MATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS 

OF MUNICIPAL SOLID WASTE INCINERATORS (MSWI)

Matthias Achternbosch

Institut für Technikfolgenabschätzung und Systemanalyse

Forschungszentrum Karlsruhe

P.O. Box 3640

D-76021 Karlsruhe, Germany

Ulf. Richers

Forschungszentrum Karlsruhe

Zentralabteilung Technikbedingte Stoffströme

P.O. Box 3640

D-76021 Karlsruhe, Germany

ABSTRACT

The aim of this study is a comparison of four different flue gas cleaning systems of municipal solid waste

incinerators (MSWI). The main topic of the investigation is the relationship between type of flue gas cleaning system and investment costs. This comparison will be done with the aid of material flow analysis. The elements chlorine, sulfur and mercury are considered. In addition, the amounts of residues will be taken into consideration. As a

result of this work, a flue gas cleaning concept with a wet flue gas cleaning system equipped with fabric filter followed by a two-stage scrubber system seem to be very interesting for the construction of new plants.

INTRODUCTION

The incineration of municipal solid waste makes a contribution to waste disposal in Germany irrespective of a discussion

about waste management. Up to now, 57 municipal solid waste

incinerators (MSWI) equipped with grate furnaces are in operation in Germany with a total capacity of 13.5 Mio t waste per

year. Additionally no less than 7 new incinerators are planned.

Other technologies for thermal waste treatment like pyrolysis or

gasification were applied in 4 plants. (1)

In future, older plants have to be replaced and new incinerators have to be built to meet the requirements of the TA

Siedlungsabfall (Technical Directive for Recycling, Treatment

and Other Management of Residential Wastes). In connection

with these aspects, a question arises about the selection of adequate flue gas cleaning technologies. The partially expensive

flue gas cleaning systems installed in operating municipal solid

waste incinerators are the result of the several times upgraded

emission standards during the last 15 years and often - on top of

that - of demands to stay even far below those stringent limits.

An other reason was the demand to remain far below the existing

emission limits.

There is a lack of detailed comparison between different

flue gas purification systems including the examination of the

distribution of pollutants, the need for auxiliary chemicals, the

amounts of residue and finally additional consideration of the

investment costs.

MSWI-TECHNOLOGY AND FLUE GAS CLEANING

Grate firing systems shown in Fig. 1 are mainly used for

thermal waste treatment in Germany.

1

Fig. 1. Scheme of a Municipal solid waste incinerator with flue gas cleaning.
The waste delivered is stored in the bunker (1). A charging

system fed by a crane is used to load the incineration chamber

(furnace) (3) where the waste is incinerated on a grate. The air

necessary for combustion is fed through the grate (primary air)

or directly into the combustion chamber (secondary air). The

incineration residue named as bottom ash falls into a bottom ash

discharger and is transported by a conveyer to the bottom ash

bunker.

The heat energy of the hot flue gas is used to generate steam

in the boiler (4) above the furnace. The flue gas temperature

decreases from up to 1000°C down to the range of 200°C. The

pollutants contained in the raw gas have to be separated in the

flue gas cleaning installed downstream the boiler. The flue gas

cleaning system shown in Fig. 1 is a simple wet one chosen from

a large variety being developed to meet the emission limits. The

first device is a fabric filter (5), followed by a HCl-scrubber (7)

and a SO2-scrubber (8). The pressure drop in the plant has to be

compensated by the fan (6).

The first flue gas cleaning step is a dedusting device performed as a fabric filter (5). The addition of pulverised coke to

the flue gas upstream the fabric filter makes the separation of

PCDD, PCDF and mercury in this cleaning step possible, too.

In wet flue gas cleaning plants like the example shown in

Fig. 1 aqueous liquids are applied to separate HCl, SO2 and other

pollutants from the flue gas. In the first scrubber, an acid solution with pH 1 absorbs HCl. A neutral absorption solution must

be applied in the second scrubber for SO2 removal. The effluents

of the scrubber system are vaporised with an evaporator plant

(11) with the result of a solid residue. A Selective Non Catalytic

Reduction (SNCR) System installed in the boiler reduces the

nitrogen oxide emissions.

Other municipal solid waste incinerators in Germany with

wet flue gas cleaning are equipped with additional flue gas

cleaning steps. An entrained flow reactor or a carbon adsorber

are built downstream the scrubber system to reduce the emissions of PCDD, PCDF and mercury and instead of a SNCRSystem a Selective Catalytic Reduction (SCR) System is

installed in some cases.

