Characterization Study of Kütahya-Tunçbilek Lignite During Oxygen Enriched Combustion

Transcription

1 Characterization Study of Kütahya-Tunçbilek Lignite During Oxygen Enriched Combustion Özlem Uğuz *, Hanzade Haykırı-Açma +, Serdar Yaman + * Chemical Engineering Department, Faculty of Engineering, Marmara University, Istanbul, 34722, Turkey ozlem.uguz@marmara.edu.tr + Chemical Engineering Department, Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, Istanbul,34469, Turkey hanzade@itu.edu.tr yamans@itu.edu.tr Abstract Lignite is the most abundant energy source to generate energy in thermal power plants in Turkey. Thus, efficient and environmental-friendly use of lignite in electricity generation is of great importance. Because of this reason, clean coal technologies have been planned to achieve less emissions and increase the efficiency of the combustion in power plants. In this regard, oxygen enriched combustion (oxy-combustion) is regarded as one of the clean coal technologies, by which coal is burnt with oxygen concentrations higher than that in air. The aim of this study is to investigate the combined effects of temperature inside the furnace and oxygen ratio of the combustion atmosphere on the efficiency of coal combustion and the changes of the amounts of the C, H, N, S, O elements, mineral matter, and the morphologies of the residues which are left after combustion process. To do this, crushed and sieved lignite from Kütahya-Tunçbilek region were characterized by elemental, proximate and heating value analyses. After that, untreated sieved samples were subjected to heating at the temperatures of 200ºC, 450ºC, 800ºC under the atmospheres with the ratios of 21%O 2 +79%N 2, 30%O 2 +70%N 2, 40%O 2 +, and 50%O 2 + in the horizontal tube furnace. Residues obtained after the process were characterized by elemental, proximate, heating value analyses, followed by XRD, FTIR and SEM characterizations. According to the results, it was observed that the effect of oxygen enrichment was more pronounced at high operation temperatures. However, the ratio of oxygen is important as it causes carbonization to proceed if the oxygen concentration is low. Moreover, it was confirmed that increasing temperature and oxygen concentration in the atmosphere inside the horizontal tube furnace caused the residues to have lower contents of aliphatic, aromatic structures. However, carboxylic and etheric structures were seen to be more resistant to these parameters. SEM results indicated increasing ash formation as the treatment temperature and oxygen ratio increased. Keywords Lignite, Energy, Oxygen enriched combustion, Coal characterization, Oxygen ratio. I. INTRODUCTION Energy demand has a strong relationship with the industrialization and population growth rates. In order to generate energy, consumption of fossil fuels has the highest share among all the energy generation methods. Even though, it is an undeniable fact that burning fossil fuels is hazardous since it pollutes nature, contributes to global warming and harms human health, it is projected to stay as the main energy source in foreseeable future, more specifically in developing countries such as Turkey. Among all fossil fuel sources, coal is utilized in combustion processes for generation of energy in thermal power plants [1]-[3]. Combustion for energy production in thermal power plants is the major utilization way of coal. The efficiencies and pollutant emission levels of coal-fired power plants are critical for the economy and environment, and it can be significantly influenced by the coal combustion characteristics in boiler. Since it is well known that coal chemical structure is one of the key factors which determines coal combustion characteristics, scientists have studied the characterization tests thoroughly for the coal samples and utilized them by applying correct methods for decades [1]. However, the most important issue to be careful about is decreasing emissions and increasing efficiency while applying these coal utilization methods. Thus, governments have been promoting R&D on clean coal technology (CCT) for a more sustainable environment and economical energy generation [4]. Oxygen enriched combustion is one of the clean coal technology methods by which coal is combusted in an air atmosphere enriched in oxygen (>21%by