Tackling High-Chlorine Corrosion in Straw Fuels: Grate Material Upgrades and Anti-Corrosion Coating Processes for the Corrosion-resistant biomass incinerator
In agricultural and forestry waste-to-energy projects, crop straw has become a mainstream fuel due to its low collection cost and wide distribution. However, its high chlorine content—ranging from 0.5% to 2%—presents a severe challenge to incineration equipment.
When traditional grates are exposed to chlorine-containing flue gas and alkali metal salt deposits over long periods, they frequently suffer from oxide scale spalling, grate bar perforation, and even fractures, drastically shortening maintenance cycles. In this scenario, a Corrosion-resistant biomass incinerator specifically designed for complex fuels shifts from being an "optional upgrade" to a "mandatory requirement" to ensure continuous operation.
High-chlorine corrosion is not a simple chemical reaction; it is the coupled result of gas-phase chlorination, molten salt erosion, and thermal stress fatigue. To protect equipment lifespan and operational safety in straw incineration scenarios, breakthroughs must be made in both material metallurgy and surface protection processes.
The Destructive Logic of Chlorine Corrosion: Why Ordinary Grates Fail Rapidly
During straw combustion, chlorine primarily exists in the flue gas as HCl gas and alkali metal chlorides (KCl, NaCl), triggering an "active oxidation-chlorination cycle" in high-temperature zones:
High-Temperature Gas-Phase Corrosion: When the grate and wall temperatures are between 470–600°C, the protective $Fe_3O_4$ film on the metal surface is destroyed by $Cl_2$ or HCl. This generates gaseous or low-melting-point metal chlorides (like $FeCl_2$). These products migrate outward, release chlorine gas in oxidizing environments, and form oxides. The chlorine acts as a catalyst, continuously "transporting" the base metal away, leading to deep, sustained corrosion.
Molten Salt Erosion: KCl and NaCl in the flue gas easily condense on the grate surface between 600–700°C, forming low-melting-point eutectics. These melt, destroying the integrity of the oxide film and accelerating the loss of alloy elements. This molten alkali metal chloride corrosion is exceptionally fierce given straw's high-potassium, high-chlorine nature.
Coupled Thermal Corrosion and Erosion: Hard particles like Si and Ca associated with straw ash peel away surface protective layers under the action of grate vibration and airflow. This repeatedly exposes fresh metal to the corrosive atmosphere.
Ordinary cast iron or carbon steel grates often show significant thinning and cracking within 3–6 months in such environments. Conversely, a Corrosion-resistant biomass incinerator utilizes systematic matching of materials and coatings to extend lifespan by 2 to 3 times or more.
Metallurgical Upgrades for Grate Materials: From Gray Cast Iron to High-Alloy Heat-Resistant Steel
The first step in combating a high-chlorine environment is increasing the content of alloying elements like Cr, Ni, and Si in the base material to form a stable, self-repairing high-temperature oxide film that inhibits chlorine penetration. Mainstream upgrade paths for the Corrosion-resistant biomass incinerator include:
High-Chromium Heat-Resistant Cast Iron (Cr15–Cr28): Increasing chromium forms a dense $Cr_2O_3$ film on the surface, significantly reducing the inward diffusion rate of chlorine and oxygen. Simultaneously, its high hardness and wear resistance withstand straw ash erosion, making it suitable for fixed or reciprocating grates in medium-temperature zones.
Austenitic Heat-Resistant Steel (e.g., 310S, 253MA, HK40): 310S contains roughly 25% Cr and 20% Ni, offering excellent resistance to high-temperature oxidation, sulfidation, and chlorination between 600–900°C. 253MA strengthens grain boundaries with rare earths and nitrogen, improving thermal fatigue resistance for vibrating grates facing severe temperature fluctuations.
Silicon-Aluminum Alloy Modified Cast Steel: Adding 1.5%–2.5% Si promotes the formation of a silicon-rich surface oxide layer, inhibiting internal carburization and chlorination. Aluminum micro-alloying further raises the thermal corrosion threshold.
High-quality manufacturers configure materials by temperature zones: using high-nickel austenitic steel or high-chromium cast iron in the main combustion zone, and medium-alloy heat-resistant steel in the pre-combustion and ash-discharge zones to balance costs and prevent economic waste.
Surface Protection Engineering: Triple Barriers Defeating the "Chlorination Cycle"
Relying solely on base materials is insufficient under extreme straw combustion conditions. Advanced surface engineering is the "armor" of the Corrosion-resistant biomass incinerator:
HVOF (High-Velocity Oxygen Fuel) Cermet Composite Coating: Spraying WC-Co or $Cr_3C_2$-NiCr powders onto the grate forms a dense layer (porosity <1%, bond strength >70 MPa) with hardness exceeding HRC 60. This blocks chloride ion penetration and resists ash wear.
