CO2 Combustion with Partial CO2 Removal from

Jan 12, 2012 - Scheme of O2/CO2 Combustion with Partial CO2 Removal from Recycled Gas. Part 2: High Efficiency of In-Furnace Desulfurization. Hao Liu*...
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Scheme of O2/CO2 Combustion with Partial CO2 Removal from Recycled Gas. Part 2: High Efficiency of In-Furnace Desulfurization Hao Liu,*,† Hong Yao,‡ Xing Yuan,† Siwei Dong,† Takashi Ando,§ and Ken Okazaki§ †

College of Energy, Soochow University, Suzhou 215006, People’s Republic of China State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China § Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan ‡

ABSTRACT: Through experiments on desulfurization, CaSO4 decomposition, and system approach using theoretical analysis, the in-furnace desulfurization in an O2/CO2 combustion system with partial CO2 removal from recycled gas was investigated. The results revealed that the SO2 concentration increased with the CO2 removal ratio and could be much higher than conventional combustion in air. This high SO2 concentration came from the enrichment effect of gas recirculation, at a high gas recirculation ratio of 0.989. The system desulfurization efficiency also increased with the CO2 removal ratio. Under the conditions investigated, the system efficiency of in-furnace desulfurization could be as high as 88.5%. The system desulfurization efficiency for the new scheme could be 6−10 times higher than conventional combustion in air. With this new scheme, easy CO2 recovery and efficient in-furnace desulfurization could be realized simultaneously. the system. Duan et al.9 studied sulfur evolution from coal combustion in the O2/CO2 mixture. Their results show that COS was preferentially formed during the coal pyrolysis process in the CO2 atmosphere rather than in the N2 atmosphere. When the temperature was above 1000 K, sulfate in the CO2 atmosphere began to decompose because of the reduction effect of CO, which came from the CO2 gasification. Chen et al.10 investigated calcination and sintering characteristics of limestone under an O2/CO2 combustion atmosphere. They found that the specific pore volume and specific surface area of CaO calcined in an O2/CO2 atmosphere were less than those of CaO calcined in air at the same temperature, whereas the pore diameter of CaO calcined in an O2/CO2 atmosphere was larger than that in air. Manovic et al.11 studied sulfation and carbonation properties of hydrated sorbents in a fluidized-bed CO2 looping cycle reactor. Nevertheless, up until now, studies aimed at desulfurization in O2/CO2 pulverized coal combustion were still limited12,13 and there exist many unknowns concerning in-furnace desulfurization in O2/CO2 pulverized coal combustion. Most of the previous work about CaSO4 decomposition was conducted with thermogravimetric analysis (TGA),14−17 which suffers from many limitations, owing to its low gas throughput and its dependence upon weight change as the sole response normally measured. Research on CaSO4 decomposition aimed at desulfurization in O2/CO2 pulverized coal combustion is obviously necessary, but few reports are currently available. In O 2 /CO 2 coal combustion, both SO 2 and CO 2 concentrations are much higher than in conventional coal combustion, owing to the recirculation and enrichment of flue

1. INTRODUCTION O2/CO2 coal combustion has an advantage of easy CO2 recovery because of its high CO2 concentration in the exhausted gas. The CO2 concentration in the flue gas may be enriched up to 95%, and an easy CO2 recovery therefore becomes possible.1−4 Moreover, there is possibility of efficient in-furnace desulfurization in O2/CO2 pulverized coal combustion, which has been impossible in conventional pulverized coal combustion. In-furnace desulfurization in conventional pulverized coal combustion was investigated during the late 1960s and the early 1970s with limited success.5 Its desulfurization efficiency is usually low, typically around 20% according to the operation condition of the boiler. The main reason to account for the very low desulfurization efficiency of in-furnace desulfurization in conventional pulverized coal combustion is the drastic decomposition of CaSO4 formed from the desulfurization reaction, owing to the high temperatures in pulverized coal combustion boilers. However, in O2/CO2 pulverized coal combustion, the SO2 concentration inside the furnace is very high, owing to the recirculation of flue gas, and consequently, an ideal desulfurization condition is provided. Furthermore, CaSO4 decomposition is also significantly inhibited, owing to a high SO2 concentration inside the furnace. All of these factors contribute to efficient in-furnace desulfurization in O2/CO2 coal combustion. Desulfurization in O2/CO2 pulverized coal combustion has been studied by some researchers. Tan et al.6 found that the SO2 concentration in an O2/CO2 case was 3−4 times higher than the corresponding air case because of the enrichment effect of flue gas recycling and reduced volume of flue gas. Croiset et al.7 and Kiga et al.8 conducted experiments and observed that the conversion of sulfur in coal to SO2 decreased in O2/CO2 combustion compared to traditional air combustion because a considerable amount of sulfur was absorbed in ash in © 2012 American Chemical Society

