Catalytic removal of NOx, VOC and dioxin - Topsoe · Catalytic removal of NOx, VOC and dioxin Page...

Catalytic removal of NOx, VOC and dioxin

by:

Anders Bo Jensen Hans Jensen-Holm

Martin Schröter Haldor Topsøe A/S

Originally presented at Pollutec 2000, 17-20 October 2000, Lyon, France

Catalytic removal of NOx, VOC and dioxin Page 2 of 17

Abstract The SCR DeNOx process is based on a selective catalytic reduction of nitrogen oxides, NOx, by ammonia forming harmless nitrogen and water vapour. Due to a very high and cost efficient removal of NOx, this technology has been successfully supplied to a variety of industries, rang-ing from the power generation sector through marine diesel engine propulsion to the chemical and pharmaceutical industries. The CATOX and REGENOX catalytic combustion technologies for removal of VOCs from indus-trial off-gases offer high efficiencies and attractive economical aspects, including very low or zero consumption of energy, low investment costs and twice as long equipment lifetimes com-pared to thermal oxidation units. As a result, some 250 units have been supplied to a vast num-ber of industries, including the chemical and pharmaceutical industries, off-set printing, varnish and paint manufacture, food processing, polymer curing etc. As legislation on air emissions is gradually being tightened within EU and elsewhere, the de-mand for flexible abatement technologies is increasing. The versatile catalyst-based technolo-gies offer promising solutions when different catalysts are brought together to provide for simul-taneous removal of various pollutants. Already, the DeNOx and CATOX technologies have been combined for reduction of emissions of toluene, ammonia, CO and NOx in the chemical industry, for removal of NOx, carbon monoxide and ethylene in CO2 fertilisation plants for green-houses and for removal of NOx and dioxin in waste incineration plants.

Introduction Within the EU, and throughout the world, existing legislation regulating emissions to the air is gradually being tightened. At the same time, new limits are being introduced on emissions which were earlier not regulated. This has led to an increasing requirement for flexible and effi-cient emission abatement technologies. Based on extensive research programmes, Topsøe has over the past three decades developed a series of catalysts and processes for air cleaning, including the SCR DeNOx, CATOX and REGENOX processes for removal of NOx and VOCs in flue- and off-gases. These catalyst-based abatement technologies offer the required flexibility and have been combined and refined to fulfil an ever increasing demand from various industries for treatment of more complex off-gases with emissions of NOx, VOCs, CO, dioxin etc. A number of industrial examples will be discussed in this paper following an introduction to the individual technologies.

Information contained herein is confidential; it may not be used for any purpose other than for which it has been issued, and may not be used by or disclosed to third parties without written approval of Haldor Topsøe A/S.


The SCR DeNOx process The selective catalytic reduction (SCR) process is in general the most widely applied process for the reduction of nitrogen oxides in flue gases and exhaust gases generated by burning of fossil fuels in power generation plants. For reduction of nitrogen oxides, an ammonia source is injected into the flue gas at a tempera-ture of typically 300 – 420°C. The mixture of ammonia and flue gas passes through a catalyst where the nitrogen oxides are converted to nitrogen and water vapour. The nitrogen oxides, which primarily consist of NO and to a minor extent, NO2, are converted according to the follow-ing reaction schemes: 4 NO + 4 NH3 + O2 → 4 N2 + 6 H2O NO + NO2 + 2 NH3 → 2 N2 + 3 H2O 6 NO2 + 8 NH3 → 7 N2 + 12 H2O The conversion of nitrogen oxides does not create any secondary pollution as the products formed are only nitrogen and water vapour, which are already present in the atmosphere in large quantities. The conversion efficiency is typically 90% but may be as high as 99%. Reducing agent

The ammonia source for the reduction of NOx may be anhydrous ammonia, aqueous ammonia or an aqueous solution of urea. Anhydrous ammonia is considered the cleanest and most cost-effective reducing agent, but storage of anhydrous ammonia under pressure may imply a poten-tial hazard. On the other hand, urea solution is considered safe to handle and may therefore be the preferred reducing agent for SCR DeNOx units in residential areas. Process layout

