Applicability of the PEMS technique for simplified NOX monitoring on board ships

The performance of a predictive emission monitoring system (PEMS) as a technique for NOx monitoring on medium speed marine diesel engines has been evaluated for 16 similar engines on four different ships. The PEMS function tested measured O2 concentration in the exhaust gas, engine load, combustion air temperature and humidity, and barometric pressure to calculate the NOx concentration. Emission measurements were carried out by means of a conventional continuous emission monitoring system (CEMS) and the measured NOx concentrations were compared with those calculated by the PEMS function. For 11 of the 16 engines, the average error between measured and calculated NOx concentration was <10% of the calibration range (1725 ppm). In addition, 10 of the engines displayed correlation coefficients between measured and calculated NOx as 0.90 or higher. For two of the ships, the predicted NOx concentrations from all engines on board gave good agreement with those measured (2.6–4.7% and 2.6–8.0% average error). In other cases however, the performance of the PEMS function was poor e.g. the four engines of ship D showed average errors of 10.3–17.7%. Although similar engine models, fuel and load characteristics were compared in the tests, the specific NOx emissions at steady-state loads used varied from 12.6 up to 15.8 g k−1Whcorr. Although a single PEMS function may prove universal and adequate for calculating NOx emissions from similar engines on board the same ship, an engine specific PEMS function is recommended. The form of the PEMS function, i.e. using exhaust O2 and engine load as inputs, is however likely to be applicable to most propeller-law diesel engines. Bearing in mind the performance criteria for using PEMS at land-based installations, the results from this study are promising. Viewed as a single data set of 56 h with 16 separate engine comparisons between CEMS and PEMS, the data set shows a relative accuracy of 14.5% i.e. within the 20% requirement of the US Environmental Protection Agency. In light of the increased interest and international guidelines for continuous NOx monitoring on board ships, the PEMS technique can offer a simple but cost-effective option.