Received: September 27, 2011 Revised: January 11, 2012 Published: January 12, 2012 835

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Figure 1. Diagram of the new scheme of O2/CO2 combustion proposed in this work.

gas, which may lead to different desulfurization behaviors. The desulfurization phenomenon is also more complicated. To estimate the desulfurization efficiency in O2/CO2 pulverized coal combustion, a system approach must be included. In this work, a new scheme of O2/CO2 combustion with partial CO2 removal from recycled gas is proposed (Figure 1). A mixture of flue gas without CO2 removal and a fraction of flue gas devoid of CO2 is recycled. Thus, the recycled gas has been subjected to partial CO2 removal. Here, CO2 removal can be realized with sorbents, such as activated carbon, limestone, and metal oxides, or through amine-based CO2-capture technology. When CO2 is partially removed from the recycled gas, a high gas recirculation ratio is needed, so that there is enough CO2 to dilute the feeding O2, i.e., to compensate for the removed CO2 and satisfy the initial O2 concentration and O2 amount at the entrance of a combustor needed for combustion. That is why the recycle ratio increases with an increasing CO2 removal ratio. In such a system, the gas recirculation ratio is very high to maintain the O2 concentration at the furnace entrance and the stoichiometric O2 amount necessary for burnout of coal. Although the recycled gas is only partially depleted in CO2, CO2 in the exhausted flue gas can be completely removed if necessary, just as in an existing O2/CO2 combustion system. Removal of CO2 from the flue gas can be realized through liquefaction (cooling/pressurization) or adsorption with solid sorbent (activated carbon, monoethanolamine, etc). Through experiment and system approach, the possibility of high efficiency of in-furnace desulfurization was investigated. Moreover, the dependence of desulfurization efficiency upon the CO2 removal ratio and temperature was clarified. The desulfurization efficiency was compared between the new scheme of O 2 /CO 2 combustion and conventional air combustion.

Figure 2. Schematic diagram of the fixed-bed reactor. to prevent catalytic oxidization of SO2 at high temperatures. Mixed gases entered the reactor at the top of the quartz tube and were heated as they moved downward. The pre-experiment demonstrated that, at a sample weight of 0.2 g, the gas flow rate had no effect on desulfurization when it was above 6.7 × 10−5 m3/s. To minimize mass-transfer limitations between the bulk gas and sample layer, a gas flow rate of 1 × 10−4 m3/s was adopted. Moreover, a very thin layer of sorbent dispersed in a quartz wool substrate was used to enable homogeneous contact between the sorbent and reacting gas and to achieve differential conditions for valid kinetic data. The reaction rate was calculated from the difference between the SO2 concentration from the reactor without sorbent and the SO2 concentration with sorbent. The solid sorbent used in this work was prepared through calcination of limestone (Table 1). Table 2 summarizes the conditions for experiments on the desulfurization reaction. At the beginning of

2. EXPERIMENTAL SECTION 2.1. Desulfurization Reaction. The desulfurization reaction kinetics, in both a CO2-enriched atmosphere and a conventional low CO2 atmosphere, was investigated in a fixed-bed reactor (Figure 2). We used a fixed-bed reactor because it was able to obtain a high degree of desulfurization reaction to derive the effective diffusivity of SO2 in the CaSO4 product layer, owing to the long residence time available. A quartz tube, with a 0.02 m inner diameter and 0.65 m length, was used

Table 1. Constituent of the Sorbent Precursor (wt %)

836

CaCO3

Ca(OH)2

MgO

P2O5

moisture

99.10

0.00

0.16

0.09

0.00

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Table 2. Experimental Conditions for the Desulfurization Reaction temperature (K) O2 concentration (vol %) SO2 concentration (mol/m3 at 1500 K) CO2 concentration (vol %) Ar mean diameter of sorbent particles (m) sample weight (g) total gas flow rate (N m3 s−1) total pressure (Pa)

