Combustion of Single Coal Char Particles under Fluidized Bed

Jul 21, 2010 - Figures 5 and 6 report the carbon burning rate (calculated with eq 3) as ..... coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions Fue...
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Ind. Eng. Chem. Res. 2010, 49, 11029–11036

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Combustion of Single Coal Char Particles under Fluidized Bed Oxyfiring Conditions Fabrizio Scala* and Riccardo Chirone Istituto di Ricerche sulla Combustione, Consiglio Nazionale delle Ricerche Piazzale Vincenzo Tecchio 80, 80125 Napoli, Italy

The fluidized bed combustion of single coal char particles was investigated at high CO2 concentrations, typical of oxyfiring conditions, at different bed temperatures and oxygen concentrations. The conversion rate of the char particles was followed as a function of time by continuously measuring the outlet CO and O2 concentrations. Char gasification tests were also carried out under 100% CO2 at different temperatures to quantify the importance of this reaction and to extract a suitable kinetic expression. This expression was then combined with a correlation for the mass transfer controlled particle burning rate to simulate the experimental conversion rate data. The calculated carbon consumption rate was an excellent fit to the experimental data for all the operating conditions. Results showed that carbon combustion dominates particle conversion at high oxygen concentrations and low temperatures, while carbon gasification contributes to a comparable extent at high temperatures and low oxygen concentrations. Even under fluidized bed oxyfiring conditions oxygen boundary layer diffusion controls the combustion rate, and the main combustion product is CO2 rather than CO. Introduction Coal-based power generation is responsible for roughly 70% of the global CO2 emissions related to power generation and for about 20% of the global CO2 emissions. New technologies are under investigation with the aim of reducing greenhouse gas emissions from coal power plants, with particular attention to CO2. In particular, technologies that can accommodate capture and sequestration of carbon dioxide are of primary interest, since they can easily retrofit existing plants. Within the available options, oxyfiring (or oxyfuel) technology appears to be relatively mature. Oxyfiring can produce an almost pure CO2 outlet stream, by using pure oxygen instead of air for coal combustion, so that the costs of CO2 separation from the flue gas can be substantially reduced.1 Flue gas is partly recycled back into the furnace to control the combustion temperature. As a consequence, the combustion chamber environment experienced by the coal particles contains practically no nitrogen, while carbon dioxide is present at high concentration levels. It has been recently pointed out that research is still needed to obtain a more fundamental understanding of the differences between oxyfiring and conventional air-fired combustion.1 One particular issue regards the combustion characteristics of coal char in an O2/CO2 atmosphere. From a general point of view, the reactions involving carbon-containing species that can occur during the oxy-combustion process are C(s) + O2 f CO2

(A)

1 C(s) + O2 f CO 2

(B)

C(s) + CO2 f 2CO

(C)

1 CO + O2 f CO2 2

(D)

where reactions A and B represent the carbon combustion reactions, reaction C represents the carbon gasification by CO2, * To whom correspondence should be addressed. E-mail: scala@ irc.cnr.it.

and reaction D is the homogeneous CO oxidation reaction. It has been suggested that under oxyfiring conditions (especially at the high temperatures experienced in pulverized fuel combustion) carbon gasification by CO2 (reaction C) might contribute significantly to the char mass loss.1,2 Most of the research activity carried out to date in oxyfiring has concentrated on pulverized coal boilers.2 Recently, the application of this technology to circulating fluidized bed (CFB) coal boilers has been examined.3-7 CFBs appear to be particularly suited for oxyfiring conditions because of the fuel flexibility and better temperature control (which allows to reduce the amount of recycled flue gas). The feasibility of CFB coal oxyfiring has been successfully demonstrated in pilot-plant tests, and no particular technological barrier appears to exist for implementing this technology in the near-term. However, the combustion characteristics of coal char in an O2/CO2 atmosphere under conditions typical of CFB combustion have not been investigated in depth so far. A preliminary investigation was performed in a lab-scale fluidized bed at 850 °C and showed that char combustion was mostly controlled by external diffusion of oxygen through the particle boundary layer.8 The particle temperature was approximately equal to the bed temperature up to an oxygen concentration of 2%; it was higher for larger oxygen concentrations. Both CO2 gasification of char and homogeneous CO oxidation appeared to be non-negligible. In this work, we pursue further the analysis by studying the fluidized bed combustion of single large coal char particles under simulated oxyfiring conditions at varying inlet oxygen concentrations and bed temperatures. The burning rate of the particles was followed as a function of time by continuously measuring the outlet CO and O2 concentrations.8 Gasification tests were also performed under 100% CO2 conditions at different temperatures in order to extract a kinetic expression for gasification of the specific char used in the experiments. This expression was combined with an available correlation for the mass transfer controlled particle burning rate to simulate the experimental

