Treatments of polluted emissions from incinerator

Treatments of polluted emissions from incinerator gases: a succinct review

Pierre Le Cloirec

Reviews in Environmental Science and Bio/Technology ISSN 1569-1705 Volume 11 Number 4 Rev Environ Sci Biotechnol (2012) 11:381-392 DOI 10.1007/s11157-012-9265-z

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Author's personal copy Rev Environ Sci Biotechnol (2012) 11:381–392 DOI 10.1007/s11157-012-9265-z

REVIEW PAPER

Treatments of polluted emissions from incinerator gases: a succinct review Pierre Le Cloirec

Published online: 18 February 2012 Springer Science+Business Media B.V. 2012

Abstract Due to the incomplete mineralisation of some organic compounds during the incineration of municipal solid waste, gaseous emissions are loaded with a large amount of particulate matter, undesirable elements and toxic molecules. Typically, an incinerator of urban solid waste produces large flows of hot gaseous emissions to be purified before being released into the atmosphere. In this paper, treatments of flue gas from a typical municipal waste incinerator are described. The first step is an energy recovery system through heat exchangers to make steam or hot water. Steam is used to produce electricity via a turbine. The economic balance of the total system is very often dependent on the heat recovery. The second step involves particle removal technologies. Different systems are available such as cyclones, scrubbers, electrostatic precipitators or baghouse filters. The third step is the removal of numerous molecule families such as acid compounds (SOx, HCl, HF), nitrogen oxides (NOx), metal species and many organic compounds. The latter include dioxins, furans and volatile organic compounds. Some treatment processes are described according to the pollutant family.

P. Le Cloirec (&) Ecole Nationale Supe´rieure de Chimie de Rennes, CNRS, UMR 6226, Avenue du Geńe´ral Leclerc, CS 50837, 35708 Rennes Cedex 7, France e-mail: [email protected]

Keywords Incinerator Gas treatments Particle removal Acid gas SOx NOx Volatile organic compounds (VOC)

1 Introduction From an environmental perspective, recycling materials is the preferred choice. However, large fractions of industrial and domestic solid waste have to be treated by different methods such as biological or thermal processes. Among these, a range of options is available: landfill, composting, thermal treatments, with matter or thermal valorisation. Giusti (2009) reviewed waste management practices and their impact on human health in a large number of countries such as the European Union (EU), the USA, Russia, China and countries of the Organisation for Economic Co-operation and Development (OECD). A large proportion of solid waste is incinerated. For example, in France in 2008, about 32% of the 31 million tonnes of municipal solid waste produced per year was incinerated in 128 plants. The main advantages of incinerators are a reduction in mass and volume and a significant energy recovery (power and thermal) (Chandler et al. 1997). In 2006 in France, an energetic valorisation produced about 3,700 GWh (power) and 7,400 GWh (thermal) (Ademe 2008). However, incineration has gained a poor reputation due to the release of acid gases and organic pollutants (especially dioxins and furans) into the atmosphere. An evaluation

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Rev Environ Sci Biotechnol (2012) 11:381–392

of their environmental impact and a better control of emissions are both required. In recent years, European regulations (2000/76/EC and modifications in 2008) on air pollution prompted the closure of old plants and influenced drastically the design standards of recent and new installations. This legislation aims to prevent the negative effects of gaseous emissions on human health and their impact on the environment by a large reduction in pollutants from waste incinerator flue gas. However, few studies give the real performances of flue gas treatment processes. We can mention the works of Achternbosh and Richers (2000) and Chevalier et al. (2003) and, more recently, a review concerning the strategies and technologies for treating municipal flue gas (Vehlow and Delager 2011; Lens et al. 2006). In these published papers, several cleaning systems are compared and discussed in terms of auxiliary chemicals, amount of residues and investment cost. Table 1 compares pollutant levels in raw and treated flue gas with the discharge standards according to the European Directives (2000/76/EC). These data show that the flue gases must be cleaned before being discharged into the atmosphere. The main objective is first to present a general approach to gas treatment in a municipal waste incinerator. Then, based on an example of a typical incinerator with a multi-step flue gas treatment, some cleaning processes are described according to the pollutant family to be removed.

