| Combustion of residential solid fuels（coal and biomass）is an important emission source of atmospheric carbonyl compounds（CCs）.CCs is an important precursor of ozone and secondary organic aerosol（SOA）.In recent years,the research on the emission characteristics of CCs from residential solid fuel sources is increasing,but the influence on fuel types and combustion conditions（e.g.combustion temperature）and the relationship between the emission of CCs and the pyrolysis characteristics of fuel need further study.This paper consists of three parts.Firstly,the pyrolysis analysis of nitrogen and air atmosphere is carried out for each experimental fuel to provide basic data for the study of its combustion process and emission mechanism.Secondly,two kinds of bituminous coal（Lingwu coal and Xuzhou coal）and two kinds of biomass（rice straw and larch）are taken as the objects,and the combustion dilution sampling system of quartz tube furnace is combined at seven combustion starting temperatures（300～900oC,100oC gradient）.Finally,five kinds of straws（rice,corn,wheat,cotton,rape）and five kinds of firewood（larch,poplar,birch,ash,rubber wood）were selected as the research objects to collect and analyze the emissions of CCs at 400oC and 800oC and to explore the impact of fuel types.The preliminary conclusions are as follows:（1）The pyrolysis（nitrogen atmosphere）and combustion（air atmosphere）of biomass and coal can be divided into three stages:drying,pyrolysis or combustion,carbonization or burnout,respectively,correspond to the slight weight loss,rapid weight loss and near stable stages of fuel quality.Specifically,the temperature range from room temperature to 900oC（heating rate of 10 oC/min）,in the nitrogen atmosphere,straw is divided by 150oC and 550oC,and the weight loss rates of the three stages are8%,56%and 5%respectively;firewood is divided by 200oC and 480oC,and the weight loss rates of the three stages are 9%,65%and 9%respectively;bituminous coal is divided by 340oC and 600oC,and the weight loss rates of the three stages are 7%,16%and 7%respectively.In the air atmosphere,the average temperature dividing points of straw are 220oC and 520oC,and the weight loss in three stages are 14%,77%and 2%respectively;the temperature dividing points of firewood are 200oC and 500oC,and the corresponding weight loss rates are 9%,89%and 1%;the temperature dividing points of bituminous coal are 300oC and 650oC,and the weight loss rates in three stages are8%,84%and 0.The main reason for the difference in pyrolysis behavior between fuels is the difference in thermal stability of their internal components（e.g.side chain fracture,rearrangement and cyclization）,which can help to explain the generation and emission mechanism of pollutants in the process of residential combustion.（2）Combustion temperature significantly affects the emission characteristics of CCs.At 7 starting temperatures,the emission factors of carbonyl compounds(EFCCs)of each fuel showed a single peak trend,but the temperature points of the peaks were different.The maximum value of Xuzhou coal（548.0 mg/kg,500oC）is 7.5 times of the minimum value（73.4 mg/kg,900oC）;the maximum value of Lingwu coal（678.2±111.4 mg/kg,600oC）is 3.9 times of the minimum value（173.0 mg/kg,900oC）.The maximum value of rice straw（2332.7 mg/kg,400oC）is 17.5 times of the minimum value（133.2±3.6 mg/kg,800oC）;the maximum value of larch wood（932.2 mg/kg,500oC）is 3.3 times of the minimum value（278.8 mg/kg,900oC）.It is interpreted in combination with modified combustion efficiency（MCE）.At low temperature（300～600oC）the fuel is mainly in incomplete combustion（MCE<0.9）,and at high temperature（700～900oC）it is mainly in complete combustion（MCE>0.9）.The EFCCs of Xuzhou coal at low and high temperature were 499.0±62.2 mg/kg and 106.9±29.1 mg/kg respectively,while those of Lingwu coal were 518.2±106.7mg/kg and 213.6±54.2 mg/kg,respectively.The EFCCs of rice straw at low and high temperature were 1887.5±448.8 mg/kg and 421.2±426.9 mg/kg,respectively,while those of larch wood were 718.8±210.1 mg/kg and 422.1±158.2 mg/kg.The emission of CCs mainly occurs in the braising（incomplete combustion）stage with low combustion efficiency.（3）The type of fuel also has an important influence on the emission characteristics of CCs.The EFCCs of Xuzhou and Lingwu coal were 303.0±277.2mg/kg and 365.9±215.3 mg/kg,respectively;and those of rice straw and larch wood were 1154.4±1036.8 mg/kg and 570.5±209.8 mg/kg,respectively.The average EFCCsof biomass（862.4±412.9 mg/kg）was 2.8 times higher than that of bituminous coal（334.4±44.5 mg/kg）.In terms of composition,the formaldehyde EFs（195.0±94.2mg/kg）of biomass is 2.9 times that of coal（69.4±7.8 mg/kg）,acetaldehyde（349.7±360.9 mg/kg）is 3.6 times that of coal（96.1±19.6 mg/kg）,acetone（79.2±30.3 mg/kg）is equivalent to coal（82.6±11.3 mg/kg）,aromatic aldehyde（87.1±13.5 mg/kg）is 3.4times that of coal（25.3±3.4 mg/kg）.The emission characteristics of 10 kinds of biomass were compared:the EFCCs of straw（1622.6±406.3 mg/kg）was slightly higher than that of firewood（1576.9±868.5mg/kg）,and the proportion of formaldehyde,acetaldehyde and acetone in the CCs of each fuel was over 60%.（4）The aromatic aldehydes with special properties were discussed.The aromatic aldehydes emitted from coal mainly come from three aspects:the direct volatilization of aromatic aldehydes contained in coal,the thermal decomposition and oxidation of aromatic compounds in the process of coal heating,and the free radical reaction（thermal condensation polymerization reaction）.The contribution of cellulose,hemicellulose and lignin in biomass to aromatic aldehydes is different.The acetaldehyde produced by pyrolysis of hemicellulose and cellulose condenses to form four carbon fragments and cyclizes with the intermediate product of olefin aldehydes to form aromatic aldehydes.The aromatic aldehydes contributed by lignin at 300～530oC mainly come from the direct cracking of lignin structure.After 530oC,the ring opening fracture of benzene ring and the cyclization of small molecular fragments together affect the dynamic change of aromatic aldehydes emission factors. |
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