Energy & Fuels 2007, 21, 3385–3387
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The Effect of Upgrading Processes on Combustion Characteristics of Berau Coal Datin Fatia Umar,*,† Binarko Santoso,† and Hiromoto Usui‡ R&D Centre for Mineral and Coal Technology, Jalan Jenderal Sudirman 623, Bandung 40211, Indonesia, and Department of Chemical Science and Engineering, Kobe UniVersity, 1-1 Rokkodai-cho, Nada-ku, Kobe-shi, Hyogo 657-8501, Japan ReceiVed February 1, 2007. ReVised Manuscript ReceiVed July 4, 2007
Indonesian coal was upgraded by upgraded brown coal (UBC), hot water drying (HWD), and steam drying (SD) processes to study the combustion characteristics of this coal. The differential thermal–thermogravimetric analysis (DTA–TG) was carried out to obtain some combustion parameters, such as, ignition temperature (Tig), maximum combustion rate temperature (Tmax), maximum combustion rate (Rmax), char burn-out temperature (Tbo), and ash yield. The differential thermal analyzer (DTA) curve can be used to estimate the heat released during the combustion process, which corresponds to the high heating value of the coal. The results show that the combustion characteristics of HWD and SD processes are much better than those of the UBC process. This is because the UBC process was conducted at lower temperature and pressure.
1. Introduction Many coal resources are found almost all over the Indonesian islands, and most of them are classified as lignite and subbituminous coals, which are referred to as low-rank coal. The coal resources are approximately 61 billion tons,1 and some of them have been mined since 1892.2 To maintain the world consumption of coal increased year by year, the utilization of low-rank coal should be taken into consideration extensively. However, the utilization of low-rank coal is restricted by factors like high moisture and volatile matter contents, low heating value, high propensity to low-temperature oxidation, and spontaneous combustion. Upgrading of low-rank coals in order to reduce the moisture content is an interesting work in the coal research area, since this effort can suppress the low-temperature oxidation, self-heating, and spontaneous combustion during transportation and storage. Various dewatering and upgrading processes have been developed since the 1920s.3 Among them are upgraded brown coal (UBC), hot water drying (HWD), and steam drying (SD) processes. This paper reports the effect of the upgrading process by the UBC, HWD, and SD processes on combustion characteristics. Understanding the combustion characteristics of coal would help design and maintain boiler, maximize burning efficiency, and assist in reducing carbon particle emissions. Thermal analysis techniques have been widely used. The thermal analysis data can be applied not only for the characterization of different coals, * To whom correspondence should be addressed. Fax: 62-22-6038-027. E-mail:
[email protected]. † R&D Centre for Mineral and Coal Technology. ‡ Kobe University. (1) Hadiyanto. Anatomi sumber daya batubara serta asumsi pemanfaatan untuk PLTU di Indonesia. Seminar Nasional Batubara Indonesia. Pusat Sumber Daya Geologi, Bandung, 2006 (In Indonesian). (2) Sigit, S. Coal development in Indonesia, past performance and future prospects; In Proceedings of Seminar Coal Technology and the Indonesian Needs; Mangunwijaya, A. S., Hasan, O.; Eds.; Jakarta, Indonesia, 1980. (3) Suwono, A.; Hamdani. Upgrading the Indonesian’s low rank coal by superheated steam drying with tar coating process and its application for preparation of CWM. Coal Preparation 1999, 21, 149–159.