volume). Reducing N 2 concentration or increasing O 2 concentration in the oxidizer stream leads to the enhanced temperatures inside the boiler, thus bringing about higher efficiency rates and improved stability of flame. Because of this and many more benefits of oxygen enriched combustion method, it can be applied in industrial applications which require high temperatures such as glass melting and cement kilns [5]. The associated objectives of this study are to investigate the binary effects of temperature and oxygen concentration by applying oxygen enriched combustion, determination of the required temperature and oxygen concentration in the horizontal tube furnace, and discussion of the effectivity of the combustion process from the characterization tests of coal burnout samples. II. EXPERIMENTAL STUDIES A. Sample and Sample Preparation In this study, lignite sample from Kütahya-Tunçbilek (KT) region was collected and crushed with a hammer. After letting the crushed sample dry under air atmosphere for 24 hours, it 360

2 was ground with a ring mill and the particle sizes of coals were decreased less than 250 µm with Retsch AS2000 sieves. B. Horizontal Tube Furnace Studies 5 grams of sieved KT coal sample was filled uniformly inside the quartz sample holder which was placed in the middle of the tube inside the horizontal tube furnace. Afterwards, the concentration ratios of O 2 and N 2 (21%O 2 +79%N 2, 30%O 2 +70%N 2, 40%O 2 +, and 50%O 2 + ) were adjusted by the valves connected to the O 2 and N 2 cylinders inside the furnace atmosphere. The total flow rate was 100 ml/min. After waiting 20 minutes for furnace atmosphere to come into equilibrium with the adjusted concentrations, parameters such as final temperature (200ºC, 450ºC and 800ºC), heating rate (10ºC/min), and the hold time at the final temperature (60 minutes) were introduced to the PID controller and heating process took place. After heat treatment under the arranged concentrations of oxygen and nitrogen at selected temperatures, burnout samples were left to cool down to the ambient temperature. C. Characterization Tests Proximate analyses (TA Instruments SDT Q600), ultimate analyses, heating value determination tests (IKA C2000) were done on the sieved coal samples. XRD analyses were performed by using Panalytical X Pert Pro PW 3040/60 model X-Ray diffractometer with a Cu X-ray target under the conditions of 40 kv and 40 ma. FTIR spectroscopy (Perkin Elmer FTIR Analyzer) was applied to identify organic bonds within the structure of all samples. Morphologies of samples were determined with the aid of SEM Analyzer, Jeol JSM with the magnification of 600X. III. RESULTS AND DISCUSSION Ground and sieved KT samples were heat treated at 200ºC, 450ºC and 800ºC under the atmospheres having concentrations of 21%O 2 +79%N 2, 30%O 2 +70%N 2, 40%O 2 +, and 50%O 2 +. Proximate analyses of untreated KT sample and burnouts at given temperatures and atmospheres are presented in Table I. TABLE I PROXIMATE ANALYSIS RESULTS Conditions M% VM% FC% A% Sieved coal sample ºC dry air ºC- 30%O 2,70%N ºC-40%O 2, ºC-50%O 2, ºC-dry air ºC-30%O 2, 70%N ºC-40%O 2, ºC-50%O 2, ºC-dry air ºC-30%O 2, 70%N ºC-40%O 2, ºC-50%O 2, According to Table I, heat treated KT samples at 200ºC had the lowest moisture percentages among almost all samples, while volatile matter (VM) percentage didn t have a considerable change in comparison with the untreated sieved KT sample. Increased fixed carbon (FC) percentage at 200ºC can be explained with the carbonization process. At 450ºC, partial oxidation took place. As the oxygen ratio of the treatment atmosphere increased, burning proceeded easier. Moisture percentage of the samples obtained after treatment at 450ºC were higher than those of 200ºC due to the evolution of fissures and cracks in the coal structure. Increasing ash content can be given as an evidence of the burning process inside the furnace. At 800ºC, the effect of oxygen ratio was more evident, since there was a sharp decrease in the amount of fixed carbon and increase in the amount of the ash, as the oxygen concentration share changed from 21% to 50%. However, the increase in the amount of fixed carbon for the sample treated at 800ºC under the atmosphere of 21%O 2 +79%N 2 can be explained with the occurrence of carbonization. Ultimate analysis results of the