Tenova TRGX burners on California Steel Industries new walking beam furnace

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1 Tenova TRGX burners on California Steel Industries new walking beam furnace Tenova has designed and built a new slab reheat furnace at California Steel Industries facility in Fontana, California. The furnace uses the latest in burner design and operation, achieving exceptionally low NOx emissions and high operating efficiency. Authors: Massimiliano Fantuzzi and Jared Kaufman Tenova SCENARIO CSI produces ~1.64Mt/yr of hot rolled, cold rolled, pickled and oiled, and galvanised steel products and electric resistance welded pipe via a hot strip mill route. The rolling capacity of the hot strip mill is ~400t/hr. In 2007, CSI proposed to revise its Title V permit by installing a new reheat furnace #5 and a new selective catalytic reduction unit and also requested a condition to limit the combined fuel usages for existing furnace #4 and proposed #5 in order to meet California Environmental Quality Act (CEQA) requirements. The Tenova Core-designed reheat furnace #5 is shown in Figure 1. r Fig 1 Walking beam furnace #5 The furnace industry in recent years has devoted much research effort into the development of new combustion techniques, seeking to achieve both reduced emissions and better thermal performance. This is partly a consequence of the Kyoto Protocol and also to mitigate the effect of rising fuel costs. Thus, energy efficiency and very low pollutant emissions are converging goals that guide investments in both new reheating furnaces as well as for furnace revamps. International and local environmental standards are becoming ever more restrictive and, in some areas with intense human and industrial concentration, such as California, pollutant emission limits are extremely low. An overview of the different technologies applied to industrial reheating furnaces for achieving energy saving and reducing pollutant emissions and the design criteria adopted for the new walking beam furnace at California Steel Industries (CSI) facility in Fontana, California is described. TECHNOLOGY ASSESSMENT The challenge was to select the most suitable combustion technology for the most profitable production in terms of high target product quality, consuming the minimum amount of energy, while complying with environmental regulations, and achieving minimum CAPEX. Minimum energy consumption is normally synonymous with minimum OPEX. The technology candidates are basically conventional recuperative furnace, regenerative furnace or oxy-fuel furnace. Hybrid solutions, heating by zones, are also possible with intermediate performances. A traditional furnace is an arrangement with cold air instead of hot air recovered by means of a central recuperator. Each solution has pros and cons, as reported in Table 1 for a representative furnace at 1,250 C. A recuperative furnace generally involves common collection of all furnace combustion exhaust gases and passing them through a central recuperator, which is used to preheat the combustion air before it is distributed to the burners. By preheating the air, a portion of the energy that would be exhausted is recovered and returned to the process, thus increasing combustion efficiency (see Figure 2). The next alternative for improving combustion efficiency is to fire with oxygen instead of air. As air is ~80% nitrogen, 96

2 Solution Characteristics Pros Cons Recuperative Furnace Oxy-Fuel Furnace Regenerative Furnace Central metallic recuperator: air preheat up to 650 C Oxy-fuel burners using oxygen instead of air Equipped with coupled regenerative burners preheating air up to 1,100 C r Table 1 Technology assessment Medium CAPEX Less maintenance Easier pressure control Possibility of flameless combustion Less scale formation Bigger furnace Low CAPEX Exclusion of nitrogen Theoretically zero NOx emissions Possibility of flameless combustion Higher heat transfer rate Compact furnace Very high air preheating Low NOx with flameless combustion Compact furnace Less efficient unless with long recuperative zone Limited air preheating Cost of oxygen Limited or no heat recovery at the stack Difficulty in preventing air infiltration Higher scale formation Refractory thermal stresses More maintenance Higher CAPEX Higher NOx emissions unless using flameless combustion Increased scale formation More maintenance by firing with pure oxygen, the volume of waste gases is significantly reduced, thereby reducing the amount of energy exiting the furnace stack. Regenerative burners work in pairs (see Figure 3) with typical cycles of secs, and are controlled by automatic valves installed into the combustion air drops, which are dual-purposed as flue gas ducts. One burner pulls hot flue gases from the combustion chamber through the cold regenerator media (often ceramic balls). As the gases flow through the media bed, its temperature is raised to about 1,200 C. Meanwhile, the flue gases exit from the regenerator at about 280 C, before flowing to the stack. While the burner is in exhaust mode, the gas valve and combustion air valve are closed. Meanwhile, the other burner is fed with cold combustion air, which passes through the previously heated hot regenerator, and the air temperature is raised to about 1,100 C. Flue gas valves are shut. Once the regenerator achieves its switching temperature, the burner switching valves are reversed. A comparison of the technologies above must be carried out using specific targets. One of them, related to specific consumption, is the available heat defined as the total thermal input (ie, heating value of the fuel plus the contribution of the air and/or fuel preheating) less the energy carried out by the hot exhaust gases. The a r Fig 2 Recuperative furnace scheme 97

