INTRODUCTION

Introduction

NO_x is the generic term used to refer to oxides of nitrogen. The common oxides of nitrogen present at the temperatures, pressures and concentrations encountered in an acid plant are NO and NO₂. The oxides of nitrogen are pollutants, and local jurisdictions regulate allowable NO_x emissions levels in stack gases.

NO₂ in particular can cause a visible brown colour in stack gases if the concentration is high enough. As a useful rule of thumb, the brown colour will be visible in a stack if the NO₂ concentration in ppm is greater than 2400 divided by the stack diameter in inches (3). Therefore, for example, 25 ppm of NO₂ will just be visible in an 8-foot diameter stack (corresponding to 50 ppm NO_x at a 1:1 ratio of NO:NO₂. A point of concern is that as the volumetric capacity of acid plants increase, the visible limit of NO_x in the stack gas will decrease. The 8-foot stack diameter is equivalent to an acid tonnage of 1600 mtpd with a 50 ppm visibility limit while a 12 ft diameter stack relating to an acid capacity of 3500 mtpd has a visibility limit of only 32 ppm.

The maximum permitted concentration of NO_x in the product acid is being reduced through marketing pressures. While concentrations of up to 30 ppm have been acceptable in the past, more recent specifications levels have been for not more than 5 ppm of NO_x expressed as NO₃.

Another concern for NO_x emissions is the acid sulphate sludge which is formed throughout the acid plant as a by-product of the reaction between acid mist and carbon steel. This sludge is manually cleaned from equipment during the scheduled maintenance shutdown. Unfortunately this acid sludge also absorbs NO_x which is then emitted as brown fumes when the sludge comes in contact with humid atmospheric air. The TLC requirement for NO₂ is less than 3 ppm in many jurisdictions which requires complete personnel protection, self contained breathing apparatus, and mobile NO_x scrubbers.

NO_x Formation

• Furnace Temperature - Thermal NO_x,
• Oxygen Usage - Thermal NO_x,
• Fuel Type and Usage - Fuel Bound NO_x,
• Feed Type - Feed Bound NO_x,

Furnace Temperature - Thermal NO_x: The formation of thermal NO_x occurs through the reaction of nitrogen and oxygen according to the reaction [1] below at temperatures above 1000o C.

The rate of NO_x generation is a function of temperature and oxygen concentration; consequently metallurgical plants potentially may receive more NO_x in the feed gas as the primary smelting units generally operate at higher temperatures than Sulphur Furnaces.

Oxygen Usage -Thermal NO_x: The increased use of oxygen enrichment in smelters and acid regeneration plants tends to increase the quantity of NOx formed due to the effect of higher temperatures and higher oxygen concentration (4). Using oxygen enrichment the NO_x content peaks in the 35% to 50% (vol.) oxygen range. Beyond this level, the quantity of nitrogen present and available to form thermal NO_x decreases.

Fuel Type and Usage - Fuel Bound NO_x; Organic bound nitrogen present in some fuels, particularly coal and heavy fuel oils, is readily converted to NO_x in the combustion process. This conversion is difficult to prevent when using coal or heavy fuel oil.

Feed Type - Feed Bound NO_x: Some feeds to furnaces and smelters contain organic nitrogen compounds. For instance Methyl Methycralate Spent Acid consists of almost 35% of ammonium sulphate and some concentrates, particularly Zinc concentrates, contain significant quantities of organic nitrogen compounds. While the operating conditions in an Acid Regeneration Furnace can be adjusted to minimise the production of NO_x, the operation of the Smelter/Roaster is controlled more by metallurgical considerations. However the formation of NO_x in the off-gas is ensured in either case.

NO_x is formed by reactions of oxygen with nitrogen from any of the above sources in two equilibrium steps:

Temperatures above about 1000 ºC, such as in a smelter or a furnace, result in the formation of NO only by reaction [1]. The quantity of NO formed increases with increasing temperature. NO is a Henry s law gas with a very low boiling point and it cannot be captured in the gas cleaning system of an acid plant. As gas containing NO is cooled, NO₂ begins to form by reaction [2]. However the NO oxidation equilibrium reaction is slow at gas cleaning system pressure, temperature and oxygen concentration, and relatively little NO₂ will be formed before the gas enters the contact section. It has previously been reported (5) that a small amount of NO₂ in the gas used to produce liquid SO2 from a metallurgical acid plant caused high acidity in the final SO2 product, due to a catalytic reaction with moisture to form H₂SO₄.

