Combustible Matter Conversion in an Oxy-fuel Circulating Fluidized-Bed

Sep 5, 2012 - ABSTRACT: Studies on combustible matter conversion during oxy-fuel ... The paper is focused on carbon and combustible sulfur behavior in...
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Combustible Matter Conversion in an Oxy-fuel Circulating FluidizedBed (CFB) Environment Tomasz Czakiert,* Waldemar Muskala, Sylwia Jankowska, Grzegorz Krawczyk, Pawel Borecki, Lukasz Jesionowski, and Wojciech Nowak Institute of Advanced Energy Technologies, Czestochowa University of Technology, 42-200 Czestochowa, Poland ABSTRACT: Studies on combustible matter conversion during oxy-fuel combustion in a circulating fluidized-bed (CFB) environment are presented. A CFB facility has been modified to be ready for operation in an oxy-fuel mode, which means an elevated partial pressure of oxygen in a gaseous atmosphere of O2/CO2. The maximum thermal load of the unit is estimated at 0.1 MW, and the main dimensions of combustion chamber are as follows: a height of 5 m and an inner diameter of 0.1 m. Polish bituminous coal was burnt. The paper is focused on carbon and combustible sulfur behavior in a combustion chamber that runs under oxy-fuel CFB conditions. The analysis is based on sampling of in-furnace flue gases and calculations of carbon and sulfur conversion ratios. The effect of both oxygen fraction in O2/CO2 mixtures and excess oxygen was studied. An increased concentration of CO2 visibly inhibits fuel conversion and does not affect SO2 formation. A higher excess oxygen accelerates burnout of combustible matter; however, the value of 1.15 seems to be sufficient with respect to efficient combustion. The described investigations are the first part of the core work scheduled in the project “Advanced Technologies for Energy Generation: Oxy-combustion Technology for PC and FBC Boilers with CO2 Capture”. inner diameter hot cyclone. Solids separated in the cyclone return to the combustion chamber via a 0.075 m inner diameter downcomer and a non-mechanical loopseal. The unit is provided with three heaters around the combustion chamber. These heaters are used to heat the unit to (or beyond) the ignition point of the fuel being used. Primary gas (PG) is supplied through a ceramic grid at the bottom of the combustion chamber. This gas can be preheated with the help of two heaters (HE and PGH) installed in series. Secondary gas (SG) can be supplied optionally through a common-rail fitted with three nozzles at the level of 0.55 m above the grid. The oxygen fractions in PG and SG can be different because of two independent flow mixers. Both oxygen and CO2 are provided from gas cylinders, instead of flue gas recirculation. The beginning of the CO2 line just behind the cylinders, i.e., pressure-reducing station, is electrically heated, which prevents the gas supply system from becoming frozen. Fuel is fed continuously by a screw feeder located on a return leg, between the combustion chamber and the loopseal. Bed material can be fed by an additional screw feeder at the bottom of the combustion chamber. A single fabric baghouse filter, installed downstream of the cyclone, is used for ultimate flue gas particulate cleanup. The baghouse can be bypassed if necessary. The flue gas leaving the baghouse (or bypassing the baghouse) is vented with the help of an induced draft (ID) fan to the atmosphere, through a stack. The test rig is equipped with a developed data acquisition system for temperature (T) and pressure (P) measurements, as well as a number of ports for flue gas (FG), bottom ash (BA), fly ash (FA), and circulating material (CM) sampling. 2.2. Test Conditions and Measurements. Round sand of particle diameter dp = 0−1000 μm in size and Sauter mean diameter d3,2 = 224 μm was used as start-up bed material. The absolute density of sand was 2562 kg m3. The total mass of circulating solids was mbm = 5 kg. For the tests, O2/CO2 mixtures with different fractions of oxygen, i.e., CO2,ig = 21, 25, 30, and 35 vol % (first part) and CO2,ig = 35 vol % (second part), were used as fluidizing gas/oxidant. The use of bottled

