Aug 12, 2021
The new Valmet Recovery Ash Analyzer reduces unplanned boiler shutdowns and provides guidance for ash treatment and sootblowing needs. Valmet Ash Balance Advisor is an additional option to optimize ash treatment for significant cost savings in sodium make-up.
In recent years, the drive to increase power generation from recovery boilers has been boosted by environmental factors and the growing focus on renewable energy sources, in addition to increased pulp mill efficiency. Unlike boilers developed solely for power generation, recovery boilers have lower steam parameters (temperature and pressure) and thus lower steam to electricity ratios. Although the ratio of kg of generated steam per kg of liquor solids has actually increased by more than 20 percent in the last decade, limiting factors for the recovery boiler are the fuel/ash properties and the corrosion resistance of superheater materials for these conditions.
As pulp mill chemical recovery becomes more efficient, with minimized emissions, effluents and waste streams, the accumulation of non-process elements (NPE) in the liquor circulation creates considerable challenges. For the recovery boiler, the most potentially harmful NPE are chloride (Cl) and potassium (K), which can cause severe corrosion of the recovery boiler’s heat exchangers in high concentrations. They also lower the melting temperature of the boiler fly ash and increase the potential of the flue gas passages becoming fouled or plugged.
Ash treatment provides the means to decrease the level of harmful Cl and K, while retaining valuable sodium and reducing environmental impact, with less ash removed from the closed process than ash dumping. Unfortunately, ash treatment is typically based on infrequent and incomplete laboratory tests, with no standard criteria for when to start treating or dumping ash.
Sodium make-up is very expensive, and treating too much ash or treating ash with low efficiency significantly increases operating costs. However, if insufficient ash is treated, the recovery boiler can suffer plugging, increased consumption of sootblowing steam and in the worst-case, superheater corrosion and an increased need for water washes, with associated production stoppages. More frequent knowledge of ash composition is the key to an improved recovery boiler and ash treatment plant controls, leading to higher boiler efficiency and availability.
The recently introduced Valmet Recovery Ash Analyzer offers an innovative online solution for measuring fly ash composition at the electrostatic precipitator (ESP). It continuously measures the complete chemical balance of carbonate (CO3), sulfate (SO4), chloride (Cl), potassium (K), and sodium (Na) concentrations, as well as calculating the critical ash melting temperatures that determine fouling and corrosion risks.
Valmet Ash Balance Advisor software provides an online view of ash balance and boiler operation parameters to give an operating target for ash treatment volume, while keeping sodium losses and operational costs at an optimal level. The result is an enhanced removal of chloride and potassium from the recovery loop with an optimized cost balance between fresh chemical make-up and electricity generation, while extending boiler cleaning intervals.
Text Nigel Farrand
New solutions for improved recovery boiler availability
The Valmet Ash Balance Advisor and Recovery Ash Analyzer join a growing list of process analyzers and optimizers for the recovery line from Valmet. Valmet is also a designer and manufacturer of recovery line equipment that includes some of the world’s largest recovery boilers. Valmet offers a unique insight into and wealth of experience in recovery operations, controls and process optimization. Expert support, both local and remote via the Valmet Performance Center, can be supplemented with advanced analytic and data discovery services for maximal process enhancement.
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