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IFRF Combustion Journal Article Number 200303, July 2003 ISSN 1562-479X

Waste Incineration European State of the Art and New Developments Prof. Dr.-Ing. Klaus Göerner University of Essen Institute for Environmental Process Engineering and Plant Design Leimkugelstr. 10 45141 Essen, Germany Corresponding Authors: Prof. Dr.-Ing. Klaus Görner University of Essen Institute for Environmental Process Engineering and Plant Design Leimkugelstr. 10 45141 Essen Germany Tel.: +49 201 183-7511 Fax: +49 201 183-7513 E-mail: [email protected] URL: http://www.luat.uni-essen.de

IFRF Combustion Journal - 2 - Goerner Article No 200303 July 2003

ABSTRACT

In the last decade waste incineration was dominated by environmental requirements and later

by economic constraints. Hard legal requirements led to extreme technical solutions

especially for the flue gas cleaning to reach very low emission values. Typically these values

are lower than for fossil fired plants. 4-staged flue gas cleaning became standard (de-dust, de-

SOx and de-HCl, de-NOx and removal of trace elements) and the whole plant became similar

to a chemical production plant. The costs for incineration increased continuously and reached

values of more than 230 Euro/ton of waste. New legal requirements for landfill in Germany

changed this development within the last 5 years. The content of unburned material was to be

reduced to 3 or 5 % (depending on the category of the landfill), but with a transition time to

2006. Costs of 20 Euro/ton of waste for landfill changed the whole market and waste

incineration plants were forced to look actively for their fuel at a very low price level.

Intermediate over-capacities were the consequence, which led to a straightforward

optimisation of process steps. At the same time the lower calorific value of normal municipal

waste increased with the consequence of increasing combustion temperatures and a dramatic

increase of problems like corrosion, slagging and fouling in the furnace and in the flue gas

pass. Technical answers are water cooled firing grates, modified furnace geometries,

optimised secondary air injections and more suitable wall materials like ceramics or cladding

of normal boiler steels. All these modifications made mathematical modelling and simulation

more attractive even for practical applications. Some examples will demonstrate these new

solutions.

For new fields of research the tendencies will be shown. One complex of these is the dust

particle behaviour as a function of particle composition and gas species concentrations with

respect to slagging, fouling and corrosion behaviour; the other one is the development of new

ceramic materials and the application in furnaces.

Keywords: waste, incineration, grate furnace, emissions, European emissions legislation


TABLE OF CONTENTS ABSTRACT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 TABLE OF CONTENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 WASTE MANAGEMENT LEGISLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

European Community (EC) Developments National Developments (Germany)

STATE OF THE ART FOR MUNICIPAL WASTE INCINERATION. . . . . . . . . . . . . . . . . 7

Grate Firing Systems for Municipal Waste Incineration Furnace Geometries Secondary Air Injection Boiler Tube Lining

PROBLEMS ENCOUNTERED IN ACTUAL INCINERATION PLANTS. . . . . . . . . . . . . 12

Changing Waste Quality Increasing Combustion Temperature Corrosion Slagging and Fouling Primary Pollutant Formation

MATHEMATICAL MODELLING AND SIMULATION (CFD). . . . . . . . . . . . . . . . . . . . . 16

Gas Phase Modelling Modelling of the Waste Combustion Process Particle Behaviour

ACTUAL DEVELOPMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Modifications of the Grate Modifications of the Secondary Air Injection Modification of the Boiler Tube Lining Cladding of the Boiler Tubes

ADVANCED CONTROL BY NEURAL NETS AND FUZZY LOGIC . . . . . . . . . . . . . . . 22

Fuzzy Logic Concept (FL) Artificial Neural Net Concept (NN)

ACTUAL PROJECTS AND RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Influence of the Furnace Geometry Optimisation of the Secondary Air Injection Influence of the Refractory Lining Influence on Corrosion, Slagging and Fouling Operational Optimisation by Fuzzy Control Analysis and Optimisation by Artificial Neural Nets

CONCLUSIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 SYMBOLS / ABBREVIATIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30


INTRODUCTION The amount of municipal waste in the EU countries can be seen from Figure 1 as total value

per country and year and as amount per capita per year [1]. From that figure a very wide

range can be detected for the different countries. The average value for all countries is 500 kg

municipal waste per capita per year.

The distributions of the various waste treatment methods in the EU15 countries differ over a

wide range, too. The incineration or thermal treatment forms a big part of 20 % as an average

value. In Denmark roughly 50 % of the municipal waste is incinerated.

The number of incineration plants with energy recovery is indicated in Table 1 [1].

