Fire is a common cause of injuries for people in the United States, and as a matter of fact, people all around the world. Statistic shows that there were 1.3 million fires in the United States in 2014, with over 3,000 deaths and 15,000 injuries. Additionally, there was an astonishing 11.6 billion dollars of lost due to fire. These fires could range from house fires, vehicle fires to forest fires. Nonetheless, there are many fire extinguishing agents used to extinguish the fire. Bromotrifluoromethane, also known as Halon 1301 was used as a fire extinguishing agent until January 1, 1989 when the Montreal Protocol on Substances that Deplete the Ozone layer was put into effective. From then on, the production of Halon 1301 was banned and researchers all around the world did experiments to identify a replacement for Halon 1301 that suppressed fire as effectively and had no toxicity for the environment or any after effects.
Sodium Bicarbonate (NaHCO3) was proven to be an effective fire suppressant and replacement for Halon 1301. Today, it is a commonly used fire suppressant agent in Class B and C fire extinguishers for regular dry chemical. Through various experiments, it was concluded that sodium bicarbonate was an effective fire extinguishing agent in comparison to other fire extinguishing agents such as Halon 1301 (CF3Br), nitrogen (N2), potassium bicarbonate (KHCO3) and iron pentacarbonyl vapor. The reasoning behind sodium bicarbonate’s fire suppressant effectiveness, or any fire suppressants effectiveness is due to the particles or agent’s chemical, thermal and physical effects. The decomposition of sodium bicarbonate in high temperature into sodium hydroxide leads to a decrease in heat capacity and enthalpy, thus leading to flame extinguishment. Additionally, the effectiveness of a particle could also be affected by the temperature at which it decomposes, and the particle sizes of agent used. Sodium bicarbonate’s effectiveness as a flame suppressant and the mechanisms that lead to extinguishment are explained by various studies.
In an experiment conducted by Anthony Hamins in 1998 on the flame extinguishing effectiveness of sodium bicarbonate in a cup burner, it was concluded that sodium bicarbonate (NaHCO3) is an effective agent in extinguishing flames compared to Halon 1301 (CF3Br) and Nitrogen (N2). The cup burner had been often used to test the effectiveness of gaseous agents on suppressing fire but never with powder suppressants. As a result, Hamins modified a cup burner to be able to test the ability of a solid powder or particle in a conflating non-premixed flame. Fire extinguishing agents were incrementally added to the oxidizer stream until the flames extinguished. Reactant streams flowed parallel to each other so that the flame was stabilized and two dimensional (Hamins, 1998). Through his experimentations, Hamins (1998) acknowledged that in comparison to the other agents used in the experiment, sodium bicarbonate was three times more effective than Halon 1301 on a mass basis and six times more effective than Nitrogen. It took a range of 4-6 agent mass percent of sodium bicarbonate to extinguish flames burning propane, heptane, JP-8, JP-5 and 83282 and 5605 hydraulic fluids while Halon 1301 took over 15% and Nitrogen over 20% (Hamins, 1998). The effectiveness of sodium bicarbonate or any fire extinguishing agent, further explained by Hamins could be due to physical and nonphysical chemical kinetic nature such as heat capacity, dilution, phase change, dissociation, degradation, preferential diffusion and enthalpy losses associated with radiative emission (Hamins, 1998). The combination of decomposition and phase changes played a crucial role for sodium bicarbonates’s effectiveness since 10% of the enthalpy, or total heat content in a system, abstracted from one mole of NaHCO3 is due to the decomposition of 0.5 moles of sodium carbonate (Na2CO3), carbon dioxide (CO2) and water (H2O) (CITE). The decomposition of the alkali metal lead to sodium hydroxide which served as a catalyst to chemically destabilize the flame. NEED MORE
Furthermore, in another experiment conducted by Chelliah, Davis, Krauss and Wanigarathne (2000) on fire suppression by particulates containing metallic compounds, similar results were obtained. The experiment had two purposes, to model development effort based on sodium bicarbonate particles and the development of a super effective fire suppressing particles, “where a highly effective metallic compound is encapsulated in a porous solid particle” (Chelliah et.al, 2000). Chelliah et. al hoped to answer questions regarding sodium bicarbonate and metallic compounds such as whether or not particle size affected the effectiveness of the agent and to quantify the physical, chemical and thermal mechanisms of involved in the process of extinguishment. The experiment was done on a counterflow of non premixed methane and air with a steady laminar flame (Chelliah et.al, 2000). Sodium bicarbonate particles were separated into different size groups, ranging from 10um to 60um, and added to the air stream at a relatively steady rate. Results showed that on a mass basis, sodium bicarbonate powder is about 2-10 times more effective in suppressing fires than the banned Halon 1301 (CF3Br) while iron pentacarbonyl vapor is about 60 times more effective. Chelliah et. al discovered the mechanisms involved for sodium bicarbonate particles flame suppression abilities, like Hamin’s explanation, is due to its decomposition into oxides of sodium (Na) in two stages. In the first stage, at 534K, NaHCO3 decomposes to a solid sodium carbonate (Na2CO3), water (H2O) and carbon dioxide (CO2). Then at the second stage, the sodium carbonate formed decomposes at 1170K which leads to the formation of sodium oxide (Na2O) and carbon trioxide (CO3) but because of the presence of water, sodium hydroxide is formed (NaOH) (Chelliah et.al, 2000). The newly formed sodium hydroxide particles are the chief cause to extinguishment. The sodium hydroxide promotes catalytic recombination of radical species needed for flame propagation (Chelliah et.al, 2000). Thus, the effectiveness of sodium bicarbonate particles in flame suppression is strongly due to the “homogenous catalytic radical scavenging by NaOH formed”, and partially due to the thermal effects.