1. INTRODUCTION Oxy-fuel combustion is one of the options for capturing CO2 from power plants, among so-called pre-combustion (i.e., gasification process) and post-combustion (i.e., absorption process) CO2 capture technologies.1 The crucial factor that differentiates oxy-combustion from air-firing is the CO2-based gaseous atmosphere, which strongly affects the combustion process, i.e., kinetics, heat transfer, and hydrodynamics. Although the process carried out in pulverized coal (PC)fired units is relatively well-comprehended,2 oxy-fuel combustion in circulating fluidized-bed (CFB) boilers still needs attention. Although the commissioning of the world’s first full pilot-scale 30 MWth oxy-fuel CFB unit was completed in 2011,3 the operating data are still not fully available. Thus far, the most advanced investigations in the field of oxy-fuel combustion in a CFB environment have been carried out by Anthony and coworkers, at two facilities of 100 and 800 kW.4,5 A certain targeted study in a 330 kW test rig has also been conducted by Ahn et al.6 Recently, great effort was made, and up-to-date knowledge of oxy-fuel combustion was collected by Zheng.7 However, there are still no data from multi-level measurements inside a furnace. Therefore, the interest in carbon and sulfur behavior under oxy-fuel CFB conditions was the main motivation to conduct the research described in this paper. Moreover, the aim of this work is to provide a background for further studies that are going to be focused on fuel nitrogen conversion. 2. EXPERIMENTAL SECTION 2.1. Test Rig. The investigations were carried out on a 0.1 MWth CFB test rig modified to operate under oxy-fuel conditions (Figure 1), which means an elevated partial pressure of oxygen in a gaseous atmosphere of O2/CO2. The part that has been mainly modified is a gas supply system. The unit consists of a 0.1 m inner diameter and approximately 5.0 m tall combustion chamber, connected to a 0.25 m © 2012 American Chemical Society

Received: March 8, 2012 Revised: September 5, 2012 Published: September 5, 2012 5437

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Figure 1. Oxy-fuel CFB test rig (0.1 MWth). CO2 instead of a flue gas recycle was intended in this stage of research, because the main intention is to study the combustible matter behavior in an O2/CO2 atmosphere. However, the next step is the research with real recycle, where C and S conversion overlaps the destruction of recycled C- and S-based flue gas compounds and further interactions. Therefore, the data that are presented in this paper are going to be used as a reference, which allows for comprehension of the process as a whole. The total gas flux was Fa = 21 × 3600−1 m3 s−1 standard temperature and pressure (STP) (including gas to the loopseal of Fls = 3 × 3600−1 m3 s−1 and gas to the fuel feeder of Fff = 1 × 3600−1 m3 s−1), and the PG/SG ratio was 70:30. The superficial gas velocity was varied in a range of v = 1.81−2.09 m s−1 in the bottom section (below SG level) with a dense phase and in a range of v = 2.56−3.05 m s−1 in the upper section (above SG level) with a dilute phase. The difference in gas velocity results from the gas staging and the different values of the mean temperature estimated for the different regions as well as from the assumption that the whole flux of Fff gets to the combustion chamber, whereas the flux of Fls splits into 2/3 and 1/3, which get to the combustion chamber and downcomer, respectively. The excess oxygen was kept at the level of λ = 1.25 (first part) and at levels of λ = 1.05, 1.15, 1.25, and 1.35 (second part). Bituminous coal of dp = 0−2500 μm in size and d3,2 = 438 μm was used as fuel (properties given in Table 1). The fuel flux was Ff = 3−4.5 × 3600−1 kg s−1.

Table 1. Properties of Fuel (on an Air-Dried Basis) bituminous coal −1

lower heating value (LHV) (kJ kg ) Proximate Analysis moisture (wt %) volatile matter (wt %) fixed carbon(by difference) (wt %) ash (wt %) Ultimate Analysis C (wt %) Scomb (wt %) H (wt %) N (wt %) O(by difference) (wt %)

21888−22728 15.0−16.0 30.6−32.7 39.7−42.5 10.3−13.8 55.6−58.0 1.25−1.31 3.74−3.86 0.85−0.90 8.82−10.01

The temperature in the combustion chamber was varied from 1030 to 1282 K, depending upon both the oxygen fraction in the inlet gas and excess oxygen (see also Figure 3). The pressure drop in the combustion chamber was kept at approximately 2500 Pa (Figure 2). Standard sheathed thermocouples of NiCr−NiAl and differential pressure transducers with an output signal of 4−20 mA were used for 5438

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Table 2. Height of Temperature and Pressure Measurement Points and Flue Gas Sampling Ports height of T measurement point (m)