but also for the evaluation of combustion performance at high temperatures and heating rates. One of these techniques is differential thermal analyzer (DTA) and thermogravimetric (TG) analysis. DTA and TG, monitoring of the rate of mass loss and the relative mass loss in air, respectively, as a function of temperature, have been shown to be effective tools for studying coal combustion behavior.4 Since only a small sample size is required in the analysis, the burning profile is most useful for evaluating the burning properties of fuel when only small samples are available or when it is impractical to test large quantities of fuel in an existing installation.5 2. Experimental Procedures In this study, a low-rank coal from Berau, East Kalimantan, was used. This coal is characterized as sub-bituminous coal with an inherent moisture content of 18.03%. The coal sample was upgraded by the UBC, HWD, and SD processes. 2.1. Upgrading Processes. The UBC process was conducted in the UBC pilot plant at Palimanan, Cirebon, West Java, with 5 tons/day capacity. The UBC process consists of five main sections, namely, coal preparation, slurry dewatering, coal–oil separation, oil recovery, and coal briquetting.6 The raw coal was ground to a grain size of under 3 mm and was mixed with kerosene and low sulfur wax residue (LSWR) to prepare slurry. The mixing ratio of fine coal to oil is about 1:1. The addition of LSWR of about 1% was very important to prevent the re-absorption of moisture. The slurry was sent to a dewatering vessel through a vaporizer and, the moisture of coal is decreased by heating. The dewatered slurry and the water that was evaporated were separated in the gas–liquid (4) Crelling, J. C.; Hippo, E. J.; Woerner. Combustion characteristics of selected whole coals and macerals. Fuel 1992, 71, 151–158. (5) Ma, S.; Hill, J. O; Heng, S. J. A thermal analysis study of the combustion characteristics of Victorian brown coal. J. Therm. Anal. 1989, 35, 1985–1996. (6) Daulay, B.; Salim, A. D.; Otaka, Y.; Deguchi, T.; Makino, E.; Rijwan, I.; Umar, D. F. Pilot plant of upgraded brown coal (UBC) at Palimanan, Cirebon: a challenge of utilizing lignite in Indoneisa. Proceedings of the 4th International Conference and Exhibition on Coal for Power and Economic DeVelopment; Jakarta, 2003.
10.1021/ef070061j CCC: $37.00 2007 American Chemical Society Published on Web 10/03/2007
3386 Energy & Fuels, Vol. 21, No. 6, 2007
Umar et al.
Table 1. Characteristics of the Raw and Upgraded Coalsa upgraded coal analysis Proximate: inherent moisture, wt % ad ash, wt % ad volatile matter, wt % ad fixed carbon, wt % ad Ultimate: carbon, % daf hydrogen, % daf nitrogen, % daf total sulphur, wt % daf oxygen, % daf calorific value, MJ/kg, adb a
standard
raw coal UBC 4.81
SD
HWD
1.35
1.58
ASTM D 3173-00
18.03
ASTM D 3174-00 ASTM D 3175-01
7.76 45.38
3.28 0.85 1.11 49.05 42.96 43.81
by difference
46.86
47.67 56.19 55.08
ASTM ASTM ASTM ASTM
D 3178-89 D-3178-89 D-3179-89 4239-02
75.40 8.69 2.12 0.74
71.59 77.15 76.05 6.82 5.31 5.27 1.12 1.21 1.05 0.52 0.56 0.42
by difference ASTM D 5865-04
13.05 21.84
19.95 15.77 17.21 26.27 29.59 29.84
ad, air-dried basis; daf, dry ash-free basis.
separator. The oil that was evaporated was reused as the oil that was recovered by condensation and recycled for the next slurry dewatering. Oil recovery greater than 99% could be achieved from material balance of the whole process. UBC powder was discharged from the outlet of the dryer at about 170 °C as the primary product. To make the transportation easy, the UBC powder needs to be briquetted by using a double roll briquetting machine without the addition of a binder.7 The HWD and SD processes were conducted on laboratory scale by using an autoclave with 5000 mL/batch capacity. In the HWD process, the pulverized raw coal (ca. 1500 g) was mixed with water (1:1) and was heated at a temperature of 300 °C for 1 h as the optimum condition of process.8 In the SD process, the crushed and screened coal of -2 + 1 cm in size was heated at temperature of 275 °C by steam for 1 hour.9 The atmosphere in the autoclave both for HWD and for SD processes was purged and 10 kg/cm2 pressure was provided by N2. The autoclave was heated at a rate of 3–4 °C/min to reach the temperature of 300 °C (for HWD process) and 275 °C (for SD process) and pressure of about 12 MPa. 2.2. Coal Characteristics. The characteristics of the raw and upgraded coals were analyzed according to the ASTM Standard as can be seen in Table 1. 