Gas Technology: Oxy-Gas firing grows in popularity
Systems Offer Flexibility, Lower Emissions
Industrial energy users are attracted to natural gas as a premium fuel that is available at very attractive market prices. Most energy experts believe that position is unlikely to change soon. Industrial furnaces that fire natural gas can combine the benefits of natural gas with the further advantages of oxy-fuel firing. The combination gives them even lower emissions, increased energy input from existing furnace spaces, lower flue gas volumes, and fuel savings ranging from 10% to 50%. This solution is worth considering.
Why It Makes Sense
Disregarding water vapor, atmospheric air contains about 78% nitrogen and only 21% oxygen. Thus, systems for air-fuel combustion need to be designed to handle a large volume of air to support combustion. Air intakes, blowers, furnace spaces, heat recovery accessories, emissions treatment systems and exhaust piping all have to be sized for much greater volume than if combustion were supported by oxygen only.
NOx Emission Reduction
Eclipse Combustion, Inc. is a worldwide manufacturer of industrial heating systems, including burners. According to Todd Ellerton, Customer Insight Manager at Eclipse, a major advantage of oxy-fuel over air-fuel is the potential for reduction of total NOx emissions. He notes, “Since nitrogen is removed as combustion air is eliminated, total NOx produced per ton of product is reduced 80% to 95%.
However, he points out, the concentration of NOx in the flue gas may actually increase, as the total volume of flue gases is reduced significantly. NOx in oxy-fuel combustion comes from infiltration of air in and around the furnace, plus some N2 can be present in both the natural gas and oxygen.” Ellerton explains that with industrial oxygen, some nitrogen is inherent in the process itself. He adds, “With air fuel combustion you typically have a 10 to 1 fuel ratio, but with oxy-fuel it is only 2 to 1.”
Adds Furnace Capacity
Another advantage of oxy-fuel is where industrial furnaces that use air-fuel firing are at the peak of their capacity, and greater energy output is needed. By converting these to partial or complete oxy-fuel firing, the furnace output is increased without the capital expenditure of a complete furnace replacement. Product throughput in the furnace can be increased.
Glass Industry a Leader
According to Ellerton, one of the first and most prominent users of oxy-fuel technology has been the glass industry. Glassmaking furnaces are major energy users, and operators are attracted to the higher combustion efficiency offered by burning natural gas with oxygen. These potential energy savings are quite variable, and depend on the type of furnace, the type of burner, and the degree to which the air-fuel burners are replaced.
Ellerton indicates, “Depending on the furnace operation and the efficiency of the operation, fuel savings can range from slightly over 50% to only 10%, so all the variables need to be reviewed prior to deciding if oxy combustion is a viable option.” He reminds owners that that they will be saving some expense for the fuel, but it is also necessary to pay for the oxygen.
He estimates that 30% to 35% of glass furnaces are using oxy-fuel systems today. He notes, “Of the various market segments within the glass industry, the fiber glass segment has been the strongest adopter. Over half of these manufacturers are estimated to be using oxy-fuel.”
Burner Improvements for the Glass Industry
Technology for oxy-fuel burners continues to advance. One recent introduction is the Eclipse PrimeFire 400, which was jointly developed by Eclipse Combustion, the Gas Technology Institute and the U.S. Department of Energy. This burner was designed to generate a certain amount of soot in the flame. This produces a more luminous flame and thus better heat transfer through radiation. This increased flame luminosity improves the performance of glass furnaces.
Conversion Trend Continues
Praxair, Inc is a major supplier of industrial gases and gas generation equipment. According to Pravin Mathur, Global Market Director, Steel and Combustion for Praxair, the trend of increasing conversion to oxy-fuel firing in the metals industries is continuing. He notes that because of low natural gas prices, the current motivation for conversion for pure energy cost savings may not be as strong, but there are other motivations. “Historically, customers considered converting from air-fired systems to oxy-fuel to save fuel costs. However given the low price of natural gas today, the main driver now is that oxy-fuel systems allow the customer to increase capacity, or to quickly substitute from more expensive fuels to natural gas without major capital outlay costs.”
