The most harmful non-process substances for a recovery boiler are chlorine and potassium, which in high concentrations can cause severe corrosion in the superheater tubes of the recovery boiler. They also lower the melting temperature of the boiler fly ash and increase the possibility of blockage of the flue gas channels.
Valmet has seen environmental factors, the increasing use of renewable energy and the increase in efficiency of pulp mills as driving the use of boilers for electricity generation in recent years. Compared to boilers developed solely for electricity generation, recovery boilers have lower steam parameters, such as temperature and pressure, and thus clearly worse steam-to-electricity ratios. Although the amount of steam produced per kilogram of liquor dry matter has increased by more than 20 percent over the past decade, fuel and ash properties and the corrosion resistance of superheater materials are still limiting factors for recovery boiler operation.
Valmet has introduced two new products, Valmet Recovery Ash Analyzer and Valmet Ash Balance Advisor, to prevent downtime due to superheater corrosion and boiler blockage, as well as to provide cost-effective ash handling and guidance on the need for chimney sweeping.
"Ash treatment can reduce harmful chlorine and potassium levels while retaining valuable sodium and sulfur. At the same time, environmental impacts can be reduced by removing the right amount of ash from a closed process. Until now, ash treatment has typically been based on infrequent and often incomplete laboratory tests on ash chemistry, without constant comparison with boiler flue gas and steam temperatures," says Timo Laurila
“Adding sodium and sulfur is very expensive, and excessive or inefficient ash treatment significantly increases operating costs. If the ash is not treated sufficiently, the recovery boiler can become clogged and the steam consumption in the flue gas cleaning can increase. In the worst case, the superheaters will corrode quickly, and the need for water washings that require production interruption due to blockages will increase.”
Valmet believes that regularly measured ash chemical composition, combined with boiler flue gas and steam temperatures, are key to controlling recovery boiler operation and ash handling. They can be used to improve recovery boiler efficiency and availability. Therefore, the Valmet Recovery Ash Analyzer continuously measures ash composition, including carbonate, sulfate, chlorine, potassium and sodium concentrations. It also calculates critical ash melting temperatures; T0, T15 and T70, which describe the corrosiveness and stickiness of ash. The Valmet Ash Balance Advisor software provides guidance on ash handling rates and steam temperatures. The guidance helps to achieve an optimized cost-effective ash handling rate, control sodium and sulfur losses and minimize operating costs. The Valmet Ash Balance Advisor also continuously calculates the critical stickiness range in superheaters for which it is profitable to increase the flue gas cleaning efficiency. This enhances the removal of chloride and potassium from the recovery cycle between chemical addition and electricity production to achieve an optimal balance, while also extending the boiler cleaning intervals.
