Study On Speciation Conversion Of Arsenic And Bioavailability Of Phosphorus During Co-firing Of Sewage Sludge With Biomass

Recovery and use of phosphorus from fly ash remaining after sewage sludge(referred to as sludge)incineration have become a promising trend in the development of sludge treatment technology.However,high contents of heavy metals in the fly ash are the core issue affecting the bioavailability of phosphorus.To realize the source control and harmless stabilization of heavy metals in combustion process is crucial to the breakthrough of sewage sludge incineration and phosphorus utilization technology.Based on existing utilization technologies of sludge incineration,a new idea that co-firing biomass with sludge to promote the capture and realize the chemical state transformation of a typical heavy metal arsenic(As)and improve the bioavailability of phosphorus has been put forward in this paper.The mineral matters in biomass provide chemical adsorption sites for arsenic.The migration and conversion of arsenic and phosphorus during co-firing of sludge with biomass were studied systematically in a 5 kW bubbling fluidized bed and vertical tubular furnace experiment bench in the paper.The interaction mechanism between the main mineral components in biomass and arsenic and phosphorus in sludge was revealed.The method of both reducing arsenic toxicity and improving phosphorus bioavailability in the sludge incineration fly ash was mastered.The main research results are as follows:(1)Addition of biomass into municipal sludge facilitates the fixation of gaseous arsenic compounds in bottom ash in the combustion process.Mineral compounds such as Ca,Fe,and A1 compounds in biomass can provide additional reactive sites for the transformation of As3+ to As5+ in fly ash from municipal sludge incineration,and the availability of mineral active sites in cotton stalk and con stalk is higher than that in wood and wheat straw.As3+ in co-firing fly ash is completely oxidized to As5+ at cotton stalk mixing ratio greater than or equal to 40%,dramatically reducing the arsenic toxicity in fly ash.Abundant pore structures in co-firing fly ash can provide more reaction contact area for the speciation transformation of arsenic,promoting the formation of various arsenates.(2)Effect mechanism of fluidized bed operating parameters on arsenic speciation transformation was investigated.Compared with the flue gas temperature of 610 ? and 320 ?,arsenic is more easily captured in the fly ash at about 460 ?.Low temperature of 320 ? is adverse for the capture of arsenic in the fly ash.Chemical oxidation of As2O3(g)is continuously conducted during the flue gas cooling process,at 610-320 ?.Higher incineration temperature facilitates the migration of arsenic into fly ash and flue gas.Gaseous arsenic is concentrated in fly ash predominantly by the condensation and physical adsorption of As2O3(g)at the temperatures from 800 0C to 900 0C.Chemical oxidation of As2O3(g)is favored by forming various arsenates(As5+)at 950 0C.With the O2 concentration in flue gas increasing from 1%to 5%,some arsenic compounds in municipal sludge,such as AS2S3,can react with O2 to produce As2O3(g),and then more As2o3(g)is captured in the fly ash by the inherent mineral compounds like CaO in fuels through the condensation and physical adsorption.At O2 concentration more than 5%,sufficient O2 can promote the oxidation reactions between As2o3(g)and various mineral compounds to form the corresponding arsenates.(3)Promotion effect of Ca-based compounds in biomass on arsenic speciation transformation was verified in a vertical tubular furnace.Compared with corn stalk,Ca-based compounds in cotton stalk can provide more reaction adsorption sites for the migration and transformation of arsenic,promoting the conversion of As3+ to arsenic bound with calcium(As5+)in the bottom ash.The three Ca-based compounds can provide chemical adsorption sites for the speciation transformation of arsenic during municipal sludge incineration,and the sequence of capture and oxidation capacity for gaseous AS2O3 is ranked as CaO>CaSiO3>CaSO4,Compared with CaO/CaSO4 additive,CaO/CaSiO3 additive can provide more Ca-based adsorption sites for valence conversion of arsenic.Compared with CaSiO3 and CaSO4,CaO has a higher reaction rate with AS2O3.Both higher incineration temperature and reaction residence time can effectively stimulate the conversion of As3+ to As5+ in bottom ash.(4)Addition of biomass especially cotton stalk into municipal sludge can effectively facilitate the transformation of non-apatite inorganic phosphorus(NAIP)to apatite phosphorus(AP)during incineration in a bubbling fluidized bed,improving the bioavailability of phosphorus in fly ash.The Ca,Cl,and Mg compounds in biomass provide additional reaction adsorption sites for conversion of NAIP(such as AIPO4)to AP(such as Ca2P2O7,Ca5(PO4)3Cl,and Ca10K(PO4)7)in fly ash.Conversion of NAIP to AP is enhanced with the increase in incineration temperature.At different incineration temperatures and residence time,addition of CaO into municipal sludge during incineration promotes the conversion of phosphorus to Ca2P2O7 and Ca3(PO4)2 in the bottom ash.Adding CaO/KCl into municipal sludge facilitates the conversion of phosphorus to Ca3(PO4)2,Ca5(PO4)3Cl,and Ca10K(PO4)7 in the bottom ash,verifying the promotion effect of CaO and KC1 in cotton stalk on phosphorus speciation transformation during municipal sludge incineration.AIPO4 can react with CaO to form Ca2P2O7 or Ca3(PO4)2 at 900 ?,and AIPO4 can also react with CaO and KCl to generate Ca5(PO4)3Cl and Ca10K(PO4)7,revealing the primary reaction pathway between CaO and KC1 in cotton stalk and phosphorus in municipal sludge during incineration.(5)Compared with municipal sludge,the chemical constituents in industrial sludge are more complex.During industrial sludge incineration alone,the inherent Ca,Fe,Al,and other inorganic compounds in the raw material provide sufficient reactive sites for chemical oxidation of As2O3(g)in flue gas,facilitating the complete transformation of As3+ to As5+ in fly ash.Si,Al,and other compositions in biomass stimulate the stabilization of arsenic into aluminosilicates in fly ash,decreasing the arsenic toxicity in fly ash.Phosphorus in the fly ash from industrial sludge incineration is mainly present in the form of P-Ca/Mg compounds,exhibiting high bioavailability.The Ca,Cl,and Mg compounds in biomass promote the conversion of NAIP compounds like AlPO4 to AP compounds(Ca5(PO4)3Cl,Ca4Mg5(PO4)6,and Mg3(PO4)2)during industrial sludge incineration.The phosphorus solubility in 2%citric acid of the fly ash from co-firing of industrial sludge with different types of biomass is enhanced.The phosphorus solubility in fly ash increases from 78.33%to 83.30%with the cotton stalk mixing ratio increasing from 0%to 50%,which is beneficial for the recovery and reuse of phosphorus in fly ash.