Research On The Species Of Sulfur In Limestone And SO₂ Release And Capture In Preheater Environment

Industrial SO₂ emission is one of main reasons resulting in environmental disasters in China,such as smog and acid rain.According to the data from the National Bureau of Statistics,cement industry is the third largest SO₂ source in China,after coal-fired power and steel making industries.Although numerous policies and limitations have been issued by national and provincial governments,the SO₂ emission of cement industry has not been properly controlled until now.Generally,fuel is considered as the dominant source of SO₂ in flus gas,whereas most of SO₂ can be capture by fresh CaO generated from limestone,indicating strong SO₂ capture capacity of the precalciner.Since the decomposition temperature of sulfides lies in 400⁶00?,sulfides in raw meal will decompose and release SO₂ in preheater.Unfortunately,raw meal here present neglectable SO₂ capture ability,resulting in high SO₂ emission in flus gas.Limestone,accounting for approximate 85%of raw meal,contains a small amount of sulfides?typically is FeS₂?,therefore limestone is the dominant source of SO₂ in flus gas.However,the species of sulfur in limestone used in cement plant is unclarified,and the relationship between the species of sulfur in limestone and the SO₂ concentration in flus gas has not been reported,as the decomposition of sulfides and SO₂ release are very complex in preheater environment.In the present study,characteristics of limestone samples collected from high SO₂ emission cement plants were analyzed.In this thesis,the relationship between the species of sulfur and SO₂emission was correlated according to analysis on 81 limestone samples collected from high SO₂emission cement lines.A special device was designed to simulate cyclone preheater of NSP cement line,then the pyrolysis of pyrite as well as SO₂ release were followed.The feasibility of catalytic desulfurization technology was examined by both laboratorial experiment and industrial trial.Main conclusions are summarized as follows:The relationship between sulfur and SO₂ emission established according to characterization of 81 limestone samples and corresponding SO₂ emission.Low-SO₂-emission?SO₂?500 mg/Nm³?,intermedium-SO₂-emission?500 mg/Nm³?SO₂?1000 mg/Nm³?,and high-SO₂-emission?SO₂?1000 mg/Nm³?cement lines accounted for 40%,40%and 20%respectively in 124 cement lines investgated,corresponding to low-sulfur-limestone?SO₃?0.4%?,intermedium-sulfur-limestone?0.4%<SO₃?0.7%?,and high-sulfur-limestone?SO₃>0.7%?respectively.The SO₂ concentration in flue gas increased with the increase of total sulfur contents in limestone,which can be expressed as:Y=1480.28X-63.87?raw mill on?,Y=2268.66X-104.65?raw mill off?.Sulfur in limestone is existed mainly in the form of pyrite,small amount of calcium sulfate is also found in some limestone samples.Calcium sulfate usually lead to relative lower SO₂ concentration compared to pyrite.euhedral,disseminate,framboidal,and metasomatic pyrites were observed in limestone,resulting in decreased SO₂concentration in the flue gas.Pyrolysis of pyrite and SO₂ release in simulated preheater environment were investigated by employing a multistage entrained flow reactor.With the increase of heating stage,heating temperature,and the decrease of particle size,the decomposition ratio of pyrite increased gradually.the decomposition ratios of pyrite with D₅₀=10?m after 1s and 3s heating were 50%and 90%respectively.Whereas the change of O₂ concentration within 8%has minor influence on the decomposition of pyrite.Stable pyrite transform to metastable state after 310?heating,and core-shell structure with radial cracks was observed after 500?and 620?heating,indicating pyrite decompose to pyrrhotite and then pyrrhotite oxidize to iron oxides from outside to inside.The decomposition of pyrite and oxidation of pyrrhotite proceed in different layers of core-shell particle at the same time,and the oxidation of pyrrhotite started from cracks due to easier O₂ available.Influences of desulfurizers and catalyzers on the efficiency of SO₂ removal in preheater environment were investigated,and the feasibility of catalyst-desulfurization technology was validated by industrial trial.The SO₂ capture capacity of CaCO₃ and CaO was very poor,as the sulfur contents of absorbents after desulfurization product were 0.07%and 2.01%,respectively.In contact,Ca?OH?₂ presented a better efficiency in SO₂ capture,as the sulfur content of Ca?OH?₂ absorbent can be as high as 12.35%.The oxidation of SO₂ to SO₃ can be accelerated by addition ZnO,V₂O₅ and TiO₂,while CeO₂ prompted the transformation of Ca?OH?₂ to CaSO₃,all of which lead to significant increase in desulfurization efficiency.Industrial experiment showed that the desulfurization efficiency Ca?OH?₂ can be improved after the addition of ZnO,V₂O₅,TiO₂ and CeO₂,and SO₂ concentration of flue gas dropped sharply from1000mg/Nm³ to 200mg/Nm³.Catalytic desulfurization changes the path of sulfur migration in cement kiln and most of sulfur is reserved in clinker at last,according to sulfur balance of cement kiln,SO₂ in flue gas was captured and then solidified into clinker.This research provides fundamental data for SO₂ emission of cement lines and the sulfur species in limestone.The pyrolysis of pyrite and SO₂ release in preheater environment were clarified.The results laid a firmly theoretical foundation and technical support for SO₂ emission reduction,and will promote the application and population of desulfurization technology in cement industry,resulting in an important environmental,social,and economic benefits.