Table 3. Conditions for the Experiment on CaSO4 Decomposition

1013−1363 10 1.57 × 10−2 (1920 vol ppm) 80 and 0 as balance 2.83 × 10−5 0.2 1 × 10−4 1.013 × 105

temperature (K) O2 concentration (vol %) SO2 concentration (mol/m3 at 1563 K) CO2 concentration (vol %) Ar residence time (s) mean diameter of CaSO4 particles (m) CaSO4 particle supply (g/s) total gas flow rate (N m3 s−1) total pressure (Pa)

the reaction, the product layer of a sorbent particle was so thin that the diffusional resistance was negligible and the process was considered to be chemically controlled. The rate constants, i.e., the activation energy and pre-exponential factors, were derived from an Arrhenius plot of experimental data at the beginning of the reaction. On the other hand, the experimental data from a low conversion ratio to a high conversion ratio of sorbent were used to obtain the effective diffusivity of SO2 in the CaSO4 product layer. The reaction rate constants were derived to be ks = 6.76 exp(−54000/RT) and 15.27 exp(−51900/RT) for the desulfurization reaction in a CO2-enriched atmosphere and a low CO2 atmosphere of conventional combustion, respectively. Moreover, the effective diffusivity of SO2 in the CaSO4 product layer of a sorbent particle, expressed in a semi-empirical formula, was derived to be De = 4.34 × 10−4 exp(−14000/T). 2.2. Experiment on CaSO4 Decomposition. An entrained flow reactor (Figure 3) was used to investigate CaSO4 decomposition. The reactor consisted of a quartz tube, with a 0.035 m inner diameter and 0.79 m height, heated by an electric furnace. Mixed gases entered the reactor with the particles at the top of the quartz tube and were heated as they moved downward through the reaction zone. The CaSO4 particles were fed with a vibration bowl feeder. The gas was sampled with a water-cooled stainless-steel probe (8 mm in inner diameter) to ensure that the temperature inside the sampling probe decreased rapidly. A suction thermocouple (probe) was used to avoid the possible error caused by radiation and other factors. The experimental conditions for CaSO4 decomposition are listed in Table 3. The SO2 concentration was measured online with a SO2 analyzer after dust and

1400−1620 0−30 0−0.015 (0−1800 vol ppm) 0 −100 as balance 0.59−0.62 7.65 × 10−5 0.017−0.033 1.0 × 10−4 1.013 × 105

moisture removal. The result revealed that the decomposition rate of CaSO4 decreased with the increase of the SO2 concentration, which suggested that, in O2/CO2 pulverized coal combustion, the CaSO4 decomposition could be significantly inhibited because the flue gas recycling caused a high SO2 concentration inside the furnace. Experiments were conducted at various temperatures in the same atmosphere to obtain the activation energy and pre-exponential factor of CaSO4 decomposition. Besides, we conducted experiments at various O2 concentrations (Ar as balance), at constant temperatures, to obtain its effect on CaSO4 decomposition. Experiments were also conducted to obtain the dependence upon CO 2 and SO 2 concentrations. Our experiments revealed that the rate of CaSO4 decomposition decreased with O2 and SO2 concentrations, because they are products of CaSO4 decomposition. However, the rate of CaSO4 decomposition increased with the CO2 concentration. This result comes from the fact that CO formed from CO2-enhanced CaSO4 decomposition through reaction CaSO4 + CO = CaO + SO2 + CO2 when the CO2 concentration increased. By referring to the expressions by Fuertes and Fernandez,17 we regressed our experimental data with the data-fitting method and found that the kinetics of CaSO4 decomposition can be described as follows:

Figure 3. Diagram of the entrained flow reactor. 837

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where CO2/CO2 = (SF + SR)/M is the SO2 concentration at the furnace entrance during O2/CO2 pulverized coal combustion, with M being the flue gas amount in O2/CO2 pulverized coal combustion (mol/s). As a system, the desulfurization efficiency in O2/CO2 pulverized coal combustion is defined as

dX = K (T , X , Pi) dt a b c = (3k /(r0ρ(1 + A1PO + A2PSO + A3PCO ))) 2 2 2

(1 − X )2/3

(1)

where X is the molar fractional conversion of Ca from CaSO4 to CaO, t is the time (s), k is the reaction rate constant of CaSO 4 decomposition on the apparent area basis (mol m−2 s−1), r0 is the initial radius of the CaSO4 particle (m), ρ is the particle density (mol/ m3), T represents the temperature (K), PO2, PSO2, and PCO2 refer to the molar fractions of O2, SO2, and CO2, respectively, A1, A2, and A3 are coefficients, and a, b, and c are exponents with respect to PO2, PSO2, and PCO2, respectively. An expression of the reaction rate constant, k = 1.07 × 108 exp(−307000/RT), was obtained from our experiments at various temperatures. The experimental results obtained under various O2 concentrations (at constant temperatures) were used to obtain A1 and a in eq 1. Similarly, A2, b, A3, and c were derived from the data under various SO2 and CO2 concentrations, respectively. The coefficients and exponents in eq 1 were derived to be A1 = 5.07, A2 = 633.0, A3 = −0.43, a = 0.78, b = 1.06, and c = 1.0.