10.1021/ie100537a  2010 American Chemical Society Published on Web 07/21/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 Table 1. Properties of Snibston Coal Proximate Analysis, % (as Received) moisture ash volatile matter fixed carbon

14.6 4.0 35.2 46.2

Ultimate Analysis, % (Dry and Ash-free Basis) carbon hydrogen nitrogen oxygen sulfur char density (kg/m3)

Figure 1. Experimental apparatus: (1) gas preheating section; (2) electrical furnaces; (3) ceramic insulator; (4) gas distributor; (5) thermocouple; (6) fluidization column; (7) gas suction probe; (8) stack; (9) cellulose filter; (10) membrane pump; (11) gas analyzers; (12) personal computer; (13) manometer; (14) digital mass flowmeters.

carbon conversion rate data and to disclose the relative importance of gasification versus combustion. Experimental Section Apparatus. A circular stainless steel atmospheric bubbling fluidized bed reactor 40 mm ID and 1 m high was used for the experiments (Figure 1). The gas distributor was a 2 mm thick perforated plate with 55 holes 0.5 mm in diameter disposed in a triangular pitch. A 0.6 m high stainless steel column, containing a number of steel nets for gas preheating and mixing, was placed under the distributor. The fluidization column and the preheating section were heated by two semicylindrical electric furnaces. The temperature of the bed, measured by means of a thermocouple placed 40 mm above the distributor, was kept constant by a PID controller. Temperature variations during the runs were always within (1 °C of the set point. Gases were fed to the column via two high-precision digital mass flowmeters/controllers (accuracy (1% full scale). Each flowmeter/controller was calibrated with a bubble flowmeter. Gases were supplied from two cylinders containing carbon dioxide and oxygen. The top section of the fluidization column was left open to the atmosphere and the exit gas was sucked by a hood. A stainless steel probe was inserted from the top of the column to convey a known fraction (0.06 m3/h) of the exit gas directly to the gas analyzers. A high efficiency cellulose filter was inserted in the line to avoid dust entrainment into the analyzers. The probe, 2 mm ID, was positioned 0.6 m above the distributor, approximately at the axis of the column. The absence of any gas leakage and/or suction to/from the surrounding environment in the sampling probe and line was carefully checked. A NDIR analyzer (accuracy (1% full scale) was used for online measurement of CO, and a paramagnetic analyzer (accuracy (2% full scale) was used for O2 concentration in the exhaust gases. The measuring range of the O2 analyzer was 0-10% (v/v) and for CO was 0-1000 ppmV. Data from the analyzers were logged and further processed on a PC. Materials. Snibston bituminous coal was selected as the test fuel. Table 1 reports the fuel properties. Char from this coal

