Removal from Recycled Gas - American Chemical Society

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Efficient Desulfurization in a New Scheme of Oxyfuel Combined with Partial CO2 Removal from Recycled Gas and MILD Combustion Hao Liu,*,† Yanfei Chao,‡ Siwei Dong,† Xing Yuan,† Zhao Zeng,† Takashi Ando,§ and Ken Okazaki§ †

College of Energy, Soochow University, Suzhou 215006, China Shanghai Electric Power Generation Engineering Company, Shanghai 201100, China § Department of Mechanical and Control Engineering, Tokyo Institute of Technology, Tokyo 152-8552, Japan ‡

ABSTRACT: A new scheme of oxyfuel combustion combined with partial removal of CO2 from recycled gas and MILD combustion was proposed. Through experiments and theoretical analysis including the Monte Carlo method, the characteristics of in-furnace desulfurization in this new scheme was investigated. It was found that as the initial O2 concentration decreased, the gas recirculation ratio, SO2 concentration, and global efficiency of in-furnace desulfurization increased. On the other hand, the gas recirculation ratio, SO2 concentration, and global efficiency of in-furnace desulfurization increased as the CO2 removal ratio increased. The practical residence time of SO2 in oxyfuel-MILD coal combustion increased to about five to thirteen times as long as that of conventional pulverized coal combustion. The contributions of oxyfuel combustion, partial removal of CO2 from recycled gas, and MILD combustion, to the high desulfurization efficiency, were almost the same importance. At CO2 removal ratio = 11% and initial O2 concentration = 16%, the gas recirculation ratio became as high as 99.12%, i.e., only 0.88% of the flue gas was exhausted to the atmosphere. The corresponding system desulfurization efficiency was as high as 95.8%. It was demonstrated that this new scheme can realize extremely high in-furnace desulfurization efficiency in addition to easy CO2 recovery. by Park et al.4−6 They investigated the flame structure and NO formation through the effects of CO2 addition to either fuel- or oxidizer-side in H2−O2 counterflow diffusion flame, the effects of H2O or CO2 addition to either fuel- or oxidizer-side in CH 4 −O 2 −N 2 counterflow diffusion flame, and in gas combustion of low calorific heating value. Turbulent nonpremixed CH4/H2 jet flames issuing into a heated and highly diluted coflow, which emulated those of MILD combustion conditions, were investigated by Medwell et al.7 (2007). Decreasing the coflow O2 level is shown to lead to a suppression of OH as a result of the reduced temperatures in the reaction zone. Associated with the drop in OH levels is a broadening of the OH layer. A higher oxygen level leads to increase in reaction rates, thereby deviating from the MILD combustion regime. An experimental and computational investigation of a labscale burner, which can operate in both flame and MILD combustions and is fed with methane and a methane/hydrogen mixture, was carried out by Galletti et al.8 (2009). The modeling results indicate the need of a proper turbulence/ chemistry interaction treatment and rather detailed kinetic mechanisms to capture MILD combustion features, especially in the presence of hydrogen. Mancini et al.9 analyzed the IFRF semi-industrial-scale experiments on MILD combustion of natural gas. They found the temperature rise and the increasing NO concentrations along the fuel jet are almost exclusively due to the entrainment of hot combustion products. Schaffel et al.10 analyzed the IFRF experiments on MILD combustion of

1. INTRODUCTION Flameless oxidation1 or MILD (moderate and intense low oxygen dilution combustion),2 or High Temperature Air Combustion (HiTAC2), is a technology that emits very low pollutant emissions, especially thermal NOx and CO are lowered to residual values, while maintaining a high thermal performance of the system. It is probably the most important achievement of combustion technology in recent years. Within a decade or two, it has been developed from laboratory tests to industrial applications which certainly is an extraordinary progress as for an energy technology. Under the special conditions of the combustion regime the reactions take place in a volume sustained by the hot medium above the self-ignition temperature, and it is not possible to observe any visible flame or luminous effect; that is why the regime was named “Flameless oxidation”. The essence of this technology is that fuel is oxidized in an environment that contains a substantial amount of inert gases, resulting in very low NOx emissions even if the combustion stream is preheated to temperature in excess of 1000 °C. Chemical reactions take place in almost the entire volume of the combustion chamber and uniformity of both the temperature and the chemical species fields is the characteristic of this technology. To obtain adequate knowledge of the main features of MILD combustion and creating them, Katsuki and Hesegawa3 reviewed the latest development of MILD combustion and its applicability to industrial furnaces. They focused on the effects of heat-recirculating combustion under highly preheated air conditions (1200−1600 K). They found that intense mixing of the combustion air with burnt gases in the furnace, produced by high momentum ejection of combustion air, lowers the flame temperature and yields distributed reactions. The valuable studies have been performed © 2013 American Chemical Society

