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Limestone Calcination with CO2 Capture (III): Characteristics of Coal Combustion during Limestone Decomposition Yin Wang,*,†,‡ Shiying Lin,§ and Yoshizo Suzuki† National Institute of AdVanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100190, China, and Japan Coal Energy Center, 3-14-10 Mita, Minato-ku, Tokyo 108-0073, Japan ReceiVed December 17, 2008. ReVised Manuscript ReceiVed March 4, 2009
In this study, the combustion characteristics of coal in CO2/O2 and steam/CO2/O2 atmospheres were investigated during limestone decomposition in a continuously operating fluidized bed reactor for CO2 capture. The results show that the variations and concentrations of CO, CH4, and H2 in the exhaust gas of the reactor in the steam/CO2/O2 atmosphere were smaller than those in the CO2/O2 atmosphere. Because of steam dilution, the CO2 concentration in the bed was lower in the steam/CO2/O2 atmosphere than that in the CO2/O2 atmosphere, and the effect of differential pressure variation on limestone decomposition in the fluidized bed was less pronounced in the steam/CO2/O2 atmosphere than that in the CO2/O2 atmosphere. Additionally, N2O emission was detected only in the CO2/O2 atmosphere, and the conversion of N to NO in the steam dilution atmosphere was of a smaller magnitude than that observed in the CO2/O2 atmosphere. We also found that the conversion of S to SO2 in the steam/CO2/O2 atmosphere was lower than that observed in the CO2/O2 atmosphere. The contents of sulfur, SiO2, and Al2O3 were much higher in solid samples located in the cyclone than in the overflow holder in both steam/CO2/O2 and CO2/O2 atmospheres. Finally, the hydration and carbonation reactivities of CaO produced in the steam/CO2/O2 atmosphere were better than those produced in the CO2/O2 atmosphere.
1. Introduction The reduction of anthropogenic CO2 emission is becoming increasingly urgent because CO2 contributes to global warming. One method for reducing CO2 emission is to capture and sequestrate CO2 before it is released into the atmosphere. Calcined lime (main component, CaO) can be used to capture CO2 in the exhaust gas1 or in the reactor2,3 during the utilization of fossil fuels. That is, calcium oxide (CaO) absorbs CO2 to yield calcium carbonate (CaCO3), and the CaCO3 is then thermally decomposed to CaO, releasing nearly pure CO2 for sequestration (eq 1). CaCO3 f CaO + CO2
(1)
To obtain a nearly pure CO2 stream from CaCO3 decomposition, CaCO3 should be decomposed with fuel combustion in CO2/O2 or steam/CO2/O2, or with hot solids or heat pipes. Steam was added to the atmosphere to reduce CO2 partial pressure, and the reduction of CO2 partial pressure can decrease the decomposition temperature of CaCO3. In previous studies,4,5 limestone (main component, CaCO3) has been decomposed in * To whom correspondence should be addressed. Phone: +86-10-62621607; fax: +86-10-6255-0073; e-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ Chinese Academy of Sciences. § Japan Coal Energy Center. (1) Gupta, H.; Fan, L. S. Ind. Eng. Chem. Res. 2002, 41, 4035–4042. (2) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada M. Proceedings of 10th International Conference on Coal Sciences; Taiyuan, China, 1999; B-24. (3) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2002, 81, 2079– 2085. (4) Wang, Y.; Lin, S. Y.; Suzuki, Y. Energy Fuels 2007, 21, 3317– 3321.
a fluidized bed calciner in a CO2 atmosphere when the bed temperature was raised above 1293 K, whereas with 60% steam dilution in the atmosphere, limestone has been decomposed at comparatively lower temperatures, such as 1193 K. Reducing the decomposition temperature of limestone is helpful for increasing CaO reactivity. In fact, the heat for decomposing CaCO3 can be supplied by combusting fossil fuels, such as coal and natural gas, in a calciner. Because coal is less expensive than natural gas,6 coal is often used for such combustion schemes. To obtain a highconcentration CO2 stream, the atmosphere of coal combustion should be CO2/O2 or steam/O2, and the characteristics of coal combustion with limestone decomposition in these two atmospheres should be investigated. Many researchers studied coal combustion without limestone decomposition in CO2/O2, and Buhre et al.7 reviewed oxy-fuel combustion technology for coalfired power generation in a CO2/O2 atmosphere and reported that the characteristics of pulverized coal combustion and char burnout occurring in CO2/O2 differ from those occurring in air (N2/O2). Khraisha and Dugwell8 studied coal combustion with limestone calcination in N2/O2 containing ∼16.2 vol % CO2 by means of a suspension reactor and reported that the release of CO2 from limestone had a slight negative effect on the coal (5) Wang, Y.; Lin, S. Y.; Suzuki, Y. Energy Fuels 2008, 22, 2326– 2331. (6) Natural Gas Market Overview: Oil, Coal and Gas Prices; Federal Energy Regulatory Comission: 2009. Available at: http://www.ferc.gov/ market-oversight/mkt-gas/overview/ngas-ovr-oil-ngas-coal-pr.pdf (accessed 3/2009). (7) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Prog. Energy Combust. Sci. 2005, 31, 283–307. (8) Khraisha, Y. H.; Dugwell, D. R. Chem. Eng. Sci. 1992, 47, 993– 1006.