A typical layout of an SCR DeNOx unit based on the use of anhydrous ammonia as reducing agent is shown in Figure 1. The anhydrous ammonia is stored in the storage tank at ambient temperature and at the corre-sponding vapour pressure (approx. 10.5 barabs at 25°C ambient temperature). From the storage tank the liquid ammonia flows by its vapour pressure to the evaporator, which can be heated by hot water, steam or electricity. A controlled flow of evaporated ammonia is then passed to the NH3/dilution air mixer, where the ammonia is diluted with air supplied by the dilution air blower before injection into the hot flue gas stream. The ammonia vapour is diluted with air to about 6 vol -% in order to eliminate the risk of ammo-nia ignition when injected into the hot flue gas. The upper and lower explosion limits for diluted



ammonia vapour are 15 vol% and 28 vol% respectively. Secondly, the dilution with air improves the mixing of the ammonia vapour and the flue gas. A static mixing element is located in the flue gas duct before the inlet of the reactor to ensure a homogeneous mixing of the flue gas and the diluted ammonia vapour. It is important to obtain a homogeneous mixing in order to attain a high efficiency of the SCR process and minimise the NH3 slip (unused NH3) from the SCR reactor. A gas distributor plate at the inlet of the SCR reac-tor is designed to provide a uniform distribution of the gas mixture over the entire cross section of the reactor. As the homogeneous mixture of flue gas and ammonia vapour passes through the channels of the catalyst, the nitrogen oxides are converted through the catalytic reactions described above.

Figure 1 Typical flow sheet for the SCR DeNOx process based on anhydrous ammonia as reducing agent Ammonia flow control

The flow of ammonia is controlled by a flow controller including a flow transmitter and a control valve. The set point of the flow controller, which controls the position of the control valve in rela-tion to the actual measured flow, is given by a process computer, which is also fed with informa-tion about the actual flue gas flow ("feedforward" signal) and the NOx concentration at the reac-tor outlet ("feedback" signal). If the inlet concentration of NOx varies, continuous measurement of this concentration is also included. These data are compared by the PLC with pre-programmed parameters for different flue gas flows, related NOx emission levels and required NH3/NOx ratio as measured and calculated during commissioning of the unit. On this basis, the



PLC continuously calculates the required flow of ammonia and adjusts the set point of the flow controller accordingly. The SCR DeNOx catalyst

The proprietary Topsøe SCR DeNOx catalyst type DNX® constitutes the heart of the SCR De-NOx process. The DNX® catalyst is a corrugated monolithic catalyst based on a glass-fibre rein-forced TiO2 carrier in which the catalytic active metal oxides, primarily V2O5, are homogeneously distributed. The carrier provides a vast and stable surface area. The monolithic structure is made with different channel sizes for different particulate loadings of the off-gas to be treated. By using a catalyst with small channels the necessary catalyst volume will be reduced owing to the higher surface area, but at the same time the pressure drop across the catalyst will increase as well as the risk of dust deposition on the catalyst.

F The main featudeactivation raous full-scale uhigh-dust catalonstration of thSO2 oxidation, The special feaweight than theconstruction oflyst which is ex

Information contained herein purpose other than for which or disclosed to third parties w

igure 2 Standard element of the Topsøe SCR DeNOx catalyst type DNX®

res of the Topsøe DNX® catalyst are its generally high activity and stability (low te) as well as low pressure drop characteristics. This has been shown in numer-nits and by demonstration plant test programmes carried out on both low- and yst types, and by comparison with other commercially available catalysts. Dem-e high-dust catalyst has also shown that the catalyst has a unique low activity for which provides reduced corrosion and fouling of downstream equipment.

tures of the manufacturing process result in a catalyst with a considerably lower traditionally used steel plate and extruded ceramic catalysts. This allows the

lighter SCR DeNOx reactors. The manufacturing process also results in a cata-ceptionally resistant to thermal shocks because the catalyst geometry and mate-

is confidential; it may not be used for any it has been issued, and may not be used by ithout written approval of Haldor Topsøe A/S.


