1

Design and Operation of a Modular Plant for Biomass Usage and Thermal Treatment of Wastes

Goerner, K.*°, Keldenich, K.**, Klasen, Th.*, Gillmann, P.* * University of Duisburg-Essen, LUAT, 45141 Essen, Germany

** Fraunhofer Institute UMSICHT, 46047 Oberhausen, Germany ° Correspondent author: Leimkugelstr. 10, 4514 Essen, Germany, Tel.: +49 201 183-7510,

e-mail: klaus.goerner@uni-duisburg-essen.de

Paper prepared for: Power-Gen Europe 2004 – Barcelona, Spain – 25-27 May 2004

CONTENT

1 ABSTRACT 2

2 INTRODUCTION 3

3 DESCRIPTION OF THE MARS® PLANT 4 3.1 Over all scheme of the plant 4 3.2 Technical Concept of the Combustion Unit 5 3.3 Flue Gas Cleaning 7 3.4 Analysers and Process Instrumentation 8 3.5 Concept for Modular Operation Plants 8

4 EXPERIMENTAL WORK 10 4.1 Investigated Fuels 10 4.2 Results for a Special Waste Fraction from Industry 10

5 MATHEMATICAL MODELLING AND SIMULATION 13 5.1 Gas Phase Modelling 13 5.2 Modelling of the Waste Combustion Process 13 5.3 Description of the Plant and the Simulated Operation Case 14 5.4 Simulation Results 14 5.5 Particle Behaviour 17

6 CONCLUSIONS 18

7 SYMBOLS / ABBREVIATIONS 19 6

8 REFERENCES 19

2

1 ABSTRACT The decentralised usage of biomass in small units is very common. For the thermal treatment

of wastes (municipal wastes, hazardous wastes and special fractions of wastes) normally

large centralised units are the standard solution in Europe. In this project a modular concept

for the complete spectrum of fuels, as indicated above, was developed. The corresponding

test rig consists of a water cooled grate firing system, a steam generator and a complete flue

gas cleaning system. Full scale plants based on this concept will have a thermal load of 5, 10

and 15 MW. The water cooled grate firing concept allows the combustion of fuels with a

large variety of lower calorific values (LCV) between 6-8 and up to 18-20 MJ/kg. The post

combustion chamber is designed for minimum temperatures of 850 °C and a residence time

of 2 s. Thus, also very restrictive emission limits for carbon monoxide and hydrocarbons can

be achieved by primary measures. The flue gas cleaning in the pilot and demonstration plant

is very flexible and consists of a cyclone, bag filter, catalytic/adsorptive mercury removal and

a final adsorption stage. Depending on the fuel and the requirements to the emission limits,

the flue gas cleaning can be adapted. The modular concept guarantees low costs and small

commissioning time periods.

The pilot and demonstration plant is localised at the University of Duisburg-Essen in Essen

and is operated by the chair of environmental process engineering and plant design (LUAT).

The Fraunhofer Institute UMSICHT coordinates the whole project and the commercial

marketing.

For different fuels detailed experiments have been carried out in the meantime. Some

characteristic results will be presented.

Parallel to the experimental investigations for different fuels and process variables the

LUAT-Institute develops mathematical models for thermal treatment processes and carries

out simulation calculations. Thus, the conditions in the pilot plant can be analysed and

optimised (first aim) and also predictions and optimisations for full scale plants can be

achieved (second aim). The first intention is very important for the model validation. The

second allows a design of a plant which is tailor-made for a special fuel under specific

requirements (technical or legal).

In this paper experimental results as well as simulations will be presented.

3

2 INTRODUCTION

For a thermal treatment of municipal wastes large centralised incineration plants are widely

used in Germany. In general, the used technique is a grate firing system [1-4].

