Eichi TANAKA*
Sho KUDO*
Takayoshi KAWAGISHI*
*Ebara Environmental Plant Co., Ltd.
Since the introduction of the feed-in tariff (FIT system) for renewable energy, the certified capacity of biomass electric power generation has been on the rise, so to handle this, we must ensure the stable and sustainable procurement of woody biomass fuels. In order to maintain the market competitiveness of biomass electric power generation facilities despite the rising unit price of fuel, it is necessary to improve electric power generation efficiency and to reduce the LCC of biomass electric power generation facilities. Ebara Environmental Plant Co., Ltd. is striving to develop and introduce new technologies to improve and reduce the LCC of the electric power generation efficiency of internally circulating fluidized-bed boiler, which is core equipment for biomass electric power generation facilities. The introduction of new technologies under development in recent years will make various types of fuels available in addition to woody biomass fuels and achieve superiority in fuel procurement.
Keywords: Wood biomass electric power generation, ICFB, State of the art technology
Since Japan's Feed-in Tariff System for Renewable Energy (FIT System) came into effect in July 2012, the certified capacity of biomass electric power generation has been significantly increasing. The current FIT System broadly classifies biomass fuels into:
• methane fermentation gas;
• unused wood biomass;
• general wood, etc.;
• biomass liquid fuel;
• construction material waste; and
• general waste and other biomasses.
The certified capacity of general wood, etc. is the largest among the fuels for biomass electric power generation, and it jumped in 2017 and 2018 from about 2.95 million kW at the end of fiscal 2015 (March 2016) after the enforcement of the FIT System, while the other fuel categories showed only a slight increase. As of April 2019, it had almost tripled, reaching about 8.73 million kW.1), 2) This rapid increase in certified capacity probably resulted from last-minute certifications before the change of the purchasing unit price of electric power from large-scale biomass electric power generation facilities in the fixed price system to a bidding system following a modification to the FIT System.3), 4) There are no expectations for another such rapid increase in certified capacity.
However, as long as the Japanese government sets medium- and long-term independence from the FIT System as a future goal of renewable energy, electric power generation operators will always face the problems of fuel procurement and electric power generation cost. The amount of fuel required to cover the certified capacity, which was far beyond woody fuel output in Japan, was already perceived as a problem before the rapid increase in certified capacity, and cases using imported fuels are increasing rapidly nowadays. Year by year, it is becoming more difficult for the electric power generation business to stably procure fuels at a sustainable cost.5)
As mentioned earlier, there are various types of biomass fuels, for which thermochemical conversion and biochemical conversion are available to use them as energy. Boilers and steam turbines, gas engines, gas turbines, and fuel batteries are used as electric power generation systems. Biomass electric power generation technology is a combination of fuel, usage of energy, and an electric power generation system, and it comes in a wide variety of combinations.
The internal circulation fluidized-bed boiler supplied by Ebara Environmental Plant Co., Ltd. (hereinafter referred to as “Ebara”) is a fluidized-bed boiler that can be applied as the core technology of combustion electric power generation (thermochemical conversion + boiler and steam turbine) plants generally adopted as biomass electric power generation systems using woody fuel. This report introduces the internally circulating fluidized-bed boiler and new technologies intended to improve its electric power generation efficiency and reduce LCC.
The inside of the furnace of the internally circulating fluidized-bed boiler is separated into a
combustion chamber and heat recovery chambers on both sides. Some of the bed material (sand) bursts out due to the active swirling flow motion inside the combustion chamber, and it enters the heat recovery chambers, where the sand exchanges heat with the imbedded heat transfer tubes while moving downward, before finally returning to the combustion chamber from the lower portion of the deflector (Figure 1).
Figure 1. Outline of internally circulating fluidized-bed boiler structure
Inside the combustion chamber, the sand subjected to an active swirling flow protects itself from local high or low temperatures, makes various types of fuels treatable, and can easily discharge large incombustible substances. This makes it possible to procure a wide variety of fuels.
The biomass electric power generation market faces the issues of stable procurement of fuels and the rise of combustion prices. Increasing the amount of electric power generated, or electric power generation efficiency, is an effective approach to maintaining the market competitiveness of biomass electric power generation facilities. The temperature of steam is directly related to electric power generation efficiency, which improves as the temperature of steam rises. The combustion electric power generation plants, as described in Chapter 1, tends to raise the temperature of the main steam to improve electric power generation efficiency.
