The flow microreactor, shown in Fig. 2, was also employed for performing TPD (Temperature Programmed Desorption) tests in order to determine the amount and the rate of desorption of carbon surface complexes that allows the evaluation of the active surface area (ASA) of fresh and partially burned off samples. In this case the tests were carried out by rising up the temperature from 293 Ktol373Kata rate of 20 K/min in a 500 cm3/min (STP) nitrogen flow. The concentration of CO and C02 evolved from the carbon surface were measured on line with the mentioned NDIR analysers.
Typical TPD runs were performed on fresh samples or on partially burned off samples. After a given oxidation time at fixed temperature and oxygen concentration, the reaction was frozen by lowering the temperature down to 473 K. When neither carbon monoxide nor carbon dioxide were detected by the analysers, the oxidising stream was replaced by the nitrogen stream and the temperature was raised up at the rate of 20 K/min. TPD tests yielded the temperature profile of desorption of carbon-oxygen complexes from the char surface. The initial mass of char was about 200 mg. During the oxidation, oxygen molar fraction in the gas fed to the reactor was 0.1, but in any case the reactor was operated under differential conditions with respect to the oxygen. The oxidation rates as well as the conversion degree of carbon (X) in each oxidation step were calculated from the values of carbon oxides concentrations in the outlet gases. Carbon mass balance was verified in the range 95-100%.