Additional variations of wet flue gas cleaning result from

the demand for a waste water free plant operation. To achieve

this, a spray dryer partial in combination with a further dedusting

device is employed.

In Germany not only wet flue gas cleaning systems are in

use for cleaning the flue gases generated by MSWI. Semi-wet

and semi-dry systems are in existence, too. In a semi-wet cleaning process, a slurry of a calcium compound, general calcium

hydroxide, is used for separation of HCl, SO2 and other pollutants. In a semi-dry process, a pulverized calcium compound is

injected after cooling the gas by injecting water. In both cases, a

solid product is formed, which have to be separated from the

flue gas by using a fabric filter or electrostatic precipitator.

APPROACH AND SYSTEM BOUNDARY

The material flow calculation can be carried out with data

and information from technical scale MSWI, but earlier work (2,

3) had shown differing material flows for identical flue gas

cleaning due to different plant operation. For that reason the

material flows calculated in this work are based on a model plant

with typical fuel and furnace parameters of technical scale plants

in Germany. Additional information about each flue gas cleaning

step is taken from the literature or is obtained from plant manufactures and operators.

The system boundary selected for balances comprises the

entire flue gas cleaning plant. The area of coverage begins

downstream the boiler and ends with the stack. The flue gas, the

auxiliary chemicals required and the resulting residues were

taken into consideration. The results described are limited to

SO2

, HCl and mercury. The values given are related to 1 ton (tW)

of waste burned and calculated for the chemical elements. This

was necessary to take into account the different chemical compounds of pollutants generated due to the chemical reactions in

the in the flue gas cleaning plant.

BALANCES OF THE FLUE GAS CLEANING SYSTEMS

This section contains a short description and the corresponding balances of 4 flue gas cleaning systems considered in this

study. For the separation of the pollutants in the model plants A

and B a wet flue gas treatment process is applied. In the case of

plant A relatively simple construction is considered. In contrast

plant B is more complex with a spray dryer and a fine cleaning

stage upstream the stack. Plant C is considered to operate with a

semi-wet system whereas plant D is equipped with a semi-dry

flue gas cleaning system.

Plant A

The flue gas cleaning system of plant A corresponds to the

flue gas cleaning system presented in Fig. 1. It consists of a fabric filter and a two stage scrubber system. For the balances it is

assumed that the SNCR process has no influence on the material

flows. Figure 2 shows the balance of chlorine.

According to the model calculations the dusty raw gas

downstream the boiler transports 5890 g/tW chlorine in all plants

considered in this study.

The fabric filter removes 720 g/tA chloride bound in the filter ash. The main amount of chlorine (4587 g/tW) is absorbed in

the first scrubber. The second scrubber removes only small

quantities (569 g/tW) of chlorine from the flue gas. The waste

water treatment of the scrubber liquids occurs outside the system

boundary. The clean gas which leaves the stack contains

14.1 g/tW of chlorine.

The sankey diagram in Fig. 3 shows a different distribution

of sulfur in the flue gas cleaning. The balance starts with

1265 g/tW sulfur in the dusty raw gas downstream the boiler.

Only small amounts of sulfur (70 g/tW) are removed in the

first scrubber, whereas large amounts of sulfur (621 g/tW) are

absorbed from the flue gas in the second scrubber which uses a

NaOH solution. The waste water from the scrubber system is

externally treated as mentioned above. According to the calculations 14.1 g/tW of sulfur pass the stack to the atmosphere.

The Fig. 4 shows the expected distribution for mercury

arisen from the properties of mercury. The calculations indicate

that the dusty raw gas contains 1.65 g/tW of mercury. In the

incineration process most of the mercury is passed to the flue gas

as HgCl2 and is taken up to 90 % (1,485 g/tW) by coke injected

into the gas upstream the fabric filter. The absorber liquid in the

first scrubber removes 0.138 g/tW of mercury from the flue gas.

Only very small amounts (0,0081 g/tW) of mercury are absorbed

in the second scrubber. As mentioned, the waste water treatment

2 Achternbosch MATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS
and the evaporation of the scrubber effluents is made externally.

Only very low amounts of mercury remain in the clean gas.

Plant B

Plant B is equipped with a more complex wet flue gas cleaning system shown in Fig. 5. For cleaning the flue gases an electrostatic precipitator, a spray dryer, a second electrostatic precipitator, an two-stage scrubber system and a carbon adsorber are

used. The first electrostatic precipitator separates the dust from

the raw gas. The heat content of the raw gas is used to evaporate

the neutralized effluents generated by the scrubber system. The

resulting salt residues are precipitated in the second electrostatic

precipitator. The gaseous acid pollutants are then absorbed in the

two-stage scrubber system. The effluents are fed back to the

spray dryer. After a SCR process the flue gas passes a carbon

adsorber as fine cleaning stage.

