2006
Energy & Fuels 2005, 19, 2006-2013
Performance of Chinese Coals under Conditions Simulating Entrained-Flow Gasification Baomin Wang, Xiaoyu Li, and Shisen Xu Thermal Power Research Institute, Xi’an, People’s Republic of China
N. Paterson,* D. R. Dugwell, and R. Kandiyoti Department of Chemical Engineering, Imperial College, London SW7 2AZ, United Kingdom Received January 12, 2005. Revised Manuscript Received April 11, 2005
A novel, two-stage entrained-flow gasifier is being developed by the Thermal Power Research Institute in China. In the present work, the factors that affect the pyrolysis and gasification reactivity of 14 potential coals for this plant have been studied, under conditions that simulate the temperature, pressure, and particle heating rates in the proposed gasifier. The work was performed using a high-temperature, high-pressure, variable-heating-rate wire-mesh apparatus (2000 °C, 3.0 MPa). The trends in reactivity, measured for three of the coals at laboratory scale, and a 20-kg/h (small) pilot plant in China were similar. The use of the wire mesh reactor (WMR), coupled with a residual char reactivity measurement, using a thermogravimetric analyzer, has proved useful in characterizing coals selected for entrained-flow gasification. The data have allowed the comparison of reactivities within the suite of coals. The results showed a general trend of declining reactivity with increasing rank. However, the scatter in the data, as in most such cases, does not allow the construction of simple correlations to help predict the performance of individual coals; therefore, the reactivities of individual coals must be evaluated experimentally. The pilot gasifier that is under construction is intended to achieve almost-complete conversion of the coal injected into both stages. The results suggest that overall conversions in the hightemperature first stage of the reactor will be only marginally sensitive to sample reactivity. However, the second stage of the pilot reactor is expected to give an exit gas temperature of ∼900 °C. It is likely that the reactivity of the coals injected into the second stage will have a greater effect on achieving the desired high conversions. The ash melting temperature and slag viscosity for the same suite of coals have been assessed using existing mathematical models, which were primarily developed for combustion-based applications. This has shown that the ash and slag properties will vary widely, depending on the composition of the ash. Unsuccessful attempts were made to measure the viscosity of a slag sample from a pilot-scale gasifier in China under the appropriate conditions. However, the work has highlighted the difficulties involved in making valid measurements for gasifier slags, which should help to guide future work.
Introduction Currently, coal provides more than 23% of the world’s total primary energy and generates ∼38% of the world’s electricity.1 The use of coal for power generation is projected to increase by 60% by 2030.2 However, the increased use of coal gives rise to several issues of serious environmental concern, including the increased release of fossil-derived carbon dioxide, particulates, and trace elements. Cleaner coal technologies (CCT) have been developed and demonstrated to minimize these emissions. O2/steam-blown entrained-flow gasifiers have been successfully demonstrated in integrated gasification combined cycle (IGCC) projects in Europe (Shell and Prenflo technologies) and the United States (Texaco * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Key World Energy Statistics; International Energy Agency: Paris, 2003. (2) World Energy Outlook 2002; International Energy Agency: Paris, 2002.
technology).3 There are also plans to build a demonstration IGCC plant (300-400 MW) in China, and this plant is intended to be based on entrained-flow gasification technology. There is experience with the Texaco slurryfed system in China, but the cold efficiency is lower, because of the losses incurred through the use of the water in the slurry. Dry feed systems are inherently more efficient but have not been tested in China. The final choice of gasification technology has not been made yet; however, it is likely to be based on a dry feed gasifier, because of its higher cold gas efficiency. Both commercial and in-house designs are under consideration. The benefits of the application of IGCC technology in China are clear, as it provides a route to minimize the impact of the increasing energy needs within that country and on the global climate. Currently, the Thermal Power Research Institute (TPRI) in Xi’an, (3) Bee´r, J. M. Prog. Energy Combust. Sci. 2000, 26, 301-327.
10.1021/ef050014w CCC: $30.25 © 2005 American Chemical Society Published on Web 08/31/2005
Performance of Chinese Coals under Gasification
Figure 1. Schematic diagram of the wire mesh reactor (WMR).
