Biofuels GS 2 Measuring Course Part II, DTU, Feb 2 6, 2009 Experiments in the entrained flow reactor

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1 Biofuels GS 2 Measuring Course Part II, DTU, Feb 2 6, 2009 Experiments in the entrained flow reactor Frida Claesson (ÅA) Johanna Olsson (CTU) Kavitha Pathmanathan (NTNU) Samira Telschow (DTU) Liang Wang (NTNU) Hao Wu (DTU)

2 Tables of content 1. Introduction Experimental Experimental setup Experimental performance Results and Discussion Temperature of flue gas and deposit Fuel feeding rate and concentrations of O 2 and CO Concentrations of NO, SO 2 and CO Ash and carbon balance Fuel carbon burnout Deposits SEM analysis Conclusions Appendix A Fuel properties Appendix B Calculation procedure...21 Excess air ratio Flue gas flow Carbon balance Ash balance Deposit flux Ash flux Appendix C Averages and deviations

3 Appendix D EDX analysis

4 1. Introduction In an entrained flow reactor it is possible to simulate the environment of a high temperature combustion boiler which is suitable for studying heterogeneous reactions with short residence time. In these experiments we have investigated the influences of unstable fuel feeding and addition of PVC in bituminous coal (COPRIB) combustion, compared to a reference case with stable fuel feeding of pure COPRIB coal. The influences on the gaseous emission, carbon burnout, deposition formation, and fly ash properties have been studied. Chemical attack caused by deposits formed on super heaters and water tube wall is considered to be the primary cause of corrosion in coal combustion plants. The deposits are formed by species, by condensation on the surface, or attaching as fly ash particles from the flue gas. Chlorides of the alkali metals, sodium (Na) and potassium (K) and the heavy metals, are the species believed to be the most of chemically aggressive and the ones primarily responsible for corrosion. The ash deposits may be from low-melting chlorides of alkali metals and heavy metals, of which many mixtures have melting points below 500ºC. 2. Experimental 2.1 Experimental setup The setup is about 7 m in total height and consists of a down fired reactor, a fuel particle feeder, a gas dosing system with precise mass flow controllers, a gas pre-heater, a gas and particle sampling system, a deposition system, and a data collecting system. The down-fired reactor has a length of 2 m and an internal diameter of 8 cm. The reaction chamber is electrically heated by 7 (5 operational) heating elements and the maximum operational temperature is 1500 ºC. To ensure a comparable residence time for the different experiments, the primary air flow rate is maintained at 13 Ndm 3 /min and the total air flow rate is 95 Ndm 3 /min. Upon injection the fuel particles are mixed with preheated secondary air (900 ºC). To maintain a similar excess air ratio throughout all three experiments, the fuel particle feeding rate is controlled by an adjustable gravimetric screw feeder. The pulverized fuel is injected into the reactor together with the primary air through a water cooled, as demonstrated in Figure 1. Combustion of the fuel particles takes place in the down-fired reactor, and the temperature of the heating element is set to o C. In the bottom of the reactor there is a flue gas outlet with a size of 4x8 cm. At this point, deposition measurements are performed. The deposit is made of stainless steel and has a length of 10 cm and an outer diameter of 1 cm. The deposition surface temperature (585 ºC) is controlled by adjusting the temperature of the air pre-heater and the heating tape connected to the. The majority of the flue gas passes the deposit before entering the ventilation system. The flue gas temperature is kept at 800 ºC by means of a propane flame burner in front of the deposition. 4

5 Before the flue gas outlet, a gas and particle sampling system is connected. A small sample is extracted through a water cooled. From this sample, PM2.5 is separated with a cyclone and fine fly ash particles are collected on a polycarbonate membrane with a pore size of 0.1 µm. After the cyclone, part of the flue gas is further cooled to condensate the water and lead to the gas analyzer where CO, CO 2, O 2, NO and SO 2 are measured continuously. Large ash particles from the combustion are collected in a bottom ash chamber. Figure 1 Schematic drawing of the entrained flow reactor used in the deposition experiment. The fuel for these experiments is: Case 1: COPRIB coal (properties given in Appendix A) Case 2: COPRIB coal Case 3: COPRIB coal with 10 wt% PVC The first experiment is a reference case with stable fuel feeding while the second experiment is performed with unstable fuel feeding. The unstable fuel feeding is achieved by removing a small obstacle on the vibrator in the feeding system which is designed to reduce the 5