η = (removed S)/(fuel S) = 1 − (exhausted S)/(fuel S) = ((SF + SR )ηloc )/SF Deriving eqs 3, 4, and 7 yields

η = ηloc /(1 − (1 − ηloc )α) = (R *Ca/S X )/(1 − [1 − R *Ca/S X ]α)

1 ∂ ⎛⎜ 2 ∂C ⎞⎟ ∂C r De = ∂r ⎠ ∂t r 2 ∂r ⎝

(2)

yielding the recycled sulfur

SF(1 − ηloc )α SR = 1 − (1 − ηloc )α

(3)

recycled S) (4)

where X is the molar fractional conversion of sorbent and R*Ca/S is the local Ca/S molar ratio inside the furnace during O2/CO2 pulverized coal combustion.

R *Ca/S = (Ca in sorbent) (5)

Correspondingly, the system Ca/S molar ratio is defined as RCa/S = (Ca in sorbent)/(fuel S). The SO2 concentration in the bulk gas (C0) is related to R*Ca/S and X as

C0 = CO2 /CO2(1 − ηloc ) = CO2 /CO2[1 − R *Ca/S X ]

De

∂C = hD(C0 − Cs) ∂r

De

∂C = ksC ∂r

at r = rf

at r = r0

(10)

(11)

where De is the effective diffusivity in particle, the subscripts s and f refer to the particle surface and interface between the product layer and unreacted core, respectively, C is the SO2 molar concentration (mol/m3), r is the particle radius (m), and hD is the mass-transfer coefficient of SO2 in gas film (m/s). Moreover, hD was obtained from Sherwood number Sh = 2r0hD/D = 2.0 + 0.6Re1/2Sc1/3, where Reynolds number Re = 2r0U/ν, Schmidt number Sc = ν/D, U is the linear velocity of the gas stream flowing past the particle (m/s), ν is the kinematic viscosity (m2/s), and D is the diffusivity of SO2 in the gas film (m2/s). Considering the low Reynolds number in our case, Sh = 2.0 was taken to derive hD. The initial condition was given by C = 0 at t = 0. The molar fractional conversion of sorbent from CaO to CaSO4, X, was calculated as X = 1 − (r − r0)3. Meanwhile, decomposition of CaSO4 was considered simultaneously using the kinetics obtained in this work. The above-mentioned single-particle model and system approach were combined to investigate the desulfurization in the newly proposed O2/CO2 coal combustion scheme, using our own kinetic data: the kinetics of the desulfurization reaction and CaSO4 decomposition kinetics obtained from our experiments. The system approach was based on an initial diameter of the sorbent particle of 2.83 × 10−5 m, an initial O 2

ηloc = (removed S)/(the sum of fuel S and

/(the sum of fuel S and recycled S)

(9)

with boundary conditions given by

where SF is the flux of fuel S (mol/s), SR is the flux of recycled S (mol/s), α is the (recycled gas amount)/(total gas amount), gas recirculation ratio, and ηloc is the local desulfurization efficiency inside the furnace during O2/CO2 pulverized coal combustion defined as

= R *Ca/S X

(8)

where X, the molar fractional conversion of sorbent, is a very important item to derive the system desulfurization efficiency in O2/CO2 coal combustion. To obtain X, a single-particle pore diffusion model was proposed. This model assumes that the desulfurization reaction is first-order with respect to the SO2 concentration at the surface of the unreacted core and the desulfurization reaction CaO + SO2 + 1/2O2 → CaSO4 takes place at the surface of a shrinking core of unreacted CaO within the particle. The unsteady mass conservation equation is given by (in particle)

3. EFFICIENT IN-FURNACE DESULFURIZATION IN THE NEW SCHEME OF O2/CO2 COMBUSTION 3.1. System Approach Using Theoretical Analysis on Desulfurization. The modeling of desulfurization in O2/CO2 pulverized coal combustion is based on the global balance of all material. The mass balance of sulfur in an O2/CO2 pulverized coal combustion system is given as

