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Full Oxygen Combustion in Glass Furnaces

Original Paper | Published: 8th March 2015
Author: kakugy Guo
16k Accesses | 114 Citations | 329 Altmetric | Metrics

Abstract

This article discusses the full oxygen-assisted combustion, full oxygen combustion mechanism in glass furnaces, their development trends, and the structural changes in furnaces caused by changes in combustion-assisting media. Full oxygen combustion is also discussed in the context of environmental management, including its role as an effective measure against atmospheric pollution and greenhouse effects. It is recommended that corresponding guiding policies be formulated during the preparation of the “Eleventh Five-Year Plan,” and the trial implementation of “full oxygen combustion technology” should be conducted to gain experience for wider adoption.
Keywords: Full oxygen combustion, full oxygen-assisted combustion, “Zero-Emission Furnace,” greenhouse effect, NOx, refractory wear on silica brick, high crown technology, energy conservation, atmospheric pollution, Kyoto Protocol, Montreal Protocol.
This article is dedicated to the technology workers involved in the glass industry and the formulation of the “Eleventh Five-Year Plan.”

Full Oxygen Combustion In Glass Furnaces Introduction

Over the past 20-plus years of reform and opening up, China’s national economy has rapidly developed and garnered global attention. The glass industry, encompassing flat glass, electronic glass, glass fiber, daily glass, optical glass, and more, has also experienced rapid growth. Taking float glass as an example, by the end of 2004, 126 float glass production lines had been established and put into operation, with a total production exceeding 300 million weight boxes and a daily melting capacity of 52,930 tons. There were an additional 51 production lines either under construction or planned. Various fuels, including coal, coal tar, heavy oil, natural gas, and electricity (in small quantities), are used for glass melting. Currently, the average energy consumption for melting one kilogram of glass (flat glass) in China is in the range of 1500-1800 kcal. Based on this unit energy consumption, the glass industry is undoubtedly a major energy consumer. As global oil prices rise, China’s oil imports are increasing year by year. According to research, China’s oil import rate is expected to reach 55.4%, 57.4%, and 59.7% by 2010, 2015, and 2020, respectively, far exceeding the theoretical control threshold of 30%. According to the International Energy Agency’s forecast, China’s external dependence on oil is expected to reach 74% by 2030. Most glass furnaces in China primarily use heavy oil as fuel. Therefore, the control of overall energy consumption in the glass industry, especially for high-energy-consumption glass furnaces, is an urgent priority from both an energy-saving and cost-saving perspective.

The Kyoto Protocol came into effect on February 16, 2005, and on July 27, 2005, the United States, Australia, China, India, and South Korea signed the Asia-Pacific Clean Energy Development and Climate Change Partnership Agreement, also known as the “Vientiane Partnership.” Both agreements call for the protection of the global environment.

China currently ranks second in the world in terms of greenhouse gas emissions and is projected to surpass the United States to become the top emitter. The extent of air pollution in China is a growing concern (as reported in The New York Times on October 30: China’s next likely surge is in polluted air). Rough estimates suggest that one-third of China’s territory is affected by acid rain. The Chinese government must now recognize its domestic and international responsibilities in the environmental arena.

The “Eleventh Five-Year Plan” outline proposed by the Communist Party and the state has identified energy conservation and environmental protection as key issues to be addressed during this period. Strict control of atmospheric pollution, reduction of greenhouse gas emissions through new regulations and technologies, is already a well-established policy.

As the glass industry has developed, the demand for product quality has continued to rise, and fuel costs have increased. This has led scientific and technological workers to continually explore and improve various aspects of glass production, including the core component – the “glass furnace.” The combustion system is no exception, and significant progress has been made in this area.

In addition to the increasing concern about the world’s dwindling energy supply and the exploration of various energy-saving measures, attention is also being given to the human living environment. Measures to address the various waste gases emitted by furnaces are being taken. Besides the attention given to “SOx” produced by burning high-sulfur fuels, there is a significant amount of NOx generated during combustion with air as the oxidizer. This NOx contributes to photochemical atmospheric pollution and the greenhouse effect, affecting the global human living environment, and as such, it deserves attention.

