Design Calculations for a Selective Catalytic Reduction Unit - Development of a Simulation Model and Design Algorithm for a Marine Selective Catalytic Reduction Unit for IMO Tier III Compliance
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- Institutt for marin teknikk 
The global emission of nitrogen oxides (NOx) from the shipping industry was approximately 15% in the period from 2007-2012, i.e. shipping is a major source of these emissions. Since 2000, the International Maritime Organisation (IMO) has gradually introduced more stringent regulations limiting the allowable NOx emission from ships. The current regulation, or Tier, was put into force in 2011 and is known as IMO Tier II. This regulation applies for all newbuilds, worldwide. The next level, IMO Tier III, will be introduced in 2016 and will apply to all ships sailing in designated NOx Emission Control Areas (NECAs). IMO Tier III represents the most significant step in reduction so far, with 70-80% reduction from IMO Tier II level. While engine manufacturers have been able to comply with the Tier II regulations with internal abatement techniques, compliance with Tier III level is only possible with aftertreatment if using conventional fuels.Selective Catalytic Reduction (SCR) is one of the most promising technologies for achieving IMO Tier III compliance. SCR is an external abatement technology based on injecting a reactant (typically an aqueous urea solution) into the exhaust stream. The urea solution is evaporated and decomposed to ammonia (NH3) in a mixing duct. The ammonia and nitrogen oxides then flow into a catalytic converter where the catalyst (usually Vanadium) lowers the activation energy so that the main SCR reactions can happen at the exhaust temperature. After reacting with the reactant, only nitrogen gas and water are emitted to the atmosphere. Depending on operating conditions, a small amount of NH3 is also emitted. This is known as the NH3 slip. With the new IMO Tier III regulation being introduced from 2016, DNV-GL anticipates that customers will approach them from yards, shipowners, engine manufacturers and SCR-system producers to include the SCR-unit in the certification of the main engine. The present thesis has focused on developing a modelling tool capable of doing design calculations for an SCR unit. This tool is meant to be an aid in certification or procurement processes of SCR systems. To be able to perform design calculations, a simulation model of an SCR reactor is developed. This model is developed on the basis of a literature study on the field of control oriented SCR modelling, and is based on a mean value model published in (C. M. Schär, Onder, & Geering, 2006). An SCR reactor is usually a monolithic (honeycomb) structure, with a large number of parallel reactor channels in the axial direction. The monolithic structure is approximated by modelling a horizontal flow channel, and assuming uniform radial distribution. By assuming that the SCR cell acts as a perfect heat exchanger, an energy balance is obtained based on the heat transfer from the exhaust gas to the ambient via the catalyst bed. By using key assumptions and defining which chemical reactions to include, mass balances are developed. By defining the SCR cell as a control volume in the axial direction, mass balances for the urea decomposition, ammonia concentration, NOx concentration and the surface coverage are obtained. The energy balance and the four mass balances now make a set of 5 coupled partial differential equations (PDEs) for the SCR cell, which means a total of 5 states for the model. The PDE system is then simplified into a system of ordinary differential equations (ODEs). This is done by discretising the axial flow channel into two cells in the axial direction and applying the lumped parameter assumption (assuming that gas concentrations and temperatures are homogenous in each cell). Additionally simplifications are done by assuming that the urea flow, NOx concentration and the ammonia concentration can be calculated statically by algebraic equations. This results in an ODE system with two states per SCR cell, i.e. total of four states for the model. The simulation model is implemented into a design algorithm by using Matlab. The design algorithm estimates the required converter volume and feed ratio (NH3in/NOxin) of ammonia through an iteration process. The target of the iteration process is to achieve IMO Tier III compliance, without exceeding an NH3 slip of 10 ppm. The iteration process gradually increases converter volume and decreases the feed ratio until the targeted NOx conversion and NH3 slip are achieved. For each new step, the program simulates the IMO load cycle and calculates the cycle value for evaluation against the Tier III limit. The data required as input from the user to the model is the engine load cycle with exhaust gas mass flow and temperature. A case study dimensioning a reactor for a Wärtsila 12V38 engine was performed. The resulting converter volume from the iteration process was 1.5 m3, which gives a space velocity of 31 000 h-1. The optimal feed ratio was 0.91 and the resulting NH3 slip 10 ppm, with a urea consumption of 121 L/h. The IMO cycle value was 2.49 g/kWh, which is below the Tier III limit (2.5 g/kWh) for this engine. The simulation model of the SCR unit was initially developed for a smaller diesel engine. Marine engines are significantly larger, meaning that the model parameters may be different, which again affects the accuracy of the model. Further work could include laboratory tests to calibrate the model. The developed model and design process is nevertheless able to give an estimate of the required reactor size, performance and chemical consumption for a given engine. This is proved by comparing model results with published values from engine and SCR-system manufacturers.