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Flue Gas Cleaning

The final unit of the incineration plant is one of the most important parts as it has the objective of cleaning the air pollutants produced. Each ton of incinerated waste produces solid pollutants, like dust and heavy metals, and approximately 4000 to 6000 m3 of flue gases (Bilitewski et al., 1997), mainly carbon monoxide (CO), nitrogen oxide (NOX), sulfur dioxide (SO2), nitrous oxide (N2O), hydrogen chloride (HCl) and hydrogen fluoride (HF), among others. Combinations of several individual cleaning components are utilized to provide an effective overall flue gas treatment system, with the actual number of possible flue gas treatment combined systems reaching 408 (European Commission, 2006).
Particle Removal
Disposal Mechanisms
Dry / Conditioned Dry Processes
Wet Processes
Introduction and Discussion of the Different Techniques
Optimization of Flue Gas Cleaning System
The Potential of Lime Based Systems
The Potential of Sodium Bicarbonate Systems
Substitution of Spray Absorption to Pure Steam Cooling
PTU Process Development
Multiple Additive Injection
Nitrogen Oxide Removal
1. Particle Removal
The first step of the flue gas treatment is to remove the solid particles, which size ranges from 1μm to 1mm. Three removers are nowadays very common in use, namely cyclone, electrostatic precipitator or fabric filter (Figure 1). "Inside the cyclone, the gas swirls around an immersed tube, and the particulates are carried by inertia to the cylinder wall, from where they exit through the conical section on the bottom while the clean gas exits through the top" (Bilitewski et al., 1997). Electrostatic precipitator utilizes high voltage to electrically charge the particles contained in the flue gas by making contact with ions and electrons, once the particles are charged they move toward the precipitation electrode, when the process is finished the power supply is suspended and a hammer hits the precipitator to push the dust down. Fabric filter work like a household vacuum cleaner. The raw gas passes through a filter which allows air to flow through, but retains particulate materials. The particles remain in the filter until compressed air is blown in the opposite direction, cleaning the filter and making the dust fall down from where it was collected.

Figure 1: Components and operations of three common solid particle removers (Bilitewski et al., 1997).
Electrostatic precipitators and fabric filters have a similar operational area, with the latter showing a slightly better performance for particles smaller than 1μm (Figure 2). Cyclones alone cannot fulfill the emission requirements now applied to modern waste incinerators. Cyclones, however, can be utilized as a complement to other flue gas treatment systems (TWGComments, 2003 and Bilitewski et al., 1997).

Figure 2: Operational Areas of Dust Removing Processes (TWGComments, 2003 and Bilitewski et al., 1997).
2. Gaseous Contaminants Removal
All current technologies for this removal are based on either absorption or adsorption. Absorption means that the flue gas is mixed with additives that react with the contaminant gases and transform them into non-polluting products, while in adsorption processes the molecules of the contaminants attach to the surface of adsorbents and the non-polluting air is allowed to pass through. The removal of the pollutants could be achieved by the application of (conditioned) dry or wet processes. Each method has its advantages and disadvantages, and therefore which one to choose depends on the location and the specific conditions. In table 1 the basic characteristics of dry and wet systems are compared.
Wet systemsDry Systems
Low additive amount (SR 1), respectively low residues amountHigher additive amount (SR 1.6 up to <2), respectively higher residues amount
Expensive additives (e.g. NaOH)Cheaper additives (e.g. Ca(OH)2)
High removal capacity for HCI and SO2Good removal efficiency
Selective removalNon-selective removal
Multistage systemSingle stage system
Waste water treatmentSimple and insensitive components
Need a pre-dust collectorDo not need an additional dust collector
Higher dust/Aerosol emissionLow maintenance effort
PCDD/PCDF-removal with ADIOX grids and AV-injectionVery good removal efficiency for heavy metals and PCDD/PCDF on AV
High space needNo wet stack is necessary
Table 1: Characteristics of dry and wet flue gas treatment systems (Karpf, 2010).
In the following context, attention is mainly paid to the cleaning of hazardous acidic gas pollutants. Since most facilities are equipped with a semi-dry or dry process, its disposal basics will be explained in more detail here than that of wet system.
2.1 Dry / Conditioned Dry Processes
Dry scrubbing processes are those flue gas cleaning processes, which need neither wet scrubber nor electrostatic precipitators (as pre-deduster). The disposals of the dust and hazardous materials take place simultaneously in a fabric filter. Here a powdery reagent (sodium bicarbonate (NaHCO3) or lime (Ca(OH)2) is blown into the flue gas stream and is deposited later in a tissue filter or a bag filter. Through their chemical reactions the gaseous pollutants, hydrogen chloride (HCl), hydrogen fluoride (HF) and sulfur dioxide (SO2), are bound to the reagent. Due to this it is also called sorption or chemistry sorption.

