*Article* **Two-Stage Continuous Process for the Extraction of Silica from Rice Husk Using Attrition Ball Milling and Alkaline Leaching Methods**

**Ji Yeon Park 1,2, Yang Mo Gu 1,2, Seon Young Park 1,2, Ee Taek Hwang <sup>3</sup> , Byoung-In Sang <sup>2</sup> , Jinyoung Chun 1,\* and Jin Hyung Lee 1,\***


**Abstract:** A two-stage continuous process was developed for improved silica extraction from rice husk. The two-stage continuous process consists of attrition ball milling and alkaline leaching methods. To find the optimum conditions for the continuous process, the effects of alkaline leaching parameters, such as the alkaline solution type and reaction conditions, on the silica extraction yield were investigated in a batch process. The use of NaOH showed a slightly higher silica yield than KOH. The optimum reaction conditions were found to be 0.2 M, 80 ◦C, 3 h, and 6% (*w*/*v*) for the reaction concentration, temperature, duration time, and solid content, respectively. Attrition ball milling was used to make micron-sized rice husk particles and to improve the fluidity of the rice husk slurry. The two-stage continuous process was performed using optimum conditions as determined based on the results of the batch experiment. The two-stage continuous extraction was stably operated for 80 h with an 89% silica yield. During the operation, the solid content remained consistent at 6% (*w*/*v*). The obtained silica was characterized using inductively coupled plasma–optical emission spectrometry (ICP–OES), X-ray diffraction (XRD), and the Brunauer–Emmett–Teller (BET) method.

**Keywords:** silica; rice husk; alkaline leaching; continuous process; biomass; bio-based material

#### **1. Introduction**

Rice is a major agricultural product across the world, and its annual production was approximately 996 million tons in 2018 [1]. Rice husk accounts for 20% of rice byproducts [2] and has various applications in different industries, e.g., (a) as an industrial fuel for paddy processing and in the generation of process steam in power plants; (b) as a fertilizer and substrate or pet food fiber; (c) as an ingredient for the preparation of activated carbon or substrate for silica and silicon compound production, and (d) as raw material for brick production [3,4]. Rice husk is composed of approximately 70–80% organic substances such as cellulose, hemicellulose, and lignin, and the remaining 20–30% comprises inorganic compounds [5,6]. A major inorganic component is silica, which accounts for approximately 95% of the inorganic compounds. The silica in rice husk is amorphous and has a colloidal state in water. Silica is an industrial material that is highly utilized as an additive for catalysts, insulation, toothpaste [7], coating solutions [8,9], and cosmetics [10]. The use of "biosilica" (rice husk-derived silica) as an alternative for silica in various industrial applications would mitigate high energy consumption, natural resource depletion, and greenhouse gas emissions [3].

Two approaches are used to extract silica from rice husk: combustion and chemical treatment. Direct combustion is the most popular method and is conducted in open fire

**Citation:** Park, J.Y.; Gu, Y.M.; Park, S.Y.; Hwang, E.T.; Sang, B.-I.; Chun, J.; Lee, J.H. Two-Stage Continuous Process for the Extraction of Silica from Rice Husk Using Attrition Ball Milling and Alkaline Leaching Methods. *Sustainability* **2021**, *13*, 7350. https://doi.org/10.3390/su13137350

Academic Editors: Dirk Enke, Hossein Beidaghy Dizaji, Volker Lenz and Thomas Zeng

Received: 6 June 2021 Accepted: 29 June 2021 Published: 30 June 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

stoves or boilers. During burning, rice husk is oxidized, resulting in ash products. This is the simplest method to obtain inorganic compounds from rice husk. The inorganic compounds, so-called rice husk ash, can be converted into soluble sodium silicate by reacting with aqueous sodium hydroxide [11]. Lee et al. used sulfuric acid to remove organic compounds from rice husk before combustion [5]. Sulfuric acid dissolves most celluloses and hemicelluloses, which are discarded by separating liquids from solids. The acid treatment improved the purity of silica finally obtained. Hincapié-Rojas et al. obtained submicron silica particles from rice husk by subsequent treatments of combustion, acid leaching, and mechanical ball milling [12]. Souza et al. compared the use of hot organic acid and boiling water before combustion to obtain high-quality silica [13]. Chemical extraction is adopted for environmentally friendly extraction. The chemical extraction method consists of acid treatment and an alkaline leaching step [14]. In the chemical extraction method, several chemical routes are used to achieve highly efficient silica extraction [14–17]. Zulkifli et al. combusted rice husk to obtain rice husk ash, followed by acid and alkaline leaching [17]. Chun et al. directly treated rice husk with sulfuric acid to leach metallic impurities, followed by combustion to remove organic residual compounds; afterward, high-purity silica was dissolved in sodium hydroxide to control the size and pores in silica particles [14]. In their study, 99.8% silica purity was obtained. Costa and Paranhos converted rice husk to rice husk ash (RHA) by combustion. The rice husk ash was dissolved in concentrated sulfuric acid, followed by treatment with alkaline solution to obtain sodium silicate. Nanosilica particles were synthesized by precipitation using phosphoric acid [15]. Song et al. employed the Taguchi method to obtain surfactant-free synthesis of high surface area silica nanoparticles from rice husk [16]. The Taguchi method was efficient for designing factorial experiments with a minimum number of experiments. Regardless of the specific chemical route used, the alkaline leaching step is critical for obtaining high-purity silica from rice husk [6].

This study developed a two-stage continuous silica extraction process from rice husk using attrition ball milling and alkaline leaching methods. A continuous process has several advantages over a batch process, namely production of a narrow specification product, reduced production cost, and increased productivity. Rice husk has a very low density, within the range 90–150 kg/m<sup>3</sup> [18], and conveying is usually conducted by a pneumatic conveying system [19]. In this study, rice husk was ground into micron-sized particles and mixed with a sodium hydroxide solution to make a rice husk slurry. The rice husk slurry can be easily conveyed by a fluid pump and continuously reacted to leach silica from rice husk. In addition, alkaline leaching was performed under an atmospheric pressure, which is safe to apply in a rice mill where rice husk is generated. Therefore, this study is an initial step toward the field application of a silica extraction process using rice husk. The circular bioeconomy has gained attention as a key concept for sustainable technical cycles. The circular bioeconomy focuses on the valorization of biomass in integrated production chains and making use of residues [20]. Currently, biomass valorization focuses on valorizing the organic fraction of biomass [21]. However, the valorization of ash content is also important and has the potential to extract more value from biomass. In this respect, this study is worthwhile to extend the area of biomass valorization and, ultimately, promote the facilitation of circular bioeconomy.

#### **2. Materials and Methods**

#### *2.1. Materials*

Rice husk was kindly supplied by a rice processing facility in the Chungbuk region, Rep. Korea, which was harvested in 2019. Sodium hydroxide powder (97%), acetic acid (99.5%), and potassium hydroxide (93.0%) were purchased from Daejung Chemicals & Metals Inc. (Goryeong, Korea). Sodium hydroxide powders were dissolved in distilled water and used in the experiments; the others were used as received without further purification.

