Two-Stage SCR System Reductant Metering for NOx Conversion
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Solution Overview
Problem
Two-stage SCR catalytic converter systems face limitations in achieving sufficient NOx conversion rates over the entire operating range due to restricted catalytic coating quantities in the SCR exhaust-gas treatment unit near the engine, leading to inadequate NOx emission reduction.
Innovation Solution
A method for controlling the reductant metering process based on the temperature of both SCR components, allowing for simultaneous charging and loading of both components with reductant, with specific temperature-dependent target filling-level characteristic lines to optimize NOx conversion and prevent reductant breakthrough.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the quantity of catalytic SCR coating in the SCR exhaust-gas treatment unit near the engine is increased, then the NOx conversion rate is improved, but exhaust-gas counter-pressures become unacceptable
Solution Approach 1:
The SCR system is divided into two separate units: an SCR exhaust-gas treatment unit near the engine with limited catalytic coating, and a downstream SCR catalytic converter with additional coating. This segmentation allows the first unit to handle initial NOx conversion without excessive backpressure, while the second unit provides additional conversion capacity, resolving the contradiction between conversion rate and counter-pressure.
Solution Approach 2:
The solution extends the SCR treatment from a single spatial location to a distributed two-stage system along the exhaust flow path. By adding the downstream SCR catalytic converter as an additional dimension of treatment, the system achieves higher overall NOx conversion without concentrating all catalytic material in one location, thus avoiding excessive counter-pressure.
2Productivity
If the reductant metering process is optimized for the SCR unit near the engine, then early NOx conversion is improved, but the downstream SCR catalytic converter may receive insufficient reductant
Solution Approach 1:
The reductant metering process is made dynamic and temperature-dependent, with different target filling levels assigned to different operating conditions. The control system continuously adjusts the reductant metering rate based on real-time temperature measurements from both SCR units, ensuring optimal reductant distribution across varying engine loads and temperatures, thus balancing early conversion with downstream reductant availability.
Solution Approach 2:
The system implements feedback control by continuously monitoring the temperatures of both the SCR exhaust-gas treatment unit and the downstream SCR catalytic converter. Based on these temperature feedback signals, the control unit dynamically adjusts the reductant metering rate to maintain appropriate filling levels in both units, ensuring that reductant is appropriately distributed to meet NOx conversion requirements at both stages.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances NOx conversion capacity by ensuring effective storage and catalytic conversion of reductant, reducing undesired emissions and allowing for efficient operation across varying engine conditions.
Implementation Method 1
catalytic converter systems that function according to the principle of selective catalytic reduction (SCR). These systems comprise at least one SCR catalytic converter that, in the presence of a reductant that has been added to the exhaust gas, usually ammonia NH3, converts the nitrogen oxides that are present in the exhaust gas into nitrogen and water
Implementation Method 2
NH3 storage materials that reversibly bind ammonia as a function of the temperature. In particular, metal ammine storage complexes are known in this context such as, for example, MgCl2, CaCl2 and SrCl2, which store ammonia in the form of a complex compound so as to be present, for example, as MgCl2(NH3)x, CaCl2(NH3) or SrCl2(NH3). The ammonia can be released from these compounds once again through the input of heat
Implementation Method 3
it can be obtained from a precursor compound, for example, urea in the form of an aqueous solution or solid pellets by means of thermolysis and hydrolysis
Implementation Method 4
it can be obtained from a precursor compound, for example, urea in the form of an aqueous solution or solid pellets by means of thermolysis and hydrolysis
Data Source
AI summary
The invention relates to an SCR catalytic convertor system and a method for operating a reductant metering process of an SCR catalytic converter system (14) of an internal combustion engine (10), wherein the SCR catalytic converter system (14) has a first SCR exhaust emission control device (16), an SCR catalytic converter (18) arranged downstream of the SCR exhaust emission control device, and a reductant metering device (20) for metering a reductant into the exhaust gas flow upstream of the first SCR exhaust emission control device (16), wherein the reductant metering process is controlled at least depending on a temperature of the first SCR exhaust emission control device (16) and of the SCR catalytic converter (18) arranged downstream, wherein (I) if the temperature of the first SCR exhaust emission control device (16) is greater than or equal to a predetermined minimum temperature (T_SPF min) and the SCR catalytic converter (18) arranged downstream is less than a predetermined minimum temperature (T_SCR min), the reductant metering process occurs in a first operating mode in such a way that a reductant filling level (NH3_SPF) of the first SCR exhaust emission control device (16) is less than the maximum reductant filling level (NH3_SPF_max) of the first SCR exhaust emission control device, and (II) if the temperature of the SCR catalytic converter (18) arranged downstream is greater than or equal to the minimum temperature (T_SCR min) of the SCR catalytic converter arranged downstream, the reductant metering process occurs at least at times in a second operating mode in such a way that the reductant filling level (NH3_SPF) of the first SCR exhaust emission control device (16) is greater than the maximum reductant filling level (NH3_SPF_max) of the first SCR exhaust emission control device, such that a reductant penetration passing through the first SCR exhaust emission control device (16) hits the SCR catalytic converter (18) arranged downstream.