3 r Fig 3 Regenerative furnace scheme r Fig 4 Available heat of furnaces available heat of a furnace can be used as the basis for a simple assessment of technologies from a thermal point of view. Figure 4 shows the available heat as a function of the waste gas temperature exiting a recuperative furnace (with different air preheating in the range C), a regenerative furnace (with a recirculation of 80% of waste gases) and a full oxy-fuel furnace. Natural gas is fired. The same available heat of a regenerative furnace can be obtained by a recuperative furnace with a much lower exit waste gas temperature and this can be achieved with a long recuperative unfired zone. In the same way, for a given waste gas temperature, it is evident that high air preheating of a regenerative furnace achieves higher available heat than an oxy-fuel or a recuperative furnace. Hybrid solutions can be considered simply by interpolating the curves. With the target of minimum specific consumption, the regenerative furnace solution is the most effective. For furnace #5 Tenova Core selected a hybrid solution featuring flameless regenerative burners and flameless traditional burners (with recuperation) in order to optimise energy consumption and minimise NOx emissions. THE CSI FURNACE The new walking beam furnace (see Figure 5) is designed to reheat a range of steel grades from ambient to a discharge temperature suitable for rolling. Design data are listed in Table 2. The furnace is top and bottom-fired and is end-charged by means of a charging machine and end-discharged by an extractor. Walking beams, charging/discharging machines and charging/discharging doors are hydraulically operated. The furnace is equipped with a metallic recuperator to recover heat from the furnace flue gases by heating combustion air and comprises sets of alloy tube bundles located in the waste gas flue leading to the SCR unit. Regenerative burners are used in heat 1 and heat 2 zones (see Figure 6) to improve efficiency, taking advantage of the high waste gas temperature in these zones to obtain the maximum preheat, and allowing Topic Design Range Nominal Design Value Product type Slabs Steel grade Low carbon/low alloy Low carbon/low alloy Slab thickness mm 230mm Slab width 610 1,905mm 1,245mm Slab length 4,013 5,512mm double row and >5,512mm single row 9,652mm Weight 28,122kg max. 21,655kg Charge temperature 20 C 20 C Discharge temperature 1,150 1,300 C 1,300 C Production rate 318t/h r Table 2 Design data 98

