Insight into suppression performance and mechanisms of ultrafine powders on wood dust deflagration under equivalent concentration

Highlights

• Influence of wood dust concentration on deflagration parameters was investigated.

• Suppression performance of Zr(OH)₄ on dust deflagration was better than that of SiO₂.

• Zr(OH)₄ catalyzes the formation of more residuals with smaller crystallite size.

• Suppression mechanisms of wood dust deflagration by ultrafine powder were studied.

Abstract

Flame propagation characteristics of wood dust deflagration and suppression mechanism of ultrafine powders are investigated systematically. The deflagration reaction intensity of wood dust increases firstly and then decreases with the increase in dust cloud concentration. This is due to factors such as oxygen supply, positive feedback among flame characteristic parameters. Thus, there is an equivalent dust concentration for greatest deflagration intensity. Nano-sized ultrafine zirconium hydroxide (Zr(OH)₄) and silicon dioxide (SiO₂) powder are introduced to suppress wood dust deflagration at the equivalent concentration. It is found that Zr(OH)₄ has a suppression effect of endothermic decomposition to generate zirconia (ZrO₂), dilution of oxygen and absorption of free radicals; while SiO₂ exerts suppression effect due to its high melting point and heat absorption. The suppression performance of Zr(OH)₄ is better than that of SiO₂. This is because that Zr(OH)₄ and ZrO₂ have a catalytic carbonization effect. It can not only improve thermal stability of wood particles by catalyzing production of high-temperature resistant residuals, but also promote the formation of catalytic sites to reduce crystallite size of carbon layer on wood particles surface, weakening heat and mass transfer between particles.

Introduction

Wood powder is the main by-product in wood processing and major source of biomass fuel. Pneumatic dust extraction and transportation system are inevitably applied in the handling and using processes of wood powder (Masche et al., 2018). This type of pneumatic technology can not only ensure occupational health of workplace, but also improve production efficiency (Mračková et al., 2016). However, wood particles are easily dispersed by airflow to form dust cloud in a limited space. When concentration is within explosion limit, a slight external energy may lead to an extremely serious dust explosion accident. On January 31, 2015, the initial explosion of wood particles accumulated in dust extraction system of China Inner Mongolia Xing'an Wood-based Panel Co., Ltd. caused a second dust explosion in production workshop, causing heavy deaths and injuries.

As a first-class hazardous dust, wood powder has low ignition energy and high explosion pressure rise rate (Callé et al., 2005). P. Holbrow (Holbrow, 2013) studied explosive overpressure characteristics of wood dust in a container with volume less than 0.5 m³, and the experimental value of explosion overpressure was found to be lower than the predicted value in BS EN 14491:2006. Huéscar Medina et al. (2015a,b) found that explosion pressure index (K_st) and flame velocity of Norwegian spruce dust are larger than those of Kellingley coal dust, and turbulent flame velocity during explosion is linear with K_st, indicating that flame velocity can be used to reflect reaction intensity and K_st. G. Hehar et al. (Hehar et al., 2014) studied ignition hazardous of pine dust with different particle sizes. The minimum ignition temperature of this powder on hot surface is in the range of 292.5–302.5 °C, and increases with the increment in particle size. When small size dust particle is ignited in the air, the exothermic value is higher than 5113 kJ/kg, and the released heat decreases as increasing of particle size. M.C. Lee et al. (Lee et al., 2016) investigates that smaller size and non-spherical of wood chipboard particles have higher explosion pressure and pressure rise rate. At present, the research on wood dust explosion mainly focused on the macroscopic fields such as explosion pressure, minimum ignition energy (MIE) and minimum ignition temperature (MIT), while the research on deflagration flame of wood dust is relatively limited.

