Representación en el exterior

It is clear from the presented results that the DAE based model is capable of efficiently modeling the combustion and gasification of the coal and biomass chars and their blends. It can therefore be concluded that the model is a robust and accurate method for kinetics determination. Multiple reactions were identified during the conversion of some of the chars in this study. This may be explained by the heterogeneous nature of the both the coal and biomass chars. As highlighted by Vittee (2012), the model identifies a single structural parameter for the multiple reactions identified. All the materials were successfully modeled by the RPM to accuracies in the range of RMS values of 0.0024 to 0.0081, and corresponding R2 values of 0.9996 to 0.99996. The first order reaction model yielded results in the order of RMS values 0.0154 to 0.0507 and corresponding R2 values of 0.9987 to 0.9866 respectively.

The successful application of the RPM for the modeling of the combustion and gasification of all the chars and char-blends studied serves to confirm the findings obtained by Bhatia and Vartak (1996) as highlighted in Section 2.4.2. Whilst Rafsanjani and Jamshidi (2008) state that the RPM is the most widely used in the modeling of char gasification, Bhatia and Vartak (1996) further state that the RPM found extensive application in the interpretation of gas-solid reaction rate data. It is found that the RPM is suitable for the modeling of both the gasification and combustion of coal and biomass chars.

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As observed by Vittee (2012), the use of isothermal kinetics for non-isothermal conditions or vice versa, does not yield accurate results. For coal char gasification, the kinetics obtained at a particular temperature program may not be suitable for application in another. The plots of conversion against normalized time as described by Kaitano (2007) and Njapha (2003) were applied in this study. The normalized method was not able to correctly evaluate the structural parameters when multiple reactions are taking place during the conversion. There is therefore need to further study isothermal conversion and possibly adapt the normalization to multiple reaction behavior.

The 90:10 coal and biomass blends by heat input, corresponding to 6 and 4% biomass char input by mass for grass and pine char respectively, show no or little synergetic effects during both combustion and gasification. The RMS values of the relative errors are smaller for combustion of this blend than for gasification. This is expected since the biomass component in the char is so small. However, as the composition is increased to 50:50 coal and biomass blends by heat input, the char mass contribution in the blends increases to 35 and 26% for grass and pine char respectively. This is demonstrated by Table 7-8 in Section 7.1.4. The different DTG curves and increase in the RMS values imply the possibility of synergetic effects are observed with RMS error values of 0.0129 and 0.0239 for the grass and pine blend respectively. For both blends, an increase in the maximum reaction rate was observed in magnitudes of 15% and 40% from the calculated curves for coal-grass and coal-pine 50:50 blends respectively. These findings are similar to those observed by Duong et al. (2010). Duong et al. (2010) blended biomass (woody biomass and switch grass) and bituminous coal during combustion and gasification. Duong et al. (2010) states that the blending of the biomass with the coal for the most part increased the reactivity of the blends beyond that predicted by the additive method applied on Section 7.3.2.

The compensation effect has been applied to the observed grouped kinetics with the assumption that the structural parameter has negligible effect on . The results obtained show considerable consistency of the theory with minimal deviations in observed for the 90:10 coal and biomass blends. A decrease in the during the combustion of the 50:50 coal-biomass blends is observed in the magnitudes of 2% and 1.1% for grass char and pine char blends respectively. It may then be concluded that the addition of biomass to the coal char during combustion leads to a reduction in . The theory also shows that grass char and pine char have significantly lower ‟s during

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both the combustion and gasification processes as compared to coal char. From Section 6.5 the high catalytic alkali oxide content in the ash contained in the pine char as compared to that in the pine would imply that the pine char is typically the most reactive component. The results obtained confirm this presumption and show that pine char is more reactive than grass char during both combustion and gasification. The pine char presents an 6% lower than that of coal char whilst the grass char presents an 4% lower than that of coal during combustion. During gasification, the pine char appears to be reacting with an 11% lower than that of coal char and grass char with an 6% lower. It is therefore concluded that the compensation effect is a suitable method of kinetics analysis. The high vitrinite coal char is also less reactive than the two biomasses studied. Pine char is the most reactive material followed by grass char and finally the coal char, during both combustion and gasification, in terms of activation energy.

The structural parameters identified were in the range of 8.3 to 18.9 and were found in line with those observed in literature. Similar structural parameters were also identified for a single material in the two different gas atmospheres.

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