4 r Fig 5 Furnace longitudinal view r Fig 6 Combustion scheme these zones to operate in a highly efficient manner. SCR is used to reduce NOx in the waste gases before they are discharged to atmosphere and a waste heat boiler is used to recover some of the available heat from the waste gases after passing through the recuperator and before passing into the stack. A fully integrated process control system monitors and controls all of the process functions of the furnace according to the typical architecture with Level 0 (field mounted apparatus), Level 1 (PLC for the control functions of the combustion and mechanical equipment) and Level 2 (supervisory computer-based system to determine steel temperature using an online real time mathematical model of heat transfer). The combustion system is designed to provide sufficient heat input to maintain the design production rate with 90% skid pipe lining, and uses low NOx flameless and regenerative burners (see Figure 7). The flameless burners were designed specifically for the application. The furnace is divided into four main zones: preheat, heat 1, heat 2 and soak zones. All the zones are then further divided, top and bottom, as well as the soak zone being divided east and west, to give ten temperature control zones. The products of combustion from the flameless hot air burners and the remaining 20% from the regenerative burners exit the furnace through the charge end uptake flue, and pass through a recuperator, which is used to preheat the combustion air. The regenerative burners exhaust 80% of their own products of combustion, with the aid of an exhaust fan, through a media box, which also preheats the combustion air for the regenerative burners. The now cooler regenerative exhaust joins the products of combustion exiting the recuperator and together they enter an SCR, waste heat boiler and a furnace pressure damper, before exiting to the atmosphere by means of an exhaust fan. The burners in the heat 1 and heat 2 zones are regenerative, and burner capacity is greater than the a 99

5 # Zone Burner Firing Burner No. of Zone Type Pattern Input (MW) Burners Input (MW) 1 Preheat Top Recuperative Side Preheat Bottom Recuperative Side Top Heat 1 Regenerative Side 5.86/pair Bottom Heat 1 Regenerative Side 5.86/pair Top Heat 2 Regenerative Side 3.22/pair Bottom Heat 2 Regenerative Side 3.22/pair Top Soak East Recuperative Roof Top Soak Center Recuperative Roof Top Soak West Recuperative Roof Bottom Soak East Recuperative Side Bottom Soak West Recuperative Side TOTALS r Table 3 Burner summary theoretical capacity required to allow for normal operating upsets encountered in mill operation. The burners installed the different zones of the furnace are summarised in Table 3. The amount of NOx produced at the design condition is 58ppm for recuperative burners and 100ppm for regenerative burners. Pilot burners The main burners are lit using electrically ignited pilots in the bottom heat 1, bottom heat 2 and bottom soak zones. After the pilots have been lit and flames verified, the main burners can be lit. Once any other zone temperature has exceeded the auto ignition temperature, the main burners in that zone can be lit. r Fig 7 Tenova regenerative flameless burners Cascade control Combustion control is carried out by a cascading control system whereby the burners are progressively shut off, beginning with those closest to the charge end as a percentage of turndown or flow. This method of combustion control is also used for the preheat zone. As furnace production rate decreases and heat demand decreases, burners are shut off in steps starting from the charge end of the furnace in order to heat the stock as late as possible. This effectively lengthens the unfired section of the furnace at low production rates, saving fuel and creating less scale than a conventionally fired furnace zone. Also, by reducing the number of burners firing, the remaining burners have a higher flow-rate than if the whole group of burners were turned down. This gives improved flame geometry and heat distribution across the furnace at the turned down conditions. Recuperator In order to preheat the combustion air for the preheat and soak zones, a convection type recuperator is located in the waste gas duct. The recuperator heat 100

6 r Fig 8 Tenova TRGX temperature field inside the test chamber exchange surface comprises tube bundles arranged in a two-pass, airside, cross waste gas flow configuration. The tubes are arranged vertically with the combustion products flowing in a direction perpendicular to the tube body. Combustion air is preheated up to 560 C in the nominal condition. Fans Three combustion air fans are dedicated to the recuperative burners, three to the regenerative burners and a dilution air fan is installed to protect the recuperator. A hot exhaust fan pulls the product of combustion through the regenerative media boxes. There is a main stack fan through which all combustion gases are evacuated. Combustion air system This includes all necessary equipment such as manual, flow control and on/off valves, stainless steel bellows type expansion joints, flexible connections and metering devices. Each temperature control zone has an independent stainless steel concentric orifice plate and high temperature wafer style butterfly flow control valve. Burners that turn on/off which are part of the cascading control, have wafer style butterfly valves with rack and pinion type pneumatic actuators with spring return. Those burners that do not have actuated valves have a manual butterfly valve with locking handle. For the regenerative burners, each line, except for the 1st stage air on non-piloted burners going into the burners has a cycle valve, which is controlled automatically with a solenoid or electrically operated. r Fig 9 Test furnace Fuel systems The fuel used is natural gas. The main system header includes a pressure reducing station with manual bypass and a three-valve block and bleed which automatically closes. The vent valve opens if a critical situation develops concerning one or more of the a 101