6 fluctuation in fuel feeding. To understand the influence of unstable fuel feeding on the overall combustion and deposition perform is important since the obstacle has to be removed when fuels such as coal and waste mixture are fed. The third experiment is, as shown above, co-combustion of COPRIB coal and PVC (10 wt%) with stable fuel feeding. Pure PVC particles bought from Sigma-Aldrich Inc. are used in the experiment, and the estimated properties are given in Appendix A. The water contents of COPRIB coal and PVC are measured in a Sartorius MA40 electronic moisture analyzer before experiments. According to pervious analysis, the average particle size of the COPRIB coal is about 50 µm, which is slightly larger than that of the PVC particles. 2.2 Experimental performance When working with this set up, some safety guideline are prudent. The entrained flow reactor is a high temperature reactor hence there are many hot surfaces that requires thermal resist gloves. Whenever handling the pulverized fuel, ashes, deposits and/or aerosols, protective mask and gloves are obligatory. Due to the height of the rector there are several floors where tools and equipment are being handled and therefore a safety helmet is mandatory. Before each experiment, gas analyzers are calibrated. Filters, collectors and deposition are pre-weighted and heated to the desired temperature. In order to reduce the impact of the fuel feeding, the feeding rate is increased stepwise every fifth minute by a step of 0.1 kg/h until desired feeding rate is reached. At this point, the propane burner is turned on and adjusted to maintain the flue gas temperature at 800 o C. The deposition is exposed to the flue gas when the fuel feeding rate is stabilized. The experimental time is 90 minutes. This time is pre-determined to collect an adequate amount of deposit and ash samples for analysis but not overloading the deposit thus causing significant loss of deposit material through falling. 6

7 3. Results and Discussion 3.1 Temperature of flue gas and deposit Figure 2. A schematic view of the different thermocouples positions around the deposition. T36 and T40 are positioned in the bottom of the, whereas the other thermocouples are positioned in the middle of the Average values and deviations for the subsequent figures are presented in Appendix C. Since it normally takes some time for the temperature and concentration profiles to become stable, only the data obtained 15 minutes after the beginning of experiment is taken into account in calculating the average values. 900 Temperature (oc) Time (min) T 35_Deposition T 36_Deposition T 37_Flue gas T 38_Deposition T 39_Deposition T 40_Deposition Figure 3. Temperature profiles for case 1 7

8 Temperature (oc) Time (min) T 35_Deposition T 36_Deposition T 37_Flue gas T 38_Deposition T 39_Deposition T 40_Deposition Figure 4. Temperature profiles for case Temperature (oc) Time (min) T 35_Deposition T 36_Deposition T 37_Flue gas T 38_Deposition T 39_Deposition T 40_Deposition Figure 5. Temperature profiles for case 3 From Figure 3-5 it can be concluded that the temperatures are kept stable when the measurements start, 15 minutes after experiment start. Temperatures recorded by different thermal couples also show the temperature distribution on the. During the experiments deposits were formed on T37, which might lead to inaccuracy in the flue gas temperature measurement. Therefore the deposits on T37 need to be removed frequently during the experiments. 8

9 3.2 Fuel feeding rate and concentrations of O2 and CO Concentration (%) Feeding rate (g/h) CO2 concentration O2 concentration Feeding rate Time (min) Figure 6. Fuel feeding rate and concentrations of O2 and CO2 for case Concentration (%) Feeding rate (g/h) CO2 concentration O2 concentration Feeding rate Time (min) Figure 7. Fuel feeding rate and concentrations of O2 and CO2 for case 2 9

10 Concentration (%) Feeding rate (g/h) CO2 concentration O2 concentration Feeding rate Time (min) Figure 8. Fuel feeding rate and concentrations of O2 and CO2 for case 3 The feeding rate for the first two cases was almost the same, while in case 3 the feeding rate was slightly higher. The calculated excess air ratio for case 1, case 2 and case 3 was 1.480, and respectively (calculation procedures are given in Appendix B). As shown in Figure 6-8, the concentrations profiles for O 2 and CO 2 are stable and this indicates that the combustion conditions was stable throughout the experiments. 3.3 Concentrations of NO, SO2 and CO Concentration (ppm) SO2 concentration CO concentration NO concentration Time (min) Figure 9. Concentrations of NO, SO2 and CO for case 1 10

