development of an improved secondary air system using cfd./media/downloads/conference_papers... ·...

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.


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


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


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).


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


Figure 6 Velocities at furnace side wall. A cold recirculation zone is created over the wear zone.


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.




Figure 12 Temperatures on refractory surface. Picture to the left is with conventional wear zone.



Figure 13 Furnace after refurbishment

Figure 14 Furnace after 14 weeks in operation

Figure 15 ACÅ after 10 weeks in operation




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

development of an improved secondary air system using cfd./media/downloads/conference_papers... ·...

Documents