Biomass as an energy source contributes to reducing dependence on imported fossil fuels (coal, gas), while at the same time bringing benefits to countries where biomass fuels are an emerging source of renewable energy. Understanding the combustion behaviour of fuels is essential for the efficient and safe design and operation of equipment. Similarly, understanding the mineral content is crucial, as it significantly impacts the dynamics of the power generation system. The energetic use of biomass has many benefits, including social, economic and energy security. The considerable diversity of plant species useful in the energy sector is associated with a large diversity in terms of physical properties (hardness, specific gravity, moisture, porosity) and chemical composition of the raw materials obtained from them. This has a significant impact on the calorific value of the biomass but also on the combustion process and the condition of the heating equipment in which it is carried out. Although biomass is environmentally friendly, its use can create many problems that can lead to a sudden interruption of boiler operation, for example, as a result of the complex process of ash deposition on the convection surfaces of heat exchange. This process occurs in a wide range of flue gas temperatures, surfaces and depends on the properties of the ash as well as the design and operating conditions of the boiler. Problems associated with the presence of ash in biomass combustion systems (such as grate boilers and drop-in or retort burners) are linked to the formation of agglomerates from partially molten ash at high temperatures, as well as the deposition of ash in low-temperature zones of the boiler’s heat exchange surfaces, particularly in convective sections. These phenomena contribute to accelerated metal corrosion, increased emissions, and complications in the utilization and disposal of ash waste from such biomass-fired systems. Some negative effects of increased ash deposition as a result of biomass combustion on the efficiency and operation of the combustion system can be presented as follows:

1. Decrease in boiler efficiency. Increased ash deposition, along with changes in the properties of ash deposits containing low-melting compounds [K, Na, S, and calcium (Ca)], leads to the formation of a coating on the particles of bottom ash. This coating is partially molten, which at a later stage results in the sticking and adhesion of particles, ultimately leading to ash agglomeration in the combustion chamber. In addition, the deposition of melted or partially melted ash deposits on the heat exchange surfaces slows down the heat exchange in the boiler, which is manifested by a decrease in the thermal efficiency of the combustion chamber and its performance.

Thanks to the knowledge of the chemical composition and physical properties of ash, it is possible to predict the tendency to form deposits on boiler elements and their potential for corrosion, erosion or abrasion. The behavior of ash in the system depends to a large extent on the fuel used. To achieve a stable energy generation system, it is essential to thoroughly understand the requirements of the fuel used in order to adapt the combustion technology — especially when it comes to biomass derived from industrial waste or energy crops, which due to their chemical composition exhibit different behavior during combustion than other types of biomass (Nunes et al., 2016; Wei et al., 2022).

The aim of this study is therefore to analyze and synthesize information related to the quantity and chemical composition of ash from various types of herbaceous biomass, as well as to examine the issues associated with deposit formation and ash deposition in heating systems that burn such biomass, based on the current state of knowledge in the relevant literature. These issues are related not only to the sintering and slagging of bottom ash, but also to the chemical composition of solid biofuels, which is of great importance in the case of using various types of biomass. This study may help to systematize knowledge regarding the impact of ash on the combustion process and emissions in heating systems, while also indicating solutions aimed at minimizing problems related to bed agglomeration and fouling of heat exchangers in heating equipment.