2 General approach to gas treatments Typically, an incinerator of urban solid waste produces large flows of hot gaseous emissions to be purified before being released into the atmosphere. Table 1 Example of composition of raw and treated flue gas in a domestic waste incinerator (adapted from Chevalier et al. 2003). Values expressed in a gas at 9% O2 and 11% CO2

• • • •

Carbon dioxide (CO2): 7–10% Oxygen (O2): 6–12% Nitrogen (N2): 60–70% Water vapour (H2O)g: 12–18%

Due to the incomplete mineralisation and the presence of inorganic matter, solid slag is produced and released at the bottom of the incinerator system. Municipal solid waste incinerators contribute to the emission of greenhouse gases (GHG) such as CO2 and N2O. However, a separate collection seems to play a role in significantly reducing GHG (Calabro 2009). The gaseous emissions, at a temperature close to 1,000C, are loaded with a large amount of particulate matter, undesirable elements and toxic molecules. Table 2 gives the concentrations of pollutants present in a flue gas. Several air pollution control systems are available. However, similar specific treatment processes are found. The first step is an energy recovery system through heat Treated flue gas Wet or droplet type processes (mg/Nm3)

Pollutant

Raw flue gas (mg/Nm3)

Discharge standards European Directives 2000/76/EC (mg/Nm3)

Dust

5,000

10

HCl

1,400

10

3–5

1

0.02–0.45

HF SO2 CO NOx Hg ? Cd (particles and gases) Pb ? Cr ? Cu ? Mn

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Figure 1 presents a classical diagram of a waste incinerator and flue gas treatment processes. In this case, the incinerator has two chambers: one for a primary combustion and the second for post-combustion. Air is added in order to obtain good oxidation of organic matter into CO2 and H2O. Fluidised bed combustors are also widely used for domestic and industrial waste treatments (Saxena and Jotsi 1994). The fluidisation number ranges between 3 and 11 Umf (where Umf is the minimal fluidisation velocity) with an optimum value of about 5 Umf. The optimum air factor is in the range of 0.8–1 (Johari et al. 2011). Flue gas recirculation in the incinerator combustion chamber is an alternative operating technique offering economic, energy and environmental advantages (Liuzzo et al. 2007). The flue gas volume compositions for the major components are as follows:

5 200 12.7 418

7–25

50

23–96

–

0.2–13

200

2–418

1.3

0.1

0.001–0.03

74.7

0.5

0.06–0.4

Author's personal copy Rev Environ Sci Biotechnol (2012) 11:381–392

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Fig. 1 Classical diagram of a typical municipal waste incinerator and flue gas treatments (adapted from Fino et al. 2005)

Table 2 Pollutant composition of raw flue gas in a domestic waste incinerator (adapted from Damien 2004) Pollutant

Concentration

Dust

2–5 g/Nm3

CO

20–80 mg/Nm3

Dioxins and furans

1–4 ng/Nm3

Hydrocarbons

5–30 mg/Nm3

Total metal species (Zn, Pb, Cu, Sn, Ba, Cd, Mn, Cr, Hg, Ag, Ni, As, V) HCl

90–100 mg/Nm3

HF

0.5–3 mg/Nm3

SO2

30–300 mg/Nm3

NOx (NO, NO2 and N2O)

250–400 mg/Nm3

1–2 g/Nm3

exchangers to make steam or hot water. Steam is used, in this example (Fig. 1), to produce electricity via a turbine. The economic balance of the total system is very often dependent on the heat recovery. The second step involves particle removal technologies. Different systems are available such as cyclones, scrubbers, electrostatic precipitators or baghouse filters. The third step is the removal of numerous molecule families such as acid compounds (SOx, HCl, HF), nitrogen oxides (NOx), metal species and many organic compounds. The latter include dioxins, furans, and volatile organic compounds (VOCs).The final step in the treatment of flue gas is a specialised process to reduce an individual or family component. In order to decrease the treatment cost, systems combining several processes in one reactor or new simpler routes were proposed by Werther (2007). Desulphurisation and mercury removal have been especially studied to reduce limestone consumption. An optimisation of municipal solid waste incineration and flue gas cleaning systems is required

