Takahiro EGUCHI*
Takashi NAKAJIMA*
Moriaki ISHIWATA*
*
Ebara Environmental Plant Co., Ltd.
Ebara delivered a waste incineration plant to Kitaakita City, Akita Prefecture, at the end of March, 2018. The plant was constructed as rebuilding of the existing Kitaakita City Clean Recycle Center. It has a combustion capacity of 50 t/16 h for municipal solid waste and sludge, and it uses the SDP type fluidized-bed incinerator. The plant has been operated with continuing stable high-temperature and a low air ratio to successfully reduce CO and NOx emissions, and it achieved to reduce environmental impact. In addition, the plant actively utilizes thermal energy generated from waste incineration to supply hot water and heat inside the building. The plant is expected to incinerate waste sanitarily, stably and economically for a long period over the next 20 years.
Keywords: Fluidized-bed, Waste incinerator, Sludge incineration, Carbon monoxide, Nitrogen oxides
Kitaakita City constructed a new fluidized-bed waste incineration plant with a capacity of 50 t/d (25 t/16 h × 2 units) due to the aging of the existing waste incineration plant (50 t/d) that began operation in 1992. The new waste incineration plant was constructed on the premises of the existing one and began operation as a collective treatment facility for municipal solid waste and sludge.
Since it is a semi-continuous-type waste incineration plant operating 16 hours a day, an SDP-type fluidizedbed waste incinerator was selected and adopted, as it is easy to activate and deactivate and can be reactivated with a hot start after deactivation during the night with the aid of the storage heat of the fluidized sand inside the incinerator. Cutting-edge knowledge and expertise on fluidized-bed waste incineration plants was integrated into the design of the waste incineration plant.
Our technology for fluidized-bed incinerators has its roots in the sterile disposal plant (SDP)-type fluidizedbed incinerator technology that we introduced from overseas. Since 1978, when we delivered our first SDP-type incinerator intended for municipal solid waste to Suzu City, we have delivered SDP-type incinerators to 13 waste incineration plants. To date, we have delivered a total of 44 SDP-type incinerators to waste incineration plants, including industrial waste treatment facilities for sludge and other substances.
In general, with small incinerators, the combustion performance is greatly affected by variations in the volume of waste supplied into the incinerator. SDP-type incinerators, on the other hand, can ensure a stable supply of waste because the crushed waste is compressed, supplied into the fluidized sand layer, and gradually scraped by the fluidized sand. This technology enables even small incinerators to maintain stable combustion. The most remarkable characteristic is that the fluidized-bed waste incinerator can also achieve mixed combustion of waste and sludge, which is waste with a low heat value, at a high ratio. This is the reason why Kitaakita City adopted the SDP-type incinerators.
In March 2018, the waste incineration plant was completed and began operation as the energy recovery promotion facility of the Kitaakita City Clean Recycle Center. Kitaakita City has a regional plan for establishing a sound material-cycle society, and the aim of the waste incineration plant is to be a facility rooted in the community to help realize a sound material-cycle society, as promoted by Kitaakita City, and to contribute to local society.
Figure 1 shows an appearance of the waste incineration plant, and Figure 2 its overall layout.
Fig. 1 Appearance of Kitaakita City Clean Recycle Center
Fig. 2 Overall layout of Kitaakita City Clean Recycle Center
Utilizing the fluidized-bed incinerator’s characteristic capability to incinerate diverse kinds of waste, the waste incineration plant intakes sludge discharged from human-waste and sewage treatment plants and stably mixes and combusts it with municipal solid waste.
The thermal energy generated in the process of incineration treatment is recovered by the waste incineration plant, which uses the waste heat for hot water and air heating within the plant. The waste incineration plant is also friendly to the surrounding environment and to the community. For example, it recycles all the wastewater produced inside the plant and adopts a wastewater closed system that can prevent wastewater from being discharged externally.
The flow of the units of the incineration plant is shown in Figure 3.