Ash is a solid residue resulting from the complete oxidation of fuel. It is formed from inorganic (mineral) components during combustion. Inorganic material contained in biomass can be divided into two fractions: by-products that are added to fuels during its processing (sand or clay from harvesting, storage) and inherent material that acts as nutrients fulfilling basic biological functions for plants and is more finely distributed in the fuel (atomically dispersed). The elements that make up ash include: aluminum (Al), Ca, iron (Fe), K, magnesium (Mg), Na, phosphorus (P), silicon (Si), copper (Cu). In the literature on the subject, the components of ash have been divided into macroelements: Ca, K, Si, Mg, P, Na, S and microelements: Al, Fe, manganese (Mn), lithium (Li), barium (Ba), boron (Br). These elements in ash occur in the form of oxide compounds: SiO₂, aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), manganese oxide (Mn₃O₄), titanium oxide (TiO₂), CaO, magnesium oxide (MgO), sulfur trioxide (SO₃), phosphorus pentoxide (P₂O₅), sodium oxide (Na₂O), K₂O. Basic compounds of ash include: CaO, MgO, K₂O, Na₂O and Fe₂O₃. Examples of acidic components of ash are: SiO₂, Al₂O₃, hydrochloric acid (HCl), S, sulfur suboxide (S₂O), P₂O₅ and TiO₂. When the acidic and basic components of ash react, a salt or mineral is formed. The chemical composition of ash is most commonly expressed as the percentage content of these oxides. The content of ash-forming elements in biomass fuels varies greatly depending on the fuel and its type. The presence and percentages of the above-mentioned oxides and elements in the ash are important in terms of their behavior in the combustion process of the biofuel in which they occur in the ash, which results in an impact on the operation of heating devices and pollutant emissions (Lokare, 2008; Odzijewicz et al., 2023; Tortosa-Masiá, 2010). In ash, we distinguish eutectic compounds, which are a mixture of two or more salts that together have a lower melting point than the compounds themselves. The molten alkali salt (KCl, potassium sulfate (K₂SO₄) is deposited on the heat exchanger surface and can transfer the metal oxide to the salt. They are characterized by the eutectic number, which quantitatively expresses how the alkali metals and alkaline earth metals present are related to each other (Stauber-Alfredsson, 2018). In the characterization of ash, it is also important that the alkali metals (responsible for the emission of fine particles) or alkaline earth metals (responsible for the increase in the melting temperature of the ash) (AAEMs-Alkali and Alkaline Earth Metals) form zeolites. This is a group of hydrated aluminosilicates of alkali or alkaline earth metals (mainly Na, K, Mg and Ca) with pores of uniform diameter, regular internal cavities and channels of discrete sizes and shapes. The Ca and Mg group is the strongest in bottom ash, constituting more than 50% in all fractions and fuels. Alkali metal and alkaline earth metal species (AAEMs) are the main components of biomass ash and can positively influence biomass combustion by acting as a catalyst. On the other hand, high concentrations of ash and alkali metals (K, Na, Ca, magnesium (Mg), etc.) in biomass cause ash-related problems through condensation and reactions, such as slagging (mainly related to alkali chlorides and sulfates), agglomeration (mainly dependent on the chemical and melting properties of the coating, which are sensitive to the relative amount of calcium and potassium in the fuel), corrosion (mainly related to flue gases containing chlorine (Cl₂), HCl, NaCl (aerosol), KCl (aerosol) and other species that accelerate the oxidation of metal alloys) and contamination. Alkali metals such as K and Na together with Cl are the most important elements in biomass ash that affect undesirable processes. During fuel combustion, reactive compounds evaporate and alkali metals can form aerosols (potassium hydroxide (KOH), KCl, K₂SO₄, NaCl, sodium sulfate (Na₂SO₄) in the gas phase. Alkali metal aerosols can react with each other or other ash components (SiO₂, Fe₂O₃) to form eutectic mixtures. Alkali metals can also occur in the solid phase in the form of silicates (potassium silicate (K₂Si₂O₅), sodium metasilicate (Na₂SiO₃) and aluminosilicates (potassium aluminum silicate (KAISi₃O₈), nepheline (KAISiO₄), albite (NaSi₃AlO₈) (Yu et al., 2021; Zhang et al., 2018; Royo R. et al., 2022; Jiao et al., 2023; Oladejo et al., 2020; Mlonka-Mędrala et al., 2020).

Ashes resulting from biomass combustion are classified into bottom ash and fly ash. Bottom ash captures evaporated potassium, which combines with silica present in the bed material, causing its agglomeration. Agglomeration occurs when ash particles form layers around the bed material particles (mainly quartz sand, which consists of silica, potassium and calcium). It occurs as a result of chemical reactions, the presence of a liquid phase and sintering in the solid state. Fly ash from biomass combustion, on the other hand, is very diverse in terms of chemical composition and the type of material from which it was obtained. In addition, the formation of SO3 also depends on its chemical composition. During combustion, inorganic compounds of biomass undergo chemical and physical changes. Volatile compounds (K and Na are volatile elements but Cl, carbon dioxide (CO₂) and S are considered to be particularly volatile) are released into the gas phase (released ash is formed) while non-volatile (Al, Ca, Fe, Mg and Si) ash particles (these are usually large ash particles) remain on the furnace grate forming bottom ash or forming a coarse fraction of fly ash (Laxminarayan, 2018; Kowalczyk-Juśko, 2017). Elements contained in ash can also cause problems during combustion in the boiler. Despite the fact that the ash content in biomass is much lower than in coal, their different origins and differences in chemical composition affect the operation of heating devices causing increased slag and ash deposition in the furnace or increased wear of metal boiler elements due to corrosion. Therefore, it is a very important issue to assess the content of individual mineral components in the ashes resulting from the combustion of biomass (Zając et al., 2018). The characteristics of chemical compounds present in the ash are presented in Table 1.

The sum of basic oxides is an important parameter for ash assessment. Based on the literature review, Tables 2, 3 present the % share of chemical compounds for different types of biomass.

Based on the data presented in Tables 2, 3, it can be concluded that the chemical composition of biomass ash varies significantly. The composition of grass ash has the highest content of SiO₂ (34%–86%) and K₂O (50.4%) with moderate amounts of CaO (1.9%–17.7%) and P₂O₅ (0.03%–7.2%) and small amounts (<8%) of other elements. Rice husk ash has a similar content of SiO₂ (91.42%) and rice straw (74.57%) to grass ash but a lower content of K₂O (3.71%) and other chemical compounds. In addition, the lowest content of K₂O is shown by miscanthus (2.66%), pine sawdust (3%) and woody biomass pellets (3.87%).

Willow, Virginia mallow, rapeseed straw, soybean pellets, multiflora rose, chamomile pellets contain much less SiO₂ (respectively: 0.02%, 3.20%, 5.05%, 5.46%, 6.74%, 7.74%) than grasses, rice husks (91.42%), rice straw (74.57%), wheat straw pellets (60%) and agricultural residue pellets (28%–75%), but more CaO (45.92% - soybean pellets). However, the highest CaO content is found in poplar (57.33%) and willow (46.09%).