rial prevent accumulation of thermal stress, as experienced with either brittle ceramic structures or by the use of incompatible materials like ceramics coated on steel plates. The operating temperature range for the DNX catalyst is 200 - 550°C. However, for units clean-ing flue gases containing SOx, the latter will have an influence on the minimum operating tem-perature due to the risk of condensation of ammonium sulphates on the internal catalyst sur-face, i.e. the higher the content of SOx, the higher the minimum operating temperature. The DNX® catalyst is manufactured as sheet-steel enclosed elements with the standard dimen-sions, D x W x H, 466 x 466 x 572 mm (element type A) and 466 x 466 x 322 mm (element type H), as illustrated in figure 2. The sheet-steel enclosure is designed so that the elements can be easily stacked without further gasketing between the layers. The DNX catalyst is available in more than 60 different versions to fulfil the specific requirements of a wide range of applications.

The CATOX process The CATOX catalyst and technology were developed for catalytic combustion of primarily VOCs from a variety of industrial off-gases as illustrated in Figure 3.

Figure 3 Types of industries where the CATOX catalytic combustion technology has been applied, the types of pollutants involved and their impact on the environment



The gross reactions involved in the catalytic oxidation step are the same as those taking place when oxidising the compounds thermally at typically 800 – 1000°C. However, with the use of a catalyst, as an effective means of reducing the energy barrier, the reactions take place at a much lower temperature of 300°C or lower. At the same time, the catalyst is formulated to mini-mise the formation of undesirable by-products. The result is clean combustion, as illustrated with the catalytic oxidation reactions for CO, benzene (C6H6) and TCDD (2,3,7,8-tetra-chloro-dibenzo-dioxin): CO + ½ O2 → CO2

C6H6 + 7½ O2 → 6 CO2 + 3 H2O C12H4O2Cl4 + 11 O2 → 12 CO2 + 4 HCl The CATOX process offers very high conversion efficiencies of up to 99.9%. Process layout

The CATOX process is a simple process based on recuperative heat exchange as illustrated in Figure 4.

Figure 4 The CATOX process



The polluted off-gas is taken by a blower from the polluting process and led through a tubular heat exchanger where it is heated to the necessary catalyst inlet temperature, typically 280 - 300°C. After the heat exchanger, the off-gas passes through a start-up burner, where additional heat is supplied if required. The start-up burner may be gas or oil fired or alternatively an elec-trical heater. In the reactor, the off-gas passes through the catalyst bed where the VOCs are burnt and the temperature increases proportionally to the concentration of VOCs in the polluted off-gas. The temperature increase generated by combustion varies according to the concentration and type of VOCs present. The temperature increase usually varies between 15 - 30°C per g/Nm3. The hot cleaned gas is led through the shell side of the heat exchanger, where it delivers its heat to the incoming polluted off-gas. Finally, it is sometimes possible to extract further useful energy from the cleaned off-gas before it goes to the stack by making hot air or hot water. In some cases the concentration of combustibles is so high as to allow for a net production of en-ergy from the catalytic combustion unit. The CATOX process is suitable for off-gases with medium or high (typically 2 – 10 g/Nm3) or varying concentrations of VOCs. The process typically offers a heat efficiency of up to approx. 70 – 75%, and minimum some 2 – 3 g/Nm3 of VOCs in the off-gas are required for autothermal operation (no support firing). If the concentration of VOCs drops below a minimum level, the support heater automatically adds the required heat.

The REGENOX process In many applications the off-gas to be cleaned has a rather low content of combustibles, and although the CATOX process has higher energy efficiency than for instance the thermal com-bustion process, it cannot operate under such conditions. Therefore, Topsøe developed the REGENOX process. Process layout

The REGENOX catalytic combustion process is based on regenerative heat exchange, as illus-trated in Figure 5. The polluted off-gas is taken by a blower from the polluting process and sent into a reactor with two beds, each consisting of a layer of heat absorbing material (inert ceramic spheres) and a layer of catalyst. The two catalyst layers, which are located closest to the central zone of the reactor, take care of the combustion. The two heat absorbing layers are working as a heat exchanger/heat accumulator.



