Inorganic matter characterization in vegetable biomass feedstocks☆
Introduction
Biomass is an abundant, renewable source of energy and chemicals whose increasing uses demand development of characterization techniques specifically adapted to this type of material. Inorganic matter present in biomass plays an important role in the various utilization processes of this alternative feedstock. As in the case of solid fossil fuels, many effects of inorganic constituents are regarded as negative, including environmental and technological problems. For example, alkaline metals, sulphur or chlorine may be released during thermal transformations of biomass, causing hot corrosion and/or pollutant emissions into the atmosphere [1], [2]. On the positive side, inorganic constituents of biomass can act as catalysts (or catalyst precursors) for pyrolysis and gasification reactions; this occurs, for instance, with KCl during wheat straw pyrolysis [3]. In general, the yields of pyrolysis products depend heavily on the nature of the mineral matter present [4]. In the case of coal many other effects of mineral matter on conversion processes such as pyrolysis and hydropyrolysis [5], liquefaction [6], [7], [8] and combustion [9], [10] are well-characterized. However, only recently has the increasing interest in industrial biomass applications motivated similar studies of lignocellulosic matter.
As in the case of coal, the nature and concentration of the various inorganic phases present in biomass are much more closely related to technological uses than results from conventional (high-temperature) ash characterization. In a previous work from one of our laboratories [11], the advantages of low-temperature ashing (LTA) in an oxygen plasma to selectively oxidize the organic matter of olive stones have been shown. Using the inorganic matter residue from this treatment, a methodology based on the joint use of characterization techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM), previously applied to coals [12], [13], allowed one to identify the main inorganic constituents of olive stones, a biomass feedstock with a very low ash yield (0.6 wt.%).
LTA is generally regarded as a technique able to isolate the inorganic fraction of coals with little alteration of its original form, but a series of sources of error have been pointed out as well [14], [15], [16], [17], [18]. A possible alternative is organic matter oxidation with air at a higher temperature (medium-temperature ashing, MTA), but there remains the possibility of further reactions and phase transformations of the inorganic constituents at the temperature involved.
In this paper we compare the reliability of the LTA and MTA isolation techniques in terms of avoiding parasitic phenomena during isolation of inorganic constituents from biomass. To this end, we have used XRD and FT-IR spectroscopy to characterize inorganic constituents in a suite of four biomass products submitted to LTA and MTA treatments. Although the primary objective is to optimize the methodology for inorganic matter characterization in biomass, this work is expected to contribute to fulfil equivalent objectives in the field of coal since there are very few reports on the application of MTA to solid fossil fuels.
Section snippets
Materials
Four types of vegetable biomass were used in this work:
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Apple pulp (AP). It is a solid residue from pressing apples (the fruit of Malus domestica) in the manufacture of apple juice and cider. At present this residue has few uses, but a possible application as a feedstock for activated carbons has been recently reported [19], [20].
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Olive cake (OC). It consists of solid and liquid residues from pressing olives (the fruit of Olea europaea) to extract olive oil with the exception of olive stones,
Results and discussion
Elemental analyses—which are more fairly compared on a dry and ash-free (daf) basis than on a dry (db) basis—indicate that apple pulp and olive cake are richer in C and H and poorer in O than the two primary biomass products (Table 1). Otherwise, the four studied biomass materials differ relatively little in composition; notice, for example, the comparable volatile matter yields (70–79 wt.% db; 78–87 wt.% daf).
Chemical analyses of inorganic elements are given in Table 2. It can be noticed that
Conclusions
XRD and FT-IR techniques provide a comprehensive characterization of inorganic constituents present in the biomass materials studied. Low-temperature ashing in an oxygen plasma has the disadvantage of producing abundant artifactic nitrates and sulphates. Medium-temperature ashing in air avoids these parasitic phenomena but the higher temperatures involved provoke other transformations in the mineral constituents of plants. The use of both concentration techniques has allowed us to securely
Acknowledgements
Financial support from II PRI Asturias (projects PB-TDI97-03 and PB-MAT98-01) and CICYT (project OLI96-2144) is gratefully acknowledged.
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