RELATIONSHIP BETWEEN SO2 EMISSIONS AND ...

RELATIONSHIP BETWEEN SO2 EMISSIONS AND PRECIPITATOR DUST COMPOSITION IN RECOVERY BOILERS Honghi Tran Pulp & Paper Centre University of Toronto Toronto, ON, Canada

Wm. James Frederick, Jr. and Kristiina Iisa Institute of Paper Science and Technology Atlanta, GA, USA

Alarick Tavares Domtar Eddy Specialty Papers Espanola, ON, Canada

Roberto Villarroel Votorantim Celulose e Papel, Unidade Luis Antonio, Sao Paulo, Brazil

ABSTRACT The relationship between SO2 emissions and precipitator dust composition and its implications on tube fouling were examined based on data obtained from a number of field studies conducted at kraft mills. The results show that while SO2 emissions and the carbonate content in the precipitator dust are strongly related to each other, there is no or little correlation between SO2 emissions and chloride and potassium contents. The results also imply that sulphation reactions involving alkali hydroxide and chloride vapours, SO2, O2 and H2O occur mainly in the oxidizing region before the superheater entrance. Once carbonate and chloride fume particles have been formed in the boiler, they will be unlikely to be sulphated in-flight. As long as the dust contains some carbonate, it will unlikely contain liquid acidic sulphates and thus will not be sticky.

INTRODUCTION Sulphur oxides (SO2 and SO3, customarily referred to as only SO2) are formed mainly as a result of oxidation of reduced sulphur compounds in recovery boilers. Due to their ability to form sulphates with alkali compounds, SO2 has a significant effect on the composition of fireside deposits and precipitator dusts [1], and consequently on the fouling and corrosion conditions in recovery boilers [2,3]. While the quantity of SO2 emitted from a recovery boiler can be related to bed temperature and liquor sulphidity [1], how SO2 may affect the compositions of deposits and precipitator dust, and ultimately the fouling condition of heat transfer surfaces, is not well understood. The objective of this work is to examine the relationship between SO2 emissions and precipitator dust composition, and the implications on tube fouling based on data available in the literature and on results obtained from field studies conducted at several kraft mills.

TAPPI Engineering Conference, Atlanta, GA September 2000

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SO2 EMISSIONS The SO2 concentration in the stack gas, on a volume basis, typically is less than 1 ppm for modern boilers that operate at high bed temperatures. For boilers that operate at low bed temperatures, the concentration is higher, 200 - 300 ppm. In such boilers, it is not uncommon to have brief excursions of SO2 emissions that exceed 1000 ppm, particularly during an upset in firing conditions in the lower furnace. Within a boiler, the SO2 concentration is also known to vary from location to location, depending on the local temperature, O2 concentration, and on the types of alkali compounds that are present in the flue gas. Hiner [4] reported that the SO2 concentration in a Babcock and Wilcox recovery boiler decreased from 800-1200 ppm at the superheater entrance to 200-300 ppm at the boiler bank entrance, and then remained at the same level in the boiler stack. Similar observations were also made in a field study on an ABB-CE recovery boiler at Domtar Eddy Specialty Papers in Espanola, Ontario. In this study, the SO2 concentration was measured at several locations in the boiler using a portable gas analyzer. As shown in Figure 1, the concentration of SO2 at the black liquor gun elevation averaged about 3200 ppm; it was somewhat lower at the bullnose elevation, 2400 ppm, then decreased markedly to 30 ppm in regions downstream of the superheater entrance. The sudden disappearance of SO2 near the superheater entrance observed in these studies is intriguing. It provides insight into how sulphation reactions occur in the boiler and how SO2 is removed from the flue gas, as will be discussed later.

Figure 1. Average SO2 Concentrations (in ppm) in the Flue Gas at Various Locations in a Recovery Boiler during a test at Domtar Eddy Specialty Papers, Espanola, Ontario. TAPPI Engineering Conference, Atlanta, GA September 2000

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PRECIPITATOR DUST COMPOSITION Since SO2 can react with alkaline compounds and oxygen in the flue gas to form alkaline sulphates (Na2SO4 and K2SO4), its concentration in the flue gas is expected to have an effect on the composition of the precipitator dust. The concentration of SO2 depends greatly on the sulphur input to the boiler increases and the bed temperature in the lower furnace. Boilers with Increased Sulphur Input Figure 2 shows the chloride content and pH of 5% aqueous solutions of precipitator dusts over a three-year period from a recovery boiler at a coastal kraft mill, plotted against white liquor sulphidity [5]. Each data point is a monthly average value. The chloride content decreased from 7 to 2.5 wt% Cl, as the white liquor sulphidity increased from 25 to 35% (on AA). The pH also decreased in the same period, from 10 (alkaline) to 6.6 (slightly acidic). The change in pH value corresponds to a decrease in carbonate content from less than 2 wt% CO3 to zero.

Figure 2. Relationship between White Liquor Sulphidity and Chloride Content of Dust and on pH of Dust Aqueous Solution [5]. Data are monthly average values.

Since increasing liquor sulphidity likely results in a higher SO2 concentration in the flue gas, the trend between increasing white liquor sulphidity and decreasing dust chloride content in Figure 2 indirectly suggests that SO2 plays a role in purging chloride from the dust according to the following reaction [6]: 2 NaCl(g)+ SO2(g) + ½ O2(g) + H2O(g)  Na2SO4(l) + 2HCl(g) ....(Reaction 1)


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Someshwar [7,10], in his study of HCl and SO2 emissions from various types of recovery boilers, found that boilers that had high SO2 emissions also had high HCl emissions. The finding is consistent with Reaction 1, which occurs more readily as the SO2 concentration in the flue gas increases. The relationship between SO2 concentration and the chloride content of precipitator dusts, however, was not clear, particularly for boilers with high SO2 emissions, > 100 ppm (Figure 3).