Figure 2. Pressure profile along the height of the combustion chamber.

temperature measurements and pressure measurements, respectively. More details on fluid dynamics and thermal conditions can be found in the paper.8 Fourier transform infrared (FTIR) (for CO2, CO, SO2, and SO3), electrochemical (for H2S), and paramagnetic (for O2) methods were employed for the measurement of the flue gas components considered in this paper. The following ranges of wavenumbers are used for the identification and quantification of CO2, CO, SO2, and SO3, i.e., 926−1150, 2000−2200 and 2540−2590, 1050−1250, and 895−1380 cm−1, respectively. The whole flue gas sampling line (including probe, hose, and cell) is electrically heated, and the temperature is kept at 453 K. The length of the flue gas line is 5 m, regardless of the location of the sampling port. The FTIR method for monitoring of flue gas from oxy-fuel combustion has also been used by Sanchez et al.9 For H2S measurement, the instrument is calibrated using a gas mixture of synthetic air and H2S (55 ppm). The sampling frequency for all above-mentioned variables was 1 Hz. The measurement time was established at 3600 s (360 s for H2S) and starts after stable operating conditions in the combustor are achieved, which means bed change as well. Thus, the bed material consisted only of the fuel ash and a small fraction of unburned fuel, because no additive is fed during the tests. The list of temperature and pressure measurement points as well as flue gas sampling ports is given in Table 2. 2.3. Calculation Methods. Conversion ratios (CRs) of carbon (Cfuel) to CO2 and CO as well as combustible sulfur (Scomb fuel ) to SO2, SO3, and H2S were used to study Cfuel and Scomb fuel behavior during oxyfuel combustion in a CFB environment. The data were obtained from a multi-level sampling of in-furnace flue gases conducted in accordance with the above-described methodology. That analysis method allows us to follow the progress in conversion of carbon and sulfur along the whole height of the combustion chamber. The factors of CR were proposed and successfully used in previous investigations reported in the paper.10 The general assumptions are that (i) the distribution of C and S within volatile matter as well as char is uniform and (ii) the carbon conversion ratio to CO2 and to CO corresponds to the burnout ratio of combustible matter of fuel (the burnout ratio equals 100% when the fuel is completely incinerated). Moreover, CO2 in flue gas is considered to derive from both carbon oxidation and CO2 supplied in an inlet gas that forms a background for calculations. Thus, the values of CRC → CO2, CRC → CO, CRS → SO2, CRS → SO3, and CRS → H2S can be calculated using eqs 1−3, where “i” means the number of sampling port, “out” means the outlet of the combustion chamber, and square brackets mean the concentration of flue gas components expressed as a percentage (CO2) or parts per million (CO, SO2, SO3, and H2S). For instance, the conversion ratio of combustible sulfur contained in fuel to SO2 is

height of P measurement point (m)

height of FG sampling port (m)

below grid

below grid

T1, −0.07

P1, −0.07

combustion chamber T2, 0.02

combustion chamber P2, 0.01

FG3, 2.50

T3, 0.20

P3, 0.065

FG4, 4.88

T4, 0.30

P4, 0.20

flue gas duct

T5, 0.51 T6, 1.49 T7, 2.50 T8, 3.41 T9, 5.21 cyclone T10, 5.37 return leg T11, 0.65 T12, 1.14 T13, 2.76 flue gas duct T14, 5.77

P5, 0.51 P6, 1.45 P7, 2.44 P8, 3.42 P9, 4.63 return leg P10, 0.55 P11, 1.03 P12, 2.69 flue gas duct P13, 5.86

FG5, 5.91

CR S → SO2 =

combustion chamber FG1, 0.43 FG2, 1.45

height of other points (m) grid, 0.00 combustion chamber return solids entrance, 0.28 solids feed point, 0.35 secondary gas distribution point, 0.55 fuel feed point, 0.65 viewpoint, 3.15

SO2 Sfg, i CO2 + CO C fg,out comb Sfuel C fuel

(1)

where the molar ratio of S (fixed in SO2)/C (fixed in CO2 + CO) in flue gas is SO2 Sfg, i CO2 + CO Cfg,out

