Existence Form of Potassium Components in Woody Biomass

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Existence Form of Potassium Components in Woody Biomass Combustion Ashes and Estimation Method of Its Enrichment Degree Norio Maeda, Tomonori Fukasawa, Takaaki Katakura, Munechika Ito, Toru Ishigami, An-Ni Huang, and Kunihiro Fukui Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03090 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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Existence Form of Potassium Components in Woody Biomass Combustion Ashes and Estimation Method of Its Enrichment Degree Norio Maeda 1,2), Tomonori Fukasawa 2), Takaaki Katakura 2), Munechika Ito 1), Toru Ishigami 2)

, An-Ni Huang 3), Kunihiro Fukui *,2)

1)

Environmental Engineering Department, TAKUMA CO., LTD., 2-2-33 Kinrakuji-cho,

Amagasaki, Hyogo 660-0806, Japan 2)

Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University,

1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan 3)

Department of Chemical and Materials Engineering, Chang Gung University, Taoyuan 33302,

Taiwan

Keywords: Combustion ash; Particle size distribution; Size classification; Potassium; Woody biomass; Enrichment; Fertilizer

ACS Paragon Plus Environment

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ABSTRACT: Woody biomass is a growing renewable energy source, and the emitted combustion ash may serve as valuable materials. In this study, the relationship between the potassium concentration in combustion ash and the particle size was investigated for several types of biomass ashes with different morphologies. The form of the potassium components in the ash was also analyzed in order to examine this relationship. From the results, a method to estimate the degree of potassium enrichment was proposed. The potassium concentration first decreases rapidly with increasing mass median diameter and then continues to decrease gradually, although this relationship depends upon the properties of the combustion ash. It was also found that almost all the potassium component in the ash existed as relatively small particles composed of crystallized hydro-soluble potassium compounds, such as KCl, K2SO4, and so on, although the particle size and crystalline phases varied according to the type of power plants. The classification process was implemented in an actual biomass power plant, and demonstrated to produce combustion ash with enriched potassium. The enrichment factor calculated from the size distributions of the potassium component particles and other components qualitatively agreed with the enrichment factor obtained experimentally for all types of ashes. Ash with potassium concentration above 300 mg/g could be successfully acquired on an industrial scale by collecting the finer ash selectively, using a size classification equipment constructed at the biomass power plant.


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1. INTRODUCTION Due to the increasing severity of global climate change, energy conservation and renewable energy sources have grown more and more important [1,2]. The key to mitigate global warming is reducing the concentrations of greenhouse gases such as carbon dioxide in the atmosphere [3]. In 2012, the government of Japan established the Feed-in Tariff Scheme for Renewable Energy [4,5], which can expedite the utilization of woody biomass for electric power generation. As a result, the generation capacity of woody biomass power plants in Japan rose to 2.5 million kW in 2014. In order to further increase the total power generation capacity to 3.8 million kW using 6 million m3 of biomass fuel [6], the government of Japan also promotes the construction of relatively small-scale biomass power plants (less than 2.0 MW of output) [6,7]. After the combustion of biomass fuel, the inorganic components are captured by the cyclone separator (or the bag-filter, etc.) in the power plant as biomass combustion ashes. These ashes must be disposed for reclamation at the last disposal site in Japan. The cost of this step should be reduced in order to promote the construction of small-scale woody biomass power plants [8]. Accordingly, new reuse loop of the woody biomass combustion ash must be developed on an industrial scale. The biomass combustion ash has been proposed as potential soil amendment [9], concrete additive [10,11], adsorbent material [12], and others [13,14]. However, these applications are still in the research phase. Furthermore, particularly in Europe, there have been numerous studies investigating the chemical compositions and physico-chemical characteristics of various biomass combustion ashes [15-18]. These investigations revealed that the biomass ashes could have quite different physico-chemical properties. The dependence of the chemical composition of combustion ash on