Figure 3. Relationship between SO2 Concentration in Stack Gas at 8%O2 and Chloride Content in Precipitators Dusts from Various Types of Recovery Boilers [7,10]. In two trials conducted at the Votorantim Celulose e Papel (VCP) Unidade Luis Antonio mill in Brazil to examine the effect of burning concentrated non-condensible gases (CNCG) in recovery boilers on the boiler performance, it was found that while burning sulphur-containing gases had no effect on boiler performance, it had an effect on dust composition [8]. Similar results were also obtained in another trial at the VCP Jarcarei mill. Figure 4 summarizes results of these three trials. The sulphate content of the dust during the period when CNCG was burned (ON) was higher than that during the time when CNCG was not burned (OFF). The carbonate content, on the other hand, was lowered by about the same amount gained by the sulphate content. The chloride content decreased only slightly. No effect of high sulphur input on potassium content was found.


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Figure 4. Effect of CNCG Incineration on Precipitator Dust Composition [8]. Trials #1 and #2 were conducted at different times in the same recovery boiler. Trial #3 was conducted at another VCP mill.

Boilers with Constant Sulphur Input In the above studies, high SO2 concentrations in the flue gas resulted from high sulphur input to the boiler (i.e. high liquor sulphidity and CNCG burning). However, in most boilers, CNCG is not burned and the as-fired black liquor composition on a dry basis is relatively constant. A high SO2 concentration in the flue gas results mainly from a low bed temperature in the lower furnace. A study was conducted in such a boiler at a kraft mill in British Columbia, Canada in November 1998 to determine if there is any correlation between SO2 emissions and dust composition. At this mill, the white liquor sulphidity was high, 34-35% on AA, and the black liquor solids content was low, 65-66%. The SO2 emission from the boiler was typically below 50 ppm. However, due to the low solids content and the high sulphur content of the as-fired black liquor, the boiler occasionally experienced high levels of SO2 emissions.

In this study, a total of 22 precipitator dust samples were collected at the precipitator inlet using a dust sampling probe, over a two-day period during which frequent excursions of high SO2 emissions occurred. The samples were analyzed and their composition was plotted against the hourly-averaged SO2 concentration around the time of sampling. As shown in Figure 5, no change in chloride and potassium contents with SO2 concentration was observed. The carbonate content, however, decreased from 6 to 2 wt% CO3 at a SO2 concentration below 5 ppm to 0.8 wt% CO3 at about 300 ppm SO2.


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Figure 5.

SO2 Emissions vs Precipitator Dust Composition from a Recovery Boiler. Broken line is a linear regression of the carbonate data.

The no-correlation between SO2 emissions and dust chloride content observed in this study is in disagreement with an earlier study by Vakkilainen et al [9]. They reported that the chloride content of the precipitator dust in a recovery boiler decreased from 0.55 wt% Cl at 100 mg/m3 SO2 (about 45 ppm on a volume basis) to nearly zero at 1730 mg/m3 SO2 (740 ppm). The disagreement may be due to the relatively lower SO2 concentration in this study compared to that in the Vakkilainen et al’s study. IMPLICATIONS The above field studies, together with laboratory studies on the sulphation kinetics of alkali compounds [10,11], provide an insight into how sulphation reactions occur in the boiler, and how they may contribute to deposit accumulation and fouling problems in the back end of the boiler. Sulphation Reactions In the region from the lower furnace to the bullnose elevation, alkali carbonates (Na2CO3 and K2CO3) are unlikely to exist in the gas phase due to their instability at high temperatures, i.e. above 1000oC (1830oF). Thus, the sulphation reactions involve mainly hydroxide vapours (NaOH and KOH) according to Reaction 2, and vapours of chloride (NaCl and KCl) according to Reaction 1 for sodium compounds. 2 NaOH(g)+ SO2(g) + ½ O2(g)  Na2SO4(g) + H2O(g)


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.... (Reaction 2)

The rates of Reactions 1 and 2 may be affected by the rate of SO3 formation (Reaction 3), which thermodynamically occurs more favourably at lower temperatures and in higher O2 concentrations. This means that Reactions 1 and 2 occur more rapidly to form alkali sulphates in the oxidizing zone of the boiler than in the reducing zone. They are consequently likely to be responsible for the removal of the majority of SO2 from the flue gas in regions before the bullnose (Figure 1). SO2(g) + 1/2 O2(g) = SO3(g)

.... (Reaction 3)

In the superheater region, the flue gas temperature varies from about 850oC (1560oF) at the superheater inlet to about 600oC (1110oF) at the generating bank inlet. Carbonate and chloride fume particles are expected to form, respectively, as a result of carbonation reaction between hydroxide vapours and CO2, and as a result of condensation of chloride vapours. If SO2 still remains in the flue gas, it will react with the resulting carbonate and chloride fume to form solid sulphate fume (Reactions 4 and 5). These reactions, however, occur at a much slower rate than Reactions 1 and 2, because the SO2 concentration is much lower in this region and fume particles are in a condensed form [10]. Na2CO3(s,l) + SO2(g) + ½ O2(g)  Na2SO4(s) + CO2(g)

..... (Reaction 4)

2 NaCl(l)+ SO2(g) + ½ O2(g) + H2O(g)  Na2SO4(s) + 2HCl(g)

.….(Reaction 5)

From the generating bank onward, the temperature is lower and fume particles are solid, making Reactions 3 and 4 less likely to proceed at a significant rate. As a result, SO2 concentration will not change appreciably. This, along with the short residence time (