BIOS BIOENERGIESYSTEME GmbH€¦ · Efficient mixing of unburned flue gas with re-circulated flue...

CFD-based plant development

Key information

BIOS BIOENERGIESYSTEME GmbH

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BIOS BIOENERGIESYSTEME GmbH, Graz, Austria – key information regarding CFD simulations 1

Key information and references

concerning the CFD simulation activities of

BIOS BIOENERGIESYSTEME GmbH

What is CFD?

CFD (Computational Fluid Dynamics) is the spatially (and temporally) resolved simulation of flow and

heat transfer processes. Flows may be laminar or turbulent; they may be reactive or occur in a

multiphase system. CFD simulations thus constitute an excellent tool for process analysis as well for

the design and optimisation of plants.

Basic principles and general conditions

CFD simulations are applied to solve problems in various areas, for example in the automobile and

aircraft industries, the biomedical industry, electronics cooling, in chemical engineering, for turbo

machinery, in combustion processes, in heat and power generation, and for heating and cooling pipes.

In the field of energy technology CFD is being increasingly used for the optimisation of gas and oil

burners as well as for pulverised coal furnaces.

CFD modelling of biomass combustion and gasification plants is especially difficult due to the

complexity of the processes involved in the thermal conversion of solid biomass, as well as due to the

turbulent reactive flow in the combustion chamber or the gasification reactor, respectively. BIOS, in co-

operation with researchers of Graz University of Technology, Institute for Process and Particle

Engineering, has successfully developed a CFD model especially designed for the development and

optimisation of biomass grate furnaces, boilers and fixed bed gasification plants. The CFD model

consists of an in-house developed empirical grate combustion model complemented with modified and

lab-scale tested CFD sub-models (FLUENT code) for the turbulent reactive flue gas flow in the

combustion or gasification reactor. The applicability of the CFD model, as well as the reliability of

simulation results were tested at pilot-scale and industrial-scale furnaces.

The long-standing continuous co-operation of BIOS with various research institutions, such as the

Institute for Process and Particle Engineering of Graz University of Technology and the Austrian

biomass competence centre ‘BIOENERGY 2020+’ ensures that the employed models are kept at the

forefront of scientific developments.


Goals of CFD-based plant development

The goal of a CFD-aided plant development is an efficient technological development and conception

of plants aided by a spatially and temporarily resolved simulation and visualisation of the processes in

biomass combustion and gasification plants. The objectives in detail are:

For combustion plants:

Efficient mixing of unburned flue gas with re-circulated flue gas and efficient air staging

improved CO burnout, NOx reduction

Improved mixing of unburned flue gas with secondary air efficient CO burnout, reduction of

furnace and boiler volumes

Improved utilisation of furnace and boiler geometries efficient CO burnout, reduction of

furnace and boiler volumes

Reduction of local velocity and temperature peaks in order to reduce material erosion and ash

deposit formation

Sensitivity analyses as a basis for the optimisation of plant control

(e.g. influence of load, water content and air staging)

For gasification plants:

Optimisation of the gasification agent and the reactor geometry in order to achieve an as

complete as possible gasification and low tar contents in the product gas

Optimisation of flow and temperature distribution in gas cleaning units

Assessment and optimisation of the combustion of product gas

Advantages of CFD-based plant design

CFD-based plant design brings the following advantages:

Reduced emissions

Increased plant efficiency

Smaller plant design

Increased fuel flexibility

Reduced material wear

Increased plant availabilities and operating hours

Reduced consumption of operating agents for SNCR units

Reduction of development times and costs for test runs

Increased reliability of plant development

Improved basic understanding of the processes taking place in combustion or gasification

reactors


Working fields of BIOS BIOENERGIESYSTEME GmbH

BIOS BIOENERGIESYSTEME GmbH is an engineering company highly experienced in the field of

CFD simulations of plants for thermo-chemical biomass conversion and offers the following simulation

services:

CFD-based development and optimisation of plants, CFD-based monitoring (test runs plus

accompanying CFD simulations) of biomass combustion plants and boilers in the small (furnaces fired

with pellets, wood chips and wood logs, as well as stoves), medium and large scale:

Design and optimisation of

o fixed bed and grate furnaces

o wood log fired boilers

o wood log fired stoves

o pellet stoves

o pulverised fuel furnaces

Design and optimisation of boiler geometries (including convective sections: resolved

geometry of boiler tubes for small-scale plants and using a heat exchanger model for medium

and large-scale plants)

Design and optimisation of nozzles for the injection of re-circulated flue gas

Design and optimisation of nozzles for the injection of secondary/tertiary air

Optimisation of air staging

Optimisation of pressure losses in order to support fan design

Reduction of local temperature peaks by cooling the combustion chamber and optimisation of

operating conditions

Prediction of zones prone to erosion and ash deposit formation

Modelling of the formation of ash deposits and fine particulates in biomass fired boilers

Calculation of heat transfer in and the influence of deposits (slagging and fouling) on biomass

fired boilers

Calculation of residence times using different methods (Lagrange and Euler) for the optimised

design of primary combustion zones (NOx reduction via primary measures) and secondary

combustion zones (flue gas burnout), SNCR and additive injection systems

High temperature equilibrium calculations for the evaluation of ash melting behaviour

Reduction of emissions (carbon monoxide, nitrogen oxides, fine particulate emissions)

