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.
Data from 25 operating RBs were collected and analyzed in this work [1]. Of all the operating parameters, three variables had the most significant effect on the SO2 emission rates. These were: BL solids content, black liquor S/(Na2+K2) molar ratio, and the pH (carbonate content) of the fly ash. The emission decreased as the BL solids increased, between 0-around 1000 mg/Nm3 at 65% solids to between 0-around tens mg/Nm3 at 80% solids. A higher dry solids also results in a higher HHRR (Hearth Heat Release Rate). At HHRR values of higher than 3 MW/m2, very low emissions were measured. At lower values of HHRR (<2.8 MW/m2), the SO2 emission gap widened. To determine if the liquor sulfidity had a role in this, measurements from liquors within a certain S/Na range (40-45%) were examined. The graph obtained by plotting SO2 emission versus HHRR showed a clear trend, where an increase in HHRR resulted in reduced SO2 emission.
The emission rose when the black liquor S/(Na2+K2) ratio was over 35%. However, some boilers had no SO2 emission even when the ratio was over 40%, indicating that the lower furnace was operating under optimal conditions. As the Na2CO3 content of the fly ash increased (resulting in higher pH), data showed that, at a pH range of 10-12, little or no SO2 emission was observed.
When considering options such as incinerating non-condensable gases, and biosludge and soap addition to the liquor, note that the extra sulfur input to the RB will likely increase the SO2 emission.
References:
1. Wallen, J., Ruohola, T. and Aikio, A., “Sulfur dioxide emission dependency on kraft recovery boiler operation parameters”, Int. Chem. Rec. Conf, June 6-11, 2004, Charleston, SC
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Suggestion for old boilers to reduce the ratio.
Sulfur emission from the recovery boiler is a function of temperature, the added air and the liquor’s dissolved solids content. Old boilers processing high sulfidity liquors at low dry solids, and with insufficient air systems, have a higher SO2 emission rate.
The liquor S/Na ratio and the burning intensity (% solids and boiler loading), combined with efficient mixing in the air system, are the key factors in controlling the SO2 emission. To reduce emission, high dry solids firing and a modern air system is required. High dry solids firing increases the bed temperature, resulting in increased emission of alkali metal vapor (Na) and a reduced ratio of S/Na.
Hi
Would you please tell me how to interpret the ratio referred to in this article? Is it S/(2*K+2*Na) or S/(0.5*K(total)+0.5*Na(total))?
Regards
Jaco
The ratio is a mole %, analyzed on as-fired black liquor. It is mole S divided by the sum of (mole Na+mole K)