Oxy-Fuel Does it Make Sense?

What is the feasibility of oxy-fueled combustion versus the standard preheated air and natural gas combustion? The article will determined the use of oxy-fuel combustion for material products such as glass, steel and aluminum. This article will review the current preheated air combustion systems, the science behind using oxy-fuel combustion/preheated air combustion, and how the oxygen technique could work. This information is extremely important to the industrial heating industry. An increase in knowledge base for oxy-fuel combustion/preheated air combustion could “spark” a change in the technology. If oxy-fuel combustion/preheated air combustion turns out to be an economically feasible product, it could boost the production of materials, limit emissions, and reduce the cost of such materials to the general consumer.

INTRODUCTION

In the process heating industry there has typically been one way to heat steel, aluminum, and glass. The combustion of preheated air and natural gas. Although the efficiency and cost of this combustion arrangement is relatively small, the increases in energy costs have driven industrial processers to find other means of heating various products. The most obvious choice would be to reduce the consumption of these fuels by increasing efficiency with the addition of computer control logic and highly sophisticated control systems.

Using oxygen as a fuel is a relatively new process. Within the last 25 years, furnace builders have experimented with the idea. With increases in the cost of energy, it is important to determine if oxygen is cost effective. This article will compare and contrast of the difference between standard preheated natural gas and oxygen induced combustion. This comparison is achieved through basic engineering analysis and simple elemental comparisons. In addition, the information regarding cost effectiveness is in the comparison. The difference between oxygen injected natural gas and standard natural gas will be compared in efficiency, production cost, and emissions. In the last few decades, processes such as glass and oxygen induced cutting have opened light to the benefits of including commercial oxygen in natural gas. Previous energy savings have been derived from preheating air for combustion or PAC. New technology may reveal that Oxy-fuel combustion, or OFC, could be the wave or the future. It is possible that pure oxygen and natural gas will be even more cost effective (POC — pure oxygen/natural gas combustion). Do these processes have economic benefit for the process heating industry? Is there energy savings associated with OFC of POC? Are there increased profits through these energy savings? What benefits can a process achieve with OFC or POC? Is there a reduction of emissions using OFC or POC? Are these new technologies an effective means of reducing total costs through energy efficiency, production cost, and emissions? With rising costs of energy, industrial suppliers need an efficient safe way to reduce production costs and initial capital costs. Reduction in production costs can include emissions, fuel, and maintenance. The purpose of this article is to determine if the modification from standard preheated air combustion to a new and improved OFC or POC is economically practical. Due to the large capital costs of oxygen supply systems, the reduction of fuel may not be enough. The value of the additional oxygen supply system and particular injection arrangement can be costly. In addition, the safety system and standards for transporting, containing, and burning oxygen are also labor intensive and therefore expensive. In order for OFC or POC industrial heating to be economically feasible, there will not only have to be a reduction in fuel costs, but also a reduction in maintenance and emissions.

Steel is a component that provides a major component to our lives. It is used in the cars we drive, to the buildings we live in, and all the little things in between. Steel is used for power lines, pipelines, tools, and is used to secure our national defense. Making our lives convenient and protecting our families. Steel is a metal, which represents the backbone of the American society. The development of the American society, maybe even humankind, would not have been possible if it was not for the production of steel. From the smallest of weapons to the tallest of buildings, steel holds a significant economic and development position.

New technologies developed for extracting and forming can take a versatile material and make it better. New technology can make steel more cost effective. It can change the way the material is produced, used, and recycled. The formation of new steel technology can help to produce more cost efficient houses, taller buildings, and even cheaper cars. In the middle of this twenty-first century recession, it is economically important to attain new technologies in the extraction, creation, and formation of steel materials. With new technologies, industrial suppliers can their products at lower costs due to the reduction of production costs. In the case of industrial process heating, these include capital costs, energy costs, and even costs in emission reductions.

The industrial process heating industry is crucial to the steel industry. Industrial process heating provides heat to steel forms, which are rolled into specific shapes. These shapes are used for building materials such as C-channel, I-Beams, plates, rebar, and strips. A reduction in the cost of this process would significantly boost the steel industry. A new technology such as OFC or POC could possibly provide a reduction in overall production costs that could boost the production of steel.

BENEFITS

During the last few years, the American Steel Market has been troublesome. Disturbing the American steel market is the onset of new and increasingly hard emissions reductions from the EPA and the increase of foreign steel dumping. The reduction of a formation of gas called NOx, has forced industrial process heaters to find new technologies in order to produce the same amount of materials they had produced in the past. This change in regulations and economic standards could be battled with the creation of a feasible technology. OFC or POC could be an answer to help revamp the American steel industry and boost ever needed sales to the American building and housing market.