For the internally circulating fluidized-bed boiler Ebara supplies, two main approaches are used to raise the temperature of main steam, in accordance with the specifications of the facility. One is the installation of an imbedded superheater, and the other is the installation of a superheater in combustion exhaust gas.
①Imbedded superheater
Imbedded superheaters are to be installed in fluidized beds and are designed to raise the temperature of steam by using the heat of the fluidized bed. Although the immersed superheater will be exposed to severe erosion-corrosion environments, the required temperature of steam can be secured easily, regardless of variations in the boiler load because the inside temperature of the bed is kept constant. When the load varies considerably, for example, during continued electric power generation under a low load in the nighttime, it is appropriate to raise the temperature of steam with the aid of an immersed superheater. An immersed superheater is also suitable when the amount or temperature of combustion exhaust gas varies drastically as a result of changes in the used fuel or other factors.
②Superheater in combustion exhaust gas
A superheater in combustion exhaust gas uses the heat of combustion exhaust gas to raise the temperature of steam. The temperature of combustion exhaust gas varies depending on the operation load, and it greatly affects the amount of heat absorption of the superheater, but raising the temperature by a superheater in combustion exhaust gas, which does not cause erosion, is appropriate for cases such as a private operator that constantly operates its electric power generation facility under a high load and with little variation.
Ebara not only improves electric power generation efficiency by using an approach fit for the specifications of the facility to raise the temperature of steam resulting from the raising of the temperature of the main steam, but also supplies the optimal LCC electric power generation facilities for each biomass electric power generation operator.
The internally circulating fluidized-bed boiler uses sand whose particle size is adjusted for the bed material, because its particle diameter and other properties affect the required air volume for fluidization. However, biomass and other fuels contain gravel or incombustible substances that are larger in size than sand. If quantities of these substances remain in the bed, the air volume needs to be increased to maintain fluidization, but a large volume of fluidizing air causes bed material with a small particle diameter to scatter, resulting in an increase in the average particle diameter of the bed material and less smooth fluidization. Moreover, an increase in the volume of fluidizing air causes increases in the flow velocity and the amount of resultant wear of the in-bed equipment, including the imbedded heat transfer tubes. For this reason, maintenance of the average particle diameter of the bed material contributes to extending the life of the in-bed equipment and reducing maintenance costs. In addition, if the particle diameter of sand can be regulated continuously during operation, it is possible to reduce the period of maintenance associated with sand replacement and extend the life of the in-bed equipment, which should lead to considerable cost reductions.
The particle size regulator newly developed by Ebara classifies, by wind power selection, substances (gravel, etc.) that are difficult to classify with existing vibrating sieves which has a slightly larger particle diameter than sand with optimum particle diameter for internally circulating fluidized-bed boiler operation.
In general, the sand extracted from the fluidized bed of the internally circulating fluidized-bed boiler is carried to the vibrating sieve to remove incombustible substances that it may contain, and then fed into the bed and reused. Some of the sand separated and loaded into the particle size regulator can be classified into grains that are equal to or smaller than the specified particle dimeter, and grains that are larger (Figures 2 and 3, respectively). As a result, sand with a small particle diameter is reloaded into the furnace, and sand with a large particle diameter is discharged out of the system to ensure that the particle diameter of the sand inside the furnace is maintained in an optimal state for the operation of the internally circulating fluidized-bed boiler, and that sand with a larger diameter, such as gravel, is removed.
Figure 2. Before classification
Figure 3 After classification (large particle diameter)
Imbedded heat transfer tubes are a key technology for the internally circulating fluidized-bed boiler, and they play the role of maintaining the furnace bed at an appropriate temperature while covering 20 to 30% of the total amount of heat recovered of the internally circulating fluidized-bed boiler. Since the amount of heat recovered by the imbedded heat transfer tubes installed in the fluidized bed is appropriately designed and controlled, the internally circulating fluidized-bed boiler is compatible with various types of fuels, including coal, biomass, and municipal waste, but the immersed heat transfer tubes that exchange heat in the fluidized bed are exposed to severe temperatures and erosion-corrosion environments.