Figure 3: Schematic diagram of a dry FGT system with reagent injection to the FG pipe and downstream bag filtration (UBA 2001).
The dry flue gas cleaning process uses sodium bicarbonate (NaHCO3) as an additive and is independent from the flue gas moisture content. Sodium bicarbonate (NaHCO3) decomposes in the flue gas at about over 140°C into sodium carbonate (Na2CO3), carbon dioxide (CO2) and water vapour (H2O). After the transitions in the gas phase the gaseous deposition products of the sodium bicarbonate, CO2- and H2O-molecules, leave gaps or holes in the reagent particle grain; thereby sodium carbonate is formed with a high specific surface area. The new produced sodium carbonate is therefore more reactive than the normal hydrated lime.
The dose rate of reagent may depend on the temperature as well as on reagent type. The required additive amount for the deposition must not be too stoichiometric dosed, so that a 0.1 to 0.4- time stoichiometric dosage is often used. In comparison to calcium hydroxide, which has a stoichiometric value of 2, sodium bicarbonate has only one stoichiometric value corresponding to the equivalence in chemical reactions. That means, instead of 2 moles NaHCO3 only 1 mol Ca(OH)2 is required for the disposal of 1 mole divalent SO2. For sodium bicarbonate, therefore, the advantage of more desirable additive amount is compensated by the disadvantage of its stoichiometric value, so that there is no decrease in the total reagent demand. This is required to ensure emission limits are complied with over a range of inlet concentrations (European Commission, 2006).
Table 2 compares these two absorbents considering the requirements for an optimized additive used in a dry system.
Characteristics of the plant/infrastructureLimeSodium bicarbonate
Low additive costs+-
Hig residues costs-+
Tail-end SCR-DeNox-plant+1)/-+
Maximum need for heat recovery+1)/0+
Use of dry fuel (waste)+/-+
Low HF- and SO2-emission values+-
High Hg-removal+-
APC-System with puffer capacity (Emission)+-
Table 2: Requirements for an optimized additive used in a dry system (Karpf 2010).
+ Good; 0 neutral; - not so good
1) By using PTU-process

Equation 1: 2NaHCO3σ > 1 4 0 °C→Na2CO3 + CO2 + H2O

Equation 2: Na2CO3 + SO2→Na2SO3 + CO2

Equation 3: Na2SO3 + ½O2→Na2SO4

Equation 4: Na2CO3 + 2HCl→2NaCl + CO2 + H2O

Equation 5: Na2CO3 + 2HF→2NaF + CO2 + H2O

During the thermal decomposition of sodium bicarbonate the actual reagent Na2CO3 comes into life (see Eq. 1). The gaseous decomposed products CO2 and H2O will be emitted in the flue gas stream in order to decrease the total reagent mass and reduce the left residue amount compared to the dry sorption with hydrated lime.
The process requirements for effective noxious gas treatment are that the sodium bicarbonate grinding a large surface area, which is used as materials exchange area, should be provided and also a minimum residence time of 2 seconds at a temperature over 140°C must be ensured.
Using hydrated lime as the reagent leads to the following reactions with the gaseous hazardous materials mentioned above:

Equation 6: Ca(OH)2 + 2HCl→CaCl2 + 2H2O

Equation 7: Ca(OH)2 + 2HF →CaF2 + 2H2O

Equation 8: Ca(OH)2 + SO2 →CaSO3 + H2O

Equation 9: Ca(OH)2 + SO3→CaSO4 + H2O

Equation 10: Ca(OH)2 + CO2→CaCO3 + H2O

Researches and operational experiences tell us that for the contaminants deposition with hydrated lime the treatment efficiency could be improved under some specific conditions, which in particular includes the increase of the relative humidity of the flue gas.
The hydrogen chloride reacts with calcium hydroxide to calcium chloride, which exists as dihydrate in the usual temperature 130°C-150°C in the conditioned dry sorption.

Equation 11: Ca(OH)2 + 2HCl→CaCl2 + 2H2O

The HCl integration overweighs the energy preferred integration SO2, since the activation energy for the reaction in the low temperature is lower compared to SO2.
Generally the rate of dissolution in aqueous medium plays a crucial role for the treatment of acid pollutant gases (HCl, SO2, HF), with the exception of SO3, the same with the dry additive addition. For the ever available water vapour in the flue gas which builds a hydrate cover around the solid particles, the reaction kinetics is more preferred rather than the absolute dry sorption. That means the adsorption and absorption processes take place simultaneously. Thereby the hydrate covers benefit the material exchange through the gas- or particle surface and the pore diffusion through some certain dissolution effects, which at the molecule level enables fast ion reaction. For this reason the availability of HCI or CaCI2, plays an important role especially for the high SO2 dissolution efficiency, as it uses the hygroscopic properties of the calcium chloride to build the hydrate covers.
In addition, there is an intermediate reaction between the formed calcium chloride with calcium hydroxide according to Eq. 12 and 13 (Allal et al. 1998).

Equation 12: Ca(OH)2 + HCl →Ca(OH)Cl + H2O

Equation 13: Ca(OH)Cl + HCl←→CaCl2 + H2O

For the dry sorption with hydrate lime the disposition of sulfur dioxide could be improved with the presence of the hydrogen chloride in the flue gas steam. On the contrary, the disposition of the hydrogen chloride will be deteriorated under the same conditions.
It can be concluded that the intermediate product calcium hydroxide chloride which comes from the reaction of hydrate lime with hydrogen chloride (see Eq. 12) also reacts with the sulfur dioxide. The reaction rate of this reaction must be higher than that of the reaction between hydrate lime and sulfur dioxide (see Eq. 8), which can only be explained by a higher reactivity of the calcium hydroxide chloride under the unchanged reaction conditions (pressure and temperature).
Calcium hydroxide chloride does not only come from the reaction between hydrogen chloride and hydrate lime, but it also exists in the residue from the tissue filter as a result of the reaction between the excessive hydrate lime and calcium chloride.

Equation 14: Ca(OH)2 + CaCl2←→2Ca(OH)Cl

Figure 4: Operating principle of a spray absorber (UBA 2001).
2.2 Wet Systems
The hydrogen halide combination HCI and HF (also hydrogen bromide HBr and hydrogen iodide HI) are absorbed by water easily or very easily, because for the most existing exhaust gas concentrations, their equilibrium partial pressures are very low and acids are generated here. These make HCI, HBr and HI very easily and almost completely to be absorbed into water.
In contrast, the dissociation, the mentioned process HF and water, takes place very weakly, which is why HF alone cannot be deposited in water so well.

Equation 15: HF+H2O←→HF(aq) ←→H3O+ + F-

Equation 16: HCl+H2O←→HCl(aq) ←→H3O+ + Cl-

Equation 17: HBr+H2O←→HBr(aq) ←→H3O+ + Br-

Equation 18: HI + H2O←→HI(aq) ←→H3O+ + I-

Through the lime milk injection one part of the absorbed acids ions reacts with the cleaning water into salt. Generally, the pH-value controlling lime milk will be added to raise the remaining acids pH-value from 0.8 to 1.5.