11 Concentration (ppm) Time (min) SO2 concentration CO concentration NO concentration Figure 10. Concentrations of NO, SO2 and CO for case Concentration (ppm) SO2 concentration CO concentration NO concentration Time (min) Figure 11. Concentrations of NO, SO 2 and CO for case 2 From Figure 9-11 it can be concluded that complete combustion was achieved for all of the experiments, with low CO concentrations in the flue gas. In case 2 we have a lower NO and SO 2 concentration compared to the reference case 1, since unstable feeding may result in a local fuel rich condition which is favored by the reduction of NO. The low SO 2 concentration in case 2 is probably caused by the absorption of the existing deposits in the reactor as no high temperature cleaning has been performed between case 1 and case 2. In comparison with case 1, lower NO and SO 2 concentrations have also been observed in case 3. The primary reason is probably due to the reduced amount of coal fed in case 3 compared 11

12 to case 1. This reduces the overall nitrogen and sulphur present during the experiment for the formation of NO and SO 2. Furthermore, there is no nitrogen and sulphur content in PVC. Another possible reason is the fuel mixture used in case 3 contain higher volatile content than that of COPRIB coal. Thus, less fuel nitrogen may be converted to NO during the devolatilization stage in case 3. The SO 2 concentration profiles in Figure 9-11 show larger fluctuations than other concentration profiles. It may both due to the SO 2 analyzer or the unstable pressure in the feeding system. 3.4 Ash and carbon balance The ash and carbon balance shown in Table 1 and 2, was calculated with the equations in appendix B. The carbon balance is close to 100%, which indicates the quality of the experiments is high. The ash balance is lower compared to the carbon balance. This can be related to the difficulties in collecting all ash in the equipment. For the ash content detection, the ash content of the fuel is determined at 550ºC in a lab scale furnace. However, the experiment combustion temperature is approximately 1300 ºC, the calcium carbonate and some of the alkaline content will be lost due to high temperature evaporation, and hence the ash residues collected from the fuel combustion in the reactor will be lower then ash content detected in the furnace condition. Table 1. Carbon balance Table 2. Ash balance Case % Case % Case % Case % Case % Case % 3.5 Fuel carbon burnout The unburned carbon in fly ash was analyzed and the fuel carbon burnout was calculated according to the equation: A 100 A Bc = 100 A0 Ai 0 i (1) where B c is the carbon burnout, A 0 is the ash content of the dry fuel, and A i is the inert content of the fly ash. The fuel carbon burnout was high for all of the cases. In case 2 the carbon burnout was lower compared to case 1 because of the unstable fuel feeding resulting in poor fuel and air mixing, and more carbon will keep in the ash residues without burning out. In case 3 the carbon burnout is slightly decreased compared to case 1. 12

13 Table 3. Carbon burnout Case % Case 2 99,77% Case 3 99,83% 3.6 Deposits Table 4. Deposit calculation results Deposition flux (g/cm 2 /h) Ash flux (g/cm 2 /h) Deposition flux / Ash flux Case 1 0,026 0,893 0,029 Case 2 0,022 0,779 0,028 Case 3 0,030 0,759 0,040 The deposition flux and the ash flux were calculated according to the equations given in the Appendix B. The experimental results from the coal combustion in ERF indicated that the deposition rate from the increased with addition of 10% PVC (case 3). The deposits flux increased from g to g/cm 2 /h, with lower ash load in the reactor. Figure 12 shows an increase of deposits on the under the coal and PVC co-combustion condition (case 3). More Cl will be released during the combustion, because of the chemical composition of PVC. The Cl in the gas phase will easily react with hydrogen and alkali metals. The HCl will rapidly react with alkali metals in the coal to form alkali chlorides, especially sodium chloride and potassium chloride. Both sodium chloride and potassium chloride have low melting temperatures, in the range of ºC, and hence become sticky. All of the alkali chlorides will be in gas, liquid and solid phase in the flue gas. When the flue gas passes through the, the surface of the might be covered by a sticky alkali chlorides layer. This layer will capture more particles in the flue gas passing it. The blending of PVC will increase the alkali chloride in the combustion flue gas, and lead to more ash deposits on the. The unstable combustion conditions of coal seem to decreases the deposits collection rate and aspect. Figure 12 shows three depositions s used during the different experiments. In Figure 12- f, it can be seen that corrosion appeared at the deposition within a short experimental period, due to the chlorine released from PVC. 13

