Optimization of a Dual-Fuel Low-NOx Combustion System for a Tangentially-Fired Utility Boiler Operating at a High Elevation. by F. McKenty, N. Brais, M. Mifuji, L. Gravel, and Y. Sirois STAR Global Energy Forum – Houston, Tx 23-24 June, 2009 Brais Malouin & Associates Inc. 144 Barr Street, St-Laurent, Qc. Canada, H4T 1Y4 Tel: (514) 382-8866 www.bma.ca
Images courtesy of Cerrey S.A. de C.V. Av. Republicana Mexicana 300 San Nicolas de Los Garza, N.L. Mexico C.P. 66450 Industrial Boiler Manufacturer
Overview • Motivation • Objectives • Problem Description • CFD Modeling • Results & Analysis • Summary • Conclusion 3
Introduction Motivation • Stricter pollutant emission regulations are presenting new challenges to boiler and burner manufacturers in order to meet the new emission specifications. Develop new Boiler/Burner designs Retro-fit existing boilers with new combustion systems capable of meeting the emission specifications . • Physical constraints limit the positioning of the new combustion system. • Re-use existing components (fans, ducts etc..) as much possible to limit costs. • Maintain the existing boiler’s operational characteristics: • Wall heat transfer • Heat transferred to Superheaters, Reheaters and Convection Banks • Gas temperature at key locations 4
Introduction Objective • Replace the existing 32 burner (16 Natural Gas and 16 Heavy Oil #6) Tangentially Fired combustion system with a new Low NOx combustion system having 16 Dual-Fuel burners and 8 Over Fire Air (OFA ) ports. • Use CFD to optimize the combustion system’s firing angles in order to maintain the existing boiler’s operational characteristics 5
Problem description Tangentially fired boilers Operating principal: • Four or more burners located in the corners or on the boiler walls are fired tangentially to a target circle located at the center of the boiler. • Objective: • Create a rotating flow pattern in the center of the furnace. • Use the furnace as a mixing vessel. • Create a fireball in the middle of the furnace instead of several individual flames. • This type of boiler design was originally developed for coal firing in order to minimize the space required for large utility boilers. 6
Problem description Tangentially fired boilers • The size (diameter and height) of the fireball is highly dependant on the diameter of the target circle. • If the target circle diameter is too large; the diameter of the fireball could increase until it reaches the furnace walls. • If the target circle diameter is too small; the jets could impinge with one another and the rotating motion of the flow is lost. • The size of the fireball is dependant on burner jet penetration into the furnace. • Jet penetration is a function of jet momentum. • Increased jet momentum means increased penetration and higher jet velocities at the location of the target circle and vice versa. 7
Problem description Tangentially fired boilers • Increasing or decreasing the momentum of the burner jets will change the furnace aerodynamics. A target circle that was adequate for a given burner (jet momentum) may yield an inappropriate shape of the fireball if the burners are replaced with burners having jets with more or less momentum than the original burners. • Jet momentum is defined as: • • → → → → m = = ρ ⋅ = = G m V V A V ( N ) V ρ A • Density decreases with altitude. • Velocity increases with altitude for a given flow rate. • An increase in velocity will increase jet momentum. • Consequence: A target circle diameter that is optimal at sea level may no longer be adequate when the boiler is located at high altitude (2000 ft and more). 8
Problem description Tangentially fired boilers • Example: Comparison of a natural gas flame with standard target circle diameter for a single burner level T-fired boiler at sea level and at 5200ft altitude. Fireball at sea level Fireball at 5200 ft 9
Problem description Tangentially fired boilers • Example: Comparison of a natural gas flame with the target circle diameter optimized for operation at sea level for a single level T-fired boiler with the same design operating at 5200ft altitude. Fireball at sea level Fireball at 5200 ft 10
Problem description Tangentially fired boilers • Additional problems are encountered when trying to optimize the firing configuration for both Natural Gas and Heavy Oil firing: • The optimal target circle diameter for natural gas firing is most often too small for Oil firing because of the difference in the distribution of momentum in the flames. • The core of the central vortex for oil flames can become unstable. 1 st burner level - NG 1 st burner level – Oil #6 11
