Experimental Study On Combustion Behavior Of Bio-oil

Bio-oil serves as a liquid renewable fuel,considered as one of the potential alternatives to fossil fuels.However,the chemical composition and physiochemical properties of bio-oil are quite different from those of traditional fossil fuels.Bio-oil also has a unique combustion characteristic.At the same time,there are few studies on bio-oil combustion characteristic,hindering the development of bio-oil industrial application to fuel.Based on this background,the combustion characteristic of bio-oil atomization is investigated by this dissertation.In this investigation,the chemical composition and physiochemical properties of bio-oil were studied widely.Moisture,higher heating value(HHV),chemical components and elements were measured and analyzed.Bio-oil molecular weight distribution was also measured.Bio-oil flowability parameters,such as surface tension,kinematic viscosity,dynamic viscosity,were measured using rheometer and other instruments.Bio-oil thermal properties,such as specific heat capacity and thermal conductivity,were measured using thermal analyzer.These results showed that bio-oil featured an aqueous fuel including enormous oxygen-containing organic compounds,resulting in high oxygen content and low HHV of it.Bio-oil moisture was approximately 13.86%,as well as HHV was 23.25 MJ/kg.The species proportion of organic components with molecular weight less than 400 was 59.10%,whereas the mass proportion of these was merely 11.16%.Bio-oil average molecular weight was 3063.09,and quantity average molecular weight was 736.16.Bio-oil exhibited the non-Newtonian fluid property at 30℃,whereas the Newtonian fluid property was exhibited at temperature higher than 40℃.Bio-oil existed in the form of turbid liquid(suspension or emulsion).Bio-oil molecular gravity was very unstable,leading to the non-Newtonian fluid property at 30℃.As temperature increased,the intermolecular spacing increased,causing some polymer bond breakage and macromolecular substance reduction.Thus,bio-oil presented the Newtonian fluid property.Bio-oil dynamic viscosity-temperature relationship satisfied Andrade formula.Viscous flow activation energy E was approximately 60kJ/mol.Three common alcohols(methanol,ethanol and n-butanol)were selected as additives to improve bio-oil physiochemical properties.Results indicated that adding methanol could significantly improve the bio-oil fluidity.Bio-oil presented the Newtonian fluid property by adding 10 wt%methanol.The density of bio-oil/alcohol mixture was linearly related to alcohol content.With the alcohol content increasing,the mixture density decreased linearly.Viscosities of bio-oil and mixture decreased with temperature increasing.Viscosity-temperature curves of bio-oil and mixture satisfied Arrhenius model.Kinetic viscosities of bio-oil/alcohol mixture decreased with the alcohol content increasing.The modified Cargoe model was satisfied with the blending rule of bio-oil and alcohol mixture.Adding 20 wt%methanol was a considerable method for the bio-oil modification.Secondary atomization characteristics of bio-oil and bio-oil/alcohol mixture were simulated by the commercial computational fluid dynamics(CFD)software FLUENT.These findings indicated that the atomization penetration distance of bio-oil and bio-oil/alcohol mixture increased with atomization time growing.The statistical average particle size radial distribution of bio-oil and bio-oil/alcohol mixture presented a larger central statistical average particle size.With the methanol content increasing,the fuel viscosity and surface tension decreased,resulting in droplet size decrease and atomization penetration distance decrease.Bio-oil fluidity characteristics could be improved by adding more than 20 wt%methanol considerably.Atomization characteristics of mixture were significantly different from that of ordinary bio-oil.The droplet size distribution of mixture with high methanol content was more dispersive.The cyclone effect on average droplet size was negligible,but the average droplet size could stabilize quickly by cyclone.The atomization penetration distance of bio-oil/alcohol mixture decreased slightly by cyclone.Bio-oil thermal decomposition and oxidation properties were studied by thermal analysis.Results showed that the derivative thermal weightlessness(DTG)of bio-oil in air atmosphere was slower than that in nitrogen atmosphere after 225℃,indicating that oxygen had a negative effect on bio-oil DTG.After 180℃,oxygen showed a negative influence on phenolic precipitation.The thermochemical application of phenols as target products should be conducted as far as possible in air isolation condition.The oxygen effect on bio-oil residue was mainly on residue surface at 200-300℃temperature range.Bio-oil under air atmosphere was more likely to form a dense solid film on the surface.Film prevented the volatilization of gaseous productions,resulting in bio-oil DTG under air atmosphere was lower than that under N2 atmosphere.At the same time,inner gaseous productions were easily to accumulate for forming the pores,resulting in the micro-explosion in atomized combustion.Besides,pores would lead to the deformation and expansion of residue and was unfavorable to the thermal utilization(such as distillation,pyrolysis,gasification and so on).With temperature lower than 360℃,the stable aromatic ring in residue would be oxidized by air.Heating flow curves under air and N2 atmosphere of bio-oil had the same trend as temperature was lower than 300℃.These results showed above decomposition process was endothermic reaction.However,with temperature higher than 318℃,bio-oil degrading process became exothermic reaction under air atmosphere,indicating oxidation reaction was increasingly severe after 318℃.The apparent activation energy changed with various temperature on decomposition process.It indicated that the decomposition activation energy of bio-oil under air atmosphere was higher compared with bio-oil decomposition under N2 atmosphere when the temperature was above 158℃.Oxygen had a negative effect on bio-oil decomposition.Combustion experiments of bio-oil/methanol mixture were performed in a self-made device.Circumferential vortex reflux zone was observed in device inlet area due to the strong swirling flow.Fuel,oxidation agent and fuel pyrolysis products were mixed in the zone,accelerating combustion reaction development.Besides,swirling flow played a negligible role on gaseous pollutant components,whereas the components were affected significantly by air flow rate.The main function of swirling flow was to create the zone of circumferential vortex reflux for mixing fuel and oxidation agent,so as to accelerate and promote the development of combustion.However,the formation and emission of gaseous pollutant after stable combustion were adjusted by air excess coefficient.CO content in the gaseous pollutant decreased with oxygen content increasing,whereas NOx content had an opposite trend with oxygen content increasing.Meanwhile,FLUENT was used to simulate the flow field of swirling flow in combustion chamber.These results indicated the tangential velocity vector was obvious on the Y-Z section.Air diffused to the sides on the X-Y section after leaving the inlet area,which formed a flow distribution similar to a hollow cone.The relative pressure inside the hollow cone was low,causing the X-axis fluid to flow back and form a circumferential vortex reflux zone.Moreover,mixed fuel combustion was simulated by FLUENT.It could be found that when the swirling coefficient unequaled to 0,the distribution of CO content had a significant contour.The swirl could make the flame form a remarkable outline and eventually stabilized fire.