Article pubs.acs.org/EF
Modeling of Limestone Sulfation for Typical Oxy-Fuel Fluidized Bed Combustion Conditions Margarita de las Obras-Loscertales, Luis F. de Diego,* Francisco García-Labiano, Aránzazu Rufas, Alberto Abad, Pilar Gayán, and Juan Adánez Department of Energy and Environment, Instituto de Carboquímica (ICB-CSIC), Miguel Luesma Castán, 4, Zaragoza, 50018, Spain ABSTRACT: A model based on the shrinking core model is proposed and evaluated to predict the behavior of limestones during the sulfation reaction under oxy-fuel firing mode in calcining conditions at typical time scales and particle sizes of circulating fluidized bed combustors (CFBCs). This model is chosen due to its simplicity in order to incorporate it in an easy way into available global CFBC models. It is observed that the model predicts properly the evolution of the sorbent sulfation conversion assuming that the first step of the sulfation reaction is controlled by gas diffusion through the porous system of the particle and the second step is controlled by gas diffusion through the CaSO4 product layer. The kinetic parameters needed in the model are determined for two limestones. For that, sulfation reaction tests were carried out in a thermogravimetric analyzer and in a batch fluidized bed reactor.
1. INTRODUCTION
sorbent can calcine or not. This is due to the characteristics of the thermodynamic equilibrium of calcination.
The emission of pollutant gases into the atmosphere from fossil fuel combustion to obtain electric power is one of the main causes responsible for the major environmental problems. Currently, an important environmental problem is the greenhouse effect, carbon dioxide (CO2) being the main contributor. The technologies of CO2 capture and storage (CCS) are very promising to minimize the release of this gas into the atmosphere. Oxy-fuel combustion is a CCS technology that consists of burning a fuel with a mix of pure O2 (≈30 vol %) and CO2 recirculated from the outlet gas stream to control the operating temperature. With this technology, the CO2 concentration in the flue gas may be enriched up to 95% (dry basis), and therefore, an easy CO2 recovery becomes possible. The fossil fuels, as coal, are characterized by containing sulfur. During the combustion process, the sulfur reacts to form mainly sulfur dioxide (SO2), which is another undesirable pollutant gas that should be removed. For CCS technologies, the SO2 elimination of the flue gas stream is focused on complying with CO2 stream quality requirements to transport and storage.1 Although oxy-fuel combustion is commonly carried out in pulverized coal (PC) boilers, fluidized bed combustors (FBCs), and specially circulating fluidized bed combustors (CFBCs), are also very appropriate for this combustion system. This technology has the advantage that external solid heat exchangers can be used to extract heat from the combustion process. This allows a significant reduction of the amount of recycled flue gas required for combustion temperature control.2 Moreover, desulfurization of combustion gases can be produced in situ by supplying a low-cost calcium-based sorbent, which allows burning fuels with a high sulfur content. Calcium-based sorbents have been extensively used in the past to remove SO2 at high temperatures. Depending on process conditions (temperature and CO2 partial pressure), the © 2013 American Chemical Society
CaCO3 ↔ CaO + CO2
(R.1)
Under conventional combustion conditions with air (low CO2 partial pressure), the operating conditions are always calcining conditions because the temperature in the system is above the calcination temperature, and thus sulfation of calcined sorbent, the so-called indirect sulfation (R.2), takes place.3 CaO + SO2 +
1 O2 ↔ CaSO4 2
(R.2)
However, under oxy-fuel combustion conditions, the CO2 partial pressure in the boiler is higher than that in air combustion, and as a result, the calcium-based sorbent can operate in calcining or noncalcining conditions. If the temperature in the system is below the calcination temperature, the sorbent operates in noncalcining conditions and direct sulfation (R.3) takes place. CaCO3 + SO2 +
1 O2 ↔ CaSO4 + CO2 2
(R.3)
Thermogravimetric analyzers (TGAs) and batch fluidized bed reactors (b-FBRs) are the most usual techniques used for the sorbent characterization, all of which try to reproduce the environment existing inside the boiler. The TGA permits the study of gas−solid reactions for long time test in differential operating conditions, and the b-FBR allows the sorbent characterization in similar conditions to those existing in fluidized bed combustors, such as simultaneous calcination and sulfation, attrition, thermal shock, and crackle. The sulfation reaction model has been the subject of many experimental and theoretical investigations. There are several Received: December 20, 2012 Revised: February 13, 2013 Published: March 4, 2013 2266