The intense internal reactions occurring in biomass ash at elevated temperatures generate a large amount of eutectic compounds, which significantly reduce the melting point of the ash resulting in an increased possibility of ash particles adhering from the gas flow. Additionally, ash formed from herbaceous biomass sinters at low temperatures (below 850 °C) and causes the formation of slags. Hence, wood-based products are the most commonly used biomass fuels, containing up to 40% less ash compared to other agricultural biomass raw materials, e.g., straw (Laxminarayan, 2018; Li et al., 2023; Generowicz et al., 2023; Rodriguez et al., 2020; Repić et al., 2019; Ion et al., 2023; Zapata et al., 2023; Dyjakon, 2012; Vaniyambadi, 2015; Yongtie et al., 2018; Zeng et al., 2018; Odzijewicz et al., 2023; Nzihou and Stanmore, 2013; Royo J. et al., 2022; Sarbassov et al., 2017; Schulze et al., 2020; Laxminarayan et al., 2017; Meka et al., 2022; Lalak et al., 2016; Xu et al., 2021; Thomas et al., 2018; Tesfaye et al., 2022; Niu et al., 2016; Zhai et al., 2021). The PN-EN ISO 21404:2020–08 standard (update of the CEN/TS 15370-1:2006 standard) is a document presenting a method for determining characteristic temperatures during the melting of complex inorganic components of solid biofuel ash at high temperatures. The characteristic temperatures defined in this standard are used to compare the tendency of ashes of different types and qualities of solid biofuels to form molten deposits or in assessing the susceptibility of the bed to agglomeration during heating (Stałe, 2025).

The composition of ash affects its softening and melting temperatures. The melting characteristics of the ash (Table 4) are an important factor in determining the propensity of fuels to form molten or partially melted slag deposits on the furnace wall surfaces of all combustion systems and can influence the nature of fouling deposits that occur on heat exchangers and other surfaces of biomass-fired boilers. The melting of most fuel ashes is a complex phenomenon that is best described by a melting curve, which shows the mass percentage of molten ash as a function of temperature. Two temperatures are considered in the ash melting curves and are used to describe the behavior of the ash:

- T15 temperature, at which 15% of the ash material melts. This is the temperature at which the surfaces of the particles or slag deposits begin to become sticky and susceptible to incoming particles,

There are a number of complex thermodynamic models that can accurately predict the ash melting curve of a given fuel. For coal combustion, slagging and fouling indices have been developed to obtain and indicate the propensity of fuel ash to form deposits during combustion based on its chemical composition. When biomass is co-fired with coal, all the components of most biomass ash, mainly alkali metals and alkaline earth metals, are strong fluxing agents for aluminosilicate systems. The introduction of biomass can result in a lower melting point and therefore an increased slagging potential (Loo and Koppejan, 2008).

One of the very important parameters during each biomass combustion process is the temperature at which ash shrinkage (sintering) begins. This is the temperature at which ash shrinks, i.e., the temperature at which ash begins to show a tendency to slagging. The ash sintering temperature for different types of biomass varies as follows: wheat straw 840 °C, rapeseed straw 640 °C, sunflower husk pellets 810 °C, rye straw 730 °C, oat straw 770 °C, mustard straw 630 °C, corn straw 650 °C, hemp straw 640 °C, sorghum stalks 760 °C, tobacco stalks 570 °C (Zhu et al., 2014; Cichy, 2013; Chen et al., 2022).

The alkali content of biomass creates potential problems related to bed agglomeration as well as fouling and corrosion of heat exchangers. An example is herbaceous fuels, which contain silicon and potassium, the main components in fly ash formation. Compared to other biomass fuels, they are also rich in chlorine (Cl). These properties create problems with ash deposition and formation at high or moderate combustion temperatures. The main sources of these problems are:

1. Reaction of alkali with silica to form alkali silicates, which melt or soften at low temperatures (below 700 °C) depending on composition.

The sintering and softening temperatures can differ significantly for the same type of herbaceous biomass, depending on the chemical composition of the ash, which is related to the plant species, cultivation method and soil type. Characteristic ash softening temperatures for some types of biomass are: rye straw 1,002 °C, wheat straw 998 °C, barley straw 980 °C. Alkaline material plays a major role in both processes. The mobility of the alkali material is defined as its ability to contact other materials and is measured using chemical extraction techniques. The presence of alkali metals and alkaline earth metals (AAEMs) in biomass has been found to have a significant effect on ash deposition, sintering, agglomeration, contamination, slagging and corrosion (Gogolev et al., 2021; Biedermann and Obernberger, 2005; Xu et al., 2020; Vassiliev et al., 2013a; Vassiliev et al., 2013b; Wang et al., 2020; Garba et al., 2012; Horvat and Dović, 2018; Hartmann et al., 2013; Baxter et al., 1998; Oladejo et al., 2020; Nzihou and Stanmore, 2013). Herbaceous biomass (straw, nut husks, fruit pits, grasses) causes more problems during combustion than wood. Compounds such as potassium, sodium, whether in the form of oxides, hydroxides or organometallic compounds, tend to lower the melting point of ash mixtures containing various minerals such as SiO₂. Also, high chlorine content in the fuel increases the release of potassium in the gas phase at relatively low temperatures. This causes serious problems related to slagging, contamination and agglomeration of the bed in the boiler, which can lead to switching off the heating device (Dyjakon, 2012). In connection with the above, the chemical composition of the fuel affects the rate of ash deposition from biomass fuels. It was also found that the process of deposit formation from herbaceous grass or miscanthus is about six times faster than from wood. Woody biomass is rich in Ca, Si and K compounds. Determining the melting behavior and release to the gas phase of biomass fuel ash is of great importance, therefore it is important to study herbaceous biomass fuels in order to characterize the ash composition. From the point of view of biomass combustion, a high content of K and Ca is unfavorable, because they easily react with other elements (e.g., Si) to form alkalis with very low melting points (approx. 700 °C). In turn, an increased content of K can increase the slagging potential. In contrast, ashes with lower alkali and higher Ca content exhibit easier to manage slagging and contamination problems.

