Oxygen-rich combustion and its application in reheating furnaces

Steel reheating is an energy-intensive process that requires uniform temperature distribution within the reheating furnace. Historically， heat exchangers have been used to preheat the combustion air and thus save energy. Recent innovations include oxygen enrichment (O2) and the use of regenerative burners， which provide higher preheating air temperatures than heat exchangers. These processes have limitations such as aging equipment， decreasing energy efficiency over time， high maintenance costs， and increased NOx emissions as the air preheat temperature increases unless special equipment is used.

Three things are necessary to start and maintain combustion. They are fuel， oxygen and sufficient energy for ignition. The efficiency of the combustion process is highest if the fuel and oxygen can meet and react without any restrictions. However， in heating practice， in addition to efficient combustion， heat transfer is also a practical consideration.

The common air used for combustion contains nitrogen (N2) and argon (Ar) in addition to oxygen. In an air-fuel burner， the burner flame contains nitrogen from the combustion air. A large amount of fuel energy is used to heat this nitrogen. The hot nitrogen leaves through the chimney， resulting in energy loss. As a result， the air does not provide optimal conditions for combustion and heat transfer. The heat absorbed by the nitrogen is either wasted or recovered to achieve energy savings. Currently， the best air-fuel heating systems in reheat furnaces require a minimum of 310 megacalories per ton of steel to reach the proper temperature for rolling steel products.

Historically， the primary use of oxyfuel combustion has been for welding and cutting metals， especially steel， because oxyfuel combustion allows higher flame temperatures than can be achieved with air-fueled flames. The introduction of the innovative oxyfuel burner technology (using 100% oxygen) for reheating steel is a very new phenomenon. The concept of oxyfuel combustion was introduced by Abraham in 1982 to provide carbon dioxide (CO2) rich flue gas. Because of its potential benefits， the Argonne National Laboratory (ANL) has conducted several research activities， including techno-economic studies and pilot scale studies on the subject.

Oxyfuel is the practice of completely replacing air with industrial-grade oxygen as the oxidant source for combustion. Industrial grade oxygen is defined as a liquid oxygen supply vaporized into a gas or generated on-site. The purity of the liquid oxygen supply typically exceeds 99.99%， while the purity of the on-site generated oxygen typically ranges from 90% to 93%. The advantage of using on-site generated oxygen is that it is less expensive because the product does not need to be liquefied or transported and is delivered at a lower pressure to minimize power consumption. In integrated steel plants with steelmaking air separation plants， high purity oxygen (99.99%) can be supplied from the air separation plant via pipeline.

When industrial grade oxygen is used to avoid nitrogen， as in the case of oxyfuel combustion， not only is the combustion itself more efficient， but so is the heat transfer. Oxyfuel combustion affects the combustion process in several ways. The first obvious result is the increased thermal efficiency due to the reduced amount of exhaust gas， which is a fundamental result and is valid for all types of oxy-fuel burners. In addition， the concentration of highly emitted combustion products - carbon dioxide and water - increases in the furnace atmosphere. For heating operations， these two factors lead to higher heating rates， fuel savings， lower CO2 and NOx emissions and， if the fuel contains sulfur， lower sulfur oxide emissions. Figure 1 shows the oxygen-fuel and air-fuel combustion processes.

Figure 1 Oxyfuel and air-fuel combustion processes

 Oxy-fuel combustion has been found to differ from air combustion in many ways， including reduced flame temperature and delayed flame ignition. Many of the effects of oxy-fuel combustion can be explained by the difference in gas properties between CO2 and N2， which are the primary dilution gases in oxy-fuel and air， respectively. CO2 has different properties than N2， which affects heat transfer and combustion reaction kinetics. These differences are explained below.

Density - CO2 has a molecular weight of 44 compared to N2's molecular weight of 28， resulting in a higher density of flue gas in oxy-fuel combustion.

Heat Capacity - CO2 has a higher heat capacity than N2.

Diffusivity - The oxygen diffusivity of CO2 is 0.8 times higher than that of N2.

Radiative properties of furnace gases: - Oxyfuel combustion has higher levels of CO2 and H2O， both of which have higher emission capacity.

Oxyfuel heats steel products more efficiently and faster than air fuel. Oxyfuel has a thermal efficiency of about 80%， while air fuel has an efficiency of about 40% to 60%. The use of oxyfuel increases productivity， reduces fuel consumption and heats the steel product to the desired temperature. The use of oxyfuel also improves temperature uniformity and reduces environmental emissions.

A general advantage of replacing air with industrial grade oxygen is that the nitrogen content of the air is almost or completely eliminated from the combustion process. Reducing nitrogen in combustion increases flame temperature and combustion efficiency because the lower amount of combustion gas reduces the amount of heat that is captured from the flame and lost to the exhaust. During oxyfuel combustion， a gas consisting primarily of carbon dioxide and water is produced.

The heat transfer by oxyfuel combustion is characterized by a high emissivity (considerable concentration of carbon dioxide and water in the flame) and a reduction of the flame volume， which allows， firstly， an enhanced energy transfer to the load and， secondly， additional gains in terms of energy efficiency.

For continuous heating operations， it is also possible to operate the reheating furnace economically at a higher temperature on the inlet side of the furnace. This further increases the possible output of the reheating furnace. It has been observed that the energy efficiency of oxyfuel combustion is comparable or even better than that of a reheating furnace with a highly preheated combustion air plant. Thus， the benefits of using oxy-fuel compared to air-fuel combustion are as follows.