Plasma Cladding of High-Entropy Alloys or Ni-Cr-Si-B Systems: Utilizing high-energy beams to form a metallurgical clad layer in-situ. The dense microstructure contains corrosion-resistant phases (like $Cr_{23}C_6$, $Ni_3Si$) that maintain a stable passive film in high-temperature chlorine atmospheres, ideal for the top and side edges of grate bars in the main combustion zone.
Chromizing/Aluminizing Surface Alloying: Thermal diffusion creates a Cr-Al enriched layer (tens to hundreds of microns thick) with no obvious weak interface layer with the base metal. Its oxidation/chlorination resistance surpasses simple hardfacing and resists spalling during thermal cycling.
Nano-Modified Hybrid Sealer Primer + High-Temp Topcoat: For new or overhauled grates, microscopic pores are first filled with inorganic silicate-based nano-slurry, followed by a ceramic-powder-infused topcoat resistant to >600°C, serving as a "first line of defense" to delay initial corrosion.
Advanced manufacturers adopt a triple-protection concept: "Material Upgrade + Cladding/Spraying on Critical Surfaces + Overall Sealing Coating," achieving synergistic effects under complex straw conditions.
Supporting Optimization in Structural Design and Operation Control
Even the best materials and coatings will degrade rapidly if local high-temperature hot spots, reducing atmospheres, or excessively thick ash deposits occur. Therefore, structural and operational interventions are necessary to push the Corrosion-resistant biomass incinerator to its limits:
Grate Cooling and Wall Temp Control: Utilizing water-cooled vibrating grates or forced air-cooling structures to keep the average working temperature of the grate bars within a safe range (e.g., <450°C). This lowers the high-temperature chlorine corrosion rate and prevents localized overheating from accelerating chloride vaporization.
Air Distribution Optimization to Suppress Reducing Zones: Primary air is supplied in stages below the grate to guarantee an oxidizing environment at the bottom of the fuel bed, reducing the highly corrosive reducing atmosphere where $H_2$, CO, and HCl coexist. Secondary air enhances mixing in the upper furnace.
Soot Blowing and Ash Removal Strategies: Addressing the sticky nature of straw ash, combined steam + shockwave soot blowing prevents KCl/NaCl from slagging around the grate and heating surfaces, minimizing the contact time of molten salts.
Fuel Pre-treatment for Chlorine Reduction: Sun-drying or blending with sawdust/fruit branches keeps the straw chlorine content below 0.3%, reducing the corrosion load at the source.
BNTET Corrosion-Resistant Biomass Incinerator: A Systematic Anti-Corrosion Solution
In straw and high-chlorine biomass fuel projects, the BNTET Corrosion-resistant biomass incinerator builds its equipment system around three main lines: "Base Corrosion Resistance + Surface Barrier + Mild Thermodynamics":
Zoned Material Configuration: Grates in the main combustion zone use high-chromium heat-resistant cast iron (Cr26–Cr28) or 310S austenitic steel castings, with carbide network distribution controlled via aging and metallographic inspection.
Cladding Protection on Critical Surfaces: The top surfaces, sliding sides, and ash-prone corners of the grate feature plasma-clad Ni-Cr-Si-B alloy layers (1.5–3 mm thick, microhardness ≥HRC 50), with porosity and thermal shock testing completed in-factory.
Comprehensive Surface Treatment Chain: After casting cleaning, chromizing or aluminizing micro-alloy diffusion is applied, followed by inorganic nano-ceramic sealing primers and high-temp ceramic topcoats, forming a three-level barrier.
Water/Air Cooled Grate Structures: Depending on capacity, water-cooled vibrating grates or forced-cooling fixed grates are configured. Combined with CFD-optimized staged air supply, the average grate wall temperature is kept in a safe zone, weakening the kinetic conditions for high-temp chlorine corrosion.
Fuel Adaptation and Commissioning Services: Upon delivery, we provide recommendations on straw blending ratios, moisture, and chlorine content control. During thermal commissioning, primary/secondary air ratios and grate frequencies are fine-tuned to avoid localized reducing atmospheres and hot spots.
The high-chlorine corrosion of straw fuels cannot be eradicated by a single technological fix; it requires a closed-loop approach across corrosion mechanism understanding, metallurgical upgrades, surface coating processes, and structural/operational control.
As the demands for annual operating hours and maintenance cost control in agricultural and forestry biomass projects rise, a Corrosion-resistant biomass incinerator equipped with systematic anti-corrosion capabilities will become the mainstream industry choice. Through high-alloy material selection, plasma cladding, nano-ceramic composite coatings, and optimized cooling/air distribution, BNTET maximizes grate lifespan and reduces unplanned shutdown risks in high-chlorine/high-alkali straw environments, providing robust equipment support for your biomass-to-energy projects.
