A comparative fiber morphological analysis of major agricultural residues (used or investigated) as feedstock in the pulp and paper industry

Abstract

The suitabilities of major agricultural residues were assessed as papermaking feedstocks. All the examined agricultural residues were assumed as potential candidates for substituting hardwood fibers in mixed pulp blends from a fiber morphological perspective. Wheat, barley, rice, rapeseed, maize, sunflower, sugarcane bagasse, coconut husk, and two genotypes of miscanthus grass underwent identical maceration. The fiber length, fiber width, cell wall thickness, and lumen diameter were measured to calculate the slenderness ratio, flexibility coefficient, and Runkel ratio. The average fiber length ranged from 0.50 mm ± 0.32 mm (MG-S-02-V) to 1.15 mm mm ± 0.58 mm (sugarcane bagasse). The fiber width ranged from 10.77 μm ± 3.28 μm (rice straw) to 22.99 mm ± 5.20 mm (sunflower stalk). The lumen diameter ranged from 4.52 μm ± 2.52 μm (rice straw) to 13.23 μm ± 4.87 μm (sunflower stalk). The cell wall thickness ranged from 3.02 μm ± 0.95 μm (rice straw) to 4.80 μm ± 1.48 μm (sunflower stalk). The slenderness ratio, flexibility coefficient, and Runkel ratio values ranged between 28.08 to 58.11, 37.97 to 60.8, and 0.62 to 1.68, respectively. Wheat, maize, rapeseed, sugarcane bagasse, and coconut husk were found to be appropriate residue sources for papermaking feedstocks.

Full Article

A Comparative Fiber Morphological Analysis of Major Agricultural Residues (Used or Investigated) as Feedstock in the Pulp and Paper Industry

Dimitrios Tsalagkas,^a,b,* Zoltán Börcsök,^a Zoltán Pásztory,^a Vladimir Gryc,^b Levente Csóka,^c,d and Kyriaki Giagli ^b

The suitabilities of major agricultural residues were assessed as papermaking feedstocks. All the examined agricultural residues were assumed as potential candidates for substituting hardwood fibers in mixed pulp blends from a fiber morphological perspective. Wheat, barley, rice, rapeseed, maize, sunflower, sugarcane bagasse, coconut husk, and two genotypes of miscanthus grass underwent identical maceration. The fiber length, fiber width, cell wall thickness, and lumen diameter were measured to calculate the slenderness ratio, flexibility coefficient, and Runkel ratio. The average fiber length ranged from 0.50 mm ± 0.32 mm (MG-S-02-V) to 1.15 mm mm ± 0.58 mm (sugarcane bagasse). The fiber width ranged from 10.77 μm ± 3.28 μm (rice straw) to 22.99 mm ± 5.20 mm (sunflower stalk). The lumen diameter ranged from 4.52 μm ± 2.52 μm (rice straw) to 13.23 μm ± 4.87 μm (sunflower stalk). The cell wall thickness ranged from 3.02 μm ± 0.95 μm (rice straw) to 4.80 μm ± 1.48 μm (sunflower stalk). The slenderness ratio, flexibility coefficient, and Runkel ratio values ranged between 28.08 to 58.11, 37.97 to 60.8, and 0.62 to 1.68, respectively. Wheat, maize, rapeseed, sugarcane bagasse, and coconut husk were found to be appropriate residue sources for papermaking feedstocks.

Keywords: Agricultural waste biomass; Non-wood fibers; Papermaking potential; Morphological indices

Contact information: a: Innovation Center, University of Sopron, Bajcsy-Zs, 4, Sopron 9400 Hungary; b: Department of Wood Science and Technology, Faculty of Forestry and Wood Technology, Mendel University in Brno, Zemědělská 3, Brno 61300 Czech Republic; c: Institute of Cellulose and Paper Technology, Celltech-paper Ltd., Sopron 9400 Hungary; d: ELTE University, Faculty of Informatics, Budapest 1053 Hungary; *Corresponding author: dimitrios.tsalagkas@mendelu.cz

INTRODUCTION

Although the primary global trend is to exploit agricultural residues (AgRs) as feedstock materials in bioenergy or biorefinery fields, not all residues are suitable for such purposes (Lal 2005). For instance, while maize and sugarcane crop residues appear as promising raw materials for bioethanol or biogas production, contrariwise, rice and wheat (44% of the total global agricultural residues production), only play a minor role in the production of biofuels (Cherubin et al. 2018). Moreover, second-generation biofuels, based on lignocellulosic by-products and energy (perennial herbaceous) crops, are not expected to become economically viable or commercially available in the forthcoming years; thus, second-gen biofuels were not able to contribute to reaching the 20% renewable energy consumption EU-targets by 2020 (Elbersen et al. 2012).