Throughout history, glass furnaces have always used air as the combustion-supporting medium. Through analysis and research on existing combustion systems, it is believed that using air as the combustion-supporting medium is a significant factor contributing to high energy consumption, pollution, and a high greenhouse effect. Only 21% of oxygen in the air participates in combustion, while the remaining 78% nitrogen not only does not participate in combustion but also carries a large amount of heat into the atmosphere. Through extensive and repeated experiments, it has been found that using oxygen with a purity of ≥85% as the combustion-supporting medium has a significant effect on energy savings and environmental improvement. Energy consumption can be reduced by 12.5% to 22%, with a potential for more than 30% reduction in the future (see Figure 2). Emissions of waste gases are reduced by over 60%, and “NOx” in the waste gases is reduced by 80-90%, while particulate matter is reduced by more than 50%.

Figure 1

This system that uses oxygen with a purity of ≥85% for combustion is referred to as “full oxygen combustion.” In glass furnaces, partial installations of full oxygen combustion systems (commonly known as “Zero-Emission Furnaces” in float glass furnaces) are referred to as “full oxygen-assisted combustion.” Due to changes in the combustion system, the structure of glass furnaces has undergone a transformation. Full oxygen combustion furnaces eliminate the need for regenerators, small furnaces, and regenerative systems, making them resemble unit kilns (see Figure 1). For float glass kilns using transverse flame structure, the span of the melting section factory building can be reduced by about 2/5, and the investment in the main production line can be reduced by about 30%. For float glass kilns that adopt full oxygen combustion, there is no need for the “traditional regenerative process,” making glass melting more stable, almost approaching the ideal state. The flying batch during the melting process is significantly reduced, bubble release in the clarification zone is thorough, and the quality of glass melting is significantly improved.

Figure 2: Comparison of Energy Consumption for Full Oxygen Combustion

The differences in heat transfer processes between using air and full oxygen as combustion-supporting media are significant (see Table 1).

Traditional air-assisted combustion requires heat exchange between the flue gas and combustion air through periodic regenerator switching to recover some of the heat energy. However, during the regenerator switching process, the flame inside the kiln is momentarily extinguished, and the glass melt inevitably loses its heat source, resulting in kiln temperature fluctuations and transient pressure fluctuations as a consequence.

Typically, in air-assisted combustion, due to the structural requirements of small furnaces, they need to occupy a relatively wide position along the length of the pool wall, limiting the reasonable placement of burners. With full oxygen combustion, the burners have a different design compared to small furnaces, and their external dimensions are relatively small. This allows for a rational distribution of burners based on the melting temperature curve, allowing for burners to either align or cross each other. It’s entirely possible to automatically control the kiln temperature according to the melting temperature curve without risking damage to the kiln body. In the case of float glass kilns, it generally leads to a decrease in hot spot temperatures and an increase in the temperature of the raw material pre-melt zone. The result is that the raw materials in the pre-melt zone quickly form a thin shell due to high-temperature gas heat transfer, preventing the dispersion of fine materials. Full oxygen-assisted combustion, commonly referred to as “Zero-Emission Furnaces” for float glass kilns, involves installing “oxygen + fuel” burners on both sides of the chest walls between the “No. 1 furnace” and the front face wall. These burners are used during the middle and late stages of kiln operation and for producing special dark-colored glass. They serve to increase the temperature of the pre-melt zone, move the bubble boundary forward, reduce material loss, increase production by about 10-15%, significantly reduce bubbles in the glass, improve product quality, and restore the early-stage functionality of the kiln without the need for regenerator maintenance, saving labor, materials, and repair costs.