The internally circulating fluidized-bed boiler introduces thermal spraying technology as a measure to protect the surfaces of the imbedded heat transfer tubes, but it is also necessary to repair the thinned sprayed coating regularly. Extending the life of the sprayed coating is directly related to extending the life of the imbedded heat transfer tubes, which, in turn, contributes greatly to the stable operation of the internally circulating fluidized-bed boiler and maintenance cost reduction.
Thus, Ebara is working with Hokkaido University, the Hokkaido Research Organization, and Dai-Ichi High Frequency Co., Ltd. to develop a thermal spraying material superior in durability to the currently used material.
In the fluidized bed, the bed material, or sand, is fluidized by air blown in from the bottom, and the imbedded heat transfer tubes are exposed to a erosive environment caused by the bed material at all times. At the same time, it is exposed to the corrosive environment caused by the fuel-derived chlorine content in the bed material. Thickness reduction of tubes in environments with both erosion and corrosion shows different behavior from when the environment involves only one or the other. It is necessary to understand the erosion-corrosion mechanisms of the imbedded heat transfer tubes to efficiently extend their life, but it is difficult to perform an internal observation of the imbedded heat transfer tubes of an actual boiler in operation. Thus, we developed a fluidized bed experiment device (Figures 4 and 5) that simulates an actual boiler environment to grasp the corrosive and erosive environments in the bed more accurately on a laboratory level.
Figure 4. Fluidized bed experiment device
Figure 5. Reactor
This experiment device makes it possible to perform erosion-corrosion tests under controlled conditions of bed material temperature, metal surface temperature, and salt concentration, to visually check the inside of the boiler, and to deactivate the device at certain times and check and analyze samples.
The following findings obtained by using the test device were unprecedentedly interesting.
It is widely known that, in a chloride corrosive environment in an oxidization atmosphere, corrosion resistance tends to be improved by adding Mo to an alloy6) and a similar tendency is obtained in the corrosion test (Figure 6). However, the test device confirmed that this tendency was reversed in a erosion-corrosion environment, even in a chloride corrosive environment, and that the addition of Mo to an alloy would deteriorate erosion-corrosion resistance (Figure 7).
Figure 6. Effect of Mo in corrosion tests
Figure 7. Effect of Mo in erosion-corrosion tests
Since erosion-corrosion shows different wall thinning behavior from that by corrosion alone, it is difficult to evaluate through a test with corrosion alone.
It is also anticipated that the tendency of wall thinning will change depending on the strength of the erosion environment comprising the erosion-corrosion environment. For this reason, we are developing a thermal spraying material optimized for the internally circulating fluidized-bed boiler by accurately reproducing the erosion-corrosion environment of an actual boiler.
By arranging the imbedded heat transfer tubes close to one another, it is possible to improve the heat recovery efficiency of the entire internally circulating fluidized-bed boiler and to reduce the construction cost by downsizing the device. It is also possible to reduce the power of the plant if the particle diameter and fluidized state of the bed material and the optimum arrangement of the imbedded heat transfer tubes are clarified and the height of the fluidized sand bed can be reduced. To achieve these goals, we are conducting research on improving the heat exchange efficiency of the imbedded heat transfer tubes using a cold model test device (Figure 8) in collaboration with the Kochi National College of Technology.
Figure 8. Cold model test device
Although fuels with various properties are available for the internally circulating fluidized-bed boiler, they contain incombustible substances with various properties in addition to combustible substances. Incombustible substances, such as gravel and wire, are present even in biomass fuels and must be constantly discharged to continue stable operation. Thus, what is very important to optimize the shape of the device for the purpose of further improving efficiency or functions is to elucidate the behavior of incombustible substances in the fluidized bed.
Recently, as computation capabilities improve, numerical simulations have been utilized in all fields of the industry as well as for fluidized bed technology. Current general-purpose numerical analysis software, however, cannot perform accurate computations under conditions where the fluidized bed contains much larger coarse substances than bed material particles. Thus, experimental verification using a cold model, etc. is unavoidable at present to confirm that incombustible substances are not accumulated or retained in the design process of a device.
Under these circumstances, Ebara is committed to joint research with Osaka University, Okayama University of Science, and Hokkaido University in the EOI (Ebara Open Innovation)7) framework with the aim of establishing a simulation approach capable of accurately simulating the behavior of coarse substances in the fluidized bed and elucidating phenomena dynamically.