Calculation of the precipitation rates of fly ash particles and ash vapours in various plant

zones

Investigation/optimisation of the operating conditions of furnaces and boilers with regard to

efficiency, plant availability, partial load operation and fuel flexibility


Further CFD applications:

Application of furnace models to waste incineration plants

Simulation of biomass gasification plants

Simulation of rotary cement kilns

Calculation of heat and pressure losses in pipe networks (e.g. district heating)

Simulation of cyclones (particle precipitation, erosion tendencies)

Simulation of filters and particle separators

Air conditioning simulations in boiler houses and industrial plants


More detailed information on CFD simulations

Design and optimisation of nozzles for the injection of secondary air and re-

circulated flue gas

The design of the secondary air and flue gas nozzles is a key factor in meeting the following

requirements:

High turbulent mixing and homogenisation of the flow across the flue gas channel.

Minimisation of furnace volume (investment costs).

Reduction of excess air and flue gas recirculation ratio (efficiency, operation costs).

Reduction of CO and NOx emissions.

Reduction of temperature peaks (fouling and slagging) and flue gas velocity peaks (material

stress and erosion).

An example of a furnace geometry optimised by CFD analysis is shown in the figure below. A

significant reduction of CO emissions and temperature peaks was achieved by the appropriate

arrangement of the secondary air nozzles, resulting in optimised mixing conditions.

Biomass grate furnace equipped with a

horizontally moving grate

CO concentrations [ppmv] (above) and

temperature distribution [°C] (below) in different

cross-sections near secondary air injection nozzle


Design of the geometry of the combustion chamber

The design of the geometry of the combustion chamber is of great importance in order to fulfil the

requirements already stated for the design of the secondary air and flue gas injection nozzles.

Below, exemplary figures of a Low-NOx biomass grate furnace appropriate for a broad fuel assortment

(waste wood, wood chips, bark) are shown. This type of furnace was realised as a pilot-scale plant

and subsequently also as a large-scale plant. The combination of vertical barriers and a staged

secondary air injection leads to a highly turbulent mixing, a homogeneous flue gas distribution and a

good utilisation of the secondary combustion zone. Besides a significantly reduced and simplified

furnace volume, the following advantages could be achieved:

Strong reduction of CO

emissions

CO profiles [ppmv] in the symmetry plane of the furnace

Lowering of temperature

peaks (fouling & slagging)

Temperature profiles [°C] in a horizontal cross-section at the level of the vertical barriers and the

secondary air nozzles


Lowering of flue gas velocity

peaks (erosion & material

stress)

Profiles of flue gas velocity [m/s] in a horizontal cross-section at the level of the vertical barriers and

the secondary air nozzles


Optimised design of wood log fired stoves

The basic grate combustion model, which was developed to describe the release of the flue gas

components during biomass combustion on the grate in order to achieve boundary conditions for the

CFD simulation of turbulent reactive flow in the combustion chamber, was modified for wood log fired

stoves, wood log furnaces and wood log fired boilers operated in discontinuous batch mode. This

modified model is used to calculate time-dependent profiles of wood log combustion by a

transformation of the release profiles along the grate determined with the basic grate combustion

model. Using these time-dependent profiles, the composition of a virtual fuel consisting of the fuel

components C, H, N, O and water vapour converted during solid biomass combustion, can be

determined at any point in time. Furthermore, a mass balance and the amount and composition of flue

gas released can be calculated at any point in time during batch operation.

In order to prevent the distortion of CFD simulations by the heat storage of the stove when defining

virtual steady-state operating cases, it is necessary to determine the time-dependent profile of the heat

fluxes over the stove surface based on test runs. By balancing energy, two virtual steady-state

operating cases can be identified, which are characterised by the heat storage of the stove, which is

zero.

The following processes can be analysed with the in-house developed CFD model for wood log fired

stoves:

Flow of combustion air and flue gas in the stove and flow of convection air in the double jacket

of the stove

Gas phase combustion in the stove

Heat transfer (conduction, convection, and radiation) between gas phase, stove materials

(stove lining, metal sheets and glass sheets) and surroundings

This enables analysis of:

Velocities and temperatures of combustion air, convection air and flue gas

Path lines of air and flue gas

Concentrations of O2 and CO in the flue gas

Material and surface temperatures of stove lining, metal sheets and glass sheets (see figure

below)

Heat transfer and efficiency

Pressure losses over different plant zones


Temperature profiles at the outer surface of a wood log fired stove [°C]; 3D view of the front side (left)

and backside (right) of the stove

In the figure below the development of a new stove is demonstrated by means of the CO

concentrations. In the basic variant the emissions are rather high due to a bypass flow in the

redirection baffle of the post combustion chamber. Furthermore, the post-combustion chamber was

not insulated. In the pre-optimised variant (before the realisation as testing plant) first improvements

could be achieved by a closure of the bypass flow and an insulation of the post combustion chamber.

By these measures, the temperature in the post-combustion chamber was elevated and the CO

burnout considerably improved. A further improvement could be achieved by the optimised variant

which was realised as testing plant. Here, additional tertiary air nozzles have been installed in the rear

part of the combustion chamber, which lead to an improved flue gas burnout already in the combustion

chamber. Moreover, the CO emissions are a leading parameter for the burnout quality of the flue gas

and can be used as an important indicator concerning organic fine particle emissions from incomplete

combustion. Besides the considerably reduced CO emissions also the organic fine particle emissions

could be reduced. Finally, the excess air could be reduced, leading to a higher plant efficiency.

