ARTICLE pubs.acs.org/EF
CO2 Adsorption Inside the Pore Structure of Different Rank Coals during Low Temperature Oxidation of Open Air Coal Stockpiles Uri Green,*,†,‡ Zeev Aizenstat,† Franz Gieldmeister,‡,^ and Haim Cohen*,‡,§ †
Chemistry Institute, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel Department of Biological Chemistry, Ariel University Center at Samaria, Ariel, Israel § Chemistry Department, Ben-Gurion University of the Negev, Beer Sheva, Israel ^ TU Bergakademie Freiberg, Institut f€ur Energieverfahrenstechnik und Chemieingenieurwesen, Freiberg, Germany ‡
ABSTRACT: Steam coals are used in utilities worldwide for electrical power production. During monitoring of gases evolved via the weathering processes of large coal piles under open air storage and laboratory simulations, it has been observed that large amounts of carbon dioxide are adsorbed by the coal. Thus, one should take into account that appreciable amounts of CO2 formed by the low temperature oxidation (LTO) of the coal are trapped inside the pore structure of the coal as well as when studying LTO processes and performing kinetic evaluations of CO2 production, the amounts of CO2 released and adsorbed should be calculated. As the coals used for power production are often stored in large piles (>100 000 tons) for long periods, these piles are prone to undergo weathering which might result in self-heating of the coal piles and, in extreme cases, can spontaneously combust. Furthermore, the adsorption is also rank dependent and deviates from coal to coal. The effect of coal rank has been characterized with a South African bituminous coal and with Indonesian sub-bituminous coal which are currently in use in Israeli utilities as well as two German lignites which are consumed in German utilities. The results indicate that only the higher rank coals (bituminous and sub-bituminous) exhibit significant adsorption of CO2.
1. INTRODUCTION Coal is a complex heterogeneous organic-rich sedimentary rock composed largely from degraded plant material. Coal deposits formed periodically during the past 300 million years in various locations, in a range of geological environments, and have experienced different degrees of metamorphism (rank).1 As a result, coals of different rank, react differently to oxidative weathering processes.2 The effects of low temperature oxidation (LTO) in coal are significant for various coal applications.3 These LTO processes compose a multistage complex mechanism, and even today, it is not fully understood.4 The first stages involve physical adsorption and chemisorption of atmospheric oxygen at the surface inside the macropores. The second stage is the formation of surface oxides and hydroperoxides which can thereafter partially decompose to yield low molecular weight inorganic gases like carbon oxides (CO, CO2), water, hydrogen (H2), and also some organic gases (C1 5).5 The major gaseous product released from coals undergoing low temperature aerial oxidation processes is carbon dioxide.6 It is accepted7 that the emission of carbon dioxide is an exothermic process that is dependent on ambient temperature, oxygen concentration in the vicinity of the coal macromolecule, and the concentration of inherent oxygen functionality of the coal structure. Since the early 19th century when scientists realized that gases in the atmosphere cause a “greenhouse effect”, which can affect the planet’s temperature, researchers have struggled to understand how the level of carbon dioxide had changed in the past and how the level was influenced by chemical and biological forces. They found that CO2 plays a crucial role in climate change and r 2011 American Chemical Society
that the rising level may gravely affect our future.8 As the twentieth century saw a rapid 20-fold increase in the use of fossil fuels (between 1980 and 2006, the worldwide annual growth rate was 2%1) and 80 to 90% of the global increase in the carbon dioxide concentration is derived from the combustion of fossil fuels, solutions for reducing these emissions are paramount. One of the main options being investigated is geological sequestration.9 The ability of coal seams to store gas is a wellknown fact in coal mining.10 During the coalification process, significant amounts of methane are generated and retained permanently, if a sealing structure is present.11 Due to large surface areas within the coal matrix,12 it is safe to assume that the main storage method of the gases is indeed sorption. It is potentially important that through an adsorption/desorption process the