The calculated technical potential production from the eight major crops in the world, i.e., wheat, maize, rice, soybean, barley, rapeseed (or canola), sugarcane, and sugar beet, is approximately 3.3 Gton yr^-1 (fresh weight), as was determined by Bentsen and Felby (2010). It was also demonstrated that the Asian continent presented the largest share of crop residues production (47% of the total share), followed by the USA (29%), Europe (16%), Africa (6%), and Oceania (2%) (Cherubin et al. 2018). Bentsen et al. (2014) developed a model and pointed out that maize, rice, and wheat residues were accounting for more than three-quarters of the total production in 227 countries around the world. Likewise, Camia et al. (2018) and García-Condado et al. (2019) found that wheat, grain maize, rapeseed, barley, and sunflower residues, together constituted more than 80% of the total EU-28 residual production by using empirical models. In Asia, China and India are viewed as the two primary countries in terms of residue biomass availability, primarily in the form of wheat, maize, and rice straw in China, and sugarcane bagasse, wheat, and rice straw in India (Gregg and Smith 2010; Jiang et al. 2012; Hiloidhari et al. 2014; Chen 2016). In the USA, potential farmgate supplies of primary crop residues, e.g., maize stover, wheat, barley, oats, and sorghum straw, and energy crops, e.g., switchgrass, miscanthus, and biomass sorghum, were estimated to reach up to 214 Mt of crop residues and 729 Mt of herbaceous energy crops by the year 2040 by using the POLYSYS simulation assumption model (Langholtz et al. 2016).

For the fulfilment of the oriented global targets, set within the strategies of a sustainable and resource efficient based circular economy, it is critical to utilize alternative sources of fibrous, lignocellulosic biomass, e.g., non-wood plants, as raw materials for paper and paperboard manufacturing (Przybysz et al. 2018; Sharma et al. 2018; Jeetah and Jaffur 2021). The cell types present in non-woody plants, including AgRs, are more heterogeneous than the cell types present in woody plants. The basic structure of a non-wood plant consists of vascular bundles and parenchyma tissue and contains many types of cells with a wide distribution of cellular dimensions. Furthermore, the type and size of fibers and vessel cells greatly vary within a single plant and between species, since in grasses the same type of cell may originate in different tissues and organs of the plant, which either positively or negatively influence the pulp and paper properties (Ilvessalo Pfäffli 1995; Rousu et al. 2013). The length of short fibers, a low bulk density, a high fines content, and a high amounts of parenchyma cells and mineral substances are a few of the most important inherent features of these non-wood plant sources (Sridach 2010). Additionally, AgRs contain lower proportions of lignin compared to wood-based sources, which is beneficial during the bleaching stage of pulp production (Kaur et al. 2019).

Currently, a major portion of non-wood fibers have already been used for papermaking for a long time, especially in the developing countries of Asia, Africa, and Latin America, which may feature a shortage of wood raw materials (Reddy et al. 2014). Conventional pulping processes, e.g., soda, soda-antraquinone, and kraft, are already used for non-wood pulping, yet alternative pulping processes for non-wood pulping are more desirable. During the last years, several promising approaches in all fields of established papermaking procedures have been investigated at a laboratory or pilot scale, to overcome the limitations and challenges of non-wood pulping. These methods have constituted non-wood plant fibers as a reasonable candidate to replace wood fibers, especially replacing hardwood as the pulp feedstock (El-Sayed et al. 2020; Ferdous et al. 2020; Sharma et al. 2020a; Jahan et al. 2021).

Various non-wood plant agricultural sources have already been considered as potential pulp and paper feedstocks in the past (Ogbonnaya et al. 1997; Gonzalo et al. 2017; Saeed et al. 2017a; Gülsoy and Şimşir 2018; Lavrič et al. 2018). In addition, energy crops, and their residues, including Miscanthus spp. have also been considered as potential feedstocks for paper pulps (Cappelletto et al. 2000; Goel et al. 2000; Ai and Tschirner 2010; Albert et al. 2011). Yet most of the studies have been focused on the investigation of cereal/rice straws, sunflower stalks, rapeseed/canola straw, and sugarcane bagasse. de Assis et al. (2019) found that semi-bleached wheat straw pulps (SBWP) had intermediate FL and coarseness values, with very high fines content, which results in lower bulk and water absorbency. However, this is not a limitation of producing low quality tissue products that require an intermediate combination of water absorbency, softness, and strength and could be used to replace deinked pulp. Pulps obtained from rapeseed stalks can be used as secondary fibers, replacing recycled paper in pulp blends with virgin wood fibers (González et al. 2013). Jeetah et al. (2015) demonstrated that rice husk-bagasse pulp blends (20:80 ratio) are suitable for producing insulating boards or medium packaging cardboards for decorative purposes. According to Bates et al. (2020) triticale pulp could be used for specific categories of printing substrates or bagasse fibers can be used for rough papers like those that are used in packaging.