Table 1: Number of Municipal Waste Incineration Plants with energy recovery in the EU15 countries (1993) [1]

Country Number Capacity [1000 Mg/y]

Austria 2 340 Belgium 24 2240 Germany 49 12020 Denmark 30 2310 Finland 1 70 France 225 11330 Greece 0 - Ireland 0 -

Italy 28 1900 Luxembourg 1 170 Netherlands 10 3150

Portugal 0 Sweden 2115 1860 Spain 315 740

United Kingdom 31 3670

0

5000

10000

15000

20000

25000

30000

Austria

Belgium

German

y

Denmark

Finlan

dFran

ce

Greece

Irelan

dIta

ly

Luxem

bourg

Netherl

ands

Portug

al

Sweden

Spain

United

King

dom

Country

Mun

icip

al w

aste

[k to

n/y]

0

100

200

300

400

500

600

Mun

icip

al w

aste

[kg/

cap

y]

Municipal waste [kton/y] Municipal waste [kg/cap y]

Figure 1: Amount of municipal waste in the EU15 countries [1]


It can be concluded from those figures that incineration is an important factor for waste treat-

ment in all EU15 countries.

Cost structure of waste treatment is very different and depends on the respective nation.

Generally, in Europe the costs for landfill are half the costs for incineration. In Germany one

of the reasons for that fact is the legislation for landfill [3], which allows deposition until

2005 under lower restrictions. Starting in 2005 the requirement for unburned carbon is below

3 % or 5 % (depending on the category of the landfill) as TOC. The costs for incineration are

between 100 and 250 Euro/ton of waste whereas the costs for landfill have a range down to

20 Euro/ton of waste.

The efficiency for incineration plants with electricity production is 20 % (average) at its best

with values of 24 % (peak). The main reason for these low values compared to fossil fired

plants is low to moderate live steam temperatures below 400 to 420 °C. Below 400 °C

corrosion on the fire side of the boiler can be limited.

The overall costs are a sum of fixed and variable costs. In Germany the fixed cost block

cannot be reduced anymore. With respect to the variable costs it is the same situation for the

personal costs and the costs for the operation (rolling stock). So the only way is to increase

the mechanical throughput or the availability of the plant. Throughput has to be reduced

because of increasing lower calorific value LCV of the waste. Consequently, the availability

of the plant is the only real incentive to reduce the operational costs.

Nevertheless, operational problems in incineration plants are the corrosion of boiler tubes, the

life time or resistance behaviour of the ceramic refractory lining, slagging in the furnaces and

slagging or fouling in the radiative and convective parts of the boilers.

All these problems are points to be taken into account for reducing the operational costs for

these plants. Therefore new developments for reducing corrosion by increasing the burnout

and tools like mathematical modelling and simulation to optimise the incinerator operation

are of major interest and are able to contribute to the reduction of the problems mentioned

above.


WASTE MANAGEMENT LEGISLATION European Community (EC) Developments

Nowadays the legislation concerning waste management and treatment is about to be

harmonised in the European Community (EC).

EC waste management legislation is following the Integrated Pollution Prevention and

Control (IPPC) guide lines from 1996 [4]. The handbook on the Implementation of EC

Environmental Legislation [5] is divided into various sections or directives wherein the

following ones are important for the thermal treatment of wastes:

• Framework Directive of Wastes (75/442/EEC),

• Regulation on Shipment of Waste (93/259/EEC),

• Directive on the Landfill of Waste (91/31/EEC),

• Municipal Waste Incineration Directive (89/429/EEC, 89/369/EEC),

• Hazardous Waste Directive (91/689/EEC, 94/31/EEC)

• Hazardous Waste Incineration Directive (94/67/EEC),

• Sewage Sludge Directive (86/278/EEC),

• Waste Oil Directive (75/439/EEC),

• Batteries Directive (91/157/EEC) and

• Packaging Waste Directive (94/62/EEC).

In the following section only municipal waste is covered, not hazardous waste, sewage

sludge, waste oils or others.

It is not easy to really cover all former national laws, decrees or regulations and all different

national interests. For this reason much time is necessary to compromise over a superior

legislation in the EC. At the moment we are in this situation.

Under article 6(1) the proposal for a Council Directive on the incineration of municipal waste

[5] requires a nearly complete combustion to be achieved. “To demonstrate this, ashes and

slags arising from incineration must have a content of total organic carbon of less than 3 %.

In addition gas resulting from incineration is raised to a minimum of 850°C for at least 2

seconds.” These are exactly the same requirements as in Germany under the 17. BImSchV-


legislation [6]. In the EU-proposal a minimum O2-content in the flue gases of 6 % is not

required; however, it is found in the German requirements.

Other emission values of this proposal are indicated in Table 2.

Table 2: Emission values for municipal waste incineration plants [6,8]

Emission limits in mg/m3

species EU-standards 2005 [Ref. 8]

ARN Netherlands (1989) [Ref. 8]

BimSchV Germany (1990 / 1999)

[Ref. 6] dust 10 5 10

TOC 10 10 10

HCl 10 10 10

HF 1 1 1

SO2 50 40 50

NOx 200 / 400 70 200

NH3 10 5 -

CO 50 50 50

Cd+Tl 0.05 0.05 0.05

Hg 0.05 0.05 0.05/0.03

metals 0.5 1 0.5

Dioxins+furans (ng TE/m3) 0.1 0.1 0.1

The application of those values leads to a lower standard in some European countries and to

more restrictive ones in others.