untreated sieved KT samples and burnouts which were treated at given temperatures and atmospheres are presented in Table II. TABLE II ELEMENTAL (ULTIMATE) ANALYSIS RESULTS Conditions C% H% N% S% O% Sieved coal sample ºC dry air ºC- 30%O 2,70%N ºC-40%O 2, ºC-50%O 2, ºC-dry air ºC-30%O 2, 70%N ºC-40%O 2, ºC-50%O 2, ºC-dry air ºC-30%O 2, 70%N ºC-40%O 2, ºC-50%O 2, Results given in Table II are in accordance with the results of Table I. Carbon percentages of the samples which were heat treated at 200ºC slightly increased due to the carbonization effect. Hydrogen percentage was less when compared to the untreated sieved KT sample because of the moisture removal from the coal matrix. Even though nitrogen and sulphur amounts are fixed at this temperature, their percentage is higher than the untreated sample because of the removal of other volatile compounds. At 450ºC, carbon, and hydrogen amounts decrease due to the volatile combustion, 361

3 while oxygen amount increases, as it is not a combustible compound. At 800ºC, the changes of carbon, hydrogen, nitrogen, sulphur and oxygen were more clear due to the combustion of volatile matter and char. Almost complete combustion was achieved at 800ºC under the atmosphere having 50% O 2 and 50% N 2. Higher heating values of all samples were given in the Table III. Fig. 1 depicts the FTIR spectra of the ground KT lignite and the burnouts which were left after burning under the atmospheres having 21%O 2 +79%N 2, 30%O 2 +70%N 2, 40%O 2 +, 50%O 2 + at 200ºC, 450ºC, 800ºC. TABLE III HIGHER HEATING VALUES OF SAMPLES Conditions Higher Heating Value (HHV) (cal/g) Sieved coal sample ºC dry air ºC- 30%O 2,70%N ºC-40%O 2, ºC-50%O 2, ºC-dry air ºC-30%O 2, 70%N ºC-40%O 2, ºC-50%O 2, ºC-dry air ºC-30%O 2, 70%N ºC-40%O 2, ºC-50%O 2, 71 Based on the results which are given in Table III, it can be deduced that the data are quite consistent with the proximate and ultimate analysis results of KT samples. HHV of KT lignite can be regarded as high in comparison with most of the lignites which are mined in basins of Turkey. Ground original KT lignite had the HHV of 5917 cal/g and that value increased as the experiments were carried out at 200ºC. This increase can be explained by the moisture loss from the original sample and carbonization at 200ºC. As the oxygen share of the atmosphere inside the furnace was elevated to 50%, it was observed that HHV decreased significantly compared with the HHVs of the residues which were treated under the atmospheres with the oxygen shares of 21%, 30% and 40%, and 5.78% of the HHV of the ground original KT sample was lost. This situation means that some components such as hydrogen and carbon started to leave the coal matrix. Shifting the temperature to 450ºC and 800ºC caused serious reductions in the HHVs of the residues. The influence of oxygen concentration became more pronounced as the burning temperature got higher At 450ºC, the HHV decreases were 11.9%, 19.1%, 28.3% and 35.9% of the original ground KT sample for the residuals which were burned under the atmospheres which had the oxygen shares of 21%, 30%, 40% and 50%, respectively. At 800ºC, the differences between HHV values changed to a wide extent and the original KT sample lost 16.7%, 37.7%, 79.3% and 98.8% of its HHV when it was burnt under atmospheres which have oxygen share of 21%, 30%, 40%, and 50%. Fig. 1 FTIR spectra of the ground original KT lignite and the residues which were left after burning under the atmospheres having 21%O 2+79%N 2, 30%O 2+70%N 2, 40%O 2+, 50%O 2+ at 200ºC, 450ºC, 800ºC. According to Fig. 1, the bands which were observed at around 3699 cm -1, 3651 cm -1, 3620 cm -1 were assigned to OH vibrations because of the clay minerals, mostly kaolinite. As the temperatures increased, these bands tended to diminish. Broad bands between 3595 cm -1 and 3000 cm -1 corresponded to the occurrences of OH groups of phenols, alcohols, carboxylic acids and moisture. Because of the heat treatment 362