Apart from China and India, the amount of produced pulp from fiber sources other than wood is still limited globally. According to FAOSTAT (2019) data, more than 80% of non-wood pulp worldwide is produced in Asian countries since the 1980s. More particularly, in these countries, the average pulp production coming from fibers other than wood was estimated at around 14258000 metric tons from 1995 to 2018. In Europe, CEPI members, which constitute 92% of the European pulp and paper industries in terms of production, the total non-wood pulp production amount hardly reached 0.8% of the total pulp production (273000 tons) (CEPI 2020).

An overall evaluation of the papermaking potential of a raw material as pulp feedstock, besides its morphological analysis, requires the evaluation of the physical properties of the obtained paper handsheets, the optimization of pulping and bleaching conditions, and the pulping chemical recovery (recycling of pulping chemicals, utilization of black liquor) processes. (Kamoga et al. 2016; Jahan et al. 2021). Non-wood fibers have unlimited differences in terms of their physical and chemical properties, and they all cover various average fiber dimensions and a wide selection of cell types and sizes (El-Sayed et al. 2020).

The physical properties of any paper primarily depend on its fiber morphology, fiber-fiber bonding, pulp refining, wet pressing, and formation (Sharma et al. 2020b). The fiber anatomical dimensions greatly impact the quality and performance of the final paper product, since these dimensions are highly correlated with its physical strength, and printing quality (Pulkkinen et al. 2009; Hu et al. 2013; Pereira et al. 2016). For instance, the fiber morphological properties directly affect the runnability on the paper machine, the refining response, the pulp bonding ability, and the physical, optical, and strength properties of the paper (Gülsoy and Şimşir 2018).

The fibers length (FL) and average FL distribution of a plant are considered essential morphological features, since they have a major impact on the paper strength, paper sheet formation, and drainage. Nevertheless, FL alone is not a good predictor of paper properties (Simmonds and Hyttinen 1964; Ai and Tschirner 2010; Saeed et al. 2017a). The fiber width (FW) and cell wall thickness (CWT) are highly correlated with fiber flexibility and bending resistance. Arundo donax fibers obtained from internodes parts of the plant, presented narrower LD and less wide CWT compared to those of nodes fibers, suggesting better papermaking properties (Shatalov and Pereira 2006). Furthermore, shorter LD have a positive influence on the beating of the pulp. In contrast, thicker cell walled fibers diminish the folding endurance, the burst and tensile index, and the synergistic effect on the tear strength of a paper (Agnihotri et al. 2010; Tofanica et al. 2011; Saeed et al. 2017a).

In addition, the importance of plant fiber cell dimensions and their derived values on pulp and paper mechanical strength is well documented (Ververis et al. 2004; Nasser et al. 2015). Therefore, the morphological indices derivatives, i.e., the slenderness ratio (SR), flexibility coefficient (FC), and Runkel ratio (RR), obtained from geometrical measurements of the fibers, are often used as an evaluating criterion to assess the suitability of a plant-based source as feedstock in papermaking production.

Additionally, pulp refining, the mechanical treatment of fibers, is a necessary step conducted on the raw source to improve the pulp quality. Depending on the pulp source, pulp consistency, refining equipment, and intensity, refining differently affects the final morphology and characteristics of the treated fibers. A few of the changes during the internal and external fibrillation of the fibers due to refining are the shortening of the fiber length, fines formation, and fibers’ straightening (Gharehkhani et al. 2015). Therefore, the fiber morphological parameters, i.e., fiber length, fiber width, lumen diameter, and cell wall thickness, of the raw non-wood plant fibers are important quality factors influencing both the pulp and paper properties and are essential to predicting the strength properties of the produced paper grades.

Thus, an initial morphological evaluation of raw non-wood fiber dimensions is a needful assessment to take into consideration, regarding the properties of the produced pulp and paper. Nevertheless, up to now, there are contradictory findings when it comes to the previous extended literature. The objective of this study was to conduct a unified fiber morphological parameter analysis, by applying the same maceration treatment and calibration/measurement method on the raw AgRs. Finally, this article aimed at detecting the potential differences between the novel results of the authors and the existing literature, hoping to infer the most appropriate AgRs feedstock for the pulp and paper industry.

EXPERIMENTAL

Materials and Methods

Raw materials

For this study, the following 10 AgRs sources were investigated: wheat straw (Triticum spp.), barley straw (Hordeum vulgare), maize stalk (Zea mays), rice straw (Oryza sativa), sunflower stalk (Helianthus annuus), rapeseed (Brassica napus L.), and sugarcane bagasse (Saccharum officinarum). In addition, coconut husk (Cocos nucifera) fibers, and from the perspective of energy crops, two genotypes of Miscanthus x giganteus grass residues were examined (labelled as MG-S-O2-V and MG-S-01-P, respectively) (Fig. 1).

