Jul 5 2002
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Energy efficiency, productivity, and reduced emissions are considered as major challenges in the steel industry. Such requirements can be and obviously have been fulfilled by using oxy-fuel combustion in a variety of batch as well as continuous-type furnaces.
Roller hearth, walking beam, and pusher are examples of continuous furnaces. These furnaces are designed such that the flow of exhaust gases is counter-current to the in-coming product. Thus, the energy contained can be utilized in the pre-heat zone at the furnace entrance, thereby improving thermal efficiency.
However, the use of oxy-fuel in these kinds of furnaces leads to a step-change increase in fuel efficiency as well as productivity, that otherwise cannot be achieved through air-fuel combustion methods.
Characteristics of Oxy-Fuel Combustion
Oxy-fuel combustion offers several potential advantages that can be credited to two fundamental enhancements. First, there is a significant enhancement in the furnace’s thermal efficiency because of decreased energy losses through the exhaust gases. Second, the efficiency of radiant heat transfer is increased because of better radiant heat transfer properties, that are integral to oxy-fuel combustion.
The overall heat transfer that takes place inside a furnace can be divided into four major areas:
- The transfer of radiant heat from the flame to the product
- The transfer of radiant heat from the exhaust gases to the product
- The transfer of convective heat from the exhaust gases to the product
- The transfer of radiant heat from the furnace structure to the product
Radiation Heat Transfer from the Flame
Carbon particles present in the fuel are the source of radiation from the flame. An oxy-fuel flame typically contains fewer particles. This is because the improved thermal efficiency results in less usage of fuel for a specified heat input requirement, when compared to an air-fuel. But the overall flame radiation is also a function of the temperature variation to the fourth power between the material being heated and the flame.
This more than offsets the lower number of carbon particles since the adiabatic flame temperature of the oxy-fuel flame is about 2700 °C when compared to 1800 °C of the air-fuel. This gives an overall, higher total radiation from the flame, and results in improved radiant heat transfer in oxy-fuel combustion.
Radiant Heat Transfer from the Exhaust Gases
The radiant heat transfer from exhaust gases can be defined as a function of the molecular components inside the exhaust gases, and also the velocity and temperature of the exhaust gas stream.
Air-fuel combustion contains exhaust gases that include mostly nitrogen, with smaller quantities of water vapor and CO2. However, the oxy-fuel combustion products are water vapor and CO2. These are tri-atomic molecules and exhibit exceptional radiation heat transfer characteristics, while nitrogen cannot transfer its heat energy through radiation.
Hence, the air from oxy-fuel combustion contains mostly hot CO2, and water vapor transfers heat in a much more efficient way when compared to the mostly nitrogen-containing air-fuel atmosphere.
Convective Heat Transfer
Heat conduction inside the gases is known to be fairly poor. Hence, turbulence and velocity have a major impact on the extent of heat transfer by convection. Due to this reason, convection contributes considerably to the total heat transfer in the air-fuel fired furnace, because huge volumes of exhaust gas travel via the furnace.
But this is not so in the case of oxy-fuel because the exhaust gas volumes are decreased by more than 75%. Also, the high efficiency of radiant heat transfer by CO2 and water vapor is by far the main source of heat transfer.
Radiant Heat Transfer from the Furnace Side Walls
Radiant heat transfer of the surface of a material mainly depends on its absorptivity and emissivity. In oxy-fuel combustion, heat is effectively transferred to the sidewalls of the furnace because of the efficient transfer of radiant heat from both the exhaust gas and directly from the flame heat.
The transfer of radiation heat from the sidewalls of the furnace and back into the furnace largely depends on the surface’s emissivity and absorptivity. Moreover, the rate of heat transfer is affected by the relative positions of the radiating surface and the receiving surface. The quantitative measure of this potential is called the “arrangement or view factor.”
This is equal to the fraction of radiation emerging from the radiating surface to the receiving surface. Hence, the transfer of real radiation heat from the walls is the black body rate multiplied by the arrangement or view factor and the emissivity factor.
Benefits of Oxy-Fuel Combustion
Oxy-fuel offers the following major benefits:
- Higher furnace productivity
- Enhanced quality
- Decline in specific fuel consumption
- Reduction in emissions
These advantages are highlighted in the following sections.
Higher Furnace Productivity
Due to the enhanced heat transfer properties of oxy-fuel combustion and the enhanced thermal efficiency of the furnace, the heating rate to the required temperature is faster when not subject to metallurgical limitations. Many factors such as the temperature of the target, the charging temperature of the product, the material, the geometry of the product (specifically the ratio of its surface area to total volume), the prevailing condition of the furnace, and the geometry of the furnace govern the heating rate.