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ash particle size has also been revealed in some reports [19-26]. However, few such studies on biomass ashes have been reported from Japan [27,28]. Our previous paper [29] has revealed that, for one kind of woody biomass (Japanese cedar), the potassium concentration in the combustion ash decreases with increasing mass median diameter of the particles. Based on the results, we proposed a method to enrich potassium in the ash; the ash component with more than 35% of potassium concentration could be successfully obtained by selectively collecting the finer ash with a particle-size classifier [30,31]. We also found that this ash could be used as raw material to produce chemical fertilizers that completely satisfy the official specifications from the Fertilizers Regulation Act in Japan. However, these results have not been confirmed in other types of biomass combustion ashes. Neither has the method of potassium enrichment with particle-size classification technique been effectively adapted to an actual factory-scale plant. In the present study, we examined four biomass ash samples from various woody sources and four different furnaces. The dependence of their potassium concentration on the ash particle size was investigated. To understand the cause of the correlation, the form of potassium components in the ash was also analyzed. A factory-scale potassium enrichment experiment by using the particle-size classification technique was carried out in an actual biomass power plant. Finally, a method to estimate the degree of potassium enrichment was proposed.


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2. MATERIALS AND METHODS Four biomass combustion ash samples from different four power plants in Japan were used. Table 1 lists the corresponding furnace type, boiler output, biomass fuel type, and properties of the original ashes. Plant A used a fluidized bed combustion furnace and Oregon pine as the biomass fuel. Stoker furnaces were installed in Plants B, C, and D, which used Japanese cedar, general wood (chips, branches, sawdust), and Palm Kernel Shell (PKS) as the biomass fuel, respectively. Among the four types of ashes, that from Plant C has the lowest potassium concentration. Table 1. Specification of furnace and fuel, and ash for Plant A–D. Boiler plant Furnace type Steam temperature [°C] Rate of steam generation [kg/h] Biomass fuel type Median diameter of original ash [µm] Concentration of potassium in ash [mg/g]

Plant A Plant B Fluidized bed Stoker furnace combustion furnace

Plant C

Plant D

Stoker furnace

Stoker furnace

305

350

425

480

39,000

20,000

48,200

75,000

General wood

General wood

Oregon pine

Japanese cedar

(Bark: 75%, Raw sawdust: 25%)

(Bark: 20%, Dried sawdust: 80%)

10.0

97.2

(Branch: 40%, Chip: (Branch: 60%, Dried 40%) sawdust: 20%)

+ PKS: 20%

+ PKS: 20%

11.4

9.0

7.5

116.6

16.9

107.3

Figure 1 contains the schematic diagrams of different combustion technologies used in the biomass power plants. The combustion ash from the fluidized bed furnace was a mixture of ashes


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captured by a boiler, a multi cyclone, and a bag-filter. Plant A belonged to this method. In contrast, in the stoker furnace combustion process, the ashes collected by the boiler and the multi cyclone were returned to the stoker furnace, since the unburned component in the ash should be incinerated again. The combustion ash without the bottom ash was ejected as the mixture of ashes captured by the cooling tower and the bag-filter. This method was adopted in Plants B, C, and D. In general, the combustion temperature of the stoker furnace is about 1000 °C, which is higher than that of the fluidized bed furnace of 800 °C. The power generation output from these plants was in the range of 5,800–18,000 kW.

Figure 1. Schematic diagram of biomass power generation plants installed with different combustion methods.


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Several tens of kilograms of ash samples were collected from the four different power plants. Each collected sample was manually mixed and homogenized, and then separated to volumes suitable for various experiments and analyses using sample powder dividers. In order to investigate the dependence of ash composition on ash its particle size, the collected samples were classified by the cyclone separator shown in Figure 2 [32]. The sample ash was fed from a screw feeder, dispersed into the main aerosol flow (0.5 m3/min) by a disperser, and then introduced into the cyclone separator via the aerosol inlet. Here, the feed rate of the ash was maintained constant at 2.0 g/min. The coarse ash was captured in a dust box, and the fine one eliminated through the vortex finder was collected by a glass fiber filter. The cut size of the cyclone separator was controlled by the rate of the blow-up flow (0.05–0.15 m3/min) and the set position of the apex cone installed at the top of the dust box.

