Thanks to advances in damper drive technology, steel plant instrumentation and control engineers can now demonstrate to upper management that improved control not only saves money, but can actually make money, by enabling optimal use of blast furnace gas (BFG). However, extracting additional revenue from spent fuel requires more than just accurate sensors and feedback control algorithms: it also takes responsive mechanical technologies to implement the commands.
Within the steel industry, I & C engineers are now turning to precisely controlled damper drives to move stack dampers accurately and rapidly in response to process sensor inputs, returning the maximum amount of blast furnace gas for reuse in plant boilers to shave fuel expenses, or to resell for instant cash.
TYPE K Damper Drives from Controls International helped both U.S. Steel, Gary Works, and Rouge Steel Co of Dearborn, MI, to realize the full potential from their BFG.
BFG''s Profit Potential
BFG is a byproduct of the process of converting iron ore pellets (or sinter) and coke into molten pig iron that can subsequently be transformed into steel. High-temperature (2000°F), high-pressure (30-60 psi) air is blown into the bottom of the blast furnace. As the hot air passes through, it is chemically changed to BFG. For every cubic foot of hot air entering the bottom of the furnace through the hot air "main," approximately 1.35 cubic feet of BFG is discharged from the top of the furnace.
Still hot (250-400 °F), the BFG is channeled to cleaning equipment where dust is removed and the gas cooled. At this point, it is suitable for use as a heating fuel.
BFG contains very low amounts of combustibles (20-22% CO) and high amounts of inert elements such as Nitrogen and CO2. The calorific value of BFG is relatively low (approximately 600 Kcal/Nm3); however, 2-3 tons of BFG are generated for each ton of iron, according to American Iron & Steel Institute figures. Considering that a typical blast furnace will produce 3,000-10,000 tons of molten iron each day, its value as an energy source increases rapidly.
Typically, BFG is used within a steel-making plant for two main purposes: to fuel the stoves that drive the hot-air blowers; and to heat the boilers that run the steam-driven turbines that generate electricity for the plant. By using BFG - essentially, a free byproduct -- to offset the need for burning expensive natural gas (methane) in the stoves and the boilers, steel-plant operators can save literally millions of dollars in fuel costs.
"We use blast furnace gas to burn in our boilers -- since it''s a byproduct fuel of the blast furnaces, it''s basically a free fuel for us," says Shawn Gallagher, who handles process control for the energy division of the U.S. Steel, Gary Works. Headquartered in Pittsburgh, PA, U.S. Steel manufactures and sells a wide variety of steel sheet, plate, tubular and tin products, coke, and taconite pellets. "Blast furnace gas reduces our need to buy fuel such as natural gas and fuel oil."
Rod Meloche, instrument supervisor at Rouge Steel Co of Dearborn, MI, has had similar experience during his 30 years in the steel business. With a total annual steel-making capacity of 3.3 million tons, Rouge Steel produces high quality flat-rolled carbon steel products for the automotive industry, converters, service centers, and other related markets. (Photo shows before and after views of damper drive controls at Rouge Steel.)
"Our blast furnace gas has about one-tenth the BTU content of natural gas, but even at that, it''s a fraction of the cost of natural gas," says Meloche. "We use some of the blast furnace gas to help heat our blast furnace stoves, and the rest of it is sold to a new cogeneration power plant located adjacent to the steel plant that in turn provides electricity and process steam to Rouge Steel.
"We both win," Meloche continues. "Our customer gets a lower cost fuel for its power generation requirements and we make a little money."
The Challenge of Harnessing BFG
While saving, or making, money by using BFG might easily be taken for granted given its low caloric content, the need for precisely controlled volumes and pressures cannot be overstated. High reliability is mandatory because steam turbines depend on a continuous supply to keep running. However, fluctuations in the process of making pig iron cause the supply pressure, flow, and calorific value of the BFG to vary widely, which can result in an unstable flame.
"We regulate the BFG pressure in the common main in order to keep their boilers firing, and my job is to make sure that the flow rate stays correct," states Meloche.
"If a blast furnace kicks, it will spike the pressure and can result in the shutdown of the power plant''s steam boilers. When steam production ceases, we cannot operate our steam turbines that provide wind for our blast furnaces -- a potentially vicious circle that we like to avoid."
U.S. Steel''s Pittsburgh plant also deals with pressure variations and the risk of wasting gas.
"When the pressure gets too high in the system, these gas-furnace blast stacks open up and flare the gas," notes Gallagher. "Well, every time we flare gas, that''s free fuel going into the atmosphere."
Both engineers point out that while instrumentation is the first step in capturing and controlling BFG, the other half of the equation requires exacting damper controls.
"There are several measures involved to better enable capturing that gas, but among the most important are the butterfly valves in the bleeder stacks," continues Gallagher. "You need quick-response actuators up there so you can keep just the right amount of gas in without over-flaring."
Rouge''s Meloche also stresses the importance of sometimes elusive damper controls.