14 Figure 12. Figure Deposits collected on the : a. stable combustion of coal; b. unstable combustion of coal; c. co-combustion of coal. Probe after collection of deposits: d. stable combustion of coal; e. unstable combustion of coal; f. co-combustion of coal with 10 wt% PVC 3.7 SEM analysis The analytical results from the SEM-EDX show that in the collected cyclone ash, Figure 16 and Table 10, (large fly ash particles, in case 3), the Cl content is relatively high in comparison with the cyclone ashes from the first case. The microstructure of the fine fly ash particles (aerosol) with PVC addition, Figure 14, is consistent of agglomerates of fine particles. Similar structures can also be found for combustion of coal alone (case 1, Figure 13). The agglomeration of particles in both cases might be due to two different reasons. The first reason causing the agglomeration observed in Figure 13 and 14 is probably due to the filtration mechanism occurring in the total filter. As an additional affect, for case 3 only, the agglomeration could be also cause by the alkaline chlorides. The fine particles agglomerated together or covering the surfaces of larger particles are mainly due to the increased Cl content in the combustion. The more chlorine in the combustion process the more fine alkali chloride particles will be generated with different sizes and in different phases. These fine alkali chloride particles are easily melted and become adhesive because of their low melting points. Most of these alkali particles are going to agglomerate together and cover the surfaces of other particles when they pass through. The agglomeration and covering are related to different forces like thermophoresis force, collision and van de Waal forces. Figure 13 and 14 show that fine fly ashes particles are mostly spherical particles, as a sequence of nucleation. Comparing the SEM Figures 13-16, particles from cyclones samples are larger than the fine fly ash particle from the total filter samples. 14

15 Figure 13. SEM photography of aerosol sample particles collected from stable coal combustion Figure 14. SEM photography of aerosol sample particles collected from co-combustion of coal and PVC 15

16 Figure 15. SEM photography of cyclone sample particles collected from coal combustion Figure 16. SEM photography of cyclone sample particles collected from co-combustion of coal and PVC 16

17 Table 5. Deposits from case 1 Element Wt% Na 4,18 Al 21,46 Si 41,14 S 6,55 K 10,04 Fe 16,63 Total 100% Table 6. Deposits from case 3 Element Wt% Na 3,08 Mg 1,93 Al 22,88 Si 49,42 S 3,08 K 3,66 Ca 3,21 Fe 12,72 Total 100% Table 7. Fine filter ash from case 1 Element Wt% Na 2,00 Mg 4,31 Al 22,21 Si 37,02 S 4,05 K 4,66 Ca 6,97 Fe 19,78 Total 100% Table 8. Fine filter ash from case 3 Element Wt% Na 2,69 Mg 2,35 Al 17,26 Si 27,66 S 6,27 Cl 4,55 K 15,17 Ca 7,68 Fe 16,35 Total 100% Table 9. Cyclone ash from case 1 Element Wt% Na 2,02 Mg 2,16 Al 24,45 Si 43,15 S 1,44 Cl 0,65 K 4,73 Ca 4,11 Fe 17,16 Total 100% Table 10. Cyclone ash from case 3 Element Wt% Na 3,12 Mg 2,79 Al 22,52 Si 43,09 S 2,95 Cl 1,28 K 6,91 Ca 4,74 Fe 12,54 Total 100% From the analysis of EDX, it is observed that sodium (Na), magnesium (Mg), aluminum (Al), potassium (K), sulfur (S) and calcium (Ca) are present together in all the different operating conditions. This shows that these are the main inorganic elements found the coal. Coal contains Mg and Na in the range of 2-4 wt.%. The contain of Al in coal is between wt.%, S is 1-6 wt.% and Ca is 3-6 wt.%. The percentages of K is between 4-6 wt.% for all the samples except for sample from the particles collected on the total filter (aerosol filter) which is 15 wt.%. Cl is mostly present in the sample where the PVC was used as a part of the fuel. The chloride (Cl) amount in the sample is of 1-4 wt.%. The presence of silicon (Si) and iron (Fe) may be due to the usage of glass rod and steel collecting container during the sampling. It 17

18 should be noted that the carbon (C) and oxygen (O) contents detected by EDX are neglected in the results shown in Table 5-10 in order focus on the major inorganic elements. Comparing the samples derived from the total filter and the samples collected from the bottom of the cyclone show an increase of the Cl in the case of PVC addition to coal. In these samples a significant increase of K and a small increase of Na are observed. This might be due to the formation of NaCl and KCl. 18

