Experimental Investigation Into The Explosibility Of Hybrid Mixtures Of Flammable Gas And Dust

Hybrid mixtures become commonly accompanied with the rapid development of industry.Accidents generated by hybrid mixtures have been verified in literatures.However,the explosibility of hybrid mixtures have not be presented systematically by the existing researches.Furthermore,most of the existing investigations focused on the effects of dust explosion parameters at low gas concentration.However,studies on hybrid mixtures at high gas concentration are still less.In present paper,experiments are performed to systematically study the effect of combustible gas on dust explosion characteristics of methane/lycopodium,methane/ polyethylene,ethylene/lycopodium and ethylene/polyethylene mixtures in a standard 20 L spherical dust explosion vessel.Premixed gas-air mixtures were ejected to explosion chamber along with the dust.The hybrid mixtures were ignited under high turbulence conditions.The main conclusions are listed as follows:（1）The explosion characteristics of pure fuels were studied in the standard 20 L spherical vessel.The results show that the lower flammability limits of methane and ethylene are 4.9% and 2.7%,respectively,which are close to the results in the literatures which are measured under macroscopic static conditions.The maximum explosion pressures of methane and ethylene are 0.73 MPa and 0.84 MPa,respectively.The explosion indexes of methane and ethylene are 42.27 MPa·m·s^-1 and 75.98 MPa·m·s^-1 respectively,which are significantly higher than the values measured under macroscopic static conditions.It can be concluded that the lower flammability limits and the maximum explosion pressure of methane and ethylene are not affected by the initial turbulence.But the explosion indexes of methane and ethylene are affected by the initial turbulence significantly.The minimum explosion concentrations of 38.7 μm lycopodium dust and 19.2 μm polyethylene dust are 180 g?m-3 and 80 g?m-3,indicating that the LDPE dust is more sensitivity than lycopodium dust.The explosive intensity of two dust are all belong to the same level of St1.（2）The minimum explosion concentration of hybrid mixtures was studied.The results show that the minimum explosion concentration of the hybrid mixtures decreases obviously with the increase of the gas concentration.The mixtures of methane（or ethylene）with a concentration lower than its lower flammable limits and the lycopodium（or polyethylene dust）with a concentration of less than its minimum explosion concentration are still explosible.And the increasing explosion rate of relative theory of non-explosive area of methane（or ethylene）/dust is more than 50 %,indicating that the risk increases significantly.However,the increase explosion rate induced by ethylene is higher than that taken by methane,indicating that the influence of ethylene on the dust MEC0 is more significantly.That is to say hybrid mixtures containing ethylene is more dangerous than those containing methane.（3）The experimental results and the calculated results according to Le Chatelier’s law,Bartknecht curve and Jiang method were compared.The results show that the results predicted by Le Chatelier’s Law and Jiang method are not consisted with the measured values.Though the predicted results by Bartknecht curve are similar to the measured values,devations still exist between them,indicating that a margin should be set when using Bartknecht curve.（4）The change rule of the explosion severity of hybrid mixtures was studied systemly.The results show that the maximum explosion pressure P_max and the explosion index of the hybrid mixtures increase with the increase of the flammable gas concentration.The maximum explosion pressure P_max and the explosion index of the pure fuels and hybrid mixtures satisfy the following relationship: pure combustible gas> hybrid mixtures > pure dust.The maximum explosion indexes of the four different hybrid mixtures are all higher than 35 MPa·m·s^-1,which belong to the St3 grade.That is to say the addition of flammable gas to a dust cloud increase the explosion severity of the dust significantly,turning its severity classification from St1 to St3.（5）It is found that the flammable gas increase the dust explosion risk and sensitivity significantly at gas concentrations above the lower flammable limit.When the concentration of methane is y=2¹0% and the concentration of ethylene y≥3%,the optimum concentration of the dust decreases significantly with the increase of combustible gas concentration.While the effect of ethylene with concentrations lower than 2.7%（LFL0 of ethylene）on the optimum explosion concentration of polyethylene and lycopodium dust was not significant.