(SF + SR )(1 − ηloc )α = SR

(7)

(6) 838

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concentration of 21 vol %, an oxygen/fuel stoichiometric ratio λ of 1.2, and a coal property described in Table 4. Table 4. Coal Property parameter proximate analysis of coal (wt %) moisture ash volatile matter fixed carbon ultimate analysis (wt %, dry) C H O N S

value 4.6 10.6 41.6 43.2 70.6 5.41 10.98 1.19 0.71

Figure 5. Change of the SO2 concentration with the CO2 removal ratio in the presence of sorbent (T = 1500 K, and system Ca/S molar ratio = 5).

In the presence of sorbent, the tendency is similar to that in the absence of sorbent, although not as strong as in the absence of sorbent. This high SO2 concentration facilitated efficient in-furnace desulfurization and drastically improved sorbent use. Moreover, only a very small fraction of flue gas was exhausted to the atmosphere because of a high flue gas recirculation ratio. 3.3. Desulfurization Efficiency at Various CO2 Removal Ratios. The system efficiency of in-furnace desulfurization at various CO2 removal ratios is shown in Figure 6. It was found

3.2. SO2 Concentration in the New Scheme of O2/CO2 Combustion. Figure 4 shows the change of the SO 2

Figure 4. Change of the SO2 concentration with the CO2 removal ratio in the absence of sorbent.

concentration with the CO2 removal ratio in the absence of sorbent. A CO2 removal ratio of 0% corresponds to the existing scheme for O2/CO2 combustion. It was found that the SO2 concentration increased with the CO2 removal ratio, most significantly between CO2 removal ratio values of 10 and 15%. In particular, at a CO2 removal ratio of 15%, the SO2 concentration could be as high as 63 700 ppm in the absence of sorbent. This high SO2 concentration came from the enrichment effect of gas recirculation, at a high gas recirculation ratio of 0.989. This high SO2 concentration was over 100 times higher than conventional combustion in air (540 ppm). Figure 5 shows the change of the SO2 concentration with the CO2 removal ratio in the presence of sorbent. Similar to Figure 4, the SO2 concentration increased with the CO2 removal ratio. Although the SO2 concentration in the presence of sorbent (Figure 5) was lower than in the absence of sorbent (Figure 4) because of the desulfurization reaction, it was still much higher than conventional combustion in air. From eq 3, we can see that in the absence of sorbent, when gas recirculation becomes close to 100%, the recycled sulfur increased drastically with a nonliner relationship. When the CO2 removal ratio increases from 10 to 15%, the gas recirculation becomes close to 100%, and as a consequence, the SO2 concentration increases drastically compared to a lower CO2 removal ratio.

Figure 6. System desulfurization efficiency at various CO2 removal ratios (T = 1500 K, system Ca/S molar ratio = 5, and residence time = 8 s).

that the system desulfurization efficiency increased with the CO2 removal ratio and could be as high as 88.5% at a CO2 removal ratio of 15%. 3.4. Desulfurization Efficiency at Various Temperatures. Figure 7 shows the system desulfurization efficiency at various temperatures. For comparison, the desulfurization efficiency for combustion in air is also plotted in Figure 7. It can be seen that, in the temperature range investigated, the system desulfurization efficiency for the newly proposed O2/ CO2 coal combustion scheme was much higher than conventional coal combustion in air. Figure 8 shows the ratio of desulfurization efficiency between the new scheme of O2/CO2 coal combustion and conventional coal combustion in air. At 1500 K, this ratio was higher than at 1400 K, because CaSO4 decomposition was inhibited as a result of a high SO2 concentration in O2/CO2 combustion. This effect was pronounced at a high temperature. This work demonstrated that the system desulfurization efficiency for the new 839

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tion in this combustion system was investigated and the following conclusions were reached for the new scheme: The SO2 concentration increased with the CO2 removal ratio. It could be over 100 times higher than conventional combustion in air, when in the absence of sorbent. This high SO2 concentration came from the enrichment effect of gas recirculation, at a high gas recirculation ratio of 0.989. The system desulfurization efficiency increased with the CO2 removal ratio. Under the conditions investigated, the system efficiency of in-furnace desulfurization could be as high as 88.5%, which is impossible for conventional pulverized coal combustion in air. The system desulfurization efficiency for the new scheme could be 6−10 times higher than conventional combustion in air.

