A study on evaporation and decomposition of aqueous urea for Selective Catalytic Reduction technology

Abstract

Air pollution has serious impacts on the environment and health. One of main sources that cause the air pollution is the emission from internal engines burning fossil fuel. Urban area is more vulnerable due to dense vehicle density so that countries around the world set up emission standards to cope with the situation. Since nitrogen oxides produced during high-temperature combustion cause photochemical smog and acid rain in the atmosphere, the emission standards make a strict restriction on NOx emission. Engine manufacturers have been developing technologies to meet the regulations. One of practically available ways to meet the regulations is a technology of selective catalytic reduction using aqueous urea (Urea-SCR). Urea-SCR technology uses aqueous urea that should be converted into ammonia in the exhaust stream because ammonia is used as a reducing agent in the SCR catalyst. Water evaporation, urea hydrolysis and hydrolysis of isocyanic acid take part in the converting processes. These processes begin with the sprayed injection of urea in the stream. Some of injected droplets reach the surface of the channel and form deposits. This study is a fundamental research on chemical reactions and thermos-physical properties of urea and its role in the water solvent to understand the deposit formation. A urea pyrolysis model for temperature around 200℃ where most deposition takes place in SCR systems is presented. The reactions and their chemical kinetics among primary species such as urea, isocyanic acid, biuret, cyanuric acid, and ammonia are proposed. A two-step numerical scheme based on the model is also developed to track molar history of species. The kinetic parameters of the model are obtained by comparing measured masses with simulated ones to minimize the sum of squared errors between them. The present model is able to anticipate the amount of cyanuric acid which is a main ingredient of deposits for the temperature around 200℃ and forms polymeric complexes at higher temperature. Kinetic coefficients for urea pyrolysis are compared with other authors results to validate the model. The competition between the formation of cyanuric acid and the evaporation of isocyanic acid is shown a key factor for product deposits which can be filed up more at lower temperature. Water vaporization from aqueous urea is a first process during the conversion of urea into ammonia. Droplets sprayed into the exhaust gas stream are composed of non-volatile urea solute and water solvent. As water evaporates, relative composition and thermo-physical properties of the solution change. It is called a colligative property. Two colligative properties to understand water evaporation are boiling-point elevation and change in heat of vaporization of water from aqueous urea. The boiling-point elevation relation for water in the mixture is derived and shown to be valid by experiments. The heat supply to droplet from the surrounding gas and its distribution to evaporation, heating evaporated vapor, and heating droplet are analyzed using the proposed boiling-point elevation relation. The change in enthalpy of vaporization of water from aqueous urea is calculated considering endothermal heat for urea dissolution in water. The change is validated by experiments as well. Droplet evaporation analysis reveals that droplet temperature can increase high enough to begin urea decomposition. Since droplets convey energy to wall and their precipitation is related to drop temperature, the proposed model will be useful for calculating drop temperature and species fractions in Urea-SCR system design.