“Oxygen + Fuel” Combustion Technology Achievements

By the turn of the century, there were more than 200 full oxygen combustion kilns worldwide, with North America having 550, including small specialty glass kilns, of which about 140 were full oxygen combustion kilns. In Europe, out of the 350 existing kilns, more than 30 were full oxygen combustion kilns (excluding glass wool and specialty glass kilns). In Asia, there were more than 20 full oxygen combustion kilns. In recent years, full oxygen combustion has also been introduced in China, with full oxygen-assisted combustion in glass fiber tank kilns, thin-shell kilns, glass cone electronic kilns, and the addition of “No. 0 furnace” in float glass kilns. (See Figures 3 and Figures 4).

Figure 3: Distribution Map of Oxy-Fuel Kilns in North America.
Figure 4: Distribution Map of Oxy-Fuel Kilns in Europe

Companies like Praxair and Air Products in the United States have conducted long-term research and development of full oxygen combustion technology to develop the gas market. Over the past fifteen years, full oxygen combustion technology has been gradually improved, and companies with combustion test facilities around the world have gained many successful experiences. These include providing design reference materials, including mathematical models and technical software, for full oxygen combustion kilns, as well as equipment such as oxygen supply systems, burners, refractory bricks to support burners, self-supply systems for fuel (heavy oil, coal tar, natural gas), and self-control systems for “oxygen + fuel,” as well as various types of oxygen production equipment.

The American company Praxair also holds multiple patents in this field, such as silicon brick high-arch structure design technology and burner patents for “oxygen + fuel.” So far, full oxygen combustion has become a mature technological achievement with significant effects.

Issues Related to Full Oxygen Combustion

Here, we will discuss some relevant issues regarding full oxygen combustion in glass kilns, including oxygen purity (rich oxygen, full oxygen), temperature distribution in float glass kilns, kiln atmosphere, oxygen consumption for full oxygen-assisted combustion in the “No. 0 furnace,” and high arch technology.

  1. Oxygen Purity: In the air, oxygen constitutes only 21% of the mixture. Oxygen levels above 21%, such as 22%, are considered rich oxygen. Therefore, the term “rich oxygen combustion” is not precisely defined and can be confusing. Experimental evidence shows that for significant combustion effects, the oxygen concentration should be at least 85%. Generally, full oxygen combustion requires an oxygen concentration of ≥91-92%. Combustion with an oxygen concentration below 85% does not yield good results. Therefore, using oxygen generated by air separation equipment, which typically has an oxygen concentration below 85%, as an assist combustion medium is not sufficient for achieving significant combustion effects.
  2. Temperature Distribution in Float Glass Kilns: Due to the relatively small size of the “oxygen + fuel” burner in terms of its external dimensions, it can be distributed reasonably according to the melting temperature curve. Compared to air-assisted combustion, the typical temperature rise in the pre-melting zone of a float glass kiln (from the batch inlet to the No. 1 furnace) is about 65°C, and the temperature at hot spots drops by approximately 20°C (see Figure 5).
Figure 5
Practical experience has shown that this is beneficial for the glass melting process, including pre-melting, melting, clarification, and adjustment of the kiln atmosphere, and it slows down the erosion of refractory materials.
  1. Kiln Atmosphere: Full oxygen combustion leads to significant changes in the kiln atmosphere. The volatile reaction of alkali (NaOH) on the surface of the glass melt results in a several-fold increase in the concentration of alkali vapor (NaOH). This intensifies the erosion of the kiln crown’s silicon bricks (see Figure Six).
  2. Oxygen Consumption for Full Oxygen-Assisted Combustion in the “No. 0 Furnace”: Oxygen consumption can be calculated based on oxygen usage for the entire kiln assist combustion, which should be ≥15%, or it can be calculated based on increased glass production.
  3. High Arch Technology: To prevent the erosion of the kiln crown’s silicon bricks caused by substances like sodium sulfate (Na2SO4), Praxair in the United States has implemented high arch technology. This involves increasing the height of the kiln crown, raising the temperature in the batch charging section, and elevating the burners. These measures reduce the concentration of alkali vapor (NaOH) released and decrease the erosion of the kiln crown’s silicon bricks, leading to longer kiln lifetimes. The practical results of this approach have been very positive (see Figures Six, Five, and Seven).
Figure 6
Figure 7
Economic Analysis of Full Oxygen Combustion Technology
  1. The investment cost for the installation of the “No. 0 Furnace” for full oxygen-assisted combustion is low, and the daily oxygen consumption is minimal, resulting in significant benefits. During the operation of float glass kilns in the middle and later stages, it can increase production, improve glass quality, and eliminate the need for additional expenses such as reheating chambers, making its economic benefits very apparent.
  2. Economic Comparison of Full Oxygen Combustion (see Table Two)