NO_x Distribution in the Acid Plant Contact Section

Of the NO_x absorbed by acid, typically less than 10% enters the large flow of acid circulating around the acid towers. Instead it concentrates in the much smaller quantity of acid mist formed at the inlet to the absorber towers. It has been suggested that the presence of NO_x actually promotes the formation of quantities of acid mist (6), perhaps by reactions between NO_x and SO₃ that form particles that act as nucleation sites for the acid mist. There are several reactions involved, but the net result can be summarised as:

Although the nitrogen acid compound physically present in concentrated sulphuric acid is actually nitrosylsulfuric acid (HNOSO₄), the analytical procedure for NO_x in acid often reports NO_x present as equivalent nitrous acid (HNO₂) or nitrate (NO_3--).

The result is a concentrated nitrosylsulfuric acid solution, which is collected in the absorber tower candles. The concentration of nitrosylsulfuric acid in the candles can be quite significant. HNOSO4 concentrations in the range 5% to 20% are common, and in extreme cases the HNOSO₄ concentration may reach 40%.

Note that reaction [3] is a reversible reaction. The HNOSO₄ will be hydrolysed on contact with water, for example when candle drips are diluted, to release NO_x fumes that are noticeably brown due to the presence of NO₂. The hydrolysis is essentially complete when the H₂SO₄ / H₂O solution falls below about 60% (7). Below this strength little or no HNOSO₄ can exist in the acid. Great care must be taken in analysis of acid samples for NO_x, because dilution of the sample with water will release the NO_x.

Hydrolysis can also be a problem when the acid towers are opened during a shutdown and the acid in the candles comes into contact with moist air. A natural draft of ambient air via open manways through the candles can result in a visible brown plume that persists for days, even when the plant is shutdown! When the source of the NO_x is a regeneration furnace, it may be possible to reduce the NO_x content of the candles prior to a planned shutdown by reducing the furnace operating temperature for a few days.

There are many qualitative reports that high levels of NO_x are associated with visible acid mist plumes from stacks, from both regeneration and metallurgical acid plants. Reports are common from single absorption plants, and less common from double absorption plants. In single absorption plants, persistent plumes occur at relatively low NO_x levels, while in double absorption cases, the stack plumes seem to occur at significantly higher NO_x levels.

In reaction [3], H₂O is produced along with HNOSO₄. It would be expected that if only this reaction occurred, there would be water present in the candle drips. However, a second reaction has been reported in other systems, such as the lead chamber process that was historically used to manufacture sulphuric acid (8):

Reaction [4] consumes water and will effectively produce candle drips containing close to 100% H₂SO₄ with dissolved HNOSO4. The reaction will stop as soon as the available water is consumed. Particularly in single absorption plants, there is a considerable stoichiometric excess of SO₂ present to consume the water that would be present in the candle drips.

Go to Figure 1: NO_x Distribution with low inlet NO_x

Go to Figure 2: NO_x Distribution with high inlet NO_x (with candle segregation)

The resulting highly concentrated acid in the candles will have a high partial pressure of SO₃ and therefore will be ineffective at removing mist from the gas. In this case, any unabsorbed vapour or mist leaving the packing of the tower and entering the candles will form the usual visible white acid mist plume. If the candles are standing, it may be possible to irrigate them from below with fresh acid from the absorber circuit to keep the SO₃ vapour pressure low and eliminate any plume.

Control of NO_x in Acid Plants

Primary NO_x Reduction

Absorption Towers that are fitted with candle type mist eliminators have an effective method for trapping and diverting these mist droplets containing the absorbed NO_x.

Standing candles can be easily converted to segregate the drips by manifolding together the existing candle tubesheet drains and routing to outside the tower.

For hanging candles the individual drain legs must be manifolded together to collect the drips, and even then a significant portion of the acid can drain outside the candles and not be collected.