81.3 5.3 1.6 10.8 1.0 1040

has been reported to have the lowest propensity to attrition and fragmentation among any of the coals studied in previous investigations.9 The fuel particles were first devolatilized by dropping them in the fluidized bed with N2 at 850 °C. After 5 min, char particles were retrieved from the bed, cooled down in N2 and machined into almost spherical particles with an average size of ∼6 to 7 mm (Figure 2). The particles were further preprocessed in air for 8 h in the fluidized bed at ambient temperature and 0.3 m/s to smoothen the particle surface. The bed material consisted of 180 g of quartz sand, corresponding to an unexpanded bed height of 0.1 m. Sand was double sieved in the 500-600 µm particle size range. The minimum fluidization velocity was 0.13 m/s. The particle density of the quartz sand was 2560 kg/m3, and the bed voidage at minimum fluidization was 0.44. Procedures. In all the tests performed in this experimental campaign the inlet oxygen concentration was lower than 10% (v/v), because these are the relevant local O2 concentrations near a burning particle in a full-scale fluidized bed combustor. This consideration is based on experimental evidence reported by Leckner,10 which show that very limited oxygen penetration occurs in the bottom dense zone of an air-fired CFB combustor, where most of the char particles burn. No similar data are available for the oxy-fired case, but the above conclusions are likely to apply also in this case, since the fluid-dynamics of an oxy-fired CFB furnace is substantially similar to that of an airfired one. The experiments were performed in the fluidized bed at 800, 850, or 900 °C, atmospheric pressure, with a fluidization velocity of 0.3 m/s, which corresponds to bubbling/slugging conditions.11 An inlet gas mixture was introduced made of CO2 and O2 in the combustion tests and 100% CO2 in the gasification tests; O2 concentration in the combustion tests was fixed at 0.5, 1, 2, 4.5, or 8% (v/v). Each test consisted of the injection of one char particle, of known mass and diameter, in the fluidized bed and in the continuous measurement of CO and O2 concentrations at the outlet. To obtain the char burning rate

Figure 2. Coal char particles used for the experiments.

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Figure 3. CO and O2 concentration profiles during an oxyfiring char combustion test at T ) 850 °C, O2 ) 2% (v/v).

during the tests, mass balances on oxygen (eq 1) and carbon (eq 2) in the reactor were carried out, under the pseudostationary assumption m ˙ Oin2 +

32 in 16 out 32 out m ˙ ˙ (t) + m ˙ (t) )m ˙ Oout2 (t) + m 44 CO2 28 CO 44 CO2

12 in 12 out 12 out m ˙ (t) + m ˙ (t) m ˙ +m ˙ cons C (t) ) 44 CO2 44 CO2 28 CO

(1) (2)

where m ˙ in ˙ out are the mass flow rates of species i that i and m i enter and exit the reactor in the gas flow, respectively, and m ˙ cons C is the carbon consumption rate by the reaction, all in kg/s. Combining these two equations gives m ˙ cons C (t) )

12 in 3 out [m ˙ -m ˙ (t) ˙ Oout2 (t)] + m 32 O2 14 CO

(3)

By integrating the curve obtained with eq 3 between time zero and the end of the test (tf), it was possible to calculate the total mass of carbon consumed by the reaction during the test: mcons ) C

∫

tf

0

m ˙ cons C (t) dt

(4)

Assuming that the spherical particle burns according to a constant-density shrinking-particle model,12 the char particle diameter as a function of time is given by d(t1) ) (6(mC0 -

∫

t1

0

1/3 m ˙ cons C (t) dt)/πFchar)

(5)

where mC0 is the initial mass of carbon in the particle, and Fchar is the carbon density in the particle. Results and Discussion Char Combustion Tests. Figure 3 shows the typical concentration profiles of CO and O2 at the exit of the bed for a combustion test at 850 °C with 2% O2 at the inlet. Similar profiles were obtained at the other oxygen concentrations and temperatures, except for the two tests at 0.5% O2 (at 850 °C) and at 900 °C (at 2% O2). In Figure 3 it can be noted that just after char particle injection a peak in CO concentration was measured. This peak was most likely caused by O2 adsorbed on the particle at ambient conditions before the test. The peak