During the operation of heating devices, the main problem is their contamination with ash during the process in which a deposit forms on the colder areas of the device as a result of the condensation of particles transferred from warmer to colder areas. Fouling means the process of deposit formation on convective heat exchange surfaces. The formation of large ash deposits is caused by a relatively large amount of compounds containing alkalis such as KCl and K₂SO₄ and the low melting point of many ash components. These deposits reduce the overall heat transfer coefficient, which leads to corrosion of metal surfaces. Additionally, the behavior of ash in the combustion chamber and furnace is problematic, as it forms slag and sinter due to the effects of high temperaturę. The mechanism of slag formation consists in the melting of ash, agglomeration of ash particles (also with fuel) and sintering. Slagging is a process during which deposits form on the heat exchange surfaces of the boiler and the refractory material as a result of adhesion of viscous particles. Slag deposits consist of an inner powder layer containing silicates and alkali compounds (Repić et al., 2019; Blondeau, 2013; Rodriguez et al., 2020; Ion et al., 2023; Wnorowska et al., 2020; Vega-Nieva et al., 2015).

Slagging is a very complex phenomenon, which is not only a physical and chemical reaction but also a serious problem in the use of biomass, which affects the operation of biomass combustion devices. Slagging and deposition reduce the efficiency of biomass combustion, which can lead to damage to the boiler, reducing its output, resulting in repair costs and, above all, difficulties in the daily use of biomass fuels (Yongtie et al., 2017; Rodríguez et al., 2021; Kowalczyk-Juśko, 2017). The parameter that determines the ability of ash to slagging is the content of iron oxides. The viscosity of particles is a key parameter in the case of ashes rich in silicates. High iron content causes a decrease in ash viscosity, which is manifested by a greater tendency to form deposits. Ashes with a Fe203 content of <6% are considered to be low-slagging (Kalisz et al., 2015; Kleinhans et al., 2018; Wu et al., 2018). Slag deposits on the boiler or furnace wall are formed as a result of the action of fine ash particles containing K and Cl. Chlorine promotes the release of potassium in the biomass, causing the formation of troublesome KCl, which condenses on the heating surfaces of the boiler. Problems with ash deposition depend on the presence of elements such as Na, K, Cl and Si and Al.

Potassium present in biomass as a result of oxidation processes during combustion leads to the formation of impermeable deposits on the surface of heating devices as well as agglomeration and corrosion (Deng et al., 2019; Miccio et al., 2019; Niu et al., 2016; Wang et al., 2018c). The more alkali metals and chlorides present in biomass, the lower the ash melting temperature and therefore the greater the tendency to form slag. The formation of slag in the furnace limits the access of air to the fuel, resulting in a decrease in temperature and an increase in the concentration of carbon monoxide in the exhaust gases. It is therefore necessary to ensure that the temperature in the furnace and combustion chamber in the case of grate furnaces exceeds 650 °C when burning agricultural biomass and, at the same time, is below the ash sintering temperature in the case of retort and horizontal-feed boilers or the ash softening temperature (in the case of heating appliances with a moving grate) (Pałaszyńska and Juszczak, 2018; Arvelakis et al., 2001; Livingston, 2016). Soluble potassium, which occurs in ionic form, is released during combustion as KOH, KCl and other forms, as a result of which it is easily available for further reaction with other flue gas compounds (Niu et al., 2016; Xu et al., 2021). According to Wang L. et al. (2012) and Kassman (2012), the main components of particulate matter present in the generated flue gases are potassium chloride (KCl) and potassium sulfate (K₂SO₄), whose melting points are 750 °C and 850 °C. Systems consisting of KCl and K₂SO₄ salts can melt at a low temperature of 550 °C or even lower (Lindberg et al., 2013; Funcia et al., 2020). Potassium and chlorine are the main components causing undesirable deposits and corrosion of superheater tubes in boilers fired with solid fuels (Ǻmand et al., 2006).

