COMBUSTION PROCESS ANALYSIS IN BOILER OP-650K BASED ON ACOUSTIC GAS TEMPERATURE MEASURING SYSTEM

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1 3 rd International Conference on Contemporary Problems of Thermal Engineering CPOTE 2012, September 2012, Gliwice, Poland Institute of Thermal Technology COMBUSTION PROCESS ANALYSIS IN BOILER OP-650K BASED ON ACOUSTIC GAS TEMPERATURE MEASURING SYSTEM D. Nabagło, P. Madejski EDF Polska CUW Sp. z o.o. Research & Development Department Ciepłownicza 1, Kraków, Poland daniel.nabaglo@edf.pl pawel.madejski@edf.pl Keywords: temperature measurement, steam boiler, hard coal combustion Abstract There is increasing environmental legislation to install flue gas treatment installation which, together with biomass co-firing technology and lifetime projects of operating power units, form a challenge for the Polish power engineering industry. Therefore, one observes numerous R&D activities aiming at optimal modifications of combustion installations which affect lifetime, efficiency and reliability of power units. The first step in meeting the new legislation is to obtain full control over the burning process with the use of innovative diagnostic technologies. This paper presents the results of temperature distribution measurements in boiler OP-650k in the Rybnik Power Plant. The impact of boiler load on temperature distribution inside the furnace was analyzed during in half-yearly measurements. The paper also presents the diagnostic method based on flue gas temperature and O 2 concentration analysis, which can show the non-uniform air consumption and flue combustion inside the chamber. 1. Introduction The critical parameter that influences the operation of the boiler is temperature distribution in the combustion chamber. Obtaining comprehensive information on distribution of temperature inside the boiler is an important aspect in enabling optimization of its operation [1]. The issue of obtaining information on temperature distribution inside the boiler chamber is a technically difficult task, due to high temperatures and limitations resulting from measurement. The AGAM-Acoustic Gas Temperature Measuring System allows key combustion process information to be obtained, which can be used for boiler operation optimization. The AGAM system uses acoustic waves for temperature measurement and is based on the principle of the relation of sound waves to temperature. By precisely measuring the transit time of a sound impulse passing a known distance between the sound generator and the registering unit (piezoelectric microphones), the average temperature of the flue gas along a straight line can be accurately determined. In addition, a number of acoustic sound sources and 1

2 D. Nabagło, P. Madejski receivers can by placed around the perimeter of a furnace to obtain a two-dimensional profile of temperature distribution within the plane area [2-3]. 2. Facility description 2.1. Principle of the AGAM measuring system AGAM operates on the principle of acoustic pyrometry, which enables measurement of the time of delay of an acoustic wave and which depends on the temperature in the environment of propagation. In respect of the number of installed sensors and location of the sensors inside the furnace chamber, delay of the wave is measured on a number of sections. The relation is defined by the following formula: v speed of sound s m, R M universal sound constant molar mass of gas T temperature K, adiabatic index κ c p. cv kg, kmol R v = κ T M (1) kj, kmol K The adiabatic index and molar mass of gas are calculated based on the typical average values of volume fraction for flue gases in hard coal fired furnaces. For boiler OP-650K the parameters are equal to: κ = 1.275, N 2 = 76.5 Vol. %, O 2 = 3 Vol.%, CO 2 = 13 Vol.%, H 2 O = 7.5 Vol.%. Velocity of a wave is determined by the propagation time of an acoustic impulse, between symmetrically places transmitters and receivers. In the measurement system, the distance between the transmitter and the receiver is constant and known. The temperature is calculated as per the relation presented below: 2 l T = B τ l distance between transmitter and receiver (propagation path) m, τ propagation time (time of delay) s, B acoustic constant κ R, M The AGAM system uses for its operation an audio signal of frequency in the range of 200 to 3000 Hz. The acoustic impulse is a pneumatically generated high intensity white noise. Compressed air is used as the sound source, which is released when the electrical magnetic valve is opened. The signal generated is propagated within the environment, where it is delayed in respect of the other transmitting and receiving probes. The measurement control system registers time signals from all receivers at the same. The registered signals are sampled and further undergo an analysis of correlation on the basis of which the time of delay is identified for all directions where the probes are installed. Thanks to use of compressed air as the generating agent, the probe during sound generating is cleaned of slag deposits the boiler walls. The temperature identified for each path of the acoustic wave is an average value between the transmitter and the receiver (Fig. 2). On the basis of the measured average values of temperature, the external software will calculate distribution of temperature on the surface (2) 2

