Energy & Fuels 2007, 21, 3171–3179
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Comparison of Chars Obtained under Oxy-Fuel and Conventional Pulverized Coal Combustion Atmospheres Angeles G. Borrego* and Diego Alvarez Instituto Nacional del Carbón, CSIC, P.O. Box 73, 33080 OViedo, Spain ReceiVed June 22, 2007. ReVised Manuscript ReceiVed August 15, 2007
The combustion of coal in conventional power plants produces large amounts of CO2 which contributes to the greenhouse effect. One of the ways to approach the CO2 emissions abatement is to burn the coal in an O2/CO2 atmosphere which eliminates the need of a separation step. In this study, two coals of different rank (a high volatile and a low volatile bituminous) have been burned in a drop tube reactor using O2/N2 and O2/CO2 mixtures with increasing oxygen content from 0 to 21%. Various oxygen concentrations have been selected for each set of experiments in order to follow both the progress of combustion and the influence of oxygen content in the devolatilization behavior of coal. Results show that a higher amount of O2 in CO2 than in N2 is needed to achieve similar burnout levels. Significant differences were found in the influence of oxygen content on the devolatilization behavior of the lower and higher rank coal. The limited amount of oxygen in the reacting atmosphere resulted in volatile release inhibition for the high volatile bituminous coal, whereas the more plastic low volatile coal was hardly affected. The presence of variable amounts of oxygen in CO2 had a small influence on the char particle appearance. The chars from both the combustion series (O2/N2) and the oxy-fuel series (O2/CO2) were similar for each parent coal in terms of reactivity and micropore surface area measured by CO2 adsorption. The main difference between both series of chars relied on the surface area determined by N2 adsorption (SBET) and on the size distribution of pores which was shifted to a larger size for the oxy-fuel series. The difference between both series of chars was larger for the high volatile bituminous coal chars than for the low volatile bituminous coal chars. This might have important implications for combustion under the diffusion-controlled regime.
Introduction The combustion of fossil fuels for energy production results in the generation of greenhouse gases, with CO2 as the major contributor, which are emitted to the atmosphere. There is a general agreement on the need to reduce the emissions of CO2, although the degree of compromise of the different governments and the ways to approach the problems are rather different.1 The coal fired power plants are among the best candidates to install systems for CO2 capture because they are stationary sources emitting large amounts of CO2. There are various possible routes to concentrate the CO2 for further storage which either act after the combustion process (i.e., amine scrubbing, calcination–recarbonation cycles) or prior to combustion through the utilization of a decarbonized fuel. The combustion of coal in a nitrogen-free atmosphere, most known as oxy-fuel technology, is one of the ways to approach the problem in which the need of a CO2 separation step can be eliminated.2 In this technology, coal combustion occurs in an oxygen atmosphere diluted with recycled CO2 to reduce the boiler temperature and to ensure the volume of gas. As a consequence, the flue gases will consist mainly of CO2 plus H2O which can be easily separated by condensation. The state of the art of this technology including research findings and testing scale results has been * Corresponding author. E.mail:
[email protected]. Fax: +34 985 297662. Phone: +34 985119090. (1) Intergovernmental Panel on Climate Change. Carbon Dioxide Capture and Storage. http://www.ipcc.ch/. (2) Anheden, M.; Yan, J.; Smedt, G. de ReV. IFP 2005, 60, 485–495.
recently reviewed.3 The replacement of N2 by CO2 in the reacting atmosphere may have a number of consequences which affect different aspects of coal combustion; on the one hand, the flame propagation speed,4 flame stability, and flame temperatures are lower in the O2/CO2 environment than in O2/N2.5,6 The larger specific heat of CO2 compared to N2 seems to be responsible for the high amounts of oxygen diluted in carbon dioxidesbetween 30 and 35% depending on gas impurities5–7s required to match the air combustion temperatures. In addition, although the CO2-char reaction rate would be much slower than the O2-char reaction rate, it could also contribute to the overall efficiency of the process, particularly at the high temperatures and CO2 concentrations prevailing on the boiler.8 Despite the existence of works comparing aspects such as heat transfer, flame temperature, and reaction rate under conventional and oxyfuel technology conditions,3 there are few works focusing on differences in char structure, specific surface area, chemical reactivity, and swelling behavior which are needed for modeling.8–10 Two papers addressing these topics were presented (3) Buhre, B. J. P.; Elliot, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283–307. (4) Kiga, T.; Takano, S.; Kimura, N.; Omata, K.; Okawa, M.; Mori, T. Energy ConVers. Manage. 1997, 38, 129–34. (5) Liu, H.; Zailani, R.; Gibbs, B. M. Fuel 2005, 84, 833–840. (6) Croiset, E.; Thambimuthu, K. V.; Palmer, A. Can. J. Chem. Eng. 2000, 78, 402–407. (7) Chui, E. H.; Douglas, M. A.; Tan, Y. Fuel 2003, 82, 1201–1210. (8) Shaddix, C. R.; Murphy, J. J. 20th Pittsburg Coal conference, Pittsburgh, PA, 2003. (9) Elliot, L. K.; Liu, Y.; Buhre, B. J. P.; Martin, J.; Gupta, R. P.; Wall, T. F. Proceedings of the ICCS&T 2005, Okinawa, Japan, CD-12pp. (10) Alvarez, D.; Fernández-Domínguez, I.; Borrego, A. G. Proceedings of the ICCS&T 2005, Okinawa, Japan, CD-6pp.