During co-combustion, leaching or the use of additives, an increase in the K + Cl and Si + Al ratio in the fuel can lead to an increase in the KCl concentration, which will manifest itself in greater slag formation. On the other hand, a decrease in the K + Cl and Si + Al ratio will lead to the capture of K by Si and Al, which will result in less slagging during combustion (Baxter et al., 1998; Kassman et al., 2013; Zhu et al., 2014). Ash deposition on the heating surface takes place as a result of four mechanisms: inertial collision, condensation of evaporated inorganic compounds, thermophoresis and chemical reactions (Veijonen et al., 2003; Hernik and Wnorowska, 2022; Jensen et al., 2019; Garba et al., 2012; Yongtie et al., 2018; Shao et al., 2012; Jensen et al., 2016; Stam, 2020; Yang et al., 2017; Livingston, 2016; Laxminarayan, 2018; Vaniyambadi, 2015).

In the initial stage of deposition, submicron ash particles are transferred to the cool surface under the influence of gas temperature, regardless of inertial impact. This process is known as thermophoresis. As a result of combustion, chemical reactions such as chlorination, oxidation and sulphation occur between solid and gaseous compounds and the deposition layer. An example is silica, which reacts with volatile alkali metal, i.e., potassium, to form low-melting compounds causing slagging and fouling at normal biomass boiler temperature (800 °C–900 °C). The produced alkali silicates or calcium chlorides tend to deposit on the boiler wall causing fouling or corrosion at low melting temperature 700 °C–750 °C (Veijonen et al., 2003; Baxter et al., 1998; Kleinhans et al., 2018; Shao et al., 2012; Funcia et al., 2020).

The main operational problem of biomass-fired boilers is the deposition of fly ash on the boiler surface. Alkalis, chlorine and silicates are the main fractions of ash deposits regardless of the type of fuel. Fouling and slagging are complex phenomena dependent on:

- Transformation of inorganic ash components in fuels,

The removal of deposits can occur by several mechanisms, namely,:

- Erosion,

Erosion occurs when sharp-edged fly ash particles collide with unfused deposits, resulting in the removal of deposits by spalling or repeated deformation. Two main mechanisms are distinguished: brittle erosion and plastic erosion. The equivalence of both sub-mechanisms depends on the material properties of the particle and the wall material, the angle of impact, and the kinetic energy of the particle (Schulze et al., 2020; Stauber Alfredsson 2018; Livingston, 2016; Laxminarayan, 2018).

Debonding occurs when the stress generated (e.g., due to soot blowing or the weight of the deposit) exceeds the adhesion force at the interface of the depositing tube, causing the deposit to be removed from the tube surface. If the temperature of the outer layer of the bed is high enough to form low-viscosity slags, molten phases can flow out of the bed. Thermal shocks caused by temperature changes in the boiler can cause deposits to form due to differences in the thermal expansion coefficients of the tube, the corrosion layer, and the deposit (Laxminarayan, 2018).

It was also found that biomass with herbaceous raw materials causes increased agglomeration due to the high content of alkaline compounds. In turn, a small share of biomass in fuel ash (coal) has a significant effect on the microstructure and chemical composition of the sediments, increasing the rate at which the sediments consolidate and strengthen with increasing temperature. Alkali metals contained in biomass, i.e., K, which reacts with other ash-forming elements, while chlorine forms compounds containing alkalis, KCl causing problems related to fouling and slagging, corrosion and agglomeration on the superheater or boilers (Guo et al., 2020; Miccio et al., 2019; Shao et al., 2012; Loo and Koppejan, 2008; Kassman, 2012; Tortosa Masiá, 2010; Balint et al., 2024; Niu et al., 2016; Oladejo et al., 2020), which hinder heat transfer and thus reduce the thermal efficiency of the boiler (Guo et al., 2020; Mlonka-Mędrala et al., 2020). Excess alkali in biomass-based fuels can be released into KOH. The presence of KOH on heat exchange surfaces can be an important factor in the corrosion mechanisms of biomass combustion devices (Berlanga and Ruiz, 2013).

The influence of boiler design, operating conditions and fuel properties on ash deposit behaviour is seen by emphasizing or limiting the role of one or more of these mechanisms, thereby changing the composition, phase and properties of the deposit. Surfaces exposed to particle impact can accumulate both silica and alkali silicates at a very rapid rate. In contrast, surfaces exposed to flue gases and less to particle impact show signs of deposit accumulation, vapour condensation and sulphation of condensed vapours containing alkali. These processes lead to the formation of deposits with different properties, high reflectivity, low thickness and growth rate. Fuels containing little alkali or silica show less growth of deposits.

Studies of deposits on the heating surface of biomass boilers are most often carried out by analyzing mature deposits collected during boiler maintenance. The composition of the deposits varies depending on the distance from the boiler superheater surface. The innermost layer of deposits usually contains iron oxide, KCl and K₂SO₄. The intermediate layer contains molten KCl and other ash particles. In some deposits, a reaction between KCl and captured ash particles can occur, leading to the release of chlorine-containing gases. However, it is difficult to study the dynamic deposition process in detail by analyzing mature deposits (Zhang et al., 2020; Sandy Sharp, 2010).

The chemistry of inorganic transformations in boilers is quite complex and involves many physicochemical pathways between alkalis, alkaline earth and other inorganic and organic compounds in the fuel. Inorganic substances derived from biomass fuels are deposited in various forms on the boiler surface (Veijonen et al., 2003; Shao et al., 2012; Capablo, 2016; Repić et al., 2019; Rabaçal and Costa, 2015).

