Regardless of the motive, the first step undertaken by a furnace operator is to investigate the various technologies available. Two of the most intriguing technologies are regenerative burners and oxy-fuel burners.
The most commonly used fuel combusted in furnace applications is natural gas, which is comprised of over 90% methane (CH4). Air, comprised of about 20% oxygen (O2) and the balance nitrogen (N2), is usually used as the oxidizer. Equation 1 expresses the typical combustion reaction, firing with 10% excess air.
CH4 + 2.2 O2 + 8.27 N2 ® CO2 + 2H4O + 0.2 O2 + 8.27 N2 (1)
1 Fuel + 10.47 Air ® 8.7% CO2 +17.4% H2O + 1.7% O2 + 72.1% N2
The hydrocarbon fuel chemically changes to a collection of exhaust gases: carbon dioxide (CO2) and water vapor (H2O). The N2 in the exhaust gas came directly from the combustion air.
As shown, the process starts with chemical energy in the fuel. There is also some sensible energy in the oxidizer, proportional to its temperature. From this heat supply, process heat is used to heat the product as well as the various system losses (refractories, water, etc.). Finally, a significant amount of heat is carried out of the furnace in the form of sensible heat in the exhaust gases. This energy balance is summarized in equation 2.
Chemical energy (fuel) + Sensible heat (oxidizer) – Sensible heat (exhaust) = Process heat (useful heat) (2)
Based on equation 2, we calculate the combustion efficiency as the useful process heat divided by the chemical energy in the fuel:
Efficiency = Process heat / Chemical energy in fuel (3)
|Fig. 1. Regenerative burners|
Regenerative burners were designed with the idea of increasing the combustion efficiency by increasing the amount of sensible heat in the combustion air while reducing the sensible heat that is lost with the exhaust gas. Specifically, a media pulls heat out of the waste gases and temporarily stores it. The media is typically made up of ceramic balls or honeycombs. After the media is sufficiently heated, the combustion air is blown across it, thus pulling the heat out of the media and returning it to the combustion process. The net result is that the waste gases leave the process at a lower temperature, meaning that less energy is lost to the stack.
In practice, regenerative burners usually work in pairs (Fig. 1). In this example, burner #1 is in firing mode while burner #2 is in exhausting mode. Burner #1 is firing with its combustion air blowing across the already hot burner #1 media, resulting in air preheat temperatures of about 300°F less than the furnace-chamber temperature. Meanwhile, burner #2 is drawing the hot exhaust gases out of the furnace across the burner #2 media, heating up the media and cooling the exhaust gases. After a period of about 30-60 seconds, the burners switch. The high air preheat temperature makes the combustion process very efficient because the flames do not have to heat the combustion air to the furnace operating temperature.
A regenerative burner system involves the burners, ceramic media, media cases, switching valve, exhaust ductwork, and combustion air and exhaust blowers.
The combustion reaction for normal 10% excess-air firing is exactly as presented in equation 1.
Another approach to increasing combustion efficiency is to fire specially designed burners with oxygen instead of air. The combustion efficiency is raised because of the fact that it takes about five times more energy to heat the air to the furnace operating temperature than the oxygen requires. This is because the air is comprised of almost 80% N2, which (for the most part) is not involved in the chemical reactions.
An oxy-fuel burner system involves the specially designed burners, oxygen valves and piping. Unlike the regenerative burner case, the combustion equations are altered to account for the use of pure oxygen instead of air for the oxidizer. Operating with 10% excess oxygen, only 2.2 parts O2 are needed for every part CH4, as shown in equation 4.
CH4 + 2.2 O2 ® CO2 + 2H2O + 0.2 O2 (4)
1 Fuel + 2.2 Oxygen ® 31.3% CO2 + 62.5% H2O + 6.3% O2
The combustion efficiency, or the percentage of available process heat over the chemical heat content of the fuel, was presented in equation 3. Table 1 shows a comparison of the combustion efficiency for a range of furnace temperatures. For comparison purposes, all cases were calculated based on natural gas firing with 10% excess air (or oxygen).
As shown in the table above, the combustion efficiency for regenerative burners and oxy-fuel burners is almost identical. Each can provide significant energy savings over cold-air combustion systems and a proportional reduction in carbon dioxide (CO2) emissions.
