The imperative to achieve carbon neutrality under intensifying climate change has positioned biomass combustion as a critical transitional technology, with potential to displace 8.2 GtO₂-equivalent annual emissions from fossil fuels. However, its sustainable implementation faces dual environmental constraints: combustion-generated particulate emissions (PM2.5: 2.1–5.3 g/kg, PM10: 3.8–7.6 g/kg) and carbon monoxide (18–47 g/kg) exacerbate atmospheric pollution, while intrinsic ash behavior-particularly alkali-induced slagging-compromises 12%–18% of combustion efficiency in industrial systems (Huang et al., 2024; Kim et al., 2024). This operational paradox underscores the necessity to resolve ash transformation mechanisms, where potassium-silicate interactions dominate slag formation in agricultural residues (slagging indices >0.8), contrasting with sulfate-controlled deposition in woody biomass. Such fundamental limitations not only challenge emission reduction commitments but also hinder the technology’s scalability under Net-Zero scenarios. As a major agricultural country, China possesses abundant agricultural straw resources, particularly from corn, wheat, and rice-the three predominant contributors to national straw supply (Antar et al., 2021). These resources are predominantly utilized through three principal pathways: domestic fuel provision, agricultural fertilization, and industrial feedstock applications. Direct combustion has emerged as an effective strategy for large-scale biomass utilization, yet the characteristic high chlorine and alkali metal content inherent in Chinese crop straw induces significant technical challenges, including heat exchange surface deposition and high-temperature slagging during combustion processes. Current research efforts primarily concentrate on three technological pathways: biomass gasification, combustion, and pyrolysis (Figure 1).

Biomass resources are formally defined as heterogeneous organic substrates originating from photosynthetic conversion of atmospheric CO₂, serving as renewable feedstocks for energy production and biorefinery applications (Sukatis, Wee, and Aris 2022). Based on provenance and physicochemical characteristics, they are systematically categorized into four classes: (1) Woody biomass comprising forestry residues (e.g., bark, branches), silvicultural byproducts (e.g., sawdust, wood chips), and post-consumer timber; (2) Agricultural biomass including lignocellulosic crop residues (e.g., cereal straws, rice husks), dedicated energy crops (e.g., Miscanthus, switchgrass), and agro-industrial processing wastes (e.g., sugarcane bagasse); (3) Waste-derived biomass encompassing municipal solid organic fractions, wastewater treatment biosolids, and construction/demolition cellulose-rich debris; (4) Animal-derived biomass consisting of livestock manures, anaerobic digestion effluents, and slaughterhouse residues (Guo et al., 2024; Alami et al., 2021). These carbonaceous materials exhibit near-carbon-neutral characteristics when utilized in energy systems, as photosynthetic carbon sequestration during growth phases counterbalances subsequent thermochemical conversion emissions. Strategic advantages stem from their global distribution scalability, biomass can play two roles, namely, bio-carbon capture and storage/bio-carbon capture and storage, and compatibility with circular bioeconomy frameworks through cascaded utilization pathways (Ilmiawati et al., 2025).

Biomass is composed of 38%–50% cellulose, 23%–32% hemicellulose, 15%–25% lignin, and 5%–13% other components (e.g., inorganic species and extracts) (Table 1) (Li et al., 2025). Its elemental composition is dominated by carbon (C), hydrogen (H), and oxygen (O), with carbon (approximately 50%) and oxygen (around 40%) together accounting for over 90% of its dry material. Slagging, ash accumulation, and slagging in biomass combustion systems can be attributed to intricate interactions between design parameters (e.g., thermal gradients and gas flow dynamics) and the unique physicochemical characteristics of the fuel (Su et al., 2024). In addition, biomass contains trace quantities of minerals, either in ionic form or as chemical compounds, such as aluminum (Al), calcium (Ca), iron (Fe), magnesium (Mg), sodium (Na), potassium (K), phosphorus (P), sulfur (S), and silicon (Si). At a structural level, hemicellulose and lignin form a chemically bonded matrix that encapsulates cellulose. Most of the ash-forming constituents are chiefly concentrated within the three-dimensional network of lignin. In comparison, the existence of major mineral elements results in ash fouling and slagging towards process equipment. In the combustion process, basic metal oxides participate in complex combustion reactions and eventually form silicates, chlorides and sulfates. These compounds can be mixed with other metal oxides to form low-temperature stable blends. Then alkali earth metals may also react with or dissolve into the melt K-silicates (Bostrom et al., 2012).