#### *2.2. Alkaline Leaching Process*

Before using the rice husk, it was washed with deionized water three times and dried at 80 ◦C overnight. After drying, the rice husk was immersed in an alkaline solution (sodium hydroxide or potassium hydroxide) and thoroughly mixed to allow sufficient soaking in the solution. The sample was moved to a heating oven (ThermoStableTM "OF-105", Daihan Scientific, Wonju, Korea) set at a specific temperature for reaction over a given reaction time. After reaction, the solids were separated from the solution using vacuum filtration (Circulating Aspirator (WJ-15, SIBATA, Saitama, Japan)) and filter paper (Whatman No. 41, 20~25 µm, Maidstone, UK). To measure the leached ash, acetic acid was added to the solution to adjust the pH to 7.0, which was stirred at 300 rpm overnight. The precipitation was washed three times with deionized water at 4000 rpm for 10 min. The washed precipitation was dried at 80 ◦C overnight. The organics such as hemicellulose and lignin were leached during the alkaline leaching process and contained in the precipitation. Therefore, the washed precipitation was calcined at 900 ◦C for 6 h to remove the organics in the precipitation. The silica yield was calculated using Equation (1) below:

$$\text{Silica extraction yield} \left( \% \right) = \frac{\left( \text{Weight of ash precipitated} \times \text{silica purity} \right)}{\left( \text{Weight of Ricci husband} \times \text{silica purity} \right)} \times 100 \qquad \text{(1)}$$

To find the optimum alkaline leaching conditions, four experimental parameters—the solid content, alkaline reaction concentration, temperature, and duration—were optimized.

#### *2.3. Attrition Ball Mill*

An attrition pulverizer (Korea powder system Co., Ltd., Incheon, Korea) previously developed for lignocellulosic pretreatment [22] was used to prepare micron-sized rice husk particles. One-third of the inner space was filled with rice husk, while another third was filled with grinding steel balls (10 mm in diameter). Alkaline solvent was added to the grinding jar. The rice husk was pulverized under wet-grinding conditions at 300 rpm for 20 or 30 min. After milling, the rice husk was transferred to 1 mm shaking sieve (Aanlysette3, Fritsch GmbH, Idar-Oberstein, Germany) and shaken for 1 min to separate the pulverized rice husk particles from the grinding balls.

#### *2.4. Two-Stage Continuous Silica Extraction Process*

A schematic diagram of the continuous silica extraction process is shown in Figure 1. The continuous extraction process consists of two steps: pulverization and alkaline reaction. At the pulverization step, rice husk was pulverized to make fine rice husk particles and to increase the fluidity. In the continuous process, rice husk was pulverized in an alkaline solvent at 300 rpm for 20 min and stored in a reservoir after separating from grinding balls. The reservoir was stirred at 600 rpm using an electronic overhead stirrer (MS 3060D, MTOPS, Yangju, Korea) to prevent the rice husk particles from settling down. The rice husk slurry in the reservoir was continuously fed into a reactor using a peristaltic pump (BT100S, Lead Fluid Technology, Co., Ltd., Baoding, China). The reactor was stirred at 400 rpm and 80 ◦C. The outlet sample was collected and separated using a vacuum filter and filter paper (Whatman No. 41, 20–25 µm, Maidstone, UK) for calculating the silica yield, which was calculated as described in Section 2.2. For measuring the solid content, 10 g of the rice husk slurry was sampled at the outlet of the reservoir every 8 h. The sample was kept in a heating oven (ThermoStableTM "OF-105", Daihan Scientific, Wonju, Korea) set to 105 ◦C for 24 h. The solid content was calculated by using the weight difference before and after drying.

**Figure 1.** Scheme of the continuous silica extraction process.

#### *2.5. Analytical Methods*

The compositions of rice husk, carbohydrates and lignin, were measured according to the standard procedure provided by National Renewable Energy Laboratory (NREL) [23]. The compositions of rice husk were compared between before and after extracting to calculate the quantities of extracted carbohydrates and lignin. The inorganic chemical composition was determined by using inductively coupled plasma–optical emission spectrometry (ICP–OES; Optima 5300DV, PerkinElmer, MA, USA). X-ray diffraction patterns were obtained using X-ray diffraction (XRD; D/Max 2500/PC, Rigaku, Tokyo, Japan). The surface areas of the obtained silica were calculated from the measured isotherms according to the Brunauer–Emmett–Teller (BET) method, and the pore volumes were taken at the P/P<sup>0</sup> = 0.995 single point using a Micromeritics Tristar 3200 system (Micromeritics Inc., Norcross, GA, USA). The pore size distributions of the silica were calculated using the Barrett–Joyner–Halenda (BJH) method from the adsorption branches of the isotherms. The rheological properties of the pulverized rice husk slurry were analyzed using a stresscontrolled rotational rheometer (MCR 702, Anton Paar, Graz, Austria) with a C-PTD200 (Cup-Peltier Temperature Device).

#### **3. Results and Discussion**

#### *3.1. Optimization of Alkaline Leaching Conditions*

For extracting silica from rice husk, two alkaline solutions, NaOH and KOH, were used and their performance compared. Both are popularly used alkaline solutions and have similar properties. The performances of various concentrations of NaOH and KOH solutions were compared to assess which would be optimal for leaching. Figure 2a presents a comparison of the performances of the two alkaline solutions regarding silica leaching from rice husk depending on their concentrations. Both showed similar extracting yields, but there was a slight difference. At 0.1 M concentration, the silica extraction yields were 1% and 30% for NaOH and KOH, respectively. The use of KOH showed a higher extraction yield at 0.1 M concentration. As the concentration of the alkaline solution increased, the extracting yields increased up to a certain point. Over this point, the extracting yields were saturated and did not increase further, even when the alkaline concentration increased. When NaOH was used, the silica yield became saturated starting from 0.2 M, with an approximately 79% yield. When KOH was used, saturation of the extracting yield was approximately 77%, starting from 0.5 M KOH. Therefore, NaOH can be used at a lower concentration than KOH while obtaining a slightly higher yield.

**Figure 2.** Comparison of the silica extraction yield depending on (**a**) the type of alkaline solution, (**b**) the alkaline leaching reaction time, (**c**) the temperature, and (**d**) the solid content.

When using NaOH, similar silica extraction yields were found for 0.2 and 0.5 M— 79.3% and 79.9%, respectively. Therefore, both concentrations were tested for optimizing the reaction time, temperature, and solid content. Under the conditions of 80 ◦C reaction temperature and 6% (*w*/*v*) solid content, five reaction times—1, 2, 3, 4, and 5 h—were investigated to determine the optimum reaction time. As the reaction time increased, the silica yield increased up to 3 h (Figure 2b). At reaction times over 3 h, the yield did not increase further (Figure 2b). At 3 h reaction time, no significant difference in yield was observed between use of 0.2 and 0.5 M concentrations. The optimum reaction temperature was determined among six chosen temperatures. At the temperature of 25 ◦C, only a small quantity of silica leached from the rice husk into the NaOH solution: 1.4% and 14% for 0.2 and 0.5 M, respectively (Figure 2c). At 60 ◦C, the silica yields increased to 68.4% and 63.6% for 0.2 and 0.5 M, respectively. Over 70 ◦C, the silica yield did not significantly increase, even when the reaction temperature was increased. The yield over 70 ◦C was approximately 80%. In the reaction temperature tests, there was no significant difference between 0.2 and 0.5 M. The solid content is related to reaction volume, which determines the reactor size. As the solid content increased, the silica yield decreased because of a lack of NaOH compared to Si (Figure 2d). Typically, there was tendency for a higher decrease in silica yield for 0.2 M compared with 0.5 M. The highest silica yield was found at 6% (*w*/*v*): 79.3% and 79.9% for 0.2 and 0.5 M, respectively.

