Aldehyde Emissions from Gas Engines

Per G. Kristensen, June 1998, P9814

Abstract

(Danish text in the report)

DGC has carried out an examination of the aldehyde emission from natural gas fired gas engine based Danish cogeneration plants for the gas companies’ Specialist Committee 2.

The measurements have been carried out with two main purposes. Firstly an evaluation, whether observed odour from cogeneration plants were due to aldehydes in the exhaust gas, was required. Secondly the project was to document the emission of aldehydes from the cogeneration plants. The little experience that DGC had before the project was initiated, indicated that the emission of especially formaldehyde from the cogeneration plants exceeded the fixed limit in the Danish Environmental Protection Agency’s Guidelines No. 6.

The measurement program has been carried out in parallel with the odour measurement program, thus connected values for odour emission, aldehyde emission, and the usually measured emissions for CO, NOx and unburned hydrocarbon have been measured. Within the frames of the project nine measurements have been carried out. Furthermore the experience from aldehyde measurements that DGC has previously carried out has been included in the analysis.

It has not been possible to confirm the expected correlation between odour emission and aldehyde emission. The observed odour emission considerably exceeds (factor 40-300) what can be expected at a purely additive evaluation from the measured aldehyde concentration based on their odour threshold values in the literature. Furthermore the odour emission does not correlate with any of the measured aldehyde and keton (acetone) species. It is thus not possible to use any of the measured components as a single reference for odour emission.

On an average the emission of formaldehyde is 67mg/m3n (10% O2, dry flue gas) for engines >1 MW input effect (maximum emission 103 mg/m3n).

The acetaldehyde emission is 5.6 mg/m3n, maximum 8.8 mg/m3n (10% O2, dry flue gas) on an average.

The measured values for the formaldehyde and acetaldehyde emissions are supported by GRI based measurements on American gas engines. The emission of formaldehyde and acetaldehyde correlates with the emission of unburned hydrocarbon, which corresponds to the assumed formation mechanisms for aldehyde emissions from combustion systems.

The environmental conditions for cogeneration plants with a total input of more than 5 MW have been regulated according to the air pollution guidance from the Danish Environmental Protection Agency, as cogeneration plants of this size are registered as Chapter 5 enterprises, the so-called specially polluting activities. In the guidance reservations for emission threshold limits for combustion plants are taken, but not for imission values. If the found emission values and bulk flow from the cogeneration plants are compared to the guidance limits for industrial plants, the emission of formaldehyde and acetaldehyde would be significant above the emission limits.

For some of the cogeneration plants tested here the aldehyde emission (formaldehyde) will be the dimensioning factor for the chimney height and not the NOx emission normally used.

As to the other aldehydes and for acetone relatively small amounts 1 mg/m3n have been found. However, the sampling efficiency of the used measurement method is smaller for those major aldehydes, of which the observed emissions must be considered as guiding.

Furthermore DGC has carried out a few measurements on possible methods for reduction of the aldehyde emissions. A reduction degree of 33% with a CO oxidation catalyst has been measured, 9% at a test with ozone treatment of the flue gas and 91% reduction with an experimental catalyst for hydrocarbon oxidation.