National Developments (Germany)

As pointed out in Ref. [1], the requirements in Germany for gases coming from the

incineration of municipal waste are residence times of 2 seconds at a minimum temperature

level of 850°C and an O2-content of 6 %. Beginning in 2000, more and more incineration

plants have deviated from the 2 seconds towards 0.5 to 1 second. Also the 6 % O2 value for

the flue gases is under pressure to be removed. Under the conditions of 850 °C / 2 sec / 6%

O2 extensive examples are available for the overall operation, emission, residues and

slagging, fouling and corrosion in the flue gas passes.

STATE OF THE ART FOR MUNICIPAL WASTE INCINERATION Grate Firing Systems for Municipal Waste Incineration

Thermal treatment of waste is possible by 1-step-incineration or 2-step-gasification and

combustion, wherein incineration plays the dominant role [9].


Incinerators can be subdivided into grate firing systems, fluidised bed systems (stationary and

circulating FB´s) and fixed bed systems. In this area, grate firing systems are the common

system with a wide distribution and a long-term-experience.

Typical grate firing systems for refuse incineration are (see also Figure 2):

• roller grates,

• reciprocating grates,

• reversed feed grates and

• counter reciprocating grates.

Depending on the grate type, the furnace geometry (see succeeding section – Furnace

Geometries) and the secondary injection concept (see succeeding section – Secondary Air

Injection) have to be adopted and optimised.

Typically, the grate bars are cooled by the primary combustion air. Thus, the degree of free-

dom for the stoichiometry in the primary combustion area (furnace) is reduced. Besides the

air cooling effect is limited for very high calorific values of the refuse and the bars can be

damaged or destroyed by intense heating.

For this reason water cooled bars have been developed and successfully applied in various

plants. In these plants, the primary stoichiometry can be optimised with respect to the burn-

out behaviour or gaseous emissions such as CO and NO. However, greater complexity and a

higher susceptibility to troubles are the main disadvantages.

Figure 2: Grate types (schematic) for municipal waste incineration plants


Figure 3:Furnace

geometries(schematic) for

municipal wasteincineration

plants(a.) Parallel Flow (b.) Counter Flow (c.) Centre Flow

Furnace Geometries

Furnace geometry is one of the main features to influence and to optimise the primary

combustion. The direction of the flue gas flow in comparison to the waste transport on the

grate is one of the most important factors.

Heat (enthalpy) and also unburned gaseous species like CxHy and CO are transported by the

flue gases. The main process steps on the grate are:

• drying of waste,

• devolatilisation,

• combustion and

• after-burning of solid residues (mainly ash and slag).

For the first two processes, heat transfer from the main combustion zone is of major impor-

tance. This transport is supported by a counter flow concept (see Figure 3b). On the other

hand the burnout of the gaseous species is very well done in a parallel flow geometry (see

Figure 3a), where the flue gas is directed through the hottest zone by a distinctive nose.

In the past, 3 standard geometries for municipal waste incineration plants have been estab-

lished (see also Figure 3):

• parallel flow,

• counter flow and

• centre flow.


The main advantages and disadvantages of the different systems are tabulated in Table 3.

Table 3: Advantages and disadvantages of the different furnace geometries

parallel flow counter flow centre flow

pyrolysis gases pass through the hottest area and therefore are burnt out satisfactorily. suitable for lower LCV

energy transfer from main combustion area to the drying and gasification area suitable for higher LCV

very flexible for different heat release distributions on the grate suitable for a wider range of LCV

advantages

energy transfer from main combustion area to ignition area is just by radiation

pyrolysis gases can bypass the hottest area and may cause burnout problems

flow and mixing pattern after the passage to the 1. pass sensible to disturbances

disadvantages

A lot of variants in geometries deviating from the base concepts as discussed above are on the

market. This fact does not so much depend on real process reasons but is due to manufacturer

specific developments or patent situation.

Secondary Air Injection

By splitting up the total combustion air (having an over-stoichiometry of 1.3 to 1.8) to

primary air and secondary air; the combustion conditions in the furnace can be controlled to

give near-stoichiometric conditions. The partitioning ratio of primary to secondary air is

between 80/20 (old plants) to 40/60 (tendency for new plants). The task of secondary air is to

complete the burnout of the hydrocarbons and carbon monoxide. In addition, secondary air

can be used as a mixing device for flue gases. Depending on the different furnace geometries,

the secondary air nozzle arrangement can also be optimised.

(a) Normal Configuration (b) Tangential Configuration (c) Static Mixing Device

Figure 4: Different Secondary Air Injection Concepts


A typical position for secondary air injection is to the transition area between the furnace and

the first pass of the boiler (radiative pass, see Figure 4a). The parameters are the number of

nozzles, the nozzle diameter, the nozzle spacing and the horizontal and/or vertical inclination.

New developments are tangential nozzle arrangements – similar to the firing concepts of

tangentially fired boilers [10], (Figure 4b) or static mixing devices in the flue gas pass [11],

[12] (Figure 4c, see also succeeding sections of this paper – Modification of Secondary Air

Injection and Optimisation of Secondary Air Injection).

Boiler Tube Lining

Normal boiler steel tubes are heavily endangered by thermo-chemical attacks especially in

the high temperature areas of the furnace. For this reason, refractory lining is applied. Typical

lining materials are ceramics like Al2O3, SiO2, SiC and others in different mixtures,

combinations and bonds.