#### *3.2. Preparation of Rice Husk Slurry for Continuous Process*

For application in a continuous process, rice husk should be continuously supplied to a reactor. Rice husk has a very low density and is usually conveyed by a pneumatic conveying system. In this study, rice husk slurry was prepared to easily convey the sample by a fluid pump. To prepare the rice husk slurry, rice husk was pulverized in NaOH solution by attrition ball milling. The rice husk had a diameter of approximately 6–7 mm before milling. After ball milling, the size of rice husk was drastically reduced, the extent of which was mainly related to milling time. However, the alkaline concentration also slightly affected the size reduction. The mean diameter of the rice husk particles was 228.9 and 139.0 µm for 20 and 30 min of milling, respectively, when it was treated with 0.2 M NaOH. When 0.5 M NaOH was used, the mean diameter was 218 and 78.1 µm for 20 and 30 min of milling, respectively. The size distribution of the rice husk after 20 min of milling showed a bimodal curve, with peaks being observed at approximately 77 and 777 µm (Figure 3). However, the peak around 777 µm reduced and shifted to 77 µm as the milling time was increased to 30 min. This result indicates that the size of rice husk particles became more homogenous as the milling time increased. The increase of milling time effectively reduced the portion of larger particle sizes. The concentration of NaOH also affected particle size distribution, but not as much as milling time. When comparing 0.2 and 0.5 M NaOH with 20 min milling time, 0.5 M NaOH showed a lower peak on 777 µm and a higher shoulder on 23 µm when compared to 0.2 M NaOH. In the 30 min milling condition, both samples of 0.2 and 0.5 M NaOH showed a monomodal curve. However, the graph of 0.5 M NaOH treatment showed a higher peak than 0.2 M NaOH. This result indicates that the size distribution of the rice husk particles was affected mainly by milling time and partially by NaOH concentration.

**Figure 3.** Size distribution of the pretreated rice husk under the conditions of (**a**) 0.2 M NaOH and (**b**) 0.5 M NaOH. Dash and line indicate 20 and 30 min milling, respectively.

Figure 4 shows the rheological properties of the pulverized rice husk slurry with a 6% (*w*/*v*) solid content. The viscosity of the slurry solutions decreased as the shear rates increased. The viscosities of all samples decreased as the shear rate increased, indicating shear thinning. The untreated sample showed higher shear stress and viscosity than the treated samples (Figure 4). The shear stress and viscosity of a slurry are closely related to the particle size [24–26]. In general, the shear stress and viscosity of the slurry increased as the particle size increased, especially at low shear rates. In this study, the untreated slurry contained 6–7 mm rice husk particles. The rice husk particles were soaked with alkaline solution, meaning the larger particles could have been heavier than the smaller particles because of soaking up more of the alkaline solution and, thus, needed stronger force to be moved. Therefore, the untreated rice husk slurry showed higher shear stress and viscosity through the range of shear rate (Figure 4). As the particle size decreased, the shear stress and viscosity reduced. In Figure 4, the ball-milled samples show drastically reduced shear stress and viscosity. This indicates that the ball-milled rice husk slurries needed less force to be moved than the untreated slurry. The ball mill used in this study improved the fluidity of the rice husk slurry and made conveying rice husk easy.

**Figure 4.** Rheological properties of the rice husk slurry before/after ball milling and shown as a function of shear rate: (**a**) viscosity and (**b**) shear stress. The ball-milled rice husk slurry was prepared by ball milling for 20 min with 0.2 M NaOH.

#### *3.3. Continuous Silica Extraction Process*

The continuous silica extraction process was performed using conditions based on the results of the batch experiments. The milled rice husk slurry with 0.2 M NaOH solution was stored in a reservoir and continuously supplied to a reactor. For stable operation in a continuous process, the solid content should be steadily supplied to the reactor because the quantity of raw material for silica should be constant during the process. Initially, a separate set of experiments was performed to measure the solid content. At every 8 h, samples were collected from the outlet of the reservoir. The first sample, obtained 8 h after starting the continuous process, showed a 6% (*w*/*v*) solid content. During the period of the continuous process, the solid content was steady, approximately 6% (*w*/*v*) (Figure 5). As previously mentioned, the ball mill used in this study improved the fluidity and enabled a steady supply of the rice husk slurry.

**Figure 5.** Silica extraction yield and solid content in the continuous process depending on process time.

Initially, the rice husk slurry was reacted for 3 h before starting the continuous process. After the reaction, the yield of the silica extraction was 77% (Figure 5). After starting the continuous supply of the rice husk slurry, the silica yield increased to 90%. This could be due to extended reaction time for the initially filled sample. After starting the continuous process, the silica extraction yield slightly decreased, but reached a steady state after 24 h. After reaching a steady state, the silica yield was constant, indicating that the process was stable. In this study, the continuous silica extraction was performed for 80 h and the process was stable during the operation, with 89% silica yield.

Usually, the alkaline pretreatment was performed for delignification. Therefore, the continuous process used in this study could remove carbohydrates and lignin. The compositions of rice husk before the NaOH leaching were 52.8 wt% carbohydrates, 29.6 wt% lignin, and 12.9 wt% ash (Figure 6). The continuous silica extraction reaction used in this study leached 21.3% carbohydrates, 30.6% lignin, and 84.2% ash into the NaOH solution. After precipitation and calcination, about 89% of silica in the rice husk was recovered. This two-stage alkaline leach strategy is capable to produce a de-ashed rice husk slurry which is more suitable for further biorefinery, a silica-rich by-product for high-value applications, and an extracted carbohydrates/lignin mixture for further valorization.

**Figure 6.** Mass balance of the continuous silica extraction process used in this study.

#### *3.4. Characterization of the Silica Obtained from the Continuous Extraction Process*

The extracted silica from the continuous process was characterized after precipitation. Originally, the purity of silica in the rice husk ash was only 93.1% (Table 1). The rice husk ash contained high impurities such as CaO, MaO, and K2O. After alkaline leaching, the silica purity increased to 98.5%. The main impurity was Na2O, which increased from 0.08% to 0.96% after the NaOH leaching. Considering the increase in Na2O content after the NaOH leaching, it is possible that the sodium in the NaOH solution was precipitated and that it could be reduced by applying stringent washing steps.

**Table 1.** Inorganic composition of the raw material ash and the extracted ash.


The crystallinity of the obtained silica was investigated using XRD. The XRD patterns of the silica samples obtained from both the rice husk ash and the continuous processes showed a broad diffraction near to 20◦ , which is typical for amorphous silica (Figure 7). In the XRD pattern, no crystalline structure of silica was found, and continuous alkaline leaching did not cause any crystallinity changes. The phase of silica is determined by the combustion temperature [27]. Over certain temperature, the phase transformation starts but the crystallization temperature varied depending on the composition of rice husk ash. In this study, the calcination was performed at 900 ◦C for 6 h to remove residual moisture and volatile compounds. The calcination did not cause any crystallinity changes of rice husk silica.