according to new regulations. Desroches-Ducarne et al. (1998) presented a modelling study of the formation and destruction of pollutants (CO, NO, N2O, SO2, HCl) during incineration of municipal solid waste in a circulating fluidised bed (CFB) combustor. The predictions were in good agreement with experimental data measured in a CFB pilot plant. In order to minimise the environmental impact, Janelli and Minutillo (2007) proposed a modelling of the total system conducted by means of the Aspen Plus code (Soutudeh-Gharebaagh et al. 1998; Cimini et al. 2005). The incineration and the emission treatments are estimated by considering simplified hypotheses of combustion, post-combustion, oxidation and reduction reactions, and mass transfer: absorption and adsorption (Yang et al. 2002). The thermochemical model simulates operations such as combustion, SO2 and HCl scrubbing by sorption onto Ca(OH)2, and NOx reduction by a selective catalytic reactor (SCR) with urea injection.

3 Energy recovery The first step is an energy recovery system. The economic balance of the total system is very often dependent on the heat recovery. For example, in Taiwan, the main advantage of the development of incinerators for municipal solid waste treatment was the generation of electricity that could be consumed by the general public and industry (Kuo et al. 2008). However, special attention was paid to control the emissions of pollutants. Bergsdal et al. (2005) present different ways of organising energy systems for heat recovery from waste (Table 3). These authors show the energy potentially recovered as a function of the

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placed after the baghouse filter to reduce the flue gas temperature from about 200–80C. In this case, part of the energy is used locally through this exchanger to heat air, which is then introduced into the incinerator chamber. A third heat exchanger reheats the flue gas before the deNox process.

Table 3 Waste amounts and energy potential. Examples from Central Norway (adapted from Bergsdal et al. 2005) Waste type

Waste amount (tons)

Energy potential (GWh)

Ratio (kW/kg)

Municipal

177,100

409

2.3

Industrial

106,900

266

2.5

4 Specific pollutant treatments waste (municipal and industrial), using average heating values. In an industrial case, an efficiency of 75% has to be applied to obtain the ‘‘real’’ energy production. An average ratio gives an energy potential close to 2.4 kW/kg and a real value of about 1.8 kW/ kg of waste. In fact, the real energy recovery ranges from 0.95 to 2.34 kW/t (Damien 2004). In the case of flue gas recirculation, the simulation results indicate a greater level of energy recovery of up to ?3% (Liuzzo et al. 2007). At different steps of the treatment system, several options are available for energy recovery. In fact, the flue gas temperature has to be adjusted continuously to obtain an optimal operating value, which is a function of the specific treatment. As shown in Fig. 1, a gas– liquid evaporator is positioned after the incineration chamber to make steam (about 2 t of steam per ton of solid waste at 15 bar, 220C) which is generally used to produce electricity via a turbine. The gas temperature decreases drastically from about 1,000–240C. However, due to acid gas compounds (SO2, HCl, HF) present in the flue gas at high temperature, corrosion of the heat exchanger installations is observed and requires the regular replacement of pipes and many types of equipment. A second gas-air heat exchanger is X Rays

UV

4.1 Aerosol–particle removal A large fraction of pollution emitted by flue gases is due to high concentrations of solid particles. Figure 2 gives the size range of some liquid and solid aerosols present in gaseous emissions (Coulson and Richardson 1997). Comparisons are given with wavelengths, molecule and microorganism sizes. Several studies have been performed to determine the size distribution and number concentration of particles. As previously reported, the target particle size for regulations has moved from total suspended solid particles (\30 lm in aerodynamic diameter) through the coarse fraction PM10 (\10 lm), to the fine fraction PM2.5 (\2.5 mm) (EN 1234 1999; EN 1490 2005; Le Coq 2006). Buonanno et al. (2009) paid special attention to ultrafine particles (\0.1 lm in aerodynamic diameter) which are emitted in large quantities and represent health and environmental risks, as indicated in recent toxicological and epidemiological studies (Cheng 2003; Kreyling et al. 2006). At the stack of a specific municipal incinerator, the total concentration of ultrafine particles ranged from 1,105 to 2,105 particles/cm3 and levels were relatively stable (Buonanno et al. 2009). Visible

