Investigation Of Transfer Characteristics And Emission Control Of Alkali Metals And Heavy Metals In Co-combustion Of Coal And Biomass In Fluidized Bed

Co-combustion of coal and biomass could reduce the consumption of coal as well as effectively dispose the large amount of agricultural and forest residues.In this research,thermodynamic calculation,combustion experiment in tube furnace and fluidized bed and quantum chemical computation were conducted respectively to investigate the transfer characteristics and emission control mechanism of alkali metals(K and Na)and heavy metals(Pb,Zn,Cd and Cr)during co-combustion of coal and straw.To be specific,interaction of coal and biomass was studied,additionally capture effect of alkali metal and heavy metal by kaolin additive during co-combustion was analyzed.This work could provide experimental and theoretical support to industrial application of co-combustion of coal and biomass.Existing forms of alkali metal and heavy metal were investigated in air atmosphere by thermodynamic calculation.Results indicated that during co-combustion(mass ratio of coal to biomass=7:3),sulfate was the main existence form of alkali metals before 900°C,alkali metals then migrated into aluminosilicates and gaseous chlorides as temperature increased.Evaporation ratio of Cd,Pb and Zn increased in co-combustion.Distribution of Cr in co-combustion was basically the same with coal combustion.At low temperatures,heavy metals mainly existed in sulfates.Pb and Cd then migrated into oxides and silicates in solid phase,a portion could evaporate in the form of chlorides at higher temperatures.Pb and Cd evaporated in the form of PbO(g)and Cd(g)eventually.Zn and Cr finally existed in oxides with high boiling point and thus had a relatively lower volatility.Combustion experiment was conducted in a tube furnace at 600?1000°C.It could be found that co-combustion reduced the release of K significantly.To be specific,release ratio of K was30%at 1000°C,much lower than the value of 48%in biomass combustion.Co-combustion also relieved slagging tendency.When coal had high Ca content and biomass had high Si content,the most notable reaction detected by XRD was the combination of potassium silicate and Al compounds,forming stable aluminosilicates in bottom ash.Similar migration characteristics were seen in Na during combustion,but the actual value of evaporation ratio during co-combustion was slightly higher than predicted value.Release of heavy metals like Cd,Zn,Pb and Zn was promoted by the increasing temperature,meanwhile the insoluble part increased in solid phase.In co-combustion,release of Pb and Cr increased slightly.However,evaporation ratio Cd decreased significantly compared with coal combustion,from 45.1%to 33%at 1000°C.Co-combustion was also conducted in a bubble fluidized bed combustor.Results indicated that co-combustion could improve coal combustion quality and reduce CO and SO₂ emission effectively.Alkali compounds in biomass could react with SO₂ in flue gas and form sulfates in fly ash.As the temperature rose,content of alkali metals in flue gas firstly increased and then decreased,the values of evaporation ratio were higher than the values obtained in tube furnace combustion.To be specific,release ratio of K at 850°C reached 36.9%in co-combustion.Meanwhile content of alkali metals in bottom slag and bed material increased.Co-combustion resulted in the enrichment of Zn in fly ash by transforming its chloride into oxides and Zn content in fly ash was in the range of 50%-60%in the whole temperature range.Cd and Pb were enriched into bottom slags because alkali metals in biomass had a higher affinity to Cl and could cause slight melting.Additionally,Cr was observed to migrate from bottom solids to gas and fly ash in co-combustion compared with coal combustion.To further reduce the contents of alkali metals and heavy metals in gas phase,5wt% of kaolin was selected as additive during combustion.It could be found that a portion of kaolin lost the hydroxyl and turned into metakaolin at higher temperatures.During co-combustion of coal and biomass,K removal ratio was slightly higher than the predicted value based on the value obtained in combustion of individual fuel and kaolin,it reached the maximum value of59%at 800°C.However,the ratio was lower for Na.Comparing with the capture effect of kaolin in coal combustion,it was worse for Zn and better for Cr during co-combustion.However,Pb and Cd capture effects were improved before 850°C while slightly weakened after this temperature point.At 950°C Pb removal ratio reached its maximum of 69%.On the whole,capture effect of Pb and Zn was better than Cd and Cr by kaolin.Quantum chemistry was applied to investigate the adsorption configuration of alkali and heavy metal compounds on the surface of kaolin and metakaolin respectively.It could be concluded that the adsorption of chloride was derived from covalent bond/hydrogen bond between H atom and Cl atom and ionic bond or covalent bond between O atom and metal atom.After the adsorption of KCl,the subsequent adsorption energy of another compound was increased.To be specific,adsorption energy of PbO increased from-136.4 kJ/mol to-173.7kJ/mol after the adsorption of KCl.After turning to metakaolin,the adsorption energy of every compound was significantly improved.For chloride,covalent bond was formed between Cl atom and Al atom while covalent bond or ionic bond could be formed between metal atom and O atom.Different with kaolin,after the adsorption of KCl,the subsequent adsorption energy of another compound was decreased.To be specific,adsorption energy of PbO increased from-495.5 kJ/mol to-221.3 kJ/mol after the adsorption of KCl.This could explain the variation of capture effect of heavy metals like Pb by kaolin in different stages during co-combustion.