**Figure 7.** X-ray patterns of silica samples obtained from rice husk ash (black) and the continuous process (red).

The surface area and pore volume of the silica obtained from the continuous process were 1.973 m2/g and 0.004 cm3/g, respectively (Figure 8). A previous study reported that the presence of metal impurities, such as Na and K, causes surface melting and agglomeration in the particles during combustion [28]. The surface melting and agglomeration led to reduced surface area and pore volume. In this study, the content of Na2O in the silica obtained increased due to the NaOH leaching and it could be the reason for low surface area and pore volume. This study did not control the structure of silica. Therefore, an additional process is required to obtain a highly specific surface area or well-defined nanostructures.

**Figure 8.** N<sup>2</sup> physisorption isotherms of silica obtained from the continuous process.

#### **4. Conclusions**

A process for two-stage continuous silica extraction from rice husk was successfully developed using alkaline leaching and attrition ball mill methods. The alkaline leaching

conditions obtained in batch experiments were employed in the continuous process. The attrition ball mill treatment was used to obtain a rice husk slurry, which improved fluidity of the sample. By applying alkaline leaching conditions and the ball mill-treated rice husk slurry, continuous silica extraction from rice husk was stably operated, with 89% silica yield and 6% (*w*/*v*) solid content for 80 h. An improvement in silica purity was obtained from the continuous process, which increased to 98.5% when compared to rice husk ash. The continuous process did not change the crystallinity or surface properties of the silica.

The continuous extraction process developed in this study would be beneficial for product uniformity and process capacity. It is very easy to operate once the system has been set up. Therefore, we expect that this method can be used in the field for the mass production of rice husk silica.

**Author Contributions:** Conceptualization, J.H.L. and B.-I.S.; methodology, J.H.L. and J.C.; validation, E.T.H. and B.-I.S.; formal analysis, J.H.L. and J.C.; investigation, J.Y.P., S.Y.P. and Y.M.G.; data curation, J.Y.P.; writing—original draft preparation, J.Y.P. and J.H.L.; writing—review and editing, J.H.L. and J.C.; project administration, J.H.L.; funding acquisition, J.H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, and Forestry (IPET) through the Agro and Livestock Products Safety Flow Management Technology Development Program, funded by the Ministry of Agriculture, Food, and Rural Affairs (MAFRA) (319109-02).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article* **Wood Ashes from Grate-Fired Heat and Power Plants: Evaluation of Nutrient and Heavy Metal Contents**

**Hans Bachmaier \*, Daniel Kuptz and Hans Hartmann**

Technology and Support Centre in the Centre of Excellence for Renewable Resources (TFZ), Schulgasse 18, 94315 Straubing, Germany; Daniel.Kuptz@tfz.bayern.de (D.K.); Hans.Hartmann@tfz.bayern.de (H.H.) **\*** Correspondence: Johannes.Bachmaier@tfz.bayern.de; Tel.: +49-9421-300-160

**Abstract:** Ashes from biomass heat (and power) plants that apply untreated woody biofuels may be suitable for use as fertilizers if certain requirements regarding pollutant and nutrient contents are met. The aim of this study was to examine if both bottom and cyclone ashes from 17 Bavarian heating plants and one ash collection depot are suitable as fertilizers (*n* = 50). The range and average values of relevant nutrients and pollutants in the ashes were analyzed and evaluated for conformity with the German Fertilizer Ordinance (DüMV). Approximately 30% of the bottom ashes directly complied with the heavy metal limits of the Fertilizer Ordinance. The limits were exceeded for chromium(VI) (62%), cadmium (12%) and lead (4%). If chromium(VI) could be reduced by suitable treatment, 85% of the bottom ashes would comply with the required limit values. Cyclone ashes were high in cadmium, lead, and zinc. The analysis of the main nutrients showed high values for potassium and calcium in bottom ashes, but also relevant amounts of phosphorus, making them suitable as fertilizers if pollutant limits are met. Quality assurance systems should be applied at biomass heating plants to improve ash quality if wood ashes are used as fertilizers in agriculture.

**Keywords:** wood ash; fertilizer; heat and power plants; heavy metals; nutrients; German fertilizer legislation

#### **1. Introduction**

Combustion of wood in heat (and power) plants generates solid residues in the form of ashes [1,2]. In the Federal State of Bavaria (i.e., Southeast Germany), a total of 30,000 to 60,000 t/a of wood ashes from untreated wood accumulates each year from plants with an installed capacity of more than 1 MWtherm (calculated from the 2018 Energy Wood Market Report of the Bavarian State Institute of Forestry (LWF)) [3]. Due to the physical and chemical properties of these combustion by-products, suitable utilization strategies might be recommended for their use as raw materials in the bioeconomy.

Depending on the point of origin of wood ashes in the heat (and power) plant, a distinction can be made between different ash fractions. The ash accumulating in the boiler is called "bottom ash" or "coarse ash". In most cases, the ash from the heat exchangers is also considered as part of the bottom ash. After the hot flue gas passes through the heat exchanger, the air is usually cleaned by a cyclone in which the "cyclone ash" (also called "coarse fly ash") is separated. If the plant has an electrostatic precipitator, a fabric filter or a flue gas condensation system, a third ash fraction, i.e., the so-called "filter ash" (also called "fine fly ash") or the "condensate sludge" is generated [1]. The following article focuses on bottom ash and cyclone ash from grate-fired boilers as these are the most common ash fractions in Bavarian heat (and power) plants.

The chemical composition of individual ash fractions depends on the fuel quality and the plant technology [1,2]. Chemical elements such as plant nutrients (e.g., Ca, Mg) or pollutants such as heavy metals vary in wood fuels depending on the species, but also on bark content, the share of green biomass (i.e., needles/leaves), growing conditions, the

**Citation:** Bachmaier, H.; Kuptz, D.; Hartmann, H. Wood Ashes from Grate-Fired Heat and Power Plants: Evaluation of Nutrient and Heavy Metal Contents. *Sustainability* **2021**, *13*, 5482. https://doi.org/10.3390/ su13105482

Academic Editors: Dirk Enke, Hossein Beidaghy Dizaji, Volker Lenz and Thomas Zeng

Received: 12 April 2021 Accepted: 4 May 2021 Published: 13 May 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

degree of external contamination or dry or wet ash removal from the combustion unit [4–9]. Major and trace elements are volatile to varying degrees at temperatures that prevail within the combustion chamber [1,10–12]. For instance, heavy metals such as Cd, Pb, Zn, and Hg are highly volatile, while elements such as Cr or Cu have low volatility. Therefore, the elements accumulate differently in bottom, cyclone or filter ash [10,13–15].

The material use of wood ashes as raw materials for the bioeconomy poses certain challenges [16–18]. Complex legal frameworks, difficult ash logistics due to decentralized accumulation, fluctuating product qualities and aspects of storage and occupational safety are just some of the points that must be considered in this context [11,19–21]. In addition, many utilization pathways are still under development or at the pilot stage [14,18,22,23] and are not applied regularly. Consequently, ashes from biomass heat (and power) plants are usually not perceived as by-products of the energetic use of wood. Thus, it is often not regarded as a valuable intermediate for further processing but is classified as waste that must be disposed at significant costs [24]. A survey conducted under the AshUse-project showed that in the opinion of the heating plant operators the challenges for implementing recycling of a functioning material include legal uncertainties, fluctuating ash qualities and low economic revenues. In addition, operators often lack knowledge about quality management strategies, such as how a defined ash quality can be reliably maintained and verified [21]. The planned further work at TFZ will therefore focus on the area of quality management in the production of wood ash at biomass heating (power) plants.

