Pretreatment Of Tobacco Waste By Water Leaching-oxidative Torrefaction And Its Effects On Combustion Characteristics

Tobacco waste is the solid waste originated from tobacco planting and production.As a kind of lignocellulose biomass,it has some disadvantages such as high moisture content,high oxygen content,low calorific value and energy density.Tobacco waste contains high content of potassium,chlorine,sulfur and ash content compared with other biomass,so it is not suitable to be used as fuel directly.Water leaching-torrefaction is a kind of pretreatment technique which can be used to deal with these disadvantages of high-ash biomass.However,the effects of different torrefaction atmosphere and processing sequence on water leachingtorrefaction process remain unclear,and it lacks the research on oxidative torrefaction characteristics of tobacco waste.First of all,the effects of oxidative atmosphere on tobacco waste's torrefaction characteristics were investigated.It is found that the CO2 in carrier gas has limited impact on torrefaction characteristics,while the O2 has great effect on it when the torrefaction temperature is above 250 ?.Oxidative torrefaction of tobacco waste can be carried out under low O2 concentration(<10%)and the torrefaction condition is recommended to be 5% O2,250 ?.Torrefaction pretreatment can effectively decrease the O content and increase C content in samples,and O/C and H/C mole ratio can be reduced to the similar value of lignite.Cellulose and hemi-cellulose are decomposed in torrefaction process,with dehydration,dehydrogenation and deoxidization reactions.Oxygen containing functional groups are reduced during torrefaction process,and the-CH2-functional group in aliphatic compounds decreases when torrefaction atmosphere containing O2.Then,the different torrefaction characteristics between raw and water-leached tobacco waste were studied.The results show that the torrefied samples originated from water-leached tobacco waste always have higher calorific value as well as energy densification factor.The differences are due to the loss of organic matters and inorganic salts during water leaching,and inorganic salts in samples can inhibit the oxidation reaction of oxidative torrefaction.When comparing water leaching-torrefaction with torrefaction-water leaching,samples upgraded by the second method have lower calorific value and higher ash content.As a result,water leaching-torrefaction is suggested to be the pretreatment method of tobacco waste.At last,the effects of pretreatment methods on tobacco waste's combustion characteristics were investigated.The results suggest that torrefaction pretreatment lower the peak of volatile combustion and raise the peak of fix carbon combustion in DTG curve.The oxygen in carrier gas can have great impact on combustion characteristics and improve burnout performance when the torrefaction temperature is relatively high(>250 ?).Water leaching can enhance ignition characteristics of both raw and torrefied tobacco waste,while the effect of water leaching on burnout performance of torrefied samples are based on torrefaction condition: the burnout performance can only be improved when torrefaction condition is severe.Three pretreatment methods are compared regarding to ignition and burnout behaviors,from best to worst: water leaching-torrefaction,torrefaction-water leaching,torrefaction(ignition);torrefaction-water leaching,torrefaction,water leaching-torrefaction(burnout).Additionally,the activation energy during combustion process is obtained by two iso-conversional methods.The activation energy at oxidative pyrolysis stage remains essentially unchanged,and the peak value occurs at fix carbon combustion stage.Water leaching can slightly decrease the activation energy of oxidative pyrolysis and increase the peak value of fix carbon combustion,while(oxidative)torrefaction can slightly lower its peak value.