Performance improvement of the dry chemical-based fire extinguishers using nanocalcium silicate synthesised from biowaste

Abstract

Herein, we investigated nanocalcium silicate (nCa₂SiO₄) prepared from clam shells and rice husks for its utilisation as a chemical agent in a fire-extinguishing mixture comprising ABC dry powder. The fire-extinguishing performance was evaluated with Class A and B fires. The prepared mixture was compared with commercial mono-ammonium phosphate powder based on different parameters, namely extinguishing time, amount of extinguishing agent used, fire temperature reduction rate, powder coating on the fuel and a reburn incident. It was found that the mixture of nCa₂SiO₄ and ABC dry powder could extinguish Class A and B fires within 10.67 and 9 s, respectively, while commercial mono-ammonium phosphate powder required 11 and 11.33 s to extinguish Class A and B fires, respectively. Thus, the mixture of nCa₂SiO₄ and ABC dry powder was more effective and less consumed as compared to commercial mono-ammonium phosphate powder (Class B only). This study demonstrates the efficacy of nCa₂SiO₄ to improve the performance of dry chemical-based fire extinguishers.

Keywords

Dry chemical fire-extinguishing agent nanocalcium silicate clam shell rice husk commercial mono-ammonium phosphate powder

Introduction

Dry chemical-based fire extinguishers are in high demand for their capability to extinguish various types of fire. The ABC dry powder (ADP) is composed of mono-ammonium phosphate.¹ A dry chemical-based fire extinguisher is not effective on Class B fires because of cooking fuel; thus, a protein foam-based extinguisher is preferred to extinguish such fires. A dry chemical-based fire extinguisher coats the fuel surface with chemical powder to separate oxygen from the fuel, eventually interrupting combustion.² However, the temperature of fuel is sufficient to reach a fire ignition zone. Therefore, even a spark can start the fire again, which is called reignition.³ Therefore, dry chemical-based extinguishers need to be improved to address this issue. Herein, the objective of studying the fire-extinguishing performance of the CAP were compared with those of commercial mono-ammonium phosphate powder (CMAP) for extinguishing Class A and B fires. Two biowastes, namely Baby Clam’s shells from a food processing industry and rice husks from agricultural industry, were used to prepare an extinguishing mixture where one of the constituents was ADP. The materials were converted to nanocalcium silicates (nCa₂SiO₄), a low-density, being stable material providing good fluidity for dry chemical fire extinguishing agents that can be stored for a long period of time. It is then mixed with ADP to improve the fire performance of ADP, which were then mixed with the ADP to improve the fire-extinguishing performance of ADP. As nCa₂SiO₄ is odourless, non-toxic and has a low thermal conductivity with high resistance to heat,^4,5 it is also environment friendly as compared to the conventional dry chemical.⁶ Moreover, nCa₂SiO₄ and silicon dioxide (SiO₂) scatter as residues during fire extinguishing and served as nutrients for plant growth.⁷ Since nCa₂SiO₄ is synthesised from biowastes, the production cost is relatively low. A large amount of shells, approximately over 7 million tonnes, is obtained from food processing industries every year. These shells are discarded in landfills or into the ocean.⁸ The annual rice production yields a large amount of rice husks of approximately 482 million metric tonnes.⁹

Experimental

Materials

Hydrochloric acid (HCl, 35%) was purchased from Fisher Chemical™, sodium hydroxide (NaOH) was purchased from KemAus™ and ADP was purchased from welfare, China.

Preparation of nanocalcium silicates (nCa₂SiO₄)

Calcium oxide (CaO) preparation from clam shells

Clam shells (10 kg) were washed, dried, ground using a shell crusher and sieved through a 60 mesh. The shells were then calcined at 800°C for 2 h.

Silica preparation from rice husks

Rice husks (100 g) were washed using distilled water, dried at 105°C for 24 h, refluxed in 1 M HCl at 70°C for 2 h and rinsed with distilled water of pH 7. The husks were then dried at 105°C and burnt at 650°C for 6 h to produce 50 g of silica. The obtained silica was placed in a beaker and 500 mL of 1 M NaOH was added. The solution was heated and stirred for 1 h to precipitate. Finally, the residue was filtered to obtain sodium silicate solution.

