H+H SCR Technology for lower NOx emissions
  • SCR Controlstrategies - Open Loop, Closed Loop, Feedforward, feedback

First of all, we would like to define some basic terms of control technology and, in the next step, explain the relations between the various possibilities in SCR technology.

Finally, we explain these terms in accordance with the SCR Guidelines MEPC291(71), the IMO's framework for implementing Tier III certification.

What is a control system?

The definition of "control system" is as follows:

A system of devices that controls, commands or regulates the behaviour of other devices or systems to achieve a desired result. This is achieved through control loops that are used to maintain a process variable at a designated set point.

Any control system has to have a clear mathematical relationship between the input and the output of the system.

Examples of controls in daily life are a refrigerator, an air conditioner, an iron and many operations in a car - like cruise control.

What is a Control Loop?

Control loops are installed/programmed in various applications to maintain process variables (PV) at the desired set point (SP).

To illustrate the concept, let's take a simple example for the urea flow control of the SCR system.
A liquid pressure of 6 bar is present at our urea control valve. We feed the urea valve with a 4....20 mA signal and the valve thus moves to a defined open position. This means that as long as the mA signal is present, a certain amount of urea is supplied to the process. At 18 mA, more urea is fed than at 8 mA, but we never know how much urea is actually injected.

This is a classic example of open loop or feedforward control.

This system can be further improved by installing a flow meter. The urea flow meter measures the difference between the actual amount of urea/reductant and the desired amount of urea.
If there is a difference between the actual and the set point, the controller will operate depending on the deviation between the two values. This difference is called the "error" of the system, this error signal is returned to the controller to adjust the 4...20mA signal for valve activation. I.e. the valve opens further or closes accordingly.

Since the signal loop from the input to the output and the error feedback signal form a closed circuit, this type of control system is referred to as closed loop or feedback control.

NOx Controller

Now we have explained on the basis of the urea control what an open-loop and closed-loop strategy is. What does this mean for NOx control?
An open-loop and alternatively closed-loop control can also be installed for NOx control.

Open-Loop - Feedforward NOx Control System

With open-loop control, we use the fact that the NOx production of engines is reproducible via the load signal. NOx reduction is based on a chemical equation that requires a certain amount of urea (NH3) to reduce a quantity of NOx.
Conversely, this means that the required urea set point can be calculated from the load signal of the engine. In simple terms, at a certain engine load, a required urea set point is transferred to the urea controller. The urea controller controls the urea valve and the urea/reduction agent quantity is injected into the exhaust gas flow.The amount of urea injected results in a certain NOx reduction, if more urea is injected, more NOx is reduced (up to a certain technical/chemical limit), if less urea is injected, less NOx is reduced.

However, we never know the level of the real NOx value - a classic open loop control.

Closed-Loop - Feedback NOx Control

In this control system, a NOx analyser is installed. This measures the NOx value at the output of the catalytic converter and can thus calculate an "error signal", a deviation between the NOx setpoint and the actual value. This signal is fed back to the SCR control unit and the required amount of urea/reductant is adjusted.

This design is used when very low NOx emissions or high conversion rates are required. From a technical and regulatory point of view, closed-loop NOx control is not necessary for compliance with IMO Tier III NOx values.

The SCR Guidelines - MEPC 291(71)

Let us first take a look at how and where the MEPC applies the systematics of NOx control.
Paragraph 3.8.1. points out that the state of the catalytic converter can be checked by means of a closed-loop/feedback control. However, this has nothing to do with the operation of the SCR system itself; this is purely about monitoring the catalytic converter. I.e. the NOx measurement is to be used to determine whether the necessary conversion rates are still being achieved and whether the catalytic converter needs to be replaced. For this purpose, the NOx Technical File must clearly specify under which circumstances the catalyst is still "good" or already "bad".

MEPC 291(71) - 3.8.1.

factors related to the deterioration rate of SCR performance, e.g. exchange condition for SCR catalyst blocks and recommended exchange time of SCR catalyst blocks:

where a feedback or a feed forward reductant control strategy is incorporated with a NOX measurement device, this is acceptable as a means of monitoring catalyst condition/degradation. The exchange criteria of catalyst blocks against the reading of the NOX measurement device is to be specified by the applicant as well as the maintenance, service, and calibration requirements for the NOX measurement device;

It should be noted, however, that only NOx measurement devices that comply with the guidelines in the NOx Technical Code 2008 are approved. Alternatively, a proof of equivalence can be provided for the IMO Tier III test.