Minoru SASAKI*
Teruyuki OKAMOTO*
*Ebara Environmental Plant Co., Ltd.
In Japan, waste treatment facilities have been changing along with the living environment and social needs of the times. In addition to their intended purpose of sanitation treatment of refuse, they are now required to play more multifaceted roles, including highefficiency energy recovery and utilization as disaster prevention bases. Of such roles, here we will focus on waste power generation technology, which is expected to respond to the growing demand for a stable electric power supply as renewable energy. In this report, we will also introduce the history of high-efficiency power generation technology in Japan, Ebara’s efforts toward such technology, recent survey results and solutions to issues, some recent cases of adopting the technology, and its future direction.
Keywords: High efficiency, Waste power generation, Stoker furnace, High temperature high pressure boiler
In the history of waste treatment in Japan, the role of waste treatment facilities has shifted from appropriate sanitation treatment of waste for the purpose of ensuring public health to the preservation of the living environment, then to the preservation of the global environment. The significance of waste treatment technology has also changed in line with such shifts, and the Great East Japan Earthquake and other devastating disasters have offered us opportunities to reconsider how waste treatment should be in Japan, how waste treatment facilities should contribute to communities as public facilities, and how energy should be used and reused. Today, waste treatment facilities are required to further strengthen their functions as bases for disaster prevention and to be used more effectively as energy centers in communities.
Under these circumstances, waste treatment facilities are recently required to fulfill a wide spectrum of capabilities, as described below.1)
• Safe and stable sanitation treatment of waste
• Prevention of environmental pollution due to emissions of hazardous substances, such as dioxins
• Recovery of energy from incineration waste heat with high efficiency
• Less power and fuel consumption
• Reduced amount of residues
• Easy operation control
It is necessary to further commit to the improvement of the recovery rate of energy from incineration waste heat, among other items, in order to promote the construction of facilities to maximize the power output and the amount of transmitted power and achieve efficient operation and improvements in response to the Paris Agreement (signed November 2016), which sets the framework and numerical goals for dealing with global warming in and after 2020, the Feed-in Tariff System for Renewable Energy (enforced in 2012), the Total Deregulation of Retail Electricity Business by the Amended Electricity Business Act (enforced in 2016), and similar policies.
In this report, we introduce the history of high-efficiency power generation technology to date, Ebara’s efforts in this area, the results of recent studies and solutions to issues, some of the latest cases of adoption of high-efficiency power generation technology, and the future outlook.
Ebara’s commitments to waste treatment trace back to 1961, when we delivered a grate-type incinerator with a capacity of 37 t/d to Aomori City. We developed a grate-type incineration system in the 1980s, and delivered our first boiler-equipped incineration plant to the Musashino-Mitaka Area Insurance Union in 1984. We were ahead of other companies in introducing a practical high-temperature, high-pressure-type boiler in 1996, and we delivered a power generation plant to the Tokachi Environmental Complex Administrative Association (Kuririn Center) in Obihiro City, Hokkaido. Featuring a steam condition of 3.82 MPaG × 400ºC and a power generation capacity of 7,000 kW, it was one of the largest power generation plants in Japan at that time. Since then, this power generation plant has been in stable operation for about 23 years while the operation cost has been cut significantly by selling redundant electric power. In 2004, we delivered a boiler-equipped power generation plant consisting of two large grate-type incinerators with a capacity of 350 t/d and two large ash melting furnaces with a capacity of 65 t/d, to the Adachi Incineration Plant in Tokyo.
At present, we are upgrading the grate-type incinerator to a new version (Figure 1) to further ensure the performance required for waste treatment facilities. This upgrade also supports combustion at a low air ratio and at high temperatures. We have delivered many facilities that are still in stable operation, dating back to the delivery of a facility to the Arakawa Clean Center in Fukushima City, the construction of which was completed in August 2008.
