Flue Gas Treatment for Removal of Pollutants and Acid Compounds

Flue Gas Treatment – State of the Art

Margit Löschau und Rudi Karpf

ContentPage

Summary2

Flue Gas Treatment Options3

Removal of Particles and Particle-Bounded Heavy Metals3

Electrostatic Separators3

Filtering Separators5

Removal of NOx6

SNCR7

SCR8

Removal of Organic Pollutants and Vaporous Heavy Metals10

Entrained Flow - Filter Layer Process10

Fixed or Moving Bed Adsorber11

Removal of Acid Compounds11

Lime-Based Processes12

Conditioned-Dry Sorption with Hydrated Lime13

Spray Absorption with Lime Slurry15

Lime Slurry Scrubber16

Sodium-Based Processes17

Dry Sorption with Sodium Hydrogen Carbonate17

Sodium Hydroxide Scrubber19

Achievable Emission Values19

Particles19

Nitrogen Oxides20

Acid Pollutants21

Dust and Heavy Metals22

Overview for Germany22

Examples for Concepts for Retrofit and New Installations23

Changing from a Wet to a Dry System23

Retrofit to an Energy Optimised Concept24

Change from Spray Absorption to Conditioned-Dry Sorption25

Concept for a New Installation26

References27

Summary

For the removal of air pollutants from the flue gas, a flue gas treatment system is required downstream the boiler. Such treatment systems consist of a system of cleaning processes for the reduction of particulate, vapour and gaseous substances in the flue gas. There are special flue gas treatment components for removal of special pollutants, however, some components are able to remove more than one pollutant.

The selection of the appropriate flue gas treatment system depends in particular on the compo- sition respectively pollution of the fuel, the resulting composition of the raw gas, the expected maximum concentrations of pollutants in the raw gas and their fluctuations and the required effi- ciency of the treatment process to meet the applicable emission limits. An overview of the flue gas treatment components for the removal of pollutants is shown in Table 1.

Table 1:Overview of flue gas treatment components for removal of pollutants

Pollutant

Apparatus/Process

Principle

Particle and Particle Bounded Heavy Metals

Cyclone

Centrifugal Force, Inertia

Fabric Filter

Filtration

Electrostatic Precipitator

Electrical Attraction

Wet Separator

Heterocoagulation

NOx

Selective Non-Catalytic Reduction (SNCR)

Gas Phase Reaction

Selective Catalytic Reduction (SCR)

Heterogeneous Catalysis

(Chemical Adsorption)

HCl, HF, SO2, SO3

Wet Flue Gas Treatment

Absorption

Semi-Dry Flue Gas Treatment

Absorption and Adsorption

Dry Flue Gas Treatment

Chemical Adsorption

Organic Pollutants and Heavy Metals

Entrained Flow Absorber

Physical Adsorption

Fixed or Moving Bed Adsorber

Flue Gas Treatment Options

Removal of Particles and Particle-Bounded Heavy Metals

For removal of particles, the following components were usually used in flue gas treatment sys- tems downstream the waste incineration:

Centrifugal Separator

Electrostatic Separator

Filtering Separator

Wet Separator

Figure 1: Flue gas treatment systems for the removal of particles

Due to their low separation efficiency, cyclones can only be used for pre-dedustig. Wet scrub- bers intend to remove acidic compounds of the flue gas, the separation of dust is just a side ef- fect (exception: venture scrubbers which are appropriate for fine dust separation). Electrostatic separators are often used in flue gas cleaning installations, mostly as pre-dedusting step up- stream of a scrubber system. However, to achieve the emission limits of the European Industrial Emission Directive, fibrous layer filter with pulse jet cleaning are the most common installation for dust removal in flue gas cleaning.

Electrostatic Separators

Electrostatic Separators are used to capture particulate pollutants by means of the electrostatic attraction. There are

Dry Electrostatic Precipitators, and

Wet Electrostatic Precipitators.

Wet ESP are often used for cleaning gases saturated with water vapor in the flue gas purifica- tion of chemical processes. As part of the FGT of thermal waste treatment plants, they are rare-

ly used, and only in combination with wet scrubbers to achieve a downstream removal of aero- sols and particulate matter.