Received: November 24, 2012 Revised: February 26, 2013 Published: February 26, 2013 1513

dx.doi.org/10.1021/ef3019063 | Energy Fuels 2013, 27, 1513−1521

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The emission characteristics of CO2, SO2, and NOx in the flue gas of coal combustion were investigated by varying the compositions and concentrations of feed gas (O2/CO2/N2) and the ratios of recycled flue gas.25 Experimental results indicate that by oxyfuel combustion technology, higher concentration of SO2 is produced as the feed gas is 30% O2/ 70% CO2 or 40% O2/60% CO2. The enhanced mass flow rates of air pollutants in such a O2/RFG combustion system are also beneficial for improving the control efficiencies of air pollution control devices. In this work, a new scheme, i.e., an oxyfuel combustion system combined with recycled gas partially removed of CO2 and MILD combustion, was proposed by us (Figure 1). A

pulverized coal. Their result demonstrated that less NO is formed from combustion of volatiles, and stronger NOreburning mechanisms exist in the MILD combustion if compared to conventional coal combustion technology. The effects of fuel mixture on the establishment of moderate and intense low oxygen dilution (MILD) combustion in a recuperative furnace were investigated by Dally et al.11 A variety of fuel mixtures using methane, ethylene, and propane were investigated, and it was found that dilution of fuel with CO2 or N2 reduced the NOx emission and made the flame inside the furnace invisible. MILD combustion is energy efficient and one of the low NOx technologies, but it does not address the problem of CO2 reduction. Oxyfuel combustion, in which the fuel is combusted in a nitrogen-free environment, is the new technology which can address simultaneously the twin demands of low NOx and low CO2 emission. In a system of oxyfuel coal combustion, the CO2 concentration in the flue gas may reach up to 95%, and an easy CO2 recovery therefore becomes possible.12−15 Our experiment and theoretical analysis revealed that the conversion ratio from fuel-N to exhausted NO is automatically reduced to less than about one-fourth of that with conventional coal combustion in air.16,17 The efficiency of in-furnace desulfurization in conventional pulverized coal combustion is usually low, typically around 20% according to the operation condition of the boiler.18 The drastic decomposition of CaSO4 formed from the desulfurization reaction owing to the high temperatures in pulverized coal combustion boilers is responsible for the very low desulfurization efficiency of in-furnace desulfurization in conventional pulverized coal combustion. However, in oxyfuel 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. Moreover, the CaSO4 decomposition is also significantly inhibited due to a high SO2 concentration inside the furnace. All of these factors contribute to the efficient in-furnace desulfurization in oxyfuel coal combustion. Investigations on desulfurization in oxyfuel pulverized coal combustion have been conducted by some researchers. The SO2 concentration in oxyfuel case was found to be 3−4 times higher than the corresponding air case due to the enrichment effect of flue gas recycling and reduced volume of flue gas.19 Experiments by Croiset et al.20 and Kiga et al.21 demonstrated that the conversion of sulfur in coal to SO2 decreased in oxyfuel combustion compared with traditional air combustion because a considerable amount of sulfur was absorbed in ash in the system. Duan et al.22 studied sulfur evolution from coal combustion in oxyfuel mixture and found that COS was preferentially formed during the coal pyrolysis process in the CO2 atmosphere rather than in the N2 atmosphere. When temperature was above 1000 K, sulfate in the CO2 atmosphere began to decompose due to the reduction effect of CO, which came from the CO2 gasification. The calcination and sintering characteristics of limestone were studied by Chen et al.23 under oxyfuel combustion atmosphere. Their results revealed the specific pore volume and the specific surface area of CaO calcined in oxyfuel atmosphere were less than that of CaO calcined in air at the same temperature, whereas the pore diameter of CaO calcined in oxyfuel atmosphere was larger than that in air. Manovic24 studied sulfation and carbonation properties of hydrated sorbents in a fluidized bed CO2 looping cycle reactor.

