| In recent years,with the global warming and the expansion of human life,the world’s forest fire situation is increasingly serious.Forest fires are caused by the combustion of forest fuel.Pyrolysis,as the first step of forest fuel combustion,is a key link in the process of forest fire.In-depth analysis of the pyrolysis characteristics and mechanism of forest combustions is conducive to the analysis of the ignition,combustion and fire spread law of forest combustions,so as to accurately assess the forest fire risk and provide basic data and theoretical support for the prevention and control of forest fires.In the present study,two typical forest fuel materials—pinus sylvestris needles and picea jezoensis needles were selected as the research objects.The effects of heating rate and environmental atmosphere on the pyrolysis characteristics of the two forest fuel materials were investigated by means of thermogravimetry,and the kinetic parameters of the two kinds of pine needles were calculated.In order to investigate the characteristic functional groups of the decomposition residues,in situ Fourier Transform Infrared（in situ FTIR）is employed.And the volatile products in helium are studied using a TGA instrument coupled with FTIR spectroscopy.The main results and conclusions are summarized as follows:（1）The influence of heating rate and environmental atmosphere on the pyrolysis characteristics of the pine needles was revealed: With the increase of heating rate,the variation trend of pyrolysis curve will gradually move to the high temperature region,and the maximum mass loss rate corresponding to the peak will gradually decreases.For pinus sylvestris needles,the average mass loss rate of the whole process in nitrogen atmosphere and air atmosphere was 0.15 %/K and 0.19 %/K,respectively.For picea jezoensis needles,the average mass loss rate of the whole process in nitrogen atmosphere and air atmosphere was 0.17 %/K and 0.20 %/K,respectively.The pyrolysis process of pinus sylvestris needles in inert atmosphere and air atmosphere can be divided into four stages.The pyrolysis process of picea jezoensis needles can be divided into four stages under inert atmosphere and three stages under air atmosphere.The residual mass in the atmosphere of air is smaller than that in the atmosphere of nitrogen.As to the average mass loss rate,the values of the two pine needles in the air atmosphere were higher than those in the nitrogen atmosphere.（2）The pyrolysis kinetic parameters of pinus sylvestris needles and picea jezoensis needles were obtained: for pinus sylvestris needles,the average values of pyrolysis activation energy under nitrogen and air atmosphere were 220.90 kJ/mol and 159.90 kJ/mol,respectively.The pyrolysis process in nitrogen and air atmosphere may be divided into four stages.For picea jezoensis needles,the average values of pyrolysis activation energy under nitrogen and air atmosphere were 208.77 kJ/mol and 167.53 kJ/mol,respectively.The pyrolysis process in nitrogen may be divided into four stages,while in air atmosphere can be divided into three stages.In addition,the pyrolytic reaction enthalpy change（?H）of two kinds of pine needles in air and inert atmosphere shows a similar variation trend with the corresponding activation energy curve.（3）The pyrolysis reaction mechanism of pinus sylvestris needles and picea jezoensis needles was analyzed: The pyrolysis reaction mechanism of the two pine needles under inert atmosphere is similar.In the first stage,the structural units of xylan in hemicellulose were dehydrated into furan or aldehydes ketones,and their side chains were broken to produce acetic acid,aldehydes and ketones,and the fatty acids in the extracted components were decomposed.H2O,CO2 and gas products containing the C=O functional group began to release.In the second stage,the hydroxy functional group of xylan in hemicellulose was dehydrated,and the pyrolysis products of cellulose chain,xylan and phenylpropane side chains in lignin were decarboxylated and decarboxylated.During the stage,the CO and CH4 gas began to release.In the third stage,cellulose and hemicellulose are further decomposed with the increase of temperature,and similar reactions such as dehydration,decarboxylation and decarbonyl continue to occur.During this stage,the change trend of gas products such as CO2,H2O,CH4,CO and compounds containing C=O group changed with the reaction rate,and reached the maximum releasing amount at around 660 K.Finally,in the fourth stage,pyrolysis enters the carbonization stage.During this stage,the aromatization of pyrolysis residues produces a large number of ketones,benzenes and phenolic compounds.Meanwhile,CO2 gas continue to be released.With the increase of temperature,the amount of volatile products such as H2O,C=O gradually decreased,while the amount of CO and CH4 remain at low levels. |