The Fig. 6 shows the balance of chlorine in plant B. For

chlorine, the mass flow of the dusty raw gas is again calculated

to 5890 g/tW. The first electrostatic precipitator separates 720

g/tW chloride, bound in the fly ash.

For the removal efficiency it is assumed that the electrostatic precipitator has the same efficiency as a fabric filter, since the

slightly higher efficiency of the fabric filter has no significant

effect on the material flows. The path of the flue gas through the

spray dryer has no influence on the chlorine flow. In total

5156 g/tA chlorine are removed in the two-stage scrubber system.

Due to the evaporation of the scrubber liquids in the spray dryer

the same amount of chlorine is precipitated as residue in the second electrostatic precipitator. The SCR process downstream the

scrubber system has no influence on the chlorine flow. The carbon adsorber removes approximately half of the chlorine flow.

Finally, the clean gas contains only 7 g/tW of chlorine.

Figure 7 shows the material flow of sulfur in plant B.

According to calculations of all model plants the raw gas downstream the boiler contains 1265 g/tW of sulfur. In the first scrubber only 70 g/tW of sulfur is absorbed. The second scrubber takes

MATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS Achternbosch 3

720 4587 569

raw gas

with fly ash

5890

14,1

5170

SNCR

fabric filter

HCl-scrubber

clean gas

residue external treatment

values given

in g/t Abfall

SO2-scrubber

Fig. 2. Chlorine balance of plant A.

560 70 621

raw gas

with fly ash

1265

14,1

705

SNCR

HCl-scrubber

clean gas

residue external treatment

values given

in g/t Abfall

SO2-scrubber

fabric filter

Fig. 3. Sulfur balance of plant A.
up 621 g/tW of sulfur. In total 691 g/tW sulfur arises as residue

after the evaporation of the effluents. The sulfur flow is not

influenced by the SCR stage. According to the model calculation

the carbon adsorber removes approximately 9 g/tW sulfur from

the flue gas. As a result the clean gas contains approximately

5 g/tW of sulfur.

The mercury balance of plant B is shown in Fig. 8.

According to the model calculation the mercury flow in the

dusty raw gas is 1.65 g/tW.

With the electrostatic precipitator 0.06 g/tA mercury bound

in the fly ash are removed from the gas. The spray dryer does

not change the mercury flow. The main amount of mercury is

absorbed in the HCl-scrubber, in which 1.33 g/tW of mercury is

absorbed. The second scrubber takes up only small amounts

(0.08 g/tW) of mercury. The effluents produced in the scrubbers

are neutralized and treated with a precipitating agent before

being piped to the spray dryer. The carbon adsorber reduces the

mercury load down to 0.019 g/tW.

Plant C

Plant C is equipped with the semi-wet process which differs

in construction and in use of auxiliary chemicals from the wet

flue gas cleaning systems discussed above. As shown in Fig. 9

4 Achternbosch MATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS

1,485 0,138 0,008

raw gas

with fly ash

1,65

0,165 0,019

SNCR

fabric filter

HCl-scrubber

clean gas

residue external treatment

values given

in g/t Abfall

SO2-scrubber

Fig. 4. Mercury balance of plant A.

N H 3

adsorber

boiler spray dryer ESP ESP stack

fly ash

coke

residue neutralisation Ca-comp.

scrubber system

residue

Fig. 5. Flue gas cleaning system of plant B.
the flue gas cleaning of plant C consists of a SNCR process and

a spray absorber followed by a fabric filter.

Pollutants are separated from the raw gas in a spray

absorber in which a slurry of calciumhydroxide is injected. The

slurry water evaporates and the calciumhydroxide reacts with the

acid pollutants. For the removal of particularly heavy metals and

PCDD/F coke has to be injected upstream the fabric filter. The

resulting salts, the loaded coke and the fly ash are separated in

the fabric filter. For the reduction of NOx in the flue gas a SNCR

process is applied. The SNCR process has no effect on the material flows of the elements considered.

As in all model plants, the chlorine balance which is shown

in Fig. 10 starts with 5890 g/tW of chlorine in the raw gas. After

the injection of the calciumhydroxide slurry, chlorine is removed

from the flue gas as calciumchloride and as part of the fly ash

respectively. In total 5867 g/tW of chlorine are obtained in the

residue. In the clean gas remains 23.5 g/tW of chlorine.