Calcium silicate preparation by hydrothermal method

A volume of 80 mL of sodium silicate solution was pipetted out into a 250 mL beaker, and 80 mL of a CaO solution was allowed to drip into the beaker. The solution stirred for homogeneous mixing. The homogeneous solution was heated using the hydrothermal method at 120°C for 3 h, and the obtained material was calcined at 800°C for 2 h to yield calcium silicates (Ca2SiO4). The compound was analysed by Fourier transform infrared (FTIR) spectroscopy and the crystalline material of nCa₂SiO₄ were analysed using X-ray diffraction analysis (XRD).

Preparation of nanocalcium silicates (nCa₂SiO₄)

Ca₂SiO₄ (4 g) was ground to a fine powder using a ball mill, sieved through a 200 mesh to form nCa₂SiO₄, which was analysed using transmission electron microscopy (TEM).

Preparation of fire-extinguishing mixture (nCa₂SiO₄ and ADP)

The mixture 20% of nCa₂SiO₄% and 80% of ADP (referred to as CAP) was added to a 15 lb (6.80 kg, EN 615:2009)¹⁰ fire extinguisher. Nitrogen was used as a propellant at a pressure of 175 psi (the maximum pressure). The CMAP was added to another fire extinguisher of the same size under the same propellant pressure conditions.

Comparison of the fire-extinguishing performance

The parameters of the fire-extinguishing performance of the CAP were compared with those of CMAP for extinguishing Class A and B fires. The parameters studied were the extinguishing time, amount of the extinguishing agent used, the fire temperature reduction rate, powder coating on the fuel and a reburn incident. All parameters were tested thrice using the analysis of variance (ANOVA).

Experimental design for extinguishing Class A and B fires

For designing the experiment to extinguish the fire from wooden crib fuels (Class A: 13A fire rating of BS EN 3 standard),¹¹ eight layers of wood timber cribs (sizing 450 mm × 450 mm × 850 mm), a 30-cm metal tong, an oil pan (sizing 20 cm × 20 cm × 6 cm), and 400 mL of n-heptane fuel were used. For each test, wooden cribs were allowed to burn for an additional 5 min after the complete burning of the fuel. A 15 lb (6.80 kg) portable fire extinguisher with nitrogen propellant at 175 psi was used for the test. The distance from the nozzle to the centre of the wooden crib was fixed at 100 cm. The fireman was placed in an upwind position to prevent a returning flame. The fire extinguisher was used to spray the dry chemicals in front of the wooden crib until the flame was extinguished. The change in temperature of the wooden crib was monitored by a type K thermocouple. Heat radiation was captured by infrared cameras at 1, 2, 3, 4 and 5 m as shown in Figure 1.

For designing the experiment to extinguish the fire from oil fuels (Class B: 55B fire rating of BS EN 3 standard),¹¹ an oil pan (sizing 1525 mm × 1525 mm × 600 mm), and 120 mL of n-heptane as the fuel were used. For each test, the fire was allowed to burn until the flame temperature was stable (approximately at the 60th second). Four thermocouples were placed 5 cm above the fuel to monitor the fire temperature changes at the tray as shown in Figure 1.

Extinguishing mechanism of CAP

When the ADP reacts with fire, it produces a large amount of nitrogen, an inert gas in the fire zone. The ammonium produced disrupts the oxygen from participating in the fuel combustion reaction. Besides, it stops oxidation reactions (which allows the fire to continue), thereby reducing the concentration of free radicals in the fire.^10,12–14 It also helps to reduce the heat released by the chemical reaction.^15,16 Free radicals produced disrupt the chain of combustion reactions.^12,17,18 At the same time, ammonia and water vapour produced separate oxygen from the fuel vapours. Phosphoric acid (H₃PO₄) covers the surface of the combustible fuel to prevent further combustion. The extinguishing mechanism is depicted in Reactions 1 to 4¹⁹

N H_{4} H_{2} P O_{4} \to^{Δ} N H_{3} + H_{3} P O_{4}

(1)

2 H_{3} P O_{4} \to^{Δ} H_{2} O + H_{4} P_{2} O_{7}

(2)

H_{4} P_{2} O_{7} + OH \to^{Δ} H_{2} O + 2 HP O_{3}

(3)