Figure 1. Diagram of the new scheme of oxyfuel combustion system combined with partial CO2 removal from recycled gas and MILD combustion proposed in this work.

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, metal oxides, or through amine-based CO2 capture technology. 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 burn-out 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 oxyfuel combustion system. This new scheme can realize super low NO emission and easy CO2 capture simultaneously. However, there are too many unknowns to be clarified for this new promising scheme, including the influences of combustion conditions (initial O2 concentration, gas recirculation ratio, etc.), coal property, sorbent property, Ca/S ratio, residence of sorbent particles, sorbent size, contributions of various possible mechanisms, etc. This paper’s target is to study the possibility of high efficiency of in-furnace desulfurization and clarify the possible mechanisms. Through experiments and theoretical analysis of the system, the in-furnace desulfurization in such a new oxyfuel combustion system was characterized in this work. To better understand the mechanism of high desulfurization efficiency in the new scheme, we would like to know quantitatively the practical residence time of SO2 molecules in such a flue gas recycled system. As an individual SO2 molecule, whether/when it is captured by sorbent, and whether/when it escapes into the atmosphere are random. The practical residence time of SO2 molecules is difficult to 1514


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The rate constants, i.e., the activation energy and pre-exponential factors, were derived from the Arrhenius plot of experimental data at the beginning of the reaction. On the other hand, the experimental data from low conversion ratio to high conversion ratio of sorbent were used to obtain the effective diffusivity of SO2 in the CaSO4 product layer. It was derived from data fitting by comparing the experimental data with those predicted with a single particle model, in which the above-mentioned rate constants and the effective diffusivity of SO2 in the CaSO4 product layer, De, were used. The value of De best fitting the experimental and predicted data, decided through trial and error method, was taken to be the effective diffusivity of SO2 in the CaSO4 product layer. We repeated the same experiments for several times to make sure the data were reliable. With the method mentioned above, the rate constants of the desulfurization reaction, in both a CO2-enriched atmosphere and a low CO2 atmosphere of conventional combustion, together with the effective diffusivity of SO2 in the CaSO4 product layer of a sorbent particle, were obtained from experiments (Table 3).

obtain directly. The Monte Carlo method helps us to obtain it. Therefore we adopted the Monte Carlo method to obtain the practical residence time of SO2 molecules.

2. EXPERIMENTAL SECTION 2.1. Desulfurization Reaction. The desulfurization reaction kinetics was investigated in both CO2-enriched atmosphere and conventional low CO2 atmosphere in a fixed-bed reactor (Figure 2).

Table 3. Desulfurization Reaction Kinetics

Figure 2. Schematic diagram of the fixed bed reactor for experiment on the desulfurization reaction.

Ks in low CO2 concn (0 vol.%) (m/s)

Ks in high CO2 concn (80 vol.%) (m/s)

effective diffusivity of SO2 in sorbent particle, De (m2/s)

15.27 exp(−51900/ RT)

6.76 exp(−54000/ RT)

4.34 × 10−4exp(−14000/T)

2.2. Experiment on CaSO4 Decomposition. The experiments on CaSO4 decomposition were conducted in an entrained flow reactor (Figure 3). For CaSO4 decomposition, we did not use the same method as the experiments for the desulfurization reaction because of their different reaction mechanisms.

We used a fixed bed reactor to obtain high degree of the desulfurization reaction and derive the effective diffusivity of SO2 in the CaSO4 product layer, owing to the long residence time available in it. Mixed gases entered the reactor at the top of the quartz tube and were heated as they moved downward. The sample weight was 0.2 g, and the gas flow rate was 1 × 10−4 m3/s. Moreover, a very thin layer of sorbent was dispersed in a quartz wool substrate 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 concentrations from the reactor with sorbent and without sorbent. The solid sorbent (CaO) used in this work was prepared through the calcination of limestone as described in Table 1. Table 2

Table 1. Constituent of Limestone (Precursor, wt %) CaCO3

Ca(OH)2

MgO

P2O5

moisture

99.10

0.00

0.16

0.09

0.00

Table 2. Experimental Conditions for the Desulfurization Reaction parameter

value

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 (Nm3/s) total pressure (Pa)