The candle drips that are segregated from the acid circuit are disposed of separately, to avoid contaminating the product acid

Candle Drip Treatment

There are several techniques for treating the drips to remove the NO_x content and return the sulfuric acid value into the acid circuit. One method (9) that has been used by Kvaerner Chemetics in two metallurgical acid plants takes advantage of the hydrolysis and heat of dilution to remove the NO_x from the drips in a controlled fashion. The candle drips flow by gravity to a PTFE lined packed column, where they are mixed with water to dilute the acid below 60% H₂SO₄. The HNOSO is completely hydrolysed, and the released NO_x is vented from the top of the column. The nitrate free candle drips gravity drain into the acid pump tank. The overhead NO_x fumes can be absorbed into water in a stainless steel packed absorber column to form a weak HNO₃ effluent solution. The process consumes no reagents or power since the acid flows by gravity. The equipment is simple, reliable, and small (a 12 packed column will treat the candle drips from a 2000 stpd acid plant).

Another method (10) involves the creation of a solution of SO₂ in weak sulphuric acid, by passing some of the process gas leaving the blower through a dilute sulphuric acid solution. The candle drips are then mixed with the SO₂ solution, and the NO_x is reduced to nitrogen by reaction with the SO₂ in the liquid. The reduction reaction leaves no NO_x or HNO₃ by-product for disposal.

Product Acid Treatment

These reagents all have in common the amine group (-NH₂), and theoretically react to reduce NO_x dissolved acid by a reaction such as:

In practice, only hydrazine and to some extent hydroxylamine are effective at the conditions of temperature and residence time typically available in an acid plant. Pure sulfamic acid or urea will reduce the NO_x only in weaker acid solutions, and by-product (NH₄)₂SO₄ may contaminate the treated product. The reducing agent must be added in excess to achieve adequate destruction of NO_x, and post treatment with H₂O₂ to destroy the excess reductant is then required. The cost of the reagent chemicals can be substantial. For instance if hydrazine is used as a reducer, and the NO_x level in the acid is high, the cost may be as high as $2.00 per ton of product acid.

Also the pure hydrazine is toxic, although the hydroxylamine or hydroxylamine sulfate are less toxic and easier to handle. Therefore, depending on local regulations, the cost of installing an engineered hydrazine storage and delivery system may be significant. In addition, when hydrazine is used it is extremely important to ensure that no excess hydrazine remains in the product acid. Often secondary hydrogen peroxide addition is employed to ensure destruction of any residual hydrazine. This further increases treatment costs.

NO_x Destruction in the Gas Phase

• Absorption in scrubbing solutions
• Selective non-catalytic reduction (SNCR)
• Selective catalytic reduction (SCR)

Absorption in Scrubbing Solutions:

Acid plant tail gases are sometimes scrubbed to remove SO₂ with alkaline solutions, such as in ammonia scrubbers or lime scrubbers. Alkaline scrubbing solutions are also known to absorb NO_x in tail gases. However at the relatively low NO_x levels present in typical acid plant stacks, the NO_x is insoluble in the alkaline scrubbing solution and the NO_x removal efficiency will be close to zero.

Hydrogen peroxide can also be used for tail gas scrubbing of SO₂ and NO_x with fairly high efficiency. However it will be most likely applicable in relatively concentrated NO_x streams. The purchase cost of the H₂O₂ will make this an expensive option. On an acid plant tail gas, the scrubber product would be a weak mixed H₂SO₄ / HNO₃ solution, which would have to be disposed of or separated by steam stripping.

Selective Reduction: The selective reduction reactions are performed using a reducing agent such as ammonia:

Selective Non Catalytic Reduction: These reactions can occur at high temperature (above about 900 °C, i.e. at the conditions of the smelter or furnace outlet) without a catalyst (SNCR). SNCR is used to reduce NO_x in power plant boiler gases. However in the typical hot smelter or furnace gas it is believed that the relatively high levels of dust and, of course, SO₂ will make the reaction largely ineffective.

Selective Catalytic Reduction: At more moderate temperatures (between 300 - 400 ºC) a catalyst is used (SCR). Figure 3 shows a schematic diagram of the SCR DeNO_x process. The process is generally the same whether applied to flue gases, exhaust gases, or to other off-gases. The main components of the SCR process consist of a reactor containing the catalyst and an ammonia storage and injection system. The ammonia can be in the form of liquid, water-free ammonia (under pressure), a 25% aqueous ammonia solution (at atmospheric pressure), or a urea solution.

