Application of oxygen in heating furnace combustion systems

The main function of a reheating furnace is to raise the temperature of semi-finished steel (billets， blooms， slabs or rounds) typically to between 1，000 degrees Celsius and 1，250 degrees Celsius until it is plastic enough to be rolled in a hot rolling mill to produce the desired cross-section， size or shape. For metallurgical and productivity reasons， reheat furnaces must also meet specific requirements and targets in terms of heating rates. In a reheat furnace， there is a continuous stream of material that is heated to the desired temperature as it passes through the furnace.

The hot rolling operation needs to obtain high quality reheated semi-finished steel at the lowest cost and at the optimum production rate for the mill. The reheating furnaces used to heat semi-finished steel in hot rolling mills consume large amounts of energy and produce large amounts of pollutants. Because of this， it is necessary to investigate ways to reduce energy consumption and pollutants， and thus reduce costs. This can be achieved by improving the fuel efficiency of the reheating furnace.

The combustion system of a reheating furnace has a significant impact on the quality of the reheated semi-finished steel and the amount of fuel required for reheating. Today， important expectations of reheating furnaces are not only to reduce pollutant emissions and energy consumption， but also to improve the quality， reliability， uniform temperature， heat flow and safety of the heated steel product as well as of the equipment and personnel. All of these are key factors that have a considerable impact on the combustion system of a reheat furnace.

The three basic elements required to start and maintain combustion are (i) fuel， (ii) oxygen and (iii) sufficient energy for ignition. The combustion process is most efficient if fuel and oxygen can meet and react without any restrictions. However， in practical heating applications， it is not enough to consider effective combustion， but also the heat transfer aspect. The following are important parameters of a combustion system in a reheating furnace

The amount of heat that needs to be transferred to the charge.

The heat generated in the furnace is not only needed to heat the charge， but also to overcome all heat losses.

Transfer of part of the available heat from the furnace gas to the surface of the heated charge.

Balancing of the internal temperature of the charge.

Loss of heat from the furnace through doors， walls， etc.

Heat carried by exhaust gases.

Emission of pollutants (e.g. nitrogen oxides， etc.) caused by exhaust gases.

Air consisting of 78% nitrogen and 1% oxygen diluted with argon does not provide optimal conditions for combustion and heat transfer. The nitrogen in the air is heated during combustion and it is necessary to recover this energy to save fuel in order to avoid the loss of energy transferred to the nitrogen.

Heat is transferred to the surface of the solid product by convection， conduction and radiation. The heat transfer inside the product is only by conduction. This means that the product surface (which varies with time during heating)， the size and material of the semi-finished steel and the internal dimensions of the furnace are all important.

For efficient and uniform heating， the gas composition and flow pattern inside the furnace are also important. Conventional， non-optimized heating strategies may be adequate at steady state， but do not provide optimal quality and cost performance in case of production interruptions， product grade or size changes， or changes in target drop temperature. In today's scenario， a solution is needed that provides the highest quality and lowest cost heating under all conditions with the least impact on the environment.

Today， due to global warming， strict environmental regulations require minimizing specific fuel consumption while reducing pollutants including nitrogen oxides (NOx). The conflicting goals of minimum energy consumption and pollutant emissions while meeting production requirements pose a challenge to operators and equipment suppliers who need to utilize all available technologies to design energy efficient， environmentally compliant reheat furnace combustion systems.

In conventional burner design， these two goals are often in conflict with each other. However， using the latest technology and burner designs with diffusion flame combustion technology， high levels of efficiency can now be achieved by preheating the combustion air and reducing NOx emissions accordingly.

