How are overboard nitrate emissions monitored?

The level of unscrubbed NOx emissions is mainly governed by the design and operation of an engine, the combustion temperature and to a lesser extent the nitrogen content of the fuel (The IMO give typical figures of 4% nitrogen for residual fuel and zero for distillate [17]).  NOx emissions consist primarily of 90 - 95% nitric oxide (NO) and approximately 5 – 10 % nitrogen dioxide (NO2). Whilst NO is not readily dissolved, NO2 is soluble and therefore likely to be at least partly removed during the exhaust gas cleaning process to form nitrate in the wash water.  When compared with the removal of SO2 however, the amount of NO2 removed is relatively small and constant for an engine burning residual fuel with an exhaust gas cleaning system. The impact of the nitrates produced has also been demonstrated to be minimal [4].

For these reasons the IMO does not require continuous monitoring of nitrate in its EGCS Guidelines, which is unlike its requirements for pH, PAH and turbidity [10]. It has however set a limit on nitrate emissions based on removing 12% of the NOx from an exhaust stream. (This is based on a hypothetical scrubber design capable of removing more NOx than the soluble NO2 fraction and gives some future proofing whilst mitigating the risk of eutrophication). Compliance has to be proven by laboratory analysis of a sample drawn during initial system certification and within 3 months of each 5 yearly renewal survey.