Dry ESP are more often part of the flue gas cleaning concept but usually not as the only dedust- ing aggregate for waste-to-energy plants as the emission limit of 10 mg/Nm3 can hardly be met. An exemplarily design of a dry ESP is shown in Figure 2:.

Figure 2: Example for a typical dry electrostatic precipitator

Source: BETH Filter GmbH: Dry Electrostatic Precipitator. Company Brochure, 2014

In the ESP, the particles are electrically charged in an electric field (20 and 100 kV) and dis- posed at the collecting electrode. The principle is shown in Figure 3.

Figure 3: Removal of dust in an electrostatic precipitator

Source: Scheuch GmbH: Dry Electrostatic Precipitator. Company Brochure

Filtering Separators

Filtration is a mechanical process for separating mixture of substances. Both mixtures of liquid and solids (suspensions) and mixtures of gases and solids can be separated. For filtering, the mixture pass through a filter medium. Through the filter medium e.g. particles will be retained from the gas stream.

The filter effect depends on the particle size and can take place on the surface of the filter me- dium, but also in the filter medium itself. Accordingly, there are:

Surface Filtration, and

Depth-Loaded Filtration (Figure 4)

Figure 4: Principe of surface and depth-loaded filtration

Source: Liqui-Filter, company material. Im Internet: http://www.liqui-filter.info/know-how/absaugen-und-filtern-von- luft/wie-arbeiten-unsere-filter.html, extracted on 1st February 2012, edited

With respect to the filter medium one distinguishes between:

Fibrous Layer Filter, and

Packed Bed Filter.

For dust removal in the flue gas of waste-to-energy plants, only fibrous layer filters which can be cleaned on-line (during operation) can be used. Fabric filters with 4–8 chambers (mostly 6), ver- tical bags and pulse jet for cleaning the filter tubes are commonly installed in waste incineration plants (see Figure 5). Filter designs with horizontal bags also exists.

Figure 5: Typical design of a fabric filter with chambers and vertical tubes

Source: Scheuch GmbH: Bauformen Filter / Abscheider. In the Internet: http://www.scheuch.com/de/filter_abscheider_impulsfilter_bauformen/, extracted April 2013

If the fabric filter operates not only as a dust collector but also as a sorption filter the filter medi- um should be always designed for a surface filtration.

Removal of NOx

For the removal of NOx two established processes exist:

Selective Non-Catalytic Reduction (SNCR), and

Selective Catalytic Reduction (SCR)

Both processes are able to remove NOx with the efficiency needed to meet an emission value of 100 mg/m3 STP. Reagents and plant configurations are shown in Figure 6.

Figure 6: Flue gas treatment systems for the removal of NOx

SNCR

In the selective non-catalytic reduction process (SNCR) nitrogen oxide (NOx) in the flue gas is reduced by reaction with ammonia (NH3) to elemental nitrogen (N2). For this purpose, aqueous ammonia solution (NH4OH) or urea solution (NH2CONH2) is injected into the hot flue gas in the first boiler pass. As a carrier medium for the reagent, pressurised air, steam or pressurised wa- ter can be used. A typical SNCR system is shown in Figure 7.

Figure 7: Example for a SNCR system

Source: Martin GmbH für Umwelt- und Energietechnik: Firmenbroschüre „Das Martin SNCR System“

NH4OH  NH3 + H2O

4 NO + 4 NH3 + O2  4 N2 + 6 H2O

2 NO2 + 4 NH3 + O2  3 N2 + 6 H2O

To avoid NH3-slip, NH3 oxidation and NO generation, the reduction reaction in the SNCR pro- cess takes place in a narrow temperature range from 850–1 100 °C, with an optimum at approx- imately 970 °C (Figure 8). For the optimisation of the injections of reagent in the right tempera- ture zone an IR-pyrometer or an acoustical gas temperature measurement can be installed. The efficiency of SNCR can be increased by increasing the stoichiometric factor for ammonia water but is limited due to the inevitable NH3-slip.