4 this broad peak also decreased in intensity. The bands observed at 2923 cm -1 and 2850 cm -1 can be assigned to asymmetric and symmetric stretching of CH groups, respectively. The band around 1700 cm -1 showed the existence of C=O vibrations in carbonyl groups. The samples exhibited bands at around 1600 cm -1 due to C=C vibrations in aromatic structures. Sharp bands between 500 cm cm -1 are attributed to the minerals. Their intensity increased as the temperature and the oxygen concentration increased because of ash formation. Fig. 2 shows the XRD analyses of untreated and heat treated samples. Ground sieved KT sample had an abundancy of quartz (SiO 2 ) and kaolinite (Al 2 Si 2 O 5 (OH) 4 ) as well as considerable amounts of pyrite (FeS 2 ), gypsum (CaSO 4.2H 2 O) and albite (NaAlSi 3 O 8 ). For the residues obtained after thermal treatment at 200ºC showed that there was no any significant change in the type of the minerals observed. However, the disappearance of the slight peak at 2θ=31.127, which is the major peak for gypsum mineral, was seen to decrease as a result of crystalline water loss. As the temperature increased further, decomposition of the aforementioned minerals took place. Kaolinite, which exhibited diffraction peaks at º and º, started to decompose resulting in the evolution of corundum (Al 2 O 3 ) compound and the peaks completely disappeared at 800ºC under 30%O 2 +70%N 2 atmosphere. Pyrite mineral underwent a reaction with oxygen to give hematite (Fe 2 O 3 ) and release SO 2. Lime (CaO) was also observed from XRD results. It showed that CaSO 4 mineral decomposed into its components resulting in the evolution of lime. A minor amount of periclase (MgO) was found from the residues obtained after burning at 800ºC. The diffraction peaks of quartz mineral increased in intensity as temperature increased, because the decomposition of quartz is very resistant to temperature. Fig. 2. XRD spectra of the ground original KT lignite and the residues which were left after burning under the atmospheres having 21%O 2+79%N 2, 30%O 2+70%N 2, 40%O 2+, 50%O 2+ at 200ºC, 450ºC, 800ºC. SEM images of the untreated and treated samples are given in Fig. 3,Fig.4, Fig. 5, Fig

5 Fig. 3. SEM image of the ground and sieved KT sample. Fig.6 SEM images of KT residues obtained after thermal treatment at 800ºC under atmospheres of a) dry air, b) 30%O 2+70%N 2, c) 40%O 2+, d)50%o 2+ (from left to right). At 200ºC, the samples formed fissures and cracks as the oxygen concentration increased. As temperature was elevated to 450ºC, particles were fragmented and ash formation took place because of the bright areas in the figure. Fragmentation and the brightness increased as temperature was 800ºC. IV. CONCLUSION Fig. 4. SEM images of KT residues obtained after thermal treatment at 200ºC under atmospheres of a) dry air, b) 30%O 2+70%N 2, c) 40%O 2+, d)50%o 2+ (from left to right) Fig. 5 SEM images of KT residues obtained after thermal treatment at 450ºC under atmospheres of a) dry air, b) 30%O 2+70%N 2, c) 40%O 2+, d)50%o 2+ (from left to right). In this study, Kütahya-Tunçbilek lignite, a high quality lignite in Turkey, was used and the combined effects of oxygen enrichment as well as temperature was investigated. The effect of oxygen enrichment was more pronounced at high operation temperatures. However, the ratio of oxygen was important as it caused carbonization to proceed if the oxygen concentration is low. Moreover, it was confirmed that increasing temperature and oxygen concentration in the atmosphere inside the horizontal tube furnace caused the residues to have lower contents of aliphatic, aromatic structures. However, carboxylic and etheric structures were seen to be more resistant to these parameters. SEM results indicated increasing ash formation as the treatment temperature and oxygen ratio increased REFERENCES [1] J. Xu, H. Tang, S. Su, J. Liu, K. Xu, K. Qian, Y. Wang, Y. Zhou, S. Hu, A. Zhang and J. Xiang, A study of the relationships between coal structures and combustion characteristics: The insights from micro- Raman spectroscopy based on 32 kinds of Chinese coals, Appl.Energ., vol.212, pp.46-56, Feb [2] S.S Daood, W. Nimmo, P. Edge and B.M. Gibbs, Deep staged, oxygen enriched combustion of coal, Fuel, vol.101,pp ,nov [3] X. Liu, M. Chen, and D.Yu, Oxygen enriched co-combustion characteristics of herbaceous biomass and bituminous coal, Thermochimica Acta,vol. 569,pp ,Oct [4] G. Guan, Clean coal technologies in Japan: A review, Chinese Journal of Chemical Engineering, vol.25, pp , June [5] U. Kayahan and S. Özdoğan, Oxygen enriched combustion and cocombustion of lignites and biomass in a 30 kwth circulating fluidized bed, Energy, vol.116, pp , Dec