2 HP O_{3} + \to^{Δ} P_{2} O_{5} + H_{2} O

(4)

The free radicals of H·, OH·and CH₃· are majorly responsible for sustainable combustion. The fire-extinguishing component, NH₄H₂PO₄, captures and terminates a large number of active radicals in the flame according to the aforementioned reactions when ADP is added to the flame. The free radical species are quickly consumed and the flame extinguishes. Combining the results of the previous study, Figure 2 schematically demonstrates the possible fire suppression mechanism of ADP.¹ It can be theoretically demonstrated from the kinetic and thermodynamic analysis of all the reaction pathways that the chemical inhibition of ADP plays a leading role in extinguishing the fire.

The role of nCa₂SiO₄ is to coat and protect the fuel. The physical and chemical reactions on heating of fire are shown in Reaction 5^10,11

nC a_{2} Si O_{4} \to 2 CaO + Si O_{2} = Δ H = - 104.4 kJ / mol

(5)

The disintegration of nCa₂SiO₄ results in the formation of CaO and SiO₂. CaO is a powdery substance that coats and sticks to the fuel surface. It is highly heat resistant and can withstand temperatures up to 2572°C. It absorbs heat (104.4 kJ/mol) to initiate the chemical reaction. SiO₂ is highly heat resistant and covers the fuel surface withstanding the temperature up to 2200°C.²⁰ It reduces the formation of fuel vapours that lowers the intensity of the fire. In addition, it prevents a reburn incident.

Results and discussion

Characterisation of nCa₂SiO₄

nCa₂SiO₄ prepared from clam shell and rice husk is brownish-grey in colour with a particle size of 50 nm, as shown in Figure 3. The FTIR spectrum shows the appearance of the O–Si–O bond at 526/cm (Figure 4). The peak at 870/cm indicates the vibration of Ca–O–C bonds, while the peaks for Si–O are observed at 690 and 1190/cm. The peak at 1430/cm indicates the vibration of the C–O bond in –CO₃.^21,22

XRD analysis of nCa₂SiO₄

The analysis of chemical compounds in nCa₂SiO₄ by XRD indicates the presence of Ca₂SiO₄ and other chemical compounds, as shown in Figure 5; the intense peaks centred around 2θ, which is the dominant peak in nCa₂SiO₄ at 32.0°.

Wooden crib fire tests (Class A)

The Class A fire-extinguishing performance test was conducted using wood cribs composed of 120 pieces, arranged in 15 layers with eight pieces per layer. The performance test followed a fire-extinguishing method of the Fire Rating 4A according to Standard EN 3 (British Standard, 2004).¹ The test results are shown in Table 1.

Table 1.

Experimental results of the wood crib fire test.

Fire model	Type	Quantity utilised (g)	Extinguishing time (s)	Characteristics of wood coating	Reburn
Wooden crib fire	CAP	4878.3^a (77.43%)	10.67	Very glazed	No
Wooden crib fire	CMAP	5706.30^a (90.57%)	11.00	Slightly glazed	No

CAP: mixture of 20% nCa2SiO4 and 80% ABC dry powder; CMAP: commercial mono-ammonium phosphate powder.

The values are significantly different (p < 0.05).

Table 1 shows the fire-extinguishing performance for Class A fire using two types of extinguishing agents: CMAP and CAP. It requires 11 s to extinguish the fire by CMAP when 0 s was the starting point of spray. The flame was completely extinguished and did not reignite, and the result is shown in Figure 6(a). The average utilisation of CMAP in the fire extinguisher tank was 5706.30 g from the total of 6300 g (15 lb) with 90.57% consumption. It required 10.67 s to extinguish the fire by CAP. It required 10.67 s to extinguish the fire by CAP. Both CMAP and CAP extinguishing time were not different. The flame was completely extinguished without reigniting, and the result is as shown in Figure 5(b). The average usage of CAP in the fire extinguisher tank was 4878.3 g from the total substance of 6300 g (15 lb) with 77.43% consumption.