10.1021/ef801105j CCC: $40.75 2009 American Chemical Society Published on Web 03/20/2009
Coal Combustion during Limestone Decomposition
Energy & Fuels, Vol. 23, 2009 2805
Table 1. Properties of Newlands Coal Proximate analysis [wt %] moisture
ash
volatile matter
fixed carbon
2.2
15.8
24.0
58.0
Ultimate analysis [daf wt %] C
H
N
S
O
69.92
4.42
1.52
0.67
23.47
Ash property [Dry wt %] SiO2
Al2O3
others
53.69
30.25
16.06
Table 2. Chemical Analysis of Limestone composition [wt %] Kuzuu (Tochigi, Japan)
FCaO
FMgO
FCO2
FSiO2
Fothers
50.60
3.98
44.32
0.52
0.58
burn-out rate. However, no information has been reported about the characteristics of coal combustion together with limestone decomposition in CO2/O2. In particular, no instances of such combustion and decomposition in a fluidized bed reactor have been reported for a steam/O2 atmosphere. In this study, the combustion characteristics of coal in CO2/ O2 and steam/CO2/O2 atmospheres were investigated during limestone decomposition in a continuously operating fluidized bed reactor for CO2 capture. We aimed to reduce the amount of emission gases (CO, CH4, and H2) in the exhaust gas of the reactor, because incomplete combustion of these gases causes heat loss and environmental pollution. The emission behavior of SO2 and nitrogen oxides (NO and N2O) and the distributions of sulfur and coal ash in the decomposition and combustion products were also investigated. Additionally, the hydration and carbonation reactivities of CaO produced by limestone decomposition were tested.
Figure 1. The continuously operating thermal decomposition apparatus.
Figure 2. Temperature and ∆P variations during coal combustion with limestone decomposition in a CO2/O2 atmosphere.
2. Experimental Section 2.1. Sample. Bituminous coal with high ash content (Newlands, Australia; Table 1) was used. The coal and limestone (Kuzuu, Japan; Table 2) were ground and sieved to 0.125-0.25 mm for the experiments. The ratio of coal to limestone was set at 1:6 (w/w) for experiments. 2.2. Experimental Apparatus. A continuously operating fluidized bed reactor was used for coal combustion and limestone decomposition (Figure 1). The apparatus consists mainly of a fluidized bed reactor, a screw feeder, and an overflow holder. The fluidized bed reactor was made of an Inconel pipe with an inner diameter of 40 mm and a length of 1000 mm. There was a gas preheater underneath the fluidized bed to preheat the carrier gas. Mixtures of CO2/O2 or steam/CO2/O2 were supplied from the bottom of the preheater, passed through the fluidized bed, and then flowed out the top of the fluidized bed reactor. A mixture of coal and limestone particles was supplied by a screw feeder, made into a fluidized bed in the reactor, and then overflowed into the overflow holder. The exhaust gas flowed out the reactor, and the water was separated from the exhaust gas by a condenser. The temperatures in the fluidized bed were measured by K-type thermocouples located at three points T1, T2, and T3. The differential pressure (∆P) between the bottom and top of the fluidized bed was measured during the experiments. In this study, exhaust gas analyzers (a gas chromatography, an infrared analyzer, and a gas detector tube) were added to the apparatus (Figure 1). 2.3. Experimental Procedure. The temperatures of the fluidized bed reactor and the gas preheater were raised by means of electric furnaces to the target values, while the CO2/O2 or steam/ CO2/O2 stream was injected at a determined flow rate. Purge gas (N2, 0.05
Figure 3. Concentrations of O2, CO, CH4, and H2 in exhaust gas during coal combustion with limestone decomposition in a CO2/O2 atmosphere.
NL/min) was injected into the overflow holder to prevent CO2 and steam in the reactor from flowing into the overflow holder. When the temperatures of the reactor reached the target values and the gas stream became stable, a mixture of limestone and coal was continuously supplied into the empty fluidized bed by a screw feeder to start limestone decomposition with coal combustion. In this study, the feed rate was maintained at 8.7 g/min. The exhaust gas was analyzed continuously by gas chromatography and infrared analysis. 2.4. Gas and Solid Analysis. The concentrations of H2, O2, CH4, CO, and N2O in the exhaust gas of the reactor were continuously measured by a micro gas chromatography (VARIAN Micro GC
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Table 3. Average Values of the Gas Emissions Under Various Experimental Conditions average value (ppm) experimental conditions CO2 9 NL/min; O2 2.58 NL/min CO2 9 NL/min; O2 2.58 NL/min CO2 9 NL/min; O2 3.45 NL/min CO2 9 NL/min; O2 5 NL/min CO2 9 NL/min; O2 2.58 NL/min CO2 5 NL/min; O2 2.58 NL/min steam 5.5 NL/min; CO2 3.5 NL/min; O2 1.93 NL/min steam steam steam steam a
5.5 5.5 5.5 2.8
NL/min; NL/min; NL/min; NL/min;
CO2 CO2 CO2 CO2
3.5 3.5 3.5 1.8
NL/min; NL/min; NL/min; NL/min;
O2 O2 O2 O2
1.93 2.58 1.93 1.93
NL/min NL/min NL/min NL/min
figure/Panel
CO
CH4
H2
Figure 3/a Figure 3/b Figure 4/a Figure 4/b Figure 4/c Figure 5/a Figure 5/b Figure 7/a Figure 7/b Figure 8/a Figure 8/b Figure 9/a Figure 9/b