was considered for carbon mass balance calculations, but was discarded for char burning rate analysis. The absence of other significant peaks or sudden changes in the slope of the CO profile confirmed that no particle fragmentation occurred during char conversion. Using the data from these curves together with eqs 3 and 5 it was possible to determine the variation of the particle diameter with time, under the assumption of a constantdensity shrinking-particle conversion pattern. The CO and O2 concentration profiles measured for the combustion test at 900 °C and 2% O2 are reported in Figure 4. Similar profiles were obtained in the test at 0.5% O2 and 850 °C. The CO profile in Figure 4 is qualitatively different from that reported in Figure 3 showing the presence of a relative maximum at intermediate conversions. As will be discussed later this is a strong indication of the relevance of the CO2 gasification reaction in these conditions. Figures 5 and 6 report the carbon burning rate (calculated with eq 3) as a function of the particle diameter for the oxyfiring combustion tests at different O2 inlet concentrations and bed temperatures, respectively. Results show that the combustion rate increases as the inlet O2 concentration and the bed temperature increase. These results can be expected both in the case of kinetically controlled and diffusion controlled combustion. Figures 5 and 6 also show that the combustion rate increases almost linearly with the particle diameter, at least at high O2 concentrations and low bed temperatures, indicating that diffusion of oxygen through the particle boundary layer is likely to be the most important mechanism controlling the burning process, as was the case during air combustion of the same char.12 This result justifies the simplifying assumption of a constant-density shrinking-particle conversion model for this char. When comparing these burning rate data with those obtained in combustion tests with the same char in O2/N2 atmospheres under the same operating conditions and in the same apparatus,12 it was noted that under oxyfiring conditions a slightly higher (∼30 to 40%) burning rate was measured for all particle diameters and oxygen concentrations. This result is consistent with those reported by Rathnam et al.2 for pulverized coals, and was explained by these authors as a possible contribution of the carbon-CO2 gasification reaction at temperatures above 1030 K. The carbon mass balance for the char particle was calculated by comparing the initial carbon mass in the particle and the

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Figure 4. CO and O2 concentration profiles during an oxyfiring char combustion test at T ) 900 °C, O2 ) 2% (v/v).

total amount of carbon consumed during the test (eq 4). For all the tests under simulated oxyfiring combustion conditions this balance had an absolute error which was always less than 5%. Char Gasification Tests. Another series of tests was performed at different bed temperatures to quantify the importance of reaction C during the tests. Outlet CO concentration profiles obtained during the gasification tests (100% CO2) at the three temperatures are shown in Figure 7. Some experiments were also performed with an O2 trap (outlet O2 concentration < 1 ppmV) in the gas feeding line to make sure that no oxygen impurity could enter the reactor, but results showed no difference with respect to the tests without the O2 trap. The gasification tests clearly show a much longer reaction time with respect to the combustion ones. Figure 8 shows the carbon consumption rate as a function of time for the same gasification tests. For these tests the curves were not plotted as a function of diameter, since in these conditions the constant-density shrinking-particle model (and in turn eq 5) is likely not to apply anymore. By comparing Figures 5-6 and 7-8 it can be noted that the gasification reaction rate is typically slower than the combustion rate (at least for oxygen concentrations larger than 1%) and that it is less dependent on the actual char particle mass (especially for the first half of the run, see Figure 8), denoting a dominating contribution of the intrinsic kinetics on the reaction rate. The presence of a maximum of the carbon consumption rate at intermediate burnoff has been often observed during gasification of different fuels under intrinsic kinetics control, and can be explained by the effect of pore widening and of the increase of accessible surface area by carbon consumption.13 The carbon mass balance calculation for the char particle in the gasification tests indicated that the error was of the order of 20% (the calculated consumed mass was always smaller than the initial mass of carbon). This suggests that particle abrasion by attrition is important during the gasification tests and that part of the char probably exited the reactor as elutriated material without reacting. This is consistent with recently reported results of carbon attrition of different fuel chars in fluidized bed under CO2 gasification conditions.14 It was shown that attrition of carbon fines from the char particles during gasification is extensive, due to a so-called gasification-assisted attrition mechanism, where carbon consumption in the particles progressively weakens the char structure by pore enlargement. The low reactivity of the generated fines under gasification conditions

