Kinetic Model of the Devolatilization Process in an Oxy-combustion Burner

B. de Caprariis1, G. Calchetti2, A. Di Nardo2, G. Levi Della Vida1, C. Mongiello2, N. Verdone1

1. Chemical Engineering Department - Università di Roma, La Sapienza - ITALY 2. Italian National Agency for New Technologies energy and Environment - ENEA Casaccia- ITALY

1. Introduction Despite coal is the most used fuel for power generation, its combustion is one of the principal sources of CO2 production due to the highest C/H ratio among fossil fuels. There are two main different techniques that have been developed to control CO2 emissions: post-combustion CO2 capture and pre-combustion treatments consisting in coal gasification and oxy-combustion. Object of this study is the analysis of the coal devolatilization process that takes place in an oxy-combustion reactor working under pressure and in flameless condition. Oxy-combustion consists in the enrichment of the exhaust gas with pure oxygen, and in using this mixture as comburent inlet flow. The flue gas is, theoretically, just composed by a mixture of steam and carbon dioxide concentrate, which is easy to separate by condensation. The oxygen excess in the inlet flow prevents the formation of non combustible compounds like carbon monoxide and PAHs (poly-aromatics hydrocarbons). Moreover the temperature profile in the reactor is homogeneous, thanks to the flameless conditions in which it operates [1]. The thermal NOx production is greatly minimized, because the temperature is below the nitrogen scission threshold. Furthermore, the non-combustible compounds are not produced, because low temperature zones are not present. Flameless conditions are reached feeding the burner with hot gas (in this case the inlet flow temperature is about 500 K) and keeping the reactor in a high turbulence state. This work is focused on the study of the coal devolatilization and on its influence on the whole combustion process. The pyrolysis is particularly important since ignition is controlled by the combustion of volatiles that evolve and successively burn. The analysis of the devolatilization process has been performed with a dedicated scientific software FG-DVC (Functional Group-Depolimerization Vaporization Cross-Linking model) [2] and supported by experimental tests with TGA (Thermogravimetry Analysis). Kinetic models of the pyrolysis have been developed and implemented into a CFD code (FLUENTTM) to analyze the burner behaviour.

2. Devolatilization process Devolatilization is a process in which coal is transformed to produce tar, light gases and char. Tar is the major volatile compound, with a composition similar to that of the parent coal and condensable. Light gases are formed by the bond breaking of the coal functional groups, CO2, CH4, H2O, CO, H2 and some light species of the paraffin (Par) and olefin (Ol) families being the principal compounds. In this work the devolatilization process has been considered as a single step reaction:

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�

coal→ x volatiles+ (1− x)char

The coal used for this work is Sulcis coal with the composition reported in Table 1. Tab. 1 - Sulcis coal composition.

Proximate analysis wt % Moisture 5.5

Ash 13.0 Volatiles 42.5

Fixed carbon 39.0 Ultimate analysis wt % daf

C 74.8 H 5.5 N 1.8 S 8.4 O 9.5

The devolatilization process has been studied with the FG-DVC software to obtain yields and production rates of all the involved species. The predicted data of the devolatilization process have been compared to the experimental ones performed with TGA, generally showing a good agreement between theory and experiment. The tests have been performed with different heating rate, and in Figure 1, as an example, the volatile yield obtained with a heating rate of 40 K/s is reported. In this case, the difference between experiment and simulation is less than 5%.

Figure 1 - Comparison between FG-DVC simulated data and experimental data.

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Ischia, June, 27-30 - 2010

The FG-DVC results, used in the combustion simulations, have been carried out considering a heating rate of 10000 K/s, characteristic value for a combustion chamber, and an initial and final temperature of 400 K and 1600 K, respectively. In the Table 2 the predicted yield values are reported. Table 2 - Devolatilisation products distribution, daf basis.

Product Distribution %C %H %O %N %S Coal 100.00 74.80 5.47 9.51 1.81 8.41 Char 54.30 91.28 0.71 - 2.14 5.86 Tar 23.30 76.79 5.24 8.72 1.89 7.37 Gas 18.80 32.84 17.23 33.35 0.93 15.65

Par+Ol 3.70 84.61 15.39 - - - To study the influence of different devolatilization models on the simulation of the combustion process, three different models have been developed and compared. They rely on the first order reaction expressed as:

�

dxdt

= k x* − x( ) (1)

where k, x and x* are the kinetic constant, the time dependent volatile yield and the total volatile yield, respectively. Kinetic parameters like the activation energy, E, and the frequency factor, A, have been valued by the linearized Arrhenius equation:

�

ln(k) = ln(A) − ERT

(2)

The first two models consider only one volatile product of devolatilization. The first one [3] assumes that kinetic parameters are constant in the whole temperature range (800K-1600K). The second, said Kobayashi model [4], considers two reaction in parallel that take place into two different temperature ranges (see Figure 2), the high temperature reaction being much faster than the low temperature one. The regressed kinetic parameters of both models are reported in the Table 3. Table 3 - Kinetic parameters for the first two models.