=

[SO2 ] 10 [CO2 ] + [CO] 4

⎛ kmolS ⎞ ⎜ ⎟ ⎝ kmol C ⎠

(2)

and the molar ratio of S/C in fuel is comb Sfuel = Cfuel

comb s fuel 32 c fuel 12

⎛ kmolS ⎞ ⎜ ⎟ ⎝ kmol C ⎠

(3)

The factors of CRS and CRC are defined in a corresponding way; thus, the equations for SO3, H2S, CO2, and CO are not given in this paper. The main advantage of the analysis method proposed here is the independence of the results from (i) changes in the moisture and ash contents in the fuel, (ii) changes in the volatile/char ratio, (iii) values of the incomplete combustion losses, and (iv) measurements of fuel flux, as well as oxygen and CO2 fluxes, because the above-mentioned parameters are excluded from the calculations (see eqs 1−3). Furthermore, the processing data that are converted into the factors of CR can be compared directly, regardless of the point of flue gas sampling. That kind of approach is not sensitive to gas staging, where flue gas compounds that are forming in the bottom zone of the furnace are further diluted in a secondary gas stream. It seems to be crucial in this study, where multi-level flue gas sampling was conducted.

3. RESULTS AND DISCUSSION 3.1. Temperature Profiles. The oxy-fuel CFB test rig (Figure 1) is not equipped with any internal or external heat exchangers (i.e., inside the combustion chamber, e.g., Wing5439

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Figure 3. Temperature distributions along the height of the combustion chamber.

Figure 4. Carbon conversion ratio: the influence of the oxygen concentration.

During the investigations described in this paper, both the temperature of inlet gas and the thickness of thermal insulation on the return leg remained unchanged. Therefore, the effect of an increased thermal load associated with both an elevated partial pressure of oxygen (Figure 3a) and changes in excess oxygen (Figure 3b) on the temperature in the unit can be observed. It follows from fixed flux of inlet gas (see also section 2.2), while excess oxygen was kept at a variable fraction of O2 in a gaseous atmosphere as well as varied at a stable fraction of O2. This approach allows us to maintain the gas velocity if only an

Walls, or externally located in the return leg, e.g., Intrex) that could be employed to control the temperature in the CFB loop. However, the thermal conditions in the unit can be changed in two ways: first, the inlet gas preheating and, second, taking an insulation off the return leg (loopseal and downcomer), which enhances heat transfer from the wall to the atmosphere and, hence, cools the circulating solids. These actions result in a temperature drop of up to 150 K that appears to be sufficient even for oxygen fractions in the O2/CO2 mixtures of up to 35 vol % (see Figure 3a). 5440

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Figure 5. Carbon conversion ratio: the influence of excess oxygen.

above the first flue gas sampling port (FG1; H/H0 = 0.088), regardless of the oxygen fraction in an inlet gas. However, the oxidation of CO is noticed only at CO2,ig of 30 and 35 vol %. The relations that can be observed above the level of FG2 (H/H0 = 0.297) for the conditions of lower partial pressures of oxygen (panels a and b of Figure 4) appear to be quite different from those noticed for higher values of CO2,ig (panels c and d of Figure 4). For instance, there is a decrease in the value of CRC → CO2 associated with an increase of the carbon conversion ratio to CO, under the conditions of a lower oxygen concentration in an inlet gas (panels a and b of Figure 4). That unexpected behavior can only be explained by the Boudouard reaction (eq 4)11

excess heat could be removed from the combustion chamber. It can be seen in Figure 3 that all profiles look alike, although an increment in the mean temperature of 130 K is observed with the increase of the oxygen concentration from 21 to 35 vol %, whereas an increase in excess oxygen from 1.05 to 1.35 results in a temperature drop of 100 K. Moreover, what can be seen is a strong increase in the temperature at the bottom zone (below the level of SG distribution points), in contrast with a relatively even profile of the temperature in the upper part of the furnace. 3.2. Carbon Conversion. Conversion ratios of C to CO2 and CO in O2/CO2 mixtures with different oxygen fractions are shown in panels a−d of Figure 4. The influence of excess oxygen is presented in panels a−d of Figure 5. However, with respect to the above-mentioned relations, the effect of both an elevated partial pressure of oxygen and excess oxygen overlaps the thermal effect here, which should be taken into consideration in all further analyses. The height of the combustion chamber in Figures 4 and 5 and further Figures 6 and 7 is expressed as a non-dimensional parameter of H/H0, where “H0” means the height of the flue gas sampling point located at the top of the combustion chamber (FG4). First and foremost, relatively high values of the carbon conversion ratio (CRC → CO2 + CO > 60%) in the bottom zone with a reducing atmosphere can be seen in panels a−d of Figure 4, where the conversion of C to CO seems to be rather considerable. Moreover, CRC → CO2 + CO increases with an increase in the oxygen concentration and ultimately exceeds 80% at CO2,ig = 35 vol %. Further, fuel burnout can be observed