Shaanxi Province, China, is constructing a novel 2436 tonne/day, two-stage dry feed coal gasifier.4 Meanwhile, the suitability of Chinese coals for use in dryfeed entrained gasifiers has been studied within a collaborative project between TPRI and Imperial College in London. A suite of coals was provided by TPRI, and these have been used to study both the reactivity and the slagging behavior of coals that could be used in the envisaged commercial plants. The reactivity of the coals has been studied using a wire mesh reactor (WMR). This was modified at the beginning of the work, to enable it to operate under the high temperatures and pressures used in entrained-flow gasifiers. The development of the reactor that enables it to operate at temperatures up to 2000 °C and pressures up to 3.0 MPa for the characterization of coal samples, under conditions relevant to entrained-flow gasification, has been discussed elsewhere.5 The present paper describes the results obtained using this apparatus to study a set of 14 coals from China. The slag viscosity has been investigated using some established models. The results obtained and their limitations are described. Experimental Section The High-Pressure Wire-Mesh Reactor. A schematic diagram of the wire mesh reactor (WMR) is shown in Figure 1. The main features and characteristics of the high-pressure version have been described elsewhere.6-9 The WMR is a laboratory-scale device used to heat a small amount of pulverized solid particles (6 mg) under computer-controlled conditions. A variety of gases can be used to study processes such as pyrolysis (helium or nitrogen), gasification (carbon dioxide or steam), and combustion (air or oxygen). Process yields are determined by the weight difference of the sample before and (4) Yongqiang, R.; Shisen, X.; Yue, X.; Chungang, C.; Juncang, X. In Conference Proceedings for the International Hi-Tech Symposium on Coal Chemical Industry and Coal Conversion, Shanghai, PRC, 2004; pp 172-174. (5) Peralta, D.; Paterson, N.; Dugwell, D.; Kandiyoti, R. Pyrolysis and CO2 Gasification of Chinese Coals in a High-Pressure Wire-Mesh Reactor under Conditions Relevant to Entrained-Flow Gasification. Energy Fuels 2005, 19, 532-537.
Energy & Fuels, Vol. 19, No. 5, 2005 2007 after testing and reported on a dry, ash-free (daf) basis. The high-pressure WMR was modified as part of this project, so that it could operate at temperatures up to 2000 °C and pressures up to 3.0 MPa. Type D high-temperature thermocouples were commissioned for use in this set of experiments. The conductor combinations were tungsten 97% with rhenium 3% (positive pole) and tungsten 75% with rhenium 25% (negative pole). This pair can be used at temperatures up to 2400 °C. The material is known to harden at high temperature; however, this did not cause any problems in this work, because a fresh thermocouple was made for each test. New ports for this set of thermocouples were installed in the reactor controllers, together with the corresponding calibration curve. An insulating sheet made of alumina was used to prevent the mesh from contacting the support plate and to avoid a short circuit. A molybdenum mesh was used to withstand the very high temperatures needed for this work. In helium, the molybdenum mesh did not suffer any physical damage, even at the highest temperature of 2000 °C, whereas in CO2, it became very brittle at this temperature. In addition, the rates of reaction were very high at 2000 °C and complete conversion of the samples occurred, which meant that differences between the sample reactivities were not apparent. Therefore, in CO2, the upper temperature was reduced to 1500 °C to preserve the integrity of the mesh and to enable the discrimination of different reactivities. The heating rate used in this work was 1000 °C/s. This is lower than the heating rate in an entrainedflow gasifier, where the rate can be as high as 5000 °C/s. However, previous work in this laboratory has shown that, in the WMR, the heating rate does not have a significant impact on the behavior at rates of 1000 °C/s and above.10 This technique is not suggested to provide fundamental kinetic data on gasification that can be applied to predicting the performance of large-scale gasifiers, because it measures the extents of reaction by pyrolysis and by gasification in CO2 only. In this work, factors such as the higher reaction rate in the presence of steam, inhibition by CO and H2, and the various dynamic influences that affect overall performance have not been studied, because our intention was to compare extents of reaction of different coals, at partial conversion. However, the method does provide a relatively simple means of establishing the order of reactivities within a suite of coals and the trends in reactivity when the operating conditions are varied. This type of information is valuable when optimizing reactor conditions and when considering the suitability of a particular coal for use under entrained-flow-gasifier conditions. Thermogravimetric Analysis. Chars formed in the WMR tests were collected and their relative reactivity was assessed using thermogravimetric analysis (TGA) (Perkin-Elmer Thermogravimetric Analyzer). The weight loss with time was measured and the indicator of reactivity (the half-life) was the time required for 50% of the initial sample mass to be lost. The samples were exposed to air, at 500 °C, and so the method measures the rate of the C-O2 reaction under chemically (6) Cai, H.