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

Figure 3. Diagram of the entrained flow reactor for experiment on CaSO4 decomposition. As is well-known, the sintering effect is strong for the desulfurization reaction, which is controlled by both chemical kinetics and diffusion, and therefore a long time to experiment is needed. However, CaSO4 decomposition is chemically controlled. Therefore a short time to experiment is enough to derive its chemical kinetics. Nevertheless, a long time residence and a corresponding decomposition amount of CaSO4 was considered and included in our theoretical calculation. The reactor consisted of a quartz tube, with a 0.035 m i.d. 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. The experimental conditions for CaSO4 decomposition are listed in Table 4. The SO2 concentration was measured online with a SO2 analyzer after dust and moisture removal. To obtain the activation energy and pre-exponential factor of CaSO4 decomposition, experiments were conducted at various

summarizes the conditions for experiments on the desulfurization reaction. To obtain the rate constants, experiments at various temperatures were conducted in both CO2-enriched atmosphere and low CO2 atmosphere. At the beginning of 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. 1515


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concentration (CO2) at the entrance of the furnace prior to combustion, the flux of recycled gas is derived as

Table 4. Conditions for Experiments on CaSO4 Decomposition parameter

value

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 particles supply (g/s) total gas flow rate (Nm3/s) total pressure (Pa)

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

RE =

(4)

where RE is the flux of recycled gas (mol/s), CO2 is the oxygen concentration at the entrance of the furnace prior to combustion, and λ is the oxygen-fuel stoichiometric ratio. Additionally, x and z must satisfy

⎤ ⎡⎛ GC WC ⎞ · + RE·x⎟(1 − φ)⎥: (λ − 1)R O2 = x: z ⎢⎜ ⎠ ⎦ ⎣⎝ 100 12

(5)

where φ is the removal ratio of CO2, defined as the fraction of CO2 removed from the recycled gas (Point B in Figure 1) with respect to the flue gas prior to CO2 removal (Point A in Figure 1). Solving eqs 4 and 5 gives the oxygen concentration in the recycled flue gas

temperatures in the same atmosphere. On the other hand, we conducted experiments at various O2 concentrations (argon as balance), at constant temperatures to obtain its effect on CaSO4 decomposition. Similar experiments were conducted to obtain the effects of CO2 and SO2 concentrations, respectively. We found that the rate of CaSO4 decomposition decreased with the O2 and SO2 concentrations, which is easy to understand because they are the products of CaSO4 decomposition. On the other hand, the rate of CaSO4 decomposition increased with CO2 concentration. This is because when CO2 concentration increased, CO formed from CO2 enhanced CaSO4 decomposition through reaction CaSO4 + CO = CaO + SO2 + CO2. Our pre-experiments with a TGA (thermogravimetric analyzer) revealed that the decomposition of CaSO4 is a shrinking core reaction, and therefore the change of reaction rate with the molar fractional conversion of Ca from CaSO4 to CaO is as follows:

dX 3k = (1 − X )2/3 dt r0ρ

λR O2(100 − CO2) CO2(1 − z)

(a + z=

c 1−φ

)

+ RE ±

(a +

c 1−φ

2

)

+ RE

c

− 4RE· 1 − φ

2·RE (6)

where a = GCWC/1200 and c = (λ−1)·RO2. The mass balance of sulfur in an oxyfuel-MILD pulverized coal combustion system is given by (Figure 4)

(SF + SR )(1 − ηloc )α = SR

(7)

(1)

Meanwhile, the concentrations of O2, CO2, and SO2 also influence CaSO4 decomposition. By referring to the expressions by Fuertes,26 we regressed our experimental data with a data-fitting method and found the kinetics of CaSO4 decomposition can be described as follows dX = K (T , X , Pi) dt =

3k (1 − X )2/3 a b c r0ρ(1 + A1PO2 + A 2 PSO2 + A3PCO2 )

(2) Figure 4. Material balance of sulfur in the new scheme of oxyfuel combustion system combined with partial CO2 removal from recycled gas and MILD combustion.