7 r Fig 10 TRGX burner (flame mode left, flameless mode right) following systems: loss of electrical power, high or low gas pressure, low combustion air pressure, cooling water flow, zone temperatures, recuperator temperatures, and low compressed air pressure. Each fuel/air ratio control zone has a manual isolation valve and automatic shutoff valve, an adjustable port type flow control valve with actuator and a stainless steel orifice plate. Each burner drop contains a female male lubricated plug valve for isolation, and automatic shutoff valves as required for cascading. Furnace pressure damper The stainless steel furnace pressure control damper is arranged to pivot on a vertical axis and the damper bearings do not require water cooling. It is driven by an electric actuator and works in tandem with the variable frequency drive-controlled main stack fan. Furnace pressure is controlled to provide a slightly positive furnace pressure to minimise air infiltration which would otherwise decrease fuel efficiency and increase NOx emissions. Selective catalytic reduction system In order to further reduce the NOx level, an SCR system has been installed in the waste gas duct and is contained within a spool piece that fits into the duct between the recuperator outlet and the waste heat boiler. The main system components are: ` Forwarding system which accepts, stores and transfers ammonia to the injection pumps. It comprises a 38,000 litre tank with a hatch, a drain, a Camlock fill connection and a vent. The tank contains two level transmitters, pressure relief valve, vacuum relief and leak detection. ` Delivery system which takes the 19% aqueous ammonia and delivers controlled amounts to the injection nozzles based on feedback from the NOx analyser. It comprises: Two injection pumps, one operating and one standby, each rated at 0.75litres/min, 690kPa and with dedicated relief valves. The pumps are driven by variable frequency drives Aqueous ammonia flow metering and flow control valve header Instrument air header with pressure control valve, solenoid valve Two ammonia dilution air fans, one operating and one on standby. Each fan is rated at 1,700 actual m 3 /h and 5kPa. The fans draw waste gas from the waste gas duct after the catalyst for mixing with the aqueous ammonia solution before injection into the waste gas duct upstream of the catalyst. ` Catalyst The catalyst is the entire section of waste gas duct that contains the catalyst and other necessary components including internally lined section of duct with test ports, catalyst (module size: (w) 560mm x (h) 380mm x (l) 380mm) and perforated plate to insure even distribution of the waste gas into the catalyst. TENOVA S BURNERS The first development stages of the TRGX burner were carried out in cooperation with the research organisation, Centro Sviluppo Materiali (CSM), by means of an iterative procedure consisting of design of the burner prototypes based on the experience gained in the engineering of the previous TSX (flameless) and TRG (regenerative) models, extensive Computational Fluid Dynamics (CFD) modelling (see Figure 8) to optimise the burner design, and testing in laboratory furnaces (see Figure 9) and full-sized industrial furnaces. The selection of the physical models has been based on extensive validation work to evaluate the performance of the different turbulence representations for simulating high velocity round jets and the combustion reactions for natural gas. Subsequently, CFD methodology was used for the optimisation of the TRGX flameless regenerative burners. The main concern in this R&D project was to maintain the high levels of air dilution while developing the reaction zone inside a compact volume. An experimental validation phase then followed where the selected burner prototypes were installed inside a test furnace in order to collect operational data and verify development activities. The laboratory tests of the TRGX burner were carried out at CSM on the modular furnace which is representative of a section of an industrial furnace. Standard furnace equipment includes several thermocouples on the roof and walls in order to monitor temperature profile. To directly verify the thermal load received by a steel charge on a real furnace, a cooled and insulated pipe monitored by recorded thermocouples had also been installed. Heat density in the modular furnace can be adjusted through the initial choice of furnace 102