CO2 + C → 2CO

(4)

which needs to take place in this region, despite a relatively low temperature. This reaction is mostly neglected in air-firing; however, Scheffknecht et al.1 indicate that it may play an important role under an elevated partial pressure of CO2, which associates with oxy-fuel combustion. Furthermore, the oxidation of C seems to disappear in the upper part of the furnace (above FG3; H/H0 = 0.512), which results in a high fraction of unburned carbon in the fly ash. The increased values of char carbon content in the fly ashes obtained from an oxyfuel CFB unit were also reported by Wu et al.12 On the one hand, it is suspected that both the high concentration of CO2 and high fraction of CO in a gaseous atmosphere strongly inhibit the fuel from being rapidly converted. On the other 5441

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Figure 6. Sulfur conversion ratio: the influence of the oxygen concentration (“free” means that the compound was not detected in flue gas).

contained in a circulating material passing through the return leg, is to be subjected to further analyses. The strongest effect of excess oxygen (panels a−d of Figure 5) can be seen in the bottom zone (FG1; H/H0 = 0.088). The value of the carbon conversion ratio CRC → CO2 + CO increases visibly with an increase of excess oxygen, to be close to 90% at λ = 1.35. However, the oxidation of carbon monoxide seems to fall behind with carbon conversion, and thus, higher fractions of CO are observed at both λ = 1.25 and 1.35. Nevertheless, almost all carbon monoxide is converted to CO2 while passing through the upper part of the furnace. The exception constitutes the case of λ = 1.05, where a rather considerable concentration of CO is noticed at the outlet of the combustion chamber. Therefore, it can be claimed that the efficient combustion needs the excess oxygen of 1.15 under the investigated conditions. The lower value of λ leads to incomplete combustion loses and can affect the problems with corrosion. Otherwise, the higher excess oxygen causes the drop in a combustion temperature, while unreacted O2 becomes an impurity in the flue gas. Moreover, an “overproduction” of oxygen that is rather expensive decreases the efficiency of the whole power system. 3.3. Sulfur Conversion. The following conversion ratios of sulfur contained in the combustible matter of fuel are shown in panels a−d of Figure 6. SO2 was found to be the main S-based flue gas compound; however, SO3 and H2S were detected in some cases as well.

hand, however, it can be expected that the combustion proceeds to a small extent at least, because of an oxidizing atmosphere and high-temperature conditions, which unfortunately is not reflected in the processing data. Nevertheless, the oxidation of CO to CO2 is observed in this region. What was found also under the conditions of higher partial pressures of oxygen (panels c and d of Figure 4) is a further progress in carbon burnout and the oxidation of CO as well, which can be observed along the height of the combustion chamber, beginning from the level of FG2. It should also be mentioned that the progress in combustible matter conversion in O2/CO2 mixtures with the oxygen fraction of 30 and 35 vol % is comparable to the results obtained for air conditions.13 Thus, it can be claimed that the substitution of N2 for CO2 in an inlet gas requires a higher concentration of O2 with reference to air. The necessity to increase the fraction of oxygen in a gaseous atmosphere from 21 to 27 vol % at least, while switching from air-firing combustion to oxy-fuel mode, was also identified by Okazaki et al.14 and Burchhardt et al.15 The value of Cunburnt corresponds to the unconverted carbon fuel that is fixed in unburned fuel particles, which can be found in the flue gas leaving the combustion chamber. Thus, the carbon balance that is limited to the combustion chamber includes both C that converts progressively to CO and CO2 along the height of the furnace and C that remains unconverted even at the level of the outlet of the combustion chamber. The whole balance, which includes C that is contained in the fluxes of bottom ash, fly ash, and loopseal ash as well as C that is 5442

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Figure 7. Sulfur conversion ratio: the influence of the excess oxygen concentration (“free” means that the compound was not detected in flue gas).

