Research paperFast pyrolysis of hot-water-extracted and soda-AQ-delignified okra (Abelmoschus esculentus) and miscanthus (miscanthus x giganteus) stalks by Py-GC/MS
Introduction
Due to many reasons, the rapidly increasing utilization of lignocellulosic feedstocks for producing renewable energy and chemicals, especially a variety of various non-wood materials, is in progress [[1], [2], [3], [4], [5], [6], [7]]. At the same time, more effective use of more versatile biomass resources is of great importance. One of the most promising integrated biorefining approaches, mainly utilized for the partial recovery of wood-derived carbohydrates, is based on different pre-treatment processes [[8], [9], [10], [11], [12], [13], [14]], such as hot-water extraction conducted prior to delignification [[15], [16], [17], [18], [19], [20], [21], [22]]. Typically, by such pre-treatments, it is possible to obtain potential by-streams and simultaneously increase the reactivity of feedstock material, resulting in enhanced pulping performance together with spent liquors that have attractive chemical compositions. Hence, by these kinds of integrated biorefinery concepts, the efficient utilization of all major feedstock constituents (cellulose, hemicelluloses, and lignin) can be considered when planning target-oriented economic processes for the manufacture of useful products from fibrous lignocellulosics.
Non-wood based raw materials, such as annual crops, can be applied as an effective fibrous alternative to the decreasing forest wood resources in most developing regions [5,23]. Potential agricultural feedstocks, such as okra (Abelmoschus esculentus) and miscanthus (Miscanthus x giganteus, a hybrid of M. sinensis and M. sacchariflorus) stalks, may offer interesting raw materials for lignocellulosic biorefineries. Okra is one of the most important vegetables and is widely grown from Asia to Africa, Southern Europe, and America. Its edible green seed pods play an essential role in the human diet by supplying carbohydrates, minerals, and vitamins [[24], [25], [26]]. Post-harvest okra stalk residues have traditionally been an unused fraction of the total harvest, although their utilization for fiber in pulps [27,28], composites [29], and ethanol production [30] has been studied to some extent. In contrast, miscanthus, as a commercial energy crop, is currently of great importance in the sustainable production of biofuel products and chemicals due to its vast production worldwide and its high dry-matter yield [[31], [32], [33], [34], [35], [36]]. Hence, its thermochemical behavior has also been studied [[37], [38], [39], [40]], for example, together with the suitability of its fiber for paper production [41,42]. However, only a limited amount of data on the detailed chemical composition of okra and miscanthus stalk is still available.
Pyrolysis is one of the thermochemical conversion methods of biomass carried out in the complete or near complete absence of an oxidizing agent (air or oxygen), typically at 500–700 °C to provide complex fractions of gases, condensable liquids (tars), and char (solid residue) [43]. In our earlier papers, we studied the thermochemical behavior of silver birch (Betula pendula) [44] and Norway spruce (Picea abies) [45] sawdust, both untreated and after various chemical treatments (hot-water extraction, delignification, and hot-water extraction followed by delignification), by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). In each case, major GC-amenable condensable products were determined and classified into several compound groups, characteristically originated from the main structural constituents (cellulose, hemicelluloses, and lignin) of these wood materials. In this comparative study, the aim was to use the untreated and analogously treated non-wood feedstocks, okra, and miscanthus stalks for the same purpose by utilizing the treatment conditions suitable for these raw materials. Also, in this case, the suitability of this analytical pyrolysis method was investigated under varying pyrolysis conditions as a rapid tool for roughly detecting changes that took place during different treatments in the relative content of each constituent of the feedstock samples.
Section snippets
Feedstock materials and their analyses
The untreated (okra, Oref and miscanthus, Mref) and hot-water-extracted (HWE) okra and miscanthus stalks (OHWE and MHWE, respectively) (<5 mm) and the soda-anthraquinone (AQ)-cooked pulps of these feedstocks (POref, POHWE, PMref, and PMHWE) were investigated.
Hot-water extraction (140 °C for 60 min) and the soda-AQ cooking experiments were carried out in a laboratory-scale, oil-heated batch digester (CRS Autoclave System 420, CRS Reactor Engineering AB, Stenkullen, Sweden) equipped with 1.25-L
Raw materials
The main chemical components (and their building blocks) in non-wood feedstocks are basically the same as those in wood feedstocks (resembling primarily hardwoods) and can be found in varying amounts depending on species (genetic differences), growing conditions, and presence of specialized tissues within individual plants [5]. However, it is still possible to detect considerable differences, for example, in both the content and composition of hemicelluloses between the different non-wood
Conclusions
Currently, one potential eco-friendly biorefinery concept is based on the hot-water extraction of fibrous non-wood feedstocks prior to sulfur-free alkaline pulping. In our present study, according to this integrated biorefinery approach, various samples of okra and miscanthus stalks (initial and hot-water-extracted, as well as those from their soda-anthraquinone delignification) were pyrolyzed (at 500 °C and 700 °C for 5 s and 20 s) by pyrolysis-gas chromatography/mass spectrometry to (a)
Acknowledgements
Financial support from the Academy of Finland, within the framework of the project IMUSTBC and from the Doctoral School in Chemistry at the University of Jyväskylä is gratefully acknowledged. We are also thankful to Mr. Yasir Iqbal, M.Sc. from the University of Hohenheim, Germany, for kindly providing miscanthus stalks for our experimental work.
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