Beside chemical protection, thermal isolation is also an important function. By varying the

coefficient of thermal conductivity or the thickness of the material layer heat extraction from

the furnace can be controlled.

The temperature in the furnace determines the primary production of species like CO, CxHy,

NO and dioxins/furans. On the other hand, residence times in the after-burning zone are

strongly dependent on the temperature levels. These facts are demonstrated in Table 4.

Table 4: Residence times in the after-burning zone as a function of the temperature

residence time flue gas temperature

[°C] after-burning zone [s]

furnace + after-burning zone [s]

> 950 0.3 1.4

> 900 1.25 2.35

> 850 2.15 3.25

> 800 3.3 4.4

So the selection of the lining material is a very important factor for the engineering of the

process.


PROBLEMS ENCOUNTERED IN ACTUAL INCINERATION PLANTS Changing Waste Quality

For operation purposes, LCV of the waste is the most important characteristics considered. It

is a function of the waste composition. During the last decade, due to the different means of

partial selection of waste fractions, the LCV has increased from mean values of about 8,000

kJ/kg to between 9,000 - 11,000 kJ/kg (depending on the country, the catchment area and the

pre-treatment of waste). Figure 5 shows the estimated mean composition for municipal waste

in the EU15 countries.

Other characteristic of the municipal wastes considered are:

• proximate analysis (water, mineral matter, fixed carbon, volatiles),

• elementary analysis (C,H,O,S,N),

• size distributions and others.

The chlorine content are also considered due the important role it plays during the

incineration process. By picking out fractions which are compostible or of inert materials like

metal, glass and ceramics, the absolute mass fraction of chlorine is also increased. This is an

important factor that affects the corrosion process.

The relationship between chlorine and sulphur compounds do not only have an impact on

corrosion, but also of high relevance in the formation/destruction of dioxin/furan. This

relationship is explained by the Deacon (R-1) and Griffin (R-2) equilibrium equations.

4HCl + O2 ⇔ 2 H2O + 2Cl2 (R-1)

SO2 + Cl2 + H2O ⇔ SO3 + HCl (R-2)

metals5%

glass7%

plastics10%

paper20%

textile and others21%

food and garden37%

Figure 5: Estimated meancomposition of the municipal waste

in the EU15 countries [1]


which determines the ratio HCl/Cl2. The Cl/S ratio in waste is different from that of chlorine

and sulphur content of coals, thus dioxins/furans play does not have a significant role in fossil

fuel combustion, but are of relevance for waste incineration.

Increasing Combustion Temperature

An increase in the LCV leads to an increase of the mean furnace temperature. Plants designed

for low LCV´s (parallel flow) have extremely high combustion temperatures, which cause

corrosion, slagging and fouling in the furnace and in the radiative, and convective passes of

the boiler (see succeeding section – Corrosion). Also sulphatisation (transition of chlorines to

sulphates) is decreased by temperatures higher than 800 to 850 °C. As a consequence, these

reactions take place in the dust layer in the boiler and cause corrosion by producing HCl or

Cl2 in the direct vicinity of the metallic boiler tubes.

At high temperatures, dust particles become liquid or partially liquid (sticky). The first effect

is used in slag tap furnaces for low volatile coals to form a molten layer with constant

thickness which protects the wall material. Sticky particles are the dominant problem. Heavy

slagging on the furnace walls leads to operational restrictions. The cleanup of the walls is

sometimes possible by soot blowers (steam or water). Very often a plant shut-down is

necessary to break-up and remove the compact solidified slag layer.

Corrosion

Corrosion in waste incineration plants has two major causes:

• direct chlorine attack and

• attack by molten salts under

dust layers.

Corrosion is a function of

temperature and can be subdivided

into low and high temperature corro-

sion. Corrosion rate as a function of

temperature is shown in Figure 6.

0 100 200 300 400 500 600 700 800

temperature [°C]

0

2

4

6

8

10

rela

tive

corr

osio

n [-]

electro-chem. corrosioncorrosion below layersorrosion in the gas phasesum

Figure 6: Low and high temperature corrosion as a function of temperature [13]


The first cause is the consequence of insufficient sulphatisation in the gas phase at high

temperatures. Subsequently the following reactions take place in the dust layer (examples,

also valid for K and Mg instead of Na) [14]:

2NaCl + SO3 + H2O ⇔ Na2SO4 + 2HCl (R-3)

2NaCl + SO2 + O2 ⇔ Na2SO4 + 2Cl (R-4)

HCl or chlorine in statu nascendi is present and leads to a direct attack.

The composition of waste incineration dust is very complex because of the great variety of

species in the plant input flow. So dust forms a eutectic system with partially very low

melting temperatures of the salts. Table 5 demonstrates this fact.

Figure 5: Ash melting temperatures for different species [14]

Pure species melting temp. °C

Eutectic systems mol-%

melting temp. °C

NaCl 801 25 NaCl – 75 FeCl3 156

KCl 772 55 ZnCl2 – 45 KCl 230

MgCl2 714 60 KCl – 40 FeCl2 355

CaCl2 772 58 NaCl – 42 FeCl2 370

FeCl2 676 90 PbCl2 – 10 MgCl2 460

FeCl3 303

49 NaCl – 51 CaCl2 500

It can be seen that up to the convective pass of the boiler, the melting temperature of particles

or dust is possible and will also take place [15, 16].