Biomass feedstock ash composition and its physicochemical attributes—especially the levels of alkali/alkaline earth metals (AAEMs), chlorine (Cl), and silicon-sulfur compounds—serve as critical determinants of combustion efficiency and operational reliability. The rapid release of volatiles from biomass, coupled with the intermittent feeding of biomass fuels and uneven spatial distribution of fuel particles, leads to inconsistency between the timescales of chemical reactions and diffusion. This results in inadequate mixing of fuel and oxygen, thereby reducing combustion efficiency (Ma et al., 2025). In particular, AAEMs (e.g., potassium, sodium, and calcium) combined with chlorine lower ash fusion temperatures through eutectic interactions, whereas silicon-sulfur compounds (such as SiO₂-SO₃ complexes) intensify ash agglomeration. Collectively, these constituents are recognized as key drivers of the interconnected processes leading to slagging, fouling, and high-temperature slagging, thereby prompting the development of novel material and process solutions (Yao et al., 2020). Operational challenges related to ash manifest via four primary mechanisms: slagging induced by alkali metals, agglomeration from silicate melts, particulate deposition dynamics, and slagging mediated by chlorine. Furthermore, elevated concentrations of chlorine and alkali metals (notably K and Na) in combustion environments intensify ash deposition, while the low melting points of alkali metal compounds facilitate particle adhesion and bonding (Tiainen, Daavitsainen, and Laitinen 2002). Under elevated thermal conditions, alkali metals and their derivatives follow two distinct migration routes: one involving direct deposition onto furnace surfaces via inertial impaction, and another characterized by vapor-phase transport with subsequent heterogeneous condensation onto downstream heat-exchanger surfaces, forming submicron fly ash particles. Addressing the complex multiphase evolution of biomass ash remains a significant obstacle to expanding clean energy systems. Figure 2 illustrates the trend in publications over the past 5 years focusing on biomass for diverse applications, thereby highlighting its emerging potential and growing scholarly attention. Biomass combustion technology-an essential pillar of decarbonization strategies-has garnered significant interest owing to its sustainable feedstock foundation and carbon-neutral life cycle. However, large-scale industrial deployment is impeded by persistent operational challenges, including slagging, fouling, and high-temperature corrosion. These issues stem from the migration, transformation, and interactions of alkali metals, chlorine, and sulfur, which collectively govern the dynamics of ash fusion and deposition. Although previous studies have partially unraveled ash evolution pathways, high ash containing biomass demonstrates several challenges when applying it as a biofuel owing to the increase in the capital and operating costs (Hess et al., 2019). Biological co-combustion, leaching, and adding additives have been identified as the most effective deashing treatment methods.

This review is to systematically elucidate the thermochemical interactions of alkali metals, chlorine, and sulfur in biomass ash systems and their roles in slagging mechanisms. Additionally, it evaluates mitigation strategies, including leaching pretreatments and aluminosilicate additives, to address ash-related operational challenges. By clarifying the slagging mechanisms and quantify alkali metal migration pathways, this advances the optimization of biomass combustion systems for enhanced durability and efficiency. The review provides a foundation for developing scalable solutions to mitigate ash deposition and corrosion, thereby supporting sustainable biomass energy integration into decarbonization frameworks.

In the process of biomass energy utilization, biomass ash is an important factor affecting the utilization process. Due to the high content of alkali metals, alkaline earth metals and chlorine in biomass fuel, After the alkali metal sulfate, chloride, carbonate and silicate enter the gas phase successively, they condense on the surface of fly ash particles, which reduces the melting point of fly ash and increases the viscosity of fly ash. Under the action of the air flow in the furnace, they bond on the heating surface and form the slag (Werkelin et al., 2010).

Figure 3 illustrates the processes of ash generation and modification during biomass combustion. Biomass combustion (Jia and Lighty 2012; Dai et al., 2025), volatile organometallic species (e.g., chromium [Cr] and vanadium [V]) are initially released during the ignition of biomass particles and the subsequent formation of char. Thereafter, alkali and alkaline earth metals-specifically potassium (K), sodium (Na), and calcium (Ca)-along with trace volatile elements such as mercury (Hg), arsenic (As), and selenium (Se), are liberated through surface diffusion from the char matrix. When the gas-phase temperature declines, these compounds undergo nucleation, resulting in the production of submicron aerosol particles via gas-to-particle conversion. Additionally, a portion of these volatiles condenses onto the surfaces of the residual fly ash. Elevated levels of alkali metals, particularly potassium (K), precipitate ash-related operational challenges, including high-temperature corrosion and slagging phenomena, owing to the synergistic interplay between nucleation-condensation processes and secondary chemical reactions.

Although both slagging and fouling represent ash deposition processes in biomass boiler flue gas systems, they operate via fundamentally different mechanisms and are influenced by varying temperature regimes. Slagging manifests in high-temperature regions (above 1,000°C) via viscous flow sintering of molten ash, a process propelled by the phase transformation of alkali-rich silicates and aluminosilicates. Conversely, fouling prevails in lower-temperature areas (below 800°C) through the dry deposition of unmolten particulate ash (Shao et al., 2012). The unmolten ash is defined by a melting point that considerably exceeds the temperature of the deposition surface. Such deposits undermine the efficiency of slag removal, diminish thermal performance, and markedly curtail the operational lifespan of the boiler (Indrawan et al., 2020; Corona et al., 2020). Compared to coal-fired boilers, biomass systems exhibit a greater susceptibility to slagging and fouling, attributable to their elevated levels of alkali metals and chlorine (Rong et al., 2017). Chlorine promotes the volatilization of potassium as KCl vapors, which subsequently condense into submicron aerosols. These aerosols then deposit by means of thermophoresis-a migration driven by temperature gradients-and inertial impaction, ultimately forming tenacious layers on the surfaces of convective passes.

