Adsorption and oxidation of NO to NO2 over a renewable activated carbon from coconut

Abstract

The NOx health and environmental problems make necessary to reduce this gaseous emission from different sources. Furthermore, its increase in the last years and the difficulties to remove it with after-treatment systems already in the market make more urgent the development of new techniques. The purpose of this investigation is to study the low temperature catalytic oxidation of NO to NO2 and its adsorption over a renewable activated carbon (AC) from coconut shell. The present research presents the results of experimental work carried out using a laboratory scale reactor to investigate the low temperature catalytic oxidation of NO. Activated carbon was housed in the reactor and tests were carried out with different reactor sizes, different activated carbon forms and shapes, different gas mixtures at different temperatures and different levels of humidity to simulate dry and wet particulate-free diesel engine exhaust gas. The effects of addition of ozone in the gas on the NO oxidation were also explored. Gas analysis upstream and downstream of the catalytic reactor was carried out in all cases during the charge and regeneration of the AC. An extensive literature review in conjunction with measurement of some properties of the activated carbon helped to understand better its characteristics and behaviour. The results of this study indicate that in the case of dry gas, the activated carbon initially acts as an adsorber and only after operation of several hours, the NO oxidation that takes place in the reactor results in increased NO2 levels in the product gas. The NO conversion is affected by the activated carbon form and reaction conditions including temperature, humidity, oxygen, NO, CO2 content in the inlet gas, temperature, space velocity, linear gas velocity, residence time, reactor shape, AC pretreatment and lifespan. Water vapour has a detrimental effect on the conversion of NO to NO2 before the AC reaches the steady-state conditions. On the other hand, ozone is effective in converting NO to NO2 at room temperature. This research has developed some findings not studied or reported by other researches before and confirms and/or complements results reported in the literature review by other groups, which will benefit the development of a renewable after-treatment system of NOx emissions.