So, with almost identical performance in energy efficiency, the deciding factor shifts to other criteria.
|Fig. 2. Heating comparison|
One of the touted benefits of oxy-fuel burners is the improved process heating performance. The higher concentrations of tri-atomic molecules such as CO2 and H2O, as shown in equations 1 and 4, result in a higher emissivity, which increases the radiant-heat transfer. On the other hand, the much larger flame envelope resultant from regenerative burners also increases the radiant-heat transfer to a slightly lesser extent than the oxy-fuel case. Sample heating model analysis results comparing the two are shown in Fig. 2.
Pressure control is essential to a furnace operation to ensure quality heating and energy efficiency. If regenerative or oxy-fuel burners are installed in addition to an existing combustion system, the exhaust-gas system will need to be expanded to handle the extra volume of combustion gases or else the furnace pressure will increase significantly. If these new burners are to be installed as replacements for existing burners, however, the impact on pressure control is not as simple. By their nature, the oxy-fuel burners release less waste gas, which may require downsizing the exhaust system so as to prevent a reduction in furnace pressure. On the other hand, replacing cold-air burners with regenerative burners requires a modification because the vast majority of furnace waste gases are exhausted through the burners, requiring some additional ductwork, valves and fans.
Surface oxidation, or scale loss, is an inherent issue in product heating. It is dependent on residence time, surface temperature and furnace atmosphere. While the time and temperature aspects are dictated by the heating requirements of the product, the furnace atmosphere can vary widely depending on the furnace equipment and control. The typical components of a furnace atmosphere are categorized in Table 2.
The rate of surface oxidation is dependent on the relative partial pressures (volumetric proportions) of these constituents. As equations 1 and 4 show, the percentages of oxidizing gases are significantly higher in the oxy-fuel burner case. This results in more scale loss, as shown in the heating-model analysis results in Fig. 3.
|Fig. 3. Surface oxidation comparison|
As with the other criteria, an evaluation of the emissions of oxy-fuel and regenerative burner systems depends on the specifics of the project. Obviously, a furnace completely fired with oxy-fuel burners will emit practically zero NOx. This is because there is no N2 in the combustion reactions unless the furnace has air infiltration. Otherwise, if the oxy-fuel burners are installed in conjunction with air-fired burners, the NOx emissions will increase as the N2 already present in the furnace atmosphere (supplied through the air-fired burners) will form NOx in the oxy-fuel burner flames. With regenerative burners, higher air preheat results in higher flame temperatures, which then results in higher NOx emissions. Recent advancements in burner designs have contributed to reducing the NOx emissions from both oxy-fuel and regenerative burners.
When comparing the capital costs associated with a combustion-system upgrade, the regenerative burner system is typically more expensive to install. That is because it involves the burners, media and cases, piping, valves, ductwork and fans. The oxy-fuel system requires new burners, piping, valves and an oxygen-supply skid.
One of the most common deciding factors in upgrading a combustion system is the resultant operating-cost impact. Typically, energy-saving projects are undertaken with the idea of paying for the project with natural gas savings. In the case of regenerative burners, this is clear. With oxy-fuel combustion, the verdict will depend on the oxygen-supply situation. Many plants operate with an oxygen contract with a gas supply company that provides “free” oxygen that would otherwise be unused. In this case, there is no additional operating cost for the oxygen. However, if the plant does not already have an oxygen contract, one must offset some of the natural gas savings with additional oxygen costs.
The most significant drawback of regenerative burners is the additional maintenance costs. There are two sources of additional maintenance in the regenerative burner system. First, the media must be cleaned or replaced regularly because it picks up contaminants from the exhaust gas. The other maintenance items are the switching valves, whose actuators will fail after a certain number of cycles. An oxy-fuel combustion system will have minimal maintenance impact.
As this article demonstrates, oxy-fuel and regenerative burner combustion upgrade projects offer significant energy-savings possibilities. However, the decision as to which type of upgrade is best for a particular operation depends on numerous other factors, ranging from process considerations to cost structures. IH
For more information: Contact Jared S. Kaufman, P.E., VP of technical services, Tenova CORE, Cherrington Corporate Center, 100 Corporate Center Drive, Coraopolis, PA 15108; tel: 412-262-2240; fax: 412-262-1308; e-mail: email@example.com; web: www.tenovacore.com