He notes that oxy-fuel combustion in some metal smelting also reduces NOx emissions, CO2 emissions (due to fuel use reduction), and also reduces flue gas volumes. In some cases this allows effective use of low-value fuels that otherwise might be flared as waste.
Supplement Existing Fuel-Air Furnaces
Mathur points out, “It is possible to convert specific zones of existing furnaces to oxy-fuel, or to add oxy-boosting burners as required at strategic locations in addition to air-fired burners.” Mathur explains that this type of flexibility is attractive for industrial furnaces in the metals industries that have periods of high load interspersed with lighter loads.
Other Industrial Applications
According to Susan Brownlow from The Linde Group, another major industrial gas supplier, her firm is also noticing a growing use of oxy-fuel firing in other industries. She points out, “In the steel industry, oxy-fuel is used to enhance the melting process in electric arc and rotary furnace processes. Here it provides a substantially increased flame temperature. The higher the flame temperature, the faster the scrap melting, with fuel savings and lower CO2 emissions.”
She emphasizes that increasing demands in the metallurgy and glass industries and new market challenges have called for accelerated development work in oxy-fuel technology. She notes, “Some of the most innovative and successful outcomes have been flameless combustion, direct flame impingement (DFI), and low-temperature oxy-fuel technology for the aluminum industry. New technologies to increase even further the benefits of oxy-fuel, including new burner firing methods for the glass industry, have seen enthusiastic uptake.”
Expanding Into New Markets
She explains that process industries are continuing to seek ways to become more energy-efficient. “The embracing of oxy-fuel technology is likely to expand to applications within the non-traditional oxy-fuel industries, such as refineries and petrochemical plants. The utilization of increased oxygen in production over air-fuel solutions will enable major advantages in terms of reducing fuel consumption and increasing capacity and throughput, with the added benefit of lowering direct and indirect greenhouse gases to help meet increasingly stringent limits.”
She adds that the use of oxy-fuel combustion also substantially increases the thermal efficiency of a furnace. This creates greater furnace output capacity and increased production — sometimes as much as a 50% increase.
Sources of Oxygen
Depending on the size of the oxygen requirement, it can be supplied in bulk as a liquid, or can be produced with on-site plants. A system called variable pressure swing adsorption (VPSA) is useful for industrial processes where significant amounts of oxygen are needed and high purity of the oxygen is not a requirement. VPSA systems are often used in the glass and steel industries.
Both liquid oxygen and support for an on-site oxygen generation plant can be supplied by several large national or international companies, including Praxair, and Linde. These types of companies are also a good resource in converting an existing system to oxy-fuel, or in designing a new plant. They can help owners analyze the potential benefits and have experts with in-depth experience with and knowledge of these operations.
Entire Burner Replacement Required
According to Ellerton from Eclipse Combustion, the conversion from an air-fuel combustions system to an oxy-fuel system will require a complete burner replacement. “Oxy-fuel burners are of a different design than air-fuel burners; it is not possible to simply insert an oxy-fuel element into an air-fuel burner.” He adds that each situation needs to be studied.
Taking the Step
Ellerton stresses the need for a careful review of each opportunity for conversion. “Oxy-fuel combustion is not the answer to all applications, so it is very important to determine if it is a viable alternative. That said, in some processes that cycle, where loads are taken from cold to hot and melted, going from air combustion to oxy combustion not only reduces fuel use but can reduce cycle and heat-up time. This is where the operator can see significant cost reductions.”
Add to this the potential for reduction in total NOx emissions and the ability to reduce the plant’s carbon footprint by reduction in fuel use, and the benefits of oxy-fuel can be important. Before replacing or adding air-fuel combustion systems, it can be valuable to take a look at the oxy-fuel option.