4. Flame images-based biomass-coal combustion characterisation

The efficiency of pulverised fuel depends on several parameters. The commonly applied, lowemission techniques of pulverised coal combustion use recirculation vortexes that lengthen the paths of the coal grains passing through the flame to minimise generation of thermal oxides of nitrogen (NOx). To make pulverised coal combustion more efficient and environment friendly, it is necessary to measure its key parameters. The information taken from the output (exhaust gas collector) is delayed and averaged. In Ref. [42], several combustion diagnostic direct techniques are presented; the most of them are impossible to utilise under industrial conditions or are expensive. Fast and minimally invasive optical methods allow using image processingbased information in process control system [43].

The study was conducted for three exact thermal power values (250, 300 and 400 kW) at the

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The tests covered two fuel mixtures containing 10% and 20% of biomass (straw), respectively. The research assumed that biomass physical properties (like particle size, inherent moisture, etc.) as well as all the image acquisition parameters (such as frame rate, camera gain and exposure time) remained unchanged. Flame images acquisition covered different fuels mixtures in every variant of the combustion facility. In order to guarantee online algorithm controllability, the images pixel amplitude was limited to 0–255 range due to 8-bit grayscale conversion. The flame area within each frame of the acquired image sequence was determined on the basis of pixel amplitude to distinguish the flame as far brighter than any other registered objects within the field of view of the borescope. Thus, a sum of all the pixels contained within the bright region defined the flame area. Coordinates of flame area centre (x, y) are calculated as the mean value of the line or column coordinates, respectively, of all flame area pixels. Flame contour length was defined as a sum of all boundary pixels, assuming that the distance between two neighbouring contour points parallel to the coordinate axes

Changes of flame area that were obtained for fuel mixtures with 10% and 20% content of biomass obtained for different values of thermal power and excess air coefficient are presented in Figures 2 and 3, respectively. Every combustion state defined by set of constant values of

Raise of thermal power of combustion facility causes increasing of flame area, as shown in

Rise of thermal load also affects coordinates of the flame area centre, especially the x-coordinate for coal with 10% of biomass only indicating that the distance between flame front and burner nozzle increases (Figure 2c). For the other fuel mixture tested, the flame

object's output and certain excess air coefficient (0.65, 0.75, and 0.85).

Figure 1. View of combustion facility: (a) left side, (b) right side with the installed camera.

Pth, λ, and biomass content was represented by 2000 images.

position was more stable (Figure 3c and d).

is rated 1.

Figures 2a and 3a.

Combustion tests were done in a 0.5 MWth (megawatt of thermal) research facility, enabling scaled down (10:1) combustion conditions with a swirl burner. The cylindrical shape combustion chamber has the following dimensions: 2.5 m long and 0.7 m in diameter. There is a low-NOx burner mounted horizontally at the front wall with 0.1 m in diameter. The stand has the necessary fuel supply systems: primary and secondary air, coal and oil. Previously prepared pulverised coal is dumped into the coal feeder bunker. Additionally, after passing through the feeder, straw is mixed with coal.

Two lateral inspection openings on both sides of the combustion chamber provide image acquisition. The CMOS sensor-based high-speed camera was placed near burner's nozzle (see Figure 1), because this area was considered as the crucial one. The 0.7 m length borescope was engaged in the transfer of the flame images from the inside of the combustion chamber. The camera acquired up to 500 frames per second at its maximal resolution (1280 1024 pixels). The optical system was cooled with water jacket. Additionally, purging air was used to avoid dustiness of optical elements of the probe.

To comply with standards, the combustion test included the following steps. First, the combustion chamber was warmed up by burning oil. After reaching the proper temperature level, the oil supply was switched off [44], and coal and biomass mixture supplied by the primary air was delivered to the burner. While the primary air was used for fuel feeding, the excess air coefficient was determined through the secondary air flow.

Several variants were taken into consideration, where thermal power (Pth) and excess air coefficient (λ) were set independently for known biomass content. It is notable that the λ was defined as quotient the mass of air to combust 1 kg of fuel to the mass of stoichiometric air.