Equation 19: Ca(OH)2(s, aq) + 2HCl(aq) →CaCl2(aq) + 2H2O(aq)

Equation 20: Ca(OH)2(s, aq) + 2HF(aq) →CaF2(aq) + 2H2O(aq)

The wet processes are usually found in the hazardous waste- and household waste combustion facilities. The exhaust gas cleaning equipment consists of a two-stage scrubber, where the second one removes SO2.
The figure below shows a typical 2-stage wet scrubbing system. The number of scrubbing stages usually varies from 1 to 4 with multiple stages being incorporated in each vessel (European Commission, 2006).

Figure 5: Operating principle of a 2-stage wet scrubber with upstream de-duster (European Commission, 2006).
3. Introduction and Discussion of the different techniques
As already described in Chapter 2, all systems have advantages and disadvantages, which need a consideration in the process selection.
The wet flue gas cleaning technology has a great disposal potential for the acidic exhaust gas components until the halogenated bromide. Because the requirements for flue gas cleaning in waste- and EBS-incineration plants are stricter and broader, more cleaning stages or components are required, which leads to the construction of the multistage wet system. Additionally in Germany the thermal waste treatment plants must achieve zero waste water emission, thus the remaining cleaning water must be reprocessed and vaporized.
As the multistage systems are very energy-intensive and costly, in the last 10 years only the (conditioned) dry sorption facilities have been established, most of which are single-stage equipment (Figure 6).

Figure 6: Operation range of single stage dry absorption systems (Karpf 2009).
Using the single-stage conditioned dry sorption technology, the emission limits are met when the raw gas concentrations are up to 2.500 mg/m³N HCI and 2.000 mg/m³N SO2 under the economic conditions, as shown in the following examples.

Figure 7: NID-system in the EBS-plant Romonta (Alstom Power).

Figure 8: Conditioned dry sorption in MHKW Ludwigshafen (Margraf 2006).

Figure 9: HCl/SO2-concentrationen in raw and clean gas in the quench operation (MHKW Ludwigshafen).
As figure 9 shows, the high HCI concentration in the flue gas increases the HCI concentration in the clean gas under the limitation of 5 mg/m³, with a middle stoichiometric value of 2. As additive the normal hydrate lime is applied with a specific surface area of 18 m²/g.
If the input concentrations of pollutants are significantly higher than the average level of 1.500 mg/m³ HCl and 600 mg/m³ SO2 in waste incineration plants, the single-stage facilities will technically and economically reach their process capacity.
According to the concentration of HCI at the entrance of flue gas cleaning and also the generated residue disposal costs, the limitations of the different applications are researched and illustrated in figure 10.

Figure 10: Appliance limitations of different raw gas cleaning technologies according to the HCI concentration and the residue disposal (Frey 2007).
Under the examined fundamental conditions, as described by Frey in 2007, the combination of dry and wet processes or a wet process are more suitable when the increasing HCI concentration at the entrance is over 2.500 mg/m³ (as loaded continuously).

Figure 11: Operating principle of a 2-stage conditioned dry scrubber with NaOH additive.
The effluent in the scrubber is used as an evaporative cooler for the flue gas conditioning in the downstream boiler.
When the additional requirements on the attainable emissions are imposed, the multistage systems are necessary, as illustrated in figures 12 and 13.
Halving the HCI limit to less than 5 mg/m³N means that a multistage system is needed in any circumstance. In contrast, if other limits are halved, the multistage system will not be mandatory.

Figure 12: Operating principle of multistage flue gas Systems.