-Y.; Guell, A. J.; Chatzakis, I. N.; Lim, J.-Y.; Dugwell, D. R.; Kandiyoti, R. Combustion Reactivity and Morphological Change in Coal Chars: Effect of Pyrolysis Temperature, Heating Rate and Pressure. Fuel 1996, 75, 15-24. (7) Cai, H.-Y.; Megaritis, A.; Messenbock, R.; Dix, M.; Dugwell, D. R.; Kandiyoti, R. Pyrolysis of coal maceral concentrates under pfcombustion conditions (I): changes in volatile release and char combustibility as a function of rank. Fuel 1998, 77, 1273-1282. (8) Gu¨ell, A. J.; Kandiyoti, R. Development of a Gas-Sweep Facility for the Direct Capture of Pyrolysis Tars in a Variable Heating Rate High-Pressure Wire-Mesh Reactor. Energy Fuels 1993, 7, 943-952. (9) Messenbo¨ck, R. C.; Dugwell, D. R.; Kandiyoti, R. Coal Gasification in CO2 and Steam: Development of a Steam Injection Facility for High-Pressure Wire-Mesh Reactors. Energy Fuels 1999, 13, 122-130. (10) Pipatmanomai, M.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Investigation of Coal Conversion under Conditions Simulating the Raceway of a Blast Furnace using a Pulsed Air Injection Wire Mesh Reactor. Energy Fuels 2003, 17, 489-497.
2008
Energy & Fuels, Vol. 19, No. 5, 2005 Table 1. Coal Samples from China sample
coal source
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Yanzhuo Beishu Yanzhuo Yangchun Shen-Hua Houjitu Datong mixed Wujialu Shen-Mu mixed Shanxi Beigou Shen-Mu Daliuta Huating Neimeng Jiangyu Huangling Pingdingshan Yangquan
controlled conditions. The sample (1-2 mg) was heated to 50 °C under an inert gas atmosphere and the initial weight recorded. The sample was then heated to 500 °C, under the inert gas atmosphere and the weight allowed to stabilize. The inert gas supply was then switched to air to initiate the test. The sample was held under these conditions until >50% of the sample had reacted. The temperature was then raised to 850 °C, to combust the remainder of the sample. The final weight was the weight of ash in the sample. Longer reaction times showed less-reactive samples. Coal Samples. A list of the coal samples provided by TPRI is shown in Table 1. The samples were received in either pulverized or lump form. They were sent to an accredited contractor for crushing to a size of -212 µm and analysis. The samples were then returned to Imperial College, so that the 125-150 µm fraction could be obtained, by sieving. This was the fraction used in the tests in the WMR. The analyses of the coal samples are shown in Tables 2 and 3. The data show a wide variation in the composition of both the organic and inorganic parts of the coals. This implies that there would be variations in the performance of the plant using these coals, as well as a need to match the conditions with the coal being used. The volatile matter contents vary over a range of 9.7%44.8%, and these are accompanied by carbon contents in a range of 92.9%-80.4%. This shows that the suite contains coals ranging from high rank to low rank. Most of the sulfur contents vary between 0.3% and 1.6% (i.e., low-to-medium sulfur contents); however, there is one sample with a sulfur content of 5% (i.e., a high sulfur content). The low-sulfur coals could probably be used without downstream sulfur removal; however, the higher sulfur contents would require some form of gas cleaning, if the IGCC is to be classified as clean. The ash content also showed a wide variation, between 5.8% and 39.7%. The ash content imposes a thermal load on the gasifier, which can lead to substantial downstream deposition, and forms a slag, which must be removed from the gasifier in the molten form. The viscosity of the slag is an important criterion, and this is dependent on the composition. Clearly, a knowledge of the ash content and composition will be vital at the design stage to ensure that the gasifier can cope with the likely flows, but also so that the behavior of the slag can be checked against the desirable properties for the process. The ash constituent analysis shown in Table 3 shows that the composition of the ash varies widely, particularly the concentrations of iron and calcium. This implies there will be considerable variations in the viscosity of the slags formed from the suite of coals. Slag Sample. A sample of gasifier slag was provided by TPRI and was formed in their pilot-scale gasifier. The composition of the slag is shown in Table 4. Work Program. The suite of 14 coals has been tested in the WMR under pyrolysis and gasification (in CO2) conditions. Temperatures of 1500 and 2000 °C and a pressure of 3.0 MPa were used for the pyrolysis tests. The gasification tests were conducted at a temperature of 1500 °C and pressures of 2.0 and 3.0 MPa. Each test was performed in duplicate.