Figure 2. Schematic diagram of ash classification experimental set-up and dimension of cyclone separator.


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The sample ashes were analyzed for their potassium concentration, using a protocol that is in full compliance with the “Testing Methods for Fertilizer” standard by the Food and Agricultural Materials Inspection Center (FAMIC) in the Ministry of Agriculture, Forestry, and Fisheries of Japan [33]. Details of this analysis method can be found in our previous paper [29]. The morphology and the constituent elements of the ashes were observed and measured by fieldemission scanning electron microscopy (FE-SEM, Hitachi, S-5200) with energy-dispersive Xray spectroscopy (EDX, EDAX, Genesis XM2). The crystalline phases were identified using Xray diffraction (XRD, Rigaku, MiniFlex600). The particle size distribution was measured by a laser diffraction-scattering particle-size analyzer (Horiba, LA-920), while the ash particles were completely dispersed in ethanol or distilled water by a sonicator.

3. RESULTS AND DISCUSSION 3.1. Form of potassium in combustion ash and its dependence on particle size Figure 3 shows the relationship between the mass median diameter of the classified combustion ash particles and the potassium concentration (K2O basis) in them for each power plant. Here, the particle size was measured using ethanol as the dispersion medium. For all cases, the potassium concentration in the ash increased as its median diameter decreased, particularly in the range of below 10 µm. However, the dependence of the potassium concentration on the particle size also varies a great deal among the four power plants. Namely, the ash particles from Plant B with a median diameter of about 4 µm contained more than 450 mg/g potassium. In contrast, the relatively smaller ash particles from Plant C (median diameter < 1 µm) had much lower


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potassium concentration. Furthermore, for Plants A, B, and D, combustion ashes with potassium concentrations more than 250 mg/g could be acquired successfully by size classification of the ash. Ashes with this level of potassium or higher could be used as raw material for fertilizers, because the potash ore used to produce chemical fertilizer in Japan has a potassium concentration of 200–300 mg/g.

Figure 3. Relationship between the mass median diameter of classified combustion ash and its potassium concentration (K2O basis) for various power plants.

Figure 4 shows the XRD peak patterns of the combustion ashes from the four plants. The crystalline phase of SiO2 could be detected in all samples except that from Plant B. This may be due to mixing of the fluidized bed sand or furnace refractory into the ashes and natural Si in the woody biomass. All types of ashes contained the crystalline phases of CaCO3, KCl, and K2CO3. In the ashes of Plants B and D, the crystalline phase of K2SO4 could be also found. Although the


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crystal structures of these potassium components are different among the four plants, all of them have hydro-soluble crystalline phases.

Figure 4. XRD peak patterns of original combustion ashes discharged from Plants A–D.

In order to confirm the aqueous solubility of the potassium component in the ash, 5.0 g of the ash was completely dispersed into 250 mL of distilled water by a stirrer, and then the ash was removed by filtration and dried at 100°C. XRD peak patterns of the washed ash samples are shown in Figure 5. Peaks of KCl, K2SO4, and K2CO3 almost disappeared from all washed ashes. The crystalline phase of CaCO3 remained in all samples, and its peak intensity is higher than that of the original ash because the content of CaCO3 increased relatively after the soluble component was dissolved in water. Therefore, almost all the potassium component existed as crystallized hydro-soluble materials in all types of ashes. This was further confirmed by the observation that the amount of potassium in the filtrate (in distilled water) measured by ICP (SII, SPS-3000) is in


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close agreement with that in the ash analyzed by the standard method from FAMIC. It was also confirmed that the cation concentrations of Si, Na, Ca, and Mg, which are the main components of the combustion ash, were much lower than that of potassium. These facts mean that when distilled water was used as the dispersion medium, only particles of water insoluble components (such as CaCO3) were measured in the particle size distribution. On the other hand, it is wellknown that ethanol could not dissolve any components in the combustion ash. Therefore, the following equation based on the mass balance theory could produce the particle size distribution of water soluble particles, most of which should be composed of potassium components such as KCl, K2SO4, and K2CO3.