Figure 1. View of combustion facility: (a) left side, (b) right side with the installed camera.

to robust adaptive controllers due to [28, 29], but this requires a large number of fuzzy rules to achieve satisfactory approximator. To cope this problem, in [30–33], Takagi-Sugeno fuzzy approximator (TSFA) is involved. The invertible fuzzy approximated input matrix needs to be imposed in case of MIMO systems [34–36]. Furthermore, some examples of combining fizzy adaptive and sliding mode control can be found in [37, 38]. The examples of robust fuzzy adaptive control schemes with guaranteed H∞ control performance for a specific class of

The efficiency of pulverised fuel depends on several parameters. The commonly applied, lowemission techniques of pulverised coal combustion use recirculation vortexes that lengthen the paths of the coal grains passing through the flame to minimise generation of thermal oxides of nitrogen (NOx). To make pulverised coal combustion more efficient and environment friendly, it is necessary to measure its key parameters. The information taken from the output (exhaust gas collector) is delayed and averaged. In Ref. [42], several combustion diagnostic direct techniques are presented; the most of them are impossible to utilise under industrial conditions or are expensive. Fast and minimally invasive optical methods allow using image processing-

Combustion tests were done in a 0.5 MWth (megawatt of thermal) research facility, enabling scaled down (10:1) combustion conditions with a swirl burner. The cylindrical shape combustion chamber has the following dimensions: 2.5 m long and 0.7 m in diameter. There is a low-NOx burner mounted horizontally at the front wall with 0.1 m in diameter. The stand has the necessary fuel supply systems: primary and secondary air, coal and oil. Previously prepared pulverised coal is dumped into the coal feeder bunker. Additionally, after passing through the

Two lateral inspection openings on both sides of the combustion chamber provide image acquisition. The CMOS sensor-based high-speed camera was placed near burner's nozzle (see Figure 1), because this area was considered as the crucial one. The 0.7 m length borescope was engaged in the transfer of the flame images from the inside of the combustion chamber. The camera acquired up to 500 frames per second at its maximal resolution (1280 1024 pixels). The optical system was cooled with water jacket. Additionally, purging air was used to avoid

To comply with standards, the combustion test included the following steps. First, the combustion chamber was warmed up by burning oil. After reaching the proper temperature level, the oil supply was switched off [44], and coal and biomass mixture supplied by the primary air was delivered to the burner. While the primary air was used for fuel feeding, the excess air

Several variants were taken into consideration, where thermal power (Pth) and excess air coefficient (λ) were set independently for known biomass content. It is notable that the λ was defined as quotient the mass of air to combust 1 kg of fuel to the mass of stoichiometric air.

4. Flame images-based biomass-coal combustion characterisation

MIMO nonlinear systems can be found in [39–41].

242 Adaptive Robust Control Systems

based information in process control system [43].

feeder, straw is mixed with coal.

dustiness of optical elements of the probe.

coefficient was determined through the secondary air flow.

The study was conducted for three exact thermal power values (250, 300 and 400 kW) at the object's output and certain excess air coefficient (0.65, 0.75, and 0.85).

The tests covered two fuel mixtures containing 10% and 20% of biomass (straw), respectively. The research assumed that biomass physical properties (like particle size, inherent moisture, etc.) as well as all the image acquisition parameters (such as frame rate, camera gain and exposure time) remained unchanged. Flame images acquisition covered different fuels mixtures in every variant of the combustion facility. In order to guarantee online algorithm controllability, the images pixel amplitude was limited to 0–255 range due to 8-bit grayscale conversion. The flame area within each frame of the acquired image sequence was determined on the basis of pixel amplitude to distinguish the flame as far brighter than any other registered objects within the field of view of the borescope. Thus, a sum of all the pixels contained within the bright region defined the flame area. Coordinates of flame area centre (x, y) are calculated as the mean value of the line or column coordinates, respectively, of all flame area pixels. Flame contour length was defined as a sum of all boundary pixels, assuming that the distance between two neighbouring contour points parallel to the coordinate axes is rated 1.

Changes of flame area that were obtained for fuel mixtures with 10% and 20% content of biomass obtained for different values of thermal power and excess air coefficient are presented in Figures 2 and 3, respectively. Every combustion state defined by set of constant values of Pth, λ, and biomass content was represented by 2000 images.

Raise of thermal power of combustion facility causes increasing of flame area, as shown in Figures 2a and 3a.

Rise of thermal load also affects coordinates of the flame area centre, especially the x-coordinate for coal with 10% of biomass only indicating that the distance between flame front and burner nozzle increases (Figure 2c). For the other fuel mixture tested, the flame position was more stable (Figure 3c and d).