Basic geometry

(tot = 2.3)

Optimised geometry

(tot = 2.0)

window

entrance of

flushing air

flue gas

exit

combustion

chamber

post-combustion

chamber

tertiary air

nozzles

wood logs

redirection

baffle

transition

5000

4750

4500

4250

4000

3750

3500

3250

3000

2750

2500

2250

2000

1750

1500

1250

1000

750

500

250

0

Iso-surfaces of CO concentrations [ppmv w.b.] in the flue gas in the vertical symmetry plane of a stove

Modifications: closure of opening in the redirection baffle; additional tertiary air nozzles; larger

transition to the chimney and insulation of the post-combustion chamber


Optimised design of wood log furnaces

The basic grate combustion model, which was developed to describe the release of the flue gas

components during biomass combustion on the grate in order to achieve boundary conditions for the

CFD simulation of turbulent reactive flow in the combustion chamber, was modified for wood log fired

stoves, wood log furnaces and wood log fired boilers operated in discontinuous batch mode (for further

information see section “Optimised design of wood log furnaces”). Here, virtual steady-state operating

cases are balanced with the empirical model for which CFD simulations of flow, gas phase combustion

and heat transfer can be performed for any point in time during batch operation.

The CFD simulations enable an analysis of:

Quality of penetration of the primary combustion zone (space filled with log wood) with primary

air

Utilisation of the secondary combustion zone, mixing of flue gas with secondary air and CO

burnout

Velocity and temperature peaks for the best possible reduction of material erosion and ash

deposit formation

Heat transfer in the primary and secondary combustion zones of the furnace as well as in the

boiler tubes as a basis for the optimisation of thermal efficiency

Pressure losses over different plant zones

Results of a CFD analysis of a wood log furnace are shown in the following figure. The diagram on the

left shows path lines of the primary air coloured by gas temperature. The path lines allow the

penetration of the space filled with log wood to be analysed and optimised in order to achieve good

and even combustion of the wood logs and to avoid bridging. The diagrams on the right show

calculated CO concentrations in different cross-sections of the secondary combustion zone, which

serve as a basis to analyse and optimise the mixing of flue gas with secondary air and the utilisation of

the secondary combustion zone.

Path lines of primary air in the primary combustion zone coloured by gas temperature [°C] (left); CO

concentrations [ppmv] in a horizontal cross-section (top right) and in a vertical cross-section (bottom

right) of the secondary combustion zone


Simulation of pulverised biomass furnaces / entrained flow reactors

Models have been developed for the combustion of thermally thin and thick biomass particles in order

to utilise CFD for the design of pulverised combustion and gasification units. The models for thermally

thick biomass particles take the intra-particle mass and heat transfer into account and thus enable a

more accurate prediction (compared to available commercial CFD particle models) of the combustion

process of the solid biomass particle and particle temperature. The model for entrained flow

conversion was successfully tested by a comparison with measurements for a pulverised biomass

flame and successfully applied to the simulation of a pulverised wood furnace. This model allows the

qualitative description of particle combustion along particle trajectories and thus provides qualitative

information about the flow and combustion processes in the furnace, making it ideal for the

development and optimisation of pulverised biomass furnaces.

The simulated CO concentrations in the vertical symmetry plane of the basic design (left) and the

optimised design (right) are shown in the following figure. The results show that a considerable

improvement of CO burnout in the furnace can be achieved by modifying the design of the nozzles for

the supply of secondary air and re-circulated flue gas as well as the operating conditions.

Basic design Optimised design

Fuel supply with carrying airFlue gas re-circulation nozzles

Secondary

air nozzles

1 nozzle pair

closed

1 nozzle pair

closed

CO concentrations [ppmv] in the vertical symmetry plane of a pulverised wood furnace


Investigation of furnace cooling systems

As mentioned before, moderate and well controlled flue gas temperatures in the furnace are important

in order to prevent slagging and deposit formation. Additional measures like cooled walls or tubes are

recommended especially for dry fuels (waste wood) and biomass fuels with a high content of alkali

metals (straw).

The figure below shows the temperature distribution in a biomass grate furnace for waste wood

combustion with path lines of re-circulated flue gas injected by the lower nozzle row, indicating high

turbulent mixing and increased flue gas temperatures. The highest flue gas temperatures near the wall

are expected in the primary combustion zone and the regions around the flue gas and secondary air

injection nozzles. Cooled furnace walls are recommended for this region in order to lower temperature

peaks and to prevent slagging.

Temperature profiles [°C] in different horizontal cross-sections at the level of re-circulated flue gas and

secondary air injection; the furnace wall section between and around the nozzles is cooled

Explanations: SA…secondary air nozzles; FGR…flue gas re-circulation nozzles

SA

FGR

FGR


CFD simulation of boilers including the convective section

CFD simulation also proved to be a powerful tool for the design of biomass boilers. Optimisation of

flow results in an improved utilisation of the boiler volume, enhanced heat transfer and a more even

temperature distribution, thereby also reducing deposit formation.