A direct attack of CO is of minor relevance, but by the local oxygen or CO concentration

(characterising over- or under-stoichiometric conditions) the phase equilibrium of the slag is

changed as clearly shown in Figure 7.

part

ition

[%]

temperature [°C]

Na SO (s, l)2 4

part

ition

[%]

temperature [°C]

Na SO (s, l)2 4

3 Vol.% O2 1 Vol.% CO

Figure 7: Phase diagrams for slag as a function of O2/CO concentration [17].


Slagging and Fouling

The most helpful criterion to discriminate between slagging and fouling is the possibility of

cleaning. Fouling layers can be cleaned easily whereas slagging forms hard layers with

unequal thickness in the areas of interest.

Slagging and fouling is an interaction of the particle phase (ash, mineral matter, unburned

carbon) with the furnace walls. So, this process can be influenced by the flow pattern in the

continuous phase and the particle behaviour and/or qualities at the time of interaction.

The flow situation in the furnace and the radiative pass are directly influenced by the furnace

geometry (see succeeding section Influence of Furnace Geometries) and the secondary air

injection concept (see succeeding section Optimisation of Secondary Air Injection). The most

effective strategy to avoid particle to wall collisions is an equally distributed flow field in the

radiative pass and the avoidance of recirculation zones.

Particle-to-wall collisions cannot be completely avoided. So, particle composition, particle

temperature and stickiness are the dominant factors influencing slagging and fouling.

Primary Pollutant Formation

Primary pollutants can be subdivided into:

• nitrogen oxides (NOx),

• sulphur oxides (SOx),

• carbon monoxide (CO),

• hydrocarbons (CxHy),

• dioxins / furans,

• HCl, Cl2,

• heavy metals and

• particulate matter (dust).

Primary formation of dust is a function of incoming waste size distribution, the partition of

small particles, the primary air velocity and the type of the grate (intensity of poke).


Sulphur oxide, chlorine and heavy metal concentrations in the raw flue gas are a function of

the input stream and cannot be influenced by primary measures (dry additive sorption is not

really a primary measure).

Nitrogen oxides, carbon monoxide and dioxins / furans can be influenced to some extent by

oxygen concentration, residence time, temperature and mixing / turbulence (3T-rule: time-

temperature-turbulence). Increasing values of all 4 qualities reduce the CO concentration, but

increase the NOx. For minimising dioxins / furans oxygen concentration should be low and

temperature and residence time high.

Typical values for primary emissions (exit of furnace) are:

• NOx 200 to 400 mg/m3N

• dioxins / furans 0.2 to 0.6 ng TE/m3N.

NOx formation can be primarily influenced by air staging (primary to secondary air) or by

selective non-catalytic reduction (SNCR) injecting ammonia or urea.

by “DE NOVO” synthesis the values for dioxins / furans are increased respectively

depending on the flue gas cleaning concept. The main influencing factors are oxygen

concentration, dust (particle) concentration, concentration of catalysts (compounds of metals

or heavy metals), temperature and residence time. In the flue gas pass, the main place of

dioxin / furan synthesis (DE NOVO) is in the bag filter or in the electrostatic precipitator.

Cleaning devices for NOx and dioxins / furans must be implemented to reach the allowed

emission values. Catalysts are used for NOx (reduction) and for dioxins / furans (oxidation).

Adsorption with coal char (brown and hard coal) is also very common, but needs waste

management for this medium.

MATHEMATICAL MODELLING AND SIMULATIONS Gas Phase Modelling

Mathematical modelling and numerical simulation of fossil fired systems have reached a high

level and are used in a wide range of applications in research and development [18]. This is


valid for gas, oil and coal dust firing, but not for fixed beds or fluidised beds because of the

problems in describing the solid phase (dispersed phase) and its interaction with the gas phase

(continuous phase). Therefore for municipal waste incineration plants the description of the

furnace and the radiative and convective parts of the boiler is possible. The boundary

conditions at the fixed bed surface are problematic because of the variability of the waste

properties and the chemical and thermal interaction between the primary combustion air and

the waste. Model developments specially take place in this area and are described to some

extent in the succeeding section (Modelling of the Waste Combustion Process) [19, 20, 21].

Modellin of the Waste Combustion Process

Fixed bed modelling has been carried out by different groups [19, 23, 24, 25]. The main

difference in the models is the level of approximation:

• zero-dimension model (integral assumption for the heat release and the released

species)

• one-dimensional modelling (heat release and species concentrations profiles over the

grate length),

• two-dimensional modelling (also estimated profiles in crosswise direction),

• two-dimensional modelling (also burnout progress in the vertical direction, that is in

the fixed bed depth) and

• fully three-dimensional modelling.

Crosswise direction profiles are a consequence of unequally distributed waste mass flow or

waste qualities. In general this information is an empirical input and does not lead to a model

of universal validity.