"We have pressure sensors on the lines of both of our blast furnaces to monitor pressure changes under different applications," adds Meloche. "We grab one signal closer to the source, so that if a spike in pressure occurs, the sensor detects it immediately. It then feed-forwards control to the damper and the valve in the stacks so that we can govern the pressure in the line. In case of a surge, the damper is supposed to quickly open before the brunt of the pressure reaches our customer."
The problem is that many older damper drives do not respond to sensor inputs quickly enough to control the escape of valuable BFG.
"We had 60-year-old drives on our bleeder stacks," recalls Gallagher. "Once we got a new generator, the response was way too slow. They were flaring too much gas out the stacks. We needed to upgrade."
In the case of the Rouge plant, slow damper response was costing them revenue.
"Our old electric damper drive was a workhorse, but it wasn''t very effective at regulating gas main pressure," recounts Meloche. "Everybody knew that if we were taking our furnaces down, we had to call the power plant so they could make adjustments, to avoid losing the flame in their boilers."
Responsive Damper Drives Meet the Need
In many cases, the only thing separating potential BFG cost savings from being fully realized, versus going up in smoke, is the linkage and actuator that controls the flue-stack damper to limit the escaping gas.
"Damper actuation for process control is always a concern in our industry, since it is how we regulate pressure in the common main," states Meloche. "If you don''t have good control on your process, you can''t function a lot of times -- it''s basically that simple. In our case, we needed a damper drive that could open this valve quickly, and it had to be powerful because it''s on a 60 in. diameter pipe."
In applications within the steel industry, a damper drive should be capable of very small movements that can respond to 0.25% and 0.50% demand signal changes. The drive and connecting rod linkage movement must be totally free of any unnecessary backlash and deadband in order to move quickly.
"We knew it was time for new drives," says U.S. Steel''s Gallagher. "So when a local distributor told us about these new, accurate drives from TYPE K, we ended up going with them."
"It was our local sales rep [Great Lakes Process Controls] who told us about the TYPE K actuator," seconds Meloche. "We checked into it, and it looked good. The rep then took measurements and we ordered it."
TYPE K Damper Drives, a division of Controls International Inc., is the world leader in the manufacture of final control drives that thrive in extreme environments -- temperatures (between -40°F and 300°F), heavy vibration, and fly ash -- while providing continuous duty service (more than 3,600 starts per hour) with smooth, accurate, and repeatable damper positioning (an overall linearity of less than 0.5% and a hysteresis of less than 0.3%).
TYPE K can warrant its drives for three years or 4 million operations because the rotary actuator has only one moving part, thereby eliminating the linear-to-rotary conversion and the grinding gears of other drives. Seven different sizes are available to handle torque output ratings ranging from 90 ft lb to 20,832 ft lb (based on 100 psi supply air).
Of significance to specifying engineers, TYPE K retrofit drives are custom designed for each application, requiring no field fabrication. Pedestal mounts are designed to match the existing drive footprint dimensions and output shaft location (X-Y-Z axis). After the drive is fully assembled, calibrated, and cycle tested, it is then shipped ready for "drop-in" installation and operation.
Installation According To Plan
Given that plant managers now must often do the same amount of work that previously took two or three individuals, the need for turnkey, drop-in solutions has clearly increased.
"One of the reasons we chose the TYPE K was because they said they would exact retrofit it to what was already had," affirms U.S. Steel''s Gallagher. "Turns out, it was an exact retrofit. In September 2001, we had the new drives rated at 5,000 ft lb torque installed in the number one, two, and four blast furnace bleeder stacks. In every case, it fit right in."
The installation at the Rouge plant occurred in June 2002, with a TYPE K actuator rated at 2,250 ft lb torque placed in the main bleeder stack.
"We already had a nitrogen supply line at the location where the actuator was going to go, and we had our control wires [4-20mA signal] and terminations ready," says Meloche. "So when the TYPE K actuator arrived, we were all set. Amazingly, installation took place while we were running both blast furnaces and supplying BFG to the power plant.
"We were able to quickly pin the valve, remove the old actuator, hoist up the new one, bolt it up, and do some stroking for position. Then we turned it on and away she went. From start to finish, the installation took just four hours because everything was accurately predesigned," adds Meloche.
Taking Results To the Bank
A modeling study by Walsh and Mitterer at the University of Maryland''s Dept of Mechanical Engineering, Institute for Systems Research, examined the policy of trading the use of natural gas for less expensive blast furnace gas. The model compared the thermal efficiency based on optimal control. Depending on how the various energy sources are valued, new optimal controls reduce operating costs anywhere from 12-31%. This is really "big bucks," concluded the researchers.
In the case of the Rouge plant, the new damper controls helped improve its sale of BFG to its utility customer.
"Since we installed the TYPE K damper drive, we have had no problems with that part of our operations," says Meloche. "We''re so impressed with how it has responded; it''s just a matter of time before we''ll be putting the TYPE K damper drives in other areas of the plant. The fact that it can handle a 5 ft diameter valve is pretty impressive, so I''m sure it can handle anything else we have."
U.S. Steel was equally satisfied with their changeover.
"We are definitely happy with the new installation," echoes Gallagher. "I''d have to talk to some other people about the exact amount of gas furnace fuel that we are saving, but it is very substantial, that''s for sure."