Combustion of Polymers in a Fluidised Bed Reactor

Transcription

1 Archivum Combustionis Vol. 30 (2010) no. 4 Combustion of Polymers in a Fluidised Bed Reactor J. Połomska *, W. Żukowski **, J. Zabagło ** * Faculty of Environmental Engineering, **Faculty of Chemical Engineering and Technology Cracow University of Technology, 24 Warszawska St., Kraków, Poland asiapolomska@gmail.com, pczukows@pk.edu.pl, zabaglo@chemia.pk.edu.pl Polymers are used to produce plastics and nowadays there are lots of everyday usage products made from polymers. Consequently there are lots of waste polymers in municipal solid waste as well. Combustion of waste polymers is a very practical method for the total disposal of them from the environment but although the typical chemical composition of natural fuels, polymers are very specific fuels and it is not easy to burn them giving complete combustion products, that are CO 2 and H 2 O. In the following work the method of the thermal decomposition and combustion of some polymers with using fluidised bed technology is proposed. The thermal decomposition and combustion processes of PC, PET, PC/ABS and PS were realized in the laboratory bubbling fluidised bed reactor. The polymers were burnt with propane in the sand bed which temperature reached 900 o C. The identification of the selected components in the flue gases and the visual process observations by using two digital cameras were made. A few gas analysers applied different chemical sensors for monitoring the concentrations of O 2, CO 2, CO, VOCs and NO x. The digital cameras made the films with the different frequencies of the frames per second and the different areas of the reactor were observed. The performed experiments enabled the studies of the kinetics of the investigated processes. 1. Experimental Site The experiments of the co-combustion processes of PC, PET, PC/ABS and PS with propane in the laboratory bubbling fluidised bed reactor took place. The reactor was the vertical oriented cylindrical quartz tube of the height of 500 mm, the diameter of 96 mm, and the wall thickness of 3 mm. The tube rested on the flat perforated Cr/Ni steel distributor, which thickness equalled to 1 mm. The perforation was made as the sequence of the circular holes spread in the quantities of 6.25 per 1 cm 2 and the diameters of the holes equalled to 0.6 mm. The bed was the quartz sand of the particle sizes from mm to mm, and the total bed mass was 300 g. During the experiments, the temperature of the fluidised bed, measured by two Cr/Ni-Ni thermocouples, located 20 mm and 50 mm above the distributor, was about 900 o C. The combustion process of propane was carried out with the air excess of about 50 %, the mixture of air and propane was supplied from the reactor plenum chamber to the fluidised bed, the streams of the gases equalled to dm 3 /s of air and dm 3 /s of propane. The laboratory position was equipped with four gas analysers for measuring

2 400 J. POŁOMSKA, W. ŻUKOWSKI, J. ZABAGŁO the concentrations of the selected components in the flue gases, and two digital cameras, which enabled to some precise observations of the processes: Ecom-SG Plus gas analyser: O 2, CO, NO, NO 2, SO 2 (electrochemical method) MRU Vario Plus gas analyser: O 2, CO, NO, NO 2, SO 2 (electrochemical method), CO 2, VOCs (infrared radiation method) Horiba VA-3000 and PG-250 gas analysers: CO 2, CO, N 2 O, SO 2 (infrared radiation method), O 2 (electrochemical method), NO, NO x (chemiluminescent acid method) J.U.M gas analyser: total VOCs (flame ionization detector method) Canon HV20 digital camera: 25 fps, 632 x 1440 pixels Samsung SC-HMX20C digital camera: 250 fps, 336 x 448 pixels Fig. 1 The laboratory fluidised bed reactor with the accompanying devices The data from the gas analyser devices were registered in the frequency of 1 Hz and were saved on the computer hard disc. One digital camera (Canon) enabled the continuous video film recording and the observations of the whole reactor, while the other (Samsung) made only ten-second video clips and it was mainly set to the perspective on the fluidised bed and just a small space above, the part of the reactor freeboard. The co-combustion