Go to Figure 3

The ammonia is evaporated in an electrically, steam, or hot water heated evaporator and is subsequently diluted with air before the mixture is injected into the process gas duct. The injection of the ammonia/air normally takes place through a system of nozzles in order to achieve a uniform mixing of the ammonia with the process gas. A static mixer may be placed in the gas duct to further improve mixing. This mixing is important so that the resulting gas-ammonia mixture has the uniform NH₃/NO_x ratio required to ensure efficient removal of NO_x and to minimise the NH₃ slip ("leakage") from the SCR reactor.

The normal operating temperature of the catalyst is between 300 ºC and 425 ºC but through the use of special catalysts the SCR process can be applied at temperatures down to 200 ºC and up to 550 ºC.

Based on reactions [8] and [9] above, any ammonia slip from the DeNO_x operation results in reformation of NO_x resulting in a reduction in DeNO_x efficiency. Therefore, design of the ammonia injection system and catalyst bed is critical if high NO_x reductions are required. SCR s have been successfully applied to treat gas streams in a number of industries including power and chemical plants. In fact, Selective Catalytic Reduction (SCR) of NO_x by ammonia over vanadia/titania type catalyst is by far the most important commercial process today for removal of NO_x. However, the process to date has had only limited application on a commercial scale in acid plants.

Application of SCR Technology in a Sulphuric Acid Plant

• Before gas cleaning in the hot gas
• Before the stack
• After gas cleaning and before the dry tower
• In the dry gas section

Before gas cleaning in the hot gas: The gas at this point is hot and in the ideal temperature range for the operation of a SCR unit. Unfortunately, except in some special cases, the gas will contain significant quantities of dust and catalyst poisons such as arsenic, fluorides and chlorides that make this location unacceptable.

Before the stack: Another obvious location for the DeNO_x catalyst is immediately prior to the acid plant stack, as the gas constituents at this location would be roughly similar to hundreds of other SCR applications. However, this location has some serious disadvantages:

• The NO_x will be absorbed into the product acid in the acid plant, requiring an expensive chemical treatment, and potentially requiring the use of a hazardous reagent on site;
• NO_x will build up in any residues throughout the plant as well as the acid plant candles, creating maintenance difficulties;
• If candle segregation is sufficient to maintain product acid quality (unlikely if high NO_x is present), the candle drainings must be disposed of safely, or treated by yet another system;
• The gas must be re-heated following the final absorption tower to a minimum of 200oC, requiring auxiliary fuel firing in a double absorption plant, or several exchangers in a single absorption plant.

After gas cleaning and before the dry tower: This location avoids the first three disadvantages mentioned for the tail gas location above in so much as it occurs before the contact section. However the major disadvantage of having to heat the gas from near ambient temperature still exists. This heating requirement is further compounded by having to heat a wet SO₂ gas stream introducing extreme and probably insurmountable problems in materials of construction.

In the dry gas section: The location that eliminates these problems is in the dry gas section of the contact plant before the first catalyst bed. The benefits of this location are:

• The gas is hot and within the required temperature range.
• The NO_x is destroyed before it encounters any acidic residues where it can be absorbed to create later problems during maintenance.
• The NO_x is reduced to a level so that candle drip segregation is unlikely to be required and consequently eliminating the disposal or treatment problem.
• The NO_x is reduced to a level so that product treatment is not required and eliminating the requirement to use potentially hazardous reagents.

Also, this location is virtually free of SO₃, so there is minimal risk of a reaction of SO₃ with ammonia resulting in the production of ammonia sulphate. This reaction would lead to fouling and equipment plugging as well as higher ammonia consumption. The DeNO_x reaction results in a small increase in the moisture content of the gas stream. Any resultant dew point and mist considerations need to be, and are, addressed.

DeNO_x Catalyst

One drawback of locating the DeNO_x catalyst before the converter is the high level of SO₂ in the gas stream. A major ingredient of the DeNO_x catalyst is vanadium pentoxide, similar to conventional DeSO_x catalyst. Therefore, the catalyst must be specially formulated to ensure that it has high activity for DeNO_x operation.

SCR catalysts are normally based on a titanium dioxide (TiO₂) carrier, with combinations of oxides of vanadium, molybdenum, and tungsten, deposited on the carrier, as the active material.

The catalytic reaction is essentially a surface reaction. In order to maximise the activity the catalyst must provide a large external surface area and is therefore manufactured with a large number of parallel channels arranged in a honeycomb structure or in parallel plates. The size of the channels depends on the content of particulate matter (dust) in the gas and typically ranges from 3 mm (dust-free gas) up to 10 mm (dust contents of > 30 g/Nm,sub>3). The catalysts are either available as extruded ceramic bodies, coated metal plates, or impregnated, corrugated ceramic fibres.