makes the loss of carbon by fines elutriation much more significant than that typically found under combustion conditions. Analysis of Char Gasification Kinetics. The carbon consumption rate data for the gasification tests have been analyzed under the assumption that the reaction rate is completely controlled by intrinsic kinetics. This assumption is justified by the low temperature (e900 °C) of the tests, by the 100% CO2 environment in the gas phase (implying limited CO2 diffusion resistance from the gas bulk to the particle) and by the shape of the curves in Figures 7 and 8, as discussed before. Since only tests at 100% CO2 concentration were performed, we did not consider the variation of the carbon dioxide concentration in the kinetic expression. The carbon consumption rate as a function of time for a gasification reaction can be correlated with the following expression:

( )

m ˙ gasif ) mC0A(X)k0 exp C

Ea RT

(6)

where k0 is the pre-exponential factor, X ) 1 - mC/mC0 is the carbon conversion degree (in which the effect of attrition was taken into account by subtracting the total carbon loss from the initial carbon mass), A(X) is a nondimensional term accounting for the variation of reactivity with conversion (as a consequence of the evolution of the internal porosity and surface area), and Ea is the activation energy. In the present analysis the value A(X) ) 1 was arbitrarily set at a particle conversion degree X ) 0.1. By taking the experimental particle gasification rate at this specific conversion for the three temperatures investigated, the Arrhenius analysis gave an activation energy value Ea/R ) 35400 K, which is very similar to values reported in the literature for other coal chars,15 and a pre-exponential factor k0 ) 2.4 × 109 s-1. Finally, by fitting eq 6 with the above parameters’ values to the gasification experimental data, the following functional form of A(X) was found to satisfactorily fit the curve shape: A(X) ) 0.457 + 4.185X - 4.657X2 + (0.015/X)

(7)

valid for X > 0.005. Analysis of Char Combustion Kinetics. A simplified approach was used to analyze the oxyfiring combustion rate data. Carbon combustion was assumed to be completely

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Figure 5. Measured and calculated carbon consumption rates as a function of particle diameter during oxyfiring char combustion tests at different O2 concentrations, T ) 850 °C.

controlled by boundary-layer oxygen diffusion. This assumption was based on the previous analysis of the combustion rate curves, and on the fact that air combustion of the same char under comparable operating conditions was demonstrated to be controlled by boundary-layer oxygen diffusion alone.12 Under external diffusion control (assuming a vanishing O2 concentration at the surface of the char particle) and under the pseudostationary assumption, the carbon consumption rate by combustion for a spherical carbon particle can be expressed as m ˙ comb ) πdShDO2COout2 C

(8)

where Sh is the particle Sherwood number, DO2 is the O2 molecular diffusivity and Cout O2 is the oxygen outlet concentration. This expression is based on the following additional assumptions: (a) carbon combustion occurs following reaction A, and negligible quantities of CO are produced; (b) the oxygen

concentration in the bulk of the bed equals the outlet measured concentration. The first assumption has been verified under air combustion conditions12 but is somewhat arbitrary here, since no data are available in the literature under oxyfiring conditions. However, the present results strongly support this assumption as will be more evident later on. The second assumption is justified by the differential nature of the experiments with respect to the oxygen concentration: oxygen conversion degree in the reactor was always below 5%. The O2 molecular diffusivity in the gaseous mixture used in the present experiments was evaluated to be DO2 ) 1.67 × 10-4 m2/s at 850 °C and was corrected for the influence of temperature for tests at 800 and 900 °C. The particle Sherwood number was calculated by means of an accurate correlation recently reported in the literature and based on experimental data gathered in the same experimental apparatus and under similar operating conditions.11 In that work the mass transfer

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Figure 6. Measured and calculated carbon consumption rates as a function of particle diameter during oxyfiring char combustion tests at different bed temperatures, O2 ) 2% (v/v).