IR

micro-waves, radar

smokes fumes

fly ashes

soots virus

pollen bacteria

macromolecule

dust

molecules 10 -6

0.0001 1 Å 1 nm

hair

smog 0.01

cloud -fog 1 1 µm

drizzle

rain 104

100

1mm

Particule size (µm)

Fig. 2 Comparison between sizes of different materials (adapted from Coulson and Richardson 1997)

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The choice of the best technologies is principally dependent on the particle size (Peukert and Wadenpohl 2001). In a classical system, electrostatic precipitation is carried out. The gas stream, with a velocity ranging from 0.7 to 1.3 m/s, passes through a series of discharge electrodes and particles are collected. Residual particles are removed with a baghouse filter (fabric filter). This two-step system gives an optimum performance by reducing not only the total mass but also the total particle number emission. The performances of such a system enable a total particle number emission of between 105 and 106 particles/cm3 (Maguhn et al. 2003). An estimation of particle concentration gives values ranging from 5 to 100 mg/Nm3 depending on the age of the installation. The electroprecipitator and the baghouse filter are really efficient both in terms of particle removal and for the elimination of some dioxins or other toxins adsorbed onto the surface of suspended solids. To minimise pollutant release, the outlet concentrations have to be less than 5 mg/Nm3. In many cases, a fabric filter is used alone with good performances (Davies 1973; Brown 1993). The filter modules are designed according to the particle characteristics (size, nature), operating conditions (velocities, mass flow, head loss) and performances (Thomas et al. 1999; Davis 2000; Del Fabro et al. 2002). Fino et al. (2005) proposed a multifunctional filter to treat flue gases. Dust filtration and the catalytic removal of NOx and VOCs are performed together in a catalytic filter based on the combination of a high temperatureresistant polymeric bag filter enclosing a catalyticallyactivated ceramic foam structure. Some attempts to combine the filtration of particles and the removal of VOCs onto activated carbon fibre cloth have also been proposed (Rochereau et al. 2007, 2008). A cyclone, working at 750C, is mentioned as the first step in the flue gas treatment system in an industrial pilot plant to control the particle level before release into the atmosphere (Rio et al. 2007). In some cases, a cyclone is used to cool flue gas with water to a suitable temperature before it is mixed with a dry sorption agent or basic reagent to remove acid gaseous compounds or metal species such as mercury (Chandler et al. 1997). However, some innovative systems of filtration need to be proposed to remove particles present in gaseous emissions at very high temperature.

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4.2 Acid gas treatment Flue gas cleaning processes depend on the properties of the compounds present in the emissions. Due to the acid character of some pollutants (SOx, HCl, HF) generated during the incineration of solid waste, a neutralisation is generally performed with lime, hydrated lime or other basic reagents (NaHCO3, Na2CO3) in a dry or wet scrubber at relatively low temperatures (Stein Atkinson Stordy Ltd 1996; DeNevers 2000; Yan et al. 2003). Although the acid–base reactions using lime seem to be simple, with final products such as CaSO4, CaCl2, and CaF2 and some intermediate compounds (CaSO3, CaOHCl), the mechanisms of the reaction are complex with mass transfers between the gaseous phase and solid particle surface, reagent and product diffusion, acidic-basic reactions, and water evaporation. The kinetics of hydrated lime and HCl at varying flow rate, with temperatures ranging from 170 to 400C and HCl concentrations from 600 to 1,200 mg/Nm3, were studied by Yan et al. (2003). Depending on the flow rate and in the absence of internal diffusion, the reaction was found to be first order with respect to HCl concentration. Verdone and De Filippis (2004) showed that sodium-based sorbents were more efficient than calcium-based sorbents towards HCl and SOx at a temperature range of 100 to 600C. These authors also found a partial reduction of NOx concentration. Simultaneous removal of SO2 and NO from flue gas was performed using NaOCl solution as the absorbent solution in a bubble reactor (Zhao et al. 2011). Mechanisms and reaction pseudoorders were investigated in different operating conditions. Mass transfer in the droplet column was studied in the presence of solid particles to determine their negative impact on the overall performances of the process (Muller et al. 2001). Wet or dry scrubbing processes have been developed as a key stage in the treatment of acid gases. Different methods are proposed: a spray system, droplet column, dry scrubber, vertical Venturi reactor, or fluidised bed. Industrial data for SO2 removal in a dry scrubber show a drastic decrease in efficiency between 120 and 200C with performances of [90% and about 30% removal, respectively (Heap and Atkinson 1996). Generally, for a dry scrubber, a separation system is required to remove particles. Examples of emission levels are presented in Table 4.