=

(1)

where Dp [µm] is the particle diameter, and FK(Dp) [%] is the cumulative undersize of water soluble component (mainly potassium components) particles. FEtOH(Dp) [%] and FWater(Dp) [%] are the cumulative undersize analyzed using ethanol and distilled water as the dispersion medium, respectively. RK [-] represents the mass fraction of the potassium component to the original ash.


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Figure 5. XRD peak patterns of various combustion ashes after washing with distilled water.

Figure 6 shows the measured cumulative undersize distribution of water insoluble component (mainly calcium component) particles, and the calculated cumulative undersize distribution of water soluble component (mainly potassium component) particles from calculation. Both particle size distributions differed completely depending upon the plant type. The water insoluble component has relatively larger particle size. However, the two size distributions are almost the same in the range below 10 µm, of which the mass median diameters are also approximately 15– 20 µm. On the other hand, it can be found that the mass median diameter of water soluble component particles is remarkably different among the four plants. That is, the median diameter decreased from about 9 to 2 µm in the order of A > C = D > B. In particular, the water soluble component of Plant B contained a number of submicron sized particles. Here, it is thought that the potassium component particles in the ash were generated via vaporization in the high-


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temperature exhaust gas from the furnace. Therefore, the size of these particles should be relatively small. Their size is determined not only by the potassium concentration in the exhaust gas (which depends on the potassium concentration in the biomass fuel), but also by parameters in the process flow: the temperature of the exhaust gas, the cooling rate, etc. [29]. Accordingly, the size distribution of potassium component particles may vary widely among the samples that were obtained from different power plants using different biomass fuels. These size analysis results revealed that the different dependencies of potassium concentration on the ash particle size from different power plants are due to the variation in the size distribution of soluble potassium component particles among the samples.


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Figure 6. Cumulative particle size distribution for water insoluble component and potassium component in ash.

To investigate the form of potassium component in the combustion ash, SEM observation of the morphology of original ashes and their element analysis by EDS were conducted. The results are shown in Figure 7. Figure 7(a) reveals that potassium component in the ash from Plant A


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contains single KCl particles (such as Particle 2) about 3 µm in diameter, which is almost the same size as the spherical CaCO3 particle (Particle 1). In Figure 7(b) (Plant B), very fine K2CO3 particles (0.5 µm in size, such as Particle 2) adhered on the agglomerated particles (Particle 1) that contain K2CO3 and CaCO3 with a diameter of about 5 µm. Similarly, in the case of Plant C, very fine K2CO3 particles (0.8 µm, Particle 2) are deposited on the spherical Particle 1 (6 µm) consisting of Al, Si, and Ca. Figure 7(d) showed that very fine particles (0.8 µm, Particle 2) consisting of K2CO3 and K2SO4 adhered on the agglomerated particles (Particle 1, about 3 µm) containing K2CO3 and CaCO3. Here, the low detected phosphorus component in Figure 7 might be due to the soil attached to the biomass fuel. SEM observation and elemental analysis at higher magnification were also carried out in the same manner, and the results produced the forms and the particle size range of potassium components in the combustion ashes in Table 2. The average sizes of the potassium component particles shown in Table 2 are in qualitative agreement with the median sizes in Figure 6(b) (Plants A > C = D > B). Furthermore, the observation that the combustion ash from Plant B contained submicron sized potassium particles at the highest fraction is also consistent from both analysis methods. In each combustion ash, some of the crystalline phases identified by XRD measurement were also detected in SEM-EDS analysis. Accordingly, these experimental findings lead to the conclusion that the potassium component in the combustion ash mainly exists as relatively small single particles of crystalline potassium compounds, although the particle size and crystalline phases depend upon the kind of power plant. As mentioned earlier, this may be due to the potassium concentration in the biomass fuel and the process conditions (the combustion temperature, cooling rate of the exhaust gas, and so on).