In steel plants there are many types of reheat furnaces in operation. The structure of a regenerative furnace consists of several zones. The reheating furnace is usually designed with several heating zones， i.e. (i) preheating zone， (ii) heating zone， and (iii) soaking zone. The semi-finished steel pieces are fed into the preheating zone and slowly moved to the heating and soaking zones in sequence. The steel piece is heated to roughly the target temperature in the preheating and heating zones and immersed in the soaking zone to maintain a uniform temperature throughout the steel piece， which is heated primarily by radiant heat transfer from the surrounding gas. Each zone has a different purpose and each zone is usually controlled by a separate burner， even though the products of combustion come out of the flue through the previous zone. The air to fuel ratio is usually set to produce a desired level of excess oxygen in the flue gas. The aim is to ensure that all fuel is burned in the reheat furnace， but at the same time to avoid reducing the heating efficiency of the furnace by too much combustion air.

The multi-zone structure of the reheat furnace makes combustion optimization very difficult because of the interactions between zones， the changing product requirements， the changing extraction rate and the behavior of the extracted bars themselves. There are many issues that can cause drift in the final results. Some of these problems are as follows.

Inaccurate measurement of gas or air

Variations in air humidity

Large amounts of fuel or oxygen migrating from another area of the furnace

In the case of preheating air through a heat exchanger， there may be (i) leaks in the piping or heat exchanger， (ii) misalignment of the temperature correction factor， and (iii) seasonal variations

Wear or damage to valves and actuators

Leaks in the furnace

In addition， each zone requires a fixed stoichiometric ratio depending on the capacity of the burner and the required excess oxygen level. The ratios may vary from zone to zone for two reasons

The excess oxygen required for each zone may be different due to the relationship between oxygen levels， temperature and scale formation.

The burner turndown rates may need to be adjusted for different burners due to different mixing capabilities.

Often， the operator of a reheat furnace does not know if the oxygen is at or near the desired set point because there is no real-time process feedback. The consequences of operating a furnace with this level of uncertainty can be severe. Considering the burner reactions and theoretical combustion products， stoichiometric deviations from the set point may occur. If the actual oxygen content in the flue gas exceeds the setpoint， efficiency is reduced and unnecessary fuel costs are increased. If the actual oxygen content is below the setpoint， then the carbon monoxide content increases， which creates unsafe operating conditions and reduces efficiency due to unburned fuel flowing out of the furnace. In addition， the air-to-fuel ratio controller in the reheat furnace is constantly adjusting its set point in order to meet the changing furnace demands， so the oxidation state is always changing. In practice， reheat furnaces experience some degree of incomplete combustion. There are several problems that can lead to poor mixing. These problems are given below.

Burner efficiency

Rotational speed

Unparalleled air and fuel velocity

Ratio control out of whack

Air leakage from the furnace

Incomplete combustion due to poor mixing can also lead to the coexistence of carbon monoxide and oxygen. In practice， it is common for carbon monoxide and oxygen to co-exist. Optimal operation of reheat furnaces therefore requires real-time combustion product data via suitable sensors.

Heat exchangers are commonly used in reheat furnaces as waste heat recovery units to achieve high thermal efficiency and energy savings. The recovered waste heat is used to preheat the combustion air， which is then fed to the burner. The preheated air leads to energy savings and good combustion performance. However， the disadvantages include the incorporation of a large-scale waste heat recovery system. In addition， the temperature of the preheated air is usually only about 600 degrees Celsius to 700 degrees Celsius at most.

Recently， there have been two major developments in the field of reheating of semi-finished steel products. These developments are (1) high temperature air combustion， and (2) oxygen fuel combustion.

High Temperature Air Combustion

High Temperature Air Combustion (HiTAC) technology utilizes preheated air in excess of 1000 degrees Celsius. The use of this combustion technology in reheat furnaces allows steel mills to make a significant contribution to the simultaneous reduction of energy consumption as well as to the reduction of CO2 and NOx emissions. The use of HiTAC technology also allows for a reduction in the physical size of the reheat furnace compared to conventional types of furnaces.

The basic concept of HiTAC technology is to maximize the recovery of waste heat by means of a high-circulation regenerator and to control the mixing of highly preheated combustion air with the combustion gases to produce a uniform and relatively low-temperature flame.