A study on the effectiveness of aerosols in extinction of propane/air counterflow diffusion flames done by Fleming, Reed, Sheinson, Williams and Zegers (1998) showed that flame extinguishment by dry chemical agents is caused by both thermal and chemical mechanism. Fleming et.al investigated the effectiveness of solid aerosols such as potassium bicarbonate (KHCO3) and sodium bicarbonate as a halon 1301 replacements and flame suppression agents. The experiments were conducted in a counterflow diffusion burner with a 1cm i.d tubed separated by 1cm (Fleming et.al, 1998). Different particle sizes of potassium bicarbonate and sodium bicarbonates were delivered to the bowl-shaped propane air flames in the air flow. Extinction effectiveness and concentrations were determined for each sample as a function of the strain rates of uninhibited flame (Fleming et.al, 1998). From experimentation, Fleming et.al’s results showed that potassium bicarbonate powders were about 2.5 times more effective than sodium powder. Potassium bicarbonate, being more effective could be due to its thermodynamic properties. The suppression of a methane fire was is often achieved by removing heat from the flame, meaning the heat capacity and endothermic reactions such as a vaporization, dissociation and decomposition decreases (Fleming et.al, 1998). However, powder particles often add heat capacity to the flame. Free scavenging processes involving homogenous gaseous by-products of agent decomposition and heterogeneous reactions would also hinder the combustion process (Fleming et.al, 1998). Catalytic reactions involving the metal atom, in this case Potassium and Sodium, are often exothermic meaning they would add heat to the flame. Under the conditions of thermochemistry though, the radical scavenging is able inhibit the flame more than the other reactions leading to flame suppression and extinguishment. Moreover, Fleming et.al (1998) explains the difference in effectiveness between potassium and sodium is due to their decomposition temperature and enthalpies. Potassium bicarbonate decomposes between 100 and 200 degree celsius while sodium bicarbonate decomposes at 270 degree celsius. Therefore, if both particle are under similar strain rate conditions and are similar particle sizes, then potassium bicarbonate would take less time to reach its decomposition phase causing it to suppress flames at a faster, more effective rate (Fleming et.al, 1998).
In addition to chemical and thermal effects affecting the effectiveness of sodium bicarbonate as a flame suppressant, the particle size of the agent had been found to affect the suppression effectiveness significantly. Chelliah et. al’s study (2000) showed that all particles of sodium bicarbonate was able to lower the extinction strain as more agents were added. Particles between 10-20um however, showed the lowest extinction strain rate indicating that it is more effective as a suppressant compared to other particles of the same size. Conversely, another study found that particles between 0-10um were the most effective (Hamins et.al, 1994). Results show that the 20-30um particles were more effective than those of 10-20um. The results were almost opposite of Chelliah et.al’s study (2000). The difference in results could be accounted for by Chelliah et.al using a counterflow non-premixed methane and air while Hamins et. al used an opposed flow diffusion flame burner (OFDF). In general, it could be concluded that smaller size particles of sodium bicarbonate are more effective in suppressing fire.
In conclusion, sodium bicarbonate is an effective alkali metal containing particle for fire suppressant and extinguishment. Compared to other fire suppressant agents, sodium bicarbonate is often one of the most effective. It is about three times more effective than the once popularly used fire suppressant Halon 1301 and six times more effective than nitrogen. Iron pentacarbonyl vapor and potassium bicarbonate however is more effective than sodium bicarbonate as a flame suppressant. The effectiveness of a flame suppressant often depends on the chemical and thermodynamic properties of the particle. For sodium bicarbonate, its decomposition into oxides of sodium and phase changes plays a crucial role in flame suppression. The two stages that sodium bicarbonate decomposes into leads to sodium hydroxide, which is a catalyst that promotes recombination of radical species needed for flame extinguishment and as a result, flame extinguishment. As explained by Fleming. et. al. fire suppression is achieved by decreasing heat capacity and endothermic reactions, lowering the heat of the flame. Many homogenous gaseous by-products of decomposition and heterogeneous reactions along with reactions involving the metal atom tend to add heat to the flame though. Fortunately, the radical scavenging is able to inhibit the flame more, leading ultimately to flame extinguishment. Lastly, flame extinguishment of a particle could also be affected by the physical properties of the particle such as their decomposition temperature, enthalpy and particle size. Smaller particles and lower decomposition temperature and enthalpy of an agent would lead to faster flame extinguishment, and an effective flame suppressant.