Addition of kaolin as potassium sorbent in the combustion of wood fuel – Effects on fly ash properties
Introduction
Forest felling residues are now commonly used as fuel which introduces a risk that the supply of nutrients and essential trace elements to the new generation of trees will be depleted [1], [2]. However, the produced ashes are enriched in most of these minerals which makes ash recycling to the forest a favourable option. Wood ash has been shown to be advantageous for the growth of trees and plants, especially on peat soil [1], [3], [4]. The ash utilisation may pose some problems, such as a too rapid increase of soil pH induced by the strongly alkaline ash [5]. To overcome such problems ash stabilisation processes, such as self-hardening, thermal treatment or granulation, have been developed [6], [7].
The combustion of forest residues and other biomass fuels is often hampered by ash sintering, fouling and corrosion mainly caused by formation of volatile KCl [8]. Kaolin (Al2Si2O5(OH)4) has been shown to be an efficient absorbent for different species of potassium, as well as for other metals in combustion processes [9], [10], [11], [12]. The binding of potassium in aluminium silicates may be favourable also for the utilisation of the ash as a nutrient source in the forest. Normally the potassium salts in the ash are still soluble after the stabilisation [5], [13] and it would be advantageous if it could be bound in less soluble forms. However, the addition of kaolin increases the Al content in the ash and since large concentrations of Al3+ in the soil water can harm the root systems of the vegetation [14], [15] it was considered important to investigate if this, and the possible release of the trace metals bound in aluminium silicate forms, is an obstacle for the utilisation of kaolin as fuel additive. In a project carried out earlier kaolin was added in full scale wood fuel combustion during a test period. Therefore it was possible to get ash samples produced within the same settings, differing only in the presence of kaolin. The kaolin was concentrated in the fly ash due to its small particle size [16] and could be expected to affect only the properties of the fly ash, not those of the bottom ash. Therefore, this work was focussed on the properties of the fly ash.
Section snippets
Materials
The ashes were produced in a 26 MWth and 9 MWel CFB boiler fired with forestry residues and operated at full load. The kaolin applied in the tests was Intrafill™ by Imerys Minerals Ltd which has a small particle size (55% < 2 μm). The amount of kaolin added was adjusted so that the stoichiometric ratio between kaolinite and potassium was one. A detailed description of the combustion tests has been given by Davidsson and co-workers [16]. Fly ash samples were collected from the electro filter
Results and discussion
The element concentrations in the original ashes are shown in Table 1. The results for the two ash samples taken before kaolin addition were very similar, so the mean values are shown as ash A, whereas the results for the ashes produced during kaolin addition are presented separately as B and C. The kaolin clearly adds Si and Al to the fly ash composition. However, the concentrations of many other elements are lower in the ashes from the kaolin addition than in the ash samples from normal
Conclusions
In the hardening/agglomeration process the addition of kaolin reduced the initial setting time of the ash–water mixture and the presence of kaolin also stabilised the ash structure.
For most elements the presence of kaolin did not affect the leaching behaviour and the leaching of trace elements was generally small. However, for Al and Cd, kaolin addition seems to increase their leaching, whereas kaolin addition decreases the leaching of K at high pH values.
Based on the results obtained here,
Acknowledgements
This work has been funded by the Swedish Energy Agency and the Adlerberts research foundation, Göteborg, which is gratefully acknowledged. The authors also want to express their gratitude to Daniel Nilsson and Brita Ohlsson for their assistance with laboratory work.
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