3 Combustion of Polymers in a Fluidised Bed Reactor 401 processes of the polymers were realized discretely, with some time spaces between the following appearance of the polymer samples in the fluidised bed reactor. The polymer samples were thrown into the hot fluidised bed from the open top of the reactor, where the flue gas probe was located. The quantities of the polymer particles, that created one polymer sample, were from three to five, and the total masses of the burned polymer samples were about from 100 mg to 500 mg. After the throwing of the polymer particles into the reactor, they sunk in the hot fluidised bed and mixed with the sand turbulently. The burning processes of the polymers contributed to some blazing flames created above the bed, in the reactor freeboard space. The stages of the experiments of the co-combustion processes of propane with the polymers brought about the lower concentrations of O 2 and the higher concentrations of CO 2, CO and VOCs in the flue gases, when compared with the only propane combustion. It was the proof that the oxidation processes arisen from the additional combustible substances and the co-combustion of them with the gaseous hydrocarbon fuel took place. 2. Description of Results The combustion of polymers have been studied by some authors yet [1-3] but the precise description of the thermal decomposition and the oxidation processes during combustion such polymers as PC, PET, PC/ABS and PS in fluidised bed reactors, that are analysed in this work, have not been described so far. The transparent quartz reactor tube made the visual observations of the investigated processes possible. The analyses of the pictures from the recorded video films enabled the studies of the combustion process kinetics. Fig. 2 The fluidised combustion in the sand bed: a) of propane, just after the ignition of the propane-air mixture; b) of propane, the bed temperature: 900 o C; c) of propane with the polymer sample (PC), the bed temperature: 900 o C

4 402 J. POŁOMSKA, W. ŻUKOWSKI, J. ZABAGŁO Fig. 3 The masses of the burned polymer samples with the quantities of the polymer particles in each sample, the concentrations of O 2, CO 2, CO, NO x and VOCs in the flue gases (according to Horiba and J.U.M. gas analysers) and the intensities of three colours - red, green, blue - in the pictures from the recorded video film (Canon digital camera) During the experiments, all the polymer particles of each specific polymer sample were thrown through the open top into the reactor at the same time moment. The thermal decomposition of the polymer particles started in the hot fluidised bed where each polymer particle was heated up by the direct heat transfer from the hot sand particles. The high temperature of the fluidised bed contributed to the depolymerisation processes and the bubbles around each polymer particle were created. Fig. 4 The polymer particle with the bubble of the thermal decomposition products around it in the fluidised bed and the flames above the bed

5 Combustion of Polymers in a Fluidised Bed Reactor 403 These bubbles were made up of the thermal decomposition products of the analysed polymers. It was observed that the bubbles of the thermal decomposition products around the polymer particles were very similar in their sizes. It was also noticed that the created bubbles were destroyed from time to time and then the thermal decomposition products flowed up through the hot sand bed and got out from it going to the freeboard. The bubbles were destroyed partially or totally by tearing them away from the surface of each polymer particle. In other words, not always the whole volumes of the bubbles were torn away and flowed above the bed, sometimes only some small upper parts of the bubbles were torn away giving the small blazing flames above the bed. When the thermal decomposition products of the polymers got out form the fluidised bed, the air diffused into the flames that were created from these substances. The air contributed that the oxidation processes of these thermal decomposition products of the polymers proceeded, and the flames vanished during going up. The created flames from these thermal decomposition products were gradually smaller when the burning polymer particles decreased during the process. During the experiments, from three to five polymer particles, that created each specific polymer sample, were burnt simultaneously in the fluidised bed, so it was very difficult to estimate the time spaces between the following formations of the flames from one polymer particle. All the applied devices enabled two types of data receiving. One type included the chemical data from the flue gas analysers, and the other referred to the recorded video films that were obtained for the observations and the visualization of the processes. Fig. 5 The emissions of CO during fluidised combustion of the polymers with propane in the sand bed of the temperature 900 o C and 50 % of air excess Fig. 6 The emissions of VOCs during fluidised combustion of the polymers with propane in the sand bed of the temperature 900 o C and 50 % of air excess

6 404 J. POŁOMSKA, W. ŻUKOWSKI, J. ZABAGŁO Fig. 7 The maximum concentrations of CO during fluidised combustion of the polymers with propane in the sand bed of the temperature 900 o C, calculated to 11 % of O 2 in the flue gases Fig. 8 The maximum concentrations of VOCs during fluidised combustion of the polymers with propane in the sand bed of the temperature 900 o C, calculated to 11 % of O 2 in the flue gases The flue gas analysers identified and gave the quantitative results of the selected chemical compounds. It is clearly seen in the figure that presented the time course of the experiments, that after the appearance of the polymer samples in the reactor, the concentration of O 2 in the flue gases decreased because of the consumption of it for the oxidative reactions. Simultaneously the emissions of CO 2, CO and VOCs increased and the time of the presence of these additional components in the flue gases could be compared to the time of the presence of the flames in the reactor freeboard space. When the emissions for all the analysed polymers were compared, the similar relationships between the maximum concentrations and the total emitted amounts of CO and VOCs could be observed but there were greater emitted amounts and concentrations of CO than VOCs. The precise studies of the sequences of the video pictures enabled to make the observations that the flames were created periodically and during the burning processes of the polymers in the fluidised bed there were some stages with the presence of the flames and without the flames above the bed. These observations were possible because of the technical parameters of the applied digital cameras. The frequency of 250 fps during the recording of the video clips enabled some more careful observations that were not possible in case of 25 fps video film. With using the digital camera with the time space of 4 ms between the following frames, the observations of the following individual flames were possible, while according to the video pictures with the time space of 40 ms could only be made the estimations of the co-combustion process duration. The obtained data made the possibility to estimate the whole time of each polymer sample co-combustion process duration by the direct observations of the time periods with the presence of the flames