Problem description Tangentially fired boilers • Combustion of the fuel increases the gas temperature in the burner jet and causes the expansion of hot combustion gases. Dual-Fuel (54% Oil – 46% NG) Firing Temperature (K) 12
Problem description Tangentially fired boilers • The expansion of the gases causes local acceleration in the jet in the ratio of about 5/1. Dual-Fuel (54% Oil – 46% NG) Firing Velocity Magnitude (m/s) 13
Problem description Tangentially fired boilers • The local acceleration is more pronounced and localized for heavy oil burners because all the fuel is concentrated in front of the oil gun. Dual-Fuel (54% Oil – 46% NG) Firing Velocity Magnitude (m/s) 14
Problem description Tangentially fired boilers • The jets from Oil flames usually have much higher momentum and penetration than Natural Gas flames. • It is obvious that the optimal target circle diameter will be different depending on the fuel. • The problem is compounded when operating at high altitudes because the effect on gas firing does not vary proportionally to the effect on oil firing. 15
Problem description Tangentially fired boilers • The firing angles should be determined according to the burner design, fuels to be fired and the altitude at which the unit will operate. • The target circle for dual fuel burners must be a compromise between the optimal firing angles for Natural Gas and the optimal firing angles for Heavy Oil. Oil #6 Firing Natural Gas Firing Burner Level 1 - Velocity Magnitude (m/s) 16
Problem description Tangentially fired boilers • Even when the optimal firing angles for Natural Gas have been modified to accommodate oil firing, the final firing angles for Natural Gas yield a much smaller vortex diameter in the center of the furnace. • Reducing the vortex diameter helps keep the reacting regions away from the furnace walls. NG-Firing – Original sea level firing angles NG-Firing – Optimized firing angles Burner Level 1 - Velocity Magnitude (m/s) 17
Problem description Tangentially Fired 300MW Utility Boiler •Boiler Characteristics: • 300 MW Utility Boiler • Gross Heat Input : 800MW (2725 MMBTU/hr) • Steam Generation : 907,000 kg/hr (2,000,000 lbs/hr) • Altitude : 1722 m (5649 ft) • Project: • Replace the existing combustion system (32 burners : 16 NG, 16 Oil) with a new 16 Dual-Fuel tilt-burner Low NOx firing system. • New Combustion System characteristics: • 16 tilting burners • 8 OFA ports • The firing system must be able to operate with: • Natural Gas • Heavy Oil #6 • Dual Fuel Firing (NG and Oil) • 25% FGR • 12% Excess Air 18
Problem description Tangentially Fired 300MW Utility Boiler : New Combustion System : • Most of the NOx produced in this furnace are the result of thermal NOx formation. • Thermal NOx formation requires: • High temperatures (T>1800K). • The presence of sufficient quantities of oxygen and nitrogen. • Limiting Thermal NOx formation in this furnace is achieved by: • Fuel Staging at the burner level • Fuel and air are injected in such a way as to minimize the presence of high temperature zones and high concentrations of oxygen at the same place. • Furnace Staging • The overall Fuel/Air mixture at the burner level is maintained fuel-rich in order to minimize NOx formation by denying the reaction the necessary oxygen. • The remainder of the combustion air is injected at the OFA level once the temperature of the combustion products has decreased. Secondary combustion, in excess air, at the OFA level of the CO remaining in the combustion products therefore takes place at a lower temperature and produce less thermal NOx. 19
Problem description Tangentially Fired 300MW Utility Boiler : New Combustion System : CFD investigation of the new combustion system: • Determine the optimal tangential burner firing angles to: • Ensure stable fireball aerodynamics for all fuels fired at the given altitude. • Ensure adequate mixing of Combustion Products and OFA for complete combustion. • Avoid flame impingement on the superheaters and the furnace walls. • Ensure that the furnace heat transfer characteristics are similar to those of Cerrey’s performance run predictions (idealized design cases) for each type of fuel firing by comparing theoretical and CFD predictions of Furnace Outlet Plane (FOP) gas temperatures. 20
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