where X is the molar fractional conversion of Ca from CaSO4 to CaO; t = time (s); and k is the reaction rate constant of CaSO 4 decomposition on apparent area basis (mol/(m2·s)). An expression of the reaction rate constant, k = 1.07 × 108 exp(−307000/RT), was obtained from our experiments at various temperatures. The experimental data obtained under various O2 concentrations (at constant temperatures) were used to derive A1 and a in eq 2. Similarly, A2, b, A3, and c were obtained from the data under various SO2 concentrations and CO2 concentrations, respectively. As a result, these coefficients and exponents in eq 2 were derived to be A1 = 5.07, A2 = 633.0, A3 = −0.43, a = 0.78, b = 1.06, and c = 1.0, respectively. 2.3. System Approach Using Theoretical Analysis on Desulfurization. The modeling of desulfurization in the new scheme of oxyfuel-MILD pulverized coal combustion is based on the global balance of all material. If the feed rate of coal is GC (g/s), then the stoichiometric amount of O2 necessary for burn-out of the coal is

R O2

G ⎛W W W ⎞ W = C ⎜ C + H + S − O ⎟ (mol/s) ⎝ 100 12 4 32 32 ⎠

yielding the recycled sulfur

SR =

SF(1 − ηloc )α 1 − (1 − ηloc )α

(8)

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

ηloc =

(removed‐S) * / S·X = R Ca (the sum of fuel‐S and recycled‐S)

(9)

* is the where X is the molar fractional conversion of sorbent, and RCa/S local Ca/S molar ratio inside the furnace during oxyfuel pulverized coal combustion:

(3)

where WC, WH, WS, and WO represent the carbon, hydrogen, sulfur, and oxygen contents of coal (wt %), respectively. Next the fractions of CO2 and O2 in the recycled flue gas (x and z, respectively) will be evaluated. To satisfy the requirement of oxygen

* /S = R Ca 1516

(Ca in sorbent) (the sum of fuel‐S and recycled‐S)

(10)


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r = particle radius (m); hD = 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; Reynolds number Re = 2r0U/ν; Schmidt number Sc = ν/D; U = linear velocity of the gas stream flowing past the particle (m/s); ν = kinematic viscosity (m2/s); D = diffusivity of SO2 in gas film (m2/s). Considering the low Reynolds number in our case, Sh = 2.0 was taken to derive hD. The initial condition is given by C = 0 at t = 0. The molar fractional conversion of sorbent from CaO to CaSO4, X, is calculated as X = 1 − (r−r0)3. Meanwhile, the decomposition of CaSO4 is considered simultaneously by using the kinetics obtained in this work. The above-mentioned single particle model and system approach were combined to investigate the desulfurization in the new scheme of oxyfuel-MILD coal combustion by using our own kinetic datathe kinetics of the desulfurization reaction and CaSO4 decomposition kinetics obtained from our experiments. Even though the residence time in Table 3 is short, in our calculation we use 8 s of residence time to consider the decomposition of CaSO4. The system approach was based on the coal properties described in Table 5, an initial sorbent particle diameter of 2.83 × 10−5 m and oxygen-fuel stoichiometric ratio λ of 1.2. Equations 14−16 in our single particle model are mass conservation equation and boundary conditions for SO2. Solving these equations gives the SO2 concentration at the surface of unreacted CaO core and the radius of the unreacted shrinking core. Furthermore, the molar fractional conversion of sorbent was obtained from X = 1 − (r−r0)3. Meanwhile, the decomposition of CaSO4 was considered by subtracting it from X. The local Ca/S molar ratio inside the furnace during oxyfuel-MILD pulverized coal combustion, R*Ca/S, was obtained according to the sorbent amount fed to the combustor and the sum of fuel-S and recycled-S. Finally the system desulfurization efficiency was determined through eq 13. The residence time of SO2 was estimated with the Monte Carlo method.

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 * / S·X ] C0 = Coxyfuel ‐ MILD(1 − ηloc ) = Coxyfuel ‐ MILD[1 − R Ca

(11)

where Coxyfuel‑MILD = (SF+SR)/M is the SO2 concentration at the furnace entrance during oxyfuel-MILD pulverized coal combustion, and M = flue gas amount in oxyfuel-MILD pulverized coal combustion (mol/s) . As a system, the desulfurization efficiency in oxyfuel-MILD pulverized coal combustion is defined as

η=

(SF + SR )ηloc (removed‐S) (exhausted‐S) =1− = (fuel‐S) (fuel‐S) SF (12)

Deriving eqs 8, 9, and 12 yields η=

ηloc 1 − (1 − ηloc )α

=

* / S·X R Ca * / S·X ]α 1 − [1 − R Ca

(13)

where X, the molar fractional conversion of sorbent, is a very important item to derive the global desulfurization efficiency in oxyfuel-MILD coal combustion. To obtain X, a single particle pore diffusion model was proposed (Figure 5). This model assumes the