methane is displaced and can be displaced and replaced by injecting carbon dioxide.13 Thus, while enhancing the production of methane at the same time, the coal can trap carbon dioxide. The adsorption properties that the coal exhibits depend on numerous factors, though primarily on partial pressure of the gas, temperature, and the coal macromolecule structure. In open pile storage of coals, there is a gradual emission of carbon dioxide as a result of these LTO processes.14 However, it has also been noted in previous work6 that several bituminous coals exhibit the ability to adsorb an appreciable part of the carbon dioxide formed under these conditions. As the emission of CO2 from the piles is also controlled by diffusion, it is possible Received: June 16, 2011 Revised: August 9, 2011 Published: August 10, 2011 4211
dx.doi.org/10.1021/ef200881w | Energy Fuels 2011, 25, 4211–4215
Energy & Fuels
ARTICLE
Table 1. Properties of Coalsa proximate analysis, wt % typeb moisture
Ashc
VMdbd
Table 2. Surface Area of Coalsa sample
ultimate analysis, wt %, dbd C
H
O
S
CV, J 3 g
1
rank
C, wt %
BET surface area, m2/g
SA
bituminous
74.19
3.92
IN
sub-bitumionous
73.25
5.45
SA
1.20
13.77
28.27
74.19
4.10
5.59
0.46
28 416
HA
lignite
66.12
2.35
IN
2.61
10.65
36.38
73.25
4.63
9.05
0.67
28 564
LUS
lignite
63.24
4.98
HA
34.53
5.09
52.39
66.12
4.32
23.65
0.16
25 323
LUS
10.57
4.16
52.78
63.24
4.50
27.24
0.20
24 516
a
VM = volatile matter; CV = calorific value. b SA = South Africa; IN = Indonesia; HA = Hambach, Germany; LUS = Lusatia, Germany. c The ash is water free. d db = dry basis.
that the conditions for room temperature adsorption might exist. We have decided to study three ranks of coals: a bituminous (South African = SA) and a sub-bituminous (Indonesian = IN). The third type were lignites from Germany (one from Hambach, HA, and a second from Lustatia, LUS). Each coal has a unique macrostructure which enabled us to better determine the extent of this adsorption effect.
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals and gases used throughout the study were of AR grade and supplied by Aldrich, Fluka, Merck, or Maxima. The water used throughout the study was purified by a Millipore column unit and had a resistance of at least 10 Mohms/cm. 2.2. Coal. Experiments in this study were carried out with three ranks of coals: bituminous, sub-bituminous, and lignite. The bituminous coal was from South Africa, the sub-bituminous coal was from Indonesia, and the lignite coals were from Germany. The South African bituminous coal used in this study serves as the major fossil fuel in coal fired power plants in Israel (more than 60% of the coal consumption). However, the sub-bituminous Indonesian coal is also fired in the Israeli utilities. The lignites are used in German utilities. The selected properties of all the coals are presented in Table 1. The experiments were performed in sealed glass vials (40 mL) used as batch reactors. The reactors were charged with coal (particle size, 74 μm e X e 250 μm) in an air atmosphere and heated at 55 95 C for various periods in a N€uveoven model FT 300. Experiments to determine oxygen concentration effect on carbon oxide emissions were also conducted (prepared in the same manner), however, in an argon or oxygen atmosphere. All the coals in the present study were prepared by grinding and separating by grain size via sieves. The coal samples were dried in a Heraeus vacuum oven model VT6060 for 24 h at 60 C. The weights of the coal samples were measured with a Mettler analytical balance model AE 100. 2.3. Gas Chromatography (GC). The amount of the CO2 in the reactors was determined using a gas chromatograph (Varian model 3800) equipped with a thermal conductivity detector and a flame ionization detector connected in series. The gases were separated on a carbosieve B 1/8 in., 9 ft ss column using a temperature programmed mode. The experimental error in the GC determination is (5%. The gaseous atmosphere was sampled (1 mL samples) after the reaction with gastight syringes (Precision Syringes, model A2), and the composition was measured in the gas chromatograph. The gases that could be determined are hydrogen, nitrogen, oxygen, carbon dioxide, carbon monoxide, methane, and ethylene. The argon gas present is not separated from oxygen; thus, the value determined for oxygen contains also ∼0.93% argon gas. As the reactions studied are gas/surface reactions, the reproducibility of the results is not good. Therefore, each experiment has been carried
a
Measured with N2 at 77 K for 1 g coal particles