In view of this potential, BIOS BIOENERGIESYSTEME GmbH has carried out an R&D project in order

to develop a CFD model for heat exchangers. This model allows the flue gas flow within the tube

bundles of the convective heat exchanger section to be included in the CFD optimisation process. A

detailed simulation of convective heat exchangers would be impossible in most cases, as the high

spatial resolution needed to resolve the geometry of interest could not be covered by computer

capacity.

The simulation models developed are able to predict the flue gas flow field including the pressure

losses, as well as the heat transfer within the convective part of the boiler. Furthermore, the maximum

temperatures at the flue gas side of the heat exchanger tubes are calculated, which are the crucial

factor in the process of formation of solid deposit layers. Special attention is paid to the influence of

radiative heat transfer on tube rows exposed to increased thermal radiation.

Consequently, the CFD simulation gives better and more valuable information for tube bundles in

highly inhomogeneous flow fields, e.g. tube bundles positioned in regions of strong flow deflections,

than conventional heat exchanger design methods based on one-dimensional assumptions. The

following advantages for the optimisation of biomass boilers can be summarised:

optimisation of flow through the heat exchanger tube bundles

decision basis for arranging the evaporator tube bundles upstream the superheater sections

identification of surfaces prone to formation of sticky ash deposit layers

optimised positioning of soot blowers to improve the functionality of the boiler cleaning system

optimisation of steam parameters to increase the efficiency of electricity production

The CFD model developed expands the capabilities of the present state-of-the-art commercial CFD

software and is currently available for the most relevant boiler types (water tube steam boilers, thermal

oil and fire tube boilers). Additionally, the model is able to simulate the flow of both primary and

secondary heat carriers. It can thus be used to optimise heat transfer and flow field for both flue gas

and water side in a fire tube boiler. Exemplary simulation results for different biomass boilers are

shown in the following.


(a) Flue gas temperature distribution [K] and (b) maximum temperature [K] at the flue gas oriented

surfaces of fouled heat exchanger tube bundles in the convective section of a biomass fired steam

boiler.

The figures above show CFD results of a real plant, which was selected for validating the CFD models

developed by BIOS BIOENERGIESYSTEME GmbH. The inhomogeneous inlet flow into the

convective part of the boiler leads to a strong fluctuation of the maximum tube surface temperatures at

the first tube rows of the evaporator section. Deflections of the flue gas flow in front of the tube bundle

heat exchangers can be optimised by means of CFD simulations in order to achieve a more even

distribution of the incoming flow and a reduction of temperature peaks.

Temperature profiles [°C] in different horizontal cross sections of a thermal oil-boiler (radiative section)


Profiles of the flue gas side wall temperature [°C] of the radiative heating surfaces of a thermal oil

boiler at different stages of fouling from clean walls (left) to walls covered with a considerable layer of

fly ash deposits (right)

The figures above show simulation results of flue gas temperature profiles in the radiative section of a

thermal oil boiler as well as simulation results based on an investigation of the effective heat transfer in

the radiative section of the boiler under real operating conditions. The increased wall temperatures

due to deposit layer formation lead to a dramatic reduction of the heat transfer in the radiative boiler

section. This should be considered in boiler dimensioning and should be prevented as far as possible

by an appropriate design and by appropriate automatic cleaning facilities.


Computational grid (above) and flue gas temperature profiles [°C] in the symmetry plane of a fire tube

boiler; a) simulation of the convective section using the CFD heat exchanger model; b) detailed

simulation of the convective section with spatially resolved tubes.

The figure above shows the results of a validation study of the developed CFD heat exchanger model

for fire tube boilers. A comparison of a detailed boiler simulation with a simulation using the CFD heat

exchanger model was performed for this study. The detailed simulation, used as a reference, was

carried out with spatially resolved tubes (convective section). However, this is only possible for very

small boilers. In the simulation using the CFD heat exchanger model the tubes were considered by the

model (compare figures above). The comparison shows good agreement of the simulation results for

both methods (see flue gas temperature shown). An additional comparison with experimental data of

pressure losses and heat transfer in the boiler also showed good agreement of calculated and

experimental results. The CFD heat exchanger model can thus be applied as a design tool for fire tube

boilers.


Simulation of ash deposit and fine particulate formation in biomass furnaces

and boilers

It is of the utmost importance to avoid the formation of deposits in biomass furnaces and boilers. A

model accounting for deposit formation is currently under development in a R&D project. Furthermore,

the reduction of fine particulate emissions is gaining increasing importance due to continuously stricter

emission limits and an increasing market demand concerning „new“ biomass fuels with enhanced ash

contents like short rotation coppice and agricultural residues. A model accounting for deposit and fine

particulate formation is currently under development in a R&D project. At the present stage of CFD

modelling, the time-dependent formation of deposits at furnace walls and boiler walls can be predicted.

The model currently considers the impaction of fly ash particles (silicate and salt particles) depending

on their stickiness, as well as the condensation of ash forming vapours at furnace and boiler walls.

Furthermore, the formation of fine particulates (basic model) and their deposition on furnace and boiler

walls can be investigated. In addition, the erosion of the deposition layer by coarse fly ash particles

can be studied. This model is characterised by high flexibility regarding the biomass fuel used, a

detailed consideration of the ash chemistry and reasonable computing time even if applied for

engineering applications.

