Study On PM₁₀ Emissions During The Combustion Of Agricultural And Forestal Biochar Produced By Thermal Pretreatments

The massive combustion of fossil fuels around the world leads to the aggravation of the greenhouse effect.There has been a research trend to utilize the available resource of agricultural and forestal biomass while coal is still used as the main energy source in China.Biochar,as a solid product of the thermal pretreatment of biomass,not only possesses the carbon neutralization and renewability of biomass energy,but is also greatly improved in its characteristic using as fuel compared with biomass,which is expected to be a substitute for coal.With the country's strict environmental policy,the emission of combustion pollutants should be paid attention to.Previous studies have shown that gas pollutant emissions are less problematic during biochar combustion,but there are few studies involving PM₁₀ emission characteristics during biochar combustion.Therefore,in this thesis,PM₁₀ emission characteristics of typical agricultural and forestal biochar prepared by laboratory-scale or semi-industrial-scale thermal pretreatment were systematically studied.First,the relationship between the content of inorganic elements in various occurrence forms and the PM₁ and PM_1-10 emissions was revealed,and the PM₁₀ formation mechanism during the combustion of biochar was also explored.Sequential extraction were performed on typical agricultural and forestal biochar prepared from willow and wheat straw,and the combustion experiments were carried out on drop-tube furnace.It was found that PM₁emissions were almost linear with K/Cl/S in water-soluble and ammonium acetate-soluble forms.PM_1-10 generated from the combustion of forestal biochar was mainly composed of Ca-containing minerals,and its emissions were almost linearly correlated with the Ca content in each occurrence form.For agricultural biochar,PM_1-10 was mainly composed of Si-containing minerals.The removal of inorganic elements in various forms would reduce the formation of silicate,meanwhile the resulting coalescence would be weakened.The impact of them on PM_1-10 emissions were reversed.Under the comprehensive effect,PM_1-10emissions were less related to the content of water-soluble and ammonium acetate-soluble inorganic elements.Second,the PM₁₀ emission characteristics during the combustion of agricultural biochar prepared by laboratory-scale thermal pretreatments were explored.Three common pretreatments including torrefaction,slow pyrolysis and hydrothermal carbonization were compared using rice straw as raw material.The single or co-combustion experiments of all samples were performed on drop-tube furnace.The selected coal were bituminous coal with high Si and Al content,and the blending ratio of biomass/biochar was 20%and 50%.The results showed that a central mode occurred between 0.3-1?m,so PM₁ was divided into PM_0.3 and PM_0.3-1 for discussion.Based on ash input,PM_0.3 emissions decreased from 20.22mg/g(raw straw)to 9.17 mg/g(torrefaction),6.61 mg/g(slow pyrolysis),and 1.15 mg/g(hydrothermal carbonization).PM_0.3 was mainly formed by the homogenous nucleation and agglomeration of KCl.The reduction ratio of PM_0.3 emissions had an obvious positive correlation with Cl release rate during the pretreatment;PM_0.3-1 was mainly formed by the heterogeneous condensation of KCl,its emissions were much smaller than PM_0.3,but the change trend was similar to that of PM_0.3.Based on ash input,PM_1-10 emissions decreased from 49.97 mg/g(raw straw)to 41.88 mg/g(torrefaction),30.02 mg/g(slow pyrolysis),and15.20 mg/g(hydrothermal carbonization).Under the effect of the phosphates and silicates,Si-containing spheres could aggregate to form PM_1-10,and the change of PM_1-10 emissions was determined by the content of ash and inorganic elements.During blend combustion,the difference between the calculated value and the measured value of PM₁₀ was mainly concentrated in PM_0.3.The interaction between aluminosilicates in coal and alkali and alkaline earth metals in biomass would reduce PM_0.3 emissions,and the reduction ratio was significantly negative correlated with Cl release rate during the pretreatment.Then,using rice straw and sesame stalk as raw materials,the PM₁₀ emission characteristics during the combustion of agricultural biochar prepared by the compound pretreatments:water washing after carbonization,water washing before carbonization,hydrothermal carbonization were explored.In addition,the effects of changing the water washing conditions to high liquid-solid ratio/long washing time on PM₁₀ emissions reduction caused by pretreatments were investigated.The samples were subjected to combustion experiments in drop-tube furnace.The PM₁ emissions of agricultural biochars produced by three pretreatments were 0.14-0.77 mg/g,which was almost equivalent to that of bituminous coal.Compared to raw biomass,the reduction ratio is determined by the removal effect of Cl+S and Na+K during the pretreatment,hydrothermal carbonization performed best,followed by water washing before carbonization and water washing after carbonization.Increasing the water washing time and the liquid-solid ratio could further increase the removal rate of inorganic elements,thereby further reducing the PM₁ emissions.The reduction ratio of water washing before carbonization even increased to a level equivalent to hydrothermal carbonization.With respect to PM_1-10 emissions during the combustion of agricultural biochar prepared by the compound pretreatments,they were determined by the type of biomass,the change of inorganic elements content and the change of the degree of fragmentation during combustion.Finally,the PM₁₀ emission characteristics during the combustion of forestal biochar prepared by mid-temperature pyrolysis(pyrolytic polygeneration base)of semi-industrial level were explored.The single/co-combustion experiments of poplar pyrolysis char were carried out on drop-tube furnace.The selected coal was high Si bituminous,and the blending ratio of biocahr was 20%,50%and 80%.During single combustion,the PM₁ emissions from biochar were basically the same as that of coal,0.78-0.82 mg/g.PM_1-10 emissions from biochar were 5.6 mg/g,which was slightly higher than that of coal(4.1 mg/g).During blend combustion,the dilution effect and chemical reactions of high-melting Ca-containing minerals in biochar would cause more minerals in coal to enter PM_1-10,while large-sized minerals in coal could adhere to small-sized minerals in biochar into PM₁₀₊,reducing the amount of minerals entering PM_1-10 from biochar.Under the combined effect,PM_1-10emissions during blend combustion only increased obviously compared to the theoretical value in the case of 20%biochar blend.The concentrations of PM₁ during the combustion of coal,biochar and blend fuels were low and close to each other.The formation of the inner layer fouling were dominated by small particles,thus,the growth rate of the inner layer fouling during the combustion of coal,biochar and blend fuels were relatively close.