Physical characteristics of the wooden crib s after the tests

The coating of wood with fire-extinguishing agents after the test is shown in Figure 6. A thin layer coating of CMAP residue was found on the wooden crib (Figure 6(a)). However, some area was not well-coated and did not have sufficient heat resistance to the proximity flame. Thus, it caused a high mass flux from the surface ( $\overset{\cdot}{m}$ ). Phosphoric acid, SiO₂ and calcium oxide in ADP acts as a coating on the fuel surface and is also acting as an insulator to prevent oxidation process to decrease the chain reaction of fire. The coating of CAP was also formed during the fire on the wooden crib (Figure 6(b)). The wood surface attained a lower temperature after converting to CaO and SiO₂ as both are heat-resistant substances. It caused a low mass flux from the surface ( $\overset{\cdot}{m}$ ) making the fire difficult to spread. The wood was barely damaged. We found that the coating of CAP on the wooden crib did not burn as fuel.

Figures 7 and 8 show 13A fire-extinguishing tests. The fire was ignited in the fuel pan below the wooden crib. The fire was allowed to burn until the fuel was completely burnt. The flame was allowed to burn for an additional 1 min before conducting the fire extinguisher tests. This is considered the starting point of the fire-extinguishing test (0 s). The distance between the wooden cribs ranged at about 1 m and the injection was directed from the base of the fire to the centre of the fire, as shown in Figure 8.

Figure 9 shows that the wooden crib temperature dropped after 1 min of starting the fire. The temperature was measured using Type K thermocouples at the lower, middle and top positions of the wooden crib. At 0 s, CMAP was added to the testing fire having a temperature of 1490°C. The temperature profile is as shown here: 1 s = 900°C, 2 s = 500°C, 4 s = 300°C, 5 s = 100°C, 6 s = 80°C, 7 s = 60°C, 8 s = 50°C (the fire was reignited), 9 s = 40°C, 10 s = 39°C and the fire died with no reignition. By the end of the 11th second, the temperature of the wooden crib was just 27°C. Figure 10 shows the addition of CMAP to the fire at the wooden cribs. The fire in the wooden crib began to extinguish at the eighth second. The flame completely died at the 11th second.

Figure 11 shows the fire-extinguishing test using CAP for a Class A fire. The temperature profile is as follows: 0 s = 1500°C; 1 s = 700°C, 2 s = 500°C, 3 s = 300°C; 4 s = 70°C, 5 s = 30°C; 6 s = some part of the fire was extinguished and the temperature reached 30°C and 7 s = the fire was completely extinguished with the temperature at 27°C.

Figure 12 shows the time during the seventh to eighth second interval when the temperature is between 27°C and 29°C and the flame is extinguished. By the ninth second, the fire at the wooden crib is completely extinguished with the temperature dropping to 27°C. Some layers of the wood are not damaged by the flames because CAP forms a coating on the wood surface. The disintegration rate from solid (wood) to gas (fuel vapour) for this coated wood is lower as compared to the normal burning wood.

Pool oil fire test

The fire-extinguishing performance of Class B fire (oil and gas) by CAP and CMAP is shown in Table 2. The flame from the oil pan after 1 min of burning was completely extinguished within an average of 9 s by CAP. CMAP required an average of 11.33 s to put off the fire under the same conditions. CAP was more effective and 2.33 s faster than CMAP. Table 2 displays that the average CMAP used in fire extinguishing is 4613.5 g from the total substance of 6300 g (15 lb) with 73.20% consumption. CAP used is 2184.70 g from the total substance of 6300 g with 34.7% consumption. It explains that CAP used was 38.5% less than CMAP.

Table 2.

Experimental results of the pool oil fire test.

Fire model	Type	Quantity (g)	Extinguishing time (s)	Coating presence	Reburn
Pool oil fire	CAP	2184.70^a (34.70%)	9^a	Very glazed	No
Pool oil fire	CMAP	4613.5^a (73.2%)	11.33^a	Slightly glazed	No

CAP: mixture of 20% nCa2SiO4 and 80% ABC dry powder; CMAP: commercial mono-ammonium phosphate powder.

The values are significantly different (p < 0.05).

Figure 13 shows the presence of oil–water coating in the fuel pan. CMAP contained a light coating on the surface of the fuel pan after the fire was extinguished. The fuel temperature of the CMAP test was 42°C, which was higher than that of CAP. There was a white-top coating of CAP on the surface of the fuel pan that played a significant role in reducing the flame temperature of the surface. The temperature at the fuel surface extinguished by CAP was 26°C.