Calculated thickness (mm) of deposit layers in furnace and radiative boiler sections (fire tube boiler) of

a biomass grate furnace (fuel: waste wood) after 1 hour of operation


Calculated deposit mass flux (kg/m2s) of coarse fly ash particles to the walls of furnace and radiative

boiler section (fire tube boiler) of a biomass grate furnace (fuel: waste wood) after 1 hour of operation

Calculated condensation mass flux (kg/m2s) of K2SO4 to the walls of furnace and radiative boiler section (fire tube boiler) of a biomass grate furnace (fuel: waste wood) after 1 hour of operation.

The figures above exemplify the deposit build-up calculated for a waste wood fired grate furnace

including the radiative section of the fire tube boiler. The separate visualisation of the different deposit

formation mechanisms (e.g. deposit build-up by impaction of coarse fly ash particles or by

condensation of different ash forming vapours (e.g. K2SO4)) can be mentioned as a major advantage

of this deposit formation model. The simulation results agree qualitatively with observations made in

the biomass grate furnace investigated as well as in a number of other biomass fired boilers.


sum of fine particles formed

0%

20%

40%

60%

80%

100%

measurement simulation

chemical composition of the

fine particles

primary

air

supply

SCZ

heat

exchanger

0%

20%

40%

60%

80%

100%

measurement simulation

Cl

S

Na

KPCZ

secondary air nozzles

14.3

13.5

12.7

11.9

11.1

10.3

9.5

8.7

7.9

7.2

6.4

5.6

4.8

4.0

3.2

2.4

1.6

0.8

0.0

14.3

13.5

12.7

11.9

11.1

10.3

9.5

8.7

7.9

7.2

6.4

5.6

4.8

4.0

3.2

2.4

1.6

0.8

0.0

1

2

Total fine particle concentrations [mg/Nm³ dry flue gas, 13% O2); (left) and chemical composition of

the fine particles (right)

Explanations: 70 kW pellet boiler; 1…first particle formation; 2…particle formation starts to dominate at

the entrance into the heat exchanger; PCZ…primary combustion zone; SCZ…secondary combustion

zone

In the figure below the simulation results concerning fine particulate formation in a 70 kW pellet boiler

are shown. In the PCZ, the flue gas temperature and the wall temperatures are too high for a direct

wall condensation or fine particle formation. The first formation was predicted at the exit of the PCZ.

Simultaneously to the fine particle formation, condensation occurs at the cooled walls. The highest

deposition mass fluxes have been calculated on the opposite side of the SCZ. In this region, mass

transfer coefficients as well as concentration gradients at the wall are high in comparison to the other

regions. In the heat exchanger, the condensation flux strongly decreases and the formation of fine

particles dominates. For the purpose of model check, the simulation results have been compared with

measurements during test runs. The predicted fine particle emissions are in good agreement with the

measurement values (simulated: 9.92 mg/Nm³; measured: 7.65 mg/Nm³). Moreover, the predicted

chemical composition of the fine particulate emissions is in good agreement with results from chemical

analyses. Concluding, the selected results of a number of validation simulations showed that already

at the present state of development the model is able to predict fine particulate formation and

emissions even at a quantitatively acceptable level. Hence, the model under development can be

already applied as efficient tool for the development of new low-dust combustion technologies since it

predicts local fine particle formation in dependence of relevant influencing parameters and thus leads

to a better and deepened understanding of the underlying processes.

The ash deposit and fine particle formation model is currently being further developed by means of

deposit probe experiments in cooperation with BIOENERGY 2020+ GmbH. Moreover, an enhanced

model for the prediction of ash deposition formation on convective heat transfer surfaces is currently

being developed. The model is based on the CFD heat exchanger model and the deposit formation

model described above. Additionally, models for a more accurate description of the release of ash-

forming vapours during biomass combustion on the grate as well as the formation of fine particulates

(regarding the distribution of particle sizes) and their influence on ash deposit formation will be

developed. The extension of deposition formation modelling to convective heat exchanger sections is

of special importance, since ash deposit formation causes problems in these regions especially if they

are exposed to high temperatures (e.g. superheaters).


CFD simulation of NOx formation in biomass furnaces

The reduction of NOx emissions is a very important issue due to ever stricter emission limits. In order

to meet these requirements an extensive amount of R&D work has already been performed. This work

focuses on the implementation of N-release functionalities and a NOx formation model in the CFD

routines of BIOS in order to develop an efficient design and prediction tool. This model consists of:

Extension of the empirical grate combustion model to include the most relevant NOx precursor

species NO, NH3 and HCN in the empirical combustion model

Eddy Dissipation Concept for turbulence / chemistry interactions

Detailed chemical kinetics (Kilpinen 92) and reduced kinetics (Kilpinen 97-skeletal)

ISAT (in-situ adaptive tabulation) algorithm for run-time tabulation of the reaction kinetics

(reduction of CPU time)

Simulated mole fraction profiles of NH3 (right) and NO (left) in the symmetry plane of a pilot-scale

biomass grate furnace and comparison of measured and simulated NOx emissions at boiler outlet

The figure above shows the comparison of measurements and simulations for the investigated

biomass grate furnace with air staging (fuel investigated: fibreboard) concerning NO (main NOx

component in a biomass furnace) and NH3 (usually the most important NOx precursor in a biomass

furnace). Very good qualitative and quantitative agreement was obtained between the measured and

simulation results regarding NOx as well as the precursors NH3 and HCN for two different operating

conditions of the furnace (oxygen-lean and oxygen-rich conditions in the primary combustion zone). In

order to save CPU time, a reduced mechanism was also applied and validated for lab-scale flames

and for fixed bed biomass grate furnaces of different scale. Good qualitative agreement between

simulated and measured NOx emissions was achieved for all applications with lower CPU time.