Oxyfuel combustion results in a significant increase in available heat (total energy input minus energy lost to the exhaust) compared to air-fuel combustion. The increase in available heat is directly related to the reduction in energy consumption.

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

Increasing the heating rate leads to higher throughput. The practical limits to the increase in output depend on the heat absorption capacity of the load and the time and temperature at which the load is heated. Experience from various oxyfuel units is that in most operations， product yield can be increased without increasing the furnace temperature set point， except for those reheating furnaces that already meet the set temperature ramp limits. In addition to the increase in available heat， higher oxyfuel flame temperatures and the radiation potential of the combustion gases have a positive impact on heating capacity and productivity.

Since radiant heat transfer depends on the fourth power of the temperature difference from the source to the receiver， oxyfuel combustion leads to a significant increase in the radiant potential of the flame to the load. The combustion products of oxyfuel combustion are also a better source of radiant heat transfer. This is because most of the combustion products of air fuels are nitrogen， which is not as effective a radiant heat transfer mechanism as carbon dioxide and water vapor， which make up most of the products of oxyfuel combustion.

Reduced furnace emissions - Oxyfuel combustion has significantly lower exhaust emissions. Total exhaust emissions with oxy-fuel are typically 70 to 90 percent less than total exhaust emissions with air fuel. The most obvious result of using oxyfuel combustion is a reduction in fuel consumption. With reduced fuel consumption， CO2 emissions are also reduced for a given period of time or per unit of load heating. 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.

The higher concentration of pollutants in the flue gas makes separation easier.

The flue gas is mainly carbon dioxide， making it suitable for sequestration

In addition to the benefits mentioned above， the option of using oxyfuel can sometimes result in lower capital investment compared to other methods of improving efficiency， such as heat exchangers or emission control equipment. Oxyfueling allows for compactness of all installed piping and flow paths without the need for any heat exchange or regenerative heat recovery devices. It also greatly reduces the physical size of the burner， furnace and flue gas ducting， and eliminates the need for electric vent fans. Also， the combustion air blower and associated low frequency noise problems are avoided. In addition， in some cases， the conversion to oxyfuel combustion results in less scale loss due to better control and shorter heating times.

Oxy-fuel flames have higher temperatures and are smaller in size and length than air-fuel flames. Oxyfuel flame characteristics need to be considered when designing an oxyfuel burner system for steel reheating. In general， steel heating requires a uniform temperature distribution to avoid localized overheating or underheating of the product. The type and location of the oxyfuel burner depends on the type of furnace and the proximity of the flame to the steel product.

Although of great benefit to efficiency， oxyfuel combustion has a low product volume and requires special attention when designing the combustion control system. Proper control of the combustion ratio is critical to the steel heating 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 along 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.

Changing the furnace atmosphere with oxy-fuel has no adverse effect on scale formation and in some cases has proven to be a benefit. The higher partial pressures of CO2 and H2O in the combustion products provide a more efficient heat transfer mechanism， allowing for increased heating rates， thus reducing the time factor for scale formation. In addition， a comparison of air-fueled and oxyfuel combustion showed that the characteristics of scale formation changed with oxyfuel combustion. The scales formed on the surface of the steel were thinner than those formed by air-fuel firing. The reason for the change in scale characteristics is thought to be due to the fact that the atmosphere of oxyfuel combustion rapidly produces a thin， dense oxide layer that prevents further oxidation and scale formation.

Flameless oxyfuel combustion

In recent years， the term "flameless oxyfuel combustion" has been adopted. This expression conveys the visual aspect of the type of combustion， i.e.， the flame is no longer visible or easily detected by the human eye. Another description might be that combustion is 'extended' in time and space - it spreads out in large volumes， which is why it is sometimes called 'volumetric combustion'. Such a flame has a uniform and lower temperature， but contains the same amount of energy.

In flameless oxyfuel combustion， the flame is diluted by the hot furnace gas. This lowers the flame temperature to avoid the production of hot nitrogen oxides and to achieve a more uniform heating of the steel.

In flameless oxyfuel， the mixture of fuel and oxidizer reacts uniformly through the flame volume at a rate controlled by the partial pressure and temperature of the reactants. Flameless oxygen fuel burners effectively disperse the combustion gases throughout the furnace， ensuring more efficient and uniform heating of the material even when a limited number of burners are installed - the dispersed flame still contains the same amount of energy， but is distributed over a larger volume. The lower flame temperature significantly reduces the formation of low NOx. Low NOx emissions are also important from a global warming perspective; the so-called global warming potential of NO2 is almost 300 times greater than that of CO2. It is also feasible to use fuels with low calorific value， which has been emphasized recently， for example with blast furnace roof gas.

Oxyfuel burners have always been powerful and compact， and the new generation of flameless oxyfuel burners maintains their compact design to facilitate the exchange of already installed oxyfuel burners and to facilitate the retrofit of air-fuel units. In addition， flameless oxyfuel combustion not only adds more advantages， but opens up new applications， all of which support a significant reduction in environmental impact.

In steel plants where oxyfuel combustion technology has been implemented， the following results are being obtained

Up to 50% increase in throughput capacity of regenerative furnaces

Fuel consumption savings of up to 50%.

Carbon dioxide emissions reduced to 50%.

Reduction of NOx emissions

Reduction of scale losses during reheating

No negative impact on the surface quality of the steel

Positive impact on the temperature uniformity of the steel

The ideal heating profile recommended by the control system can be achieved more easily

Fewer fumes from the furnace chimney， which significantly improves the environment of the plant