In this joint research project, we are conducting multi-faceted research14) (Figure 11) intended to elucidate the dynamic mechanism governing floating-sinking phenomena of objects in a fluidized bed in addition to accuracy verification of computation models through the integration of research utilizing numerical simulations9) (Figure 9) based on the computation approach (FPM: Fictitious Particle Method)8) developed by a group from Osaka University, experimental research11) by a group from Okayama University of Science working on basic research on the development of dry specific gravity difference separation technology using solid-gas fluidized beds and floating-sinking phenomena of objects in a powder bed10), and research based on simultaneous non-contact measurement of the positions, postures, and acting forces of objects in a fluidized bed using the sensor system12) (Figure 10) developed by a group from Hokkaido University.
Figure 9. Example of numerical analysis of the behavior of a coarse sphere in the fluidized bed9)
Figure 10. Sensor12)
Figure 11. Experiment device14)
This report introduced the current situation of wood biomass electric power generation and the internally circulating fluidized-bed boiler, and gave an overview of some new technologies related to the internally circulating fluidized-bed boiler. In addition, the FIT System is changing from initial purchasing at fixed prices to the introduction of bidding,2) and plants with higher electric power generation efficiency are required. Ebara will continue to be committed to further developing internally circulating fluidized-bed boiler technologies in collaboration with specialized institutions of various fields and fulfilling these needs.
1) Biomass Industrial Society Network (NPO), Biomass White Paper 2018.
2) Current Situation of Renewable Energy in Japan and Overseas and Drafts of Issues in the Procurement Price Calculation Committee for the Present Fiscal Year (Agency for Natural Resources and Energy)
https://www.meti.go.jp/shingikai/santeii/pdf/038_01_00.pdf
3) Website for disclosure of information about the Feed-in Tariff System for Renewable Energy (Agency for Natural Resources and Energy)
https://www.fit-portal.go.jp/PublicInfoSummary
4) Feed-in Tariff System for Renewable Energy (Agency for Natural Resources and Energy)
http://www.enecho.meti.go.jp/category/saving_and_new/saiene/kaitori/fit_kakaku.html
5) Basic Survey for Promoting the Introduction of New Energy, etc. for FY2018 (Agency for Natural Resources and Energy)
http://www.meti.go.jp/meti_lib/report/H30FY/000087.pdf
6) Y. Kawahara, Y. Kaihara: Recent Trends in Corrosion-Resistant Tube Materials and Improvements of Corrosion Environments in WTE Plants, Corrosion 2001, Paper No. 01173 (2001).
7) Manabu Tsujimura: What is the Ebara Way of Open Innovation? – Why Has the Ebara Way of Open Innovation Succeeded? – EBARA Engineering Review 255, 4 (2018).
8) Tsuji, T. et al.: Fictitious Particle Method: A Numerical Model for Flows Including Dense Solids with Large Size Difference, AIChE J., 60, 1606 (2024).
9) Tsuji, T. et al.: Destabilization of Object Sedimentation in Solid-Gas Fluidized Beds at High Wind Velocity – Experiments, Radio Sensors, Numerical Analysis – (3), Society of Chemical Engineers, Japan 50th Autumn Meeting (2018).
10) Oshitani, J. et al., Anomalous Sinking of Spheres Due to Local Fluidization of Apparently Fixed Powder Beds, Physical Review Letter, 116, 068001 (2016).
11) Oshitani, J. et al.: Destabilization of Object Sedimentation in Solid-Gas Fluidized Beds at High Wind Velocity – Experiments, Radio Sensors, Numerical Analysis – (1), Society of Chemical Engineers, Japan 50th Autumn Meeting (2018).
12) Harada, S. et al., Direct Measurement of Fluid Force on a Particle in Liquid by Telemetry Systems, Int. J. Multiphase Flow, 37, 898 (2011).
13) Harada Shusaku et al.: Destabilization of Object Sedimentation in Solid-Gas Fluidized Beds at High Wind Velocity – Experiments, Radio Sensors, Numerical Analysis – (2), Society of Chemical Engineers, Japan 50th Autumn Meeting (2018).
14) Yoshimori, W. et al., Non-invasive Measurement of Floating-Sinking Motion of a Large Object in a Gas-Solid Fluidized Bed, Granular Matter, 21, 42 (2019).
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