The regenerative burners used in HiTAC technology have unique combustion characteristics. These characteristics result in efficient and clean flames with uniform temperature and heat flux profiles. These characteristics lead to increased productivity and improved product quality， as well as a gentler environment for the furnace components. HiTAC technology allows for lower operating costs.

HiTAC technology provides significantly higher flame stability， higher heat transfer， and low heat loss from the stack (waste heat) in all fuel-air mixtures， including very lean fuel mixtures. The method provides a way to recirculate heat from the high temperature side (burning gas) to the unburned mixture side using appropriate heat exchange methods. Preheating gives additional enthalpy to the unburned mixture without dilution by combustion products.

With HiTAC technology， the flame characteristics are significantly different， the flame is stable， emissions are reduced and significant energy savings can be achieved. The flame color was found to be very different from the normally observed blue or yellow color. Under certain conditions， blue-green and green flames have been observed using typical hydrocarbon fuels. Conversely， flameless (or colorless) oxidation of the fuel has also been observed.

The following are the main features of HiTAC technology

Hydrocarbons are burned at very high air preheating temperatures using a regenerator in the burner.

The sensible heat of the exhaust gas product is used to heat the combustion air (at temperatures greater than 1000 degrees Celsius). The technology attempts to preheat the air close to the target furnace temperature.

The exhaust gas temperature is about 150 degrees Celsius to 200 degrees Celsius.

Most of the energy in the extracted fuel is used for the heating process.

There is a fuel saving of 50% (replacement of cold air) and 30% (replacement of air from the heat exchanger).

There is a very uniform heat distribution in the furnace.

Very low NOx production.

In a regenerative burner， there is a pair of configurations where each burner circulates between combustion and exhaust. The combustion air is circulated between two sets of channels in the burner. It enters through one group and gains heat from the regenerative material in the burner. Gases from the combustion products pass through the other set of outlets， heating the regenerator material to high temperatures. In the next cycle， combustion air and combustion products exchange paths.

Figure 1 illustrates the HiTAC concept and compares it to conventional reheat furnace combustion. If direct combustion occurs between the fuel and the high temperature fresh air， an extremely high temperature flame is usually produced in the furnace. Due to the modified furnace geometry， not only the extinguishing of the base flame occurs by shear motion of the high velocity inlet air， but also the combustion gases (BH) in the air must be diluted by separating the fuel and air inlets prior to combustion. It is important to note that these are conditions under which ordinary combustion cannot be sustained in air at ambient temperature. In addition， fuel injected separately into the furnace also entrains combustion gases in the furnace， if any， and during this preparation phase some changes in the fuel occur， such as pyrolysis， decomposition and vaporization of the liquid fuel. Weak combustion reactions may occur between the fuel and the entrained products (B*F)， while the main combustion takes place in the mixing zone of fuel and dilute air， where a large amount of combustion gases (B*F*BH) are present. Due to the high recovery of combustion gases resulting in low oxygen concentrations， flame changes may produce an expanded reaction zone where relatively slow reactions may occur. In established combustion without preheated air， direct combustion between fuel and fresh air (F*A) occurs in the near field of the burner. Thereafter， in the downstream part of the flame， some combustion in a diluted state may occur due to the entrainment of recirculated combustion gases by the incoming combustion air. The combustion near the burner (F*A) shows the highest temperature in the furnace， where most of the NOx emissions from the furnace are formed. However， combustion in this area is critical to maintain combustion in the furnace， and if this part goes out， the entire flame cannot exist.

Fig. 1 Mixing and combustion concept with HiTAC technology

Despite the use of highly preheated air， the average and instantaneous peak temperatures of HiTAC are much lower than those of normal combustion.

Oxyfuel combustion system

Oxyfuel combustion is the complete replacement of air as the oxidant source for combustion with oxygen， which can be generated by cryogenic or adsorption technology. The general advantage of replacing air with oxygen is that the amount of nitrogen brought into the combustion process along with air is almost or completely eliminated. Reducing nitrogen in combustion increases flame temperature and combustion efficiency because the lower volume of combustion gas reduces the amount of heat that is captured from the flame and lost to the exhaust. Therefore， the advantages of using oxy-fuel combustion over air-fuel combustion are as follows.

