Stationary diesel exhaust treatment by blending discharge plasma/ozone with industry wastes: a study on abatement of NOx and THC

Abstract

Increased usage of fossil fuels, especially diesel, has made a large impact on the environment in the form of rise in global temperature, increased acidity in the rain water, decreased yield in vegetation and numerous health related issues. Diesel has become a foremost and inevitable source of energy in day to day life, be it in stationary engines or in automobile engines. In the past three decades the usage of diesel has doubled up, particularly in third world countries like India, resulting in increased soot and gaseous particle emission. Of importance is the emission of oxides of nitrogen (NOx) and total hydrocarbons (THC), together accounting for 50% of NOx/THC emission. Though there exists an efficient system for controlling the solid soot particulate of diesel exhaust, the same for handling the gaseous pollutants is still at large. Therefore, any effort towards treating these gaseous pollutants efficiently and cost effectively is a welcome step before letting the same into the atmosphere. The treatment involved in controlling these gaseous pollutants can be met at engine level (pre-combustion) or at the exhaust stream (aftertreatment/post-combustion). While the former technique has limited scope and been saturated with engine design modifications, the latter is currently being handled by catalyst/adsorbents. The latter scenario is more or less similar, be it engine exhaust or industrial exhausts, from the point of treating gaseous pollutants. However, the usage of catalysts/adsorbents has several drawbacks such as short life, high cost, storage, leakages and limited efficiency thereby motivating the researchers to look for alternate means of mitigating these gaseous pollutants. It is at this juncture, seeing the success of high voltage driven electric discharge-based precipitators, the thought of exploring the chemical potential of this electrical discharge plasma (also known as non-thermal plasma, NTP) came up almost three decades ago for controlling the gaseous pollutants at the downstream of the exhaust. Across the globe there was a spur in this NTP treatment of gaseous pollutants in a controlled condition and many successful reports came out at laboratory level. It was realized that NTP mainly results in oxidation of the pollutants due to the oxygen rich environment of the exhaust thus, necessitating usage of additional treatment involving catalysts/adsorbents. Since the exercise of introducing NTP is to provide an alternative to the commercially available expensive catalysts/adsorbents, the attention was shifted to utilize materials which are available abundantly and at a lesser price. The solid industrial waste is one such material satisfying this requirement and is being explored in the current thesis work. In the current thesis work, gaseous pollutants from a stationary diesel engine exhaust were exposed to electrical discharge plasma shower in a carefully controlled laboratory condition. Oxides of nitrogen and total hydrocarbons are the two components that were studied amongst the gaseous pollutants. Since NTP is known for oxidation of the pollutants, in the current work, the exhaust was treated with discharge plasma/ozone injection and the oxidized pollutants were then adsorbed in pellets made out of solid industrial wastes such as fly ash, red mud, oyster shell, coffee husk, foundry sand and rice husk, the latter two being explored for the first time for their adsorption properties. The barrier plasma was either volume discharge type or surface discharge type during the study. The thesis then progresses through utilizing a novel way of treating the exhaust by cascading the barrier discharge plasma with ozone injection and vice-versa to enhance the overall oxidation of the gaseous pollutants be it NOx or THC. It was observed that among the solid industry wastes studied, the red mud and foundry sand showed better NOx removal efficiencies compared to oyster shell, coffee husk, rice husk and fly-ash, when cascaded with plasma treated exhaust. Further, foundry sand and red mud (as catalyst) performed equally well in controlling increased concentrations of NOx (associated with higher loading of the engine) in the post-plasma treated exhaust. Combined plasma+industrial waste-based adsorbents provide an efficient and economic option for NOx mitigation in diesel exhaust with appropriate scaling. Combined plasma+ozone-based technique provides a possible option for reduction/conversion of THC in diesel exhaust. This approach is first of its kind in the NTP fraternity. The results have been discussed at length in this thesis from the point of possible reaction pathways associated with conversion/reduction of NOx/THCs under plasma/O3 injection. The trapped NO2 in the adsorbents can be used as potential raw material for nitric acid/fertilizer industries. Through this research work another pathway for managing the ever-accumulating solid waste has been shown