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Proceedings Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
Cini Foundation, Venice, Italy; 12 - 15 November 2012
2012 by CISA Publisher, Italy
Development of an improved secondary air
system using CFD.
Thomas Norman, Paw Andersen, Ole Hedegaard Madsen
Babcock & Wilcox Vølund A/S, Falkevej 2, DK-6705 Esbjerg Ø, Denmark
SUMMARY: A CFD case study of the Aars Municipal Solid Waste (MSW) CHP plant in
Denmark was conducted to investigate the possibility of improving the capacity of the plant and
to ensure compliance with European Union (EU) environmental legislation. The initial approach
was to refurbish the plant with a new grate, a new inconel protected water cooled wear zone and
an upgraded system for the secondary air supply. Two cases were analysed, the operation prior to
modification and the modified situation. The modified situation demonstrated the ability to meet
the requirements of increased capacity but it also revealed very high temperatures on the
refractory protected furnace walls. Consequently a new system for applying secondary air
through openings in the water cooled wear zone combined with a number of nozzles in the
ceiling was developed using CFD as the main design tool.
1 INTRODUCTION
BWV uses CFD to optimise the furnace and boiler design with respect to a large number of
critical factors such as velocities, particle impingement, oxygen level, temperature and surface
temperature. Factors such as the locations of air injection nozzles, wall heat transfer properties
and local geometry features are therefore all investigated thoroughly during the design phase
using CFD as one of the main tools. For BWV CFD is a very efficient tool for evaluating
different design alternatives which otherwise are too expensive, too time consuming or
impossible to test in an operating plant. Babcock & Wilcox Vølund has more than 70 years of
experience in designing waste-to-energy and bio-fuel plants and we have been using state-of-the-
art CFD design tools since 1996. CFD is today one of the major keystones in our technology
design. On-going development of our CFD models is carried out in co-operation with Lund
University, Aalborg University, The Technical University of Denmark and the Danish industrial
research centre Force Technology. Babcock & Wilcox Vølund uses CFD analysis to deal with
issues such as: Longer lifetime through optimum flow and temperature conditions, Redesign due
to changing calorific values of fuel, Increasing plant capacity, in some cases by as much as 25%,
Injection of cooling water, leaching water or sludge, Optimum design of combustion air system,
Optimum burner position, Verification of residence time, Emission control.
The purpose of this study was to develop a concept for an improved secondary air system for a
MSW plant. The new secondary air system solves a growing problem caused by very high
temperatures on the furnace walls and consequently prevents extensive problems caused by
melted slag.
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
2 BACKGROUND
The engineering approach applied to the present problem with respect to mesh generation and
simplifications of boundary conditions and sub-models is not believed to enable precise
prediction of chemistry and flow field with a high level of details. The analysis therefore focuses
on trends. A CFD analysis in a problem-solving project typically consists of two cases: First a
base case, which is a CFD analysis of the plant with current operation and design. Secondly a
modified case, where features are modified and tested to determine their ability to obtain the
required results.
2.1 Boundary conditions
2.1.1 The bed
The top of the waste layer is modelled as a series of mass flow inlets. The waste bed was
modelled using the external bed model developed by Jørgensen and Swithenbank (1997), which
is based on waste specification, calorific value, the waste mass flow and primary air flow. The
processes in a burning refuse bed include: Drying, Ignition, Pyrolysis, Gasification, Solid-phase
combustion, Gas-phase combustion and the process is mainly controlled by heat and mass
transfer.
CFD domain
Fuel Feed
Primary Air
Underfire AirClinker
discharge
Pyrolysis
Char
Oxidation
Char Gasification
Char
Drying
H2OH2, CH4, CxHy
CO2, CO
CO2, O2
CO2,
CO
Ash
Radiation
Particle release
Particle
Combustion
Gas Phase
Combustion
Figure 1 Coupling of bed model and CFD model
The output from the bed model includes values for gas temperature, mass flow of gas, species
concentrations, particle mass flow and temperature. These are read into the fluent model as
boundary conditions for the mass flow inlets on the top of the waste layer on each grate section.