Hence, the NOx formation model can be applied to simulations for the performance of sensitivity

analyses concerning the influence of furnace geometry as well as plant operation and air staging on

NOx formation.


NOx emissions TFN/TFNin

PA

PCZ

secondary

air nozzles

SCZ

NOx reduction

300

280

260

240

220

200

180

160

140

120

100

80

60

40

20

0

1.00

0.95

0.90

0.85

0.80

0.75

0.70

0.65

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Iso-surfaces of NOx concentrations [ppmv w.b.] (left) and of the local TFN/TFNin ratio [-] (right) in a

vertical cross-section through the axis of a 20 kW underfeed multi-fuel boiler at nominal load

Explanations: fuel: straw; fuel-N = 0.54 wt.% d.b.; λtotal= 1.71; λprim = 0.69; TFN (total fixed nitrogen):

sum of all moles of nitrogen contained in NO, NH3, NO2, HCN and N2O; TFN/TFNin: TFN in the flue

gas related to TFN released in the fuel bed and introduced via the recycled flue gas (TFNin)

The figure above exemplifies the simulation results for a 100 kW multifuel furnace. It is shown that the

formation and reduction of nitrogen oxides mainly takes place in the PCZ and in the region of the

secondary air nozzles. Looking at the TFN/TFNin ratios it is possible to identify the regions of NOx

reduction (the smaller the ratio, the more NOx precursors (HCN, NH3 and NOx) are reduced to N2). In

the example shown below it can be seen, that NOx is mainly formed in the outer, oxygen-rich region of

the PCZ and that it is reduced in the inner, oxygen-lean region of the PCZ.


Simulation of cyclones, particle separators and filters

The range of simulation services of BIOS also includes the CFD-based design and optimisation of

cyclones, particle separators and filters. In this case the simulations cover flow and temperature

distributions, the calculation of particle separation rates as well as of particle impaction and erosion

rates on the walls.

The following figure shows simulation results for a cyclone of a circulating fluidised bed combustion

plant. The comparison of erosion simulation results with observations (photo) showed good qualitative

agreement of the calculated zones of high erosion rates with locations at the plant where strong

erosion was observed.

Erosion rates observed at the wall of a circulating fluidised bed furnace (left) qualitatively compared

with calculated erosion rates (right)


Ongoing enhancements and future objectives of CFD simulations

Gas phase combustion

Future developments should include models for the consideration of gas streaks caused by

channelling in the fuel bed. Furthermore, the models for gas phase combustion should be adapted to

low-turbulence zones in biomass furnaces (especially relevant for small-scale furnaces), in order to

improve the prediction accuracy.

Ash deposit and fine particulate formation in biomass fired boilers

Simulations and case studies demonstrated the necessity of a more fundamental description of ash

related problems such as slagging of the combustion chamber and aerosol formation. The in-house

deposit formation model for biomass fired boilers is thus being further developed and coupled with the

CFD heat exchanger model within a collaborative project of BIOS and BIOENERGY 2020+ GmbH.

This enables the modelling of ash deposits in the combustion chamber and the whole boiler including

the convective sections. An improved model for the formation of fine particles forms another essential

extension of the simulation routines.

Corrosion in biomass fired boilers

Material corrosion of steel surfaces in biomass combustion and boiler plants is of major importance

especially when firing biomass fuels with high contents of chlorine, sulphur and alkali metals (waste

wood as well as agricultural fuels) but also for conventional wood fuels (wood chips, bark) with respect

to increasing the steam parameters and thus the efficiency of future biomass CHP plants. Therefore,

within the framework of the large FFG project BioCorrSim (http://www.ffg.at/ausschreibungen/modsim-

computational-mathematics-3-ausschreibung) which is coordinated by BIOENERGY 2020+, basic

models for the prediction of the local corrosion potential in biomass fired combustion and boiler plants

in dependence of relevant influencing parameters will be developed. The simpler approach will be an

empirical model, which describes the corrosion potential in dependence of relevant influencing

parameters like the molar 2S/Cl ratio, flue gas temperature and surface temperature. The second and

more sophisticated CFD-based model considers transport processes and chemical reactions between

the steel surface, surrounding deposit layer and gas phase for the most relevant high temperature

corrosion processes in biomass combustion plants. Both corrosion potential models will then be linked

with an existing and comprehensive CFD based deposit formation model, which provides the local

values of e.g. flue gas temperatures and species concentrations as input values for the corrosion

potential models and further allows for a 3D simulation of the local corrosion potential in dependence

of the influencing parameters. These new models will enable the 3D simulation of the local corrosion

potential in the plant in dependence of influencing values like fuel and furnace temperature.