In some cases, improved quality can be observed because of improved characteristics. This is due to, for instance, improved temperature uniformity achieved while heating the product. This is the outcome of having an excellent circulation of the exhaust gases in the furnace and a natural outcome of having exhaust gases containing H2O and CO2; both H2O and CO2 have excellent radiant heat transfer characteristics.
Secondly, reduced scale formation has been observed, specifically with stainless steel. This was mainly due to better control of the furnace atmosphere provided by the oxy-fuel combustion, together with the reduced time at the temperature required by the product to reach the set-point temperature.
Decline in Specific Fuel Consumption
The enhanced heat transfer properties and better thermal efficiency cause a reduction in the requirement for a particular fuel to heat products to a specified temperature. This again depends on the aforementioned factors.
Reduction in Emissions
Results demonstrate that the reduction in fuel-borne emissions, like particulates and SOx, widely follows the reduction in specific fuel consumption. In a similar way, the CO2 formation is reduced according to the reduction in specific fuel consumption. The Zeldovitch mechanism is used to regulate the formation of thermal NOx. While the concentrations of oxygen and nitrogen play a major role, the temperature itself is the true critical aspect in the extent of NOx formation.
Thermal NOx emissions can be minimized through precise flow control equipment, good furnace pressure control, and uniquely designed low NOx burners. Methods to achieve this are discussed in the following section.
The formation of NOx is the challenge that has driven the development of most of the novel oxy-fuel burner and control technologies as well as the latest furnace zoning strategies currently being used in heating furnace applications.
NOx is formed either by the combination of nitrogen contained in the fuel during combustion with free oxygen, or by the reaction of free nitrogen in the air with the available oxygen, or the so-called thermal NOx.
In an ideally airtight furnace, the formation of NOx arising from the small quantity of nitrogen present in the fuel, would usually be insignificant. But in practice, air can leak into the furnace and supply a major source of nitrogen for NOx formation.
The generation of thermal NOx is a function of the flame temperature and the available oxygen and nitrogen concentrations. One aspect that is instantly clear is the dramatic effect of the temperature on the extent of NOx formation (NO2 formation can be disregarded at temperatures over 1000 °C).
As an example, at 1 bar total pressure and 1500 °C, 4% N2 and 2% O2 would result in approximately 280 ppm NOx at equilibrium. This matches with 41 to 45 mg NOx/MJ based on the fuel type. The data points indicate flue gas analyses made from two sets of measurements. These measurements were performed on a furnace with a wall temperature of approximately 1300 °C and an estimated maximum flue gas temperature of 1400 °C.
Overlooking NO dissociation during the quenching of a gas sample, and recalculating NO2 to NO, the data points would reflect the concentrations of NO in the furnace atmosphere for enormous sets of partial pressures for nitrogen and oxygen.
It must be remembered that during the measurement periods, the furnace was exposed to heavy leakage of air reflected by maximum values. Additionally, the concentration of oxygen was maintained comparatively high (~5%) as needed for the specific steel grade being treated. But with the square root of (pO2 x pN2) below 0.03 bar, the ensuing NOx emissions are fairly low.
Perhaps, NO is produced mainly at the high temperatures in the vicinity of the flame and subsequently dissociated at the lower temperatures of the surrounding atmosphere in the furnace. The NO concentration, thus obtained, will rely on various other factors, including residence time and the temperature pattern of the furnace.
While the kinetics are quite complicated, it can be concluded that there is a strong association between pO2 x pN2 and NO, implying that very low NO values can be achieved by precisely controlling the burner stoichiometry, and tightening the furnace to reduce the amount of air that leaks from the furnace.
Low NOx Burner Development
At present, a lot of controversy surrounds NOx regarding the way it is measured. When oxy-fuel burners are used, the amount of flue gases produced is reduced significantly. However, this can increase the concentration of the NOx in the exhaust gases owing to the reduced overall volumes of the flue gas.
The formation of NOx is very much a function of temperature. Below 1425 °C (2600 °F), the formation of NOx is not dramatic, but above this threshold temperature, a rapid increase takes place. For this reason, the recent low-NOx burner developments mainly focus on decreasing the flame temperature as a means of decreasing NOx.
An air-fuel burner is used to reduce the flame temperature, and this is achieved by firing with surplus air or by re-circulating the exhaust gases, or a combination of both. When a small amount of surplus air is fired in the region of 5%–7%, NOx formation increases as a result of the additional oxygen and nitrogen available. But at maximum levels, the surplus air is more than enough to cool the flame to such an extent that it begins to suppress the formation of NOx.