Vertical burnout progress is described by Swithenbank [24], but needs data on waste trans-

port, waste qualities and additional assumptions on the combustion behaviour in the fixed

bed.

Assuming one-dimensional heat release, species concentration profiles can take into account

empirically the grate system in combination with furnace geometry, position of fire (main

combustion zone), waste quality (LCV, water content, volatile content and others). Results

shown in the succeeding section (Actual Projects and Results) are based on this approach.


A fully three-dimensional modelling is strongly dependent on a large number of waste

qualities and empirical information.

Particle Behaviour

Local particle concentration can be modelled in the Eulerian (continuous phase) and

Lagrangian (discrete phase) framework.

Euler-Euler-description reduces the numerical effort, but is reduced in significance and

accuracy related to thermo-chemical behaviour.

Euler-Lagrange-description can generally take into account:

• turbulent interaction between particulate and gaseous phase by Monte-Carlo-

modelling [18],

• equilibrium assumptions for the particle and gas phase by thermo-chemical modelling

[17],

• kinetically dominated reactions in the gas and the particle phase,

• particle size distribution depending qualities like emission coefficients, pore size

distributions and/or turbulent interaction to the gas phase,

A very high numerical effort is the major disadvantage of this method. Typical results are

shown in the succeeding section (Actual Projects and Results).

ACTUAL DEVELOPMENTS Modifications of the Grate

The main disadvantage of an air cooled combustion grate is that the primary air has the role

of both the combustion air and the cooling air. So, both functions cannot be decoupled. By

applying water cooled grates, the combustion air can be reduced which is advantageous to the

primary NO production. The thermal load of the grate can be heavily increased. Furthermore,

the furnace temperature is reduced by extracting heat from this area. A typical air- and water-

cooled grate and the cooling circuit can be seen in Figure 8.


A small disadvantage of a water cooled grate is a slightly more complex system which can

lead to problems of availability. The water cooling system is integrated into the air preheating

for heat recovery reasons.

Modifications of the Secondary Air Injection

In addition to the measures of optimising the secondary air injection described in the previous

sections (State of the Art… Secondary Air Injection and Mathermatical Modelling and

Simulation) the following methods are partially in use (see also Figure 9):

• rotating cylinder (with additional secondary air nozzles),

• fixed prism (with additional secondary air nozzles) and

• steam injection (instead of secondary air injection).

The rotating cylinder (see Figure 9) is problematic in some constructive details and in

operation. The prism is implemented in the municipal waste incineration plant in Bonn,

Germany [23, 27, 28] and shows very good operation results such as a higher burnout of

hydrocarbons and CO and a lower fouling tendency in the radiative and convective passes of

the boiler. Steam instead of air injection at the secondary air ports increases mixing and

therefore the burnout behaviour, but is more expensive in operational costs.

Figure 8: Air- and Water- cooled grate systems [26]


From these three solutions the prism has the highest potentials for bigger plants with a large

cross section (large penetration depth) at the secondary injection level.

Modification of the Boiler Tube Lining

In Figure 10, the different linings for boiler tubes are indicated. The following arrangements

are possible:

• finned tubes with a dust layer (15Mo3 plus layer of fouling and/or slagging layer;

Figure 10-a),

• finned tubes with cladding layer (building-up welding by Inconel 625) – Figure 10-b,

• finned tubes with castable refractory (metallic pins with ceramic cups for mechanical

form-closing, Figure 10-c),

• finned tubes with castable refractory (metallic foot lugs for mechanical form-closing,

Figure 10-d),

• finned tubes with luted ceramic plates (Figure 10-e) and

• finned tubes with air cooled ceramic plates, partially luted (Figure 10-f).

Depending on the desired heat extraction from the furnace and the heat transfer in the

radiative part (lower section) the refractory material (thermal heat conductivity) and thickness

can be adopted.

Generally, plates have a denser structure in comparison to castable refractory linings and

therefore have a higher diffusion and, in consequence, a higher chemical resistance.

Overall view of the furnace and the transition area

(a.) Rotating Cylinderand Air

(b.) Prism and Air (c.) Steam Injection instead of Air

Figure 9: Advanced Secondary Air Injection Concepts


The thermal heat conductivity of different materials can be taken from Table 6.

Table 6: Thermal heat conductivity of different refractory linings

refractory material thermal heat conductivity [W/(mK)] at 800 °C

Refrax 20 plates 20

SiC 70 refractory castable 4 - 6

SiC 90 refractory castable 6.3 – 7.5

Sillimanit (Andalusit) plates 1.74

compact refractory castable 1.83

For practical calculation the thermal heat conductivity of the dust layer as well as its emission

coefficient are of importance. From those values, the radiational interaction between the wall

surface and the gas phase can be calculated. This gives a realistic surface temperature

assumption, which is important for the calculation of the gas phase temperature as well as for

the particle (dust) layer temperature and, in consequence, for its behaviour (stickiness).

Cladding of the Boiler Tubes

Cladding is a building-up or plating welding of the boiler tubes by nickel based alloys. Thus,

the mechanical and chemical qualities are shared. The tube material provides the mechanical

strength whereas the welded covering layer ensures the chemical resistance.