The collected municipal solid waste is temporarily stored in the waste pit, then loaded into the waste loading hopper by the waste crane, crushed by the two-axis crusher, and finally fed into the incinerator by the waste feeder.
The sludge is carried out of the human-waste treatment plant and the three sewage treatment plants located in the city, input into the sludge receiving hopper, and then fed into the incinerator by the pump under pressure. Figure 4 shows a cross-section of the fluidized-bed incinerator.
Inside the incinerator, the waste and sludge are incinerated at high temperatures of 850 °C or more, and incombustibles are discharged from the lower portion of the incinerator together with the fluidized sand and separated from the fluidized sand by the vibrating screen. Then, iron is separated by the magnetic separator, and is stored in and carried out of the iron storage banker, while other incombustibles are stored in and carried out of the non-combustible storage banker.
The exhaust gas discharged from the incinerator is cooled to 350 °C by water injection in the gas cooling chamber and then to 180 °C in the air preheater and the air heater for exhaust gas cooling. Calcium hydroxide and activated carbon are injected into the cooled exhaust gas, and the dust, acid gases, and dioxins are adsorbed and removed by the bag filter. The incinerator’s combustion control prevents the generation of nitrogen oxides, and no chemicals are used for that purpose.
Table 1 shows the specifications of the major units of the Kitaakita City Clean Recycle Center.
Fig. 3 Flow of units
Fig. 4 Cross-section of disposal plant (SDP)-type fluidized-bed incinerator
| Receiving and supply equipment | |
| Waste pit | Capacity: 2500 m3 |
| Waste pit crane | Semi-automatic crane × 1 |
| Combustion equipment | |
| Incinerator | Semi-continuous-type fluidized-bed incinerator (SDP type) Treatment capacity: 25 t/16 h × 2 incinerators |
| Combustion gas cooling equipment | |
| Gas cooling chamber | Water injection cooling method |
| Exhaust gas treatment equipment | |
| Exhaust gas cooling method | Shell and tube-type air preheater |
| Dust collection method | Bag filter |
| Denitration method | Combustion control method |
| HCI/SOx removal method | Dry method (calcium hydroxide injection) |
| Measure against dioxins and mercury | Activated carbon blow-in method |
| Equipment utilizing waste heat | |
| Hot water generator | Used in waste incineration plant (hot water, air heating) |
| Ash unloading equipment | |
| Incombustibles, iron | Carrying out by banker (Iron is separated by magnetic separation.) |
| Incinerated fly ash | Chemical treatment by chelate Carrying out by banker |
| Wastewater treatment equipment | |
| Waste pit drain | Injection treatment in incinerator (without discharge) |
| Plant wastewater | Circulation and reuse in waste incineration plant (without discharge) Injection treatment in incinerator and gas cooling chamber (without discharge) |
| Domestic wastewater | Discharged into the river after treatment by a combined -type private sewage treatment system Circulation and reuse in waste incineration plant (without discharge) |
The exhaust gas values measured in the performance test are shown in Table 2. All exhaust gas values are below the guaranteed values, proving that the units have satisfactory performance.
The waste incineration plant ensures a stable supply of waste into the incinerator by supplying crushed waste into fluidized sand layer. It also realizes slow combustion by reducing the temperature of the fluidized bed, and realizes low-air-ratio, high-temperature combustion through high-speed control of the combustion air volume using the laser-type O2 meter installed at the outlet of the incinerator. If the air ratio is decreased to incinerate municipal solid waste with uneven properties, the peak of carbon monoxide (CO) generation is likely to occur. In contrast, the production of nitrogen oxides (NOx) can be controlled by reducing the air ratio. Maintaining low concentrations of both CO and NOx requires an optimum air ratio, mixing and agitation in the secondary combustion chamber, and stable internal temperature in the incinerator.
As shown in Figure 5, the waste incineration plant operates at an oxygen concentration of approximately 4.3 % wet (air ratio: 1.46) at the outlet of the incinerator, and the CO is kept at an average concentration level as low as 7 ppm, although there is a slight peak. The operation of the waste incineration plant fulfills the guaranteed value of NOx through combustion control alone, without using ammonia water or other chemicals.