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Table 2. Existence form and particle size range of potassium component in combustion ashes from Plants A–D. Plant A

Plant B

Existence form

KCl

K2CO3

Particle size range [µm]

2~6

0.8 ~ 2

Plant C KCl K2CO3

Plant D K2CO3 K2SO4

~ 0.8

0.8 ~ 3

Figure 7. SEM-EDS analysis results of combustion ashes from Plants A–D.

Figure 8 shows the relationship between the mass median diameter of the classified ash and the enrichment factor EE [-], which was calculated from the relation between the potassium concentration in the ash and the mass median diameter shown in Figure 3. Here, EE was defined as the ratio of the potassium concentration in the finer ash collected in the glass fiber filter to that in the original ash fed into the cyclone separator. For all power generation plants, the potassium


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concentration in the ash increased as its median diameter decreased. As in the relationship of Figure 3, the enrichment factor increased as the median diameter decreased. However, the particle size dependence of the enrichment factor was opposite to that of the potassium concentration in the ash, and the most pronounced variation of the enrichment factor occurs in the combustion ash from Plant C. In addition, the particle diameter where the enrichment factor rapidly increases is approximately 40 µm for Plant C and about 15 µm for the other plants. These differences, which could not be observed in Figure 3, were thought to be due to the low potassium concentration in the sample from Plant C. In other words, it could be said that the enrichment factor depends on three variables: the potassium concentration in the original ash, the size distribution of the potassium component particles, and the cut size of the classification.

Figure 8. Relationship between the mass median diameter of classified combustion ash and enrichment factor for various power plants (K2O basis).


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Based on the above findings, a method for estimating the enrichment factor was investigated. Assuming the ash could be perfectly classified at the cut size Dpc [µm], which means that all ash particles smaller than Dpc were collected as enriched potassium finer ash, the values of cumulative undersize of potassium and water insoluble component particles at Dpc were expressed by FK(Dpc) [%] and FWater(Dpc) [%], respectively. Hence, the enrichment factor could be obtained by the following equation.

=

(2)

The calculated EE is plotted in Figure 9 as a function of two variables: the mass fraction of potassium component to the original ash (RK), and the cumulative undersize of water soluble component particles at Dpc (FK(Dpc)). Here, Dpc and FWater(Dpc) were assumed to be 4.0 µm and 10% from the size distribution shown in Figure 6(a), respectively. These values corresponded to classifying the ash samples at 4.0 µm. It can be found that EE increases monotonically with increasing the cumulative undersize value of water soluble component particles FK(Dpc), which corresponds to decreasing the size of the water soluble particles, and decreasing the mass fraction of potassium component to the original ash RK. The conditions of lower RK and higher FK(Dpc) also provided a faster increase in the enrichment factor.


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Figure 9. Estimated enrichment factor as a function of the mass fraction of potassium component to the original ash, and the cumulative undersize at the cut size.

For all four ash samples from Plants A–D, the enrichment factor was evaluated by the same method assuming that the potassium-rich finer ash particles 2.5 µm in median diameter were obtained. The comparison between the measured enrichment factor and the estimated ones for potassium-rich finer ashes from the four plants is shown in Figure 10. The calculated enrichment factor did not quantitatively agree with the measured values, possibly because the actual classifier (e.g. the cyclone separator) could not separate particles perfectly at a cut size but has Tromp's curve with a gradual slope instead. However, since the difference in the enrichment factor among the four plants could be qualitatively reproduced by the calculation results, this method was considered to be effective in practical use.


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Figure 10. Comparison between measured and estimated enrichment factor of enriched potassium finer ashes from Plants A–D (Median diameter: 2.5 µm).

