Furnaces used in annealing and pickling (A&P) lines for stainless steel strip are generally designed as catenary furnaces. The strip is heated from cold to temperatures in the range of 2000 F (1095 C) using free flames. In most cases, the combustion products are withdrawn on the strip inlet side, preheating the strip in an unheated furnace section. This preheat section is required to improve fuel efficiency and to lower exhaust gas temperatures prior to a central air preheater. Production increases are often limited by:
- Decreasing fuel efficiency with increasing firing rates
- Limitations for the heat exchanger for combustion air preheat
- Limitations in the flow capacity of the exhaust system
- Increasing NOx emissions
All these limitations can be overcame by the use of autoregenerative burners.
Regenerative burner concept
A major difference of the autoregenerative burner concept is to withdraw the exhaust gases locally and use them for combustion air preheating within the burner body (Fig. 1). That enables to control furnace zones independent from neighbor zones and eliminates the requirement for an unheated preheat zone.
The large surface area of ceramic honeycomb regenerators allows lowering the temperature of exhaust gases from temperatures well above 2000 F at the hot face to below 300 F (150 C) after passing through the regenerators. This low temperature allows integrating the switching valves directly into the burner housing. Every ten seconds, the flow direction of exhaust and combustion air is reversed, and the combustion air picks up the energy from regenerators, reaching temperatures of more than 1800 F (815 C). Exhaust-gas losses are minimized to increase fuel efficiency.
Combustion with highly preheated combustion air generates exceedingly high NOx emissions if no special precautions are taken. The use of the FLOX(r) (WS Wärmeprozesstechnik GmbH) principle allows restricting NOx formation to levels lower than those from cold air or moderately preheated air burners. In the FLOX mode, temperature peaks of flames are avoided and combustion occurs very smoothly (a homogenous temperature distribution) at low noise levels and often without a visible reaction zone (Fig. 2). Because the FLOX mode is only possible when the furnace temperature is above self-ignition temperature, the burner must be capable of burning with a stabilized and supervised flame at lower temperatures. This low temperature flame mode can also be activated for peak power demand, doubling the gross capacity for a so-called boost mode.
Since the switching valve, mechanical and electrical over-temperature protection and other controls are preinstalled (Fig. 3) on the burner body, furnace design, installation and start up are straightforward and do not require more effort than installing simple cold air burners.
The burners are controlled on/off in a sequential firing mode. The flow rates are either 0% or 100% and drawbacks of turned-down firing are avoided.
Case study
Recently, an A&P line at Columbus Stainless Ltd. in South Africa was upgraded, using regenerative burners. The main data of the line are:
- Length of the line: 253 m (830 ft)
- Length of the furnace: 36 m (118 ft)
- Number of furnace zones: 6 zones (zones 0 to 5)
- Reference strip: 3 mm gauge; 1,500 mm wide (0.12 in.; 59 in.)
- Process temperature: 1150 C (2100°F)
- Burner quantity, power: 100 REGEMAT burners
- Burner capacity: 680,000 Btu/hr (boost mode: 1,360,000 Btu/hr)
- Increase in production: from 40 t/hr to 70 t/hr
An important issue for the retrofit project was furnace downtime, which needs to be kept to a minimum. For this reason, the retrofit was carried out in steps. In a first step, 26 burners were installed into zone 0 with the furnace off two days, followed by three days for burner installation and two days for furnace commissioning. Retrofitting zones 1 to 5 with 74 burners required eight days for installation and two days for commissioning. Burner installation and start up went smoothly, and the target production rate was achieved in a short period. An analysis of the fuel efficiency has not yet been conducted, but is expected to be below 1000 ft3/t of natural gas, well below the previous consumption.
Conclusions
Continual changes in energy costs and environmental restrictions require improved combustion equipment, and many manufacturers are developing new equipment and engineered systems to meet these needs. The use of advanced equipment can lower costs and meet emissions standards while increasing production simultaneously as shown in the case study discussed above.
The author thanks Columbus Stainless Ltd. (South Africa) and Engineered Thermal Systems Ltd. for their fruitful cooperation.