Co-combustion Of Bituminous Coal And Pickling Sludge In Pulverized Coal Boiler:Combustion Characteristics And Pollutants Emissions

Pickling sludge is produced from the stainless steel production process.It contains heavy metals Cr(Cr3+,Cr6+),Ni,Fe and chemical components CaF2,CaO,CaSO4,etc.It is classified as hazardous waste(HW17 surface treatment waste).Pulverized coal boiler has the high-temperature environment which is suitable for eliminating the harmfulness of hazardous waste.Besides,it also has purification equipment for flue gas and fly ash.Therefore,co-combustion of hazardous waste in pulverized coal boiler has the advantages including making full use of existing boiler and environmental protection equipment,and eliminating the need for specialized waste incinerator.Funded by the Special Fund for the National Environmental Protection Public Welfare Program(Grant 201209023-4),the study of co-combustion of bituminous coal and pickling sludge was carried out,and the main research content included blended fuel combustion characteristics,pollutants emission,heavy metals emission and partitioning,40kw pilot test,and the solution design and simulation for co-combustion in a 300MW boilers.This study mainly includes the following five parts:(1)In Chapter 2,the combustion characteristics and leaching toxicity of blended fuels were studied.TG-FTIR analysis was carried out for the mixture of bituminous coal and different proportions of pickling sludge.The results indicated that the addition of less than 10%pickling sludge into bituminous coal did not have an obvious influence on the combustion process.The leaching results indicated that the leaching rate of Cr in the pickling sludge was significantly reduced by co-combustion process.Compared to the National standard(GB 5085.3-2007),the co-combustion fly ash was not classified as hazardous waste when the proportion of pickling sludge added was less than 10%.(2)In Chapter 3 and 4,the gaseous pollutants emission,and the heavy metals emission and partitioning characteristics were studied in a drop tube furnace.The results indicated that co-combustion of<10%pickling sludge did not have an obvious influence on the combustion efficiency.The influence of different pickling sludge proportion(0-10%)and different furnace temperature(1100-1400?)on gaseous pollutants SO2,NOx,HF,HCl,PCDD/Fs and heavy metals emissions was carried out.With the temperature rising,SO2,NOx and HCl emission increased while the HF emission is temperature-insensitive.With the increasing share of pickling sludge,SO2 and HF emissions increased while NOx and HCl emissions decreased.No obvious effect of temperature and the share of pickling sludge on the total TEQ of PCDD/Fs was found,and its concentration was between 3.27-6.54 ng/m3.The concentrations of heavy metals in flue gas meet the national standard(GB 18484-2001)under all experimental conditions.Besides,the partitioning of Cr,Ni,Mn,As,Pd,Cd,Cu,Sb and Sn in flue gas,fly ash and bottom ash was studied.Based on the bottom ash mass percentage,the above 9 heavy metals were divided into three different classes:less-volatile heavy metals Cr,Ni,Mn and As;semi-volatile heavy metals Pb,Cd,Cu and Sb;and irregular heavy metal Sn.(3)In Chapter 5 and 6,the effect of chlorine,sulfur,moisture and ash content in blended fuels on the partitioning of Cr,Ni,Mn,As,Pd,Cd,Cu,Sb,and Sn were studied in a high temperature horizontal tube furnace.The results indicated that more than 99%of Cr and Ni were retained in bottom ash under all experimental conditions and not affected by these four additives due to the non-volatilty and stability of Cr,Ni and their compounds.With the increasing addition of NaCl,the percentage of As,Cu and Pb in bottom ash decreased significantly and the Mn content in fly ash increased slightly.With the increasing addition of FeS2,Cu and Pb percentage in bottom ash increased significantly due to the incomplete decomposition of the generated CuFe2O4 and PbSO4 under high temperature.With the increasing addition of water,a higher percentage of As moved into the fly ash from bottom ash,while the proportion of Pb in the fly ash decreased obviously.With the increasing addition of kaolin,the As partitioning showed a fluctuation trend,while Pb and Cu distribution in bottom ash had an upward trend.Mn and Ni reacted with Al2O3 and SiO2 in kaolin and formed Mn2SiO3(s),Mn3Al2Si3O12(s)and Ni2SiO4(s),etc.The changes of bottom ash were also studied by XRD analysis,and the results indicated that a large amount of Ca2Al2SiO7 generated under 5%NaCl addition.More CaSO4 generated with FeS2 addition due to the sulfation reaction.Under water and kaolin added conditions,no obvious change of bottom ash composition was found.(4)In Chapter 7,the pilot test of co-combustion of bituminous coal and pickling sludge was carried out in a 40kw pilot scale drop-tube furnace.The results indicated that co-combustion of Shenmu coal and<20 wt%pickling sludge did not decrease the combustion efficiency in this experiment.With increasing proportion of pickling sludge in the blended fuels,NOx and HCl emission concentrations were basically stable,while the SO2 and HF emission increased significantly.To make the concentration of heavy metals in flue gas meet the national standard(GB 18484-2001),under the experimental conditions with a high efficiency cyclone separator,the pickling sludge amount used in co-combustion must be limited to<10 wt%.Compared to the national standard(GB 16889-2008),due to the high leaching concentrations of heavy metals Cr,Ni and Pb,the co-combustion fly ash needs further processing before being landfilled disposal.(5)In Chapter 8,the solution design of co-combustion of bituminous coal and pickling sludge in a 300MW pulverized coal boiler was carried out,and the adopted co-combustion method was to mix the two fuels before the coal mill.By using BESS 5.0(General boiler design calculation system)and fluent simulation,the effect of co-combustion on the boiler operating conditions under different operations was studied,including different pickling sludge addition ratios and different furnace loads.The variation of temperature and velocity field,and the changes of CO,NOx and SO2 concentration field under different operations were obtained,which can provide useful data for the future industrial applications.