Table Two: Cost Comparison between “Air + Fuel” and “Oxygen + Fuel”

Note:
a. This table uses a 700T/d float glass kiln as an example.
b. Clinker (crushed glass) accounts for 15% of the batch.
c. Heavy oil is priced at ¥3000 per ton.
d. Oxygen is priced at ¥0.35 per standard cubic meter.
e. Environmental expenses and incentives are not considered.
f. Maintenance expenses for “Air + Fuel” kilns are not included.
g. Savings in construction and equipment investment due to “Oxygen + Fuel” technology are not included.

The investment in the oxygen supply station for a 700T/d float glass production line is covered by the gas supplier. The oxygen price includes the cost of electricity.

From this simplified calculation, it can be seen that due to the rising cost of energy (heavy oil), the cost difference between the two options is only ¥1.85 per box. If we consider the improvement in glass quality resulting from using “Oxygen + Fuel” technology, which raises the quality from construction grade to automotive grade, it can increase the selling price of 5mm glass per square meter by ¥1-1.5. Additionally, with favorable government policies promoting the application of new environmental protection and energy-saving technologies, the economic benefits of adopting full oxygen combustion are favorable.

How to Apply Full Oxygen Assistance and Full Oxygen Combustion

There are two practical options for applying the “Oxygen + Fuel” scheme:

  1. Install a full oxygen-assisted combustion system with a “No. 0 Furnace.” In principle, all float glass kilns currently operating in China can be retrofitted with a “No. 0 Furnace,” especially for kilns that have been in operation for 3-4 years in the middle and later stages. Adding a “No. 0 Furnace” can restore the kiln’s production capacity in the early stages, and glass quality significantly improves. For newly commissioned kilns, it is more reasonable to reserve a space for the installation of the “No. 0 Furnace” before construction and install it after the kiln is in operation. For some large glass conglomerates with three or more float lines in one location, a single oxygen production system can be used to save equipment investment.
  2. Design a full oxygen combustion kiln. Since the cost of producing oxygen is higher than that of using air, a feasibility analysis should be performed when choosing to apply the full oxygen combustion scheme. Choose a reliable oxygen production unit (e.g., vacuum pressure swing adsorption can produce oxygen with a concentration of 90-91% or higher), and also consider whether there is a “liquid oxygen source” nearby as an emergency backup oxygen supply base. It is generally advisable to locate it near the intersection of highways and within 300-400 kilometers of the liquid oxygen factory.

For cold modifications of existing float glass company float lines, if the full oxygen combustion scheme is adopted, it can also save investment in reheating chambers, small furnaces, and reduce maintenance costs for the firing system, while improving product quality and yield.

Calculating operating costs: When calculating, each item should be considered, including oxygen expenditure, fuel savings, increased revenue from higher product quality levels, financial income from investment savings, and the cost of extending the kiln age.

From the perspective of operational benefits, for horseshoe flame kilns, unit kilns, cross-firing kilns, etc., high-value-added kilns should implement full oxygen combustion. For float glass melting kilns, high-value-added ultra-thin glass, ultra-transparent glass melting kilns, and large to medium-sized production of high-end float glass melting kilns should be selected. From a long-term perspective, once energy prices continue to rise, glass pollution fees increase, and the national incentive measures for emission reduction of flue gases are implemented, the promotion of full oxygen combustion becomes imperative.