First model Second model T range (K) 800-1600 800-1000 1000-1600

A (s-1) 2.033 104 4.852 108 1.264 102 E (J/mol) 5.550 107 1.265 108 7.246 106

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Fig. 2 Linearization of the Arrhenius equation to obtain kinetic parameters for the second

model. The data are split into two lines, to describe the discontinuity of the kinetic parameters.

The third model takes into account the kinetic parameters of each volatile species separately and considers their variation with temperature. The volatile yields and kinetics data are presented in the Table 4. Table 4 - Third model volatile yield and kinetic parameters.

Specie Yield (wt%) A (s-1) E( J/mol) 1.44 1012 2.07 108

CH4 3.20 2.68 105 7.89 107

2.65 1010 1.80 108

CO 3.36 4.42 105 9.44 107

1.31 109 1.34 108

Tar 23.30 2.68 105 5.99 107

1.81 102 2.96 107

CO2 0.95 9.30 104 7.09 107

1.95 1011 1.86 108 H2O 5.24 1.95 1011 7.52 107 2.68 105 2.18 108 H2 1.69 1.80 104 7.40 107

3. CFD simulation The modelled burner is a cylindrical reactor with a diameter of 1.2 m and a length of 5.5 m with a capacity of 1.6 MWth and working at 4 bar. The coal is injected in slurry form with a water mass content of 50% and the inlet comburent mixture flow is partially swirled. The coal

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Ischia, June, 27-30 - 2010

particle size distribution is between 80 µm and 850 µm. In the Table 5 the inlet conditions and the operative parameters are reported. Table 5 - Inlet conditions in CFD simulations.

Coal flow rate 274 kg/h Air flow rate 1870 kg/h

Recycled gas temperature 510 K O2 43%

CO2 34% H2O 21% Air stream composition (wt %)

N2 2% Three different simulations in Fluent environment were performed. Coal combustion has been modelled with the Eddy dissipation model [3], the radiative heat transfer with the Discrete Ordinate model and the Realizable k-ε model has been used for the turbulence. The first devolatilization model has been implemented with the single kinetic rate mode, and the second one has been simulated with the two competitive rate model. Fluent does not allow to define different kinetic parameters for more than one devolatilizing specie, unless the user defines its own function. To avoid this problem, six different coal flow rates have been defined, one per each devolatilizing species, and for each flow rate a different kinetics has been defined. The total coal inlet flow rate has been divided according to the volatile yields. The volatiles formation rate throughout the reactor according to the three models is shown in Figure 3. It is evident that with the first model (blu line) the gaseous species are formed principally at the burner end, result explained because this simplified model does not take into account the faster kinetics at high temperature. On the contrary, an uniform distribution of volatiles formation rate is obtained with the second and the third model.

Figure 3 - Volatile formation rate comparison between the three kinetic models.

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To compare the results, the temperature profiles have been considered. In Figure 4 it can be seen that with the first model a homogeneous temperature profile is not reached, so that the flameless condition appears to be not obtained. This behaviour is due to the fact that, as al-ready evidenced, most part of the volatile combustion occurs at the end of the reactor. The most homogeneous profile is reached with the second model. Finally, the third model also shows a good agreement with the flameless condition.

Figure 4 - Temperature profile comparison between the three models.

4. Conclusions A great influence of the devolatilization modellization has been found. The second and the third model represent well the flameless combustion. The third model better reflect what hap-pens in the real devolatilization process but its representation is based on some simplification. The most important one is that only one volatile compound devolatilizes from a single coal particle, while in the real process all the volatiles are formed by the same particle. In this way the competitive reactions and the transport phenomena involved in all particle gas evolution are not taken into account. The next step will be to develop an user define function to model more precisely the devolatilization process. 1. Cavaliere A., de Joannon M.: Progress in Energy and Combustion Science, 30:329 (2004). 2. Solomon, P.R., Hamblen D.G., Carcangelo R.M., Serio M.A., Deshpande G.V.: Energy

and Fuel, 2:405 (1988). 3. Backreedy R.I., Habib R., Jones J.M., Pourkashanian M., Williams A.: Fuel, 78:1745

(1999). Combustion Symposium Proceedings: 4. Kobayashi H., Howard J.B., Sarofim A.F.: Proceedings of The Combustion Institute,

16:411 (1977).

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