The distribution of sulfur is shown in Fig. 11. The injection

of the calciumhydroxide slurry causes the precipitation of

1242 g/tW sulfur and 23.5 g/tW of sulfur are emitted into the

atmosphere.

The Fig. 12 shows the material flows of mercury in plant C

which starts with 1.65 g/tW of mainly gaseous mercury in the

dusty raw gas. In the spray absorber the injection of calciumhydroxide causes no significant separation of mercury. The addition

of coke allows the separation 1.63 g/tW of mercury from the flue

gas. This results in a mercury load of 0.019 g/tW in the clean gas.

Plant D

Plant D uses a semi-dry process for the removal of pollutants in the flue gas. The construction of this system is similar to

the semi-wet flue gas cleaning described above. It consists of a

SNCR process, a cooler and a fabric filter.

For the reduction of NOx a SNCR process is used. In the

cooler the flue gas is cooled by water injection to generate optimal conditions for pollutant separation. In a next step, a calcium

compound and coke are injected in form of dry powder into the

conditioned raw gas. The pollutants react with the calcium comMATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS Achternbosch 5

Fig. 6. Chlorine balance of plant B.

Fig. 7. Sulfur balance of plant B.

ESP

HCl-scrubber

SO2-scrubber

SCR

carbon

5890

adsorber

720

5170

5156 569

14.1

7

7.1

clean gas rawgas,

fly ash

fly ash residue residue values given

in g/t waste

spray dryer

ESP

4587

neutralisation

ESP

HCl-scrubber

SO2-scrubber

SCR

carbon

adsorber

1265

560

705

70 621

14.1

9.4

4.7

clean gas raw gas,

 fly ash

fly ash residue residue

values given

 in g/t waste

neutralisation

spray dryer

ESP

691
pound or are adsorbed by the coke. The solid residues are separated in the fabric filter.

The model calculations have been performed in such a manner, that the material flows of chlorine, sulfur and mercury in the

semi-wet process and the semi dry process are the same. But it is

important, that a higher amount of neutralisation agents is

required in the semi dry process to achieve this aim.

As a result, there is no difference in material flows of the

elements considered in plant C and plant D. Therefore the distribution of the elements can be seen in the sankey-diagrams of

plant C, too.

AMOUNTS OF RESIDUE - COMPARISON 

OF THE PLANTS

The amount of residues in the flue gas cleaning is controlled

by the consumption of auxiliary agents for the neutralisation of

acid pollutants and the fly ashes. In this study NaOH and

Ca(OH)2 are used in the wet process and only Ca(OH)2 in the

semi-wet and conditioned semi-dry process. The model calculations in this study assume an amount of fly ashes of 16 kg/tW.

The use of coke in the fabric filter and carbon adsorber as well

as the precipitation agent for the removal of heavy metals in the

scrubber effluents are taken into account.

The consumption of neutralisation agents depends on the

process used for the separation of the acid pollutants. Semi-wet

and semi-dry processes have a need for different stoichiometric

ratios to remove the pollutants. The stoichiometric ratio is the

quotient of chemical equivalent of neutralisation agents to chemical equivalent of acid pollutants. If the stoichiometric ratio

increases, an excess of neutralisation agents is indicated and this

results in an increase of the residues. The Table I compiles the

stoichiometric ratios used in this study and the values listed in

the literature.

The higher emissions calculated for plants C and D can be

avoided by a higher stoichiometric ratio as shown in Table I.

6 Achternbosch MATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS

Fig: 8. Mercury balance of plant B.

E-Filter

HCl-scrubberSO2-scrubber

SCR

carbon

adsorber

1.65

0.06

1.59

1.33

0.083

0.177

0.158

0.019

raw gas clean gas

fly ash

fly ash residue residue

values given

 in g/t waste

neutralisation

spray dryer

ESP

1.413

Fig. 9. Flue gas cleaning system of plant C.
MATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS Achternbosch 7

Fig. 10. Chlorine balance of plant C.

5867

raw gas

with fly ash

5890

23,5

SNCR

fabric filter

clean gas

residue

values given

in g/t waste

spray absorber

Fig. 11. Sulfur balance of plant C. fabric filter

1242

clean gas

raw gas

with fly ash

1265

residue

23,5

values given

in g/t waste SNCR

spray absorber

Fig. 12. Mercury balance of plant C.

1,63

raw gas

with fly ash

1,65

0,019

SNCR

fabric filter

clean gas

residue

values given

in g/t waste

spray absorber
The use of calcium compounds which have an improved activity

are not considered in this study due to relatively high costs.

