| Low rank coal reserving in abundant is important reserve energy in China.Its efficient upgrading and clean use have important energy strategic significance.But,Upgraded low rank coal is very easy to re-absorb moisture and results in reduction of production efficiency and increase of transportation costs.After rain or fall of snow,coal stored in the high-humidity environment is more likely to cause spontaneous combustion.Upgrading technology lacks economy and safety.Technology industrialization and popularization application for low rank coal are hard to be carried out.Aiming to moisture re-absorption and spontaneous combustion of upgraded low rank coal,the basic laws are studied in this paper.The difference about moisture re-absorption as well as spontaneous combustion was studied.The contents and results of this study are mainly as follows:1.Moisture re-adsorption behavior of upgraded low rank coalTwo kinds of representative low rank coal(lignite/long flame coal)selected were thermal upgraded at 105~900oC in a fixed bed reactor.The characteristics of moisture re-adsorption of upgraded low rank coal in different ambient temperature(10~40oC)and humidity were systematically studies by humidity chamber.Results are showed below.(1)With upgrading temperature rising,the re-adsorption moisture amount reduced in RH(relative humidity)?40%,however moisture content for sample upgraded at 900 oC remarkably improved and reached 12% in RH rising,the re-adsorption moisture amount sorption equilibrium time.Samples upgraded at 400~600oC is hardly impacted by ambient temperature/RH.Moisture re-adsorption difference of lignite upgraded at different temperature was more remarkable comparing with long flame coal.(2)Equilibrium moisture content(EMC)was raised with increase of RH.EMC in an environment of RH?40% gradually decreased with upgrading temperature rising,but it in an environment of RH?60% firstly reduced and then rose.EMC respectively reached minimum for lignite upgraded at 600 oC and long flame coal upgraded at 400 oC.When RH exceeds 80%,EMC for lignite upgraded at 105~900oC is over 8%.(3)Particle size(0~3mm)and ambient temperature presented negligible effect on re-adsorption ability.Particle size just obviously affected samples of high moisture amount.The smaller particle size is,the higher EMC is.(4)Comparing with the same particle size samples upgraded by the fixed bed,EMC of thermal fragmentation samples upgraded by the rotary kiln is lower.Thermal fragmentation results that ash was rich in small size particle.EMC became smaller with particle size decreasing.2.Isothermal adsorption/ adsorption kinetics model of moisture re-adsorptionMoisture re-adsorption process was discussed by isothermal adsorption and adsorption kinetics model.(1)Bound water and free water widely distributed on samples upgraded at 105/400 oC,however free water amount on samples upgraded at 600/900 oC sharply increased when adsorption potential lowed the inflection point of the characteristic curve.(2)Type II adsorption isotherms for samples upgraded at 105/400 oC and raw coal were fitted by Dent model well,however Type V isotherms for samples upgraded at 600/900 oC can be fitted by DS2 model.The primary site adsorption capacities reduced when upgrading temperature rose or coal rank increased(lignite?long flame coal).The low capacity of primary site adsorption for samples upgraded at 600/900 oC means that re-adsorption moisture mainly came from pore filling.The secondary site adsorption amount firstly decreased and then increased,the amount respectively presented minimum for lignite upgraded at 600 oC and long flame coal upgraded at 400 oC.(3)With 85% of EMC in first 4 hour,re-adsorption process went through the fast/low speed phases,which is fitted by the pseudo second-order adsorption kinetics model could well(R2?0.999).The initial adsorption rate is higher for samples owing the lower initial moisture content,but samples upgraded at 400 oC presented the lowest initial adsorption rate.For EMC higher samples possessing abundant oxygen-containing functional groups or prosperous pore structure,it is easy that water film quickly produced on sample surface,which resulted to lower adsorption rate constant.3.Structure/properties changes of upgraded low rank coal and moisture re-adsorption mechanism researchChanges of coal quality,decomposition of hydrophilic oxygen-containing functional groups,and damage effect of pore structure were revealed by proximate analysis,ultimate analysis,scanning electron microscope(SEM),nitrogen adsorption,peak fitting of FT-IR.The effect of structural changes on moisture re-adsorption was explicated.The description model for the mechanism of moisture re-adsorption was proposed to.(1)With thermal upgrading temperature rising,O content reduced and the highest equilibrium moisture content(MHC*)first decreased and then increased.Their relationship is conform to the equation of.O content of 9% corresponded to the lowest value of MHC*(4.5%).The correlation of Vdaf and MHC* is more noteworthy than Ad.(2)Deep fold and collapse presented on upgraded lignite,but shrink and fracture presented on upgraded long flame coal as upgrading temperature increasing.Pore volume and specific surface area gradually went up for lignite.Moreover pore volume reached 134.19×10-3cm3/g for lignite upgraded at 900 oC.However,the values firstly slightly decreased and then increased.Pore volume cut down to 23.69×10-3cm3/g for long flame coal upgraded at 400 oC.Mesopore of 20~50nm developed in lignite after thermal upgrading,and micropore rapidly climbed up in samples upgraded at 600/900 oC.Upgraded long flame coal possessing more 2 2 3y12.47 0.60x0.16x1.37 10 x-(28)--(10)?2~10nm pore,pore distribution change of which is not as significant as lignite.