A Review of Particulate Matter Emission Factors for Gas Firing in Combustion Units at Refineries
Report no. 2/25: Particulate matter is defined as either filterable (FPM) or condensable (CPM) and by size: FPM2.5 refers to filterable particles with aerodynamic diameters ≤2.5 μm and FPM10 those ≤10 μm. Total filterable PM is referred to as total suspended particulates (TSP), or more commonly as ‘dust’. The US EPA assumes that all PM from gas-firing, both filterable and condensable, is less than 1.0 μm in diameter.
A primary use for emission factors (EFs) is the development of PM emission inventories in the absence of measured data. PM emissions from refinery fuel gas (RFG) combustion are estimated, by default, using an EF derived by the US EPA from tests on natural gas (NG) fired industrial combustion units.
A review of published and oil company test data on FPM emissions from RFG-firing has been undertaken. From six tests on five different refinery combustion units, it has been shown that the size profile of total suspended particulates averages 58% FPM2.5 and 99% FPM10. This challenges the widely held assumption that particles from gas-firing are less than 1.0 μm in diameter. However, the tests were all undertaken at one refinery. It is recommended, therefore, that a programme to test different types and sizes of combustion units firing fuel gas at a number of refineries is initiated to establish the validity of the assumption and to compare the average ratios of FPM2.5/TSP and FPM10/TSP with the measurement results above.
The EF for TSP derived from a data set of 42 tests on 28 different RFG-fired units is 0.71 g/GJ. This is of the same order of magnitude as the common EF for FPM2.5, FPM10 and TSP of 0.89 g/GJ for NG-firing in the US EPA AP-42. This is widely replicated for refinery gas firing, for example in the EMEP/EEA Air Pollutant Emission Inventory Guidebook (GB). The latter provides guidance to EU Member States on inventory development.
Indicative EFs for FPM10 and FPM2.5 have been derived from the data set of six tests on five RFG-fired units. The EF for FPM10 = 0.7 g/GJ and that for FPM2.5 = 0.41 g/GJ. That for FPM10 is of the same order of magnitude as that in the EMEP/EEA Guidebook. That for FPM2.5, however, is less than 50% of the value of that in the Guidebook.
There were insufficient data for NG-firing in refinery units to establish if the assumption that the size of particles from natural gas combustion is limited to <1.0 μm is valid.
The only published EF for CPM emissions from gas-firing has been shown in an American joint industry/intergovernmental test programme to be conservatively high due to an artefact with the test method used in its derivation. Although the test programme indicated that CPM emissions are negligible relative to FPM from gas-firing, the US EPA has not, as yet, updated the EF for CPM emissions from natural gas fired combustion units.
A series of test campaigns employing ISO standard method 25597 which uses dilution sampling is required to generate robust CPM EFs for both natural gas and RFG-firing.