Wood ashes are already used to some extent as raw materials for various purposes in Germany and in other European countries, and to some extent also in Northern America, depending on the technical, economic, and legal circumstances and considering environmental aspects. A relatively widespread application is the use of ashes as a fertilizer or as an additive for fertilizer production for agricultural and forestry applications [11,15,20,25–29]. In Germany and Austria, suitable ashes are also added to composts [30,31]. In Austria, this pathway is limited to a very low blending rate of 2% ash to the compost, which makes this process uneconomical and no relevant quantities of ash are recycled via this route [30]. Instead, approximately 40% of the annually produced ash in Austria is processed by the cement and building materials industry [30]. The use of ashes in road construction has successfully been tested in research projects in Austria and Finland [22,32,33]. Tejada et al. (2019) [34] investigated wood ashes as a source of raw materials in urban mining.

Heavy metal contamination, and in particular Cd in ashes, is seen as a major concern in the use of ashes as fertilizers [16,35,36]. Limit values in other European countries are considerably higher, especially for Cd [19]. There are regulations on specific parameters of ash in individual countries. For example, in Germany, there is a limit value for Cr(VI) for application on arable land [11,37], and in Denmark there is a conductivity limit value for the eluate from ashes [38].

Many authors document the chemical composition of ashes used as fertilizers or soil conditioners [10,11,19,20,24,26,28,29]. This is carried out by considering national conditions regarding prevailing plant technology and legal requirements for the application of the ashes. Due to the higher limits for heavy metals in ashes for fertilizer purposes in many countries [19], higher contaminated mixed bottom and fly ashes or ashes from fluidized bed combustion plants are also sometimes used as fertilizers [13,15,39,40]. In terms of composition, these are often not comparable to the predominantly pure bottom ashes from grate-fired furnaces, as they mainly occur in the study area [21] and are eligible for use as fertilizer and soil conditioner under the German fertilizer law. Ash qualities of German biomass heating plants are documented by Reichle et al. (2009) [16], Wilpert (2016) [11], and Schilling (2020) [10]. Wilpert (2016) [11] and Schilling (2020) [10] examined bottom ashes that were collected in the context of a quality assessment of bottom ashes for fertilizing purposes. Therefore, rather less contaminated ashes than average may have been used. The values of Reichle et al. (2009) [16] originate from before 2003, without giving any further details on the sampled plants. Data on the ordinary ash quality of combustion

plants according to the current state of the art are therefore missing. However, these data are necessary to estimate the bioeconomic potential of an increased ash utilization.

Due to the high solubility of calcium oxide (CaO) in the ash, a pH shock is feared when spreading in the forest, which negatively affects the soil flora and soil fauna. Therefore, the ash often is pretreated before application. This process, the so-called "ash stabilisation" or "ash hardening", includes the addition of water followed by a storage period of several months. Moistening and contact with atmospheric carbon dioxide causes a variety of chemical transformations. Most importantly, the easily soluble calcium oxide (CaO) transforms into the poorly soluble calcium carbonate (CaCO3) [11,12,24,38]. In large piles, this reaction occurs only on the surface if there is no mixing [12,28].

In Germany, wood ash is mixed with lime dolomite and is then used for soil improvement on arable and forestry land [11,41,42]. Wilpert (2020) [11] points out that the use of wood ash-lime mixtures is particularly recommended where improved potassium supply is desired and the alkaline effect is present. Since the solubility of the alkali salts remains high even after ash hardening, the hardened ash should be protected from rain during storage in order to prevent nutrient leaching. Another positive effect of humidifying the ash and storing it for several months is the conversion of any toxic chromium(VI) into the harmless chromium(III). Schilling (2020) [10] and Polandt-Schwandt (1999) [9] observed this effect in the case of ash from combustion plants with a wet ash discharge system where the hot ash was placed in a water bath and then discharged moist. Pelletizing and granulation of wood ash also serve to reduce the reactivity of the wood ash. Auxiliary materials such as cement or organic binders can be used in this process [28,43]. Pelletized or granulated ash can be applied with conventional fertilizer spreaders [40]. Moistening of the ash is not recommended if the ash is to be used as a substitute for quicklime— e.g., in road construction. In this case, the ash must be stored dry [33].

Many authors emphasize the liming effect of ashes and mixtures with ashes on agricultural and forest soils [11,24,28,44]. Katzensteiner et al. (2011) [45] describe the plant availability of calcium and potassium from wood ashes as "high", magnesium availability as "medium" and phosphate availability as "low". "Low" in this context means that less than 10% of the total phosphate from wood ashes is available to the plant in the year of application. In pot experiments, Kebli et al. (2017) [46] and Maltas et al. (2014) [47] demonstrated the uptake of potassium from wood ash in ryegrass and sunflowers. In the case of sunflowers, P uptake from the ashes was also observed.

An important prerequisite for approval as a fertilizer in Germany is compliance with the heavy metal limits in the German Fertilizer Ordinance (DüMV) [42] and, if applicable, the minimum nutrient contents required, depending on the fertilizer. If the ash is mixed with biowaste or compost, the limit values of the German Biowaste Ordinance (BioAbfV) [48] must also be complied with in certain cases [31]. Table 1 summarizes these limit values. The DüMV also contains limit values for organic compounds (perfluorinated tensides, dioxins and dioxin-like substances). These compounds are usually absent in ash from biomass heating (and power) plants [10] and were not investigated in this study. Currently, both bottom ashes and cyclone ashes (if the cyclone is not the last precipitation unit in the plant) may be used for fertilizer production according to DüMV. Compared to the application of wood ashes on farmland, 50% higher heavy metal limit values apply to the application on forestry land. The Cr(VI) limit only applies to ash fertilization on arable land.

Much uncertainty remains around the variability of wood ashes among plants or within the same plant and which of these ashes might be suitable for application as fertilizers in agriculture or for liming of forest soils. The aim of this study was to assess the range and average values of nutrients and pollutants in ashes from individual Bavarian biomass heat (and power) plants. This is an important prerequisite for an increase in ash utilization as it is in in line with the Bavarian bioeconomy strategy [49].


**Table 1.** Limit values or maximum contents (in brackets) for wood ashes according to the current German Fertilizer Ordinance (DüMV) and Biowaste Ordinance (BioAbfV) (d.b. = dry basis).

For this purpose, mainly bottom ashes but also mixtures of bottom and cyclone ashes (due to individual ash handling at certain plants) and pure cyclone ashes from heat (and power) plants with a thermal output of more than 1 MW were sampled and analyzed. The cyclone ashes are used for comparison with the bottom ashes and for an estimate of how the distribution of ash constituents could be influenced by plant operation.

#### **2. Materials and Methods**

A total of 17 biomass heat (and power) plants with an installed thermal capacity between 0.8 and 31.6 MWtherm, as well as one centralized ash collection depot of several small heating plants (plant ID 18), were selected for sampling. Table 2 gives an overview of the sampled heat (and power) plants with thermal outputs, fuels used, ash samples and plant IDs. The quality of the ashes varied due to different fuels, plant types or operating parameters of the furnace. At 17 sites, pure bottom ash could be sampled. At one plant (plant ID 1, TFZ), pure cyclone ash was sampled too. At five points, mixtures of bottom ash and cyclone ash could be sampled, leading to a total of 50 ash samples (Table 2). Depending on the ash management procedure, the storage duration of bottom ashes at the heating plants varied considerably and ranged from a few days to several weeks. For ten plants, sampling took place in two different heating seasons (winter 2018/2019 and winter 2019/2020). Seven plants and the central ash collection depot were sampled only once (*n* = 1). At the heating plant of TFZ (plant ID 1), a series of a total of 20 ash samples was obtained over an entire heating period (12 × bottom ashes, 8 × cyclone ashes). For the general analysis of variability between plants, mean values on ash quality per plant were calculated, while individual samples were used to assess heterogeneity within one plant.