One study found that the K, S and Cl of biomass fuel gradually decreased with the temperature rising to 1,200°C, and their ash content also gradually decreased (Johansen et al., 2011). Then they remain either unchanged or decrease slightly between 1,200°C and 1,400°C. Cl concentration is essentially zero at 815°C. However, between 815°C and 1,200°C, K concentration decreases because of the volatility of KOH and K2SO4 aerosols. Meanwhile, inorganic S is released in the form of SO2 by the decomposition of K2SO4. All these conditions result in the continuing decrease of the ashing ratio as temperature increase until 1,200°C.

Deposits on the walls of tubes emerge from both solid-phase mineral conversions and vapor-phase condensation processes. The resulting layers, typically rich in alkali sulfates (such as K₂SO₄) and chlorides (such as KCl), possess adhesive qualities attributable to their low eutectic melting temperature, thereby fostering secondary agglomeration through capillary forces. Controlled drop-tube furnace experiments conducted by Wang et al. demonstrated that a synergistic interaction between sulfate and chloride compounds critically influences ash deposition kinetics in biomass combustion systems. At combustion temperatures exceeding 800°C–900°C, gaseous KCl participates in heterogeneous gas-solid reactions with SO₂ (Wang L. et al., 2012).

When flue gas temperatures drop below 500°C, thermophoretic effects combined with turbulent diffusion drive the condensation of alkali-enriched aerosols, thereby initiating the formation of a sticky slag layer on heat exchange surfaces. This viscous film subsequently augments the adherence potential for additional particulate deposits. A portion of these alkali aerosols condenses onto fly ash particles, either creating tacky surfaces or reacting with refractory oxides to produce eutectic mixtures with melting points below 700°C. Upon further reduction of the gas temperature to below 300°C, alkali vapors traverse a sequence of phase changes: an initial phase (comprising condensation, nucleation, and coalescence) yields submicron aerosols (less than 1 μm), whereas a secondary phase, marked by oxidation, re-nucleation, and coalescence, produces coarser fly ash particles (greater than 10 μm) (Bostrom et al., 2012). Irrespective of their surface viscosity, coarse particles predominantly deposit onto the primary slag layer via inertial impaction. In contrast, fine particles enriched with potassium, sodium, chlorine, and sulfur (e.g., KCl, K₃Na(SO₄)₂) tend to concentrate on surfaces due to their elevated surface-to-volume ratios, thereby enhancing the adsorption of silicon- and aluminum-rich coarse particles through electrostatic and van der Waals interactions (Nordgren et al., 2013). The lower adhesion efficiency between fine and coarse particles triggers secondary enrichment processes, ultimately producing self-organized, stratified slag formation via a hierarchical deposition mechanism. This process elucidates the heterogeneous microstructure characteristic of combustion deposits in industrial biomass boilers. In summary, the initial slag layer is composed of fine ash particles alongside coarse particles that display only partial adhesion. Owing to the surface’s low operational temperature and substantial thermal resistance, the layer remains non-adhesive even as its temperature approaches, but does not exceed, the ash fusion point. When ash-laden flue gas impinges upon the slag layer, inertial forces facilitate the primary deposition of particles that are softened or partially molten. As the slag layer accumulates, its surface temperature eventually exceeds the melting threshold, thereby transitioning into a molten state. Thereafter, fly ash particles coalesce to form a dense coke layer through secondary deposition. Gravitational shedding occurs when the deposit’s weight overcomes its structural cohesion or adhesion to the furnace wall (Zhou et al., 2013; Liu et al., 2011). Ultimately, the interplay between layer accumulation and gravitational shedding establishes a dynamic equilibrium that stabilizes the deposit thickness (Figure 4) (Niu et al., 2014).

The application of high-ash biomass in thermochemical combustion systems poses significant technical and economic barriers, stemming from elevated capital expenditure and operational expense linked to ash management, accelerated equipment degradation, and unplanned maintenance interruptions (Hess et al., 2019). Three principal approaches-co-processing with low-ash feedstocks, leaching processes (e.g., water or acid pretreatment to diminish alkali concentrations), and additive integration (e.g., aluminosilicates to sequester mobile alkali species) have demonstrated efficacy in addressing ash-related inefficiencies. Selecting the optimal methodology necessitates a comprehensive evaluation of feedstock-specific ash composition (e.g., alkali metals, chlorine, silica ratios), ash production rates, regulatory limitations (e.g., particulate emission thresholds), and site-dependent variables, including reactor design and operational adaptability (Bostrom et al., 2012).