2.3. Combustion Characteristics. The DTA–TG test was carried out by using a DTG-60 SHIMADZU apparatus. The raw and upgraded coal samples were milled to less than 75 µm in size. The size was chosen because it is used at pulverized fuel power plants. The sample weight of 5 mg was placed in a platinum cell, air flow rate was 25 mL/min, and heating rate was 10 °C/min. Maximum experimental temperature was 800 °C. From the DTA–TG curves, a number of combustion parameters can be derived, such as, ignition temperature (Tig), maximum combustion rate temperature (Tmax), maximum combustion rate (Rmax), char burn-out temperature (Tbo), and ash yield.10 Ignition temperature (Tig) is an important characteristic of coal combustion, especially for low-rank coal due to its high intensity of spontaneous combustion.11 The ignition temperature is taken as the extrapolated onset temperature of the first peak of the DTA curve, which also corresponds to the temperature at which the TG curve departs from the base line. Ignition temperature in this work corresponds to the Tig of the volatile matter. (7) Umar, D. F.; Usui, H.; Daulay, B. Change of combustion characteristics of Indonesian low-rank coal due to upgraded brown coal process. Fuel Process. Technol. 2006, 87 (11), 1007–1011. (8) Umar, D. F.; Usui, H.; Daulay, B. Effect of processing temperature of hot water drying on the properties and combustion characteristics of an Indonesian low-rank coal. Coal Preparation 2005, 25 (4), 313–322. (9) Umar, D. F.; Usui, H.; Daulay, B. Upgrading of Indonesian lowrank coal by steam drying method. Indonesian Min. J. 2005, 8 (1), 21–26. (10) Umar, D. F.; Usui, H.; Daulay, B. Characterization of upgraded brown coal. Coal Preparation 2005, 25 (1), 313–322.
Figure 1. DTA curves for the raw and upgraded coals.
Tmax is the temperature at which the maximum rate occurs, and Rmax from the DTG curve relates to the maximum combustion rate. The peak value of Rmax of the DTG curve gives an indication of the intensity of combustion. Mass loss on the TG curve represents the amount of coal burned out. Rmax is defined as in eq 1: Rmax ) [d(TGA) ⁄ dt]T)Tmax
(1)
Char burn-out temperature (Tbo) is defined as temperature at the minimum DTA peak after Tmax, while ash yield is a ratio of the sample weight remaining at the temperature of 800 °C to initial stage, as given by eq 2: ash yield ) TGAf ⁄ TGAi × 100%
(2)
3. Results and Discussion As can be seen in Table 1, the inherent moisture of the upgraded coals was significantly decreased. The HWD and SD processes reduce moisture content more than the UBC process does. This could be understood, because the UBC process was operated at lower temperature and pressure compared with the HWD and SD processes. Consequently, the specific energy of all of the upgraded coals was significantly increased. DTA curves of the raw and upgraded coals are shown in Figure 1. The curves illustrate the heat differentiation that was released during the test. There are three DTA peaks for all of the raw coals. The first and second DTA peaks appearing at around 60 °C (endothermic) and 330 °C (exothermic) are due to the vaporization of moisture and combustion of volatile matter, respectively. However, the third peak at around 420 °C represents the combustion of char.5 For all the upgraded coals, the first peaks were significantly decreased because of the removal of moisture during the upgrading process. The second peaks, which correspond to the removal of volatile matter, of the upgraded coal that was upgraded by the UBC process were not obviously changed, because the UBC process was conducted at mild temperature. For the upgraded coals obtained by the HWD and SD processes, the second DTA peaks were slightly decreased. Furthermore, the third peaks of all of the upgraded coals were significantly increased. The increase of the third DTA peaks of all of the upgraded coals demonstrated that major heat release in combustion of upgraded coals had taken place.
