A thermogravimetric method for the measurement of CO/CO2 ratio at the surface of carbon during combustion

Wenting Hu,Ewa Marek,Felix Donat,John S Dennis,Stuart A Scott

doi:10.1016/j.proci.2018.05.040

Wenting Hu, Ewa Marek + Show 3 more

Open Access

https://doi.org/10.1016/j.proci.2018.05.040

Copy DOI

Abstract

This work presents a new method of measuring the CO/CO2 ratio at the surface of carbon particles during combustion. This thermogravimetric method deduces the ratio of CO to CO2 by comparing the rate of consumption of carbon with the rate of oxidation of an external reference material with fast oxidation kinetics, in this case Cu. The method is useful when combustion is controlled by external mass transfer, commonly encountered in large-scale processes. The viability of this method has been demonstrated experimentally with graphite and a lignite char. It was found that in an atmosphere of ∼ 1% O2, the graphite produced CO2 between 700 and 900 °C whilst the lignite char produced a mixture of CO and CO2 between 700 and 800 °C with the proportion of CO increasing with temperature, and above 850 °C, only CO was produced. It was also found that for this particular lignite char, the ratio of CO/CO2 increased with decreasing pO2 in the environment.

Highlights

IntroductionFor char particles larger than 2 mm in diameter or so, the combustion is most likely to be controlled by external mass transfer [2,3]
After oxygen was introduced into the system (t = 1200 s), the rate of combustion remained constant for a sustained period of time in both cases and the rate only slowed down in b) when the mass of graphite fell below 0.7 mg
This behaviour is typical of the reaction being controlled by external mass transfer of O2 to the surface of the graphite

Summary

Introduction

For char particles larger than 2 mm in diameter or so, the combustion is most likely to be controlled by external mass transfer [2,3]. In pulverised coal-fired boilers, where the size of the coal particles is considerably smaller, the burning of the char could be limited by external mass transfer, owing to the high reaction temperature giving very rapid intrinsic kinetics [4]. The combustion of large spheres of carbon has often been used to measure the mass transfer in a fluidised bed, as discussed by Scala [5]. There are a number of reasons why the measured rate of combustion cannot always be used directly to deduce the rate of mass transfer [6].

Results

Discussion

Conclusion