The gas from the bed model is a mixture of O2, CH4, CO, H2O and CO2.
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
2.1.2 Walls
Heat transfer to the walls is mainly dependant on the quantity and quality of ash deposits.
Nevertheless the type of refractory and or weld overlay also has an influence when the ash layer
is thin. Heat transfer properties of furnace and boiler walls take this into account through
adjustment of heat transfer coefficients based on BWV’s modelling experience.
Figure 2 Heat transfer as function of ash layer thickness
2.1.3 Sub models
The following Fluent sub models are normally applied:
Table 1. Fluent submodels used in CFD model
Physics Sub model Comment
Radiation heat transfer Discrete Ordinates Model
(DO).
Support particle radiation
Turbulence RNG k-ε model Fluent default model parameters
Particle transport Discrete Random Walk
Model
Lagrangian approach where Fluent
integrates the instantaneous fluid
velocity at any current position of
the particle
Boundary layer Wall function Fluent Standard Wall Function
Gas phase combustion Eddy-Dissipation Model
or Arrhenius reaction rate
Homogeneous reactions for a
simple methane reaction
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
3 THE SERVICE PROJECT
The service project described in this article is related to boiler 1 on a small plant called Aars
Fjernvarme (Aars District heating) which is situated in the northern part of Denmark. The plant
was built in 1985 with a nominal capacity of 3.5 t/h at a lower heating value of 9.2 MJ/kg.
Increasing heating values up to 11.3 MJ/kg caused problems achieving the capacity.
Figur 3 Aars Fjernvarme
Consequently a refurbishment was carried out installing an updated version of the grate, water
cooled wear zones and new secondary air system. The objective of the refurbishment was to
make the plant able to achieve the capacity.
Table 2. Load situation for the boiler
Before
refurbishment
After
refurbishment
Nominal capacity t/h 3.5 3.5
Lower heating value MJ/kg 9.2 10.1
Thermal load MW 8.9 9.8
Oxygen level % dry 11 9
The increased thermal load is obtained by the installation of the water cooled wear zone.
A number of refurbishments of smaller plants hold a challenging situation because the goal in
most cases is to increase the fuel capacity or thermal load. In some cases both goals are required.
Consequently the heat load in the furnace is increased and the risk of very high temperatures
becomes evident. Many of the smaller plants are built without water cooling of the furnace walls.
The walls are typically covered with refractory or air cooled tiles.
When non water cooled furnaces is refurbished to increase the capacity or to address problems
caused by increased heating value very high temperatures often becomes apparent and is creating
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
severe problems with melted slag and distorted refractory as consequence. Prior to the
development of the improved secondary air system BWV conducted a refurbishment of a plant
in Aarhus, Denmark. This refurbishment resulted in very severe problems with melted slag and
distorted refractory as it can be seen in Figure 15 and Figure 16.
3.1 The base case
BWV engineers visited the plant to determine and define the current mode of operation. The
mass and energy balance required is obtained from readings recorded in the control system
history log. General information from the plant also provides an understanding of special
operational conditions such as waste type and the location of molten slag and ash deposits.
Predictions are analysed in selected cross sections. The mesh is dense close to the secondary air
nozzles. During computation, the mesh is adapted with respect to y+ and gradients of
temperature and velocity. Position and direction of these nozzles is very important as they
control the flow field in the furnace.
A CFD analysis produced by BWV is usually based on a selection of predicted species
concentrations, temperatures and velocities examined in a large number of planes. Given the
number of planes analysed, it generally is unnecessary and of little value to present and describe
each plane to the client. We therefore usually present a more general picture to our clients.
3.2 The modified case: changing the secondary air system
The analysis of the base case demonstrates that the original secondary air system in the pre-
modified state significantly reduces the efficiency of the furnace volume.