Figure 8: NH3-reaction depending on temperature as a function of temperature

Source: Dittrich, R.; Nowag, R.: Vergleichende Beurteilung und Abscheideleistung von SNCR-/SCR-Technik. VDI- Wissensforum: BAT und preisorientierte Rauchgasreinigungstechniken, München, 2002, edited

SCR

In the selective catalytic reduction process (SCR), a catalyst is used to increase the efficiency of the reaction of nitrogen oxide (NOx) in the flue gas with the reagent ammonia (NH3). NH3 is mostly provided by the injection of ammonia water. The reaction equations are the same as for the SNCR but the catalysed process takes place at lower temperatures (160–450 °C). As am- monia water is converted almost stoichiometrically there is hardly any NH3-slip. With respect to the location of the SCR-catalyst there are four different plant configurations (Figure 9):

High-Dust-SCR

Low-Dust / High SOx-SCR

Low-Dust / Low-SOx-SCR

Tail-End-SCR

Figure 9: Plant configurations for SCR

Source: Karpf, R.; Basic features of the dry absorption process for flue gas treatment systems in waste incineration; Earth Engineering Center, Columbia University, New York, April 2015

Karpf, R.: Überblick zur Abgasreinigung. 10. Fachtagung Abgasreinigung von Feuerungsanlagen und thermische Prozesse; Haus der Technik, Essen, 25-26th June 2015

Most common for waste-to-energy installations is the Tail-End-SCR (Figure 10) which is located downstream the removal of particles and acid compounds in the flue gas and therefore ensures long lifetime of the catalyst. However, as the temperature of the flue gas after the fabric filter is usually too low for the catalytic reduction of NOx, for this configuration a reheating of the flue gas is necessary, which requires steam or natural gas. Most common are catalysts consisting of TiO2 as carrier material with V2O5 as active component and WO3 as promoter.

Figure 10: Typical Tail-End-SCR-Concept

Removal of Organic Pollutants and Vaporous Heavy Metals

For the removal of organic pollutants (mainly PCDD/F) and vapour forms of heavy metals (main- ly Hg), the following established processes exist:

Entrained Flow - Filter Layer Process

Fixed or Moving Bed Adsorber

For organic pollutants, additionally an oxidation catalyst can be used.

Entrained Flow - Filter Layer Process

In an entrained flow adsorber, the adsorbent (mostly lignite active coke) is injected directly into the flue gas and carried away with this. Organic pollutants (such as PCDD/F) and heavy metals in vapour form (such as Hg) are adsorbed on the surface of the adsorbent and removed with the particles in the downstream fabric filter. On the filter bags of the fabric filter layer of dust and ad- sorbent is developed which contributes to the adsorption processes. In Figure 11 the basic prin- ciple of the entrained flow - filter layer process is shown.

Figure 11: Principle of an entrained flow - filter layer process

The advantage of this process is that it can be perfectly combined with conditioned-dry removal of acid compounds where usually a reactor and a fabric filter are part of the system too. So the technical equipment can be minimised.

Fixed or Moving Bed Adsorber

Fixed or moving bed adsorbers are usually located downstream of a wet FGT process (Figure 12). The flue gas flows through a granular bed of an adsorbent. For technical and economic reasons, mostly lignite coke is chosen. The separation of organic pollutants and vaporous heavy metals is based on the principle of physical adsorption.

Figure 12: Example for a moving bed adsorber with four three-layer active coke bed

Source: Thermische Abfallbehandlung Lauta GmbH & Co. oHG: Aufbau und Funktion: Aktivkoksfilter. In the Internet: http://www.t-a-lauta.de/aufbau-funktion/aktivkoksfilter.html, extracted 23rd January 2012, edited

Removal of Acid Compounds

For the removal of acid compounds, mainly HCl and SO2 downstream waste incineration, there are the following three established flue gas treatment systems in Europe:

wet.

semi-dry and

dry

Usually, calcium or sodium based absorbent are used as reagent for the removal. In Figure 13, the further subdivision of the main systems is shown.

Figure 13: Flue gas treatment systems for the removal of acid compounds

Lime-Based Processes

The separation of the pollutants takes place via adsorption on the surface of calcium hydroxide particles. These particles are brought into contact with the flue gas in a variety of forms. The primary reactions take place a