Study On The Burning And PM_2.5 Emission Characteristics Of Agroforestry Biomass Based On A Cone Calorimeter（CONE）

The resource of agroforestry biomass in China is abundant.PM_2.5from the burning of agroforestry biomass,mainly straws and wood based on materials,is one of the most important sources of air pollution in China.In order to explore PM_2.5 emission characteristics of the burning of agroforestry biomass,the burning behaviors and PM_2.5emission characteristics of crop straws,planted wood and pinecones were characterized simultaneously and studied systematically based on the cone calorimeter（CONE）,combined with the LD-5H laser dust meter.The relationship between combustion behaviors and PM_2.5 emission characteristics was explored.The influences of agroforestry biomass types,combustion stages and chemical composition on PM_2.5 emission characteristics were explored.The migration rules of chemical elements from biomass to PM_2.5were revealed.The results are as follows:（1）Study on the relationship between burning behaviors of agroforestry biomass and PM_2.5 emission characteristicsThe burning behaviors of rice straw,wheat straw and corn straw were characterized based on CONE.Corn straw has the worst thermal stability among the three straws.The residual rate of straw,wheat straw and corn straw were 18.87%,17.02%and 9.75%at 500℃,respectively.The curves of heat release rate（HRR）curves showed a"unimodal"structure during the whole combustion.The peak HRR appeared in the ignition stage,which was the main generation stage of thermal degradation and flue gas.The total smoke production（TSP）of wheat straw was 0.81m²,which was 2.89 and 1.08 times that of rice straw and corn straw,respectively.The real-time PM_2.5mass concentration curves of the three kinds of straws were consistent with the extinction coefficient during the combustion process.The PM_2.5 emission factors between 60.65 and 180.91μg/g,and the trend of emission factors was consistent with result of TSP.It shows that reducing smoke production can effectively reduce PM_2.5 emission factor.PM_2.5 emission factor of wheat straw was the highest,which was 2.98 and 1.14 times that of rice straw and corn straw,respectively.The total carbonaceous（TC）components of PM_2.5 accounted for 32.95～44.68%,which was the most important component of PM_2.5.The ratio of organic carbon/inorganic carbon（OC/EC）in TC was 14.82～30.78,which confirmed that straws smoldering played a dominant role in straws open burning and PM_2.5 was mainly generated in the ignition stage.Meanwhile,the chemical composition of PM_2.5 in this study was consistent with the study of experimental results of open straw combustion,indicating that it was feasible to simulate the PM_2.5 emission characteristics of biomass combustion based on CONE.（2）Study on the PM_2.5emission characteristics of agroforestry biomass in different combustion stagesThe burning behaviors and PM_2.5emission characteristics of poplar,eucalyptus,Chinese fir and Masson pine were investigated.Due to the carbonization characteristics of wood,the HRR and K curves showed an obvious"bimodal"structure.The PM_2.5 real-time mass concentrations of the four kinds of wood showed a"trimodal"structure.The first peak of PM_2.5mass concentration appeared near the first HRR peak,and the second and the third peaks of PM_2.5mass concentration appeared on the both sides of the second HRR peak.In the second stage of"bimodal",the PM_2.5emission factor of wood was 1.34,2.97,1.66 and 1.46 times that of in the first stage,respectively.In addition,among the four kinds of wood,TSP of poplar and Chinese fir were higher,and the PM_2.5 emission factors were 200.25±3.47μg/g and 427.73±4.34μg/g,respectively.In the first stage of"bimodal",the combustion temperature was higher than the precipitation temperature of the main volatile inorganic substances in wood,so a large of volatile inorganic substances were precipitated in this stage.The water-soluble inorganic components（WSIC）of PM_2.5 in the first stage of"bimodal"had a higher proportion.The relative content of TWSIC in the first stage accounted for 7.01～17.95%,which was 1.29～1.44 times that the second stage.OC and EC were the main components of PM_2.5 in the"bimodal"stage.The first stage of"bimodal",the OC in TC is higher,and the ratio of OC/EC was 2.71～3.41.In the second stage of"bimodal",the ratio of OC/EC was 1.78～1.90.The ratio of OC/EC can be used as an index to distinguish the"bimodal"combustion stage of wood.The main alkali metal compounds dominated by K and Na,which were mainly precipitated in the first stage of"bimodal"of wood combustion.TEM-EDS analysis showed that the stages of"bimodal"of wood had important effects on PM_2.5 morphology and chemical composition.The main PM_2.5 in the first stage of"bimodal"was mainly composed of organic matter（OM）and K/Na salt,while the main PM_2.5in the second stage was mainly OM.The net-like aggregates are the main types of PM_2.5in the"bimodal"stage.The average particle size of the main PM_2.5 in the first stage of"bimodal"were30～47 nm,which was larger than that in the second stage,they tended to generate larger PM_2.5.（3）The migration rules of chemical elements from agroforestry biomass to PM_2.5The burning behaviors and PM_2.5 emission characteristics of pinecones of red pine,spruce and Japanese larch pinecones were systematically investigated.The migration rules of biomass chemical elements from agroforestry biomass to PM_2.5 were explored.With similar to planted wood,pinecones were a biomass material with carbonization characteristics,whose HRR,K curves and mass loss rate（MLR）all showed obvious"bimodal"distribution.The real-time PM_2.5mass concentration variation from the burning of pinecones were consistent with the result of"bimodal"structure trend of burning behaviors.The peaks of PM_2.5mass concentration are mainly caused by two strong smoking processes.The PM_2.5 emission factor in the whole combustion stage was 129.24～162.70μg/g.TWSIC of PM_2.5accounted for 10.29～15.40%.Red pinecones had the highest content of TWSIC in PM_2.5,15.40%.It was 1.50 and 1.45 times these of spruce and Japanese larch pinecones,respectively.Potassium is the metal cation element with the highest content in the raw material of pinecones.K⁺was the cations in PM_2.5 with the highest relative content（3.08～6.59%）.The content of K⁺in the red pinecones was significantly higher than the other two types of pinecones.In contrast to the anion component in PM_2.5 from the burning of straws and wood,the content of S element in pinecones raw material is 8.73～14.21 times these of Cl,and SO₄^2-was the most important anions in PM_2.5,accounting for 17.00～19.94%of TWSIC.The content of TC in raw materials of pinecones were much higher than that of straws and wood,and TC was the component with the highest relative content in PM_2.5（70.55～78.93%）.It was related to the abundant content of S element in pinecones.TC was the component with the highest relative content in PM_2.5,accounting for 70.55～78.93%.It was much higher than these of straws and wood.It was related to the highest content of TC in raw materials.TEM-EDS results showed that PM_2.5 from the burning of pinecones were mainly ultrafine particles（0～90nm）,among which the red pinecones occupied the highest proportion at 60～90nm.It tended to generate PM_2.5 with large particle size.The content of S element in red pinecones raw material was the highest（1495.20 mg/kg）,the inorganic components of PM_2.5 was K₂SO₄.However,the content of S element in spruce and Japanese larch pinecones were equal,the proportion of Cl element was higher,the inorganic components were K₂SO₄ and KCl.Crop straws and planted wood also had similar element migration laws,the content of Cl was much higher,the inorganic components of PM_2.5 were all KCl.Net-like aggregates were the main type of PM_2.5from the burning of pinecones In addition,no irregular PM_2.5 was found from the burning of pinecones,which was significantly different from straws and wood.