In the last years CO2-reduction came more and more into the focus of interest. Therefore the

German legislation guarantees a revenue to the electricity production for small and medium

sized plants if they use biomass as a fuel. In the meantime, the costs for the fuel (wood etc.)

have slightly increased. So it can be foreseen that a larger amount of these plants will be

retrofitted to other fuels or wastes. For this reason, a high flexibility with respect to the fuel

quality spectrum (calorific value, harmful substances and others) is expected for these plants.

On the other hand, smaller units always have higher specific investment and operation costs in

relation to large units. This implies the need for the development of a plant concept with low

specific investment costs.

Therefor, a plant concept had been developed to fulfill all these conditions. The main

characteristic items are:

• Modularisation of the various components

• Standardisation in the thermal loads (5,10 or 15 MW)

• Water-cooled grate firing system for the range of lower calorific values (LCV)

from 6 to 20 MW/kg

• High burn-out for the ashes

• Flexibility in the flue gas cleaning depending on the fuel

This plant concept is called MARS®, which means Modular Plant for an Optimised

Treatment with Respect to Residues.

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3 DESCRIPTION OF THE MARS® - PLANT

3.1 Over all scheme of the plant

The pilot installation is a plant consisting of two main parts with the following components:

• combustion-unit (10-staged grate, post-combustion chamber, exhaust gas cooler)

• gas cleaning unit applicable at different temperature ranges.

With the pilot plant, the following trials are in the main focus:

• combustion of solid matter (domestic waste/garbage, biomass fuel, production

residues) with low pollutant emission

• inertisation of solid matter and creation of utilisable residues

• analysis concerning afterburning of waste gases from the grate firing

• generation of flue gases with specific pollution loads, passing subsequently arranged

flue gas cleaning units allowing specific tests concerning cleaning by installed

components or by the integration of new separators.

Figure 1 surveys the main components of the plant.

Fig. 1: Flow Chart of the Test Facility

In the following sections some more informations are given for these components.

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3.2 Technical Conception of the Combustion Unit Air cooled combustion grates have the main disadvantage that the primary air is combustion

and also cooling air. So, both functions can not be decoupled. By applying water cooled

grates, the combustion air can be reduced which is advantageous to lower the primary NO

production. The thermal load of the grate can be increased heavily. Also the furnace

temperature is reduced by extracting heat from this area. A typical air and a water cooled

grate and the cooling circuit can be seen from fig. 2.

air support

fixedrow

movablerow

fixedrow

primary air

air cooled system

lifting movement

water cooled system of the air cooled system

Fig. 2: Air and water cooled grate systems [5,6]

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.

The water-cooled grate firing of the MARS® plant has been dimensioned for a thermal output

below one megawatt. The lower calorific value (LCV) of the used fuels can vary between 7

MJ/kg (e.g. domestic refuse) and 24 MJ/kg (e.g. BRAM1), bearing an average value of about

12 MJ/kg in mind at dimensioning. Full load combustion air rate is about 1600 m3/h, flue gas

1 BRAM: Brennstoff aus Müll, i.e. fuel made from waste

position "1"

position "2"

position "3"

200

200

hight of lift = 400

distributor

cooling water"in" cooling water

"out"

collector

6

rates vary between 3500 and 6000 m3/h, depending on operation and the fuel used. The grate

is 4.3 m long and 1.19 m wide resulting in a total area of 5.11 m2.

The water-cooled combustion grate including feed hopper and hydraulic slide-supply requires

an outer floor space of 7.5 m x 2.5 m and has an overall height of 5.5 m. The grate was

conceived and constructed to be containerisable. The dimensions of the container frame

amount to 8.4 m x 2.5 m with the ash pit and the ash conveyor placed underneath the

framework construction. Fig. 3 shows the structural steel work of the grate in its early stages.

Fig. 3: Structural steel work of the reciprocating grate

Four air supply zones are uniformly distributed over the length of the grate, i.e. the primary

air supply can be adjusted to the particular needs of the combustion. The refractory-lined

waste hopper is provided with a fire extinguishing device and a hydraulic flap for the

prevention of leak air intrusion resp. against leakage of flue gas. For the fuel supply onto the

grate a hydraulic slide-supply is used. The fuel is conveyed to the first grate-segment by slow

strokes of the slide.

