Modern recovery boilers (RB) operating at high bed temperatures have, typically, a low SO₂ emission. These RBs have a modern air system and, by firing liquor at high solids concentrations, create adequate burning intensity and mixing in the lower furnace. Older boilers operating at high sulfidity, low dry solids and/or a not sufficient air system will have a high SO₂ emission rate, i.e., around 1000 mg/Nm³ [1].

The molar ratio of S/Na in black liquor (BL), along with furnace operating conditions, determine the chemistry of the S and Na in the flue gas. Based on the results obtained from a flue gas survey of different boilers, the range for Na and S emissions has been examined. For RBs with a low S/Na ratio in the flue gas (high burning intensity) and a hot bed, more Na is released than needed to bind with S, resulting in little or no SO₂ emission. The fly ash will contain a high Na₂CO₃ concentration, corresponding to a high pH. RBs with a high S/Na ratio in the flue gas have low burning intensity and colder beds that do not release enough Na to capture S, resulting in more SO₂ emission and the formation of acidic sulfates, which lower the sticky point of the ash and its pH. Click to continue »