10.1021/ef700353n CCC: $37.00 2007 American Chemical Society Published on Web 10/06/2007
3172 Energy & Fuels, Vol. 21, No. 6, 2007
Borrego et al.
[ (
conversion (%) ) 1 -
Figure 1. Scheme of the drop tube reactor used for the preparation of the chars.
at the last International Conference on Coal Science and Technology held in Okinawa. The one by Alvarez et al.10 was the basis of this extended version, which also contains additional information on petrography11 and textural characterization of chars. These results will be compared with those of Elliot et al.9 who followed an approach similar in many aspects to the one reported in this work. Experimental Section 1. Coal Samples. Two coals of different rank, ground and sieved to 36–75 µm, have been selected for this study. Ultimate analyses of coals were performed using a LECO CHN600 instrument for carbon, nitrogen, and hydrogen, a LECO SC132 instrument for sulfur, and a LECO VTF900 instrument for oxygen. Proximate analyses were carried out following the standard procedures described in UNE 32-019-84 for volatile matter and ISO-1171/1981 for ash contents. Standard petrographic (maceral ISO 7404-3; 1994 and random reflectance ISO 7404-5; 1994) analyses were carried out on the coal fractions. 2. Char Preparation. Coal chars were prepared in a drop tube reactor at 1300 °C under nine different O2/N2 ratios (ranging from 0 to 21% oxygen), and also under four different O2/CO2 atmospheres (0–21% oxygen). The reactor, whose scheme is shown in Figure 1, is a furnace which surrounds two concentric alumina tubes (70 and 50 mm inner diameter, 1.30 and 1 m long, respectively). The reacting gas (600 L h-1) was injected at the bottom of the outer cylinder and was preheated while flowing upwards. When at the top of the outer cylinder, the gas was forced onto the inner tube through a flow straightener, and the gases flowed downwards and left the reactor through a water-cooled collection probe. The fuel particles were entrained (1 g min-1) by a jet of nonpreheated gas (300 L h-1) to a water-cooled injection probe placed on top of the inner tube. The estimated residence time of the particles in the reactor was 0.3 s. The chars left the reactor through the collection probe, and an extra nitrogen flow was added to the exhaust gases in order to quench the reaction and improve the collection efficiency in the cyclone. Coal burnout was calculated by the ash tracer, which is a mass balance between the ashes entering and leaving the reactor. The conversion is then calculated as (11) Alvarez, D.; Fernández-Domínguez, I.; Borrego, A. G. 57th Annual Meeting of the ICCP, Patras, Greece, 2005; p 36.
ashcoal 100 - ashcoal
)(
100 - ashchar-comb ashchar-comb
)]
× 100
The use of the ash tracer implies a number of assumptions including that mineral matter transformations occurring in the drop tube reactor (DTR) and during the proximate ash determination are similar and that the cyclone is able to recover 100% residual material both organic and inorganic.12 Both are unrealistic assumptions because typically ashes from proximate tests have a powderlike appearance indicating limited melting, whereas extensive melting occurs in the reactors and the boilers, as shown by the abundance of aluminosilicate spheres. Just a difference of 200 °C in the operating conditions of the DTR (1000 vs 1300 °C gas temperatures) has shown to have different effects on the melting behavior of the mineral matter.13 The opposite effect might be observed for minerals such as carbonates, which are completely decomposed during the proximate test but are not able to fully decompose in the short residence times of the reactor. On the other hand, liberation of fine ashes is a complex process which does not only depend on the characteristics of the parent coal and its mineral matter but also depends on the operating conditions and oxygen concentration.14 Without disregarding the limitations of the ash tracer for burnout estimation, its extended use to express conversion in combustion experiments and the good repeatability of the reactor (differences in char ash between runs around 0.6% for coals with 15% ash content have been recorded) justifies its utilization also in this work. Moreover, it is mainly used for comparative purposes between experiments performed at a single temperature varying only the reaction atmosphere, and therefore, the transformations of the mineral matter in the reactor for each of the parent coals are expected to be similar. 3. Char Characterization. Two widely used methods to determine the pore surface area of carbon from gas adsorption isotherms were applied in this study to selected char samples, using CO2 at 0 °C and N2 at -196 °C as adsorptives. The equipment used was a Micromeritics ASAP 2020 instrument. Chars were outgassed under vacuum prior to gas adsorption experiments in order to eliminate moisture or condensed volatiles, which could prevent the adsorbate accessibility. The heating rate used was 5 °C min-1 with holding temperatures of 90 °C (1 h) and 350 °C (4 h). This temperature is well below the char preparation temperature and is not expected to modify the structure of the char. CO2 adsorption isotherms were performed at 0 °C up to a pressure of 0.035 Torr, and the Dubinin–Radushkevich (D–R) equation15 was applied to the adsorption data. The Brunauer–Emmett–Teller (BET) theory was applied to the N2 adsorption data to obtain the surface area.16 These two methods can be regarded as complementary, given the difficulties of CO2 to fill large micropores and the slow diffusion of N2 in the small micropores.17 As some of the samples contained large amounts of mineral matter, the surface area data are expressed on an ash-free basis considering a surface area for the ashes of 0.8 m2 g-1. The isotherms were analyzed using the Micromeritics density functional theory (DFT) software package DFT plus. The pore size distribution was obtained in the size range 0.4–1.0 nm for CO2 adsorption and in the range 0.4–250 nm for N2 adsorption. Considering the DFT distributions, the pore volumes in the micropore and mesopore range were further split into micropores (