The characteristic feature of the reciprocating grate is the water cooling of the grate seg-

ments. By the use of internal cooling for the iron casting segments a very good heat removal

and a long service life is achieved. The cooling circuit of the whole grate is filled with

deionised water (demineralised water), which is cooled down by a cooler (The supplied air

acts only as combustion air, but not as cooling air).

7

A light oil fired burner that acts as start up burner resp. auxiliary burner is mounted at the

front side of the grate firing above the slag hopper.

The flue gas is guided in co-current flow with the combustible. At the end of the combustion

chamber, it is directed upwards into the post-combustion chamber. The ash from the grate is

thrown off into the already mentioned ash conveyor (water quench with chain conveyor) and

then passed into a container. In the lined post-combustion chamber - after secondary air

supply - the flue gas reaches a residence time of more than two seconds at a minimum

temperature of 850°C.

3.3 Flue Gas Cleaning

After leaving the post-combustion chamber the flue gas can be cooled down - if required - to

a temperature of approx. 450 °C to 550 °C by use of an air injection fan. Afterwards, the flue

gases are adjusted to approx. 180 °C by means of water spraying (quench) and evaporation.

The water quenching offers primarily a suppression of the de novo synthesis, which occurs

intensely at a temperature window between 250°C to 400°C, secondly the quenching allows

to decrease thermal stress in the succeeding machinery. Further, the agglomeration of the flue

dust is stimulated, i.e. the particles tend to become sticky because of the increase of air

humidity. This condition persists even after evaporation of the fine water droplets in the

quench unit, the removal of the enlarged dust-particles becomes easier. The water to be

atomised can be mixed with additives, the effects of which on the flue gas cleaning are to be

tested. The cooler is, just as the other flue gas cleaning units, constructed for a maximum

throughput of 12,000 m3/h at working conditions.

In the first stage of flue gas cleaning a cyclone is used for precipitation of rough dust

particles. Fine dust is being detained by the succeeding dust filter. It is equipped with 24

Kevlar®-Nomex® filter cartridges and has a whole filtering surface area of 492 m2. By this,

the separation of particulate matter from the flue gas is mainly finished.

The last two steps of the flue gas cleaning are responsible for the catalytic conversion resp.

the destruction of gaseous pollutants. The catalytic converter (Amalgator®) is equipped with

ozonisers, which generate ozone for advanced catalytic reactions. Finally, remaining heavy

metals and gaseous pollutants are removed in the adsorber unit. The adsorber acts as a so-

called “police filter”. After gas cleaning, the flue gas is transported to the chimney by an

induced draught fan.

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3.4 Analysers and Process Instrumentation

Two Fourier Transform Infrared spectrometers (FTIR), each with an additional flame

ionization detector (FID), for continuous flue gas analysis are connected to the plant. For the

measuring- and test-gas-supply of the FTIRs two gas batteries and a gas distribution device

are available. The gas samples, which are taken from the pilot plant and passed through

teflon® tubes and the gas distribution device to the FTIRs, have to be heated to avoid

condensation of acid and water. For this purpose, the delivery tubes are wrapped with electric

heating and the gas distribution device is placed in a heatable cabinet. The two FTIRs and the

FIDs allow a continuous registration of the following flue gas components: HCl, CO, CO2,

SO2, NO, NO2, NH3, N2O, HF, O2, H2O and Ctotal.

To quantify the BET-surface area of slag, ashes and additives, an AREA-meter® is provided.

For the determination of the calorific value a bomb calorimeter is available. For the recording

of measured values (flue gas composition, temperatures, …), an efficient computer with

auxiliary peripheral equipment is utilised. The complete control of the pilot plant is handled

by a PC-system (process control system, type “Siemens Simatic S7®”).