After the process has been operated for a certain time with down-flow, the upper heat accumu-lator is cooled down and the lower heat accumulator has been heated up by the heat from the upper accumulator as well as from the incinerated VOCs. Then the flow direction is reversed, and the polluted gas now enters the lower hot heat accumulator first. By changing the flow direction several times per hour, it is ensured that the necessary combus-tion temperature is maintained in the central zone where the catalyst is located in an economi-cal and highly heat efficient way. The reversal of the flow, i.e. the sequence of moving the con-trol valves, is controlled by the temperature in the reactor rather than by a simple timer. Thereby the process can easily adapt to variations in off-gas flow and VOC concentrations.

Figure 5 The REGENOX process The REGENOX process is suitable for medium or low (typically 0 – 5 g/Nm3) concentrations of combustibles. The process typically offers a heat efficiency of more than 95%, and only 0.7 g/Nm3 of VOCs in the off-gas is required to maintain autothermal operation (i.e. no energy con-sumption in the support heater). As in the CATOX unit, the support heater may either be a gas or oil fired heater or an electric heater. At too low concentrations of VOCs, the heater automati-cally adds the balance heat.



Combustion catalysts

The same catalysts are used in the CATOX and REGENOX units. As for the DeNOx catalysts, they are all developed and manufactured in-house and consist of a porous ceramic carrier ma-terial which supports the active catalyst material. The catalytically active material is either pre-cious metal like platinum or palladium, an oxide of one or more of the base metals such as cop-per, manganese, iron, cobalt or chromium, or a combination of a precious metal and a metal oxide. The metal oxide catalyst offers the advantage of being less sensitive to poisoning from small amounts of pollutants in the off-gas like sulphur, silica and phosphorus. But compared to the highly active precious metal catalysts, the volume activity is lower and the required catalyst volume therefore higher. Specialised catalysts have been developed for a wide range of appli-cations, including catalysts for combustion of halogenated hydrocarbons and hydrocarbons containing sulphur or nitrogen as well as for ammonia-containing off-gases. The catalysts are available as spheres (type CK), as illustrated in Figure 6.

Figure 6 The spherical CATOX combustion catalyst, type CK



Combination of the DeNOx and CATOX processes In a number of industrial installations, the DeNOx and CATOX catalysts and technologies have been combined for simultaneous removal of NOx, VOCs, CO and/or dioxin from more complex off-gases or for special applications. Four different examples of such installations are detailed below. Case 1: Nuclear fuel plant In 1997 Haldor Topsøe A/S supplied a combined DeNOx/CATOX unit for cleaning of a 7,500 Nm3/h flue gas stream containing methanol, ammonia, NOx and H2 to Westinghouse Electric Sweden (formerly ABB Atom AB), Västerås, Sweden. The design basis for the cleaning unit is given in the table:

Compound Unit Inlet Outlet

Methanol ppmv 0 – 10,000 100

NH3 ppmv 500 - 2,000 70

NOx ppmv 165 17

Process description

Figure 7 Flow sheet for the combined DeNOx/CATOX unit at Westinghouse Electric



The methanol containing off-gas stream (I) is pre-heated in the heat exchanger and then led through the start-up heater, after which the NOx-containing off-gases (II and III) are added. The gas streams are mixed in the static mixer element. In the first layer of DNO catalyst in Reactor 1, NOx and NH3 react according to the catalytic reactions described above and methanol is oxi-dised to CO2 and H2O. DNO is a precious-metal doped DNX® catalyst. After the DNO catalyst a fraction of the stream, fraction-1, is led directly to a second mixer and the remaining part of the stream, fraction-2, is led through the CK oxidation catalyst in Reactor 1, where the remaining methanol is oxidised to CO2 and H2O, and the NH3 is oxidised to NOx. The split streams are then reunited in the second mixer where uniform mixing of the NH3- and NOx-containing streams is established. The homogeneous mixture is led to Reactor 2, where NOx and NH3 react on the DNX® catalyst as described above and the remaining methanol is oxidised across the CK catalyst. After the reactor, the cleaned off-gas delivers its heat to the cold incoming methanol laden stream in the heat exchanger before it goes to the stack. The actual required ratio between fraction-1 and fraction-2, which controls the position of the control valves between the two reactors, is controlled by a PLC which is fed with information about the actual NH3 content in the off-gas (“feedforward”), and the NOx concentration after the DNX® catalyst in reactor 2 (“feedback”). The position of the heat exchanger bypass control valve and the duty of the heater are controlled by the temperature at the inlet of Reactor 1. Case 2: MNT/DNT effluent gas

In 1997 Topsøe supplied a catalytic cleaning unit for treatment of the off-gas from the MNT/DNT (mononitrotoluene/dinitrotoluene) plant at Nam-Hae Chemical Corporation in Korea. Streams from six different plants are jointly treated in the unit and consequently the flow rate and compo-sition of the gas vary significantly. The following design basis was specified:

Parameter Unit Inlet Outlet

Off-gas flow Nm3/h 0 – 3,000 0 – 3,000

NOx ppmv 0 – 10,000 100

NH3 ppmv 0 – 105 50

CO ppmv 0 – 7,500 200

C7H8 (toluene) ppmv 0 – 230 20



Process description

Figure 8 Flow sheet for the MNT/DNT off-gas cleaning unit at Nam-Hae Chemical Corporation, Korea The off-gas is taken in from the MNT/DNT production units to the treatment unit by means of the off-gas main or stand-by blowers. The off-gas is routed through the heat exchanger at the tube side, where the off-gas is preheated by heat exchange with the hot, cleaned gas. The off-gas is then routed through the electric heater, where the temperature is increased, if necessary, to process temperature. The electric heater is only in operation when the content of combustibles is lower than the specified content necessary for autothermal operation. From the ammonia storage tank, the ammonia vapour is diluted with air and injected and mixed into the hot off-gas stream as described earlier. In the reactor the homogeneous off-gas/ am-monia mixture passes first through the DNX® catalyst where the NOx is reduced as described above and secondly through a layer of CK catalyst where CO, C7H8 and excess NH3 are oxi-dised. The cleaned exit gas from the reactor is routed to the shell side of the heat exchanger and from there to the stack.



Case 3: The GREENOX® concept

ilisation in greenhouses is a feasible way to increase

n-

he design basis for the GREENOX® unit is given in the following table:


Experience has proven that CO2 fertgrowth. It has for many years been common practice to add CO2 to the atmosphere of the greenhouses, either in the form of purchased liquid CO2 or by admitting cooled flue gas contaiing CO2 from gas fired boilers to the greenhouses. In 1995, a power plant based on two 3.1 MW lean-burn gas engines was built at the Alfred Pedersen & Søn Aps Greenhouses in Den-mark and they got the idea to utilise the CO2 in the exhaust gas as CO2 source for the growing of tomatoes after proper cleaning. Topsøe was contacted and the process developed for this specific purpose was called GREENOX®. T

Flow Nm3/h 0 18,00 18,000

Ethylene

ppmv 35 0.5

CO ppmv 405 35

NOx ppmv 100 10

he cleaning requirement for a CO2 containing flue gas destined for a greenhouse is more d

.

rocess description

eous solution of urea as reducing agent for the process, as illustrated in

Tstringent than for flue gases in stacks. A greenhouse is to a large extent considered a closeroom, and for both human beings and plants stringent limit values exist that must be observedFor CO the lower limit is defined by the allowable limit for human beings, whereas for NOx and ethylene the lower limit is set by the plants. Ethylene is highly undesirable in the greenhouse, asit acts as a plant hormone that makes the tomatoes ripe prematurely. P