Reduced energy consumption

Increased heating rate and thus higher throughput (without increasing the furnace temperature set point)

Reduction of furnace emissions

In addition to the above advantages， the use of oxyfuel combustion can sometimes result in lower capital investments compared to other methods of increasing efficiency (e.g. heat exchangers or emission control equipment). Oxyfuel combustion may also result in less scale loss due to better control and shorter heating times.

The partial pressures of the two combustion products， CO2 and H2O， are much higher in oxyfuel combustion compared to air fuel. This improves the heat transfer rate. Since the exhaust gas is not diluted by nitrogen， the gas phase plays a more active role in the heat transfer process， not only because of the higher heat transfer conductivity and heat capacity of CO2 and H2O， but also because they are both three atomic gases with high thermal radiation.

The flow pattern of oxyfuel combustion furnaces is favorable compared to that of air fuels. Due to the absence of nitrogen and due to the fuel savings， the exhaust gas volume is reduced by 70 to 80%. As a result， the residence time of the gas is longer and there is more time to transfer the heat to the product. In fact， the product is immersed in a gaseous exhaust gas consisting of carbon dioxide and water， which means that the moist environment has a higher heat transfer capacity.

When comparing an oxy-fuel furnace to an air-fuel furnace， both set at the same furnace temperature， the material reaches the set point faster in the oxy-fuel furnace. This is due to the properties of the gas.

Compared to air-fuel combustion， oxyfuel combustion has significantly more heat available. The increase in available heat is directly related to the reduction in energy consumption and the increase in furnace output.

The increase in available heat of combustion means that less heat is lost to the exhaust gas and a greater percentage of the total energy input is left to do work in the reheating furnace. Thus， when the available heat increases， the total energy input required to do a constant amount of work decreases.

Energy savings vary depending on fuel type， available combustion ratio and combustion air temperature. Other factors， such as reduction in flue opening size and radiation losses， affect the actual energy reduction rate of oxyfuel combustion. Another effect of oxyfuel combustion to improve combustion efficiency is the ability to increase the heating rate and obtain more furnace throughput.

The practical limits to the increased throughput depend on the heat absorption capacity of the semi-finished steel material and the time and temperature at which the semi-finished steel material is heated. Experience with various oxy-fuel units has shown that in most operations， material throughput can be increased without increasing the temperature set point of the furnace， except for furnaces that already meet the set temperature ramp limits. In addition to the increase in available heat， higher oxyfuel flame temperatures and the radiant potential of the combustion gases have a positive impact on heating capacity and productivity.

Oxy-fuel flame temperatures are typically 500 degrees Celsius to 800 degrees Celsius higher than air-fuel flame temperatures. Since radiative heat transfer depends on the fourth power of the temperature difference from the source to the receiver， oxyfuel combustion results in a significant increase in the radiation potential of the flame to the material. The combustion products of oxyfuel combustion are also a better source of radiant heat transfer. This is because the combustion products of air fuels are mostly nitrogen， which is not as effective a radiative heat transfer mechanism as carbon dioxide and water vapor， which constitute most of the products of oxyfuel combustion.

In some cases， furnace production is limited by the amount of gas emissions allowed out of the furnace. Oxyfuel combustion can also be a means to reduce furnace emissions and allow for increased production capacity within the upper limit of allowable emissions.