7 Combustion of Polymers in a Fluidised Bed Reactor 405 above the fluidised bed. Besides, the studies of the intensities of three colours (red, green and blue) in the pictures could make the conclusion that red colour dominated what came from the radiation from the hot sand bed and from the hot flashing flames. The executed calculations enabled to make some conclusions: (A) the intensities of the flames above the bed depended on the total masses of the polymer samples, which were burnt in the fluidised bed; and (B) the frequencies of the flames, that appeared above the bed, depended on the quantities of the polymer particles, that created the polymer samples. Fig. 9 The visualization of the fluidised combustion of the polymers with propane in the sand bed, of the temperature 900 o C, by two digital cameras: Samsung: 250 fps, 336 x 448 pixels, and Canon: 25 fps, 632 x 1440 pixels Fig. 10 The scheme of the processing of the video frames for the calculations of the intensities of three colours - red, green, blue - in the pictures from the recorded video film (Canon digital camera)

8 406 J. POŁOMSKA, W. ŻUKOWSKI, J. ZABAGŁO The data from the digital cameras were processed according to the following algorithm: each colourful picture was divided into three matrixes that were made up of three basic colours (red, green and blue), and the sums of the numbers from 0 to 255 for each picture were made and next each sum was divided by the maximum sum that was possible. The intensities of the colours in the pictures were calculated as follows: [jpg] = [[red],[green],[blue]] rows x cols rows = height of picture in pixels cols = length of picture in pixels [colour] = [0,1,2,3,,255] max = 255 rows cols i = 1 rows j = 1 cols R G B, = i j, = i j red max, = i j green max blue max Fig. 11 The presentation of the periodicity of the combustion process of the polymers according to the intensities of three colours - red, green, blue - in the pictures from the recorded video clips (Samsung digital camera)

9 Combustion of Polymers in a Fluidised Bed Reactor 407 Fig. 12 The sums of the intensities of three colours in the pictures from the recorded video pictures during fluidised combustion of propane with PC (the sand bed temperature 900 o C) Fig. 13 The sums of the intensities of three colours in the pictures from the recorded video pictures during fluidised combustion of propane with PC/ABS (the sand bed temperature 900 o C) Fig. 14 The sums of the intensities of three colours in the pictures from the recorded video pictures during fluidised combustion of propane with PET (the sand bed temperature 900 o C) Fig. 15 The sums of the intensities of three colours in the pictures from the recorded video pictures during fluidised combustion of propane with PS (the sand bed temperature 900 o C) Fig. 16 The duration of the fluidised combustion process of the polymer samples with propane in the sand bed of the temperature 900 o C Fig. 17 The frequencies of the flames during the fluidised combustion process of the polymer samples with propane in the sand bed of the temperature 900 o C

10 408 J. POŁOMSKA, W. ŻUKOWSKI, J. ZABAGŁO In the presented graphs, the sums of the intensities of the colours in the pictures inform about the intensities of the flames during the burning processes of the polymers. The intensities of the flames above the bed were higher when the greater masses of the polymer samples were burnt in the fluidised bed, and the frequencies of the flames above the bed were higher while the greater quantities of the polymer particles were burnt simultaneously in the fluidised bed. The character of the frequencies of the flames may be the proof that the burning process of each polymer particle may proceed independently when appropriate technological conditions are maintained. Certainly the chemical structures of the burnt polymers influenced the crucial importance to the proceedings of the whole processes. 3. Conclusions The burning processes of the polymers in the fluidised bed reactor may be described in space and in time : a) in space the processes proceeded: in the hot fluidised bed where the depolymerisation processes of the polymers and the thermal decomposition processes while the oxidative pyrolysis processes took place and then above the bed, in the reactor freeboard space, where the combustion of the thermal decomposition products of the polymers in the blazing flames took place b) the intensities of the flames above the bed varied during the whole burning processes, what resulted in the various intensities of the colours, so in time the whole burning processes were periodic. References [1] Andrady A.L., Plastics and the Environment, Wiley-Interscience, Hoboken 2003 [2] Baron J., Bulewicz E.M., Kandefer S., Pilawska M., Żukowski W., Hayhurst A.N., The Combustion of Polymer Pellets in a Bubbling Fluidised Bed, Fuel 85, 2006, [3] Cullis C.F., The Involvement of Oxygen in the Primary Decomposition Stage of Polymer Combustion, Fire Safety Science: Proceedings of the First International Symposium, USA, 1986,