3.5 Concept for Modular Operation Plants

The basic concept for a future operation plant is a modular construction of the main operation

units like:

• grate firing system,

• furnace,

• post combustion chamber

• heat extraction (boiler),

• dust separator and

• further flue gas cleaning units.

First of all, the flue gas cleaning units are mainly depending on the fuel used and the allowed

emission values related to the local requirements and needs.

The heat extraction can be realised via boiler (water-steam circuit) or by a thermal oil circuit.

The last one has the advantage of a further possible integration of an ORC-process (Organic

Rankine Cycle).

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Manufacturing and assembling can be arranged according to the related boundary conditions

and the demands of the final client.

Fig. 4 gives an impression of unit arrangement.

Fig. 4: Concept for the modular operation plant with 5, 10, 15 MWth

fuel storage

grate firing unit

boiler resp. heat extraction

flue gas cleaning

fuel / waste

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4 EXPERIMENTAL WORK 4.1 Investigated Fuels In the last months the following fuels/wastes have been investigated in the MARS® plant:

• wood pellets

• wood chips

• scrap of paper + plastic scraps in different mixture fractions

• commercial wastes

• refuses similar to municipal waste

• residual materials

• residual wastes

Most of these fuels are the products of special processes or are products or residues of

material recycling plants in Germany, where this recycling is governed by national

legislation.

4.2 Results for a Special Waste Fraction from Industry

For a special waste fraction (LCV=16,000 kJ/kg) some typical results are shown in the

following figures. The aim of these trials was to determine the combustion behaviour and the

primary emissions from the combustion chamber.

Fig. 5: Temperature distribution over one day.

simulated operation case

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Fig. 5 shows the temperature transients for a typical day with fluctuating LCV and com-

bustion behaviour. It also demonstrates a typical daily operation mode for a small

decentralised plant fired by light fuel oil and with a heating period over night.

Fig. 6: Oxygen concentration over one day.

Fig. 7: Carbon monoxide concentration over one day.

simulated operation case

simulated operation case

12

The marked periods are typical for the combustion of this fuel and form the basis for the

simulation in section 5.

Typical values for unburnt material in the ash (TOC) are below 5 %. Flue gas concentrations

after the primary combustion were:

• SO2 and NOx below 200 mg/m3 and

• HCl between 1,000 and 1,500 mg/m3.

These values are just a rough indicator for the actual situation because of the fluctuating input

contents of the relevant species.

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5 MATHEMATICAL MODELLING AND SIMULATION

5.1 Gas Phase Modelling

Mathematical modelling and numerical simulation of an industrial burner (gas, oil or coal)

has been subject of several studies [7, 8]. The description of fixed or fluidised beds is difficult

because of the fuzzy character of the waste properties (solid or dispersed phase) and its

interaction with the gas phase (continuous phase). But it is possible to describe the furnace

and the radiational and convective parts of the municipal waste incinerator. The boundary

conditions at the fixed bed surface can be calculated in dependence on the mass flow rate of

the solid fuel (waste and biomass), the flow rate of the primary combustion air and the

properties of the solid fuel, e.g. composition and LCV [9, 10].

5.2 Modelling of the Waste Combustion Process

Submodels for the heterogeneous combustion of solid fuels has been developed by different

groups [11 - 14]. The main difference in the models is the stage 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

(can take into account empirically the grate system in combination to furnace

geometry, position of fire (main combustion zone, fig.7) and waste quality,

e.g. developed by Klasen [8], Krüll [15], Choi [12]),

- two-dimensional modelling:

also estimated profiles in crosswise direction,

- two-dimensional modelling:

also burnout progress in the vertical direction, which means in the fixed bed

depth (is described by Swithenbank [13], but needs data on waste transport,

waste qualities and additional assumptions on the combustion behaviour in the

fixed bed) and

- fully three-dimensional modelling.

