Koh SASAKI
EBARA Environmental Plant Co., Ltd.
We have developed image processing technology to control the amount of waste fed to a fluidized-bed incinerator. The information obtained from image processing of the waste fed into the incinerator is used to control the amount of waste fed into the system and thus improves the combustion stability of the incinerator. We have installed this technology in a fluidized-bed incineration plant and operated for one year. As a result, we confirmed that the CO concentration was reduced. This technology is expected to improve combustion stability of new projects and existing facilities.
Keywords: Feeding Control, Image Processing, Fluidized-bed Incinerator, Combustion Stability
A fluidized-bed incineration plant is mainly composed of a receiving and feeding facility, an incineration facility, a combustion gas cooling facility, a gas treatment facility, a heat utilization facility, and a fly ash treatment facility. Figure 1 shows the flow of operation for these facilities.
Figure 1 Process flow in fluidized-bed incineration plant
The fluidized-bed incinerator, which functions as the incineration facility, is characterized by having a small installation space due to its compact size, outstanding controllability of the amounts of generated power and transmitted power, and other advantages. However, since its combustion reaction is fast, fluctuations in the amount of waste fed from the receiving and feeding facility (amount of waste fed to the incinerator) and in the quality of waste will have large impacts on combustion. It is therefore necessary to feed a constant amount of waste into the fluidized-bed incinerator in order to ensure stable combustion.
However, the various kinds of waste collected at waste combustion facilities, and the resulting differences in their properties, will cause the amount of waste fed to the incinerator to become unstable, so the amount of waste must be controlled to reduce fluctuations in combustion.
In the past, the fed amount of waste was controlled based on plant operation information after the combustion of waste loaded from the feeder into the incinerator.
This paper describes a technology for improving combustion stability by performing image processing of waste while it is falling from the waste feeder into the incinerator, and controlling the fed amount of waste based on the amount of waste converted into numerical form (amount of falling waste).
In a fluidized-bed incinerator plant, waste is fed according to the process explained below.
(1) Waste temporarily stored in a waste pit is loaded into a receiving waste hopper by a crane.
(2) The waste that has been loaded into the receiving waste hopper is transported by a waste feeder (screw conveyor).
(3) A scratcher (screw blade type) finely scratches off the waste transported by the waste feeder, and the waste is then fed into an incinerator.
An image processing system arithmetically processes images of the waste falling from the scratcher into the incinerator, which are captured by a camera installed at the scratcher outlet, and outputs the amount of waste converted into numerical form. Figure 2 shows the configuration of the image processing system.
Figure 2 Configuration of image processing system
The waste feeder controls the fed amount of waste in accordance with the rotating speed of the screw. Since the waste collected at the waste incineration plant is not uniform, the filling rate of waste in the waste feeder may become accordingly higher or lower. As a result, the proportional relationship between the fed amount of waste and the rotating speed of the waste feeder may not be satisfied in some cases. Therefore, control is necessary.
In conventional operation, the amount of waste fed into the incinerator was made constant by increasing or decreasing the rotating speed of the waste feeder. This was performed by feedback control based on information on plant operation which fluctuates according to the status of combustion, including the brightness of the flame inside the incinerator, the concentration of oxygen in exhaust gas, and the drum pressure of the boiler.
With control that utilizes an image processing system, the constancy in volume of the amount of waste fed into the incinerator is improved through feedforward control of the rotating speeds of the waste feeder and the scratcher as described below based on the amount of falling waste before combustion, in addition to conventional control.
(1) Control to increase speed of the waste feeder
When the filling rate of waste inside the waste feeder becomes low and the amount of falling waste decreases, the rotating speed of the waste feeder is increased. This control operation can reduce the amount of time in which the amount of waste is temporarily insufficient.
(2) Control to decrease speed of the waste feeder and scratcher
When the filling rate of waste inside the waste feeder becomes high and the amount of falling waste increases, the rotating speeds of the waste feeder and scratcher are decreased. This control operation makes it possible to temporarily reduce the fed amount of waste only when the amount of falling waste increases, thereby preventing continuous excessive loading of waste.