Figure 14 shows the temperature curve after 1 min of fire burning in the fuel pan. The temperature was measured using a Type K thermocouple at three positions: the front, centre and back of the tray (Figure 1). The flame temperature in the test pan was 1600°C. CMAP was added at 0 s. The temperature profile is as mentioned here: 1 s = 800°C, 2 s = 400°C, 3 s = 100°C, 4 s = 100°C, 5 s = 50°C, 6 s = 40°C, 7 s = 35°C, 8 s = 30°C and the temperature was between 27°C and 29°C during the ninth to tenth second.

Figure 15 shows the test result of CAP for a Class B fire (oil fuel). The initial flame temperature of the test pan was 1600°C. The temperature decreases in the following manner: 1 s = the flame temperature of the fuel pan rapidly reduced to 100°C, 2 s = 70°C, 3 s = 54°C, 4 s = 42°C, 5 s = 33°C, 6 s = the flame was partially extinguished at 29°C and 7 s = the fire completely extinguished at 27°C.

Figure 16 shows the fire test in the fuel pan. The flame immediately increased its height when the fire extinguisher was used. This is because the gas from the extinguisher forced the sudden combustion with the surrounding air in that split moment.²³ The flame base was cut off from the fuel in the pan while using the fire extinguisher. This extinguished the fire on time. The injected powder built an isolated layer above the fuel surface to cut off the air so the fire could no longer complete its burning reaction.

nCa₂SiO₄ has flame-retardant capacity and forms an environment friendly fuel coating. The flame-retardant mechanism is due to the anti-flammability of the silicon compounds present in nCa₂SiO₄. The ADP is considered as a preventive agent of the ‘free radicals’ (formed in the combustion process, e.g., OH^–, H⁺, CO, CO₂, C₂H₄, C₂H₆) and also terminates the generation of new ‘free radicals’ breaking the chain reaction. It has been asserted that in some instances the surface area of the chemical particles allows the free radicals to link up and carry out reaction sequences. The substance can extinguish a fire by eliminating one element from the fire triangle (The fire triangle is an easily understandable model that explains the ingredients required for combustion). nCa₂SiO₄ can also reduce heat. nCa₂SiO₄ on heating deforms to a ceramic that coats the fuel, thereby preventing the heat from reaching the fuel. Furthermore, its high viscosity stabilises the flow of substances inside the tank.²⁴ nCa₂SiO₄ can coat all types of fuel surfaces; therefore, it serves as a shield to protect the heat flux derived from the heat radiation of the flame in the vicinity, which disturbs the adjacent fuel surface (chain propagation and consecutive reactions).^23,24 According to Hshieh,²⁵ nCa₂SiO₄ covering the fuel surface directly controls the fire reaction by interrupting the fire triangle. Furthermore, it can disrupt the air (oxygen) supply and reduce the combustion of a continuous fire. It reduces the probability of reaching the lower explosive limit (LEL: is defined as the lowest concentration (by percentage) of a gas or vapour in air that is capable of producing a flash of fire in presence of an ignition source).²⁵ In addition, nCa₂SiO₄ has a high heat resistance and shields the decomposition reaction of fuel vapour. The fuel emission (pyrolysis) of nCa₂SiO₄ creates a protective layer on the surface of the burning fuel. Furthermore, nCa₂SiO₄ particles are diluted and reduce thermal reflection from the ash of the combustion fuel. It also forms a physical structure such as a ceramic layer to coat the fuel.²⁶

nCa₂SiO₄ as a fire-extinguishing agent can absorb heat from the fuel. The silicate in nCa₂SiO₄ absorbs heat during hydration, thereby lowering the surrounding temperature to form hydrate silicate.²⁷ It acts as an inhibitor of fire products (portable extinguisher, fire ball, automatic dry chemical fire extinguisher systems and so on) formed during the combustion reactions. The substance is made of fine granules and can be mixed with ADP available in the market. The performance of the compound mixture can be enhanced when used in a large quantity. The compound agent has to be injected at the base of the fire to increase its performance. Furthermore, nCa₂SiO₄ can also be used in other applications such as a composite for building materials to prevent fire risks and for the manufacturing of heat-resistant and refractory materials in buildings, ships, tanks, trucks, containers or aircrafts, as it makes materials harder to ignite.