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fixed bed

main combustion zone

drying zoneafter combustion zone

c)

Fig. 8 Heat release profile over the grate length

5.3 Description of the plant and the simulated operation case

The geometry of the furnace and the boundary conditions for the calculated operation case are

presented in fig. 9.

Fig. 9 a) Furnace geometry and boundary conditions of the calculated case

b) Secondary air nozzles (d = 40 mm), ratio: 50%/50%

c) Places of temperature measurement

5.4 Simulation Results

The simulation results for the mentioned case (PA/SA: 58/52%, SA/SA: 50/50%, LCV: 13

MJ/kg, thermal input: about 1 MW) are displayed in fig. 9. The temperature distribution is

very homogeneous in the vertical plane (fig. 9a), especially after the last secondary air

injection. Temperature values over 850°C can be recognized in the whole after combustion

chamber. The O2 profile looks also equally distributed. The concentrations are nearly

a) b)

15

b) Oxygen [Vol.-%] Temperature [°C]

c)

0

CO [Mass-%] d) Path Lines

constant in 1st path (fig. 10b). This indicates that the oxidation of the incomplete products of

combustion like CO (fig. 10c) is finished directly behind the last secondary air injection. Fig.

10d visualise the flow field. It is quite uniform because of no large recirculation or dead

zones and the after combustion chamber is fully covered by the path lines.

Fig. 10 a) Temperature distribution (section: symmetry) b) Oxygen distribution (section: symmetry)

c) CO distribution (section: symmetry) d) Path Lines (starting point: bed surface)

Measurements were performed at 3 temperature sampling points and at 1 species

concentration sampling point. A comparison between the experimental and simulated data is

shown in the following figures (fig. 11a, 11b). The difference in all planes is very

satisfactorily small.

> 1300

< 300

800

a)

> 14

< 4

9

> 2

1

16

a) Temperature [°C]

1100

measured temperatures

b) Temperature [°C]

1100

measured temperatures

c) Temperature [°C] 900

measured temperatures

Fig. 11a: a-c) comparison: calculated/measured temperatures

13:00 h 1037°C 13:35 h 1042°C

13:00 h 1073°C 13:35 h 1033°C

13:00 h 866°C 13:35 h 903°C

900

1000

900

1000

T 1

T 2

800

850

T 3

17

7.5

d)

Oxygen [Vol.-%]

Fig. 11b d) comparison: calculated/measured oxygen concentrations

5.5 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 show less meaningfulness 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 [7],

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

[16],

- 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.

6.5

7.0

calculated operation case

measured oxygen concentrations

7.0

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6 CONCLUSIONS In the EC´s economy, municipal waste incineration is an important factor for waste

management and treatment.

The thermal treatment of municipal wastes is centralised in big plant units. Environmental as

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

liberalisation of the common market form pressure on the costs in this area.

Subsidies for electricity generation from biomass have generated a new market for small and

medium sized units. Using the techniques from big plants the investment costs for these

plants are too high. On the other hand the same emission limits are valid. These facts implied

the new concept of MARS® with a wide range of fuel qualities to be used and a very flexible

plant concept by modularisation.

The MARS® plant is now in operation since autumn 2003. Meanwhile, some biomass as well

as waste fractions have been tested with success. In parallel, the tool of mathematical

modelling and simulation has been developed to describe the trials and give the opportunity

to scale up the results to bigger plants.

Concepts for plants with thermal loads of 5, 10 and 15 MW are in preparation with the

industrial partners.

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7 SYMBOLS / ABBREVIATIONS

CFD computational fluid dynamics ORC Organic Rankine Cycle EC European Community PA primary air FG flue gas SA secondary air LCV lower calorific value

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[14] Beckmann, M.; Scholz, R.: Residence Time Behaviour of Solid Material in Grate Systems. 5th Europ. Conf. Ind. furnaces and Boilers INFUB, Porto, Portugal, 11./14./4.2000

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