Solid biomass conversion in fixed beds

While CFD simulations are successfully being applied for the simulation of flow and gas phase

combustion in biomass grate furnaces no engineering model for the 3D simulation of solid fuel

combustion on the grate is available so far. Therefore, within the framework of several R&D projects in

cooperation with BIOENERY 2020+ models for solid biomass conversion on the grate are being

developed and linked with the gas phase CFD models. The basic model already developed is based a

2-step approach. In a first step the movement of the particles in the packed bed is described with a

non-reacting multi-flow simulation (Euler-Granular Model). In a second step, the conversion of the

http://www.ffg.at/ausschreibungen/modsim-computational-mathematics-3-ausschreibung

http://www.ffg.at/ausschreibungen/modsim-computational-mathematics-3-ausschreibung


particles along their trajectories is calculated with a layer model for thermally thick particles with

temperature gradients inside which is embedded in a Lagrange model. This enables the 3D simulation

of the processes during solid biomass combustion in dependence of relevant influencing parameters

for the first time.

Carbon content of the ash: Simulation: 0,8 Gew.% TS Analyses: ca. 1,0 Gew.% TS

Maximum particle temperature: Simulation: 1100 – 1150 °C Measurements: ca. 1100 – 1200 °C

Iso-surfaces of flue gas temperature in the furnace axis [K] (left); particle trajectories coloured by

particle temperature [°C] – top view (middle); photo of the fuel bed taken from the top of the furnace

(right)

Explanations 20kW underfeed stoker pellet furnace

In the picture above the simulation results (flue gas temperatures and particle tracks coloured by

particle temperatures) of a 20 kW pellet furnace are shown. Moreover, a photo of the fuel bed taken

from above is depicted for the purpose of a qualitative comparison with the simulation results. The

calculated peak temperatures of the fuel particles are in good qualitative agreement with

measurements at a lab-scale packed-bed reactor. Moreover, the calculated carbon content of the ash

is in the same range as empirical values from different test runs.

In a next step a model based on the Discrete Element Method will be implemented in the simulation

routines for the purpose of a more accurate description of movement and heat transfer of the particles

in the packed bed and the linked processes of solid fuel conversion. Furthermore, release models for

nitrogen species and ash forming elements will be implemented in order to simulate the influence of

solid fuel combustion on NOx and fine particulate formation.


Selected references

CFD-based design, refurbishment and/or optimisation of furnaces, gasifiers, stoves, boilers and flue

gas cleaning systems for biomass, waste wood and sewage sludge combustion plants in order to

reduce emissions and increase the availability and efficiency of such systems:

Simulation, further development and optimisation of electrostatic precipitators for biomass

combustion plants; client: Scheuch GmbH, Aurolzmünster (Upper Austria, Austria).

Project period: 2001/2002

Simulation and support of biomass furnace and boiler design for the CHP plant Grossaitingen

(Bavaria, Germany) / Josef Bertsch Gesellschaft m.b.H. & Co, Bludenz (Vorarlberg, Austria).

Biomass grate furnace and water tube steam boiler; nominal thermal capacity: 16.5 MW biomass steam

boiler; nominal electric capacity: 5.0 MW steam turbine; fuel: waste wood; project period: 2001-2003

Simulation and support of biomass furnace and boiler design for the CHP plant of LINZ STROM

GmbH, Linz (Upper Austria, Austria).


boiler; nominal electric capacity: 7.0 MW steam turbine; fuel: untreated woody biomass including bark; project

period: 2002/2003

Simulation and support of the design and optimisation of the new "i-series" wood log fired stoves

of the company HAAS + SOHN OFENTECHNIK GMBH, Puch (Salzburg, Austria) with a nominal

thermal load of 8 kW

Wood log fired stoves; nominal thermal capacity: 8 kW stove; fuel: wood logs; project period: 2007/2008

Simulation and support of the design and optimisation of the BIOTEC series biomass grate

furnaces of the company Uniconfort srl., San Martino di Lupari (Italy)

Biomass grate furnace and fire tube boiler; nominal thermal capacity: 3.5 MW - 5.8 MW biomass hot water

boiler; fuel: untreated woody biomass; project period: 2008/2009

Simulation and support of the design and optimisation of a biomass grate furnace of the company

VYNCKE ENERGIETECHNIEK N.V., Harelbeke (Belgium)

Biomass grate furnace and fire tube boiler; nominal thermal capacity: 6 MW biomass hot water boiler; fuel:

woody biomass; project period: 2008/2009

Simulation and support concerning the reduction of erosion tendencies in the lining of the cyclone

evaporator of the biomass CFB combustion plant in Strongoli/ Biomasse Italia S.p.A., Strongoli

(Italy)

Biomass CFB furnace and water tube steam boiler including cyclone evaporator; fuel: woody biomass and

agricultural residues; project period: 2008/2009

Simulation and support of the design and optimisation of the prototype of a new pellet furnace of

Windhager Zentralheizung GmbH, Seekirchen (Salzburg, Austria)

Biomass fixed bed furnace and fire tube boiler; nominal thermal capacity: 15 kW biomass hot water boiler;

fuels: wood pellets; project period: 2007-2009


CFD based design of the prototype of a new pellet and wood chip-fired furnace of KWB Kraft &

Wärme aus Biomasse GmbH, St. Margarethen/R. (Austria); introduced into the market as KWB

TDS Powerfire 150 boiler series, received the “Energie Genie 2004” award from the Austrian

Ministry of the Environment in co-operation with the regional energy agency “O.Oe.