Combustion Characteristics of Berau Coal
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Table 2. Combustion Parameters Based on DTA–TG Analysis sample marks Tig, °C Tmax, °C Tbo, °C Rmax, mg/min ash yield, wt % raw coal UBC SD HWD
280 290 304 293
439 439 460 453
796 793 785 789
0.27 0.29 0.34 0.41
1.42 1.62 1.58 1.19
Table 3. The Calculated and Measured HHV of the Raw and the Upgraded Coals sample marks
DTA max peak, µV
A/TGAi, µV · s/mg
HHV measured, MJ/kg
HHV calculated, MJ/kg
raw UBC SD HWD
414 496 524 584
48 447 55 361 58 020 59 323
21.84 26.27 29.59 29.84
22.10 27.08 28.99 29.92
The Tbo reflecting the char characteristics was almost unchanged or was slightly decreased due to the upgrading treatments. Compared with the combustion characteristics of high-rank coal, the char characteristics of the upgraded coal are widely different from the char characteristics of high-rank natural coal.11 Since the area under the DTA curve is interpreted as the heat change during the whole processes, the DTA curve can be used to estimate heat that is released during combustion process, which corresponds to the high heating value of coal. In this study, the relationship of the calculated high heating value (HHVcalc, MJ/kg) and area under the DTA curve, A (µV · s) is represented by eq 3 with correlation coefficient of r ) 0.99: HHVcalc ) -12.73 + (7.19 × 10-5 × A ⁄ TGAi) Figure 2. TG curves for the raw and upgraded coals.
Based on this fact, it seems that the heating value of the upgraded coals was higher than that of the raw coals. The TG curves of the raw and upgraded coals illustrate the relative weight loss (Figure 2). Weight loss below 150 °C is related to the removal of moisture, and that of above 150 °C is due to the combustion of volatile matter and char. Table 2 shows the summary of combustion parameters that were obtained from the DTA–TG. The Tig value corresponds to the ignition temperature of the volatile matter. In this study, the Tig values of all of the upgraded coals were increased. The increase of Tig that was caused by the UBC process is the lowest compared with those of the HWD and SD processes. The SD process shows the highest Tig followed in decreasing order by HWD, UBC, and raw coal. The increase of Tig is ascribed to the decrease of the volatile matter, since the ignition for higher rank coals are controlled by the volatile content. However, it is difficult to cite the general comparison between higher and lower rank coals, because the ignition of low-rank coal is largely influenced by the reactivity of the oxygen. In general, those coals with low ignition temperature and high mass loss in the low temperature range can be considered as easy to ignite and burn out.7 Tmax relates to coal reactivity; reactive coal has a low Tmax. It can be seen that the Tmax of the upgraded coal obtained by the UBC process is not changed. The change achieved by HWD and SD processes are quite small, reflecting the char characteristics demonstrating almost no change. Different from Tmax, Rmax values, which indicates the maximum combustion rate, for all of the upgraded coals are higher than that of the raw coal. This demonstrates that the upgraded coals were easy to burn due to high calorific values and less moisture content. (11) Mahidin; Ogaki, J.; Usui, H.; Okuma, O. The advantages of vacuum treatment in the thermal upgrading of low-rank coals on the improvement of dewatering and devolatilization. Fuel Process. Technol. 2003, 84 (1–3), 147–160.
(3)
The calculated and observed HHVs of raw and upgraded coals are shown in Table 3. It can be seen that the HHV of all of the upgraded coals was significantly higher rather than that of the raw coal based on the higher peaks of the upgraded coals. Table 3 indicates that the HHVs that were calculated from DTA analyses differ from the HHVs that were measured using a bomb calorimeter according to ASTM standard D-3286. This might be explained by the equation deviation of eq 1 that was used. The HHV of the upgraded coals obtained by the HWD and SD processes was higher compare with that of the raw coal and upgraded coals obtained by the UBC process, based on higher peaks of their upgraded coals. Conclusion Based on the combustion characteristics that were evaluated by the DTA–TG analysis, the upgraded coals obtained by the HWD and SD processes were generally better compared with those of the upgraded coal obtained by the UBC process. These are reflected in higher Tig, Tmax, Rmax, and DTA maximum peaks that result in high calorific value. This could be understood, because the UBC process was held in lower temperature and lower pressure compared with those of the HWD and SD processes. Acknowledgment. The authors gratefully acknowledge the JSPS Ronpaku fellowship program and R&D Centre for Mineral and Coal Technology (tekMIRA) for making it possible to do this research. Thanks also go to the staff at the coal laboratory of tekMIRA for their full assistance during the laboratory tests. Supporting Information Available: Raw data from DTA–TG experiments. This material is available free of charge via the Internet at http://pubs.acs.org. EF070061J
