Straw / agricultural-residue firing

Straw and agricultural-residue firing means burning crop residues such as wheat straw, rice straw, rice husk, corn stover, sunflower husk or other field residues as boiler fuel. These fuels can reduce fossil-fuel use and create value from agricultural waste, but they are more difficult than clean wood because their ash chemistry is harsher and more variable.

The main issue is not only the amount of ash. It is the combination of alkali metals, chlorine, silica, phosphorus, soil contamination and seasonal variability.

Why straw is hard to burn

Straw commonly contains potassium and chlorine that form low-melting salts. These salts can become sticky at temperatures found in furnace exits, superheaters and convective passes. Rice husk may contain high silica, which changes abrasion and ash-melting behaviour. Field collection also brings soil, stones and fertiliser residues that vary from harvest to harvest.

Operational consequences include:

• Slagging in the furnace and on waterwalls
• Sticky deposits on superheaters and evaporators
• Chloride-induced corrosion under deposits
• Bed agglomeration in fluidised-bed boilers
• Ash abrasion in cyclones, ducts and fans
• Higher cleaning demand in hoppers and dust-handling systems

Design and operating controls

Plants manage these risks with fuel sampling, blending, lower steam temperatures, corrosion-resistant materials, additives, careful excess-air control, bed-material management, robust ash handling and cleaning systems designed around the real ash chemistry. Storage is also important: wet straw can decay, bridge, heat and feed unevenly.

Fuel contracts should specify moisture, ash, chlorine, alkali and contamination limits. A boiler designed around clean wood pellets may not tolerate straw without material, combustion and cleaning changes.

Acoustic-cleaning relevance

Sonic horns are useful where straw ash is still friable or only lightly bonded, especially in convective passes, economisers, air preheaters, cyclones, filters and hoppers. They cannot remove fused slag or reverse chloride corrosion. In Sylio-style assessments, straw firing is treated as a high-variability duty, so horn location and firing frequency are tied to fuel mix, ash analysis and outage deposit samples.

Combustion and ash behaviour

Straw firing is difficult because agricultural residues often contain high levels of potassium, chlorine and fine silica compared with clean woody fuels. During combustion, these elements can form low-melting salts and sticky fly ash that deposit on superheaters, boiler banks, air preheaters and filters. The problem is not only total ash content. The form of the alkali and chloride, the amount of soil contamination, the harvest season and the storage condition all influence deposit stickiness and corrosion risk.

Plants control these risks through fuel specification, blending, bed or furnace temperature control, staged air, additive use, sootblowing strategy and careful material selection in high-temperature zones. Moisture and bale quality also matter because variable fuel feed can cause oxygen swings, reducing pockets and unburned carry-over. In grate and fluidised-bed units, poor distribution can move the ash chemistry from a manageable coating to rapid slagging or bed agglomeration.

Maintenance and acoustic-cleaning relevance

Inspection teams watch for superheater corrosion, chloride-rich deposits, bridging in convective banks, fouled economisers, ash build-up in hoppers and accelerated baghouse pressure drop. Deposit samples are useful because a grey, friable ash and a hard potassium salt glaze need different responses. Online cleaning intervals often have to be shorter during high-straw periods than during wood or coal firing.

Sonic horns can help in cooler convective passes, ductwork, SCR areas, air preheaters and hoppers where straw ash is dry enough to fracture and move. They are not a cure for molten bed agglomeration, fused superheater deposits or active chloride corrosion. In Sylio-style surveys, straw firing is a flag to check dew point, ash chemistry, material limits and whether acoustic cleaning should be combined with fuel blending or more aggressive local sootblowing.

Related terms

• Biomass
• Alkali metals in ash
• Chloride-induced corrosion
• Slagging