| Torrefaction is a promising pretreatment technology for biomass,which is a thermo-chemical treatment at 200-300?,low heating rate(<50?/min)and in inert atmosphere to obtain torrefied biomass(biochar).Biochar not only retains the original characteristics of low sulfur and nitrogen content,high ash coke activity and net CO2 emission,but also has the advantages of higher heating value,hydrophobicity,good grindability,non-perishable and wide application range.In this paper,four kinds of lignocellulosic biomass were torrefied in a fixed bed reactor,and the pyrolysis kinetics of biochar was studied by thermogravimetric analyzer.The main work and conclusions are listed as follows:Two kinds of herbaceous biomass(com straw(CS),wheat straw(WS))and two kinds of woody biomass(poplar wood(PW)cedar wood(CW))were selected for the tested biomass.80 kinds of biochar were prepared at the torrefaction temperature of 200-300? and holding time of 10-60min.Based on the analysis results of proximate analysis,ultimate analysis,calorific value analysis,composition analysis,hydroscopicity determination and grindability determination,the effects of species,torrefaction temperature and holding time on the torrefaction characteristics of biomass were studied.The results indicated that:(1)With the torrefaction temperature and holding time increasing,the fixed carbon content,ash content,high calorific value and carbon content of biochar increase continuously,and the volatiles content and oxygen content of biochar decreases continuously.For the same biomass,the effect of torrefaction temperature on the sample is greater than holding time.Under the same torrefaction conditions,the change degree of proximate analysis and ultimate analysis components content and high calorific value of herbaceous biochar is greater than that of woody biochar.(2)The mass yield(?m)and energy yield(?e)of biochar decreased with the increase of torrefaction temperature and holding time.When the torrefaction condition is 300?and 60min,?m of biochar decreased to 46.76%,37.20%,58.60%and 56.60%respectively,and ?e decreased to 61.98%,50.37%,73.13%and 72.65%,respectively.Under the same torrefaction conditions,the ?e of the same kind of biochar was significantly higher than that of ?m,and the reduction of herbaceous biochar ?m and ?e was significantly larger than that of woody biochar.(3)The torrefaction severity index(?T)and enhancement factor coefficient(?HHV)of biochar are increasing with the increase of torrefaction temperature and holding time.There is a positive correlation between ?HHV and?T of biochar,while ?e and ?T of biochar have a negative correlation.(4)With the increase of torrefaction temperature,the hemicellulose content of biochar decreased significantly,and the change of cellulose content first increased slowly and then decreased rapidly.With the increase of holding time,the hemicellulose content of biochar decreased continuously,and the variation of cellulose content was different.Under the same torrefaction conditions,The composition decomposition coefficient(?C)of cellulose and hemicellulose of herbaceous biochar is significantly larger than that of woody biochar.(5)With the torrefaction temperature and holding time increasing,the equilibrium moisture content(?EMC)of woody biochar decreased,while that of WSC decreased and then increased.Under the same torrefaction conditions,the highest ?EMC of WSC and the lowest ?EMC of PWC.(6)The grindability of biochar improved with the increase of torrefaction temperature.At the same torrefaction temperature,the grindability of CSC and CWC continued to increase with the increase of holding time,while the grindability of WSC and PWC almost did not change.The pyrolysis process of four kinds of biochar(CS275-40?WS275-40?PW275-40 and CW275-40)was studied under nitrogen atmosphere with thermogravimetric analyzer.Based on the established three-component pyrolysis kinetic model and Gauss multi-peak fitting method,the contribution factor and kinetic parameters of the three components in biochar were analyzed.The results indicated that:(1)The maximum weight loss rate and corresponding peak temperature of woody biochar(PW275-40 and CW275-40)were significantly higher than that of herbaceous biochar(CS275-40 and WS275-40).(2)The contribution factor of cellulose in biochar was significantly higher than that of hemicellulose.The cellulose contribution factors of the four biochar(CS275-40?WS275-40?PW275-40 and CW275-40)were 0.25,0.37,0.55 and 0.47,respectively,and the hemicellulose contribution factors were 0,0.01,0.09 and 0.12,respectively.(3)Compared with the determination results of fiber analyzer,the difference between the three component contribution factors calculated based on kinetic analysis is 0.03 at the maximum.(4)The activation energy factor and pre-exponential factor of herbaceous biochar were lower than those of woody biochar. |
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