The exhaust gas volume of oxyfuel combustion is significantly reduced. The total exhaust gas volume of oxyfuel combustion is typically 70 to 90 percent less than the total exhaust gas volume of air fuel. In many cases， the reduction in exhaust volume alone can be beneficial， especially where existing pollution control equipment is limited and/or particulate emissions are a concern. One of the more important results of oxyfuel combustion is the reduction in emissions of certain exhaust gas components. The most obvious result of using oxyfuel combustion is reduced fuel consumption. As fuel consumption decreases， so do CO2 emissions over a given period of time or per unit of heated semi-finished steel material. While CO2 production is not currently a major issue， ongoing global warming and climate change suggest that stricter CO2 emission conditions may be a factor in the upcoming future. A more immediate concern for many furnace operators is the emission of nitrogen oxides. With oxyfuel combustion， the partial pressure of nitrogen in the combustion products is greatly reduced， reducing the potential for NOx formation even at elevated flame temperatures. Many factors influence the NOx emission rate of oxyfuel furnaces. The purity of the oxygen product is one such factor. However， the main factor in minimizing oxyfuel NOx emissions is furnace pressure control. Secondary air leakage combined with high oxygen combustion flame temperatures can greatly reduce the impact of oxyfuel combustion as a NOx control technology.

Oxy-fuel flames have higher temperatures with smaller volumes and lengths than air-fueled flames. The flame characteristics of oxyfuel flames need to be considered when designing oxyfuel burner systems for applications in reheating steel materials. In general， reheating of steel requires a uniform temperature distribution to avoid localized overheating or underheating in the reheated steel product. The type and location of the oxyfuel burner depends on the type of reheating furnace and the proximity of the flame to the semi-finished steel material.

The recirculation of combustion products promotes the movement of gases in the heating chamber， thus minimizing temperature differences. In addition， the recirculation of combustion gases into the oxy-fuel flame reduces the peak flame temperature and promotes a more uniform flame radiation profile to protect the product closest to the burner from being overheated. Several oxy-fuel burner designs are available to obtain recirculation， including patented nozzles and other designs that use oxy-fuel flame power to produce gas recirculation.

While providing significant benefits to efficiency， the low amount of combustion products from oxyfuel combustion requires some special attention when designing combustion control systems. Proper control of the combustion ratio is critical to the reheating process， as the products of combustion make up the heating atmosphere and ultimately affect the rate and type of scale formation. In an air-fuel combustion system， the large amount of nitrogen entering the combustion process with the air provides a damper or safety factor to prevent changes in the air-to-fuel ratio. In oxyfuel combustion， this damper is almost completely eliminated. This means that the percentage change in the oxygen to fuel ratio in oxyfuel combustion has a greater effect on the furnace atmosphere than the same change in air-fuel combustion.

Good control of the furnace atmosphere in oxyfuel combustion requires a higher degree of accuracy in the control system than in air-fuel combustion. Mass flow compensation is usually required to meet the accuracy requirements of oxyfuel combustion to maintain a good furnace atmosphere. Another important variable in oxyfuel combustion control involves the furnace exhaust and pressure control system.

The exhaust gas volume of oxyfuel combustion is 10 to 30 percent less than that of air-fuel combustion， depending on the increase in thermal efficiency. This means that existing designs for controlling furnace pressure for air-fuel combustion are in most cases insufficient to maintain good furnace pressure control when converting to oxyfuel combustion. Specifically， the control range of the pressure control apparatus for the larger flue opening and damper sizes used in air-fuel combustion is reduced to the point where it is ineffective for oxyfuel combustion of the exhaust. Therefore， it is necessary to reduce the flue opening size when converting to oxyfuel combustion or to compensate for the lower exhaust volume when designing a new reheat furnace unit. Lack of good furnace pressure control can lead to tertiary air leakage， which can provide a source of nitrogen for additional NOx formation， thus minimizing or eliminating the potential for reduced emissions. Air leakage into the reheat furnace can also affect the furnace atmosphere and create problems for steel surface quality control.

Regardless of the type of combustion system， the rate and type of scale formation is an important consideration in the operation of steel reheat furnaces. In addition to the type of steel alloy， the furnace atmosphere and heating rate are the main factors governing scale formation. In order to maintain control of scale formation and steel surface quality， good control of combustion ratio and furnace pressure is required. However， even under ideal conditions， oxyfuel combustion produces an atmosphere different from that of air-fuel combustion...