Development of natural gas utilization technology in the field of heat treatment of metals Endothermic gas generators fully powered by natural gas

Transcription

1 23rd World Gas Conference, Amsterdam 2006 Development of natural gas utilization technology in the field of heat treatment of metals Endothermic gas generators fully powered by natural gas Main author Toshihiro Kobayashi Japan

2 ABSTRACT Toho Gas is a gas utility company supplying city gas to areas centered on Aichi prefecture, where automotive and related manufacturers are the key industry. In the automotive industry, there is much thermal demand for heat treatment of metals, and use of electric heaters for heating is widespread. However, from the view of global environmental preservation, there is a great demand to reduce CO 2 emission by combustion heating of natural gas. Endothermic gas generator is one of the primary types of equipment used for heat treatment. While combustion heating is rarely used in Japan, propane or butane gas are widely used as raw materials for atmosphere gas. Toho Gas is promoting the development of gas combustion heating technology that uses the latest gas burners with high efficiency regenerative combustion, as well as atmosphere gas control technology that uses natural gas as the raw material. This paper reports on Toho Gas s development on full natural gas powering of equipment used for thermal metal treatment. * Endothermic gas generators Generators which generate a special type of gas for use as atmosphere gas during heat treatment. Gas is generated by passing a mixture of hydrocarbon gas and air through a catalyst filled reactor tube that is heated to 1,000 degrees Celsius. * Regenerative combustion burners Burners are used in pairs, where the two are alternately combusted. This is a high efficiency combustion system where when one burner is being combusted, the other is used to vent the exhaust gas, and the regenerator media embedded in the burner is used to recover wasted heat. Development of high efficiency endothermic gas generators using a regenerative combustion burner Although regenerative combustion burners are high efficiency systems, due to their special method of combustion, require engineering work specific to the shape of the furnace. We have selected two of the most commonly used type of endothermic gas generators in Japan, and in accord with the shape of the each furnace, applied an internally developed Swirling flow type regenerative combustion burner (SFRB) or Single type regenerative combustion burner(srcb), a burner which has the characteristic of being compact in size. In both burners, high heating efficiency of over 80% was achieved. This has resulted in a 35% reduction in CO 2 emissions compared to conventional gas burners. This is approximately a 40% reduction in CO 2 compared to electric heaters. Heating using Swirling flow type regenerative combustion burners Heating using Single type regenerative combustion burners Development of atmosphere gas control technology Endothermic gas is used as atmosphere gas for carburizing metals, and by controlling the oxygen concentration within the atmosphere gas, the performance of carburization (the carbon potential) is kept at a fixed level. However, the main component of natural gas is methane, and oxygen sensors have a problem of increased errors under high concentration of methane in atmosphere gas. Thus, development relating to controlling atmosphere gas was performed as follows: Demonstration on methods for controlling carbon potential of atmospheric gas Development of hybrid carbon potential control Development of a new oxygen sensor

3 TABLE OF CONTENTS ABSTRACT 1. Development of high efficiency endothermic gas generators using regenerative burners 1.1 Background and aim of development 1.2 Heating using Swirling flow type regenerative combustion burner (SFRB) Outline Principles Characteristics 1.3 Heating using Single type regenerative combustion burner (SRCB) Outline Principles Characteristics 1.4 Summary and effects 2. Development of control technology for atmospheric gas 2.1 Background and aim of development 2.2 Demonstration of methods for carbon potential (CP) control in atmospheric gas Outline Test content and results 2.3 Development of hybrid carbon potential control (HCPC) Outline Characteristics 2.4 Development of a new oxygen sensor 2.5 Summary 3. Conclusion 4. References 5. List of Tables 6. List of Figures

