SO 3 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 SO x emission. Certain amount of SO 2 is transformed into SO 3. SO 3 reacts, at lower temperature, with water vapour forming sulphuric acid  causes 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:NO x, postprocessor combustion model:probability density function (PDF) formulation reaction system:equilibrium chemistry formulation* *OK for major combustion species (except NO x and soot) but not good enough for SO 3 formation/destruction modelling! SO 3 model:model based on finite rate chemistry, implemented as User Defined Function routine

Kinetics of SO 3 formation/destruction: Recommended values for the third body reactants [M]: N 2 /1.3/, SO 2 /10.0/ and H 2 O /10.0/ K C – equilibrium constant

Mathematical model of SO 3 formation: Transport equation for SO 3 : is diffusion coefficient of SO 3 : The source term is defined as: Schmidt-Prandtl number is:  =0.7 The rate of SO 3 change for the reactions (1) and (2) is:

Results: Mathematical model was applied to simulate SO 3 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  C at the exit of the regenerative Ljungström air-heater, reported occurrence of the severe low- temperature corrosion of the generator “cold-end” surfaces.

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 SO 3 formation (and CO, NO x, 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 X SO 3 Fig. 3. Distribution of SO 3 for different combustion air excess ratios

Fig. 4. Molar fractions of a) SO 3 and O, b) CO and H 2, c) NO and mean flue gas temperature, d) SO 2 and soot vs. combustion air excess ratio  ~50% SO 3 soot [-] soot

X SO 3 Fig. 5. Distribution of SO 3 for different swirl numbers S=0.44S=0.48S=0.55 S=0.63S=0.68S=0.71

 ~30% SO 3 Sl. 6. Molar fractions of a) SO 3 and O, b) CO and H 2, c) exit flue gas temperature and heat flux, d) SO 2 and soot vs. swirl number soot [-] soot

X SO 3 d=50  md=70  md=100  md=130  md=160  m Fig. 7. Distribution of SO 3 for different fuel droplet sizes

 ~4.5% SO 3  50-75% CO Fig. 8. Molar fractions of a) SO 3 and O, b) CO and H 2, c) NO and mean flue gas temperature, d) SO 2 and soot vs. fuel droplet size soot [-] soot

Conclusion: Proposed finite rate chemistry model of SO 3 realistically describes SO 3 formation/destruction. Such a model could be used in analysis of SO 3 reduction.  Decrease of the air excess ratio reduced SO 3 production, but increased CO and H 2 (incomplete combustion).  Increase of magnitude of the swirl of combustion air, the fuel spray angle and finer spraying (smaller fuel droplet size) lowered SO 3 concentration in lesser extent than the air excess ratio, but improved combustion (reduced CO and H 2 formation).  The right strategy would be in combination of all these measures.