The strategy behind the furnace modification therefore was to change the secondary air system in
such a way that gasses are retained in the furnace for complete burnout.
BWV has achieved good results with a secondary air concept, which uses a smaller number of
nozzles positioned in the furnace roof. The modified case employed this secondary air system to
begin with but the results revealed very high temperatures on the furnace side walls as shown in
the left picture of Figure 12. Consequently a new secondary air system was developed using
CFD as the main tool to evaluate the functionality and efficiency with respect to avoiding the
high wall temperatures for different solutions.
The final concept for the new secondary air system consist of three nozzles in-line where nozzle
1 and 2 is used to control the oxygen level while nozzle 3 follows the load and is used to
maintain the flow pattern in the furnace. On larger furnaces nozzles will be placed on rows
across the furnace width. Further a part of the secondary is applied to the furnace through an
opening in the water cooled wear zone (See Figure 5) with low velocity (about 3 - 5 m/s). Due to
the low velocity of the cold air stream coming from the wear zone the air is pulled upwards by
the hot flow from the grate creating a cold recirculation at the refractory covered walls above the
wear zone (See Figure 6).
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
Figure 4 Configuration of SA – nozzles with 3 nozzle inline on the front ceiling.
Figure 5 CAD drawing of water cooled wear zone with air inlets
Nozzle 1
Nozzle 2
Nozzle 3
Inlets for cooling air in wear zone
Inlets for cooling air in wear zone
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
Figure 6 Velocities at furnace side wall. A cold recirculation zone is created over the wear zone.
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
Figure 7 Temperatures at fuel inlet. Picture to the left is with conventional wear zone. Picture to
the right is with integrated air cooling.
Figure 8 Temperatures 5 m from slag pit. Picture to the left is with conventional wear zone.
Picture to the right is with integrated air cooling.
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
Figure 9 Temperatures 4 m from slag pit. Picture to the left is with conventional wear zone.
Picture to the right is with integrated air cooling.
Figure 10 Temperatures 3 m from slag pit. Picture to the left is with conventional wear zone.
Picture to the right is with integrated air cooling.
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
Figure 11 Temperatures 2 m from slag pit. Picture to the left is with conventional wear zone.
Picture to the right is with integrated air cooling.
Figure 12 Temperatures on refractory surface. Picture to the left is with conventional wear zone.
Picture to the right is with integrated air cooling.
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
Figure 13 Furnace after refurbishment
Figure 14 Furnace after 14 weeks in operation
Figure 15 ACÅ after 10 weeks in operation
Figure 16 ACÅ after 10 weeks in operation
Figure 17 ACÅ after 10 weeks in operation
Venice 2012, Fourth International Symposium on Energy from Biomass and Waste
4 CONCLUSION
BWV has successfully used CFD as the main tool in the process of solving the problems related
to very high temperatures on furnace walls. Consequently melted slag does not create the
extensive problems, which has been the case prior to the development of the wear zone with
integrated cooling air. After the refurbish of the plant the operation is more stable with less down
time, enabling the client to improve thermal load by 10%. A patented concept based on applying
cooling air through openings in a water cooled wear zone has been developed using CFD.
5 ACKNOWLEDGEMENTS
We would like to thank the management of Aars Fjernvarme for allowing us to use their plant as
a case study for this paper.
6 NOMENCLATURE
k turbulent kinetic energy (m2/s
2)
ε rate of dissipation of turbulent kinetic energy (m2/s
3)
7 REFERENCES
The European Parliament and the Council (2000)
Directive 2000/76/EC of the European Parliament and of the Council of 4 December 2000 on the
Incineration of Waste. Article 6.1
Jørgensen, K., Swithenbank, J. (1997) CFD Simulation of Flow and Combustion in the
Vestforbrænding 5 Furnace and Boiler. Internal BWV document
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