Figure 7. System desulfurization efficiency at various temperatures (system Ca/S molar ratio = 5, and residence time = 8 s).

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-512-6787-0271. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (50936001), the Foundation of State Key Laboratory of Coal Combustion (China), and the National Key Scientific Instruments and Equipment Funding (2011YQ120039).

Figure 8. Ratio of desulfurization efficiency between the new scheme of O2/CO2 coal combustion and conventional coal combustion in air (system Ca/S molar ratio = 5).

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scheme could be 6−10 times higher than conventional combustion in air. Among the three temperatures that we investigated, the desulfurization efficiency in conventional coal combustion is the highest at 1400 K. Its desulfurization efficiency is low at 1300 K because the desulfurization reaction is difficult to occur at a low temperature. On the other hand, at 1500 K, the desulfurization efficiency is low due to significant decomposition of CaSO4 at a high temperature. That is why the ratio of desulfurization efficiency between the new scheme of O2/ CO2 and conventional coal combustion is lowest at 1400 K.

4. DISCUSSION The results of this work revealed that the new scheme of O2/ CO2 combustion proposed by us could realize easy CO2 recovery, superlow NO emission, and super-efficient in-furnace desulfurization simultaneously. Obviously, it is more promising than not only conventional coal combustion technology but also the current O2/CO2 combustion system. The high desulfurization efficiency in this scheme is obtained via sorbent injection into the furnace. Although SO2 is recycled into the furnace, it is captured by sorbent particles in the furnace rather than in the back pass through a flue gas desulfurization (FGD) facility. Therefore, the desulfurization investigated in this work still belongs to in-furnace desulfuization. Further detailed investigations on this new scheme are highly significant. 5. CONCLUSION A new scheme of O2/CO2 combustion with partial CO2 removal is proposed. Through experiments and a system approach using theoretical analysis, the in-furnace desulfuriza840

NOMENCLATURE a, b, and c = exponents defined in eq 1 A1, A2, and A3 = coefficients defined in eq 1 C = SO2 molar concentration (mol/m3) C0 = SO2 molar concentration in bulk gas (mol/m3) CO2/CO2 = SO2 molar concentration (in bulk gas) at the furnace entrance in O2/CO2 pulverized coal combustion (mol/m3) CS = SO2 molar concentration at the particle surface (mol/ m3) D = diffusivity of SO2 in gas film (m2/s) De = effective diffusivity of SO2 in the sorbent particle (m2/ s) hD = mass-transfer coefficient in the gas film (m/s) k = reaction rate constant of CaSO4 decomposition on the apparent area basis (mol m−2 s−1) ks = reaction rate constant of desulfurization on the apparent area basis (m/s) M = gas amount in O2/CO2 pulverized coal combustion (mol/s) PCO2 = molar fraction of CO2 PO2 = molar fraction of O2 PSO2 = molar fraction of SO2 r = particle radius (m) r0 = initial particle radius (m) rf = radius at the interface between the product layer and unreacted core (m) R = universal gas constant (J mol−1 K−1) RCa/S = (Ca in sorbent)/(fuel S), system Ca/S molar ratio R*Ca/S = (Ca in sorbent)/(the sum of fuel S and recycled S), local Ca/S molar ratio inside the furnace during O2/CO2 pulverized coal combustion Re = 2r0U/ν, Reynolds number dx.doi.org/10.1021/ef201464z | Energy Fuels 2012, 26, 835−841

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S = sulfur content of coal Sc = ν/D, Schmidt number SF = flux of fuel S (mol/s) Sh = 2r0hD/D, Sherwood number SR = flux of recycled S (mol/s) t = time (s) T = temperature (K) U = linear velocity of the gas stream flowing past the particle (m/s) X = molar fractional conversion of sorbent from CaO to CaSO4 or vise versa Greek Symbols

α = (recycled gas amount)/(total gas amount), gas recirculation ratio η = (removed S)/(fuel S), system desulfurization efficiency in O2/CO2 pulverized coal combustion (for conventional pulverized coal combustion, η is exactly the same as ηloc) ηloc = (removed S)/(the sum of fuel S and recycled S), local desulfurization efficiency inside the furnace during O2/CO2 pulverized coal combustion λ = oxygen/fuel stoichiometric ratio ν = kinematic viscosity (m2/s) ρ = particle density (mol/m3)

Subscripts

e = effective f = interface between the product layer and unreacted core s = particle surface

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