4 Paper 1. Development of high efficiency endothermic gas generator using regenerative burners 1.1 Background and aim of development Metals, in the aim of improving fatigue and wear properties of metals are heat treated in furnaces. Endothermic gas generators are widely used for manufacturing of atmospheric gas required in these furnaces. As these generators are peripheral equipment of heat treatment furnaces and must be compact in size, they are often powered by electricity. And the combustion space is required for gas combustion, so it was difficult to adopt combustion heating that requires a larger space than electric heater installation space. Recent demands for reduction in cost and CO 2 emissions have resulted in our development of space saving, high efficiency regenerative burners that aim to achieve significant energy saving and improved gas generation capability. We have applied Swirling flow type regenerative combustion burner (SFRB) or Single type regenerative combustion burner (SRCB) in accord with the shape of two of the most commonly used endothermic gas generators in Japan. These have been commercialized and have been introduced to the market. 1.2 Heating using Swirling flow type regenerative combustion burner (SFRB) Outline In generators which have reactor tubes placed in the center axis of the cylindrical generator body, and which have electric heaters installed on the generator wall surrounding the reactor tubes, there is only narrow space between the reactor tubes and the generator inner wall. As such, combustion of gas within this space was difficult. Swirling flow type regenerative burners have been developed for application to this type of generators, and are designed so that combustion gas flows in a swirl alongside the generator wall, resulting in even heating within a confined space. The burner capacity is rated at 80kW, gas consumption is 125,000m 3 per year inclusive of raw material gas. These generators have been commercialized in July 2004, and 15 units are due to be deployed by September Principles This burner is operated by a system where two burners combust alternatively, and where the regenerator media embedded in each burner is used to recover wasted heat for the purpose of pre-heating the air during combustion. Gas and air nozzles are placed so that combustion in a narrow space takes place in a swirling fashion around the reactor tubes. And dual burners are placed above and below the generator, perpendicularly and tangentially to achieve even heating of the reactor tube.

5 1.2.3 Characteristics This generator is a development and application of a swirling flow type regenerative burner where an even heating of objects have been achieved through formation of swirling flames within the restricted space alongside generator walls. The generator has the following characteristics: [Space saving] By placing swirling flow type regenerative burners above and below, in perpendicular, and tangentially with the cylindrical generator, an evenly distributed temperature of the reactor tube has been achieved. Moreover, by adopting a structure where the majority of the burner body is placed inside the generator wall space, the generator as a whole could be made compact in size. By using high performance insulating materials for the generator insulator, insulation performance was increased while reducing the insulation thickness, allowing a 25% reduction in size compared to conventional gas burner heating (comparable to electric heating furnace). [Energy saving (eco-friendly)] To achieve a high efficiency pre-heating of air, wasted heat in the exhaust gas is collected through the regenerator media embedded in each burner. Compared to conventional gas heating furnaces which do not reuse the wasted heat, a significant 35% reduction in energy usage and CO 2 output have been achieved (55% reduction in energy usage and 45% reduction in CO 2 compared to electric heating). 40% reduction in heat loss from generator wall through use of high performance insulating materials. [Higher performance] Increase of endothermic gas output by 1.4 times through gas heating. Decrease in heating time by 30% and cooling time by 60% through use of high efficiency regenerative burners.

6 Item Conventional electric Conventional gas Development heating generator heating generator generator Energy usage (ratio) CO 2 output (ratio) Table1 : Comparison of environmental performances of development generator and conventional generator(sfrb) 1.3 Heating through single type regenerative combustion burners (SRCB) Outline In the single type regenerative burners, regenerative combustion is performed by a single burner, allowing a generator design of compact size. However, care is required as there is a possibility of unevenness in the flow of combustion gas within the generator. This burner has been applied to generators which use U-shaped reactor tubes. By adopting a structure where combustion takes place at the center of the U-shaped reactor tube, exhaust gas from combustion flows alongside the outside of the reactor tubes and returns to the burner. This is an effort to achieve an even distribution of heat in the reactor tube. The burner capacity is rated at 116kW, and gas consumption is between 80,000 to 160,000 m 3 per year depending on the gas output capability. Since commercialization in 1996, 16 of these units have been shipped Principles This burner uses a single burner for regenerative combustion. By embedding a regenerator media inside the burner, and through use of the internal rotating disk switch, regenerative combustion takes place while the combustion air path and exhaust gas path are continuously switched. By adopting a structure where combustion takes place in the space at center of the U shaped reactor tube, combustion exhaust gas flows along the outside of the reactor tube and returns to the burner, achieving an evenly distributed heating of the U shaped reactor tube Characteristics [Space saving] By employing a singly type regenerative burner, reduction in footprint of 40% was achieved compared to conventional regenerative burners. [Energy saving (eco friendly)] The combustion exhaust gas is at 250 degrees Celsius, and compared to conventional gas heating generators which do not recover wasted heat, a 40% reduction in energy use and