Oxy-fuel refers to technology that burns oxygen with gaseous fuel. As compared to air, which contains 20.95% oxygen, pure oxygen can reach higher flame temperatures. Approximately the same total energy is produced when burning a fuel with oxygen as compared to with air; the difference is the lack of temperature diluting inert gases. The most common fuel burned in a torch with oxygen is acetylene; even though it presents special handling problems, it has the greatest heat output.

NOx is a generic term for mono-nitrogen oxides (NO and N02). These oxides are produced during combustion, especially combustion at high temperatures. At ambient temperatures, the oxygen and nitrogen gases in air will not react with each other, in an internal combustion engine, combustion of a mixture of air and fuel produces combustion temperatures high enough to drive endothermic reactions between atmospheric nitrogen and oxygen in the flame, yielding various oxides of nitrogen. In areas of high motor vehicle traffic, such as in large cities, the amount of nitrogen oxides emitted into the atmosphere can be quite significant.

A large concentration of nitrogen in an airstream is a downfall. The nitrogen in combustion gas soaks up a large portion of heat. This event causes the exhaust air to be larger and also higher in temperature. Therefore, the flue gasses are hotter and larger in volume. Increasing the amount of oxygen in the combustion airstream can have a positive effect. It can lower the amount of waste gasses and also reduce the heat leaving the stack.

The transformation from preheated combustion air, which contains only 20.9% oxygen to an oxygen-enhanced concentration, may significantly improve combustion characteristics. In oxygen enhancement, commercial oxygen (90% to 100% pure), is forced into either the air stream of directly injected into either the burner of the flame.

There are three ways to force oxygen into the combustion system:

Oxygen Enrichment — the concentration of oxygen is increased by blowing it into the combustion air. This oxygen is usually commercial grade and is mixed until the content of oxygen in the airstream is between 22% and 35%.

Oxygen Lancing — oxygen is mixed with the burner flame inside of the furnace. Commercial grade oxygen (90% to 100%) is injected with a lance through the wall of the furnace and or directly through the flame exiting the burner.

Oxy-Fuel Firing — no combustion air is needed. Natural gas is mixed directly with the commercial grade oxygen. The mixture of oxygen and gas is then burned inside the furnace.

These three forms of oxygen enhanced heating provide three different ways to modify a furnace. Oxygen enrichment uses a blower to deliver combustion air. The air then mixes with oxygen and further mixes with the fuel inside of the burner. This particular system can be retrofitted into an existing system but does require a combustion system. Oxygen lancing is a form of enhancement that can also be retrofitted. This system like oxygen enrichment requires the use of a combustion air system and a precise knowhow into the position of the oxygen lance. This is critical to the performance of the burners. Lastly, Oxy-fuel firing is a system that requires no combustion air; this system does not need a blower and all the connecting ductwork. Fuel is mixed directly with the oxygen and then burned. This type of system would also require a smaller flue and stack due to the potency and small volume of the combined gasses.

Oxygen for these types of heating comes in three different forms:

Cryogenic System — this form of oxygen is created through separating oxygen from air. This type of commercial grade oxygen is the leading supplier of liquid oxygen. It can produce grades of oxygen up to 99.5%. This form can be transferred by truck or rail. This form of delivery can be used for systems up to 100,000 scfh.

Bulk Liquid Supply System — this system is designed for small systems between 1000 to 10,000 scfh. It is also the most economic form of supply. A high grade of 99% can be reached. This type of supply is delivered in a liquid form then placed into a vaporizer to change it into a gaseous state between 100 to 250 psig. This form can then be piped into a furnace.

Adsorption System — Pressure Swing Adsorption (PSA) or Vacuum Swing Adsorption (VSA) are an onsite separation system. The nitrogen inside the captured air is removed using an adsorbed bed of synthetic zeolites. The combination is then removed using a cyclic fluctuation or change in pressure. The remaining product has an oxygen content of 90 to 95%, with some argon and remaining nitrogen. This system is designed for volumes between 10,000 and 200,000 scfh.

Membrane System — This form of oxygen is not like any of the previous. Oxygen is separated from nitrogen using a membrane. The particular membrane permeates oxygen faster than nitrogen. In a single stage separation 28 to 35% content can be reached. Because of the small size and simplicity this type of system is the most cost effective, but produces low purity content in the range of 1,000 to 15,000 scfh.

