Environmental Technology

NOx production and monitoring

Paul Gunton
Paul Gunton

09 November 2018

NOx production and monitoring

In all internal combustion engines, boilers and incinerators, it is necessary to mix air with the fuel to allow combustion to take place. Air is mostly composed of nitrogen (about 78%) and oxygen (about 21%) with a few trace gases and water vapour. The fuels themselves are a complex mix of hydrocarbons with other components depending on their type. Even within the defined ISO 8217 fuel grades there are no fixed absolutes but minimum and maximum levels for constituents of the fuel.

Different fuel types burn best at different temperatures and this along with their chemical composition and the spray pattern into the combustion chamber is instrumental in determining the exhaust gases produced. The majority of engines are at their most efficient when cylinder pressures and temperature in the combustion chamber are high and when operating at an optimum loading.

When measured in the exhaust duct of a marine diesel engine, NOx emissions comprise about 95% nitric oxide (NO) and 5% nitrogen dioxide (NO2), which is formed as NO oxidises after the engine. The formation rate of the majority of nitric oxide is dependent on peak temperatures in the engine cylinders – above 1,200°C the formation is significant and above 1,500°C it becomes rapid.

A highly efficient engine will obviously reduce the amount of CO2 produced in relation to the power produced. However, such conditions are more likely to produce NOx when burning oil fuels. Reducing the temperature or pressure will reduce the amount of NOx produced but will inevitably result in a less efficient engine.

Ensuring engines meet the NOx limits is in the first instance down to the engine maker. The engine should come with a technical file and a certificate confirming the engine complies with the relevant limits. Thereafter, the owner has a choice of three methods of ensuring the engine continues to perform as required.

The first is the engine parameter check, under which it needs to be demonstrated that all those areas that influence NOx production remain in strict accordance with the engine maker’s original test bed condition as regards components, calibration, setting and operational parameters. Adopting this may mean that no change to engine settings can be made without it being accounted for in the technical file and it may mean that use of third-party spare parts is out of the question. The parts affected would probably include all those for the fuel injection system, camshaft, valves and valve timing, pistons, heads and liners, connecting rods and piston rods, charge air system and turbochargers, plus others depending on the engine type.

While some operators are quite happy to stick to OEM spares, others prefer cheaper pattern parts and for the latter there are two options to consider, namely the simplified measurement method or direct monitoring on board. Simplified measurement entails an effective repeat of the initial manufacturer’s test-bed certification procedure at every intermediate and special survey. This may involve specialist attendance. There is, however, no requirement that all parts on the engine need to be OEM parts.

Alternatively, direct measurement and monitoring is possible, using type-approved equipment available from a number of suppliers. Monitoring can either take the form of spot checks logged with other engine operating data on a regular basis and over the full range of engine operation, or monitoring can be continuous and the data stored.

A variety of technologies are used in the monitoring systems, most of which rely on traditional gas detection techniques. As is to be expected, each of the makers believes that its equipment (or the technology used in it) is the most appropriate. No system is perfect, however, and each of them could develop faults that would affect the accuracy of the test results. Probes and sensors can become clogged, affecting accuracy either way; leaks in the exhaust system and absorption of gases are also problems that have been identified.

To overcome this problem, the monitoring equipment needs to be calibrated on a regular basis to ensure that it is functioning correctly. The reliability of monitoring systems has improved over time as their use has expanded. When there was only a need to monitor NOx emissions most of the systems in use were set up to do just that. However, now that SOx scrubbers are becoming more common, so the makers of monitoring systems have enhanced their products to cover other regulated exhaust emissions.

The new breed of monitors come with other enhancements and some have a GPS input and can be programmed to send an alarm to the bridge when the vessel is close to a regulated emissions control zone in order that arrangements can be put in hand to ensure compliance with the rules effective there.

It should be noted that the NOx limits apply to the engine and not the ship. A vessel which has replacement engines fitted will need to comply with the limits applicable at the time of the engine manufacture. There is also provision in the code for engines being obliged to comply with a higher Tier limits if the OEM produces means to make this possible. MAN is one maker that has done this for a limited number of engine types. Meeting the NOx Code limits for Tier I and Tier II has been achieved without too much difficulty and for Tier III a number of options have been explored.