Figure 1. Pattern diagram of the new grate-type incineration system
Europe has a long history of commitment to waste incineration power generation, and Japan has lagged about 20 years behind in terms of commitment to high-temperature, high-pressure boilers in particular.1) In 1965, Japan's first waste incineration power generation plant, the Nishiyodo Incineration Plant, began operation in Osaka City. With the aim of realizing high-efficiency power generation, research bodies of local governments, plant manufacturers, universities, and other bodies in Japan utilized NEDO’s Development of High-Efficiency Waste Power Generation Technology Project to work on studies and development of materials resistant to high-temperature corrosion by pursuing the improvement of technology for boilers compatible with higher temperatures and pressures2). A boiler compatible with a steam temperature of 380ºC began operation in Saitama Prefecture in 1995, and a power generation plant supporting a steam condition of 3.82 MPaG × 400ºC (Kuririn Center), which is the basis for current high-temperature, high-pressure boilers, was put into operation in 1996.
Ebara undertook the design and construction of this plant, and has been handling its operation and maintenance for more than 23 years to date.
Table 1 shows an overview of the equipment of the Kuririn Center.
It used three superheaters (SHs): namely primary, secondary, and tertiary, and made of STB410 steel, SUS309 series steel, and SUS310 series steel, respectively. Figure 2 shows the configuration of the SHs. Installed close to the soot blower to blow off depositing dust is a protector to reduce wall thinning attributable to wear. When the SHs were introduced, they were exposed to a severely corrosive environment where the peak concentration of hydrogen chloride exceeded 1,000 ppm (value converted into 12% O2), the amount of wall thinning was as low as about 0.43 mm in two years.3) Subsequently, the tertiary SH underwent its first renovation between 2005 and 2007 and its second between 2015 and 2017, serving for a period of about ten years longer than the originally scheduled period of replacement.
At the same time, some issues were found that had to be improved to achieve life cycle cost reduction, safer and more secure plant operation, and technology compatible with higher temperatures and pressures.
Figure 2. Pattern diagram of superheaters
One of the major engineering issues facing high-temperature, high-pressure boilers for waste incineration plants is high-temperature corrosion countermeasures. It is particularly important to grasp the corrosion behavior of superheater pipes, and in order to resolve this issue, a long-term investigation and appropriate design and use based on the results thereof are indispensable. The sections that follow introduce some efforts toward resolution.
Figure 3 shows a flow chart of a general waste treatment plant, and Figure 4 an example of ash depositions inside the boiler. Waste contains large amounts of ash and chlorine, the latter of which forms chlorides with alkali metals, heavy metals, etc. Although such chlorides differ in the concentration temperature range depending on the kind, they are presumed to produce high-temperature exhaust gas, they tend to concentrate in areas where the pipe wall temperature is low, and they cause a chlorination reaction mainly by a Cl2 gas, which accelerates corrosion. The melting point of ash is considered to be another factor of corrosion. Table 2 shows an example of the properties of ash depositing on a corroded area and on an intact one, and Figure 5 shows the relationship between the heavy metal contents in depositing ash and the melting point. The table and the figure demonstrate the possibility that the larger the heavy metal content in ash depositing in a boiler or superheater, the lower the melting point of the ash, resulting in an adverse effect on corrosion. It is also pointed out that exhaust gas with a higher temperature accelerates the concentration of chlorides.4)
Figure 3. Flow chart of waste treatment plant
Figure 4. States of ash depositing in boilers
Figure 5. Relationship between heavy metal content in depositing ash and melting point
The effect that the concentration of heavy metals in depositing ash has on corrosiveness is widely known, but recent studies have revealed that the properties of ash depositing in a boiler or superheater differ considerably depending not only on the properties of waste but also on the combustion method or area of the incinerator and conditions. In particular, the demineralization process inside the incinerator or boiler differs depending on the combustion method, and the difference in the rate of transition of chlorine to an exhaust gas or depositing ash is also regarded as a reason for the difference in the properties of the ash. Figure 6 shows the transition characteristics of chlorine and heavy metals in different incinerator types.