*Sustainability* **2021**, *13*, 5482



Sampling was carried out directly at the heating plants in accordance with LAGA PN 98 [50]. Thereby, it was necessary to prepare a representative sample for laboratory analysis from several individual samples of the ashes stored at their respective locations. The minimum volume of an individual on-site sample and of the laboratory sample prepared by sample combination, homogenization and sample division depends on the maximum grain size of the ash and was between 0.5 and 10 L. Fine-grained ashes have a lower required minimum volume than coarse-grained ashes. The minimum number of incremental samples results from the basic quantity of stored bottom ash or cyclone ash. For example, up to a volume of 30 m<sup>3</sup> , at least eight individual samples should be taken according to LAGA PN 98. During sampling, the individual samples were recorded photographically (Figure 1).

**Figure 1.** Sampling of bottom ash according to LAGA PN 98 [50].

To obtain the laboratory sample from the individual samples, the samples were combined and thoroughly mixed with a shovel. After that, the mixed sample was divided into four even parts. Two of the four parts were discarded. The two remaining quarters were combined again, carefully mixed and the laboratory sample of approx. 8 L was taken from the mixture. Each sampling was documented on a sampling protocol.

The TFZ heating plant was an exception regarding sampling. Here, twelve individual samples of bottom ash and eight individual samples of cyclone ash were collected to assess variability of this plant over a complete heating season. To compare variation among heating plants, results from the bottom ash analyses were combined mathematically by calculating a theoretical mixed sample for the entire heating season.

The chemical analyses were performed by Wessling GmbH, Neuried, Germany. The analysis included the following ash components with a fertilizing effect—the macronutrients calcium (Ca), phosphorus (P), potassium (K) and sulfur (S), as well as the micronutrients cobalt (Co), iron (Fe), manganese (Mn), molybdenum (Mo), sodium (Na) and selenium (Se). The following heavy metals were analyzed: arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), both as total content and as chromium(VI), copper (Cu), nickel (Ni), mercury (Hg), thallium (Th), and zinc (Zn). In addition, pH, moisture content and loss of ignition of the ashes were measured. Elemental concentrations of the ash were determined mostly according to ISO standards. The dry residue was determined according to DIN EN

12879 [51]. The ash samples were dissolved with aqua regia (DIN ISO 11466 1997-06) [52] and analyzed by plasma mass spectrometry (ICP-MS) (DIN EN ISO 17294-2 (2005-02) [53]. Cr(VI) was determined according to DIN 19734 (1999-01) [54] The pH value in the solid was analyzed according to DIN ISO 10390 (2005-12) [55] and the alkaline active components according to VDLUFA Method Book Volume II.2, Method 4.5.1 [56].

#### **3. Results and Discussion**

Table 2 summarizes the results for the bottom ashes and the mixtures of bottom and cyclone ashes. The results are given per heating plant and ordered from left to right with ascending boiler output. In this order, the different combustion plants were also provided with IDs. All plants except one used wood chips from natural wood as fuel, and one plant used wood pellets. The analysis of the ashes included heavy metals, nutrients, pH, moisture content and loss of ignition. The results refer to the dry mass and are given either as concentrations (mg/kg d.b.) or as mass fractions (wt% d.b).

#### *3.1. Quality of Bottom Ash*

First, the results of the heavy metals in bottom ashes were evaluated in more detail (Figure 2), followed by analysis of the nutrient contents (Figure 3). The mean values for the relevant chemical elements and the physical ash properties per plant are given in Table 2.

**Figure 2.** Heavy metal contents in mg/kg (on dry basis) of the 26 bottom ash samples as point clouds and as boxplots with 25% and 75% quantiles (box) and minimum to maximum values (whisker). Horizontal lines show the respective limit values according to the German DüMV and BioAbfV.

**Figure 3.** Main nutrients of the 26 bottom ashes (based on dry matter) as point clouds as well as boxplots with 25% and 75% quantiles (box) and minimum to maximum values (whisker). The numbers next to the boxplots are the mean values.

The results refer to dry basis (d.b.) and, for each element, the individual results are shown as a cloud of points and as a boxplot with a minimum and maximum. The twelve ash samples from the TFZ heating plant are included in the evaluation as one mean value to avoid weighting effects. This results in a total number of *n* = 26 for the evaluation of the variation of bottom ashes among plants. In addition, the limit values for agricultural and forestry use according to the German DüMV and the limit values of the German BioAbfV are indicated in Figure 2. Table 3 gives the results in numbers.

**Table 3.** Analytical results of heavy metal contents of 26 bottom ashes from Bavarian biomass heat (and power) plants with an installed capacity of >1 MW (d.b. = dry basis).


The limit values of the DüMV were exceeded in one sample for Pb (3%) and in three samples for Cd (8%). These exceedances apply to the DüMV limit values of both agricultural and forestry applications, although a 50% higher heavy metal content is permissible for forestry applications (Table 1). Schilling (2020) [10] examined 334 ash samples from 12 plants. He found exceedances for Pb in 1.9% of cases and for Cd in 1.6% of cases. The author documented exceedances for Cr(VI) in just 6% of cases. This deviates strongly from the values in the present study. For an application on agricultural land, a limit value for Cr(VI) of 2.0 mg/kg applies, according to DüMV. This limit is exceeded by 62% of the examined bottom ashes. Additionally, Reichle et al. (2009) [16] point out that

the Cr(VI) limit is frequently exceeded in bottom ash from wood combustion. The authors recommend paying particular attention to Cr(VI) during the recycling of wood ash.

Ten heating plants were sampled twice in the current study, i.e., during winter 2018/2019 and during winter 2019/2020, and only two heating plants complied with the limit value for Cr(VI) in both samples. These were plants with a wet ash removal system (plant IDs 4, 14, 16), whereas all other plants used dry ash removal systems. For the plants that used dry ash removal, at least one sample per heating plant exceeded the limit value for Cr(VI). In three plants, the limit value was exceeded both times. Moistening of bottom ashes provides the conditions for a chemical reduction of Cr(VI) into Cr(III) [9]. Pohlandt-Schwandt (1999) [9] and Schilling (2020) [10] state that wet bottom ashes are low in Cr(VI). Therefore, moistening of bottom ashes is already often applied as a quality management tool to improve bottom ash quality [9,57].

In contrast to DüMV, there is no limit value for Cr(VI) in the BioAbfV. However, some of the other limit values in the BioAbfV are lower compared to DüMV and some bottom ashes exceeded the values for copper (19%, *n* = 5), nickel (8%, *n* = 2) and zinc (15%, *n* = 4).

The DüMV limit value for Cd was exceeded by two plants (plant IDs 6 and 11). Nickel and copper limits of BioAbfV were exceeded in each of the heating plants that were sampled twice in one of the samples, while the BioAbfV limit value for zinc was exceeded by both samples at one heating plant. Thereby, Zn was exceeded in all three ash samples with exceeded Cd. Kovacs et al. (2018) [8] and Schilling (2020) [10] show that there is a negative correlation between the concentration of volatile metals such as Cd, Pb or Zn and the temperature in the combustion chamber. Therefore, higher temperature combustion could probably solve the problem of Cd in bottom ash. Schilling (2020) [10] observed a complete volatilization of Cd at an average temperature of above 750 ◦C in the combustion chamber. The boiling temperature of Cd is 767 ◦C.

