A large amount of incinerated ash is emitted with the construction of biomass power generation facilities, and there is an urgent need for its effective use or storage management. Incinerated ash contains a large amount of potassium and is attracting attention as a resource. However, the ash often contains harmful elements such as hexavalent chromium and fluorine [1], and there is a need to make these substances harmless.

The authors are researching a method to make incinerated ash harmless, however along with the treatment process, monitoring of the harmful substances in the ash is also an important matter. The monitoring the harmful elements enables us to control the addition rate of the reagents and also the treatment effect.

The monitoring is desired to be timely, but the official methods (JIS. K-0102) require a considerable amount of time for analysis to ensure accuracy, and it usually takes 1 to 2 weeks for results to be available. In harmless processing, it is essential to grasp the status of the harmless treatment, and measurement of the target harmful elements are required in a short time.

For this reason, we believe that it is important to have a simple analytical management method that quickly determines the concentration of harmful elements. In the field of soil contamination, attempts have already been introduced using simple analytical methods in on-site management [2]. We carried out the application of the simple methods for harmless treatment of the incinerated ash [3].

Management Flow

The flowchart of the expected processes of the harmless treatment is shown in Figure 1. Before receiving the raw ash, we conduct a preliminary analysis like checking the presence and concentration of harmful substances, and select the chemicals which are necessary to render them harmless. After the harmless treatment, the effectiveness of the treatment can be confirmed by analyzing the treated ash. In these cases, simple analytical methods are applied.

Figure 1: Management flow.

Simple Analysis Method

In this experiment, the analytical method was selected based on the principle that it could be easily performed on-site and does not require the use of expensive equipment.

The target harmful elements and their analysis methods are shown below.

Hexavalent chromium: Pack test (Kyoritsu Rikagaku Kenkyusho Co., Ltd.) [4]

Fluorine: Pack test (Kyoritsu Rikagaku Kenkyusho Co., Ltd.) [5], to prevent the influence of interfering substances, 2 mL of acetylacetone (Fuji Film - Wako Pure Chemical Industries, Ltd.) was added to 40 mL of the test solution, stirred and dissolved until it became transparent, and then analyzed

Arsenic: Detector tube method: 25 mL of the test solution was put into a gas generating cylinder, and added 4mL of hydrochloric acid (1+1), 2 mL of 10% potassium iodide solution, and 2 mL of 1% Tin (?) chloride solution. After connecting an arsine (AsH₃) detection tube (manufactured by Komyo Rikagaku Kogyo Co., Ltd.) to the exhaust pipe, 1 g of zinc powder (particle size 1 mm, manufactured by Kanto Kagaku Co., Ltd.) was added, and measured (Figure 2) [6].

Selenium: Detector tube method: 25 mL of the test solution was put into a colorimetric tube (50 mL), added 2 mL of hydrochloric acid (1+1) and 2 mL of 20% KBr solution, heated in boiling water for 10 minutes, later transferred to a gas generating cylinder. 4 mL of sodium tetrahydroborate solution (1%) was added, and measurements were performed using a hydrogen selenide detector tube (manufactured by Komei Rikagaku Kogyo Co., Ltd.) (Figure 2) [7,8].

Boron; HANNA Instruments test kit HI 38074 [8,9]

The measurement limits for the simple analysis methods which are carried out by our experiments are shown in the Table 1.

Figure 2: Detector tube method instrument.

Table 1: Measurement limits of the simple methods.

Procedure of Simple Elution Test

The procedure of the simple elution test is shown in Figure 3. In the official elution test, ash is mixed with water in a ratio of 1 part (ash) to 10 parts (water) [10]. Since the analytical sensitivity of the simple analytical method is low, in order to improve the accuracy, increasing the concentrations of the target substances are necessary. For this reason, a high solid-liquid ratio is desired in the elution process.

In this experiment, various amounts of the test ash were put into the beakers (internal volume 100mL), water was added to the reach marked line (100 mL), stirred for about 1 minute using a glass rod, and filtered using filter paper (Advantech 5A). The filtrate was analyzed by a simple method. When a test was conducted using hexavalent chromium as the target element, the appropriate mixing ratio was considered 30 g (dry ash) to 100 mL of total water volume (Figure 4).

The official elution test is operated on the dry samples, and wet sample (treated ash) has to be dried at room temperature for many days. Since the target ash is normally discharged as wet ash, it was decided that the drying process for the elution test was carried out as was necessary, taking into consideration time.

The moisture content of the test ash will have a large influence on the result of the elution test, so in the elution test, 30 g of dry ash (moisture content of about 1%), 38 g of semi-dry ash (moisture content of about 18%), 45g of wet ash (moisture content of about 33%) were respectively mixed with 100mL of water (total volume).

Figure 3: Simple elution test flow.

A management method was carried out using two types of biomass incinerated ash from different sources. The concentration of the harmful elements was comparatively low, in order to facilitate the experiment, potassium chromate or sodium fluoride reagent was added to the raw ash in advance to treatment. To make ash harmless, various agents under consideration were added to the ash, and the concentration of the harmful elements were measured before and after treatment using a simple analytical method, and compared with the official method.

Regarding hexavalent chromium, a good correlation with the official method was observed (Figure 5). In the case of fluorine, measurement of the low concentration range (below 0.6 mg/L) was difficult using the pack test method. However, although there is little data on high concentrations, it is presumed that judgments can be made at environmental standard concentrations of around 0.8 mg/L (Figure 6).

A good correlation with the positive method was also observed for boron, and the method was judged to be applicable (Figure 7).

Concerning other harmful substances, the concentrations in the incinerated ash used this time were low, and some of them were below environmental standards, so it was not possible to evaluate them.

Figure 4: Relation elution water and Cr⁶⁺ (pack test).

Figure 5: Relation official method and simple test (Cr⁶⁺).

Figure 6: Relation official method and pack test (F).

Figure 7: Relation official method and simple test (B).

We investigated a management method that can quickly determine the status of harmful elements in ash. It is considered effective to use simple analytical methods as a management method for removing harmful elements from incinerated ash, etc. Lead was not able to be investigated this time, however, analysis is possible using pack test method. Mercury is also considered possible to test for using detector tube method [9,10].

In the future, we will consider expanding the target element and introducing other simple analysis methods [11], as well as accumulate data.

Acknowledgment

The authors wish to express their appreciation to Dr. Eric Bray (Former Professor of Yokkaichi University) for his advice on making this article.

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