Original Research PaperUtilization of incineration fly ash from biomass power plants for zeolite synthesis from coal fly ash by microwave hydrothermal treatment
Graphical abstract
Introduction
The use of renewable energy sources for the generation of electric power in Japan is being promoted by the Feed-in Tariff Scheme for Renewable Energy, which was established in 2012 [1], [2], [3]. Using woody biomass for electricity generation can give a stable supply of electricity that is not influenced by the weather while also contributing to the suppression of carbon dioxide emissions and the conservation of forests. Therefore, the idea of a boiler power plant using woody biomass as a fuel has been attracted attention in recent years [4]. In Japan, the ash generated during biomass incineration is discharged from power plants and transferred to a final disposal site as industrial waste without any further use [5]. Since the costs of conveying and landfilling is particularly high (about 20,000 JPY/t), this appears to be sufficient to inhibit the spread of boiler power plants using woody biomass as a fuel, and so the development of a suitable scheme for the utilization of biomass incineration ash is necessary [6].
It has been reported that the incineration ash discharged after the combustion of biomass could be utilized for a variety of purposes, for example as a soil improving agent [7], [8], a fertilizer [9], [10], or an adsorbent material [11]. One potential way of utilizing biomass incineration ash is in the preparation of potassium-type zeolites (K-zeolites) from coal fly ash using extracted solutions of the biomass incineration ash, which exhibit pH values ranging from 11.5 to 13.0 and contain high concentrations of potassium [12], [13]. Although a number of studies have reported zeolite formation from coal fly ash using hydrothermal treatment methods [14], [15], [16], [17], [18], [19], [20], [21], a potential problem with these systems is that they require a large quantity of highly concentrated alkaline agent for the dissolution of the ions necessary for zeolite synthesis. Thus, aqueous solutions extracted from biomass incineration ash could potentially be employed as alkali sources to reduce the quantity of alkaline agent required. We have previously reported the synthesis of K-zeolites from coal fly ash via an external heating method using woody biomass incineration ash as an alkali source [22]. The zeolites were successfully prepared at a KOH concentration that was much lower than the concentration required to synthesize K-zeolite using a KOH solution alone as the alkaline source. However, long thermal treatment times were necessary to synthesize the zeolite. Therefore, shortening the thermal treatment time is necessary for practical use.
Microwaves are capable of heating matter rapidly, directly, and selectively. They generate heat by rotating the dipole of an object. In our previous studies, we have investigated the effect of microwave irradiation on zeolite synthesis from coal fly ash using pure chemicals as alkali agents, and have reported that microwave heating reduced the treatment time necessary for the reaction to go to completion [23], [24], [25].
In this study, we investigated the effect of microwave heating on the rate of zeolite synthesis from coal fly ash in an extracted solution of biomass incineration ash. Furthermore, the zeolite crystal synthesis rates achieved using the microwave and external heating methods were compared based on the investigation and an empirical equation.
Section snippets
Experimental methods
Table 1 lists the properties of the coal fly ash and biomass incineration ash employed herein. Coal fly ash (JIS Z8910, No. 10) with a median diameter of 3.4 μm was used for preparation of the zeolite framework as a source of Si and Al. Biomass incineration ash, which had a potassium concentration of 57.6 mass%, was also used. The ash was collected from the bag filter dust collector following combustion of dry sawdust, bark, and wood chips of Japanese cedar using a fluidized bed furnace in the
Results and discussion
Fig. 3 shows the XRD patterns of the powdered products obtained by microwave hydrothermal treatment using extracted solutions obtained from various amounts of biomass incineration ash. As shown, when the quantity of the biomass incineration ash extracted into the solution was ≥3.0 g, signals corresponding to the newly generated zeolite phases (i.e., phillipsite) were observed in addition to peaks corresponding to quartz present in the unreacted coal fly ash. However, when 0.3 mol/L of KOH was
Conclusions
We synthesized zeolite by microwave hydrothermal treatment of coal fly ash using extracted solutions of biomass incineration ash. The effect of microwave heating on the crystalline zeolite synthesis rate was investigated by comparison with oil-bath heating (external heating). The zeolite crystal formation rate obtained by microwave heating was larger than that seen for oil-bath heating. This is considered to be due to the local heating in the microwave heating method. The microwave heating
Acknowledgment
This work was supported by JSPS KAKENHI Grant Number JP17K06891. SEM images of product powders were obtained using FE-SEM (S-5200, Hitachi) at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University.
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