3 CPOTE 2012, September 2012, Gliwice, Poland which is measured by the system Measurements details The acoustic temperature measurement system (AGAM) has been installed on boiler K-4 (OP- 650k type) in Rybnik PP for assessment of conditions inside the combustion chamber. AGAM is measuring temperature on a 30.2 m level (Fig. 1) and is composed of 8 heads (Fig. 2) which realize the measurements on 21 paths. Figure 1: Boiler OP-650k with marked AGAM measurement level. Based on 21 paths (Fig. 2a), the maps of temperature distribution are generated from 12 zones, which are presented in Figure 2b. Figure 2: AGAM heads set-up on boiler OP-650k in Rybnik PP. Measurement paths inside the boiler (a), and location of 12 measuring zones (b). Combustion analysis was done for the half year measurement period, between July 2011 and January Analyzed boiler operation data are listed below: flue gas temperature inside boiler on level 30.2m AGAM, live steam flow, electric power, O 2 and CO concentration after boiler, over-fire air flow to OFA II and OFA III nozzles, 3

4 D. Nabagło, P. Madejski fuel (coal and biomass) mass flow, The validation of the acoustic method for temperature measurements in the combustion chamber is very difficult. The high temperature of flue gas inside the chamber renders conventional thermocouple and water-cooled thermal probe methods useless. The results obtained from AGAM system were validated based on the comparison with the temperature field calculated by the CFD simulation for similar boundary conditions and operation parameters of the boiler OP-650K Boiler OP-650 overview Boiler K-4 in Rybnik PP has a 650 t/h rate nominal live steam flow. The boiler is a steam generator for a power unit which can operate between 135 MW e (minimum stable capacity) and 225 MW e (maximum stable capacity). The boiler is supplied by 6 coal mills (ball-ring). Fuel, which is fed into the boiler via mills, consists of hard coal and biomass up to 10% mass share. The boiler is equipped with biomass direct injection installation, which has maximum capacity about 16 t/h. The boiler combustion system was last modernized in Modernization was associated with low-no x technology to meet NO x emission level 350 mg/nm 3 (Fig. 3). Figure 3: General scheme of low-no x combustion system on boiler no. 4 (a), and wind box scheme (b). General scope of modernization is listed below: modification of wind box: o low-no x burners supplied by concentrated mixture (burner level I and II), o re-burning zone was created (thinned mix burner level III), o 1 st OFA were added in the zone separating main flow of fuel and re-burning fuel jet (finally OFA I nozzles are closed), o four injection nozzles of biomass were added (S1 S4), 4

5 CPOTE 2012, September 2012, Gliwice, Poland o 3 rd stage of OFA nozzles were added at the rear wall of boiler (OFA 3 rear wall), distributors which divide PF mass flow on thinned mixture and concentrated mixture were added, installation of protective air system, 2.4. Operating data during measurements Measurements were carried out since July 2011 until January Table 1 shows the values of main operating data that were measured during analysis. Table 1: Main operating data. Parameter Value Operating period ~ 3700 h Outage period ~380 h Coal net calorific value kj/kg Biomass net calorific value kj/kg Avg. daily coal consumption 1500 t/day Avg. daily biomass consumption 260 t/day Electric power 135 MW e 190 MW e 225 MW e Live steam flow t/h t/h t/h O 2 concentration after boiler % % % Avg. flue gas temperature inside boiler C C C Figure 4 shows the electric power, live steam flow and average flue gas temperature changes during one day average of measurement. Figure 4: Average day of measurements with hour averages for electric power, live steam flow and flue gas temperature. 5

