Fugitive VOC Emissions from Storage-Tank Accessories: Comparison of Quantification Methods Based on Optical Gas Imaging

Fugitive emissions from storage-tank accessories are important sources of volatile organic compounds (VOCs) in chemical industrial parks, but the applicability of estimation methods requires field evaluation. Optical gas imaging (OGI) was used to screen about 200 selected volatile organic liquid storage tanks in a Chinese chemical park. Evident emissions were detected at 46 tanks; breathing valves (14) and foam generators (13) accounted for about 59% of detections. Twelve of the 13 foam generators were inaccessible to flame ionization detector (FID) measurements, illustrating the complementary coverage of OGI and probe-based screening. FID readings for 14 breathing valves were 2125–8423 µmol mol−1, above the numerical criterion of 2000 µmol mol−1 in amendments to GB 31570/31571/31572. Regulatory interpretation requires the applicable operating-pressure condition and facility requirements. For 30 FID-accessible accessories, annualized estimates from the OGI emission-factor and correlation-equation methods were 26.3 and 1.86 t a−1, respectively, assuming continuous release for 8760 h. The approximately 14-fold difference reflects the methods’ different category mappings and dependence on screening readings; it does not establish their accuracy against an independent mass-emission reference. The findings support attention to inaccessible accessories and further field validation of accessory-specific emission estimates.