Figures 7 to 9 show the properties of ash depositing in boilers for different types of incinerators related to chlorine and heavy metals (Cu, Zn, and Pb), which are particularly known as corrosion factors.
Figure 6. Transition characteristics of chlorine and heavy metals in different types of incinerators
Figure 7. Sampling points of ash depositing in boiler
Figure 8. Concentration tendency of chlorine in ash depositing in boilers for different types of incinerators
Figure 9. Concentration tendency of heavy metals in ash depositing n boilers for different types of incinerators
The results of the study reveal that in the grate-type and fluidized-bed incinerators, relatively large amounts of chlorine and heavy metals concentrated in ash in Path 1, where the temperature of exhaust gas was high. In the fluidized-bed incinerator, which was the lowest in the rate of transition of chlorine to exhaust gas, chlorine maintaining a high concentration exists in depositing ash in the tertiary and secondary superheaters. However, the grate-type incinerator, which showed the highest rate of transition of chlorine to exhaust gas, exhibited a tendency toward a lower concentration of chlorine in depositing ash in Path 2 and the subsequent path compared with the other types of incinerators. In the fluidized-bed gasification melting incinerator, other characteristics were also identified, such as a high concentration of chlorine in the tertiary superheater.5)
These results imply the possibility that the corrosion behavior of the boiler will change depending on the type of incinerator. In terms of exhaust gas, the concentration of chlorine is lower in a fluidized-bed incinerator. On the other hand, when it comes to depositing ash, which is presumed to have a stronger effect on corrosion, we infer that a grate-type incinerator will less cause corrosion. However, it is difficult to compare these incinerators under the same conditions—such as design conditions, waste properties, and operating status—and further quantitative comparison and study would be necessary to clarify the differences of corrosion between different types of incinerators.
We investigated the degree of the effect of the temperature of exhaust gas and the temperature of the pipe wall at a municipal waste incineration plant by using a corrosion test probe and changing the temperatures of exhaust gas and of metals.6) Figure 10 shows a pattern diagram of the corrosion test probe, and Figure 11 shows a diagram of the correlation between the temperature of exhaust gas and the Cl/S ratio.
Figure 10. Pattern diagram of corrosion test probe
Figure 11. Comparison of concentrations of elements in ash depositing on probe at different exhaust gas temperatures
The key results of the investigation are as follows:
• After a latent period of about six months, serious corrosion of 1 mm or more per year was observed with the probe that was installed in an atmosphere where the temperature of exhaust gas was 700ºC.
• The higher the temperature of the exhaust gas, the higher the Cl/S ratio in depositing ash, and the stronger the effect on corrosion.
• The Cl/S ratio was also affected by the temperature of metals in a manner similar to the temperature of exhaust gas.
The results obtained in the investigation reconfirmed that the factors that most affect the state of corrosion are the behavior of corrosive components represented by chlorine compounds and the temperature condition of exhaust gas or metals.
Needless to say, the most important thing to realize technology compatible with higher temperatures and pressures in the future is more accurate life prediction and optimum design for the intended life based on past performance data as mentioned above.
Ebara has worked to improve the accuracy of corrosion prediction based on more than 20 years of experience and achievements in high-temperature, high-pressure boilers. The new challenge we are taking on is technology compatible with higher temperatures and pressures under higher steam conditions.
We are also constructing a grate-type incinerator equipped with a 6 MPa × 450ºC-class super-high-temperature, high-pressure boiler (scheduled to be completed in December 2019) for the Kuwana Wide Area Cleanup Business Association (combustible waste incineration plant) as a new high-efficiency power generation facility for municipal waste by utilizing our experience and achievements in high-temperature, high-pressure boilers for municipal waste incineration plants in Japan and in 6 MPa × 450ºC-class boilers for private use both overseas and in Japan. This plant will be operated in Design-Build-Operate (DBO) form, and we are undertaking a package business, including 20-year plant operation services in addition to its design and construction. For this reason, we designed the plant with distinguished LCC combining initial and running costs, taking into account not only the initial 20-year period of operation services but also the extension of life to 35 years or more as the plant’s operation target.