Figure 13: Operating principle of multistage flue gas Systems.
In several studies, the procedures shown in figures 12 and 13 were evaluated and compared. Under the given boundary conditions, the concepts in figure 12, version 1 and in Figure 13, version 2 were the best options. Equipped with the infrastructure in figure 12, version 1, to use decoupled heat by the Eco after the catalyst attributes clearly the cost advantages to this version. If this was not the case and also high noxious gas was loaded, then the process in Figure 13, version 2 has obvious advantages.
For this reason, it is impossible to rely on one certain method, which appears to be the most appropriate and the best. It has to be evaluated from case to case repeatedly under the prevailing conditions.
4. Optimization of Flue Gas Cleaning Systems
To increase the collection efficiency of existing flue gas cleaning systems, various measures can be used.
4.1 The potential of lime based systems
The biggest potential of lime based systems is to increase the relative humidity.
  • Recuperative gas cooling, for example with a heat exchanger,
  • Water injection in a cooling tower associated with a steam cooling system,
  • Steam injection.
Regardless of the relative flue gas humidity, an optimized removal requires a good filter flow and the development of an adequate "filter cake" as well as a suitable recirculation of the reaction salts, especially CI-salts.
Which optimization is the most feasible and suitable should be determined from the practical application and evaluation.
4.2 The potential of sodium bicarbonate systems
  • Ensure sufficiency of high temperature,
  • Warranty of a homogeneous Distribution,
  • Realisation of sufficient residence time,
  • High surface or small particle sizes.
4.3 Substitution of Spray Absorption by Pure Steam Cooling
In MHKW Ludwigshafen hydrated lime in the form of lime milk is no longer needed since March 2006, instead dry lime is injected in front of the fabric filter. The spray absorber is operated only with water for flue gas cooling and humidification.
This conversion of flue gas conditioning resulted in the calcium carbonate content decreasing by about 20%, and the lime free content increased accordingly.
A result is yielded from a general comparison of the spray absorption with the conditioned dry process in a quench operation that in favour of the dry scrubbing there is an operation cost difference of 240,000€ per year, regardless of the maintenance and repair costs.
Measures and results of the substitution of spray absorption to steam cooling system:
  • Elimination of calcium carbonate consumption and a larger particle surface for the material exchange so as to get a better stoichiometric value.
  • Better removal efficiency, because most absorbers are over dimensioned for a pure water Evaporation, and in this case the operation temperature can be reduced to less than 140°C.
  • A heating system at the bottom of the absorber is necessary to avoid corrosion.
  • More economical operation with better or even the same treatment efficiency.
4.4 PTU Process Development
To optimize the existing systems especially in processes where the flue gas humidity is too low the PTU process (Partial dew point underflow) (Dütke 2000) represents a more meaningful way. The PTU method is characterized by that a synchronous injection of saturated steam and reagent is carried out in the flue gas stream, and thus a local dew point underflow and conditioning takes place in a defined range (mixing zone).

Figure 14: Principle of PTU Process (Karpf 2010).
In the existing dry absorption systems with lime it is not possible to lower the flue gas moisture or to reduce the temperature.
The PTU-process has already been successfully applied in:
  • EBS-plants,
  • municipal waste incineration plants,
  • hazardous waste incineration plants.
4.5 Multiple Additive Injection
In principle, the treatment efficiency can be increased via a multistage additive addition in the existing system. For example, lime additive can be added two times at different temperatures, or an extra dose of sodium bicarbonate (NaHCO3) can be added to the existing lime additive to control the exhaust gas peaks. It is also possible to decrease raw gas emission peaks in the application of TAV technology by adding limestone to the furnace.
Studies on a conditioned dry system in a waste incineration plant have shown that HCl/SO2-pollutant peaks could be effectively reduced by a small inhibition of the quench water with a water-soluble alkaline sorbent (e.g. sodium hydroxide).

Figure 15: One possibility of multiple additive injections to reduce the raw gas peaks.
5. Nitrogen Oxide Removal
Nitrogen oxides are destroyed either by selective non-catalytic reduction (SNCR) or by selective catalytic reduction (SCR). SNCR applies dry urea (CO(NH2)2) or ammonia (NH3) as reductive agents directly in the furnace. At temperatures between 900 and 1050°C the reducing agents react with the nitrogen oxides to form water and nitrogen (Equations 21 and 22).