19 4. Conclusions The experiments were carried out during good conditions, which can be concluded from the results; Stable temperature, stable gas concentrations and high carbon balance. Case 2 and case 3 both resulted in a lower emission of NO and SO 2. The carbon burnout shows that unstable feeding and co-combustion of coal and PVC only reduce the burnout slightly. The burnout is above 99% for all three experiments. Case 3 gave a different deposit on the, see Figure 12. The deposition rate also increased, and corrosion on the has been observed in this case. In case 2, the deposition rate was decreased compared to case 1. The SEM analysis shows the chemical composition of the different samples, the presence of chlorine is evident in the fly ash particles from case 3. 19

20 Appendix A Fuel properties Properties of the COPRIB coal LHV (MJ/kg wet) 27,440 Moisture (wt% wet) 5,172 Volatiles (wt% wet) 36,324 Ash (wt% wet) 6,920 C (wt% dry) 72,600 H (wt% dry) 4,910 N (wt% dry) 1,440 S (wt% dry) 0,770 O (wt% dry) 17,356 Cl (wt% dry) 0,002 Si (wt% dry) 1,800 Al (wt% dry) 0,840 P (wt% dry) 0,006 Ca (wt% dry) 0,130 Na (wt% dry) 0,046 K (wt% dry) 0,100 Properties of the PVC LHV (MJ/kg wet) - Moisture (wt% wet) 1,100 Volatiles (wt% wet) 98,900 Ash (wt% wet) 0,000 C (wt% dry) 38,400 H (wt% dry) 4,800 N (wt% dry) 0,000 S (wt% dry) 0,000 O (wt% dry) 0,000 Cl (wt% dry) 56,800 Si (wt% dry) 0,000 Al (wt% dry) 0,000 P (wt% dry) 0,000 Ca (wt% dry) 0,000 Na (wt% dry) 0,000 K (wt% dry) 0,000 20

21 Appendix B Calculation procedure Excess air ratio where: Volumetric air flow [Nm 3 /s] Molar flow of species i in the fuel [mol/s] Pressure ( Pa) Temperature (273 K) Ideal gas constant (8.314 J/(mol K)) Mole fraction of oxygen in air (21%) Flue gas flow Carbon balance where: molar mass for carbon [g/mole] mass fraction of carbon in dry fuel moister content 21

22 Ash balance mass flow of wet fuel [g/s] where: total time for experiment mass fraction of ash in wet fuel with where: extracted flue gas flow [Nm 3 /s] collected mass of ash in the cyclone [g] collected mass of ash in the extraction pipe [g] where: extracted gas flow to the filter [Nm 3 /s] collected mass of ash in the filter [g] where: total mass of collected ash in bottom of reactor [g] mass of fuel used during experiment [g] total mass of fuel used during experiment [g] 22

23 Deposit flux where: total mass of collected deposit [g] the height of the flue gas outlet [cm] Ash flux where: the width of the flue gas outlet [cm] 23

24 Appendix C Averages and deviations Table 11. Case 1 Feeding rate (g/h) T 35 (oc) T37 (oc) CO2 (%) O2 (%) NO (%) SO2 (%) CO (ppm) Average 559,9 584,8 799,8 12,7 6,4 1034,7 394,2 33,8 Average deviation 3,8 3,5 3,4 0,1 0,1 20,2 18,6 3,7 Table 12. Case 2 Feeding rate (g/h) T 35 (oc) T37 (oc) CO2 (%) O2 (%) NO (%) SO2 (%) CO (ppm) Average 560,2 583,6 801,1 12,5 6,4 985,8 256,1 22,0 Average deviation 5,3 3,4 3,2 0,1 0,1 35,0 25,2 8,0 Table 13. Case 3 Feeding rate (g/h) T 35 (oc) T37 (oc) CO2 (%) O2 (%) NO (%) SO2 (%) CO (ppm) Average 581,6 577,8 800,6 12,4 6,6 929,4 280,7 62,5 Average deviation 5,2 4,6 2,3 0,1 0,1 24,5 27,6 5,6 24

25 Appendix D EDX analysis Figure 17. EDX analysis for deposits sample particles collected from stable coal combustion Figure 18. EDX analysis for deposits sample particles collected from co-combustion of coal and PVC 25

26 Figure 19. EDX analysis for aerosol ash sample particles collected from stable coal combustion Figure 20. EDX analysis for aerosol ash sample particles collected from co-combustion of coal and PVC 26

27 Figure 21. EDX analysis for cyclone ash sample particles collected obtained from stable coal combustion Figure 22. EDX analysis for cyclone ash sample particles collected from co-combustion of coal and PVC 27