Figure 10: Different boiler tube lining (refractory linings)


To protect the metallic pipe material (typically 15Mo3) cladding is applied below the ceramic

refractory lining and in the area behind the refractory lining (metallic pipe without lining) in

the flue gas pass.

Typical welding materials can be specified as indicated in Table 7.

Table 7: Welding materials for cladding purposes [32]

composition welding material

element partition [%]

NiCr625 2.4831 Inconel 625 C < 0.03

Cr 22

Mo 9

Fe < 3

Nb 3,5

Ni rest

The most important value for resistance to corrosion is the Fe content. In the welded layer it

should be lower than 5% in any case, but the lower the better.

Fe concentration is also dependent on the welding parameters such as welding type (MIG or

MAG), welding velocity, absolute welding-current and frequency (temperature of the fused

mass), direction of welding (vertical or horizontal, upward or downward) and others. A

typical plating coat thickness is 2 mm.

In practical experiences very good results have been achieved with this method. The costs are

relatively high (2,500 Euro/m3), but give a longer operation period.

ADVANCED CONTROL BY NUERAL NETS AND FUZZY LOGIC Fuzzy Logic Concept (FL)

Mass flow, calorific value and all mechanical qualities of municipal waste fluctuate over a

wide range. For example the mean lower calorific value can be calculated by an overall

balance over the plant, defining waste as thermal input and the live steam as output. In

systems engineering such boundary conditions are described as fuzzy. The fuzzy logic theory

of Zadeh [33] can be applied to such systems very successfully. The general procedure in


fuzzy control can be demonstrated by Figure 11. After a step of fuzzification the rules

describing the behaviour of the system are implemented. The final step is defuzzification.

Beside the formal process of fuzzification and defuzzification the definition and

implementation of the “rules” is the major task.

For municipal waste incineration plants in Germany fuzzy logic has been applied with

success [34, 33] by defining up to 5,000 rules. The rules have been adjusted by interviewing

the operational staff. The rules are characterized in a “if …then …” structure, for example, “if

the oxygen concentration at the end of the boiler is too low, increase the secondary air mass

flow”. The rules describe to some extent the expert knowledge of the staff.

Some results from the optimisation of a municipal waste incineration plant are demonstrated

in the succeeding section (Influence on Corrosion, Slagging and Fouling).

Artificial Neural Net Theory (NN)

Neural Nets are able to learn the behaviour of a plant by looking at the process in- and

outputs. The transfer function can be approximated to some extent by this training process

without introducing special knowledge on the process [36, 37]. After the training period the

neural net has the “same” behaviour as the real process. This quality can be used for observer

control purposes. By inverting the model an optimisation is possible by asking the model

“what are the input variables to a special combination of output variables”.

The general structure of an neural net is shown in Figure 12. In the training phase the internal

parameters such as weighting factors between the neurones and the output (response)

function will be adapted by generic algorithms.

inpu

t e

0e rule 1

rule 2

rule n

inte

rfere

nce

.

.

.

outp

ut u

= f(

e)

Figure 11: General procedure of fuzzy control concepts.


Some results from the analysis (identification) of a municipal waste incineration plant are de-

monstrated in the succeeding section (Operational Optimisation by Fuzzy Control).

ACTUAL PROJECTS AND RESULTS Influence of the Furnace Geometry

In a simulation study the general behaviour of the different furnace geometries (see previous

section – Furnace Geometries) has been investigated for a virtual plant. The mathematical

model itself was validated by data from real plants [19, 20, 23]. For example, the temperature

distributions in vertical cross sections are indicated and show significant differences (Figure

13). This should not be interpreted as an absolute result and as a preference for one concept

but should demonstrate the characteristics of the system and the simulation possibilities.

inpu

t e

outp

ut u

= f(

e)

neural net

......

......

. . .

. . .

. . .

. . .

Figure 12: General structure of a nueral net

1300oC

0oC

(a.) Parallel Flow (b.) Counter Flow (c.) Centre Flow

Figure 13: Vertical temperature distribution for the different furnace geometries [27]


Of course the oxygen distribution at the inlet of the after-burning zone is also a function of

the heat release profile along the grate and therefore a function of the waste quality. The

problem of changing the waste quality must be solved by an intelligent control concept.

Optimisation of the Secondary Air Injection

Just as for the furnace geometry various secondary

injection concepts have been analysed:

• “normal” injection at the transition from furnace

to after-burning zone (see Figure 14),

• tangential injection and

• static mixing device (prism).

The results for the vertical oxygen distribution are

shown in Figure 15.

Normal centre flow geometry shows a mal-distribution at the entrance to the after-burning

zone. The prism acts as a static mixing device and increases the mixing. The tangential

injection leads to a better mixing.

Figure 14: Details for theinjection of secondary air by 4air ports (conventional way)

23% O2

0% O2 (a.) Normal Geometry (b.) with prism (c.) tangential injection

Figure 15: Vertical Oxygen Distribution for different air injection concepts (centre flow concept) [27]


Influence of the Refractory Lining

Refractory lining and dust layers on the finned tubes can dramatically influence the

temperature distribution in the flue gases. This is a direct consequence of the furnace heat

extraction and the modified reaction rates. Figure 16 gives an impression of these dramatic

changes.