The effluent evaporation actually lies outside the system

boundary. For comparison, it is assumed that the residues of the

external evaporation are approximately equal to the amount of

the residues of the spray dryer. The Table II compares the total

amounts of residues in all model plants.

INVESTMENT COSTS

The cost analysis was extensive and difficult. The calculation of the specific disposal costs of the model plants does not

make sense, since in real MSWI local waste management and

other conditions generate complex effects on costs.

For this reason only the investment costs of flue gas cleaning stages are taken into consideration without instrumentation

and control and additional costs respectively. For the calculations a plant with two boilers, each one equipped with a flue gas

cleaning system, and a total annual capacity of 200,000 t of

waste is taken into account as example.

In the last few years an investment cost decay of design and

manufacture of apparatus occurred. The investment costs of an

individual flue gas cleaning stage are in the range of $0.25 - $3.5

million related to our example plant. The investment costs of

complete flue gas cleaning plants are calculated by addition of

individual stages. The resulting costs for the flue gas cleaning

plants are in the range of $7 - $15 million.

CONCLUSIONS

The balances calculated for chlorine and sulfur are different

for the considered flue gas cleaning systems. The wet flue gas

cleaning systems with fine purification upstream the stack show

the lowest emissions. In this study, higher emissions were calculated in the case of semi-wet and semi-dry flue gas cleaning

plants. The higher emissions are due to stoichiometric ratios and

other model parameters defined in this study. By use of a higher

stoichiometric ratio the higher emissions can be avoided.

Nevertheless, the emission limits of legal regulations are not

exceeded in all cases.

In contrast, no such dependence on the type of flue gas

cleaning system can be seen for mercury. This fact is objectively

based on the uncertainty about the removal efficiencies of the

flue gas cleaning steps, in particular the fine purification stages.

The amount of residues depends on the stoichiometric ratio

used for separation of the acid pollutants. Other auxiliary agents

do no significantly influence the residue amount. Therefore, the

wet flue gas cleaning systems are distinguished by the lowest

amounts.

The semi-wet and semi-dry systems have the lowest investment costs. Only slight differences in the investment costs exist

between these plants. A wider range of the investment costs is

calculated for wet flue gas cleaning systems. A wet system constructed in a relatively simple manner is only slightly more

expensive than a semi-wet sorption system.

As a result of our study, a wet flue gas cleaning system,

consisting of a fabric filter and a two-stage scrubber-system is an

interesting alternative. This plant generates small amounts of

residues in connection with relatively low investment costs.

REFERENCES

1. Umweltbundesamt, “Thermische, mechanisch-biologische

Behandlungsalgen und Deponien für Siedlungsabfälle in der

Bundesrepublik Deutschland”. draw up by M. BART, B.

JOHNKE, W. BUTZ, 2. Edition 10/1999 (1999).

2. M. ACHTERNBOSCH, U. RICHERS, “Comparison of

Material Flows in Sewage-free ans Sewage-generating Flue

Gas Purification Systems of Municipal Waste Incineration

Plants”, Proceeding of 6th Annual Waste-To-Energy

Conference, May 11-13, 1998, Miami-Beach, FL (1998).

3. M. ACHTERNBOSCH, U. RICHERS, “Vergleich und

Beurteilung von Stoffströmen der abwasserfreien und abwassererzeugenden Verfahren der “nassen”

Rauchgasreinigung von Hausmüllverbrennungsanlagen”

Forschungszentrum Karlsruhe, Wissenschaftliche Berichte,

FZKA 5874, Juli 1997 (1997).

8 Achternbosch MATERIAL FLOWS AND INVESTMENT COSTS OF FLUE GAS CLEANING SYSTEMS

TABLE I

Stoichiometric Ratios

process stoichiometric ratio

used in this study

ranges in literature

wet 1.1 1.1 bis 1.4

semi-wet 2.5 2.2 bis 3.0

semi-dry 2.8 2.4 bis >3

TABLE II

Total Amounts of Residues in all Model Plants

plant residue from

neutralisation

[kg/tW]

fly ash

[kg/tW]

lime/coke

or coke

[kg/tW]

TMT-15

kg/tW]

residue from

external evaporation

[kg/tW]

in total

[kg/tW]

A external 16 1 12.6 29.7

B 12.6 16 1 0.019 29.7

C 22.4 16 1.5 - - 39.9

D 24.5 16 2.2 - - 42.7

TMT-15TM : 15% solution of trimercaptotriazine