Plasma activated/assisted solid wastes in diesel exhaust treatment: a case study with agricultural, industrial and composite wastes

Abstract

The consumption of crude oil is increasing every day particularly in developing countries like India which is the third largest consumer of crude oil in the world, utilizing on an average around 160 million liters per annum of which 30% constitutes diesel fuel. More than 50% of the NOx and hydrocarbons in the air come from the diesel exhaust affecting the health of human beings, vegetation, and environment. While the solid particulate in the exhaust is taken care to a greater extent by the mechanical filters it is the gaseous pollutants such as oxides of nitrogen (NOx), oxides of carbon, hydrocarbons (HC) etc., that need to be addressed as they cause several health-related ailments in addition to acid rain, global warming, smog etc. It is timely to work on the development of economical and efficient pollution control strategies. On the other hand, issues that is affecting our country India currently are accumulation of wastes from utility industry, mariculture industry and agriculture industry. The prominent amongst them are fly ash, red mud, foundry sand, iron ore tailings, lignite ash, rice husk, wheat straw and sugarcane bagasse etc. Accumulation of these wastes are in several million tons per annum in India. Any proposition in recycling waste is a welcoming step. Efforts are continuously being made for the past three decades to mitigate these gaseous pollutants, particularly NOx, at various levels by changing the fuel composition, engine modifications, pre-combustion techniques and post-combustion (aftertreatment) techniques. The existing post-combustion mode technique “catalyst-based converters and adsorbent based techniques” are becoming expensive owing to the short life, dependency on noble metals, more vulnerability to acidic coating, bulk usage of adsorbents etc. In this regard the application of non-thermal plasma or electric discharge plasma for pollution control aided by additional techniques is slowly gaining popularity in the past few years. Discharge plasma ionizes the atoms at normal temperature and atmospheric pressure (NTP) thus creating an oxidative environment resulting in chemical reactions such as reduction, oxidation, decomposition etc. However, among these reactions it was observed that oxidation was dominating, to a certain extent the oxidised by-products in the plasma appeared to be less hazardous to humans than to nature. This led to the redesigning of plasma reactors with the intention of enhancing the energy in the charged species favoring reduction reactions instead of oxidation ones but not without serious limitations with respect to the gas flow. Application of electrical discharges for environmental purposes lies in the basic concept of treating the pollutants, particularly the gaseous ones, with plasma excited species. It is observed that plasma alone is insufficient for the successful treatment of any of the gaseous pollutants due to the oxidative environment prevailing in the discharge plasma at atmospheric conditions. This necessitated inclusion of additional treatment technique along with plasma leading to the origin of plasma catalysis/plasma adsorption methods where in the catalytic materials were kept inside the plasma environment (plasma catalysis) or the adsorbent materials are cascaded with plasma (plasma adsorption). It should be noted here that the total cost involved in the proposed technique, should be lower than that associated with conventional catalyst-alone or adsorbent-alone techniques so that the proposed ones can be a promising economic alternative to the conventional ones. That said, cascading commercially available catalysts/adsorbent with plasma can never be a cheaper alternative. Several research works, therefore, started by blending plasma with other lab made catalysts/adsorbents but the DeNOx efficiency was not significant. Present work aimed at studying the NOx abatement in diesel engine exhaust at controlled laboratory condition using electrical discharges. The intention of the study is to provide not only an efficient but also an economical solution for reduction of the NOx pollutants. Keeping this in mind, it was decided to utilize electrical discharge technique in association with abundantly available solid wastes in India be it from industrial, mariculture or agriculture domains. Given the whole spectrum of parametric variations the thesis plan was carefully drawn to touch upon the following variations: type of corona electrodes, type of applied high voltage, type of solid wastes, type of gas treatment. Four types of electrodes were studied that include needle plate, metal film, helical wire and pipe type. Type of voltage involves fast rising repetitive pulses at 80 Hz, power frequency and high frequency AC. The solid wastes comprise of three categories namely (a) industrial/mariculture wastes that include namely iron tailings, lignite ash, red mud, foundry sand, waste tiles and copper slag, oyster shell (b) agricultural wastes including coffee husk, sugarcane waste, mulberry husk, rice husk, ragi husk, corn husk, wheat husk, pine, ground nut and areca nut husk and (c) composite wastes which include a blend of waste tiles + foundry sand, copper slag + red mud, iron tailings + waste tiles, red mud + waste tiles, foundry sand + red mud and foundry sand + iron tailings. The type of gas treatment involves treating the exhaust with only plasma/plasma catalysis/plasma adsorption/thermal catalysis utilizing catalytic properties of metal oxides present in the industry wastes or porous nature of the industry wastes. A comparison was also made by replacing the industrial waste with commercial NOx catalysts. Important contribution of this research work can be summarized as: (a) with plasma catalysis approach the NOx removal efficiency gets enhanced by a factor of 5.3-6.7 compared to only plasma. (b) with plasma adsorption approach the NOx removal efficiency gets enhanced by a factor of 4-6 compared to only plasma. (c) Fe2O3/TiO2 present in red mud can act as photo catalysts in oxidizing NO through plasma generated ethyl nitrate in the plasma cascaded red mud adsorption process (d) Amongst the agricultural wastes, ground nut husk-based pellets exhibited 83% NOx removal efficiency (e) The newly developed metal film based DBD reactor enhances surfaces discharges far better than the helical wire reactor (f) commercial catalysts performed much better in NOx removal under plasma catalysis mode when compared to thermal catalysis mode. Further, plasma catalysis with industrial wastes such as iron tailings and oyster shell yielded better/similar DeNOx efficiency when compared to that with commercial NOx catalysts thus, justifying the usage of cheaper industrial wastes instead of expensive commercial ones.