What was found also is a compound of H2S, which appears for the oxygen concentrations of 25 vol % and higher (Figure 6b). Then, there was noticed an increased concentration of H2S in a flue gas under oxygen-enriched conditions (panels c and d of Figure 6), which is unexpected and hard to comprehend. A relatively high fraction of H2S can be observed along the whole height of the combustion chamber. The high levels of H2S that are reported in this paper are most likely a scale effect, because the combustion chamber of the test rig is approximately 8−10 times lower than a commercial CFB boiler.17 However, Andersson et al.18 concluded that H2S formation may be significantly increased under the conditions of oxy-coal combustion. SO3 was detected only at the lower partial pressures of oxygen (panels a and b of Figure 6). However, SO3 appears only locally, i.e., at the level of FG2, just above the secondary gas distribution point, as well as at the level of FG4, at the top of the combustion chamber. SO3 is to be considered as a residual impurity rather than a flue gas component, because of a very slight fraction compared to SO2. Furthermore, what can be seen in panels a−d of Figure 6 is a decrease in the value of CRS → SO2 + SO3 that can be observed in the upper part of the furnace; this effect is much stronger under the conditions of an elevated partial pressure of oxygen (panels c and d of Figure 6). That behavior can only be explained by bounding of sulfur oxides with fuel ash components (e.g., eq 5),19 because no additive was fed to the furnace during the

First of all, it can be seen that the values of CRS → SO2 + SO3 + H2S (panels a−d of Figure 6) are comparable (±10%) to those of CRC → CO2 + CO (panels a−d of Figure 4) at the level of the first flue gas sampling port. This seems to confirm the assumption of the uniform distribution of C and S within combustible matter. In one case (Figure 6c), however, the calculations of the total conversion ratio of combustible sulfur give values that slightly exceed 100%. With regard to the proposed analysis methodology, this error can result from both the following (see also eq 1): (i) the conversion of S was faster than the conversion of C, although the recirculation of solids seems to reject this explanation, because most unburned fuel particles are converted together with fresh fuel in the following cycles, and (ii) the ratio of S/C in the samples of fuel that were taken during the test is unrepresentative; however, there is no evidence to suggest that it took place. Another explanation can be that sulfur that is fixed in the fuel ash, which is excluded from calculations, passed into S-based flue gas compounds under the investigated conditions. Nevertheless, this error does not affect the qualitative analysis and the tendencies that are revealed from the results. Generally, the values of CRS → SO2 are relatively high (>80%), which is expected under conditions of CFB combustion.16 Moreover, it can be seen that oxidation of sulfur is rather fast and is completed already at the bottom part of the furnace. However, the rate of sulfur conversion increases visibly with an increase in the oxygen concentration in an inlet gas and, hence, with an increase in the temperature. 5443

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gaseous atmosphere. However, the high levels of H2S that are reported in this paper are suspected to be a scale effect, because the combustion chamber of the test rig is several times lower than commercial CFB boilers. Furthermore, the phenomenon of self-desulfurization was noticed; the effect becomes stronger under conditions of an elevated partial pressure of oxygen.

tests. That phenomenon used to be called self-desulfurization, which is common in the fluidized-bed boiler technology.16 CaOash + SO2 + 0.5O2 → CaSO4

(5)

On the one hand, the oxygen-enriched conditions seem to be conducive to the final emission of SO2, mainly because of the phenomenon of self-desulfurization. On the other hand, however, the increased fraction of H2S that was noticed under an elevated partial pressure of oxygen may affect the properties of ash, i.e., by the formation of alkali sulfides (eq 6),20 as well as may contribute to the intensified corrosion of the boiler. CaO + H 2S → CaS + H 2O

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +48-34-3250945. Fax: +48-34-3250933. E-mail: [email protected]. Notes

(6)

The authors declare no competing financial interest.