3. EFFICIENT IN-FURNACE DESULFURIZATION CHARACTERITICS 3.1. Desulfurization Efficiency at Various Initial O2 Concentrations. The global efficiency of in-furnace desulfurization at various initial O2 concentrations is shown in Figure 6. The result showed as the initial O2 concentration decreased, the global efficiency of in-furnace desulfurization increased. Figure 7 shows the influence of the initial O2 concentration on the gas recirculation ratio. It can be seen that the gas recirculation ratio increased as the initial O2 concentration decreased. The initial O 2 concentration here refers to the O 2 concentration in a mixture of newly fed pure O2 and the recycled flue gas. In the case of a lower initial O2 concentration, more recycled flue gas was needed to dilute the newly fed pure O2, provided the amount of the newly fed pure O2 was the same. Therefore, the gas recirculation ratio increased when the initial O2 concentration decreased, and, consequently, the global efficiency of in-furnace desulfurization increased. Figure 8 shows the SO2 concentrations at various initial O2 concentrations (in the absence of sorbent). The SO 2 concentrations increased as the initial O2 concentration decreased, due to a higher gas recirculation ratio facilitating the enrichment of SO2 in the recycled gas.

Figure 5. Schematic illustration of single particle pore diffusion model. desulfurization reaction is first order with respect to the SO2 concentration at the surface of 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 a particle)

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

(14)

with boundary conditions given by De

∂C = hD(C0 − Cs) at r = r0 ∂r

(15)

De

∂C = ksC at r = rf ∂r

(16)

where De = effective diffusivity in particle; the subscripts s and f refer to the particle surface and interface between product layer and unreacted core, respectively; C = SO2 molar concentration (mol/m3);

Table 5. Coal Properties proximate analysis (as received, wt %)

ultimate analysis (dry, wt%)

M

ash

VM

FC

C

H

O

N

S

1.74

19.44

37.28

41.54

64.77

5.00

7.45

1.05

1.95

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Figure 9. Global efficiency of in-furnace desulfurization at various CO2 removal ratios (residence time of sorbent particles = 8 s, Ca/S molar ratio = 5, T = 1500 K).

Figure 6. Global efficiency of in-furnace desulfurization at various initial O2 concentrations (residence time of sorbent particles = 8 s, Ca/S molar ratio = 5, T = 1500 K).

Figure 10 shows the influence of the CO2 removal ratio on the gas recirculation ratio. It can be seen that the gas recirculation ratio increased as the CO2 removal ratio increased.

Figure 7. Gas recirculation ratio at various initial O2 concentrations (residence time of sorbent particles = 8 s, Ca/S molar ratio = 5, T = 1500 K). Figure 10. Gas recirculation ratio at various CO2 removal ratios (residence time of sorbent particles = 8 s, Ca/S molar ratio = 5, T = 1500 K).

In the case of a higher CO2 removal ratio, more recycled flue gas was needed to compensate the removed CO2 so as to dilute the newly fed pure O2, provided the amount of the newly fed pure O2 is the same. Therefore, the gas recirculation ratio increased when the CO2 removal ratio increased, and, consequently, the global efficiency of in-furnace desulfurization increased. Figure 11 shows the SO2 concentrations at various CO2 removal ratios (in the absence of sorbent). The SO 2 concentrations increased as the CO2 removal ratio increased, due to a higher gas recirculation ratio facilitating the enrichment of SO2 in the recycled gas. At CO2 removal ratio = 11% and initial O2 concentration = 16%, the gas recirculation ratio became as high as 99.12%, i.e., only 0.88% of the flue gas was exhausted to the atmosphere. It was this particularly high gas recirculation ratio that enriched the SO2 in the flue gas and enhanced desulfurization reaction. 3.3. Contributions of Oxyfuel Combustion, Partial CO2 Removal from Recycled Gas, and MILD Combustion. To clarify the mechanism of high efficiency of in-furnace

Figure 8. SO2 concentration at various initial O2 concentrations (in the absence of sorbent).