We introduced waste feeding control by image processing to a fluidized-bed incineration facility which we had previously delivered to, and evaluated the system in accordance with the method described below.
1）Method for evaluating the amount of falling waste
The correlation between the amount of falling waste and the status of combustion was used to calculate a time delay, and an evaluation was conducted on the validity of using the amount of falling waste as an indicator representing the amount of waste actually loaded into the incinerator.
The following were used as the indicators representing the status of combustion:
(1) Brightness of the flame inside the incinerator
(2) Oxygen consumption (= value calculated from the amount of oxygen consumed for the combustion of waste and the oxygen concentration at the boiler outlet)
2）Method for evaluating the system for control of the fed amount of waste by image processing
The facility that introduced the waste feeding control system contained two incinerator systems, one of which was operated with conventional waste feeding control (hereinafter referred to as “conventional control”) and the other with waste feeding control by image processing (hereinafter referred to as “image processing control”). The system for waste feeding control by image processing was evaluated by comparing the CO concentration in exhaust gas at the stack outlet of each incinerator system.
1）Evaluation of the amount of falling waste
Table 1 shows the correlation coefficient and time delays which correspond to the amount of falling waste calculated from operation data (12 hours) in March 2019 and the indicators representing the status of combustion. Figure 3 shows this data using a time series. The correlation coefficient between the amount of falling waste and each of the indicators (1) and (2) is 0.2 to 0.4, indicating that the correlation is weak. However, it was confirmed that after the amount of waste was increased, the brightness of the flame increased following a delay of 14 seconds, and the oxygen concentration increased following a delay of 22 seconds. From these results, it was concluded that the process by which flame is produced and oxygen consumption increases after waste is fed is expressed correctly, so the amount of falling waste is a valid indicator to represent the amount of waste actually loaded into the incinerator.
| Indicator | Correlation coefficient | Time delay |
| (1)Brightness of flame inside incinerator | 0.33 | 14 seconds |
| (2)Oxygen consumption | 0.25 | 22 seconds |
Figure 3 Time series data for the amount of falling waste and the indicators representing the combustion state
2）Evaluation of the introduction of waste feeding control by image processing
Figure 4 shows the frequency at which the CO concentration (hourly average value) of the exhaust gas at the stack outlet reached or exceeded 50 ppm during the one-year operation period from June 2019 to May 2020. For August 2019 and January and March 2020, data for only one system is shown due to a non-operating incinerator. The figure indicates the following:
(1) Compared to conventional control, image processing control reduces the probability of the CO concentration reaching or exceeding 50 ppm (hourly average value) by more than 50% on annual average, and ensures improved combustion stability.
(2) In conventional control, the CO concentration worsens in summer due to seasonal changes in the quality of waste, whereas with image processing control, operation remains stable throughout the year.
Figure 4 Probability of the occurrence of high CO concentration (hourly average)
These facts corroborate that the introduction of waste feeding control by image processing makes it possible to achieve stable waste feeding that can adapt to changes in the quality of waste, and improves combustion stability.
This paper has provided an introduction of control technology for waste feeding to waste incinerators that uses image processing.
Quantification of the amount of fed waste is considered to be effective for waste feeding control as well as other types of control related to combustion control (secondary air control etc.), so this technology is applicable to a wide range of fields.
In addition, it can be said that the risks associated with introducing this technology are small, since it can be added to existing incineration facilities without making large-scale modifications. We are determined to improve secure and safe waste incineration and treatment technologies through the utilization of this technology, not only in new incineration plants but also existing facilities.
1） Koh Sasaki, Development of Waste Feeding Control Technology Utilizing Image Processing for Waste Incinerators, Collection of Papers for the 42nd National Journal of Japan Waste Management Association Research and Case Study Presentations and Lectures (2020).