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Table 4 Example of performances obtained at a municipal incinerator; treatment with a dry scrubber (adapted from Head and Atkinson 1996) Pollutant (mg/ Nm3-11% O2)

Concentration inlet

Dust

50–100

HCl

1,000

SO2

300

Hg

0.180

Concentration outlet 3

Legislation

30

35

50

100

300

0.060

0.200

4.3 Metal species removal Metal species (Hg, Cd, Pb, Zn) are eliminated as metal hydroxides by reaction with a base sorbent in a dry or wet scrubber (see removal of acid gas) in a specific reactor or they are injected directly into the flue gas flow. The resulting solids are removed by baghouse filters. Emission factors and removal efficiencies of heavy metal species (Hg, Cd, Pb, Zn, Cr) have been investigated (Chang et al. 2003; Lin et al. 2010). The kinetics of heavy metal release and abatements have been studied in the laboratory (Abadanes et al. 2005) or directly at industrial sites. Due to their high vapour pressure, mercury compounds can remain in the gas phase at temperatures less than 180C. Table 4 shows an example of mercury removal by lime in a dry scrubber. In this case, the efficiency is close to 67%. Activated carbon appears to be very efficient (\90%) for removing Hg from flue gas (Chandler et al. 1997). The injection of a carbonaceous adsorbent (Fig. 3) removes other pollutants: dioxins and furans, VOCs. In another study, sodium bicarbonate seems to be an efficient neutralising agent for gaseous streams and also contributes to heavy metal reduction of 62% Zn

Fig. 3 SEM images of an activated carbon porous surface

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and 89% Pb. The addition of activated carbon, together with sodium bicarbonate, eliminates up to 97% Cd (Rio et al. 2007). Liuzzo et al. (2007) reported a 50% reduction in mercury emissions after modifying the operating conditions to include a flue gas recirculation in the waste incinerator. 4.4 Dioxins and furans The formation and removal of dioxins and furans (PCDD/F) are extensively described by Vehlow (2012) in this present special issue. Dioxins and furans are produced, after the incinerator chamber, at temperatures between 200 and 400C with a maximum around 300C (Tanaka et al. 1989; Hunsinger et al. 2002). This is mainly due to a de novo synthesis, i.e. a recombination of carbon (soot) and chlorine (Cl-) in an oxidising atmosphere, catalysed by copper (Cu), to form these molecule families (Hagenmaier et al. 1987; Stieglitz and Vogg 1987; Stieglitz et al. 1991; Hell et al. 2001). Lundin et al. (2011) also mentioned the important roles of Cu, Fe, Ca and S in the formation of PCDD and PCDF. The mass balance in the gaseous phase produced in a municipal waste incinerator shows that, after the combustion chamber, the concentration is less than 0.2 ng/Nm3 but increases, before the particle removal process, to a range between 1 and 5 ng/m3 (Vehlow et al. 2004). A significant part is removed by the filtration system. About 100 to 1,000 ng/kg is found in the fly ash recovered after filtration. Table 5 gives the formulae of dioxins and furans. Looking at the molecular structure and physico-chemical properties of these molecules, we can note their symmetrical


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Table 5 Names and molecular structures of dioxins and furans (PCDD/PCDF) Name

Formula

Observations

Dioxin

n [ [1,8]

Polychlorodibenzo-p-dioxin

Cl or Br

PCDD

Furan

n [ [1,8]