Powder inerting refers to a method of premixing inhibitory powders into combustible powders, leading to effectively reduce the risk of dust ignition and explosion (Amyotte, 2006; Amyotte et al., 2007). Many scholars have conducted research on powder inerting application in terms of ignition hazard, flame propagation characteristics and explosion overpressure, which may provide valuable research information for this investigation. The MIE and MIT of dust mixture containing inhibitory powder are higher than that of pure combustible dust, and its ignition sensitivity is lowered (Addai et al., 2016). Yuan et al. (Yuan et al., 2014a,b) found that MIT of micro Ti powder increases with increase in content of nano TiO₂ powder which is premixed into micro Ti powder, but the MIE of dust mixture increases significantly until the percentage of nano TiO₂ exceeds 50 % (Yuan et al., 2014a,2014b). Jiang et al. (Jiang et al., 2019a,b, 2018) found that minimum inert concentration (MIC) of inhibitory powder increases with the increment in aluminum dust concentration or decrease in particle size, and MIC also decreases significantly with the reduction in particle size of inhibitory powders such as NaHCO₃, ABC powder with NH₄H₂PO₄ as main composition. The crystalline II type ammonium polyphosphate (APP-II) with a mass fraction of 80 wt% can completely inert the micron-sized acrylate copolymer (ACR) powder, while 30 wt% and 40 wt% of APP-II can significantly increase the MIE of ACR powder (Yu et al., 2018). NH₄H₂PO₄ powder has a significant decrease in flame temperature of aluminum dust, and flame morphology becomes irregular and discrete (Jiang et al., 2019a,b). NaHCO₃ powder can reduce preheating zone thickness of aluminum dust flame, causing the change in flame structure, and the suppression effect on combustion reaction zone of flame is key factor for mitigating dust explosion (Chen et al., 2017). Sodium chloride particles can reduce flame velocity by melting and absorbing of heat, and blocking the contact between aluminum powder and air, the suppression effect is better than quartz sand and silicon carbide (Xu and Yong, 2018). Inhibitory powder has a significant suppression effect on explosion overpressure and propagation velocity of coal dust (Liu et al., 2013). When the proportion of rock dust is 90 % of coal dust, explosion overpressure, peak flame temperature and velocity of coal dust-rock dust-methane mixture are 60 %, 78 % and 55 %, respectively, of coal dust-methane (Song et al., 2018). Carbamide and fly ash cenosphere (FAC) play a synergistic role in inerting of pulverized coal explosion. Carbamide and FAC have different suppression periods, which independently reduce maximum pressure rise rate and maximum explosion pressure of coal dust (Wang et al., 2019). The incorporation of 50 wt% of 25 μm NH₄H₂PO₄ can reduce the maximum explosion pressure of corn dust to less than 2 bar, and the K_st value also decreases significantly with decrease in inhibitory powder particle size or increase in concentration (Castellanos et al., 2014).

Through investigation of the existing literature, it is found that powder inerting technology has an effective suppression effect on dust ignition and explosion, but the method of adding inhibitory powder to combustible dust may cause pollution of pristine powder, which limits the application of inerting method in industry (Myers, 2008). However, as a kind of waste generated from grinding operation, wood powder mixed with inhibitory material is not detrimental, thus it has higher practical potential. It is found that some materials have excellent effect of catalysis and flame retardant, which can significantly reduce fire, smoke and toxic hazards (Yu et al., 2019; Lin et al., 2020). In particular, some metal oxides can improve thermal stability and amount of residue of composites under high temperature condition. For example, Fe₂O₃ promotes the dehydrochlorination of chlorinated polyvinyl chloride to polyenes at 700 °C. This "catalytic carbonization" effect can effectively synthesize carbon microspheres (Gong et al., 2013). Co₃O₄ can be uniformly distributed in the mixed plastics to form a network structure, which provides a precondition for the carbonization of mixed plastics to form a spherical core/shell structure with uniform size (Gong et al., 2014a,b). During combustion process, Ni₂O₃ promotes pyrolysis products form a large amount of coke in situ, which not only reduces the release of combustible gases, but also prevents oxygen and heat conduction (Gong et al., 2014a,b). Organic modified zirconium phosphate can reduce the heat release rate and total heat release rate of composite, and increase residue after combustion, indicating that it may have the mechanism of intumescent flame retardant and catalytic carbonization. Dust deflagration and polymer combustion have similar mechanisms, and we infer that this type of metal oxides may also have well suppression performance on wood dust deflagration.

Previous studies have shown that suppression efficiency of inhibitory powder tends to improve with the decrease in particle size, and ultrafine powder represented by nano-sized powder has unique physical and chemical properties, which will become an important exploration direction in high-efficiency suppression technology. Therefore, this investigation is based on propagation behavior of wood dust deflagration flame with different concentrations in duct. Through the analysis on flame parameters, it verifies the equivalent concentration of wood dust deflagration under experimental conditions. On this basis, this work further explores the different suppression effects of nano-sized powders on deflagration flame. Through the investigation of deflagration flame parameters and residuals, different suppression mechanisms of Zr(OH)₄ and SiO₂ are analyzed.