Figure 7. CO concentration profiles during char gasification tests at different bed temperatures, CO2 ) 100% (v/v).

coefficient around few freely moving Pt catalyst spheres in the bed was measured by following the CO oxidation reaction at 450 °C at different fluidization velocities, catalyst sphere sizes, and inert bed particle sizes. Experimental data were excellently fitted by the following correlation: Sh ) 2.0εmf + 0.70(Remf /εmf) Sc 1/2

1/3

The total carbon consumption rate was considered to be the simple sum of the combustion contribution given by eq 8 and the gasification contribution: m ˙ cons )m ˙ comb +m ˙ gasif C C C

(10)

(9)

where εmf is the bed voidage at minimum fluidization conditions, Remf is the particle Reynolds number calculated at the minimum fluidization velocity, and Sc the Schmidt number.

To account for the progressive reduction of the particle volume available for gasification because of parallel combustion, eq 6 has been modified in the following expression:

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Figure 8. Carbon consumption rates as a function of time during char gasification tests at different bed temperatures, CO2 ) 100% (v/v).

m ˙ gasif ) C

( )

( )

Ea d(t) 3 mC0A(X)k0 exp d0 RT

(11)

where d0 is the initial particle diameter. The actual value of the internal carbon conversion degree is evaluated by considering that the change of conversion with time can be calculated from eq 6 as:

( )

m ˙ gasif Ea dX C ) ) A(X)k0 exp dt mC0 RT

(12)

This approach is based on the consideration that since the oxygen concentration vanishes at the particle external surface, all the internal pores are free of oxygen, so that the gasification environment within the particle during the test should be similar to that experienced in the 100% CO2 gasification tests. The following simplifying assumptions have been also made to apply eq 10 to the present tests: (a) the particle diameter as a function of time is calculated according to eq 5, as if no gasification was present; (b) homogeneous oxidation in the particle boundary layer of the CO produced by gasification has been neglected (this would possibly lead to an overestimation of the combustion rate when the gasification contribution is significant, since oxygen depletion in the boundary layer by homogeneous reaction is neglected); (c) the temperature of the particle was assumed to be close to the bed temperature. This last assumption is reasonable for oxygen concentrations up to 2%, but would introduce increasing errors for higher concentrations, especially as regards the calculation of the gasification rate. However, as it will be shown below, for large oxygen concentrations the gasification rate contribution to the total carbon consumption is very limited, so that the total error is likely to be small anyway. Figures 5 and 6 report the comparison of the calculated and experimental carbon conversion rates for all the tests at different oxygen concentrations and bed temperatures. The following results can be noted from this comparison: 1. The calculated carbon consumption rate (eq 10) fits excellently the experimental data for all the operating conditions, in spite of the above simplifications. It is here highlighted that no adjustable parameter is present in the calculation, since all the parameters’ values are calculated from independent tests.