Policy Orientation as the Driving Force for “Full Oxygen Combustion”

Summarizing the experiences of the past ten five-year plans, addressing challenges such as energy conservation (due to energy scarcity), environmental protection (compliance with the Kyoto Protocol and Montreal Protocol), market competition (increased demand for high-quality glass products), and cost reduction (rising fuel prices, environmental fees, and increased taxes) has imposed stringent requirements on the future development of glass furnace technology. The “Eleventh Five-Year Plan” has set the goal of building energy-efficient and environmentally friendly industries, explicitly targeting a 20% reduction in energy consumption as a proportion of GDP by the end of the Eleventh Five-Year Plan. Policy orientation has provided significant impetus to the development of glass furnace technology.

In summary, “full oxygen combustion” has already become a mature technology with significant effectiveness. Therefore, it is imperative to guide this technological revolution in furnaces by formulating appropriate policy directives. The following recommendations are proposed:

  1. Conduct a comprehensive nationwide survey of energy consumption, atmospheric emissions, and air pollution in the glass industry. Formulate corresponding measures to encourage the adoption of “full oxygen combustion” and other energy-efficient and environmentally friendly technologies.
  2. Develop practical new regulations based on international agreements such as the Kyoto Protocol and Montreal Protocol, as well as relevant domestic laws and regulations. These regulations should specify technical standards and environmental requirements for energy conservation, pollution control, and reduction in greenhouse gas emissions.
  3. Establish incentive mechanisms for adopting new combustion technologies, energy efficiency measures, and emissions reduction technologies. Implement policies such as tax exemptions and deductions, favorable financing terms for technology upgrades, and credit support.
  4. Create policies to reward individuals and organizations for inventing, promoting, and designing energy-efficient and environmentally friendly technologies.
  5. Recommend implementing “full oxygen combustion” technology in a new construction or cold modification project as a pilot initiative to gain experience for wider adoption.

In conclusion, policy orientation plays a crucial role in driving the development and adoption of technologies like “full oxygen combustion.” Well-designed policies can encourage energy efficiency, environmental protection, and technological innovation in the glass industry, ultimately contributing to sustainable development and improved product quality.

Conclusion

Full oxygen combustion is a mature and reliable technology that not only benefits the enterprises employing this technology but also proves effective in combating air pollution, contributing to the improvement of regional air quality. Moreover, due to the structural changes it brings to furnaces, it saves more than 30-40% of refractory materials used in regenerators and small furnaces, resulting in significant savings in construction investments. As a result, raw materials like refractories, cement, and steel, which are used in manufacturing, also save energy. From a societal perspective, full oxygen combustion offers immense energy-saving and environmental benefits and should be implemented to gain experience before systematic expansion.
Full oxygen combustion represents a revolutionary change in glass furnace technology, aligning with the requirements for establishing an energy-efficient, environmentally friendly, and circular economy.

References:
  1. Tall Crown Furnace Technology for Oxy-Fuel Firing, H. Kobayashi, K. T. Wu, G. B. Tuson, F. Dumoulin, and H. P. Kiewall, GLASS, April 2005, pp. 78-80.
  2. Kobayashi, H., K. T. Wu, G. B. Tuson, F. Dumoulin, and H. P. Kiewall, Ceramic Bulletin, February 2005, pp. 14-19; March 2005, pp. 24-27.
  3. Advances in Glass Melting Technology: Recent Advances in Oxy-Fuel Fired Glass Melting, James Yuan, and William T. Kobayashi, Praxair (China) Investment Co., Ltd.
  4. “The Next Big Thing in China Could Be Polluted Air,” The New York Times, October 30th article.
  5. “Black Reputation,” Editorial in the Financial Times, October 31st.
  6. Research Report on China’s Productivity Development, Academician Xu Shoubo, Chinese Academy of Engineering, November 19, 2005, Yangzi Evening Post, A2.
About the Authors . . .

kakugy Guo Senior Engineer

CEO & Author Introduction
Advanced Refractory Materials and Furnace Technology
Leader in Oxygen Combustion Technology
info@agrmeng.com

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