In total, only eight of the bottom ashes sampled complied with all heavy metal limit values according to the DüMV and the BioAbfV (Table 1), directly. Assuming that Cr(VI) can be sufficiently reduced by suitable treatments, e.g., by moistening the ashes [9,10], 85% of the ashes (*n* = 22) complied with the limit values of the DüMV. A total of 54% of the ashes (*n* = 14) also complied with the requirements of the BioAbfV regarding the maximum permissible heavy metal concentrations.

Bottom ashes contain many nutrients that are relevant for plant growth [11,24,26,28]. The sum of the basic components (metal oxides and carbonates [24]) and the individual values for calcium (calculated as CaO), potassium (calculated as potassium oxide K2O), magnesium (calculated as magnesium oxide MgO) and phosphorus (calculated as phosphate P2O5) are shown in Figure 3 as point clouds and box plots. Table 4 shows the results in figures together with the contents of the additional trace nutrients and other parameters.

First, a comparison is made with publications on ash quality from Germany and Austria. Since here the wood qualities and the technology of the CHP plants are quite similar to the plants investigated. Reichle et al. (2009) [16] reported average nutrient contents for bottom ash of 25 to 45 wt% for calcium oxide (CaO), 3 to 6 wt% for magnesium oxide (MgO) and potassium oxide (K2O), each, and of 2 to 3 wt% for phosphate (P2O5). In the current study, higher values were measured, especially for potassium oxide. Here, the mean value is 6.3 wt% (d.b.) and 50% of the analytical results were between 4.5 and 7.5 wt% (d.b.). Obernberger (1997) [58] also gives a higher value for K2O than Reichle et al. (2009) [16] with 6.7 wt% d.b. as the average value for the content of potassium oxide in 12 bottom ashes from the combustion of wood chips. The mean phosphate content in Obernberger (1997) [58] is 3.6 wt% (d.b.) and thus about one percentage point higher than results in this study.


**Table 4.** Analytical results of trace nutrients and other parameters of 26 bottom ashes from Bavarian biomass heat (and power) plants with an installed capacity of >1 MW (d.b. = dry basis).

The results indicate that the nutrient contents in bottom ash from wood combustion can fluctuate over a wide range of values. The pH values of the ashes examined vary between pH 12.3 (minimum) and pH 13.3 (maximum) (Table 2). They thus fluctuate quite closely around the mean value of pH 12.8 and lie within the range of pH 11 to pH 13 given by Reichle et al. (2009) [16] for wood ashes.

Most of the ashes were very dry, the median value of the moisture content is 0.5 wt%. Nurmesniemi et al. (2012) [15] also notes this value for bottom ashes. Only the two plants with wet ash removal raised the mean moisture content to 6.2 wt%. For the plants with a wet ash removal system, the moisture content varied between 21 and 33 wt%.

Most of the ashes were completely combusted and showed only a low loss of ignition, which amounted to 0.6 wt% on average and reached a maximum of 3.6 wt%. Thus, all ashes remained below the value of 5 wt%. Therefore, it can be assumed that there are no organic pollutants in the ash [16].

Looking at ash qualities that have been published beyond Germany and Austria, similar contents for CaO, MgO, P2O<sup>5</sup> and K2O have been reported by Okmanis et al. (2015) [40] and Ingerslev et al. (2011) [20]. Considerably lower nutrient levels have been published by Nurmesniemi et al. (2012) [13] and Hannam et al. (2018) [16] for bottom ashes. Except for Cr (partly originating from the steels in the combustion chamber [20], the ash constituents originate from the fuels [7,20]. These differences can therefore be partly due to different fuel compositions. However, the main causes are differences in combustion technology and different temperatures in the combustion chamber.

Table 5 correlates the bottom ash contents of the present study with the thermal power of the combustion unit classified in <1 MW, 1 to 10 MW and >10 MW. The nutrient levels of alkaline active substances (CaO), MgO, P2O<sup>5</sup> and K2O decrease with increasing furnace power due to higher temperatures in the combustion chamber. This is consistent with the research of Okmanis et al. (2015) [40] who examined the ash from heating plants in Lithuania. Additionally, Wilpert et al. (2016) [11] shares this observation and suggests a mixture of ashes from large and smaller heating plants to increase the nutrient content in fertilizers from wood ash.


**Table 5.** Chemical composition (mean value and range) of the bottom ash according to thermal power of the combustion unit.

The bottom ashes which, apart from Cr(VI), do not exceed any other limit values of the DüMV, all contain more than 15 wt% (d.b.) CaO and thus meet the requirement for a "lime fertilizer made from ash from the combustion of vegetable matter". A recycling path established in Bavaria and Baden-Württemberg consists of mixing ashes of this quality with lime or lime dolomite to form "carbonic acid lime". The ash content may not exceed 30 wt%. Theoretically, it would also be possible to mix this lime fertilizer from ash with biowaste. However, minimum nutrient content limits in the finished product of 3 wt% N, 3 wt% P2O<sup>5</sup> or 3 wt% K2O in the dry matter would then have to be met. According to Kehres (2016) [31], these contents are generally not achieved by mixtures of bottom ash and biowaste.

For a large part of the bottom ash (69%), the classification as "PK fertilizer from ash from the incineration of vegetable matter" would be possible, since at least 2 wt% P2O<sup>5</sup> and 3 wt% K2O are contained in their dry matter.

Four of the ashes (corresponding to approximately 15%) contain at least 10 wt% (d.b.) K2O and would thus fulfil the requirement for a "potassium fertilizer from ashes of the combustion of vegetable matter".

Wood ash can also be used in composting. If the resulting "organic-mineral fertilizers" are to be spread on agricultural land in accordance with DüMV, the limit values of the BioAbfV must also be met. Taking into account the exceedances of Cr(VI) according to the DüMV, a total of 54% of the bottom ash examined also complies with the limit values

of the BioAbfV. However, the limit values of the BioAbfV do not have to be met if the application takes place on land, for which the BioAbfV does not apply, such as in gardening and landscaping or if substrates or topsoil materials are produced from the mixture of ash and compost [31]. This latter recovery path would thus be possible for 85% of the bottom ash investigated, as long as a reduction in the Cr(VI) content can be assumed.

#### *3.2. Distribution of Element Loads between Bottom Ash to Cyclone Ash (TFZ Heating Plant)*

At the TFZ heating plant, the distribution of the element loads between bottom ash and cyclone ash was investigated. For this purpose, individual samples of bottom ash and cyclone ash were sampled simultaneously at eight points during the same heating period.

Volatile ash components, such as Cd, Pb, Zn and Hg, evaporate at the high temperatures in the combustion chamber [8,10,13,15,16]. For this reason, volatile components can be discharged from the hot ash bed and accumulate in the cyclone ash through condensation. This results in increased concentrations of these elements in the cyclone ash compared to the bottom ash. By using the data set of samples obtained at the TFZ heating plant, this correlation should be directly verifiable.