3.2. Desulfurization Efficiency at Various CO2 Removal Ratios. The system efficiency of in-furnace desulfurization at various CO2 removal ratios is shown in Figure 9. It was found that the system desulfurization efficiency increased with the CO2 removal ratio and could be as high as 95.8% at CO2 removal ratio = 11% and initial O2 concentration = 16%. 1518


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oxyfuel combustion, partial removal of CO2 from recycled gas, and MILD combustion, to the high desulfurization efficiency, were almost the same importance. Besides, it was revealed that extremely high efficiency of in-furnace desulfurization could be realized with the new scheme of oxyfuel-MILD combustion system. 3.4. Increased Ratio of SO2 Practical Residence Time Estimated with the Monte Carlo Method. The practical residence time of SO2 in oxyfuel-MILD pulverized coal combustion, i.e., the time interval from SO2 formation until its escaping from gas recirculation or being caught by sorbent, is of interest. However, it is difficult to be derived directly because for each SO2 molecule, its practical residence time is a random value. Therefore we estimated it with the Monte Carlo method, which is very effective at simulating random processes through a computer. A most important feature of the Monte Carlo method is the possibility of simulating random processes on a computer.27 The so-called Monte Carlo method (or Method of statistical trials) is, in essence, a system of techniques which enables us to model such processes conveniently in a machine. This method can be used effectively for various simulation,28−30 ranging from cases in which actual physical systems are simulated (describing them in some language which is specific for the particular problem given) to investigations of classical mathematical problems (such as systems of linear algebraic equations). In our work, we used this method to estimate the practical residence time of SO2. We supposed that for each SO2 molecule, the probability of being recycled was the same, and so was the probability of being caught by sorbent. The calculation procedures were as follows: (1) One SO2 molecule was chosen and one random number was generated by a computer. This random number was compared with the gas recirculation ratio to determine whether this molecule escaped from recirculation or was recycled. If it was recycled, another random number was generated and compared with the local desulfurization efficiency in furnace to determine whether the SO2 molecule was captured by sorbent or not. This procedure was repeated until the SO2 molecule escaped from recirculation or was captured by sorbent. The practical residence time was recorded accordingly. (2) Another SO2 molecule was chosen and the same procedure as step (1) was repeated. The practical residence time was estimated through averaging the practical residence time of a large number of molecules. The increased ratio of SO2 practical residence time estimated through the Monte Carlo method is shown in Figure 13. Here the increased ratio of SO2 practical residence time was defined as the ratio between the practical residence time of SO2 in oxyfuel-MILD combustion to the residence time of SO2 in conventional air combustion (one pass residence time). The practical residence time of SO2 in oxyfuel-MILD pulverized coal combustion increased to about five to thirteen times as long as that of conventional pulverized coal combustion (Case D in Figure 13). Besides, the ratio increased with temperature above 1500 K (Figure 13) owing to the decomposition of CaSO4 and decrease in desulfurization efficiency as shown in Figure 12.

Figure 11. SO2 concentration at various CO2 removal ratios (in the absence of sorbent).

desulfurization in the new scheme proposed by us, the contributions of oxyfuel combustion, partial CO2 removal from recycled gas, and MILD combustion were investigated. The curves in Figure 12 show the system desulfurization

Figure 12. Global efficiency of in-furnace desulfurization at different cases (residence time of sorbent particles = 8 s, Ca/S molar ratio = 5, Case A: air combustion, Case B: φ = 0, O2 = 21%, Case C: φ = 0.11, O2 = 21%, Case D: φ = 0.11, O2 = 16%).

efficiency at four different cases, i.e., Case A: air combustion, Case B: φ = 0, O2 = 21%, Case C: φ = 0.11, O2 = 21%, Case D: φ = 0.11, O2 = 16%, respectively. As shown in Figure 12, the desulfurization efficiency was very low in conventional air combustion (Case A). The system desulfurization efficiency for Case B was much higher than Case A (conventional air combustion). The difference between them was the contribution of oxyfuel combustion. The system desulfurization efficiency for Case C, when CO2 was partially removed from the recycled gas, was much higher than Case B. The difference between Case C and Case B reflected the contribution of partial CO2 removal from recycled gas. The system desulfurization efficiency for Case D, when partial CO2 removal from recycled gas combined with MILD combustion, reached the highest. Case D corresponded to the new scheme proposed in this article. The difference between Case D and Case C reflected the contribution of MILD combustion. Figure 12 demonstrated that the contributions of

