Factors influencing the generation of carbon monoxide in fires partially suppressed through water mist application

Abstract

Fires in the built environment typically consist of diffusion flames with inefficient mixing of fuel and oxygen, resulting in a degree of incomplete combustion and the generation of carbon monoxide. Fire suppression systems, used to control the growth and spread of fire, influence the combustion reaction process. Water droplets interacting with the gas phase chemistry of fire interrupt combustion processes and promotes generation of carbon monoxide.
Past research into mist suppression primarily focuses on optimal droplet size for heat release rate reduction, and notes a significant increase in carbon monoxide concentrations with this approach. The aim of this experimental study was to contribute to the knowledge of the interaction of water droplets on the gas phase chemistry of fire and assess the factors that influence the generation of carbon monoxide.
The experimental set up was designed to minimise the effect of the mist spray on the heat release rate of the fires. Laboratory-scale fires, having heat release rates of 40-70 kW, and consisting of gaseous, liquid and solid fuels, were subjected to mist suppression sprays. The sprays applied consisted of droplets having characteristic diameters (Dv50) of 163 287 μm, and water flow rates of 1.5-3.5 L/min. The concentration of carbon monoxide produced by fires subject to mist suppression increased by up to 250%, with minor reductions to the heat release rate.
This study represents a proof of concept to a currently largely under explored phenomenon. The findings indicate a need for closer examination of how water sprays influence toxic species production. The results raise uncertainly on the applicability of typical applied species yields when fires are partially suppressed through fine water sprays. It is recommended that fire safety engineers consider the adoption of more conservative safety factors where it could be anticipated that the suppression systems would not result in extinguishment.

Popular Abstract

Building fires pose risk to human life. Fire science, and the practice of fire safety engineering, aim to preserve life through the reduction of risk to building occupants. While exposure to flames and high temperatures seem the most dangerous elements of a fire, records show that approximately two thirds of fatalities in building fires are caused by exposure to carbon monoxide. Most of these deaths were not due to carbon monoxide poisoning, rather the side effects of carbon monoxide intake resulting in confusion or disorientation and physical incoordination, limiting a person’s ability to leave the hazardous situation.
Fire safety engineering aims to control the risk to life caused by fire by first considering the likelihood of fire to occur, and in the event of a fire, control its growth/spread and provide occupants with safe a egress route within tenable conditions to escape from the danger. A common tenability criterion considered is the carbon monoxide concentrations which occupants may be exposed to whilst they are egressing to a safe place. Carbon monoxide is generated in fires where the fuel being burnt is not provided with a sufficient quantity of oxygen to fully react. This is known as incomplete combustion. Water sprays, such as those created through sprinkler or water mist systems, are a common means of controlling the growth and spread of fires. Depending on the properties of the water spray, the droplets applied interact with the fire and fuel in a range of ways, inhibiting combustion.
In a typical assessment, the amount of carbon monoxide created by the burning of a specified fuel (known as the yield) will be calculated based on the available oxygen in the compartment (based on how well-ventilated the environment is, controlling the inflow of fresh oxygen). To date, the influence of the application of water droplets on carbon monoxide production is under-researched.
A study undertaken at Lund University through the International Masters of Science in Fire Safety Engineering program looked at the application of water droplets to fires in order to quantify the production of carbon monoxide in different scenarios. An experiment applied a mist spray (water spray consisting of fine droplets) with varying properties (including droplet size and flow rate of water) to fires of various sizes and fuel types (solid, liquid and gaseous). Variations in the droplet size and flow rate was created through use of varying the system pressure and nozzle type. Carbon monoxide production was measured before and during the application of water.
The research found that the concentration of carbon monoxide (in parts per million) generated increased by up to 250% when mist suppression was applied. Sprays consisting of smaller droplets resulted in larger changes in carbon monoxide generation due to the significantly larger total surface area of the droplets. Carbon monoxide concentrations were also observed to have larger increases with higher rates of water application. Fuels shown to generate higher levels of carbon monoxide in unsuppressed conditions were shown produce even more carbon monoxide when partially suppressed.
This research shows a proof of concept for the potential significant increase in carbon monoxide generation in environments where there is only partial suppression of the fire. Practical applications where this is of relevance are fires occurring within tunnels or large warehouses. While this study is only an initial exploration of the phenomenon, it indicates that fire safety engineers should consider adopting more conservative safety factors within their calculations of these kind of scenarios to compensate for increased carbon monoxide production.

• author: Lewis, Haydn
• supervisor: Nils Johansson
• organization: Division of Fire Safety Engineering
• course: VBRM05 20201
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Fire safety engineering, water mist, fire suppression, carbon monoxide, water, combustion
• publication/series: LUTVDG/TVBB
• report number: 5615
• other publication id: LUTVDG/TVBB-5615-SE
• language: English
• id: 9027810
• date added to LUP: 2020-09-01 14:09:48
• date last changed: 2020-09-02 13:30:15