7 CO 2 output have been achieved (55% reduction in energy usage and 45% reduction in CO 2 compared to electric heating). Improved working conditions due to reduction in exhaust gas temperature. [High performance] By avoiding local over heating of the reactor tube, and through stirring caused by the combustion gas within the generator, an even spread has been achieved for the surface temperature of the reactor tube; stable formation of gas even when the endothermic gas output is reduced to 1/3 of the rated output. This has led to reduction in soot inside the reactor tube catalyst, and thus to a reduction in the number of times of burnout operation for the purpose of removing soot. 40% reduction in generator heating time through adoption of high efficiency regenerative burners, resulting in improved productivity. Item Conventional electric Conventional gas Development heating generator heating generator generator Energy usage (ratio) CO 2 output (ratio) Table2 : Comparison of environmental performances of development generator and conventional generator(scrb) 1.4 Summary and effects In Japan, and only considering those which have electric heating, over 1000 units of the above two commonly used types of endothermic gas generators are in use. In the metal heat treatment industry today, adoption of heating through high efficiency gas combustion to existing furnaces is seen as a technical solution in reducing environmental impact and reduction of heat treatment cost. The energy saving, gas heated endothermic gas generators is an attractive solution to society, and its adoption is expected to spread further. 2. Development of control technology for atmospheric gas 2.1 Background and aim of development Endothermic gas manufactured in endothermic gas generators are used as atmospheric gas during carburization of metals, and are also termed carrier gas. In order to increase carburization from atmosphere to the carburized object, a small amount of raw material gas is added to the carrier gas to achieve an atmosphere with the desired carbon potential (CP). Controlling of CP is most frequently achieved through controlling oxygen concentration within the atmosphere gas, and a zirconia type oxygen sensor is used for measuring the oxygen concentration. These sensors have a problem of measurement error increasing with increasing methane concentration in the atmospheric gas. Thus, by developing atmospheric gas control, we promoted use of natural gas in thermal treatment equipment. Of the technology mentioned below, those which are complete are a result of a joint research between Tokyo Gas Co., Ltd., Osaka Gas Co., Ltd., and Toyota Technological Institute. 2.2 Demonstration of methods for carbon potential (CP) control in atmospheric gas Outline There are two types of controlling CP, those where carbon dioxide concentration in furnace is controlled (CP CO2 ), and those where oxygen concentration is controlled(cp O2 ). We tested to see which of the two result in a more precise control, as well as understanding the characteristics of each approach Test content and results To test carburization capability of a steel test material, surface carbon concentration and carburization structure were compared when natural gas and buthane were used as

8 enriched gas. As a result, by calculating CP using the carbon dioxide concentration in atmosphere, surface carbon concentration in all test pieces matched this CP figure, and good carburization structure was achieved regardless of the enriched gas used. For atmospheric gas with CP CO2 between 1.0 and 1.3, it was confirmed that CP can be controlled through CP CO2 1). CP O2 and CP CO2 were compared when natural gas and buthane were used as the enriched gas. As a result, the CP CO2 was identical to the real CP, however CP O2 was higher than CP CO2 and the difference became large with increasing CP CO2. This is due to the catalytic effect of the platinum electrode used by the zirconia oxygen sensor causing the hydrocarbon gas near the sensor reacting with oxygen, causing the oxygen concentration to decrease locally 2),3). Tested techniques for converting enriched gas from LPG to natural gas. Test results have shown by observing behavior of CP O2 while adjusting the amount of enriched gas so that the CP CO2 for both gases are identical, that behavior could be used to set the target CP O2. When doing so, in order to reduce the risk of insufficient carburization, CP O2 is set so that the speed in increase of CP CO2 at the beginning of carburization can be high, or by increasing the maximum amount of enriched gas. 2.3 Development of hybrid carbon potential control (HCPC) Outline Traditionally, CP control was achieved through measurement of oxygen concentration inside a furnace using an oxygen sensor. This responds well to changes in atmospheric gas concentration, but at the same time, precise control of CP was difficult. As such, by using a carbon dioxide sensor in conjunction, a hybrid carbon potential control (HCPC) was developed where high precision control (within 0.05% of target CP) of CP could be achieved 4). This has also brought a 10% reduction in treatment time resultng in improved productivity and energy savings Characteristics [Low cost and energy saving] 50% reduction in time required to reach desired CP of atmosphere in furnace. 10% reduction in full carburization time. [High precision] Combined use of an oxygen sensor with good response and a carbon dioxide sensor with good precision. High precision adjustment of CP to within 0.05% of target value is possible, bringing precision to quality control. [Effective prevention of soot generation] In the carburization process, drop in product quality due to soot is a problem. However by using an oxygen sensor with fast response, risk of soot is quickly detected and soot is effectively prevented.