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Article
reaction models available to predict the behavior of the calcium-based sorbents during the sulfation process under air combustion conditions. Some of them are based on mechanistic models in which all the processes related to solid/gas reaction are taken into account. These models can be identified as pore models4−8 or grain models,9−13 and they are able to predict the behavior of the sorbents in a range of operating conditions. However, they are also complex and their integration in a boiler model is difficult when combustion and sulfation processes are carried out simultaneously. Therefore, there are other simple models based on empirical or pseudoempirical expressions able to facilitate their integration in the sulfur retention simulation in a boiler. These models include analogies between particle sulfation and catalyst deactivation process,14−16 grain models17 or shrinking core models in two steps,18−20 shrinking core model using a variable diffusivity with the sulfur conversion,21,22 and shrinking core models with a variable inner surface.23 Liu et al.24 used a shrinking core model to fit the behavior of the sorbent sulfation under oxy-fuel combustion in PC boilers. Previous studies performed by our research group,25,26 in a TGA and in a b-FBR, on sorbent sulfation behavior under oxyfuel combustion conditions show that the highest sorbent sulfation conversions for typical FBC operating conditions are reached at calcining conditions and that the optimum temperature for sulfur retention is 900−925 °C. These conclusions were later corroborated, operating in a bubbling fluidized bed combustor with continuous feeding of coal and limestone.27 The scope of this paper is to propose a sulfation reaction model able to predict the behavior of the limestones at typical time scales and particle sizes of CFBC working under oxy-fuel operating conditions in calcining conditions (indirect sulfation). Special attention is focused on the simplicity of the sulfation reaction model in order to incorporate it in an easy way into available global CFBC models.
Figure 1. Scheme of the reaction model assumed for the limestone sulfation process.
conversion in the layer is assumed). After the layer formation, the second step begins, and it is controlled by diffusion through the product layer according to the shrinking-core model (SCM). Abanades et al.31 inferred that the residual activity of the sorbent (second step) can be responsible for an important increase in sorbent sulfation conversion for particles used during long residence time in FBC. The equations that define the process are the following: First step: Control by gas dif f usion through porous system of the particle. Equation 1 shows the general expression when the reaction in a spherical particle is controlled by gas diffusion through the porous system of the particle32 dCSO2 1 dNCaO = De 2 dr 4πr dt
(1)
The integration of this equation along the thickness layer when the reaction takes place gives eq 2 −
dNCaO ⎛ 1 1 ⎞ ⎜ − ⎟ = 4πDeCSO2 dt ⎝ rc R0 ⎠
(2)
In this work, it is assumed that the value of De is not constant during the first step of sulfation reaction and it depends on the particle porosity and tortuosity in the outer layer.
2. SULFATION MODEL It is well-known that the utilization of calcium-based sorbents for SO2 retention in FBC is not complete due to the relatively large particle sizes used and pore blockage by CaSO4 formation. Under typical FBC operating conditions, the sulfation reaction usually takes place at the external surface and around the pores of the sorbent particles. Since the molar volume of CaSO4 is higher than the molar volume of CaCO3 or CaO, the pores are blocked and the inner part of the particles remains essentially unsulfated. Many studies16,25,28−31 agree with the idea that the sulfation process is performed in two steps. The first step is fast and controlled by chemical reaction and/or gas diffusion through the porous system of the particle. The second one is slower and controlled by diffusion through the CaSO4 product layer. Figure 1 shows the scheme of the particle sulfation reaction model considered for this work. In the model, the following simplifications are assumed: the particle is spherical and isothermal, the overall particle size does not change during reaction, the calcination of CaCO3 takes place instantaneously, and only indirect sulfation reaction takes place. The first step is controlled by the chemical reaction and/or the gas diffusion through the porous system of the particle. In this step, as the reaction proceeds, the pore volume decreases due to that the molar volume of the product (CaSO4) is higher than that of the reagent solid (CaO or CaCO3), and as a result, the external pores are plugged and a thin layer is formed (a uniform