Table 3 shows an overview of the plant.
As a measure to increase the temperature of steam to 450ºC in a stable manner, we adopted a four-superheater configuration, which was formed by adding a superheater to the conventional configuration of three, and designed a boiler capable of stably operating for an extended period with the aid of life prediction results based on the behavior of corrosive components discussed thus far and long-term data.
Moreover, as a means to remove ash depositing in the boiler, we adopted a shock pulse system, which removes depositing ash with pressure waves. Figure 12 shows a pattern diagram of a pressure wave-type boiler cleaning device. This technology is designed to remove ash depositing in pipes with the impact of pressure waves that are produced by igniting a methane gas-oxygen mixed gas. This technology has been used for many applications in Europe, and its adoption is spreading as a new boiler cleaning system also in Japan because it can not only reduce the wall thinning of pipes attributable to drain attacks, which are incidental to steam-type soot blowers, but also ensure uniform ash removal in wide areas. This system is also characterized by an increase in generated electric power owing to the availability of steam, which was used to remove dust in the past, for power generation and by a significant improvement in the stability of power generation and power transmission without variations in the amount of steam supplied to the turbine during operation.
Figure 12. Pattern diagram of pressure wave-type boiler cleaning device
Besides these high-temperature, high-pressure boiler technologies, we proactively adopted approaches to increasing power output, such as the adoption of a low-temperature economizer (exhaust gas temperature: 170ºC) and the higher evacuation of turbine exhaust (8 kPa), for plant design to achieve one of Japan's highest power generation efficiencies (22.9%). When the plant starts operation, it will be the first 6 MPa × 450ºC-class grate-type incinerator for municipal waste in Japan.
Since the operation of the first 4 MPa × 400ºC-class grate-type incinerator in Japan, Ebara has long studied corrosion mechanisms and behavior. As we also have various types of incinerators, including grate-type incinerators, fluidized-bed incinerators, and fluidized-bed gasification melting incinerators, we can grasp corrosion behaviors from various viewpoints and we have more knowledge and findings helpful to the elucidation of corrosion mechanisms and to improving prediction accuracy.
Ebara is determined to improve technologies for using and reusing energy through stable operation of 6 MPa × 450ºC-class next-generation high-frequency power generation plants by utilizing such knowledge and findings and to contribute as a leading company that will continue to resolve issues facing waste treatment administration by making good use of the results of our efforts in the field of power generation by waste treatment.
1) Ryuichi Ishikawa, EBARA Engineering Review, No. 237, 2012, Current Market Situation for and Future Technology Trends in Solid Waste Treatment.
2) Masayuki Yoshiba et al., 23rd Annual Conference of the Japan Society of Material Cycles and Waste Management, 2012, Environmental Analysis of High-Efficiency Waste Power Generation Plant Boilers and Analysis of High-Temperature Corrosion Damage on Superheater Pipes.
3) Manabu Noguchi et al., 10th Annual Conference of the Japan Society of Waste Management Experts, 1999, Study of Corrosion of Boiler Superheaters for High-Efficiency Waste Power Generation.
4) Manabu Noguchi et al., EBARA Engineering Review, No. 253, 2017, Lecture on Fundamental Aspects of High-Temperature Corrosion and Corrosion Protection – Part 3.
5) Naoki Kamiyama et al., 28th Annual Conference of the Japan Society of Material Cycles and Waste Management, 2017, Study of Behavior of Corrosive Components in Waste Power Generation Boilers.
6) Naoki Kamiyama et al., 29th Annual Conference of the Japan Society of Material Cycles and Waste Management, 2018, Study of Corrosion Rates of Superheater Pipe Materials and Behavior of Affecting Factors.
This report was reprinted with some additions and corrections to the article published in Environmental Facilities No. 154 (December 2018).
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