Equation 21: 4NO + 4NH3 →4N2 + 6H2O

Equation 22: 6NO2 + 8NH3 →7N2 + 12H2O

In SCR the reductive process takes place in a catalyst, where at temperatures between 200 and 400°C a mixture of ammonia and air reacts with the flue gas to form oxygen and water (Equation 23, 24 and 25). The SCR module must be installed after the particulate material and acidic gases are removed. SNCR can achieve reduction rates of 70 percent while SCR can achieve up to 85 percent (Bilitewski et al., 1997).

Equation 23: 4NO + 4NH3 + O2 →4N2 + 6H2O

Equation 24: 6NO + 4NH3 →5N2 + 6H2O

Equation 25: 2NO2 + 8NH3 + O2 →3N2 + 6H2O

6. Stack
Once the gas is cleaned, it is analyzed for air contaminants and then expelled through the stack. The stack has to be tall in order to disperse the exhaust gases over a greater area and reduce the concentration of the remaining pollutants to the levels required by the government.

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Thermal Treatment


  • Allal, K. M., J. C. Dolignier and G. Martin. 1998. Reaction mechanism of calcium hydroxide with gaseous hydrogen chloride. Oil & Gas Science and Technology 53 (6): 871-880.
  • Bilitewski, B., G. Härdtle and K. Marek. 1997. Waste management. Berlin.
  • Dütke, V. 2000. Konditionierte Trockensorptionsverfahren [Conditioned Dry Sorption Techniques], Master thesis, University of Applied Science Gießen-Friedberg, Germany.
  • European Commission. 2006. Integrated Pollution Prevention and Control Referen [...]
  • Frey, R. 2007. Konzepte zur Abgasreinigung: Tiefe Emissionen trotz hohem Schadstoff-Input. [Concepts for Flue Gas Cleaning: High Emissions despite high Pollutant-input] Paper presented at fourth Potsdam Conference of optimization perspectives and possibilities in the thermal waste and residual waste treatment, 22-23 February, Potsdam.
  • Karpf, R. Lecture Script: Air Pollution Control. University of Applied Science Gießen.
  • Karpf, R. 2009. Abgasreinigung-Markt der Möglichkeiten. [The Alternatives in the Flue Gas Cleaning Market] In Biomasse & Abfall: Emissionen mindern und Rückstände nutzen. Verfahren & Werkstoffe für die Energietechnik,Vol. 5, edited by Faulstich, M. and M. Mocker. Sulzbach-Rosenberg: Dorner PrintConcept GmbH.
  • Karpf, R. 2010. Flue Gas Cleaning Systems Status and Trends. Paper presented at the WtERT annual meeting Europe, 12-14 October, Brno.
  • Karpf, R. and M. Elfers. 2006. New and Innovative Flue Gas Cleaning System. Water, Air and Earth 5: 2-5.
  • Margraf, R. 2006. Heutige und zukünftige Konzepte für die Gasreinigung hinter Verbrennungsanlagen für Müll und EBS unter Verwendung von Ca(OH) 2 / CaO oder NaHCO3 als Additiv aus Sicht eines Anlagenbauers. [Current and future concepts for gas cleaning following waste and EBS incinerators using Ca(OH) 2/ CaO or NaHCO3 as an additive in the perspective of a plant manufacturer] Paper presented at second conference of dry flue gas cleaning, 16–17 November, Essen.
  • TWGComments. 2003. TWG Comments on Draft 1 of Waste Incineration BREF.
  • UBA (Umweltbundesamt). 2001. Draft of a German Report for the Creation of a BREF-Document "Waste Incineration", Dessau-Roßlau, UBA.

Created by Professor Dr.-Ing. Rudi Karpf (ete.a Ingenieurgesellschaft für Energie und Umweltengineering & Beratung mbH), (2014-08-30), last modified by Professor Dr.-Ing. Rudi Karpf (ete.a Ingenieurgesellschaft für Energie und Umweltengineering & Beratung mbH), (2014-10-20)


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