Haldor Topsøe DeNO_x Catalyst

Go to Figure 4 (Structure of Haldor Topsøe's DeNO_x catalyst)

The catalyst is manufactured in monolithic blocks with 466mm x 466 mm cross section (18.3 x18.3 ) and a height of 500 mm (19.7 ), and is housed in a casing forming elements or cassettes. The elements are usually combined into modules, to facilitate handling and installation of the catalyst. The modules are tailored in size to varying design requirements. Due to the use of a fibre-reinforced type of carrier, the catalyst is very resistant to thermal shocks and erosion by particulate matter; however, the weight is considerably lower than traditional extruded or plate-type catalysts.

Incorporating the DeNO_x Bed in the Acid Plant

• Type 930 Catalyst: This catalyst has the lowest operating temperature and the highest activity. It operates in the temperature range of 300 ºC to 425 ºC.
• Type 630 Catalyst: This catalyst operates in an intermediate temperature range and activity. The operating temperature range is 300 ºC to 450 ºC.
• Type 330 Catalyst: This is the catalyst that can operate at the highest temperature and is most resistant to temperature excursions. It also exhibits the lowest activity. The operating temperature range is 400 ºC to 500 ºC.

The DeNO_x catalyst, assembled into cassettes and modules, is installed in a separate vessel. The vessel design is more robust than the typical SCR container because of the requirement for the unit to operate at a higher pressure than occurs in a power plant flue gas installation. A diagram of a typical DeNO_x catalyst vessel designed for use in a sulphuric acid plant is shown in Figure 5.

Go to Figure 5

It may be advantageous, at the same time as the DeNO_x catalyst in installed, to expand or otherwise modify the existing sulphuric acid plant. One project incorporated the DeNO_x catalyst within a new Bed 1 converter also containing catalyst for the conversion of SO₂ to SO₃. The unit also contained a new internal Hot Exchanger for additional maintenance benefits. A diagram of the combined DeNO_x and DeSO_x unit with the internal Hot Exchanger is shown in Figure 6.

Go to Figure 6

A DeNO_x removal requirement of 95% or greater is typically required for these systems. This can only be achieved if the criteria of even ammonia distribution within the gas and even distribution of the gas over the catalyst are obtained. The catalyst criteria is that flow deviations through the catalyst mass should be not greater than +/- 15%. This is achieved by performing extensive CFD analysis on the flow patterns in the catalyst vessel.

DeNO_x Performance

• >95% based on an inlet NO_x of 300 ppm giving less than 15 ppm in stack and less than 5 ppm in the acid product.
• >97% based on an inlet NO_x of 250 ppm giving less than 10 ppm in stack and less than 10 ppm in the acid product.

Experience has shown that this performance level has been achieved.

Conclusion

References

2. van Driel, J., Giesen, B., Berryman, A., Sampat, S., Enevoldsen, S., Jensen-Holm, H., 2000. Budel Zink Sets a New Standard for NOx Reduction in an Acid Plant Sulphur 00 International Conference, Calgary (2000). Preprints, pp. 359-372.

3. Control Techniques for Nitrogen Oxide Emissions From Stationary Sources . US Department of Health, Education, and Welfare, Washington (1970). P. 7-1.

4. Mole, J., Queille, P., Lawler, D.: Economical spent acid recycling in the world's largest sulphuric acid regeneration plant . Sulphur '93, Hamburg, (1993).

5. Trickett, A. A., Horsley, D.: Product quality aspects of liquid SO2 production from metallurgical off gases . Sulphur '94, Tampa (1994).

6. Origin unknown.

7. Duecker, W.W., West, J.R.: The Manufacture of Sulfuric Acid . Reinhold Publishing Corporation, New York (1959) p. 105.

8. ibid., P.112.

9. Davidson, C. M., Newman, C.J.: Removal of nitrates from smelter acid . 1983 International Sulfide Smelting Symposium. Proceedings. The Metallurgical Society of the AIME (1983).

10. Ludtke, P., Schmidt, G.: Low cost NOx reduction in sulfuric acid without addition of chemicals . Sulphur '95, Abu D