**2. Wood biomass ash characterization**

There are several factors that affect the quality and quantity of WBA obtained by using wood biomass in power plants. Based on [22], these factors can be divided into three main groups as shown in **Table 1**. According to **Table 1** and a detailed review of the literature, it is necessary to highlight (1) the type of biomass used for power generation, (2) the plant technology used, (3) the combustion temperature, (4) the location of WBA collection, and (5) the conditions of WBA storage. In the following, the influence of these parameters on the characterization of WBA is discussed in detail.

One of the factors that could have an influence on the properties of WBA is the area of biomass cultivation and the condition and type of soil [24], but this influence is not very large. From the tertiary group of influences, it appears that ash from wood


#### **Table 1.**

*Groups of influence contributing to the chemical composition of WBA (adapted from Vassilev et al. [22]).*

#### **Figure 1.**

*WBA classification based on WBA collection in power plants (adapted from Obernberger et al. [35] and Eijk et al. [36]).*

biomass undergoes certain chemical processes during its collection and disposal. The plant technology, i.e., the technology of wood biomass combustion in the power plants, as one of the factors affecting the physical and chemical properties of the produced WBA, is divided into grate combustors, fluidized bed combustors, and pulverized fuel combustors [10]. Three different types of WBA can be generated in a power plant [26, 31–34]: bottom ash (1) collected at the bottom of the chamber (bottom WBA); fly ash, which may be a relatively coarse fraction, (2) collected from cyclones or boilers; and a relatively fine fraction of fly ash, and (3) collected from electrostatic precipitators and bag filters (**Figure 1**). In some power plants bottom ash and fly ash are collected in one container as mixed WBA.

In grate combustion technology, 60–90% of the WBA from the bottom of the furnace is formed on the grate, while in fluidized bed combustion, fly WBA is the dominant ash formed [37–39]. The particles from the bottom of the furnace are larger than the fly WBA [7, 40]. This can be observed from **Figure 2**, which shows the particle size distribution of WBAs [41] and cement, and the grading curve of the bottom WBAs and aggregate (particle size 0–4 and 4–8 mm) per the combustion technology. The authors [41] proposed a cumulative grading curve for all particle sizes of the bottom WBAs as they were sieved through a 1 mm sieve to eliminate impurities and larger fractions. Grate combustion has a higher influence on the particle size distribution of the fly bottom WBAs where a generally large diversity of granulometric curve of bottom WBA compared to aggregate can be seen in **Figure 2**. Grate-fired systems are designed to cope with a degree of the sintering and partial fusion of the ash on the grate. Poor fuel distribution, relatively poor air distribution, and local high temperature on the grate can lead to the formation of relatively large ash agglomerates that reduce combustion efficiency [42]. This occurrence could lead to larger particles of the WBA sample [43]. It can also be inferred from **Figure 2** that the particles of bottom WBA from fluidized bed combustion technology and pulverized fuel combustors are smaller than those of bottom WBA from grate combustion power plants.

The WBA produced at the bottom of the combustion chamber is often mixed with mineral impurities such as sand, stones, and soil contained in the biomass, as well as sintered ash particles. In addition to the coarse and fine fraction of the fly WBA inside the plant, smoke dust of the finest fraction is also emitted together with

#### **Figure 2.**

*Particle size distribution of fly WBA (F) (published in Carević et al. [41]) and bottom WBA (B) compared to the different combustion technologies used (x: grate combustion;* ⚬*: pulverized fuel combustors; and* □*: fluidized bed combustors).*

the flue gases [35]. In fluidized bed combustion, the lower WBA consists of sand particles, mainly quartz, added during combustion, inorganic components (soil or small stones), and unburned biomass fraction [32, 38]. Modern solutions of the combustion system on the grate may include a continuously moving and water-cooled grate, which consequently means that wet ash removal is performed from the bottom of the furnace [44]. In view of the above, it is very important to know what type of technology is used and at what location in the power plant the WBA is collected to further characterize the WBA. The choice of plant technology has a significant impact on the chemical composition of the WBA: fluidized-bed technology uses additives such as quartz sand as bed material, which can have a positive impact on the chemical composition of the WBA and contributes to a high SiO2 content compared to other combustion technologies [10, 45, 46]. The morphology of WBA (**Figure 3**) mostly showed non-uniform structure, inhomogeneous particle surface, and particles with different shapes, which could lead to higher water absorption and have a corresponding negative effect on the workability of the cement composites [28, 47, 48].

**Figure 4** compares the chemical composition of 46 samples of different ash types collected from the power plants: fly, bottom ash, and mixed ash. WBA is expected to contain a higher proportion of CaO than pozzolanic oxide, the sum of SiO2, Al2O3,

*Utilization of Wood Biomass Ash in Concrete Industry DOI: http://dx.doi.org/10.5772/intechopen.102549*

**Figure 3.** *Morphology of the WBA.*

and Fe2O3 (median values for CaO were 48.61% compared to 13.49% for pozzolanic oxide for all WBA samples), indicating lower pozzolanic activity and pronounced hydraulic activity [23]. Higher alkali levels (K2O and Na2O) can also be observed,

**91 Figure 4.** *Boxplots of chemical parameters from the WBA database (N = 46) by WBA type [41, 46].*

which may be reflected in the mechanical and durability properties of cement composites with WBA [49]. This is particularly pronounced in the fly WBA samples. Alkali is an integral part of the characterization of untreated biomass and in woody biomass, alkalis are bound to the organic structure, so their higher content in WBA was expected. High alkali content can cause high porosity in the hardened cement matrix, resulting in lower strength and durability [15, 23, 25]. Since the CaO content is higher in all WBA specimens, free CaO is expected, a significant amount of which can cause volume instability (swelling) during the hydration process and the formation of cracks [33, 50, 51]. As shown in **Figure 4**, the fly WBA showed the highest median LOI value (15.3 wt.%) which is significantly higher than the maximum value allowed by EN 450-1 (Category C < 9 wt.%) [20]. Unburnt carbon and inorganic compounds can significantly affect the properties of concrete (workability, setting time, mechanical properties) [52].