In commissioning, it was confirmed that the waste incineration plant was capable of incinerating sludge by increasing the ratio of the volume of sludge to the combustion volume from the rated value of 15 % to up to 40 %. At present, the incineration waste plant operates at a sludge mixed combustion ratio of 30 %.
The fluidized-bed incinerator is suitable for mixing and treatment of substances with different heat values or properties. It is expected that the waste incineration plant will be able to respond flexibly to changes in the quality or properties of treated substances for several decades to come.
| Regulated substance | Guaranteed value | Performance test results | ||
| No. 1 | No. 2 | |||
| Dust | g/m3N※1 | 0.01 | <0.002 | <0.002 |
| Hydrochloric acid | ppm※1 | 200 | 5.1 | 1.2 |
| Sulfur oxide | ppm※1 | 100 | 1 | <1 |
| Nitrogen oxide | ppm※1 | 150 | 41 | 63 |
| Carbon monoxide | ppm※1※2 | 30 | 17 | 10 |
| Dioxins | ngTEQ/m3N※1 | 1 | 0.0013 | 0.00085 |
| Mercury | mg/m3N※1 | 0.05 | <0.00045 | <0.00077 |
※1: Value converted into dry-based O212%
Fig. 5 Changes in NOx and CO over time (during performance test)
Akita Prefecture is promoting the use of wood produced in the prefecture to foster the development of the local forestry and lumber industry and realize a sound material-cycle society. The facility design incorporates vertical lattices made from Akita Prefecture cedar square timbers on the wall surface of the entrance as well as at the front gate of the waste incineration plant, where they serve as a barrier between visitors and waste hauling vehicles (Figure 6).
Cedar boards are also used for the ceiling and wall surface of the training room (Figure 7) as an interior decoration element that brings both comfort and warmth. To contribute to the vitalization of the local community, the training room is available for local citizens to use as a meeting room.
Fig. 6 Entrance
Fig. 7 Training room
To make it easy for visitors, such as elementary school students of Kitaakita City and local citizens, to understand the units of the waste incineration plant and their operating states, there are explanation items prepared in the waste incineration plant. Not only can visitors see the inside of the waste incineration plant through the windows of the visitors’ tour route, but they can also see actual substances that are judged not suitable for incineration, as well as the fly ash produced after incineration, incombustibles, and the cloth used for bag filters, all of which are exhibited in the waste incineration plant (Figure 8).
They can also see the actual operating states on video cameras, which display the insides of the incinerators and various areas of the waste incineration plant, and real-time operation data monitors installed along the visitors’ tour route (Figure 9).
Fig. 8 Example of explanation items (exhibited items)
Fig. 9 Example of explanation item (operation data displayed)
Since the construction of the waste incineration plant was completed in March 2018, it has operated in good condition and served as the core of the waste disposal services of Kitaakita City. We concluded a contract with the city for long-term, all-inclusive operation and management of the waste incineration plant for as long as 20 years after its construction. We will continue to demonstrate consideration for the surrounding environment and a safety-minded attitude in the process of construction to operate and manage the waste incineration plant. We are determined to contribute to the local community and achieve operation and management in close coordination with the local community through sanitary, stable, and economical incineration treatment, as well as also visitor services, environmental studies, and other activities.
Finally, we would like to extend our sincere thanks to Kitaakita City and all personnel involved for their helpful advice, guidance, and cooperation throughout the construction of the waste incineration plant.
1) Hiroyuki SUZUKI, et al., “Delivery of High-Efficiency Wasteto-Energy Plant and Waste Heat Utilization Facility for Funabashi City - Contribution to Establish the Recycling-Oriented Society in the Local Community -”, Ebara Engineering Review No.254, p.47-51 (October 2017).
2) Kei MATSUOKA, et al., “Performance of the Fluidized-bed Incineration Facilities and their potential”, Ebara Engineering Review No.253, p.12-17 (April 2017).
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