In oxy-fuel combustion, an excess level of oxygen would significantly increase the rate of NOx formation without any flame cooling effect, unless impractically high levels were utilized.
Hence, in order to realize very low levels of NOx, that is, below 100 mg/MJ, focus was given to the re-circulation of the exhaust gases via the burner block to cool the flame, as well as enhance the circulation of the exhaust gases. The other major developmental area has been that of staged combustion in which a part of the needed oxygen is later added to the main combustion stream to restrict the temperature of the flame. This has resulted in a wide range of burners, both ceramic and metallic, the designs of which integrate both re-circulation and/or staged combustion to be suitable for the specific furnace being transformed.
Fuel Ratio Control
The problem of the formation of NOx in oxy-fuel combustion has driven the burner development program with respect to the latest low NOx staged combustion burner. This has also led to the need for relatively more precise ratio control when altering power levels.
In 1998, a newly developed roller hearth furnace was commissioned at the hot rolled plate division of Avesta Polarit steel plant in Degerfors, Sweden. Measuring 3.5 m wide and 35.4 m long, the oxy-fuel was mainly used to boost productivity—as the company was migrating from a batch to a continuous operation. The oxy-fuel was also aimed to decrease operating costs, enhance product quality, and reduce emissions with a specific focus on NOx.
Linde provided the oxy-fuel combustion system for the furnace, for which rigorous NOx limits of just 80 mg/MJ were imposed on it. It was also necessary to prevent reducing conditions in the furnace. To realize this, the furnace was fitted with 59 uniquely designed ceramic ultra-low NOx burners including 13 individually controlled zones to provide the highest power input of 16.5 MW.
These burners use a kind of staged combustion by adding about 50% of the oxygen for combustion via the main inlet pipe, together with the entire fuel. The remaining oxygen is injected at high velocity via holes in the ceramic block, provided around the main inlet pipe, such that the remaining combustion occurs further away from the main flame front. This helps in cooling the flame and thus reducing NOx.
After initial issues with achieving the NOx limit, a new kind of flow controller was created. These bilinear predictive controllers operate with traditional PID controllers to evaluate the non-linear characteristics, and thus reduce deviations from the set point.
Practically, this means that when the burner power is altered, the ratio is kept stable to prevent the presence of surplus oxygen in the flame during the adjustment phase. This considerably helps in attaining the required low NOx levels.
The third significant factor in preserving low NOx is to maintain a positive pressure in the furnace, and thus prevent air from being sucked in.
The requisite turn-down ratio was in the order of 10:1, implying that having very good damper control and sizing of the exhaust outlet was crucial to ensure a positive pressure. Furthermore, exclusive attention was given to the amount as well as control strategies of the zones, so that such a high turndown can be achieved with a high level of precision.
Reduced Specific Energy Consumption
In addition, the specific fuel consumption is decreased by installing the oxy-fuel burners, because reduced amounts of flue gases mean less heat is lost to the atmosphere.
As a case in point, at Bohler Udderholm in Sweden, the specific energy consumption was decreased by 45%–50%, with matching reductions in the fuel-borne emissions such as particulates, SOx, and CO2. Bohler Udderholm is a top producer of tool steels. In 1994, a decision was taken at the Hagfors production facility in Sweden to change three of the four car-bottom furnaces, from cold-air combustion to oxy-fuel.
Such furnaces are employed for charging huge ingots of up to 65 tons, which are subjected to a lengthy heating phase to achieve thorough heating to about 1200 °C–1300 °C before forging. The material experiences an initial predetermined heating phase, which can take 5–10 hours, followed by a longer soaking period.
Eight burners in the original air-fuel system were divided into two zones with an overall power capacity of 2.8 MW. Low sulfur medium-heavy oil was the fuel used. This was substituted by six metallic, water-cooled, supersonic recirculation oxy-fuel burners to ensure even temperature distribution inside the furnace. With a collective power of 2.4 MW, the oxy-fuel burners were divided into two zones and were positioned high in the furnace, to prevent the flame from directly impinging on the ingots.
Changes were made to the exhaust channel exit to allow excellent pressure control, and thus to reduce leakage of air.
The company’s main objective was to reduce fuel consumption and boost production capacity, thus enhancing efficiency. The results obtained were increased production capacity of 5%–20%, due to decreased heating times of 25%–50%. Thanks to the positive outcomes, the fourth furnace was changed in 1999, and a fifth furnace was commissioned toward the end of 2001.