Figure 2. Flame area (a), contour length (b) and coordinates of flame area centre (c and d) obtained for different states of combustion process—coal with 10% content of biomass.

Low values of the flame area and contour length as well as sudden drops of coordinates of flame centre area observed for Pth = 250 kW and λ = 0.85 point to stability problems, which occurred during combustion tests.

The studies have shown that possible unstable combustion rely to optical parameters (e.g. flame area, contour length and flame centre coordinates), similarly to higher excess air coefficients, regardless the thermal power (Figures 2 and 3). The more biomass was added (Figure 3), the sudden changes of the discussed parameters were observed. This indicates that unstable

Figure 3. Flame area (a), contour length (b) and coordinates of flame area centre (c and d) obtained for different states of

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The way the flame area was defined, directly influences on the achieved quantitative parameter values of the flame area and its contour length. Mounting the camera perpendicularly to the burner axis allowed to estimate vital information about combustion process state [44–48]. These were the distance between burner and flame ignition point [45, 48] as well as spread angle of the flame. In industrial practice for full-scale power boilers, it is hard to install the camera close to a burner, because it involves disturbances in the boiler shield. So, the alterna-

5. Robust adaptive control of co-combustion process, using optical signals

For the proper boiler's power operation, the opportunity to assess the quality of combustion is critical [49]. The combustion flow in layers influences on the speed of chemical reactions, heat

combustion is a serious problem.

combustion process—coal with 20% content of biomass.

tive camera set-up was tested.

Another important factor is variability of the flame parameters discussed, that were calculated for each combustion state.

Amount of excess air coefficient significantly affects combustion process. However, the mean value of flame area has different dependencies on λ for the different values of thermal power. For Pth = 400 kW, the flame area decreases when excess air coefficient increases for fuel mixtures with 10% and 20% of biomass.

Variability of flame contour length is almost the same as it does in the case of flame area.

Changes of the flame centre position are different for the examined variants. For biomass content of 20%, the standard deviation of the discussed parameter is greater, especially for greater λ and thermal power value.

Comparing the mean values of flame area for the same excess air coefficient, it could be observed that flame area is larger for fuel mixtures with higher biomass content. This is because biomass contains more volatile contents comparing to coal.

Figure 3. Flame area (a), contour length (b) and coordinates of flame area centre (c and d) obtained for different states of combustion process—coal with 20% content of biomass.

The studies have shown that possible unstable combustion rely to optical parameters (e.g. flame area, contour length and flame centre coordinates), similarly to higher excess air coefficients, regardless the thermal power (Figures 2 and 3). The more biomass was added (Figure 3), the sudden changes of the discussed parameters were observed. This indicates that unstable combustion is a serious problem.

Low values of the flame area and contour length as well as sudden drops of coordinates of flame centre area observed for Pth = 250 kW and λ = 0.85 point to stability problems, which

Figure 2. Flame area (a), contour length (b) and coordinates of flame area centre (c and d) obtained for different states of

Another important factor is variability of the flame parameters discussed, that were calculated

Amount of excess air coefficient significantly affects combustion process. However, the mean value of flame area has different dependencies on λ for the different values of thermal power. For Pth = 400 kW, the flame area decreases when excess air coefficient increases for fuel

Variability of flame contour length is almost the same as it does in the case of flame area.

Changes of the flame centre position are different for the examined variants. For biomass content of 20%, the standard deviation of the discussed parameter is greater, especially for

Comparing the mean values of flame area for the same excess air coefficient, it could be observed that flame area is larger for fuel mixtures with higher biomass content. This is because

occurred during combustion tests.

combustion process—coal with 10% content of biomass.

mixtures with 10% and 20% of biomass.

greater λ and thermal power value.

biomass contains more volatile contents comparing to coal.

for each combustion state.

244 Adaptive Robust Control Systems

The way the flame area was defined, directly influences on the achieved quantitative parameter values of the flame area and its contour length. Mounting the camera perpendicularly to the burner axis allowed to estimate vital information about combustion process state [44–48]. These were the distance between burner and flame ignition point [45, 48] as well as spread angle of the flame. In industrial practice for full-scale power boilers, it is hard to install the camera close to a burner, because it involves disturbances in the boiler shield. So, the alternative camera set-up was tested.