4. DISCUSSIONS To better understand the mechanism of the extremely high efficiency of in-furnace desulfurization in the new scheme proposed by us, typical sulfur balance was shown in Figure 14. This balance was based on 1.0 mol/s of flux of fuel-S, φ = 0.11 1519


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In oxy-fuel combustion, the flame temperature is lower than conventional coal combustion in air because the specific heat capacity of CO2 is higher than N2 in air, provided the O2 concentration is the same (Vol. 21%). To keep the same combustion intensity, approximately Vol. 30% O2 combustion is needed in oxy-fuel combustion. On the other hand, in MILD (moderate and intense low oxygen dilution) combustion, the combustion stream is preheated to temperature in excess of 1000 °C. Under such special conditions of the combustion regime the reactions take place in a volume sustained by the hot medium above the selfignition temperature, and it is not possible to observe any visible flame or luminous effect; that is why the regime was named “Flameless oxidation”. In this combustion system, chemical reactions take place in almost the entire volume of the combustion chamber and uniformity of both the temperature and the chemical species fields is the characteristic of this technology. This feature maintained enough combustion intensity and ensured burnout of residual char in flyash. The fraction of residual carbon in flyash was less than 0.5%.

Figure 13. Increased ratio of SO2 practical residence time at different cases estimated through the Monte Carlo method (residence time of sorbent particles = 8 s, Ca/S molar ratio = 5, Case B: φ = 0, O2 = 21%, Case C: φ = 0.11, O2 = 21%, Case D: φ = 0.11, O2 = 16%).

5. CONCLUSIONS A new scheme of oxyfuel combustion combined with partial removal of CO2 from recycled gas and MILD combustion was proposed. Through experiments and theoretical analysis including the Monte Carlo method, the characteristics of infurnace desulfurization in this new scheme were investigated. The following conclusions were reached: As the initial O2 concentration decreased, the gas recirculation ratio, SO2 concentration, and global efficiency of in-furnace desulfurization increased. On the other hand, the gas recirculation ratio, SO2 concentration, and global efficiency of in-furnace desulfurization increased as the CO2 removal ratio increased. The practical residence time of SO2 in oxyfuel-MILD coal combustion increased to about five to thirteen times as long as that of conventional pulverized coal combustion. The contributions of oxyfuel combustion, partial removal of CO2 from recycled gas, and MILD combustion, to the high desulfurization efficiency, were almost the same importance. At CO2 removal ratio = 11% and initial O2 concentration = 16%, the gas recirculation ratio became as high as 99.12%, i.e., only 0.88% of the flue gas was exhausted to the atmosphere. The corresponding system desulfurization efficiency was as high as 95.8%, and the CO2 concentration was as high as 96.9%. It was demonstrated that this new scheme can realize extremely high in-furnace desulfurization efficiency in addition to easy CO2 recovery.

Figure 14. Typical material balance of sulfur in the new scheme of oxyfuel combustion system combined with partial CO2 removal from recycled gas and MILD combustion (residence time of sorbent particles = 8 s, Ca/S molar ratio = 5, T = 1500 K).

and O2 = 16%. The recycled sulfur was enriched to 4.8 mol/s due to the high gas recirculation ratio, but only 0.042 mol/s of sulfur was exhausted to the atmosphere. Therefore, the system desulfurization efficiency was as high as 95.8%. This value was conditional, i.e., at Ca/S molar ratio = 5. In this work, we adopted Ca/S molar ratio = 5, which is large. This ratio is necessary to have a high efficiency of desulfurization, because the desulfurization reaction suffers from sintering and decomposition of CaSO4. The desulfurization efficiency increases with Ca/S ratio but not linearly because the bulk SO2 concentration will decrease with Ca/S ratio. Nevertheless, it was an obvious fact that at the same value of the Ca/S molar ratio, the system desulfurization efficiency in our new scheme was much higher than conventional coal combustion. In addition to the high desulfurization efficiency mentioned above, the corresponding CO2 concentration was as high as 96.9%, which makes CO2 recovery very easy. From the viewpoint of combustion, how to decrease the residual char in flyash is directly related with combustion efficiency. One factor is excess air ratio, i.e., feeding enough O2 required for complete burnout of char (practically close to burnout). Besides, the temperature field should be high enough so as to ensure combustion intensity.

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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)and the Foundation of the State Key Laboratory of Coal Combustion (China). 1520


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Removal from Recycled Gas - American Chemical Society

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