SO3 Reduction in the Heavy-oil Fired Furnace

Power Engineering Dept.

Faculty of Mechanical Engineering and Naval Architecture

University of Zagreb, Croatia

mr.sc. Daniel Rolph Schneider

Prof. dr.sc. Željko Bogdan

• Introduction

• Use of heavy-oil fuel, rich in sulphur, in combustors of steam generator furnaces causes increased SOx emission.

• Certain amount of SO2 is transformed into SO3 .

• SO3 reacts, at lower temperature, with water vapour forming sulphuric acidcauses low-temperature corrosion of the steam-generator sections.

• Mathematical model:coupled gas flow and liquid spray physics, non-premixed turbulent flame, Fluent code

• turbulent flow: realizable k- model• radiation heat transfer: discrete ordinates model• liquid fuel spray: discrete second phase, “particle in

cell” model• formation of the pollutants: NOx , postprocessor• combustion model: probability density function (PDF)

formulation• reaction system: equilibrium chemistry formulation*

*OK for major combustion species (except NOx and soot) but not good enough for SO3 formation/destruction modelling!

• SO3 model: model based on finite rate chemistry,

implemented as User Defined Function routine

• Kinetics of SO3 formation/destruction:

1

12 3SO O M SO M (1)

f

b

k

k

2310

1

1200.38 cm 19.2 10 exp

mol sfk RT

Recommended values for the third body reactants [M]: N2 /1.3/, SO2 /10.0/ and H2O /10.0/

2

23 2 2SO O SO O (2)

f

b

k

k

312

2

10064.95 cm 12 10 exp

mol sfk RT

11

1

fb

C

kk

K

22

2

fb

C

kk

K

KC – equilibrium constant

• Mathematical model of SO3 formation:

3

3 3 3

SOSO SO SOj

j j j

Yu Y Γ S

x x x

Transport equation for SO3:

is diffusion coefficient of SO3:

3SOΓ effΓ

3SO

The source term is defined as:

3SOS 3 3 3SO SO SO 3SOS Y M

t t

Schmidt-Prandtl number is: =0.7

The rate of SO3 change for the reactions (1) and (2) is: 3

1 2 2 2 2 1 3 2 3

SOSO O M SO O SO M SO Of b b f

dk k k k

dt

• Results:• Mathematical model was applied to simulate SO3 formation in

the furnace of a real steam generator of the 210 MW oil-fired Power Plant Sisak.

• PP Sisak burns heavy-oil fuel with 2-3% sulphur and exhibits flue gas temperatures of 135-140 C at the exit of the regenerative Ljungström air-heater, reported occurrence of the severe low-temperature corrosion of the generator “cold-end” surfaces.

PRIMARY AIR

SECONDARY AIR

TERTIARY AIR

SECONDARY AIR SWIRLER TERTIARY AIR SWIRLER

ATOMISER NOZZLE

STEAM ATOMISER

TERTIARY AIR INLET SECONDARY AIR INLET

PRIMARY AIR INLET

Fig. 2. Schematic of the burner Fig. 1. Discretization of the furnace

• two oil burners (Fig. 1) on each side-wall of the chamber• the burner consists of the axial/radial inflow type swirl generating

register and the steam atomiser (Y-nozzle)• the airflow is divided into three streams: unswirled primary stream and

then secondary and tertiary streams, which are swirled

Influences of different combustion parameters on SO3 formation (and

CO, NOx, soot) were analysed:

combustion air excess ratio,

magnitude of the swirl of combustion air,

fuel droplet size (as a function of atomising steam pressure and number of the openings of atomiser)

fuel injection spray angle

combustion air distribution (portion of primary, secondary and tertiary stream)

• Analysis:

=1.105 =1.140 =1.175 =1.210

=0.965 =1.000 =1.070=1.035

XSO3

Fig. 3. Distribution of SO3 for different combustion air excess ratios

Fig. 4. Molar fractions of a) SO3 and O , b) CO and H2 , c) NO and mean flue gas temperature, d) SO2 and soot vs. combustion air excess ratio

~50% SO3

soot [-]

soot

XSO3

Fig. 5. Distribution of SO3 for different swirl numbers

S=0.44 S=0.48 S=0.55

S=0.63 S=0.68 S=0.71

~30% SO3

Sl. 6. Molar fractions of a) SO3 and O , b) CO and H2 , c) exit flue gas temperature and heat flux, d) SO2 and soot vs. swirl number

soot [-]

soot

XSO3

d=50 m d=70 m d=100 m d=130 m d=160 m

Fig. 7. Distribution of SO3 for different fuel droplet sizes

~4.5% SO3

50-75% CO

Fig. 8. Molar fractions of a) SO3 and O , b) CO and H2 , c) NO and mean flue gas temperature, d) SO2 and soot vs. fuel droplet size

soot [-]

soot

• Conclusion:

• Proposed finite rate chemistry model of SO3 realistically describes SO3

formation/destruction.

• Such a model could be used in analysis of SO3 reduction.

Decrease of the air excess ratio reduced SO3 production, but increased

CO and H2 (incomplete combustion). Increase of magnitude of the swirl of combustion air, the fuel spray angle

and finer spraying (smaller fuel droplet size) lowered SO3 concentration

in lesser extent than the air excess ratio, but improved combustion (reduced CO and H2 formation).

The right strategy would be in combination of all these measures.