Patrice Flot, chief technical officer at CMR Group, considers the development of new natural gas sensors in the wake of the increasing specification of new engine technology for marine vessels.
Diesel engines have long been appreciated for their efficiency and capability to burn cheap residual fuels. However, the market for this technology is beginning to change, as the marine sector considers alternative solutions in the face of global environmental issues and the drive to reduce carbon emissions.
Previously restricted to LNG ships until just a few years ago, we are now seeing gas engines increasingly being specified for a wide variety of vessels, including passenger ships (under full agreement and control of the Classification Societies), as the technology continues to develop, and owners and operators recognise the fleet optimisation advantages offered.
While the growth of gas engine usage and the interconnection of the gas networks, together with a greater requirement for reliability and availability and lower maintenance, are creating new challenges are all important considerations in the continued development of engines, the issue of gas quality variation – how gas quality varies between sources, countries, regulations and even time – is also a critical consideration, presenting a challenge that new sensor technology can help to overcome.
Attempts to harmonise gas quality regulations in the EU which in turn, eases gas trading across borders, are convincing gas distributors to push for a broadening of gas quality limits to enlarge the portfolio of possible sources. Simultaneously, OEM engine builders and other gas-fuel users, are pressing for restrictive limits that are better suited for their power plants and ancillary equipment.
Natural gas is a generic designation for various qualities of gas based on the main gas component methane (CH4). As such, it covers many different qualities linked to the gas composition including the source of raw gas and refinery processes. The gas can be manufactured from synthesis technologies, natural processes, or ‘renewable power-to-gas’ technologies. Gas networks are interconnected and strategically linked to multiple sources. Because of this, the quality of gas can change at a given location and these changes could be rapid when switching from one source to another. As well, rapid changes of gas quality can occur in other situations faced by end users, such as onsite switching from a gas tank to the gas network, or vice-versa.
Also, when gas is only available in LNG tanks, switching from an empty tank to a full one can result in significant changes in gas quality – an empty tank will deliver the heaviest molecules contained in natural gas, and a full one may deliver the lightest molecules.
In marine applications, propulsion gas engines are often fed by boil-off from LNG tanks. When engine power exceeds the capacity of the boil-off flow, the additional gas flow is fed to the engine from the bottom of the tank. Boil-off is made from the lightest molecules of the natural gas, especially when the tank is nearly full. In contrast, gas delivered from the bottom of the tank comprises the heaviest natural gas molecules. Consequently, the resulting mixture quality varies with the load of the engine.
During heavy seas, load fluctuation on the propeller will typically generate rapid changes in gas quality at the engine inlet. Engine builders are acutely aware of these situations, which deeply affect the combustion inside the gas engines. Most of these gas engines are running in accordance with the Otto principle – i.e. a four-stroke engine cycle – which is sensitive to the methane index. Gas with a high methane index (80 – 90) will provide smooth combustion starting from the spark ignition device, whereas gas with a low methane index (55 – 65) enhances the risk of knocking by auto-ignition at several places inside the combustion chamber.
To counteract this, vibration sensors and combustion pressure sensors are used by engine builders to monitor high levels of knocking and reduce it to low levels by adapting the tuning of the engine. As are often the case, such alterations do not always occur quick enough to accommodate sudden gas quality changes, causing an engine emergency stop. Additional margin is usually obtained through conservative engine tuning. This includes reduced spark ignition timing before top dead centre, which is detrimental to the efficiency of the engine.
Another major difficulty for engine builders is the rate of change of gas quality over short periods of time. Because that point is not addressed by regulation, engine builders must cope with potential sudden changes in gas quality that are not always managed by current combustion control loop technologies. These control loops are based downward on combustion parameters measurement, not upfront on gas quality measurement.
To overcome the knocking issues linked to the Otto principle and rapid gas quality changes, we have seen significant strides in the development of new technology – the Near InfraRed Intelligent Sensor natural gas sensor (NIRIS NG) for instance – which has been designed to analyse the natural gas composition at the inlet of the gas engine. (Surprisingly, there hasn’t been a sensor commercially available in the industrial market to allow upfront gas quality measurement that eases the engine combustion control loop).
Compared to other gas quality analysers, the new generation of gas pipe plugged sensors from CMR use low cost optical components and proprietary signal treatment software contained within robust housing to resist vibration and heat. Light is emitted by the sensor and travels through the gas inside the supply pipe on the engine before being analysed by a detector. The output signal from the detector is a simple spectrum that is then processed by a calculation matrix to provide gas composition, and related parameters.
This matrix is specific to each sensor and is inbuilt during manufacturing at the calibration stage thanks to data base of gas mixtures. Calculation is carried out through multivariable methods like the principal component regression (PCR) and partial least square regression (PLS). Measurement data is then provided through CAN protocol to the engine. The methane number input at the ECU (Engine Control Unit) can be used by the engine builder to continuously optimise the spark ignition timing and other engine actuators.
Using the new CMR NIRIS NG sensor reduces consumption levels of natural gas-powered engines through the real time measurement of fuel quality. Directly connected to the gas feeder pipeline, the sensor is built around smart infrared hardware and data treatment software and features a CAN bus communications facility, which enables it to be upgradeable without dismantling the sensor for improved performance and retro applications.
Natural gas consumption levels can be further reduced by engine tuning closer to knocking limits due to effective fuel management strategies using sensor data. Other benefits include lower fuel analysis costs, correct engine performance and the overall alleviation of time consuming and costly damage to components due to inferior or low-grade gas fuels.
New opportunities are opening-up to gas engine manufacturers on the back of new technology. Linked to electrical actuators, the new generation of sensors permit instantaneous adjustment of the engine during a change of gas supply from one source to another of different quality, or when heavy seas place variable load on the propulsion system, requiring variable forced boil-off to be added to natural boil-off, resulting in a highly variable methane number at engine entrance.
Moreover, with the capabilities offered by a fast response time sensor, gas authorities and suppliers can harmonise gas quality better, finding new and easier ways for agreement and greater opportunities for global gas trading.