Economic efficiency

Regarding economic efficiency to ADP, the production cost per cylinder is US$11.28 per tank, whereas the production cost of CMAP per tank is US$13.85 per tank. ADP has a lower production cost than CMAP as equal to US$2.77 per tank. It shows that the cost of calcium silicate production is not very high as the material is obtained from bio waste. A large amount of shells, approximately over 7 million tonnes, is obtained from food processing industries every year. These shells are discarded in landfills or into the ocean. The annual rice production yields a large amount of rice husks of approximately 482 million metric tonnes.

Conclusion

Herein, we have synthesised nCa₂SiO₄ from clam shells and rice husks and utilised it as a fire-extinguishing material. The study compared the physical properties and fire-extinguishing performance of CAP with CMAP. The heat reduction and coating properties of CAP have also been explored. The following conclusions could be drawn: (1) The fire-extinguishing performance of CAP for Class B fires is better than that of CMAP due to phosphoric acid calcium oxide. (2) Silica is able to coat the surface of the fuel faster, causing flame knock-down or as an insulator to prevent oxidation of the fuel surface better than a fire from a pile of wood, which yield more fuel surface. (3) CAP is highly efficient with a better disintegration capability as it is flame resistant and flame retardant (phosphoric acid, calcium oxide and silica oxide). (4) CAP easily reaches the burning surface and serves as a good coating material. It coats the fuel surface (wood and charcoal) forming a heat protective layer similar to that of ceramic. (5) CAP thermally decomposes into other compounds. (6) Both silica and calcium present in CAP participate in extinguishing the fire by lowering the temperature at the burning surface and diluting the oxygen during combustion. (7) Calcium present in nCa₂SiO₄ breaks the combustion chain reaction and reduces the fuel vapour emission from type A and B fuel, thereby rapidly stopping the fire reaction. This study also demonstrated the efficient recycling of wastes from industrial processing to prepare a dry chemical powder extinguishing agent that improved the fire-extinguishing performance, reduced the temperature reduction rate and decreased the material consumption.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Science and Technology Development Agency (NSTDA) of Thailand (grant number 21004419). The authors expressed their sincere gratitude to the organisation.

ORCID iD

Nuttabodee Viriyawattana

Author biographies

Nuttubodee Viriyawattana was born in 1975. She received a degree in biology in 1998, a degree in occupational health and safety in 2015, a master's in biopharmaceutical science in 2002 and a PhD in Medical Microbiology in 2011. She is currently a lecturer at Suan Dusit University in Thailand. Her current research interests include occupational health and safety, environmental science, chemical industry and materials technology.

Surachat Sinworn was born in 1976. He received a degree in environment science in 2000, a master's in safety engineering in 2004 and a PhD in sustainable land use and natural resource management in 2014. He is currently a lecturer at Suan Dusit University in Thailand. His current research interests include occupational health and safety, environmental technology, environmental science, land use and natural resource management, agricultural technology, agricultural product and fire engineering.

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Performance improvement of the dry chemical-based fire extinguishers using nanocalcium silicate synthesised from biowaste

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of nanocalcium silicates (nCa2SiO4)

Calcium oxide (CaO) preparation from clam shells

Silica preparation from rice husks

Calcium silicate preparation by hydrothermal method

Preparation of nanocalcium silicates (nCa2SiO4)

Preparation of fire-extinguishing mixture (nCa2SiO4 and ADP)

Comparison of the fire-extinguishing performance

Experimental design for extinguishing Class A and B fires

Extinguishing mechanism of CAP

Results and discussion

Characterisation of nCa2SiO4

XRD analysis of nCa2SiO4

Wooden crib fire tests (Class A)

Physical characteristics of the wooden crib s after the tests

Pool oil fire test

Economic efficiency

Conclusion

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

Author biographies

References

Preparation of nanocalcium silicates (nCa₂SiO₄)

Preparation of nanocalcium silicates (nCa₂SiO₄)

Preparation of fire-extinguishing mixture (nCa₂SiO₄ and ADP)

Characterisation of nCa₂SiO₄

XRD analysis of nCa₂SiO₄