Studies indicate that the major release of alkaline material occurs as a result of combustion during the charring phase. The original form of the alkaline waste gases corresponds to thermodynamic estimates of product stability and vapor pressure. Deposits enriched in potassium, sulfur, and chlorine indicate that condensation and chemical reaction mechanisms play a significant role in deposit formation. The condensation mechanism enriches the deposit with condensable vapors such as chlorides. The chemical reaction mechanism converts alkali and alkaline earth metals to sulfates. Both mechanisms are dependent on the mass transfer rate. Differences between the boiler wall and convection channel deposits are evident and indicate different mechanisms responsible for ash deposition in different areas of the boiler.

In the literature on the subject, slagging and contamination indicators have been defined to determine the risk of deposits and agglomeration of biomass ash during combustion and co-combustion of biofuel in a boiler.

One of the most popular indices used as an indicator of the tendency for fuel ash contamination is the ratio of base to acid B/A to acid oxides in the ash: B / A = Fe 2 O 3 + CaO + MgO + Na 2 O + K 2 O S i O 2 + A l 2 O 3 + T i O 2 (1)

If: B/A ratio <0.75 – contamination may occur; B/A < 0.4 or B/A > 0.7 – the tendency to sedimentation is low, but if 0.7 > B/A>0.4 – high (Bapat et al., 1997; Dyjakon, 2012).

In the case of biomass, the importance of potassium is also justified. Therefore, biomass of agricultural and herbaceous origin - straw, nutshells, fruit pits, weeds and grasses cause more problems than wood. The metals potassium and sodium, whether in the form of oxides, hydroxides or organometallic compounds, tend to lower the melting point of ash mixtures containing other various minerals, such as silica (SiO₂). As a result, serious problems with slagging, fouling or agglomeration of the bed in the boiler arise, which can lead to its shutdown. Therefore, it is justified to introduce a new indicator based on the melting and sintering coefficient of SI oxides: S I = Fe 2 O 3 + CaO + MgO + Na 2 O + K 2 O + P 2 O 5 SiO 2 + Al 2 O 3 + TiO 2 (2)

If: SI index >2 – the risk of slagging is low, SI < 2 – high (Bapat et al., 1997; Basu et al., 2000) and the C_m index, which expresses the ratio of basic to acidic oxides: C m = Fe 2 O 3 + CaO + MgO + Na 2 O + K 2 O + P 2 O 5 SiO 2 + Al 2 O 3 + TiO 2 (3)

The SR slag viscosity index is also used to determine the tendency to deposition. Viscosity is one of the basic properties of liquid slags and is a key parameter for many industrial processes. It plays an important role in slag adhesion, ash fusion, viscous flow and other phenomena: S R = SiO 2 SiO 2 + Fe 2 O 3 + CaO + MgO · 100 (4)

A high value of the index means a high content of slag viscosity. If: SR > 72 – the slag has a low tendency to slagging, 72 ≥ SR > 65 – medium, SR ≤ 65 – low, SR is 72–80 – the tendency to form impurities is low, SR is 65–73 – medium and 50-65 – very high (Bryers, 1996).

The dependence of melting temperatures on basic compounds (Fe₂O₃+CaO + MgO) shows a similar character for different fuels (coals, biomass and municipal waste).

Biomass is burned in various heating devices (pulverized fuel, grate, fluidized bed boilers) where particle agglomeration may occur. In order to estimate this risk, the bed agglomeration index BAI was developed, relating the ash composition to agglomeration (fluidized bed reactors): B A I = Fe 2 O 3 K 2 O + Na 2 O (5)

If: BAI <0.15, then agglomerates are formed (Capablo et al., 2009).

Another IC index is based on the ratio of iron oxide and calcium oxide as a parameter to assess contamination: I C = Fe 2 O 3 CaO (6)

If: IC < 0.3 or IC > 3.0 then the tendency to create pollutants is low, 0.3 < IC < 3.0 – high (Ściążko et al., 2006).

A similar tendency to form deposits is described by the quotient (SA) of silicon oxide and aluminum oxide (Bapat et al., 1997). S A = SiO 2 Al 2 O 3 (7)

Another indicator is the sum of Fe and Ca oxides: Fe + Ca = Fe 2 O 3 + CaO (8)

If the sum of Fe₂O₃+CaO does not exceed 10% then the fuel has a low tendency to form pollutants.

The final parameter is the Fu fouling index, which takes into account in particular the alkaline elements (sodium and potassium). This index is presented by the following equation: F u = Fe 2 O 3 + CaO + MgO + Na 2 O + K 2 O SiO 2 + Al 2 O 3 + TiO 2 · Na 2 O + K 2 O , % (9)

The Fu index determines the tendency of the fuel to form and then sinter contaminants on heating surfaces and shows the tendency to form deposits that initiate slagging.

If: Fu ≤ 0.6 – the fuel is not susceptible to contamination, 0.6 < Fu ≤ 40 – it has a high tendency, Fu > 40 – very high tendency to the formation and sintering of contaminants (Bapat et al., 1997; Dyjakon, 2012; Kowalczyk-Juśko, 2017).