The unit utilises an aquFigure 9. By means of a frequency-controlled diaphragm metering pump, the urea is passed to a high pressure injection nozzle from where it is injected into the hot exhaust gas. When in con-tact with the hot exhaust gas, the atomised urea solution is vaporised and decomposed into NH3

and CO2. The reduction of NOx takes place on the DNX catalyst, as described above. In the layer of CKM oxidation catalyst the CO and the ethylene are subsequently oxidised to CO

®

2 andH2O.



igure 9 Flow sheet for the GREENOX ® system utilising urea solution

t the outlet of the unit the cleaned exhaust gas is continuously analysed for NOx and CO in -

y

ase 4: Dioxin and NOx removal from waste incineration flue gas

n plant in Bolzano, g-


F Aorder to ensure that the gas led to the crops in the greenhouses is of the required quality. During start-up of the unit, the exhaust gas will be passed to the stack until the levels of NOx and CO are well below the set point values as specified above. When the set point values are ob-tained, the GREENOX® control system informs the climate control system that the unit is readto deliver CO2 to the greenhouses if required. If during operation of the unit the content of CO orNOx exceeds the set point, the greenhouse distribution blowers will automatically stop and the exhaust will be passed to the stack. C

In 1996, Topsøe supplied a catalytic cleaning unit for a waste incineratioNorthern Italy, where the flue gas from two incineration facilities had to observe a stringent leislation on emissions of both dioxin and NOx. The required removal efficiency of the unit was 97% for dioxin and 86% for NOx at the following design conditions:

Flue gas flow 00 Nm3/h 117,0 117,000 Dioxin1 ng/Nm3 3 0.1 NOx mg/Nm3 500 70

1 -equivalents TCDD



Dioxins in incinerator flue gas consist of a group of chlorinated aromatic compounds, of which

rocess description

igure 10 Flow sheet for the combined NOx and dioxin removal unit at the Bolzano waste incineration

he off-gases coming from two washing towers at about 60°C are preheated in several steps.

mmonia water is used as reducing agent and injected and mixed with the flue gas as dis-the

2,3,7,8-tetra-chloro-dibenzo-dioxin (TCDD) is the best known and probably the most poisonous. Also chlorinated dibenzofuranes are often counted along with the dioxins. P

Fplant. TFirst a certain flow of hot air available from the two incinerator units is added to increase the temperature and to avoid condensation and corrosion in the system. The temperature is thenboosted further in a steam heater to about 105°C. The main blower takes the flue gas throughthe gas/gas heat exchanger and through the natural gas burner where final heating to reactor temperature takes place. Acussed above. A DNX® catalyst bed provides for the required conversion of NOx, whereas catalytic removal of dioxin takes place on both the DNX® catalyst and in a lower CKM catalyst



bed. Since both the DNX® and the CKM series of catalysts are active for dioxin removal, the choice of catalyst for a dioxin removal unit to a large extent depends on the other types of pol-lutants to be removed from the off-gas. At the same time, extensive resources are continuouslybeing invested in the development of new and improved catalyst types also for dioxin removal.

losing remarks psøe SCR DeNOx concept for removal of NOx in flue, exhaust and off-

ll of these emission producers have taken advantage of the fact that Topsøe has developed

e

t-

Presented at Pollutec 2000, October 2000, France

CThe feasibility of the Togases is very well proven with the implementation of the Topsøe SCR DeNOx catalysts and/or technology in numerous applications treating gas flows in the range from 1,000 to 2,500,000 Nm3/h. Similarly, the Topsøe oxidation catalysts have been utilised in more than 350 units, cleaning off-gases from a wide variety of industries. Aboth catalysts and process design for these technologies. This has provided a unique basis forcommanding all aspects of the said technologies and for further developing a number of ad-vanced and successful designs for simultaneous removal of NOx, VOCs, CO, dioxins, etc. Thcatalytic cleaning units provide stack emission levels that comfortably fulfil the requirements of tomorrow. With an unmatched commitment to research and development Topsøe is dedicated to continuously providing flexible, efficient and reliable solutions to emission problems, which are gradually becoming more demanding and complex as air emission legislation is being tighened all over the world.


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