Wang et al. Attempts were made to measure the viscosity of a gasifier slag, but this was not successful. As an alternative, several models were used to estimate the viscosity of the slag, which has produced a useful insight into the slag behavior. However, this approach does have some limitations and these are discussed.
Results and Discussion 1. Reactivity of the Coals. The data obtained during the tests in the WMR with the 14 coal samples are shown in Table 5. 1.1. Pyrolysis of the Coals. Pyrolysis tests were conducted in helium and the weight loss that occurred during each test was solely due to loss of material by thermal decomposition. At temperatures of 1000 °C or less, the loss is due to the pyrolysis of the organic volatiles in the coal, because the inorganic phases have a tendency to be stable. However, at higher temperatures, there is potential for weight losses to be caused by both the pyrolysis and the decomposition and volatilization of some of the mineral species present in the coal. Data have been obtained, using the WMR, at 1500 and 2000 °C at a pressure of 3.0 MPa, and these are shown in Table 5. The values under both sets of conditions are higher than the volatile matter content, as measured by the British Standard Method. The standard test is conducted at a temperature of 900 °C, and the weight loss is attributed to the loss of organic material alone. The higher losses in the WMR, at higher temperatures, may, in part, be due to a slightly enhanced loss of organic species; however, it is also possible that the results are influenced by the volatilization and melting of the some components of the mineral matter. This effect will vary with the nature of the mineral matter in the different coal samples. The values are higher at 2000 °C than at 1500 °C, and this increase is most probably due to the increased loss of components of the mineral matter, as the decomposition of organic species should be complete at temperatures well below those studied here. Both sets of data are plotted as a function of the carbon content in Figures 2 and 3. As expected, the volatile yield decreases as the carbon content increases (i.e., as the rank of the coal increases). There is some scatter in the data, which will, in part, be due to experimental errors, because these are difficult tests to conduct under the extreme conditions used. It is also likely that different levels of loss of inorganic species between different coals also will have contributed to the scatter. This is unavoidable under the conditions used. However, the overall trend will be caused by the more-dominant influence of loss of organic material. It is possible that the differences between the level of volatile release at the very high temperatures and that at the lower temperatures, when pyrolysis is expected to be complete, can be used to give an indication of the extent of volatilization of inorganic species. This possibility has not been pursued in this work but might be a useful avenue for future study, as a means of assessing the deposit-forming potential of candidate coals. 1.2. Gasification of the Coals in CO2. Gasification has been studied in CO2. In this atmosphere, the weight losses during the tests in the WMR were caused by gasification and by losses were due to thermal decom-
Performance of Chinese Coals under Gasification
Energy & Fuels, Vol. 19, No. 5, 2005 2009
Table 2. Composition of the Coal Samples from China Value component
sample sample sample sample sample sample sample sample sample sample sample sample sample sample 1 2 3 4 5 6 7 8 9 10 11 12 13 14
moisture (% ad) 3.7 ash (% db) 15.5 volatile matter (% daf) 44.8
3.6 14.4 37.2
7.6 8.5 37.8
4.0 8.8 31.2
Proximate Analysis 7.4 6.2 1.4 8.0 5.8 29.0 33.3 35.2 23.0
9.8 5.9 37.5
1.1 39.7 24.8
11.1 12.1 42.8
1.2 30.0 23.4
2.0 21.0 36.3
1.2 28.5 34.4
0.9 14.3 9.7
C (% daf) H (% daf) N (% daf) S (% daf)
82.6 4.9 1.5 0.7
79.8 4.7 1.1 0.5
82.4 4.3 0.8 1.2
Ultimate Analysis 80.1 80.0 85.2 4.1 4.4 4.6 0.8 0.9 1.2 0.6 0.5 0.6
79.4 4.8 1.0 0.4
78.3 4.2 1.5 1.3
75.3 4.2 1.0 0.3
82.7 4.2 1.1 1.3
85.5 5.1 1.4 0.8
87.0 5.2 1.6 0.6
92.9 3.4 1.5 1.6
80.4 5.3 1.4 5.0
Table 3. Composition of the Ash Composition (%m/m)a element sample sample sample sample sample sample sample sample sample sample sample sample sample sample oxide 1 2 3 4 5 6 7 8 9 10 11 12 13 14 SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O Mn3O4 P2O5 SO3 a
42.5 11.3 20.9 0.4 12.2 1.1 0.2 0.5 0.1