Wheat straw, barley straw, rapeseed straw, maize stalks, and sunflower stalks samples were collected by local farmers in the region of Győr-Moson-Sopron, Hungary. The Miscanthus x giganteus stalks, were provided by Energianoveny Ltd., (Lengyeltóti, Hungary). The rice straw, coconut husk coir fibers, and sugarcane bagasse were obtained from local producers in Vietnam, while their chemical treatment and measurement analysis were performed in Hungary. All samples were air-dried, chopped into 3 to 5 cm length pieces, and finally stored in sealed polyethylene bags until sample preparation.

Fig. 1. Raw material of the agricultural residues (AgRs) examined in this study

Sample preparation

The non-wood AgRs were macerated according to the method performed by Danielewicz et al. (2018). Approximately 2 mm x 2 mm x 5 mm sticks were cut using a sharp knife. Thereafter, the sticks were placed in vials with the maceration solution, capped, and placed in a drying oven at a temperature of 60 °C for one week. The ratio of the volume of maceration solution to non-wood samples was 100 to 1 (v∕w); the maceration solution was composed of one-part hydrogen peroxide (30% H₂O₂ solution), four parts of deionized water, and five parts of pure glacial acetic acid. In due course, the samples were washed and mixed with distilled water to separate the fiber bundles into individual fibers. The macerated solution samples consisted of an overall mixture of all plant parts, including the pith, nodes, and internodes.

Measurements and data processing

For the morphological analysis, optical microscope (OM) images were captured at x40 and x200 magnifications using a Nikon Eclipse 80i optical microscope (Nikon Instruments Inc., Tokyo, Japan). At least 200 fibers per AgR source were randomly measured. The number varied according to the density of each AgR solution. Image-Pro Plus software (version 6, Media Cybernetics Inc., Rockville, Maryland) was used for measuring the fiber morphological parameters. The OM images captured at x40 magnification were used for the FL measurements, while those captured at x200 were used for measuring the FW, LD, and CWT. The average values of the FL, FW, LD, and CWT parameters were calculated for each AgR source.

To assess the suitability of the AgRs as pulp feedstocks for paper production, the following three fiber morphological indices (SR, FC, and RR) were calculated according to Eqs. 1, 2, and 3,

SR = FL ∕ FW (1)

FC = (LD / FW) × 100 (2)

RR = 2 × CWT / LD (3)

(Ogbonnaya et al. 1997; Ververis et al. 2004; Albert et al. 2011; Mousavi et al. 2013; Saeed et al. 2017a).

RESULTS AND DISCUSSION

Fiber Dimensions Analysis

Representative OM images of the macerated fibers measured for the morphological analysis of the AgR sources are shown in Figs. 2 through 6. The OM images depict the diversity among the AgR raw materials.

Fig. 2. Optical microscope images of the cereal straw residue fibers at ×40 and ×200 magnifications

Fig. 3. Optical microscope images of the stalk residue fibers at ×40 and ×200 magnifications

Fig. 4. Optical microscope images of the rapeseed residue fibers at x40 and x200 magnifications

Fig. 5. Optical microscope images of the Asian originated residue fibers at x40 and x200 magnifications

The observed average FL values of the measured non-wood plant residues (Table 1) were found to be approximately within the 0.7 mm to 3.0 mm hardwood FL range (Simmonds and Hyttinen 1964; Ring and Bacon 1997). In addition, they were found to be within the FL range of the other investigated non-wood fibers obtained from vegetable AgR sources (Gonzalo et al. 2017; Saeed et al. 2017a). The sugarcane bagasse fibers presented the longest average FL (1.15 mm ± 0.58 mm), while the Miscanthus MG-S-02-V fibers had the shortest (0.50 mm ± 0.32 mm) average FL. As demonstrated by Marín et al. (2009) and de Assis et al. (2019), fibers with lengths ranging between 0.2 mm and 1.2 mm, as well as those with a length greater than 1.2 mm, are considered short and long fibers, respectively. Hardwood fibers usually are short ranging from 0.7 to 1.6 mm, with an average fiber length of 1 mm, while softwood fibers are much longer typically ranging from 2.7 to 4.6 mm (Elmas et al. 2018). Therefore, the assessed AgRs can be counted as having a short FL, and short length fibers result in a denser, smoother, and more uniform paper sheet formation (Ai and Tschirner 2010).

Fig. 6. Optical microscope images of the Miscanthus genotypes straw fibers at x40 and x200 magnifications

Table 1. Estimated Fiber Morphological Parameters of the Investigated Agricultural Residues (AgRs)

Table 2. Average Values of the Fiber Morphological Parameters per Agricultural Residue (AgR) Source According to Literature References