METHODOLOGY

In the paper “Oxy-Fuel — demanding but highly rewarding” people from Ovako Steel, AGA Gas, and Linde determine that OFC is a highly demanding production but has great rewards. They claim that at Ovako Steel, the installation of OFC has reduced fuel combustion by 35% and has reduced the NOx level to below 100 mg/MJ, The furnace temperature heats up to 1200C to 1250C. The heating time was reduced from 3hr to 2hr 15min. The reduction of heating time has therefore increased capacity by 30%. Although these changes seem to be rewarding they are not without downfalls and limitations. (Lars Arvidsson, Mats Gartz, Joachim von Scheele, 2003) The paper claims that the introduction of OFC and the proper control system increases the amount of maintenance. The tolerance of the control system is greater than that of POC. This means that the equipment used in order to monitor and control the

OFC system needs a greater deal of maintenance. The paper also claims that pressure transmitters, flow meters, and control valves must maintain a higher accuracy. Therefore, the devices are more expensive and require more attention. Ovako Steel states “reasons have been the need for reduced NOx emissions, higher heating capacity, and reduced fuel consumption.” These benefits, combined with the reduction of heating time, have Ovako Steel convinced that their rotary hearth furnace is a more productive and more efficient furnace. When referring to the supplier of the OFC equipment, Ovako Steel says –“It is a continuous improvement process that is mutually beneficial to both parties.”

A website called glass media online has a paper published that encapsulates the benefits of “Tall Crown Furnace Technology for Oxy-Fuel Firing.” The paper demonstrates the benefits of OFC inside a furnace built for glass melting. ‘The authors, H Kobayashi, K T Wu, B G Tuson, F Dumoulin and H P Kiewall, claim that their furnace design reduces wear to the silica refractory and reduces both SOx and NOx emissions. They also remark that they maintain high heat transfer and energy efficiency. Tall states “A comparison of the data from air fired furnaces and those form conventional oxygen fired furnaces shows that about 20% to 30% reductions of overall particulate emissions under oxygen firing.” (H Kobayashi, K T Wu, G B Tuson, F Dumoulin and HP, 2005)

Russell Hewertson’s paper “When does Oxy-Fuel Make Sense”, states that when the oxygen content in the combustion gas is above 20.9%, the combustion process is thermally more efficient. In addition, the paper states that with the reduction of nitrogen the benefits are lower NOx and particulate in the emission gas. This information is based on information from a reverb furnace. This form of heating using OFC has a downfall. Performance in reverb furnaces can be altered by the incorrect flow of OFC. The improper flow could provide uneven heating. Therefore, the correct size, flow, and distribution of the OFC system are crucial. The paper states: “Performance results using oxygen in reverb furnaces vary according to many factors. At a typical operation, production increases range from 20% to 35% along with fuel savings of 20% to 40%, reduced flue gas volume by up to 60%, and reduced total melting cost of 20%.” (Russell Hewertson, 2005)

Airproducts.com states that they have experimented with the idea of providing burners that use oxygen, air, and natural gas. They provide burners for reverb and rotary furnaces. They claim their burners “consistently outperform other burners by more than 10% in fuel efficiency and productivity. Many Air Products’ metal costumers have doubled profits and realized a two-month payback by using these Air Products’ technologies.” (Russell Hewertson, 2005)

Another article “Tall Crown Furnace Technology for Oxy-Fuel Firing” determines that not only has OFC been developed and used by the Praxair Corporation, but it has also been improved on. This means that a functioning OFC furnace for the glass industry has been developed and used successfully.

CONCLUSION

Large industrial industries have the benefit of resources and money in order to create cost effectiveness and increased profits. The results of an added oxygen process to an existing natural gas system may be cost effective. The initial cost of an oxygen system is expensive but if an existing oxygen system can be used, the cost is reduced. Cost savings resulting from the reduction of emissions and the decrease in wasted fuel may not be enough to balance the huge debt incurred by purchasing an oxygen system. This is primarily due to the extreme cautions that are required when handling commercial grade oxygen. Knowing which and what type of burners to use is just as important as how much oxygen one uses. In addition, the control system as stated before is a significant part of the fuel reduction and savings. This is also an important component to minimizing operation cost and keeping a safe environment.

If emissions are not an issue than oxygen may not be the best fit. A major cost break comes from an industrial processor producing more material. This can reduce the final cost to the consumer. New efficiencies gained by using oxygen were not attainable using preheated combustion air and natural gas. The newfound efficiency using oxygen can be attributed to two basic principles, available heat and total heat transfer. These two phenomenon’s can lead two four fundamental benefits:

  • Productivity as a result of better thermal efficiency
  • Improved Quality due to even heating
  • Reduced Specific Fuel Consumption due to better thermal efficiency
  • Reduced Emissions due to the reduction of fuel

 

BIBLIOGRAPHY

Kobayashi, H. “Tall Crown Furnace Technology for Oxy-Fuel Firing” Combustion Efficiency, April 2005, www.glassmediaonline.com

Linde- “Oxy-Fuel boosting of a pusher furnace”

When does oxy-Fuel Make Sense? Hewertson, Russell. Air Products and Chemicals, Inc., 2005, www.airproducts.com/metals

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