6 D. Nabagło, P. Madejski The average flue gas temperature varies during the day from 1100 C to 1220 C and depends on live steam flow and electric power. The highest values of average temperature are between 8:00 and 20:00, since the production of electric power is highest. 3. Temperature distribution analysis as a function of boiler load The average flue gas temperature is calculated by measuring values in each zone (Fig. 2), which belongs to cross section on 30.2 m level. The values obtained for all zones, can be interpolated and presented as a maps of temperature distribution. Figure 5 depicts the temperature distribution maps for three different boiler loads at a specific moment. Rear [mm] MWe Left [mm] Right [mm] 9000 Front [mm] Rear [mm] MWe Left [mm] 6000 Right [mm] 9000 Front [mm] Rear [mm] MWe Left [mm] 6000 Right [mm] 9000 Front [mm] Figure 5. Temperature distribution maps for three different boiler loads: 135 MW e, 190 MW e and 225 MW e. The maximum flue gas temperature for all three loads exists close to the rear wall of the boiler and its plane of symmetry. The values of maximum temperature are 1170 C for 135 MW e, 1290 C for 190 MW e and 1340 C for 225 MW e. The lowest temperatures of flue gas occur in the corners of the boiler near the front wall, and their values for loads 135 MW e, 190 MW e and 225 MW e are: 1020 C, 1120 C and 1210 C, respectively. The temperatures of flue gas calculated for the left, right, front and rear side of the boiler are 6

7 CPOTE 2012, September 2012, Gliwice, Poland presented in Figure 6, as a functions of electric power production. The electric power range in the analysis is MW e and the temperature for each value is the average of six-monthly measurements of the boiler. Figure 6. The temperature of flue gas as a function of electric power for the left, right, front and rear side of the boiler, (half-year average). The average flue gas temperature increases from 1090 C to 1270 C when the electric power is changed in the MW e range. The significant differences in flue gas temperature between the left and right side of the boiler occurs only above 200 MW e. These differences suggest irregular work of coal mills or OFA air nozzles, which can lead to reduced combustion process efficiency. The on-line measurement of temperature distribution and the differences in the combustion chamber allows for changes to be made, improving the efficiency of the combustion process. 4. Oxygen concentration analysis as a function of combustion temperature Oxygen concentration can be also an indicator of combustion quality. In the case of non-uniform combustion, O 2 concentration differences between the left and right side of the boiler can occur. The O 2 concentration and flue gas temperature can be compared to find the differences between each side of the boiler (Fig. 7). The O 2 concentration at the outlet of the boiler was measured separately for the left and right side. 7

8 D. Nabagło, P. Madejski Left side of the bolier Right side of the bolier Figure 7. The temperature of flue gas as a function of oxygen concentration for the left and right side of the boiler for three electric power loads. Based on Figure 7, we can see that there is a difference in oxygen concentration between the left and right side of the boiler. For 190 MW e and 225 MW e, the range of measured O 2 concentration on left side changes in comparison with the right side. For left side the range of O 2 concentration is between 2.1 and 3.5 %, and between 1.6 and 2.9 % on the right. This shift shows non-uniformity in the combustion process between the right and left side of the boiler. The measured flue gas temperatures are the same for each side. The minimum flue gas temperature is 1050 C, and maximum is 1350 C. For low electric power (135 MW e ) the range of measured oxygen concentration is the widest for right side (from 2.0 up to 4.8 %). For the same electric power the range of measured O 2 concentration for left side is between 2.8 and 4.8 %). 5. Conclusion Acoustic pyrometry uses the relation between sound speed and the absolute temperature of gas. Due to measurement principal, this technique can be used to determine the gas temperature in the combustion chamber without radiation faults. Acoustic techniques in boiler operation provide a new standard in diagnostics and combustion process control. AGAM combustion process analysis provides important information about combustion quality, which is the temperature distribution in a cross section of a boiler. With this information the plant operator can react precisely to optimize the combustion process. AGAM can also be used to diagnose problems with coal feeding which has an influence on temperature distribution and oxygen concentration. References [1] W. Nowak, M.Pronobis, Advanced technologies for combustion flue gas treatment, Wydawnictwo Politechniki Śląskiej, Gliwice, [2] M. Deuster, Acoustic Gas Temperature Measurement, Proceedings of Wissenforum: Temperature Measurement Technique, Aldenhoven, [3] M. Deuster, H. P. Drescher, Optimization of Coal-Fired Boilers Using Acoustic Pyrometry, Coal- Gen Europe, Warsaw,