3.2 Factory-scale potassium enrichment by particle size classification The above analysis revealed that the potassium concentration in the combustion ash could be controlled by separating the ash particles by size on the laboratory scale. Next, it was investigated whether enriched potassium combustion ash could be acquired by selectively collecting small ash particles using a plant-scale classification operation. Figure 11 shows the schematic diagram and photograph of the enrichment process set up in Plant D, which has 18,000 kW of power generation capacity when 600 t/day of biomass fuel is supplied. This enrichment process consists of an ash receiving hopper, a disperser, a highperformance mechanical centrifugation type classifier, and a bag filter dust collector. All the combustion ash discharged from the boiler plant was fed into the classifier. The classified fine ash particles were collected in the bag filter. The cut size of the classifier was easily varied by changing the rotational speed of the classification rotor in it, and the flow rates of primary and


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secondary air feeds. Namely, the particle size of the ashes collected in the classifier and in the bag filter dust collector could be controlled.

Figure 11. (a) Photograph and (b) schematic diagram of the enrichment process set up in Plant D.

Using this process, the dependence of the enrichment factor on the mass median diameter of the ash particles was investigated. As shown in Figure 12, the enrichment factor rapidly decreased with increasing mass median diameter, then it decreased more gradually. The factory-scale classification operation was shown to be able to provide a high enrichment factor almost equal to that on the laboratory scale. Since the enrichment factor and the yield of fine ash (which corresponds to the enriched potassium ash) are in a trade-off relationship, it is natural that the


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yield is lowered as the enrichment factor increases. The yields at EE = 2.8 (corresponding to a potassium concentration of 300 mg/g) was equal to 17.2%.

Figure 12. Relationship between the mass median diameter of combustion ash and enrichment factor for Plant D from factory-scale operation.

In Japan, in order to use a certain substance as raw fertilizer material, the official specifications enacted by the Fertilizers Regulation Act must be satisfied. The official specification for byproduct compound fertilizer is as follows [33]: < 0.75 ppm cadmium and < 20 ppm arsenic per 1.0% of the maximum concentration among the nitrogen, phosphoric acid, and potassium components in the fertilizer. It was confirmed that all the enriched potassium combustion ash samples satisfied this standard. Furthermore, thus enriched potassium ash can be obtained by selectively segregating the fine ash by using the size classification approach in a factory-scale commercial facility.


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4. CONCLUSIONS For combustion ash samples discharged from four different biomass power plants, the dependence of the ash particle size on its potassium concentration was investigated, and a method to enrich potassium in the combustion ash using a particle-size classification technique was proposed. Moreover, the enrichment factor was estimated from the particle size data, the mass fraction of potassium component to the original ash, and the cut size of the classifier. The results obtained can be summarized as follows: 1. The potassium concentration in combustion ashes eliminated from all biomass power plants increased as the median diameter decreased. However, this relationship varied remarkably according to the type of the power plant. 2. Almost all the potassium component in the ash existed as crystallized hydro-soluble materials, such as KCl, K2SO4, and K2CO3 for all types of ashes. However, its crystalline phases depended on the type of power plant. 3. The potassium component in the combustion ash mainly existed as relatively small single particles composed of crystalline potassium compounds, although the particle size and crystalline phases varied by the kind of power plant in this work. 4. The mass fraction of potassium component to the original ash, the particle size distribution of the potassium and other components, and the cut size of the classifier


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determine the enrichment factor. A method to estimate the enrichment factor from these data was proposed, and the estimation results qualitatively agreed with the experimental values for all types of ashes in this study. 5. Enriched potassium ash, which could be a valuable raw material for the production of fertilizer, could be obtained on an industrial scale by collecting the finer ash (i.e., smaller particles) selectively using a particle-size classification process constructed in the actual biomass power plant in this study.

AUTHOR INFORMATION Corresponding Author * Telephone: +81-82-424-7715, E-mail: [email protected]

ACKNOWLEDGEMENTS This investigation was carried out as a grant-aided project by the Forestry Agency of the Ministry of Agriculture, Forestry, and Fisheries of Japan in 2015, and was also supported by the Electric Technology Research Foundation of Chugoku in 2017 and Tanikawa Fund Promotion of Thermal Technology 2017. The authors acknowledge valuable discussions with Dr. Katsunori Noguchi (Katakura ＆ Co-op Agri) and Mr. Hidenao Matsuoka (Chugoku Mokuzai).


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Existence Form of Potassium Components in Woody Biomass

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