On the other hand, Miles et al. (1996) presented the alkali index in the following equation in order to monitor the amount of alkali in fuels and minimize the occurrence of slagging and contamination: AlkaliIndex = K 2 O + Na 2 O   ·  Ash  % / HHV (10) where: HHV (Higher Heating Value) is the higher calorific value (MJ/kg) at H = 0% and K₂O and nitrous oxide (N₂O) are expressed as a percentage of these components in the fuel composition in dry mass according to the index:

- Alkaline index >0.17 kg alkali/MJ – probable contamination.

Whereas (Pronobis, 2005) presents the following values to reduce slagging and Cl based contamination in biomass fuels, where Cl is the chlorine content (%) in the dry mass of the sample:

- Cl < 0.2 – low tendency to slagging,

In the study conducted by Kowalczyk-Juśko (2017) for herbaceous biomass, the ratio of alkaline to acidic components, showing a tendency to ash deposition, was the lowest in the case of perennial grasses (Miscanthus sacchariflorus) and prairie knotweed (Spartina pectinata Boscex Link), and many times higher in the case of Virginia mallow. Ashes from the biomass of Chinese miscanthus (Miscanthus sacchariflorus) and Japanese knotweed (S. pectinata) were characterized by a low tendency to form impurities and sediments (Table 5). At the same time, in the case of ash from Virginia mallow (Sida hermaphrodita Rusby) and Jerusalem artichoke (Helianthus Tuberosus L.), a very high tendency to sintering, slagging and sedimentation was observed, which is evidenced by significantly higher Fu, SI, BAI values.

In turn, the research conducted by Dyjakon (2012) for the slagging and contamination indicators of the boiler heating surfaces showed significant differences in the tested plant biomass, which is presented in Table 6.

In turn, in the study by Baxter et al. (2014) it was shown that the ratio of alkaline to acidic components in the leaves of Miscanthus giganteus ranged from 0.7 to 1.3, while in the stems these proportions were different. The B/A ratio was 1.3–3.2.

There are many empirical correlations that allow the assessment of ash behavior and its effect on deposition both on boiler walls (slagging) and on the edges of convection tubes (fouling). A number of slagging and fouling indices have been developed for coal fuels to obtain and indicate the tendency of fuel ash to form deposits in combustion chambers or boilers based on chemical composition, which exhibit a different behavior than biomass fuels. For example, the slagging indices for Serbian coal, which consists of 88% brown ash coals, their application to biomass fuels containing brown ash should confirm the tendency of this fuel to form deposits on boiler heating surfaces. Since most slagging indices are therefore based on the assessment of the melting properties of aluminosilicate ash from coal, their application to biomass ash systems, which are very different in chemical terms, may be problematic. In case of low-level biomass co-firing, the mixed ash still consists mainly of the aluminosilicate system, so that typical methods for assessing coal slagging can be applied. If biomass is co-fired with coal in larger quantities, all important components of most biomass ash, mainly alkali metals and alkaline earth metals, are strong fluxing agents for aluminosilicate systems. The introduction of biomass can result in a lower melting point and therefore an increase in the slagging potential. Pollution indicators of biomass ash are based on the total alkali content of the fuel. The main driving force for conventional pollutant formation in boilers is considered to be the deposition of Na and K compounds by oxidation/condensation mechanism (Loo and Koppejan, 2008; Repić et al., 2019).

Slagging, deposit formation and surface corrosion are ash-related problems that should be investigated when using fuel blends, as they can lead to reduced efficiency and availability of facilities, which results in increased energy costs. Proper selection of biomass fuels based on the chemical composition of the ash and mixing them in the right proportions can reduce the rate of ash deposition and corrosion. The mechanisms of release of inorganic compounds depend on the boiler operating conditions and staged combustion can cause local reducing conditions, which poses a risk of boiler corrosion. Corrosion is the process of dissolving metal into a more stable compound. This process occurs as a result of electrochemical reactions, oxidation and reduction reactions, involving ions and electrons, forming an oxide. The mechanism of the corrosion process caused by chlorine, HCl, Cl₂, NaCl, KCl and alkali compounds (K, Na) is the process of steel oxidation accelerated by the presence of chlorine in the flue gases, called active oxidation. The corrosion rate is temperature dependent and depends on the volatilization of the metal chloride (Lokare, 2008; Stauber Alfredsson, 2018).

In the research conducted by Royo J. et al. (2022), it was shown that in order to compensate (at least partially) the negative impact of plant components in the mixtures, it is interesting to use a higher value of excess air (λ), because it allows to lower the combustion temperature (at λ > 1), which means reducing the degree of sintering and deposition, thus improving the ash behavior during pellet combustion.

In the study conducted by Naganuma et al. (2013) the mechanism of fouling formation was investigated and it was found that alkali sulfates play an important role in increasing the adhesion force. One possible solution to the fouling problem is to reduce the adhesion force. Therefore, thermal spraying of a nickel alloy onto the heating device pipes may help achieve this.