Table 6 shows the heavy metal and nutrient concentrations in the bottom ash in direct comparison with the corresponding cyclone ash. The mean value and the standard deviation of the eight samples taken in pairs are given in each case. Pairs of mean values that differ significantly are printed in bold. Means were compared using the Wilcoxon signed-rank test. At the points where there is no standard deviation, all samples had fallen below the detection or determination limit with respect to this element. The specified detection or quantification limit was then used as the concentration. For the elements As and Hg, which also occur at very low concentrations in the cyclone ash, this can lead to a distortion in the calculation of the element loads, since this procedure means that a similarly high value must be assumed in both the bottom ash and the cyclone ash. In fact, it can be assumed that the proportion of the two volatile elements As and Hg is higher in the cyclone ash than in the bottom ash. However, the detection limit of the analysis via the external laboratory does not allow this conclusion to be drawn.


**Table 6.** Mean heavy metal and nutrient concentrations (including standard deviation) in bottom ashes and in the associated cyclone ashes from eight paired samplings at the TFZ heating plant.

**Table 6.** *Cont.*


\* Significantly different pairs of mean values are printed in bold.

The interpretation of the results in Table 4 is based on the calculated absolute element loads related to the total mass of the respective element in the ash (Figure 4). In order to make quantitative statements about how the actual loads of the individual elements are distributed between the bottom ash and the cyclone ash, it is first necessary to make reasonable assumptions about the mass ratio between bottom ash and the associated cyclone ash. For fixed-bed furnaces, a proportion of 10 to 30 wt% of cyclone ash is usually reported [45,58–60]. Fine fly ash is not considered in the following analysis.

The actual proportion of cyclone ash depends on various factors, such as the turbulence of the primary air in the combustion bed or the fineness of the fuel, for comparison of the ash fractions from wood chips or sawdust shows [1]. With these assumptions, it is possible to derive from the eight pairwise analyses of the bottom ash and the cyclone ash at the TFZ heating plant how the fractions of heavy metals and nutrients are distributed between the ash fractions. In addition to the 1:1 mixing ratio (bottom bar chart), Figure 4 shows the distribution of the loads at 10, 20 and 30 wt% cyclone ashes of the total ash.

Heavy metal compounds containing Pb, Cd, Tl, Hg and Zn, are highly volatile [8,13] and are predominantly found in the cyclone ash in all calculations. Consequently, even at the lowest assumed cyclone ash content of 10 wt% of total ash, Cd accumulates in the cyclone ash of up to 93 wt%. Should high concentrations of highly volatile elements be observed in bottom ashes that are considered for utilization as fertilizer, an increase in the temperature in the combustion bed could result in a reduction in these elements in bottom ashes and an increase in cyclone ashes.

For As, no clear effect could be seen in the data presented here. As the concentrations of As in the investigated bottom and cyclone ashes were overall very low, the limit of determination often had to be used as the concentration in the ash fractions. Cu, Cr, Ni and the main nutrients Mg, P, Ca and K are less volatile and, depending on the calculation performed here, are found in only 11 to 50 wt% in the cyclone ash. Therefore, they predominantly remain in the bottom ash.

Obernberger (1997) [58] shows basically similar element ratios between bottom ash and cyclone ash for wood chips. However, the reported concentrations in the cyclone ash were consistently lower compared to the results presented here (with exception of K and P), which may be due to different combustion chamber and cyclone temperatures of the heating plants investigated. The combustion chamber temperatures near the combustion bed are not known for the TFZ heating plant. Lanzerstorfer (2017) [13] observed that at combustion chamber temperatures between 830 and 920 ◦C, Cd, Pb and Zn accumulate in the fly ashes, while most nutrients (Ca, Mg, P2O5) remain in the bottom ash. Both Lanzerstorfer (2017) [13] and Schilling (2020) [10] note a higher volatility for potassium, which leads to K losses from the bottom ash. These increased K losses could not be observed at the TFZ heating plant, suggesting that the combustion temperatures are sufficiently high to remove the volatile heavy metals and at the same time low enough to avoid high potassium losses.

**Figure 4.** Ratio of element loads in bottom ash and cyclone ash of the TFZ heating plant at hypothetical mixing ratios with cyclone ash contents of 10, 20, 30 and 50 wt%.

#### *3.3. Quality of Mixtures of Bottom Ash with Cyclone Ash*

In some heating plants, the bottom ash and the cyclone ash are collected in the same container due to the plant design. The composition of these ashes is shown in Table 2 (right columns). All five samples of these mixed ashes exceed the DüMV limit value for Cd. Further exceedances occurred for Cr(VI) (*n* = 4), thallium (*n* = 1) and lead (*n* = 1). None of the bottom ashes mixed with cyclone ash can meet the requirements regarding the heavy metal limit values of the DüMV or the BioAbfV. They are therefore not eligible as a source material for fertilizers. These ashes are excluded from being spread on agricultural and forestry land in Germany. If the aim is to recycle bottom ashes, it is recommended that these ash fractions are collected and reused separately. When using other fuels, e.g., when firing agricultural fuels such as straw, a mixture of bottom ash and cyclone ash can often

comply with the limit values of the DüMV [42]. This is due to the generally lower heavy metal content of agricultural fuels compared to wood fuels.

#### **4. Conclusions**

The energetic use of untreated wood in biomass heat (and power) plants produces combustion residues in the form of ash. The increased use of by-products and residues contributes to the conservation of natural resources. It has been shown that the bottom ashes produced are basically suitable for use as fertilizers or as a raw materials for fertilizers despite the low pollutant limits in the German DüMV.

On average, the bottom ashes examined contained 33 wt% alkaline active components, 29 wt% Calcium (calculated as CaO), 3.9 wt% Magnesium (calculated as MgO), 6.3 wt% Potassium (calculated as K2O) and 2.6 wt% of phosphorus (calculated as P2O5).

However, quality assurance of the ashes and compliance with the relevant legal requirements due to possible exceedances of the heavy metal limits prescribed by the German Fertilizer Ordinance are crucial. The limits were exceeded in the bottom ashes for chromium(VI) (62%), cadmium (12%) and lead (4%). Mixing of the bottom ashes with cyclone ashes led, in all cases, to the heavy metal limit values being exceeded, especially for cadmium. The following measures contribute to the quality assurance of ashes for fertilization purposes:


The frequently exceeded limit value for chromium(VI) in the German Fertilizer Ordinance can be reduced by moistening and storing the bottom ashes. In this process, chromium(VI) converts into the harmless chromium(III).

The present study maps the ash quality of typical biomass heating plants according to the state of the art in Germany. The evaluation of the results is carried out according to the regulations applicable in Germany for the use of biomass ash for fertilizer purposes. Other combustion techniques, other fuels and other legal regulations may lead to different assessments.

**Author Contributions:** H.B. designed and performed the experiments, derived the models and analyzed the data. Both D.K. and H.H. contributed to the final version of the manuscript. D.K. supervised the project. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Bavarian State Ministry of Food, Agriculture and Forestry, grant number G2/KS/17/02.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Abbreviations**

BioAbfV = German Biowaste Ordinance; d.b. = on dry basis; DIN = German Institute for Standardization; DüMV = German Fertilizer Ordinance; ISO = International Organization for Standardization; LAGA = German Federal/State Working Group on Waste; TFZ = Technology and Support Centre in the Centre of Excellence for Renewable Resources; VDLUFA = Association of German Agricultural Analytic and Research Institutes.

#### **References**