De = D0ε /τ
(3)
The tortuosity is calculated using the equation of Wakao and Smith33 τ = 1/ε
(4)
De = D0ε 2
(5)
The evolution of limestone porosity in the outer layer during the sulfation reaction is expressed by: ε = ε0 − xl(1 − ε0)(Z − 1)
(6)
xl being the sulfation conversion in the product layer and Z the molar volume ratio of product to reactant. A molar volume ratio from CaSO4 to CaO (16.9 cm3/mol) of 2.72 is used in this work, considering the anhydrite II (2.96 g/cm3, 46 cm3/ mol) as the unique stable specie of the CaSO4 at high temperatures.34 From eq 6, a limit or maximum conversion, Xlim, can be reached by the product layer when all the initial porosity developed during calcination is filled with CaSO4. It is usually assumed that there is no conversion of the solid after this value unless changes in the particle size are considered. ε0 Xlim = (1 − ε0)(Z − 1) (7) 2267
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Article 2/3 2/3⎤ ⎡⎛ ⎛ tdiff2 − t1 Xs ⎞ ⎥ X1 ⎞ ⎢ = 3 ⎜1 − ⎟ − ⎜1 − ⎟ ⎢⎣⎝ τdiff2 Xlim ⎠ Xlim ⎠ ⎥⎦ ⎝
Substituting eqs 5 and 6 in eq 2 and integrating, eq 8 is obtained. It gives the relationship between the sulfation conversion of the product layer, xl, and the reaction time. ρCaO (R 03 − rc3)(R 0 − rc) ⎛ 1 ⎜ 3D0CSO2R 0rc(Z − 1) ⎝ ε0 − xl(1 − ε0)(Z − 1)
tdiff1 =
1⎞ − ⎟ ε0 ⎠
⎛ X − X1 ⎞ − 2⎜ s ⎟ ⎝ Xlim ⎠
Transition f rom the f irst step to the second step. In this model, a variable effective SO2 diffusivity through the porous system of the particle is assumed for the first step. The transition from the first step to the second step is produced when De(ε) Ds. It means that a residual porosity is remaining in the product layer when the maximum conversion is reached. Therefore, the maximum or limiting conversion, Xlim, must be redefined as ε0 − εr Xlim = (1 − ε0)(Z − 1) (18)
(8)
Equation 9 shows the relationship between the particle sulfation conversion, Xs, and the sulfation conversion of the product layer, xl, with a thickness “e” for spherical particles: Xs = xl
(R 03 − rc3) R 03
(9)
e = R 0 − rc
(10)
εr being the residual porosity
First step: Control by chemical reaction and gas dif f usion through the porous system of the particle. When the sulfation reaction is controlled by the gas diffusion through the porous system of the particle and by the chemical reaction, the theoretical time needed to reach a determined sorbent conversion is given by eq 11 t = tchem + tdiff1 (11)
3. EXPERIMENTAL SECTION 3.1. Materials. Two Spanish limestones, Granicarb and Horcallana, in a narrow particle size interval between 0.1 and 0.63 mm were used. Table 1 shows the main properties of both raw and calcined materials.
Table 1. Chemical Compositions and Physical Properties of Granicarb and Horcallana Limestones
where tdiff1 is calculated by eq 8 and tchem is calculated by eq 12 1/3 ⎛ tchem Xs ⎞ = 1 − ⎜1 − ⎟ Xlim ⎠ τchem ⎝
τchem =
Granicarb
(12)
physical properties porosity (%) apparent density (kg m−3) chemical composition (wt %) CaCO3 MgCO3 Na2O SiO2 Al2O3 Fe2O3
ρCaO R 0Xlim n bksCSO 2
(13)
τchem being the time necessary to reach the limiting conversion when the reaction is controlled by the chemical reaction. Second step: Control by gas dif f usion through the product layer. In the SCM,32 when a product layer around the particles is formed and the reactant gas diffuses through it to reach the unreacted core, it is usually assumed that the reaction is controlled by gas diffusion through the product layer. The sorbent conversion is associated with the thickness of the product layer “e” as
a
⎞ ⎛ dp ⎡ ⎢1 − ⎜1 − X s ⎟ 2 ⎢⎣ Xlim ⎠ ⎝
⎥ ⎥⎦
(14)
If there is no change in the particle size and the sorbent evolves toward a limit conversion, Xlim, the relation between the particle conversion and the reaction time is expressed by eq 15 2/3 ⎛ ⎛ tdiff2 X ⎞ X ⎞ = 1 − 3⎜1 − s ⎟ + 2⎜1 − s ⎟ τdiff2 Xlim ⎠ Xlim ⎠ ⎝ ⎝
(15)
where τdiff2 is the time needed to reach the maximum or limiting conversion, Xlim, and it is defined by eq 16:
τdiff2 =
ρCaO Xlimd p2 24DsCSO2
Horcallana
raw
calcineda
raw
calcineda
3.7 2573
49.0 1578
2.8 2601
49.8 1589
97.1 0.2 1.1
