Effect of Propene on the Remediation of NOx from Engine Exhausts

Abstract

Plasma treatment of diesel exhausts has been investigated in recent years due to its potential for remediating NOx in emissions. Hydrocarbons in the exhausts have been found to play an important role in the reaction chemistry during remediation. In this paper, we report on a computational study of the plasma treatment of simulated exhausts containing propene to investigate the effects of hydrocarbons on the conversion pathways for NOx. INTRODUCTION Increasingly stringent regulations on the emission levels of NOx in automotive exhausts has led to a reexamination of efficient methods of NOx removal. The use of low-temperature plasmas followed by a downstream catalytic converter has been found to efficiently remove NOx for select conditions [1, 2]. Dielectric barrier discharge (DBD) reactors have, in particular, been studied extensively for plasma remediation of these gases [3-7]. Accompanying experimental studies have investigated the consequences of reactor parameters such as packing material, electrode diameter, voltage, frequency, temperature and gas composition, that affect the NOx conversion process [10, 12]. Computer modeling of plasma remediation of NOx using DBD reactors has also improved understanding of the elementary processes [89,11]. Hydrocarbons in the exhausts have been found to play an important role in the NOx conversion chemistry. Earlier studies on the effects of alkenes such as ethene [11] and propene [2] have suggested that there is a positive effect of hydrocarbons on the NOx removal process. The present study investigates the consequences of propene (C3H6) on the conversion mechanism of NOx by applying a spatially homogeneous plasma chemistry model. The goals of this investigation are to quantify the reaction mechanism, and determine the consequences of hydrocarbons on the remediation process. A brief description of the model followed by a discussion of the reaction mechanisms follow. DESCRIPTION OF THE MODEL A spatially homogenous/plug flow plasma chemistry model has been developed to investigate DBD discharges for the remediation of NOx. The plasma chemistry model consists of an electrical circuit module, a solution of Boltzmann's equation for the electron energy distribution and a set of coupled ordinarydifferential equations which are integrated in time and produce the time evolution of species densities. The model uses a lookup table produced by an offline Boltzmann solver to obtain the rate coefficients of electron impact processes as a function of electron temperature. These electron impact coefficients and the current E/N (electric field/gas number density) are used to solve the electron energy equation which provides the electron temperature. A total of 750 reactions make up the mechanism for the plasma chemistry of NOx with the inclusion of propene, of which 76 are electron impact processes. The system of equations is implicitly integrated using LSODE to allow both the current pulse (10s ns) and reactor residence time (0.2 s) to be resolved. A temperature of 0.5 eV is assumed for the seed electrons and then the electron density and temperature are allowed to vary with time. Typically, the electron temperature peaks during the pulse to around 3 eV. The parameters which have been varied to determine their effect on NOx conversion are input energy, gas temperature and the inlet hydrocarbon concentration. Energy deposition was varied by changing the voltage applied to the DBD reactor. The species in the initial feed to the reactor are shown in Table 1. Although all possible species that may be in actual exhausts are not included, the input composition is representative of typical exhaust stoichiometry. All simulations were performed at 1 atm over a period of 0.2 seconds (residence time of the gas) with a single discharge pulse. A large portion of the temperature dependent reaction rate coefficients was obtained from the NIST Chemical Kinetics Database [13]. Species Inlet Concentration O2 8% CO2 7% H2O 6% CO 400 ppm NO 260 ppm H2 133 ppm C3H6 0-1100 ppm N2 Balance Table 1. Inlet concentrations to the DBD reactor. REACTION MECHANISMS AND REMEDIATION REMEDIATION VS INPUT ENERGY Earlier studies have shown that at moderate energy input and low E/N, plasma remediation in the absence of hydrocarbons dominantly results in the conversion of NO to NO2 and HNOx with a small amount of NO getting reduced to form N2 [14-15]. In order to establish a baseline for comparison to cases with hydrocarbons in the exhaust, simulations were first performed without propene. The temperature and pressure are 453 K and 1 atm respectively. The dielectric gap is 2.5 mm.