Understanding FTIR formaldehyde measurement and its influence on the RICE NESHAP rule

This paper discusses the issues of measuring formaldehyde on rich burn natural gas engines typically used in the gas compression industry in the United States. Recently, the EPA passed the final rule for reciprocating internal combustion engines to meet the national emission standards for hazardous air pollutants (i.e. RICE NESHAP rule) for both compression ignition (CI) and spark ignited (SI) engines. In natural gas lean burn engines, carbon monoxide (CO) is measured and used as a surrogate for formaldehyde and other hazardous air pollutants (HAPs). In rich burn natural gas engines, formaldehyde is directly measured in the field and used as a surrogate to represent the various HAPs present in engine exhaust. The recommended methods for the measurement of formaldehyde are EPA method 320 or ASTM D6348-03. For both of these methods, a Fourier Transform Infrared (FTIR) spectrometer is required. The FTIR is a very sophisticated instrument that is capable of measuring very low concentrations of formaldehyde. Similar to other analytical instruments, the instrument must be correctly setup to give realistic, unbiased readings. This is particularly true at low concentrations such as the levels specified in the RICE NESHAP rule. For rich burn engines greater than 500 horsepower, the rule requirement is greater than 76% formaldehyde removal efficiency or below 2.7ppmv@15% O2. This paper will discuss how FTIR formaldehyde measurement is influenced by instrument setup and the presence of other exhaust chemical components. A model gas reactor is used to simulate major exhaust gas components, giving a stable, known gas stream input to the FTIR. Step changes in the individual gas components were made and the effect on formaldehyde measurement was determined. It was determined that special care needs to be taken to avoid hydrocarbon bias in the formaldehyde measurement at a FTIR gas cell temperature of 150°C.