Polychlorodibenzo-p-furan

Cl or Br

PCDF

aromatic structures, good stability, low solubility in water but high solubility in organic solvents and fats, non-biodegradability and significant toxicity. These characteristics clearly show the need to remove these molecules from the flue gas. Buekens and Huang (1998) evaluated the techniques for controlling the formation and emission of dioxins and furans in a municipal waste incinerator: •

•

• • •

Dry/semi-dry/wet scrubber and bag filter coupled with activated carbon granule adsorption (moving bed or fixed bed process); Powdered activated carbon or mixture with base reagents injected directly into the flue gas and stopped with a dust removal system (baghouse filter). Examples of pores at the surface of activated carbon are presented in Fig. 3; Adsorption onto polymers; Catalytic decomposition by an SCR unit; Chemical treatments.

Some operating conditions and performances of different PCDD/F removal systems are summarised in Tables 6 and 7. 4.5 DeNOx Selective catalytic reduction (SCR) units are commonly used for controlling NOx emissions from domestic waste incinerators. In order to remove the NOx pollutant, catalytic oxido-reduction reactions between nitrogen oxides (NOx) and ammonia (NH3) or urea (CO(NH2)2) are carried out. Concerning NO

and NO2 reduction, respectively, the simplified reactions using NH3 are written as follows: 4NO + 4NH3 + O2 Catalyst ! 4N2 + 6H2 O

ð1Þ

2NO2 + 4NH3 + O2 Catalyst ! 3N2 + 6H2 O ð2Þ Preheated NH3 and flue gas are injected directly into the catalytic reactor (Fig. 1). Catalysts are a transition metal (platinum, palladium, vanadium), at concentrations ranging from 0.1 to 10%, covering an inert material such as aluminosilicate minerals, zeolite media or steel chips. Extensive research is currently underway to develop new multifunctional catalysts that give better performances and have a longer life. The reaction is operated in an open SCR reactor at a temperature ranging between 280 and 400C. Performances are close to 90% conversion of nitrogenous reagents into nitrogen with an NH3/NOx ratio = 2.5 (Bicocchi 1998; Popescou et al. 1998). A deNOx process is presented in Fig. 1 and more extensively in Fig. 4. For comparison, a selective non-catalytic reduction (SNCR) unit works at a higher temperature (850–1,000C) with an NH3/NOx ratio = 2.5 giving a lower NOx reduction efficiency ranging from 50 to 70% (DeNevers 2000; Davis 2000). It should be noted that during the DeSOx process with lime or sodium hypochloride, different authors observed a partial reduction of NOx (Verdone and De Filippis 2004; Zhao et al. 2011). It was also found incidentally that SCR systems decompose dioxins and furans, as mentioned previously in this paper (Buekens and Huang 1998). However, some installations in

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Table 6 Removal of dioxins and furans from flue gases by a combination of scrubber, filter and activated carbon adsorption (adapted from Buekens and Huang 1998) Process

Equipment

Operating conditions

Performances

References

Moving bed adsorber

Integral counter-current activated coke process

T = 120–165C

Emission concentration: 0.015 ng/Nm3

Maierhofer and Grochowski (1994)

Yamaguchi et al. (1994)

WKV system

Activated carbon consumption = 500 t/year Qv = 464,000 m3/h

Moving bed system

Fixed bed adsorber

Mixture: zeolite, carbon and inert material

T = 150C

Removal 98.8%

U = 1,000 h-1

Inlet: 100 ng/Nm3

Activated carbon consumption = 5 mm bed height/day

Outlet: 1.2 ng/Nm3

T = 40–100C

Removal 83%

Petzoldt et al. (1996)

Inlet: 0.3 ng/Nm3 Outlet: 0.05 ng/Nm3

Entrained flow

Wet scrubber—bag filter and activated carbon injection

Blumbach and Nethe (1994)

Removal 96.8% Inlet: 2.17 ng/Nm3 Outlet: 0.069 ng/Nm3

Fabric filter—injection of activated carbon

Carbon dosage: 50 mg/Nm 90% activated carbon is recirculated

3

Removal 95.8% Inlet: 0.24 ng/Nm3

Ruegg and Sigg (1992)

