Testing routines for emergency diesel generators vary, and can range, for example, from once per week for five minutes to two or three hours once per year. Based on such a low level of operation, it can often be demonstrated within an air quality assessment that testing is unlikely to lead to an exceedance of the annual or daily mean objectives or limit values, but further consideration may be needed for determine impacts on 1-hour mean nitrogen dioxide (NO2) concentrations. This is regularly assessed qualitatively, based on an assumption that sporadic operation cannot feasibly have a significant effect on attainment of the objective, but there is typically a lack of evidence to support this assumption. In cases where dispersion modelling is carried out, this often focuses on the Process Contribution from the generator without considering how this might interact with existing short-term concentrations (from road traffic, for example).
AQC have prepared a note to provide an evidential basis for screening out the need for detailed assessments of the impacts of generator emissions on the short-term NO2 air quality objective. The analysis has considered a number of simple hypothetical situations, which are deliberately designed to provide a worst-case assessment of emissions from the routine testing and maintenance of a single diesel generator operating no more than 18 discreet hours per year, where relevant nearby receptors are located close to a significant source of roadside NO2. A Monte Carlo modelling method was implemented to calculate the probability of an exceedance of the 1-hour mean NO2 objective occurring at the receptors.
The work provides rules to determine when NO2 impacts from generator testing can safely be disregarded, and when additional assessment is justified.
The report is available to download here.
View all news