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Panels a−d of Figure 7 show a significant increase in the sulfur conversion ratio CRS → SO2 + SO3 + H2S determined for the bottom zone that follows an increase of excess oxygen, which to some extent can be expected. It proves that the amount of free oxygen in a combustion process is the crucial factor, because an increase in excess oxygen associates in this research with a decrease of the temperature. SO3 was not detected along the whole height of the combustion chamber during the tests at CO2,ig = 35 vol % and different excess oxygen, which confirms previous observations (see also Figure 6). The lowest value of CRS → H2S is noticed at the lowest level of excess oxygen; however, it is suspected that the behavior results from higher temperatures in the furnace rather than from a deficiency of oxygen. Moreover, the reaction of combustible sulfur with fuel ash components disappears completely under the conditions of the lowest excess oxygen and the highest temperature, where the influence of the temperature that is close to 1273 K seems to be crucial. On the contrary, the most efficient selfdesulfurization is found in the test conducted at the lowest temperature and the highest excess oxygen. It can also be seen in Figure 7a that the value of total sulfur conversion slightly exceeds 100%. A detailed explanation of that kind of event is given above (see also Figure 6c).

ACKNOWLEDGMENTS

Scientific work was supported by the National Centre for Research and Development, as Strategic Project PS/E/2/ 66420/10 “Advanced Technologies for Energy Generation: Oxy-combustion Technology for PC and FBC Boilers with CO2 Capture”. The support is gratefully acknowledged.

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4. CONCLUSION The behavior of both carbon and combustible sulfur during oxy-fuel combustion in a CFB facility was studied in this paper. The formation of CO2 and CO as well as SO2, SO3, and H2S along the height of the combustion chamber was analyzed. The progress in carbon conversion in O2/CO2 mixtures strongly depends upon the oxygen fraction. The results indicate that the high concentrations of both CO2 and CO in a gaseous atmosphere, which associate with lower fractions of oxygen (21 and 25 vol %), inhibit the burnout of combustible matter. What was also found is a decrease in the conversion of C to CO2 associated with an increase of the carbon conversion to CO, which took place under the conditions of a lower partial pressure of oxygen, and hence, the appearance of the Boudouard reaction is presumed. The excess oxygen of 1.15 was established to be reasonable under the investigated conditions with respect to the efficient combustion. SO2 was found to be the main S-based flue gas compound; however, SO3 and H2S were detected in the same cases as well. The oxidation of sulfur proceeds fast, and the values of the total sulfur conversion ratio are relatively high, regardless of the temperature and concentration of O2 in a gaseous atmosphere. The oxygen-enriched environment favors the formation of H2S, whereas SO3 appears rather at a lower fraction of O2 in a 5444

NOMENCLATURE cfuel = carbon mass fraction in fuel CO + CO Cfg 2 = molar fraction of carbon fixed in CO2 and CO in flue gas (kmol m−3) Cfuel = carbon molar fraction in fuel (kmol kg−1) Cunburnt = unconverted carbon fuel CO2,ig = volume fraction of oxygen in inlet gas (%) CRC → CO = conversion ratio of carbon to carbon monoxide CRC → CO2 = conversion ratio of carbon to carbon dioxide CRS → H2S = conversion ratio of combustible sulfur to hydrogen sulfide CRS → SO2 = conversion ratio of combustible sulfur to sulfur dioxide CRS → SO3 = conversion ratio of combustible sulfur to sulfur trioxide dp = particle diameter (μm) d3,2 = Sauter mean diameter (μm) Fa = total gas flux (m3 s−1) Ff = fuel flux (kg s−1) Fff = gas flux to fuel feeder (m3 s−1) Fls = gas flux to loopseal (m3 s−1) H = absolute height of the combustion chamber (m) H/H0 = relative height of the combustion chamber mbm = mass of circulating solids (kg) P = pressure (Pa) scomb fuel = mass fraction of combustible sulfur in fuel 2 SSO fg = molar fraction of sulfur fixed in SO2 in flue gas (kmol m−3) Scomb fuel = molar fraction of combustible sulfur in fuel (kmol kg−1) Sunburnt = unconverted sulfur fuel STP = standard temperature and pressure (T = 293.15 K and P = 101 325 Pa) T = temperature (K) v = superficial gas velocity (m s−1) λ = excess oxygen [CO] = CO concentration in flue gas (ppm) [CO2] = CO2 concentration in flue gas (%) [SO2] = SO2 concentration in flue gas (ppm) dx.doi.org/10.1021/ef3011838 | Energy Fuels 2012, 26, 5437−5445

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dx.doi.org/10.1021/ef3011838 | Energy Fuels 2012, 26, 5437−5445