Energiesparverband” as well as the “Energy Globe Award 2004” (special category “most

innovative product”)

Rotary grate furnace with a cyclone combustion chamber and fire tube boiler; nominal thermal capacity: 0.15

MW biomass hot water boiler; fuels: wood chips and wood pellets; project period: 2002/2003

Simulation and support of biomass furnace and boiler design for the Kufstein CHP plant TIWAG-

Tiroler Wasserkraft AG, Innsbruck (Tyrol, Austria)


boiler; nominal electric capacity: 6.5 MW steam turbine; fuel: woody untreated biomass including bark; project

period: 2002-2004

Simulation to support the analysis and optimisation of an existing sewage sludge combustion plant

– Andritz AG, Graz (Styria, Austria)

Pulverised fuel furnace with rotary combustion chamber; nominal fuel power related to NCV: 3.7 MW; fuel:

sewage sludge; project period: 2005-2006

Simulation and support of the design of a mixed fuel furnace and boiler - Thermische

Verwertungsanlage Schwarza (TVS) in Thuringia, Germany – Oschatz GmbH, Essen (Germany).

Water cooled moving grate furnace with water tube steam boiler; nominal fuel power related to NCV: 31.0

MW; fuel: mixed fuel with paper residues (rejects) as well as waste from mechanical/biological waste

treatment; project period: 2006

Simulation and support of the development of a new multifuel furnace for woody and herbaceous

biomass fuels of the company KWB Kraft & Wärme aus Biomasse GmbH, St. Margarethen/Raab

(Styria, Austria)

Biomass grate furnace with fire tube boiler; fuel: wood chips, wood pellets, olive residues, Miscanthus etc.);

nominal thermal capacity: 8 to 120 kW biomass hot water boiler; project period: 2007-2009

Simulation and support of the development of an optimised concept of a collector for waste heat

recovery from rotary cement kiln of the company Wopfinger Baustoffindustrie GmbH, Waldegg

(Lower Austria, Austria)

Rotary cement kiln; fuel: lignite and refuse derived fuel (paper fibre residues, plastic waste, etc.); thermal

capacity recovered waste heat: 1,3 MW; project period: 2009-2010

Simulation and support of the design and optimisation of the prototype of a new pellet furnace for

low-energy houses of the company Windhager Zentralheizung GmbH, Seekirchen (Salzburg,

Austria)

Biomass fixed bed furnace with fire tube boiler; nominal thermal capacity: 1,7 to 6 kW biomass hot water

boiler; fuel: wood pellets; project period: 2009-2010


Simulation and support of the design and optimisation of a grate furnace especially designed for

peat combustion for the company POLYTECHNIK Luft- und Feuerungstechnik GmbH,

Weissenbach (Lower Austria, Austria)

Grate furnace with thermal oil boiler; nominal thermal capacity: 13 MW thermal oil boiler; fuel: peat; project

period: 2010

Simulation and support of the design and optimisation of the prototype of a new 100 kW pellet

furnace of the company Fröling Heizkessel- und Behälterbau GmbH, Grieskirchen (Upper Austria,

Austria)

Biomass fixed bed furnace with fire tube boiler; nominal thermal capacity: 100 kW biomass hot water boiler;

fuel: wood pellets; project period: 2010-2011

Simulation and support of the development of a Low-NOx furnace for „new“ biomass fuels in the

medium size range of the company Josef BINDER Maschinenbau- und Handelsges.m.b.H.,

Bärnbach (Styria, Austria) as a subcontractor of the Institute for Process and Particle Engineering,

Graz University of Technology

Biomass grate furnace with hot water or steam boiler; nominal thermal capacity: 100 kW - 10 MW; fuel: short

rotation coppice, agricultural residues (maize cobs; grass pellets); project period: 2010-2011

Simulation and support of the development of a new biomass grate furnace technology for fuels

with high water and ash contents in the size-rage from 700 kW to 13 MW for the company Mawera

Holzfeuerungsanlagen Gesellschaft m.b.H, Hard (Vorarlberg, Austria)

Biomass grate furnace with hot water boiler/ steam boiler / thermal oil boiler; nominal thermal capacity: 700

kW - 13 MW; fuel: biomass fuels with high water and ash contents (freshly harvested short rotation coppice,

wood chips with high contents of bark, needles and mineral impurities, landscape preservation wood, stools);

project period: 2011-2012

Simulation and support of the design and optimisation of different stoves of the company RIKA

Innovative Ofentechnik GmbH, Micheldorf (Upper Austria, Austria)

Wood log fired or pellet fired stoves; fuel: wood logs or pellets

Simulation and optimisation of different biomass grate furnaces of the company POLYTECHNIK

Luft- und Feuerungstechnik GmbH, Weissenbach (Lower Austria, Austria) in the medium and large

size-range

Biomass grate furnace with hot water / steam / thermal oil boiler; fuel: woody biomass fuels

Simulation and support of the design and optimisation of the next generation CHP plant based on

a hybrid biomass and solar system - EU project "Sunstore 4", Marstal (AERO, Dänemark)

Biomass grate furnace with thermal oil boiler; nominal thermal capacity: 3.24 MW biomass thermal oil boiler +

0.91 MW thermal oil economiser; nominal electric capacity: 750 kW ORC process; fuel: short rotation coppice

(willow); project period 2010-2011.

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