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10 2.4 Development of a new oxygen sensor When controlling CP of atmospheric gas in heat treatment furnace, an oxygen sensor is used to measure the oxygen concentration. However, the output of oxygen sensor is affected by the concentration of hydrocarbons deposited in the furnace. Thus, as a means to achieve precise control of CP, an oxygen sensor that is immune to being affected by content of the atmospheric gas in the furnace has been in development since April The content of development is as follows, and development is still underway: 2.5 Summary Investigation into how conventional oxygen sensors affect the measurement of furnace atmosphere. Designs on improved type of oxygen sensor. Evaluation of characteristics of improved type of oxygen sensor and improvements to maintain precision. Application of improved type of oxygen sensor to high precision atmosphere control for gas carburization using natural gas. Evaluation of improved type of oxygen sensor by using a real unit. We conducted development for controlling atmospheric gas during heat treatment of metals, and promoted use of natural gas for heat treatment equipment. The results are summarised as follows: By measuring the carbon dioxide concentration in the atmospheric gas and using that to control CP, control is possible whether natural gas or LPG is used as the enriched gas. Measurement of oxygen concentration in furnace using an oxygen sensor is affected by hydrocarbons remaining in the generator. A CP which is higher than the actual CP is obtained. As CP increases, the difference between the actual figure increases. However, if this difference can be taken into account, control using oxygen concentration can be used without any problems. This holds true even if the main component is natural gas. Use of HCPC leads to reduction in treatment time, high precision in attaining target values, and effective prevention of soot. Once development of the new oxygen sensor is complete, both high speed and high precision control of atmospheric gas will be possible.

11 3. Conclusion For endothermic gas generators, we applied Swirling flow type regenerative combustion burner (SFRB) or Single type regenerative combustion burner(srcb) depending on the shape of the furnace; as a result, we developed and commercialized a gas combustion heating with a 35% reduction in CO 2 emissions. Moreover, we conducted technical development for controlling atmospheric gas, in order to absolve a problem when using natural gas as the raw material gas for endothermic gas. Together with the heating source, we established techniques for full natural gas powering of endothermic gas generators. In the future, we will use these techniques to promote full natural gas powering of heat treatment equipment. 4. References 1. M.Okumiya, Y.Tsunekawa, K.Kurahashi, J.Takebe, A. Maeda, S.Ichinose, K.Mikami (2003). Effective Utilization of Natural Gas for Surface Heat Treatment of Steel Surface Technology Vol. 54 No.5: K.Kurahashi, J.Takebe, H.Asano, M.Okumiya (2003). Carburization atmosphere control using city gas (13A gas) Industrial Heating Vol.40 No M.Okumiya, Y.Tsunekawa, S.Ichinose, K.Kurahashi, J.Takebe, A. Maeda (2002). Effect of Residual Methane Concentration on the Carbon Potential Calculated from Oxygen Sensor in Gas Carburizing used Natural Gas for Enrichment Heat Treatment Vol. 42 No K.Kurahashi, J.Takebe, A.Maeda, M.Okumiya (2001). New methodology for controlling carburization atmosphere Industrial Heating Vol.38 No.5: List of Tables Table1 : Comparison of environmental performances of development generator and conventional generator (SFRB) Table2 : Comparison of environmental performances of development generator and conventional generator (SRCB) 6. List of Figures Figure1 : Heating by swirling type regenerative combustion burner of endothermic gas generator Figure2 : Heating by single type regenerative combustion burner of endothermic gas generator Figure3 : Diagram of hybrid carbon potential control (HCPC) Figure4 : Transition of atmosphere in furnace controlled by CP CO2 Figure5 : Transition of atmosphere in furnace controlled by CP O2 Figure6 : Transition of atmosphere in furnace controlled by HCPC