Plasma catalysis of diesel exhaust using industrial wastes: a study on NOX and THC removal

Abstract

Air pollution, caused by large scale consumption of fossil fuels such as diesel, has been the leading cause of several adverse environmental effects such as global warming, higher acidity in rainwater, lower yield of agriculture production and several health issues. Diesel has been the primary and inevitable fuel source of energy worldwide, in both stationary power supplies and automobile applications. Several developing countries like India continue to rely heavily on usage of diesel fueled machinery and automobiles, which has resulted in high soot, particulate and hazardous gas emissions. The prominent gaseous pollutants of concern are the oxides of nitrogen (NOX) and total hydrocarbon content (THC) present in the diesel exhaust. Though efficient systems have been discovered for reducing soot and particulate emissions, treatment techniques for removal of gaseous pollutants are yet to reach a similar level of progress. Therefore, research efforts aimed at identifying treatment techniques for curbing hazardous gaseous pollutants are a welcoming step towards addressing the pertinent issue of air pollution. The gaseous pollutants emitted from the diesel engine can be reduced by applying control strategies at the level of engine design (pre-combustion) or as an aftertreatment technique of the exhaust stream (post-combustion). Although the pre-combustion control strategies are limited by the possible engine design modifications, the postcombustion approach allows for greater flexibility and scope by utilizing a variety of plasma discharges, catalysts and adsorbents. One such post-combustion strategy which involves treatment of NOX/THC using non-thermal plasma (NTP) generated from dielectric barrier discharge (DBD), has yielded promising results at the laboratory level. Non-thermal plasma produces an oxidative environment containing several charged species, which include energetic electrons, excited species, ions, and radicals, at atmospheric pressure and ambient temperature conditions. Diesel exhaust exposed to such a non-thermal plasma environment has been found to cause the formation of higher oxides of nitrogen and oxidized hydrocarbon intermediates, which necessitates exposing them further to adsorbents or catalysts for effective removal of the harmful pollutants. In recent years, a treatment technique which involves filling a plasma reactor with catalytic materials that enhance reactions in the presence of plasma, referred to as plasma catalysis, has given promising results at laboratory level in terms of pollutant removal efficiency, on par with conventional thermal catalysis. The highly reactive environment produced by the interaction between reactive species in the plasma and the surface of the catalytic material can facilitate reactions that usually occur only at high temperatures in conventional (thermal) catalysis. The literature on plasma catalysis for several gas treatment applications reveals the utilization of expensive, commercially available catalytic materials. The expensive rare metals used in such catalysts and the need for replacement due to choking of the catalytic material, makes their usage an economically non-viable option. It is at this juncture that the utilization of freely available industrial wastes as potential catalysts appears to be an economically feasible alternative. Such environmentally safe and inexpensive treatment techniques for NOX/THC abatement are a desirable and welcoming option for exhaust treatment in the long run. In the current work, gaseous pollutants from a stationary diesel engine exhaust were exposed to an electrical discharge plasma in a reactor packed with pellets made from industrial wastes, in a carefully controlled laboratory condition. Oxides of nitrogen and the total hydrocarbons are the two components of the diesel exhaust that were studied as the gaseous pollutants. The pellets were made from solid industrial wastes such as foundry sand, fly ash, red mud, oyster shells, bagasse, and mulberry residue. The plasma was either volume discharge type or surface discharge type during the study. The thesis then progresses with a study of the results of NOX and THC removal through plasma catalysis and performing qualitative analysis of experiments to ascertain the dominance of plasma catalysis over other pollutant removal processes, such as plasma-cascaded adsorption and plasma-only treatment. It was observed that among the solid industry wastes studied, red mud showed better NOX and THC removal efficiencies compared to the other industrial waste pellets. Further, plasma catalysis showed moderate to significant increase in NOX and THC removal when compared to the plasma-cascaded and plasma-only methods, for all the pellets studied. This approach of using industrial waste pellets for plasma catalysis of diesel exhaust is the first of its kind in the NTP fraternity. The results have been discussed in detail along with the possible reaction pathways associated with conversion or removal of NOX/THC under plasma catalysis.