Aiming to reduce potentially harmful emissions to the environment, the recent European LED (Industrial Emissions Directive) requires that industrial installations are equipped with BAT (best available techniques). However, flexibility mechanisms are foreseen to avoid inefficiencies. One such derogation allows competent authorities to set less strict emission limit values if the installation and operation of BAT leads to disproportionate costs (i.e. abatement costs exceed related environmental benefits). The current paper aims to present an approach to quantify monetised environmental benefits related to reductions in air pollutant emissions in this context. The tool EcoSenseWeb is applied to the case of a DeNOx retrofit at a coal-fired power plant hypothetically located at three different sites in Europe, varying in terms of environmental conditions. Following environmental cost-benefit analysis principles, the DeNOx investment proves to be socially efficient at all sites. Costs tend to be disproportionate under certain conditions, e.g. lower operating hours per year and remote geographic locations, justifying case-specific derogations. The applicability of the methodology and the related tool in the process of granting derogations under the IED is explored, while noting practical and methodological limitations. Associated development needs are highlighted, at best to be addressed at EU (European Union) level in a harmonised way. (C) 2014 Elsevier Ltd. All rights reserved.