Char Bed Modernization — SHB Power Plant Engineering

Char Bed Modernization (CBM) has demonstrated significant improvements when implemented under appropriate conditions. To ensure its effectiveness, Computational Fluid Dynamics (CFD) modeling is essential for evaluating the current recovery boiler. CFD provides crucial insights into fluid flow and heat transfer within the boiler, representing the most advanced technology for modeling combustion system performance.

The primary air system, originally designed to aid in smelt drainage from the char bed, has become less relevant in modern recovery boilers, where blackouts are infrequent. Consequently, the manual operation of this system has become largely obsolete. Additionally, its design has introduced substantial operational challenges, leading to high maintenance costs and time-consuming inspections. Traditional methods for controlling the char bed often exacerbate these issues, resulting in excessive soot-blowing, flue gas passage blockages, and increased emissions, which ultimately limit the boiler’s capacity to handle higher volumes of liquor.

Given these challenges, it is increasingly recognized that reevaluating and redesigning the primary air system is essential for improving boiler efficiency and reducing operational difficulties. Addressing these issues and implementing more effective control mechanisms will help reduce maintenance costs, enhance operational efficiency, and fully utilize the system’s capabilities.

Historically, sub-optimal designs of combustion air systems have led to insufficient air penetration and mixing, often referred to as 'Lazy Air.' CBM offers a solution by enhancing primary air ports to improve combustion and char bed control. Additionally, introducing interlaced air systems at the secondary level promotes more aggressive mixing and turbulence.

Enhancing char bed stability is a key strategy for reducing risks in recovery operations. With the increase in liquor solids and larger furnace dimensions prevalent today, there is a need to reassess the traditional primary air system design. Achieving optimal boiler capacity and runtime necessitates the implementation of advanced "active control" techniques for managing char bed topography.

For this effort, it is crucial to develop a more efficient air system at the char bed. This will help achieve higher temperatures while reducing overall airflow beneath the liquor spray to lower carryover risks. To do this, more fuel must be deposited on the bed surface to improve heat release where it is most needed. Given the poor heat transfer properties of inorganics and carbon in the char bed, cooler areas will need extra heat from nearby regions, highlighting the need for effective heat management. Modernizing the primary air system will enable the char bed to handle increased liquor spray deposits and speed up their processing—drying, burning, melting, and smelting—thereby enhancing the performance of the recovery boiler.

This leads to:

Reduced liquor carryover and pluggage at convective sections of the boiler, resulting in longer run times between outages. Additionally, there is a potential reduction in sootblowing steam usage.
Increased combustion within the char bed zone due to higher fuel concentration, enhancing radiation heat transfer at a lower elevation and improving water circulation ratio.
Accelerated solid to smelt inorganic melting facilitated by higher temperatures, reducing bed inventory and enabling the operation of a flat char bed, which aids smelt drainage to the spouts.
Generation of more sodium fume at a lower elevation, which reacts with sulfur to scrub SOx emissions.
Expected higher reduction efficiency rates due to increased carbon availability at the char bed.
Enhanced resilience of the boiler to disturbances such as variations in liquor characteristics, attributed to a hotter char bed.

With full penetration, the combustion temperature in the lower furnace can increase. This adds to the stability of the char bed and increases the sodium fume generation for lower sulfur emissions. This also reduces the reduction efficiency.