(3)The order of hydrophilic oxygen-containing functional group content in upgraded samples is-OH>-COOH>-C=O.Content of –OH is higher than other hydrophilic groups and occurred the greatest impact on MHC*.Content of –OH/-COOH/-C=O and MHC* submitted the positive relationship with R2=0.82~0.95 for lignite and R2=0.82~0.95 for long flame coal.(4)The amount decreasing of adsorption sites is the primary cause of MHC* reduction in lignite upgraded at 105~600oC,which came from the hydrophilic oxygen-containing functional groups dropping off.Micropore filling in a high humidity environment is the primary cause of MHC* increasing for lignite upgraded at 600~900oC,which came from augmenting of micropore volume.MHC* for lignite upgraded at 600 oC is smallest because of hydrophobic tar covering on the surface.MHC* dominantly suffered the effect of pore volume change of 2~10nm for long flame coal.With the smallest pore volume and the surface structure of hydrophilic and hydrophobic region alternating,sample upgraded at 400 oC obtained the lowest MHC*.The peculiar structure resulted in that water molecule hardly adsorbed steadily on the surface and formed water clusters.4.Effect of moisture re-adsorption / moisture adsorbed and pre-oxidation on spontaneous combustion characteristicsFresh samples(SA)/ moisture re-adsorption samples(SB)/ moisture adsorbed and pre-oxidation samples(SC)after thermal upgrading,were tested the difference of spontaneous combustion characteristics by the experiment equipment of temperature programming.Amount of oxygen adsorbed,crossing point temperature,heating rate and spontaneous combustion liability,was surveyed.(1)Spontaneous combustion liability of SA is reduced with the upgrading temperature rising.However,the amount of absorbed oxygen at a critical temperature(70oC)for SC reduced above 60% comparing with SA.The absorbed oxygen curve for SC constituted the period of “platform” in the earlier stage of acceleration oxidation.HR(heating rate nearby CPT)of SC exceeded to fresh samples upgraded at 200~500oC.The increase for instantaneous heating rate is more quickly than SA for the acceleration oxidation stage.Spontaneous combustion liability of SC went distinctly beyond SA and was easy to spontaneous combustion.Spontaneous combustion liability of upgraded long flame coal was lower than lignite upgraded.Moisture re-absorption slightly affected for long flame coal.(2)Lignite upgraded at 200~500oC,which suffered moisture adsorbed and pre-oxidation in 2 days,presented the remarkable increase of spontaneous combustion liability comparing with SA.Lignite upgraded at 400 oC suffered moisture adsorbed and pre-oxidation in 30 days,and the spontaneous combustion liability sequentially rose to close to raw coal.(3)Lignite upgraded at 105~300oC are easier to re-adsorbed moisture,so moisture re-adsorption obviously affected on CPT comparing with SB,with the smaller difference of oxygen adsorbed amount.Samples upgraded at 400/500 oC are difficult to moisture re-adsorption,but CPT remarkably increased and oxygen adsorbed amount significantly reduced because of samples re-adsorbing a little moisture.Comparing with SB,the rising of moisture content of SC and the increase of FCC showed the positive relationship(5y0.23x9.21 10-(28)-?).5.The mechanism of effect of thermal upgrading / moisture adsorbed and pre-oxidation on spontaneous combustion characteristicsThe changes of chemical structure were analyzed by Energy Dispersive Spectrometer and difference spectra of FT-IR.The evolution of pore structure was indicated with physical adsorption experiments,and the difference of wetting heat was given in the micro calorimeter.Free radicals in situ were dynamical analyzed by X-band electron spin resonance spectroscopy(ESR).The intimate relationship of thermal upgrading,structural change,re-adsorption,moisture adsorbed and pre-oxidation,spontaneous combustion characteristics were explored.(1)Pre-oxidation increased the elemental oxygen content and the amount of oxidation active groups on the surface of samples upgraded at 105~500oC.Aliphatic side chains,oxygen-containing functional groups,new free radicals from upgrading process,and the increase of specific surface area after upgrading process intensified pre-oxidation and spontaneous combustion of the upgraded samples.Wetting heat from adsorbing moisture not only deepened the pre-oxidation and promoted to heat gather,but also enhanced spontaneous combustion liability.(2)Thermal upgrading and moisture adsorbed and pre-oxidation were beneficial to the formation of free radical.Free radical dynamic analysis in situ showed that the relative increase ratio of concentration of free radical firstly reduced and then increased,the increase ratio of SB is above SA.(3)With some active side chains fracturing,fresh sample upgraded at 105 oC continuously produced active free radicals in oxidation and temperature-rising period.But micromolecule free radicals is rapidly consumed in fresh sample upgraded at 400 oC,which could not provide a large number of new free radicals and maintain oxidation reaction after CPT.Moisture adsorbed and pre-oxidation improved the number of free radical,and moisture made new free radicals generate and oxidation leading to temperature-rising was sustained in the later period of acceleration oxidation.So spontaneous combustion liability increased for SB.Research provides theoretical support for industrialization development of upgrading and sub-quality use for low rank coal. |
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