2. Carbon combustion dominates the particle conversion at high oxygen concentrations and low temperatures. On the other hand, carbon gasification contributes to a level comparable to combustion at high temperatures and low oxygen concentrations. 3. The tests at O2 ) 8% in Figure 5 and at T ) 800 °C in Figure 6, that is, when the gasification contribution is negligible, show that eq 8 with the Sherwood number estimated with eq 9 excellently fits the experimental data. This confirms that even under oxyfiring conditions the combustion rate is dominated by oxygen boundary layer diffusion, and that the main combustion product is CO2 rather than CO, as it was assumed in the calculations. Note that if CO was assumed to be the main combustion product according to reaction B, the calculated combustion rate would have doubled. On the whole, the gasification rate at high CO2 concentrations under fluidized bed conditions is slow but not negligible at high temperatures and low oxygen concentrations, contrary to what happens under typical air combustion conditions.12 It must be noted, however, that in a real oxyfiring application the CO2 concentration in the fluidized bed is likely to be somewhat lower than that used in the present experiments, possibly leading to a reduced contribution of the gasification reaction. Acknowledgment The support of F. Raganati in the experimental tests and of S. Russo for making the photograph is gratefully acknowledged. Literature Cited (1) Buhre, B. J. P.; Elliott, L. K.; Sheng, C. D.; Gupta, R. P.; Wall, T. F. Oxy-fuel combustion technology for coal-fired power generation. Prog. Energy Combust. Sci. 2005, 31, 283–307. (2) Rathnam, R. K.; Elliott, L. K.; Wall, T. F.; Liu, Y.; Moghtaderi, B. Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/ CO2) conditions. Fuel Process. Technol. 2009, 90, 797–802. (3) Jukkola, G.; Liljedahl, G.; Nsakala, N.; Morin, J.; Andrus, H. An Alstom vision of future CFB technology based power plant concepts. Proc. 18th Int. Conf. Fluid. Bed Combust. 2005, No. 78104. (4) Czakiert, T.; Bis, Z.; Muskala, W.; Nowak, W. Fuel conversion from oxy-fuel combustion in a circulating fluidized bed. Fuel Process. Technol. 2006, 87, 531–538. (5) Saastamoinen, J.; Tourunen, A.; Pikkarainen, T.; Hasa, H.; Miettinen, J.; Hyppanen, T.; Myohanen, K. Fluidized bed combustion in high concentrations of O2 and CO2. Proc. 19th Int. Conf. Fluid. Bed Combust. 2006, No. 49.

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(6) Jia, L.; Tan, Y.; Wang, C.; Anthony, E. J. Experimental study of oxy-fuel combustion and sulfur capture in a mini-CFBC. Energy Fuels 2007, 21, 3160–3164. (7) Eriksson, T.; Nuortimo, K.; Hotta, A.; Myohanen, K.; Hyppanen, T.; Pikkarainen, T. Near zero CO2 emissions in coal firing with oxyfuel CFB boiler. Proc. 9th Int. Conf. Circulat. Fluid. Bed 2008, 819–824. (8) Scala, F.; Chirone, R. Combustion of coal char particles under fluidized bed oxyfiring conditions. Proc. 20th Int. Conf. Fluid. Bed Combust. 2009, 624–629. (9) Chirone, R.; Massimilla, L.; Salatino, P. Comminution of carbons in fluidized bed combustion. Prog. Energy Combust. Sci. 1991, 17, 297– 326. (10) Leckner, B. Fluidized bed combustion: Mixing and pollutant limitation. Prog. Energy Combust. Sci. 1998, 24, 31–61. (11) Scala, F. Mass transfer around freely moving active particles in the dense phase of a gas fluidized bed of inert particles. Chem. Eng. Sci. 2007, 62, 4159–4176.

(12) Scala, F. A new technique for the measurement of the product CO/ CO2 ratio at the surface of char particles burning in a fluidized bed. Proc. Combust. Inst. 2009, 32, 2021–2027. (13) Senneca, O. Kinetics of pyrolysis, combustion and gasification of three biomass fuels. Fuel Process. Technol. 2007, 88, 87–97. (14) Ammendola, P.; Chirone, R.; Miccio, F.; Ruoppolo, G.; Scala, F. Attrition of bed materials and fuel pellets for fluidized bed gasification application. In Fluidization XIIIsNew Paradigm in Fluidization Engineering; Kim, S. D., Kang, Y., Lee, J. K., Seo, Y. C., Eds.; Engineering Conferences International: New York, 2010; pp 575-582. (15) Laurendeau, N. M. Heterogeneous kinetics of coal char gasification and combustion. Prog. Energy Combust. Sci. 1978, 4, 221–270.

ReceiVed for reView March 8, 2010 ReVised manuscript receiVed July 6, 2010 Accepted July 12, 2010 IE100537A