8 Pollutant Emission Max hourly Max daily Max monthly Max yearly factor ppm kg kg kg kg CO ,763 69,163 NOx PM ,037 12,449 VOC ,153 13,833 SOx ,186 r Table 4 Furnace operating results [Emission factor is the relationship between the amount of pollution produced and the amount of raw material processed. In this case its value relates to the quantity of each pollutant with the fuel flow rate] length and, during the tests, controlling fuel input and air preheating. Heat extraction is also monitored through the measurement of temperature variation and mass flow rate of cooling water. Flue gases composition (O 2, CO and NOx) is measured through a suction probe installed at the exit of the furnace (before the butterfly valve) and a reliable and accurate gas analyser is fed through a heated line. A pressure transducer is used to monitor the furnace pressure, which is controlled via an air-cooled butterfly valve at the end of the flue gas duct. Figure 10 shows a pair of 1.5MW TRGX burners in two characteristic combustion modes with furnace temperatures of 1,150 C. TRGX burners operate in flameless mode when the furnace temperature is over the auto-ignition temperature of the fuel (about C for natural gas). NOx measurements indicate that the flameless combustion allows a decrease of about 60% in NOx emission compared to traditional flame mode over the whole range of tested O 2 concentration in furnace. The CO concentration in the flue gases has been found to be negligible in all of the test conditions, confirming a complete combustion process inside the furnace. Flame mode is used only during the start-up of the furnace or when temperature is below the auto-ignition value. NOx emissions are quite a bit higher than those in flameless mode. Figure 11 shows NOx emissions for the 5.1MW TRGX burners in flame mode and in flameless mode at two operating furnace temperatures: 1,150 C and 1,250 C. Average NOx emissions are not affected by the turn down while the effect of furnace temperature is limited (the combustion air temperature is not so different from the temperature of the recirculated gases). Results of the tests confirm that flameless combustion coupled with high temperature air preheating, typical of the regenerative burners, is one of the best technologies for a large reduction of both NOx and CO 2 emissions, thus matching the incoming environmental requirements of the steelmaking industry. a r Fig 11 NOx emissions for TRGX burners r Fig 12 NOx emissions upstream and downstream of the SCR 103

9 OPERATIONAL RESULTS Data relevant to the furnace emissions is summarised in Table 4. The furnace operating schedule is 24/7 for 50wks/yr, but is inactive for 16hrs/wk for hot maintenance. CONCLUSIONS Many well-known potential drawbacks (cost, complexity, control) that affect product quality, emissions, etc., and that previously limited widespread implementation of regenerative burner technology, are now being overcome by Tenova, with specific technological solutions, such as flameless technology, correct material choice, burner design, control strategy, and overall furnace design, from knowhow derived from more than 40 years experience. Tenova regenerative flameless burners combine the lowest NOx emission level with high temperature combustion air preheating, which allows an important energy saving, thus meeting the latest demands from the steel industry. Thanks to coupled gas and air staging, working in flameless mode, NOx emissions are reduced to 33-66ppm. NOx is further reduced at CSI by means of an SCR unit, giving 5-30ppm in order to comply with the local environmental regulations. ACKNOWLEDGMENTS The authors would like to thank Harry Allin, Executive Vice President, Operations and Don Barzan, Operations Planner, Hot Strip Mill, of California Steel Industries, for the support they provided for this paper. MS Massimiliano Fantuzzi is R&D Manager, Tenova Italimpianti, Genova, Italy Contact Jared Kaufman is Vice President, Technical Services, Tenova Core, Coraopolis, PA,USA COntact