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Carbon Monoxide Production in Compartment Fires: Reduced-Scale Enclosure Test Facility (NISTIR 5568)



Nelson P. Bryner, Erik L. Johnsson, William M. Pitts


The formation of carbon monoxide during room or compartment fires has been investigated using natural gas fires burning within a reduced-scale enclosure (RSE), an 0.98 m x 0.98 m x 1.46 m (w x h x d) room with a single door opening centered in the front wall. This series of 125 fires ranging in heat release rate (HRR) from 7 to 650 kW and global equivalence ratio from 0.2 to 4.2, respectively, has demonstrated that the upper layer is nonuniform in temperature and gas species, and that upper-layer oxygen is depleted for underventilated fires with high-temperature upper layers. For fires having HRR exceeding 400 kW, carbon monoxide concentrations of up to 3.5 percent have been observed in the front portion of the upper layer. Carbon monoxide concentrations in the rear were consistently lower being on the order of 2.0 percent for equivalence rateo > 2. While oxygen concentrations approached zero in both the front and rear of the upper layer for underventilated burning conditions, temperatures were generally 200 deg C to 300 deg C higher in the front of the upper layer than in the rear. Both the high temperatures and high carbon monoxide concentrations in the front of the upper layer are consistent with oxygen being transported directly into the upper layer as well as entering through the fire plume for the large fires. This oxygen appears to react with unburned fuel to form carbon monoxide, instead of being fully oxidized to carbon dioxide. As the unburned fuel is oxidized, additional energy release occurs which provides an explanation for the higher temperatures observed in the front of the RSE. The exact mechanism for transporting oxygen directly into the front portion of the upper layer is ot yet understood. The results of these RSE fires clearly indicate that higher levels of carbon monoxide can be generated in post-flashover scenarios than suggested by earlier laboratory hood experiments or earlier enclosure studies designed to generate a stable two-layer structure. Current fire models do not adequately simulate the temperature and gas species nonuniformities nor the high levels of carbon monoxide.
NIST Interagency/Internal Report (NISTIR) - 5568
Report Number


compartment fires, carbon monoxide, acute toxicity, fuel/air ratio, combustion products, fire chemistry, flashover, room fires, scale models, global equivalence ratio, oxygen concentration


Bryner, N. , Johnsson, E. and Pitts, W. (1994), Carbon Monoxide Production in Compartment Fires: Reduced-Scale Enclosure Test Facility (NISTIR 5568), NIST Interagency/Internal Report (NISTIR), National Institute of Standards and Technology, Gaithersburg, MD, [online], (Accessed April 19, 2024)
Created December 1, 1994, Updated November 10, 2018