I'm not sure if this was what you were looking for:
"If recirculating roaster gas contained only products of combustion, its weight % composition
would be: 73.2% N2, 10.2% H2O, 12.8% CO2 and 3.7% O2 for a typical natural gas burned with
20% excess air. However beans emit CO2, H2O, CO and volatile organic compounds (VOC)
during roasting. Therefore the average weight % composition of roaster gas in a recirculating
batch roaster is roughly 50.6% N2, 30.2% H2O, 16.6% CO2 and 2.6% O2. These percentages
vary during roasting, with the percentages of N2 and O2 approaching those in combustion gas
at the start of roasting. The H2O percentage peaks in the middle of roasting and those of CO2,
CO and VOC peak near the end of roasting. At the end of roasting, the VOC content of roaster
stack gas can reach 0.2% by weight. Since VOC partly break down when roaster gas passes
through the furnace, the % VOC for gas in the roasting chamber is probably even higher."
I have the report (ca. 2000) but no source name associated with it.