rue. This result is comparable with earlier findings which have shown high silica content typical for grass plants (Marschner, 1995; Ilves-salo-Pfa¨ffli, 1995). Grasses accumulate silicon as silica in epidermic cells where it protects the plant against herbivores and fungi (Jones and Han-dreck, 1965). Its role is different from that of potassium, copper, iron and manganese, which take part more in cell metabolism.
From plant parts, leaf blades accumulated the highest concentrations of minerals. Removing the undesirable minerals with the leaf blades would reduce the mineral content considerably and, at the same time, would increase the relative propor-tion of stem, the most fibre-rich part of the plant. On the one hand, sorting out the leaf blades would decrease the material usable for industry from 11 to 67% depending on plant species. On the other hand, using more stem fraction increases the pulp yield and improves the pulp quality (Petersen, 1989; Hemming et al., 1994; Pahkala et al., 1999). At the pulp mill, leaves, dust and dirt can be removed by air fractionation before cook-ing. However, in grasses the leaf sheath is usually tightly rolled around the stem, and it can be more difficult to remove than leaf blades. Mechanical pretreatment improves the quality of the pulp by increasing the bleachability of the pulp and de-creasing the fines and silica particles in the raw material. Silicon entering the process can be de-creased by pretreatment of the grass, removing 40% of the silica (Paavilainen et al., 1996b). The dewatering and drying ability of pure grass pulps can be improved by mechanical fractionation and blending the grass pulp with long-fibre soft wood pulp (Wisur et al., 1993; Paavilainen et al., 1996a,b).
5. Conclusion
reed canary grass when harvested in spring as a senescent crop and the yield level remained con-stant for at least 6 years. The main part of the fibre in reed canary grass was found in stem, and the fibre content even increased when harvested in spring. However, the contents of ash and most of the minerals studied were also high when har-vested in spring. The mineral concentrations were highest in leaf blades. By removing the leaf blades, the ash and mineral contents would de-crease considerably and at the same time, the relative fibre content could increase, thus increas-ing the value of plant material for industrial use.
Acknowledgements
The authors wish to thank the Ministry of Agriculture and Forestry and the Agricultural Research Centre of Finland for financing this study as part of the project ‘Production and use of agrofibre in Finland’. Our special thanks are due to Biometrician Lauri Jauhiainen for the comments of the manuscript, and to the Chem-istry Laboratory of MTT for the careful labora-tory work.
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