Standard castable SiC material is used as the reference case. Compared to the luted plates

their insulation effectiveness is relatively high.

Influences on Corrosion, Slagging and Fouling

The local CO concentrations near the furnace walls (flue gas side) can be used for some

preliminary statements on the direct corrosion and the equilibrium between sulphates and

chlorines in the dust layer. Therefore these distributions have been calculated and plotted in

Figure 17. As expected, the highest CO concentrations can be seen in the main combustion

area.

The reduction of CO by its reaction with secondary air is a function of the injection concept.

In general, a very fast reduction can be achieved by the counter flow and the centre flow

concept.

1300oC

0oC (a.) Castable SiC (b.) luted SiC-plates (c.) air cooled SiC plates

+ dust

Figure 16: Vertical temperature distributions for different refractory linings (conventional way) [27]


Operational Optimisation by Fuzzy Control (FC)

The daily operation of a municipal waste incineration plant is dominated by the fluctuating

quality of waste. Depending on that fact the flue gas emissions, carbon burnout in the ash,

position of the main combustion zone and others must be controlled within a narrow range.

The fuzzy control concept is based on a large variety of process variables with different

sensitivities on the output variables. One of these is the live steam mass flow. Figure 18

shows the ranking of the sensitivities of 6 variables (grey boxes) with respect to the live

steam mass flow.

As a result controllability has been increased in a special plant by implementing a set of 5000

rules.

0 mg/m3 Parallel Flow Counter Flow Centre Flow Centre Flow w/ Prism

> 5000 mg/m3

Figure 17: CO distribution near the walls for the different furnace geometry concepts [27]

ranking of variables with fuzzy control oxygen (end of boiler) temperature (end of after burning zone) feed rate (waste) primary air roller 3 secondary air primary air roller 2

ranking of variables without fuzzy controltemperature (end of after burning zone) primary air roller 2 primary air roller 3 oxygen (end of boiler) feed rate (waste) secondary air

Figure 18: Ranking in the sensitivity of process variables with and without fuzzy control with respect to the live steam mass flow [34] (bars and legend in the same sequence).


Analysis and Optimisation by Artificial Neural Nets (NN)

Implementing fuzzy rules for control purposes needs the interviewing of the operational staff

to deduce the rules. With neural nets one tries to deduce the complete set of rules or the real

process behaviour by training the neural net with measured input and output variables. This

has also been done for municipal waste incineration plants [39].

Figure 19 gives an impression of the

input and output variables for the model

of a tail gas incinerator.

The result of the training process is

shown in Figure 20 for NOx model as

an accuracy plot.

On this basis the optimisation of the process can be realised. The inversion of the trained

artificial neural net leads to a set of input variables belonging to the variables required. This

is not a real mathematical inversion but the result of a systematic variation of the input

variables.

tailgasnatural gas

primary air on tailgasprimary air on natural gas NN NOX-emissions

secondary air Modeltertiary air

flue gas recirc. on tailgasflue gas recirc. on natural gas

Figure 19: Input and output variables for the NOx modelling.

400

450

500

550

600

650

700

750

400 450 500 550 600 650 700 750

measured NOX [mg/m3]

calc

ulat

ed N

OX

[mg/

m3 ]

ideal model

R2=0,865

Figure 20: Accuracy for the NO-model as calculated against measured NOx

Figure 21: “Suggestion” of the primary air flow to reduce NO [40]

3 /h]

air f

low

[m3 /h

]

time [min]

primary airbefore optimisation

primary airafter optimisation


For a tail gas after-burning unit, the emission of NO was reduced by applying neural nets

[40]. As an example a “suggestion” of the model is shown in Figure 21, where the primary air

should be reduced and the tertiary air be increased (reduction of the primary stoichiometry).

By applying neural nets for this plant the NOx emissions could be reduced by 5 % without

changing any hardware of the plant.

CONCLUSIONS Municipal waste incineration is an important factor in the EC economy. Environmental as

well as economic constraints govern the plant operation. Harmonisation in the EC and

liberalisation of the common market especially with respect to electrical generation (as low as

2 Euro Cents/kWh) put pressure on the costs.

On the basis of a conventional municipal waste incineration plant with a grate firing system

some new technical developments and new materials or material applications have been

demonstrated.

Mathematical modelling and simulations (CFD) form a helpful tool to optimise plant

operation. Not all relevant phenomena are understood and described in full detail, but CFD-

calculations give an impression of reciprocal relationships.

New tools like fuzzy logic (FL) or artificial neural nets (NN) are front-line technologies

leading to a useful application in practice.

SYMBOLS / ABBREVIATIONS

CFD computational fluid dynamics

EC European Community

EU15 European Union member countries

FB fluidised bed

FC fuzzy control

FG flue gas

FL fuzzy logic

LCV lower calorific value


MAG metal-argon-welding

MIG metal-inert-gas-welding

NN (artificial) neural nets

PA primary air

SA secondary air

TOC total oxidisable carbon

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