Outlet: 0.01 ng/Nm3

Table 7 Removal of dioxins and furans from flue gases by SCR systems (adapted partially from Buekens and Huang 1998) Catalyst

NH3/NOx molar ratio

Operating conditions

Performances (ngTE/Nm3)

References

Pt and Au on silica boria aluminium composite oxide

0

T = 220C

Removal 96%

Sakurai et al. (1995)

U = 3,000 h-1


V2O5–WO3–TiO2

0

T = 250C

Removal increase

U = 3,000 h-1


SCR DeNox

NH3 addition

Removal 61.8% Inlet: 2.2 ng/Nm3

Tartler et al. (1996)

Outlet: 0.84 ng/Nm3 T = 230C

Removal 94.9%

Kamiyama et al. (1994)

Inlet: 0.39 ng/Nm3 Outlet: 0.02 ng/Nm3 NH3 addition

T = 325C

Removal 80%

Carlsson (1989)

Inlet: 0.059 ng/Nm3 Outlet: 0.01 ng/Nm3 Pt supported

0

T = 300–400C

Removal 66.7%

U = 20,000 h-1

Inlet: 90 ng PCDD/Nm3 Outlet: 30 ng PCDD/Nm3

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Hiraoka et al. (1989a, b)


389

Fig. 4 Schematic presentation of an SCR unit to remove NOx

specific operating conditions, especially at low temperature, increase the stack PCDD concentrations. 4.6 VOC and odour removal Due to an incomplete oxidation, VOCs may be present in the gaseous emissions. VOCs such as chlorinated molecules, aromatics or oxygenated products are generally found in flue gas at relatively low concentrations (some mg/m3). Classical treatment processes are available such as: • •

•

•

Thermal oxidation (re-burning) or by innovative systems (Masuda et al. 1995); Wet scrubber working at room temperature with different acid or base solutions (lime, sulphuric acid, bleach). Some other oxidants such as ozone have been studied in pilot units; Adsorber packed with activated carbon (Delage et al. 2000; Pre´ et al. 2002) or other adsorbents (zeolites). Modelling the breakthrough curves has been extensively published (Giraudet et al. 2009). Some new processes have been proposed using activated carbon fibre cloth (Le Cloirec and Faur 2005; Subrenat and Le Cloirec 2004, 2006; Le Cloirec 2004) or honeycomb monoliths of activated carbons (Yates et al. 2000); Biological treatments: biofilter, bioscrubber or trickling filter. These various systems have been

reviewed by several authors (Humeau et al. 2004; Delhomenie and Heitz 2005; Iranpour et al. 2005; Nikiema et al. 2007). However, there has been an increase in the number of biofilters in gas treatments compared to other biological processes. An olfactometric approach was performed by Capelli et al. (2011) at different industrial sites including two municipal solid waste treatment plants. The odour emission rates (OER) were found to range from 19,171 to 43,311 ouEs-1 where ou is odour unit. Some treatments (wet scrubbers, biofilters) are followed by dispersion into the atmosphere through a stack, reducing the offensive odours. The stack height is calculated as a function of the pollutant emissions and the presence of obstacles that could prevent the dispersion of the plume. It has to be higher than 10 m and the gas ejection velocity must be greater than 12 m/s.

5 Conclusions A general approach to gas treatment in a municipal waste incinerator is presented. Some flue gas cleaning processes are described according to the removal of particular pollutant families. Some innovative treatments are also mentioned. Nevertheless, some lines of research could be proposed as alternatives to existing specific approaches and treatments:

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•

•

• •

•


A better design of the combustion chamber and an optimisation of operating conditions to minimise pollutants produced in the flue gas. An improvement in energetic performances in terms of both the incineration of solid waste and energy recovery; The filtration of dust at high temperature could be developed using new packing media; New deNox catalysts must be found to avoid poisoning the active materials and increasing their life time thus reducing the total cost of such a treatment. In general terms, the total flue gas treatment systems have to be simplified by using multitreatment processes.

Acknowledgments A part of this review was presented at the summer school ‘‘Biological and thermal treatment of municipal solid waste’’ ETeCoS3 in Naples (Italy) on May 2–6, 2011.

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