In turn, Pałaszyńska and Juszczak (2018) examined the effect of selected biomass types (coffee husk pellets or pellets combined with wheat straw pellets and cherry stones, sewage sludge pellets, rye and corn straw briquettes, and miscanthus briquettes) on the slag formation process. The studies showed that the boiler operated correctly during the combustion of the above-mentioned types of biomass. For all types of fuel, only brittle, porous slag was formed, which was easily fragmented by the reciprocating movement of the grate pushers and then removed from the furnace. This means that the temperature in the boiler was above the ash sintering temperature and below the ash softening temperature. In addition, a significant amount of brittle deposits was observed in the combustion chamber and on the heat exchange surfaces, which required cleaning the boiler after 40 h of operation. However, most of the deposits were created by burning coffee husk pellets.

On the other hand, herbaceous biomass fuel in the form of pellets from rice husks and corn stalks was commonly used in studies (He et al., 2022) on pollutant emissions and slag properties. The studies indicate opposite trends in the change of CO and NO_x concentrations in exhaust gases. NO_x emissions generated during the combustion of corn stalks as a result of multi-layer secondary air distribution are comparable to the combustion of rice husks. The obtained research results can contribute to efficient combustion and low pollutant emissions. Therefore, NO_x emissions and ash slagging rates at the exhaust gas outlet are used as an assessment indicator to study the operating conditions for low NO_x emissions and slagging rates during the combustion of corn stalks under air distribution conditions. On the other hand, slagging does not occur during the combustion of rice husks, therefore, NO_x and CO emission concentrations serve as an assessment indicator when selecting optimal operating conditions.

Combustion of biomass, especially herbaceous biomass, can lead to the formation of deposits in heating devices burning biomass. Both straw and other herbaceous plant materials cause rapid contamination of heat exchange surfaces, also causing the formation of deposits on the surfaces of heating devices. However, this problem is not limited to straw raw materials. Other fuels that, due to their chemical composition, exhibit different combustion behavior than other types of biomass are biomass from industrial waste and energy plants, characterized by high ash content at a low melting point and high corrosion potential. The composition of the ash affects the softening and melting temperature of the ash. The value of the sintering and softening temperatures can differ significantly for the same type of herbaceous biomass, depending on the chemical composition of the ash, which is related to the plant species, cultivation method and soil type. The deposits are caused by fly ash, which is an undesirable effect of the combustion process. The chemical properties of the deposits correspond to KCl, the main compound forming the deposits. The formation of deposits also depends on the design and operation of the boiler. Also, high chlorine content in the fuel increases the release of potassium in the gas phase at relatively low temperatures. This causes serious problems related to slagging, fouling and agglomeration of the bed in the boiler, which can lead to the shutdown of the heating device. The parameter determining the ash’s slagging capacity is the iron oxide content. For coal fuels, a number of slagging and fouling indices have been developed to obtain and indicate the tendency of fuel ash to form deposits in combustion chambers or boilers based on the chemical composition, which shows a different behaviour than biomass fuels. In the case of co-firing of biomass at low levels, the mixed ash still consists mainly of aluminosilicate system, so that methods for assessing coal slagging can be used. Many existing biomass boilers are designed for exhaust gas temperatures of 900 °C or more. Lowering the combustion temperature to control deposits can lead to lower boiler operating parameters (heat exchange efficiency decreases), which has a negative impact on undesirable economic effects. Excessive fuel deposition is not only due to changes in the composition of the deposit depending on the type of raw material used, reflecting its structure, which is manifested in changing ash deposition mechanisms, but also from changes in the value of excess air during combustion. One way to reduce these undesirable phenomena is to use additives that can change the chemical composition of the ash, reduce the concentration of problematic compounds and increase the ash melting point. Natural silicates (e.g., kaolin clays) as a carrier are also an interesting raw material for the synthesis of catalysts and adsorbents. The use of natural material as a catalyst or substrate for the production of a catalyst not only reduces the costs associated with their production, but also makes the process used environmentally friendly and can make a significant contribution to sustainable development.

In recent years, a number of studies have been carried out on herbaceous plants aimed at better understanding the complex mechanism of ash transformation processes during the formation of slag in bottom ash, which also affects the release of solid particles during complete combustion. In addition, the ash melting behavior depends mainly on the relative concentrations of ash-forming elements, the total amount of critical compounds and their kinetic properties. In order to achieve sustainable and stable development, combustion technologies must be well understood and adapted to the requirements of a given fuel. Biomass, including herbaceous biomass, belongs to fuels characterized by a higher mineral content (K, P and Cl) and a high ash content at a low melting point and high corrosion potential.

Hence, as a result of the presented literature review, the following conclusions can be drawn:

1. Compounds such as K, Na, S, Ca in the form of oxides (K₂O, Na₂O, S₂O, CaO) and hydroxides (KOH, NaOH, S(OH)₂, Ca(OH)₂) tend to lower the melting point of ashes containing minerals (SiO₂), forming alkalis with very low melting points (approx. 700 °C).

In summary, available studies suggest that adding kaolin to solid fuels can reduce CO and NO_x emissions, although the mechanisms of these processes are not yet fully understood. In the case of CO, a clear reduction in emissions is observed, which may be related to the catalytic effect of kaolin. The effect on NO_x emissions is less clear and requires further research. Therefore, it would be advisable to conduct research on the effect of kaolin or urea in mixtures with selected herbal biomass on the combustion process and on exhaust gas emissions.