Investigation Of Mechanisms In Plasma-assisted Combustion And Ignition Of Dispersed Coal Particle Streams

The plasma-assisted ignition(PAI)technology has been widely used in coal-fired power plants for saving oil and minimizing pollution emission in the past few years.However,the ignition efficiency for meagre coal is not so satisfactory.The short cathode life is also an existing serious problem which limits its industrial application.Therefore,it is still necessary to further explore the mechanisms in plasma-assisted ignition.In the paper,the thermal and chemical effects in plasma-assisted coal streams ignition are preliminarily characterized.Then,the influence of plasma,on particle surface temperature,is investigated.In the end,the relationship between volatile content and PAI efficiency is also studied by a series of las-scale experiments.By applying palsma generation device on traditional Hemcken flat burner,the coal particle stream is ignited under different plasma energy.When the applied voltage is less than 3 k V,implying 900 breakdown frequency and a corona discharge phenomenon,the chemical effect plays a key role in reduction of ignition delay time becaouse of lower input plasma energy.As the applied voltage reaches at 20 k V,in which the thermal effect is prevalent,the ignition delay time further decreases about 1.2ms.The plasma transits to glow discharge with 16000 breakdown frequency.Meanwhile,the plasma-induced reduction as a function of Oxygen mole fraction is studied and it is verified that the chemical effect on ignition enhancement is more obvious under high Oxygen mole fraction condition.Then,the PAI efficiency of several typical coals(Hulunbel lignite,Xinjiang Zhundong lignite,Shenhua bituminous,and char)is explored under 600 K condtion.The ignition signal curves of both lignite and bituminous present double peak due to volatile combustion,on the contrast,the ignition signal curve of char,whose volatile content is less than 5%,has just single peak after discharge position and exhibits a decreasing trend.The plasma input energy to calorific value of lignite coal is lower than 1:6.0,while the ratio for char is approximately 1:1.2,indicating only part of char particles are ignited.Finally,we discuss the plasma effect on devolatilization process and particle surface temperature.The ignition signal is captured by ICCD camera after passing a 430 nm front-set filter.The result shows that CH* signal intensity appears to rise at 12 ms and reaches its maximum at 22 ms.Compared with non-plsama ignition experiment,the signal intensity is obviously stronger which can be explained by plasma-assisted devolatilization acceleration.Under 1200 K condition,the ignition mode transits from a heterogeneous ignition modeto hetero-homogeneous ignition mode due to the plasma discharge.Particle surface temperature is measured by RGB color pyrometry.The surface temperature increases from 900 K directly to about 2200 K,which shortens the ignition residence time to stable combustion phase for nearly 10ms(from 27 ms to 17ms).The incretion of surface temperature is an important factor,which accounts for apparent ignintion delay time reduction in plasma-assisted ignition.