Transformation Mechanism and Catalytic Effects of Sodium during Pyrolysis of Zhundong Coal

Author:Xu Lian Fei

Supervisor:wu shao hua liu hui


Degree Year:2018





Zhundong coalfield,with an enormous reserve,is an important source to guarantee sustainable coal energy supply in China.The Zhundong coal,of high volatiles content and low ash and sulfur(S)contents,is an excellent steam coal.However,its sodium content is high,leading to serious slagging and fouling problems on the heating surfaces of Zhundong coal-fired boilers.Sodium in the low-rank coal,meanwhile,is an efficient catalyst,and play an important role in improving the pyrolysis,combustion,and gasification performance of coal.High efficiency and clean utilization of Zhundong coal can be realized by rationally controlling and utilizing the abundant sodium in coal.The release characteristics and catalytic activity of Na depend on its occurrence modes.The occurrence modes of Na changes during the pyrolysis of coal,and its catalytic activity changes correspondingly.This paper focused on the transformation mechanism and catalytic effects of Na,and studied three key issues: the release and transformation of Na during pyrolysis of Zhundong coal,the catalytic effects of Na on fixed-bed and fast chars of Zhundong coal,and the effects of Na on fixed-bed volatiles and tar of Zhundong coal.All these studies provide theoretical support for the regulation,control and utilization of Na.A Zhundong coal,collected from the Wucaiwan colliery,was studied.Occurrence modes of Na in the raw coal were measured.The release of Na was investigated during the pyrolysis of coal at 500–900 °C in a fluidised-bed/fixed-bed reactor.The results show that,more than 80% of Na existed in a H2O-soluble form in the Zhundong coal.The main chemical forms of H2O-soluble sodium were NaCl,Na2SO4,and NaHCO3.As the pyrolysis temperature increased,the release of Na from coal under rapid heating condition increased sharply and reached 85% at 900 °C;while the release of Na during slow pyrolysis of coal increased slowly,and showed a low level(less than 10%)over the whole temperature range.Volatile-char interreaction increased the release of Na from char from slow pyrolysis,but its total release was still less than the release of Na during fast pyrolysis of coal.Graphene oxide was select as a model compound of char.After the reaction of graphene oxide and NaOH solution,the –COOH group in the graphene oxide was transformed to –COONa structure,and the organic form of Na was obtained.The release of Na during slow pyrolysis of sodium loaded graphene oxide was about 20% and changed little with the pyrolysis temperature.The release of Na was increased by the interreaction of volatiles and sodium loaded graphene oxide,and was further increased at higher temperature.Transformation of occurrence modes and release of Na in coals was studied during pyrolysis of H2O-and NH4Ac-soluble sodium loaded coals at 300–900 °C in a fixed-bed reactor.Results show that,the release of Na during the pyrolysis of coal in the fixed-bed reactor was lower than that in other reactors.Releses of Na from pyrolysis of four sodium-loaded coals below 600 °C were under 7%.Transformation routes and mechanism of Na during pyrolysis of Zhundong coal was revealed.H2 Osoluble sodium can interact with –COOH to form NH4Ac-soluble sodium,while NH4Ac-soluble sodium can transform to H2O-soluble sodium through the decomposition of carboxylate structures below 400 °C.The conversion direction of H2O-and NH4Ac-soluble sodium depends on the NH4Ac-soluble sodium content of coals.The transformation of Na from NH4Ac-to H2O-soluble form predominates between 400 and 600 °C.At temperatures above 600 °C,H2O-soluble sodium may interact with minerals or matrix of coal to form insoluble sodium.Various H2O-soluble sodium salts were impregnated into an acid-washed coal to eliminate the effects of inherent minerals in the raw coal.Effects of H2O-soluble sodium salts on the characteristics of chars were studied during rapid pyrolysis of sodium-loaded coals in a drop tube furnace.The study shows that,pH in the watercoal slurry increased after the addition of NaAc and Na2CO3.Massive –COONa structure was formed in the resulting coals.During fast pyrolysis of sodium-loaded coals,Na,especially –COONa,roughed the char surface,stabilized the oxygencontaining structures in char,inhibited the graphitization of char and increased crosslinking density and substituent group contents of char.Na retained in the char after the fast pyrolysis revealed significant catalytic effects.All these factors contributed to the reactivities of chars.Low-temperature gasification reactivities of chars from pyrolysis of sodium-loaded coals at 900 °C decreased in the order: H-char < Na2SO4-char ~ NaCl-char < NaNO3-char < NaAc-char < Na2CO3-char.To further investigate the catalytic effects of Na,influences of H2O-and NH4 Acsoluble sodium on the coal pyrolysis at 500–900 °C were studied in the fixed-bed reactor.And the results were analysed combining with occurrence modes of Na in chars.The study shows that,sodium loaded in the coal roughed the char surface,and increased its oxygen content.The effects of H2O-soluble sodium on the char structures were negligible,while NH4Ac-soluble sodium decreased the order of char structures.Reactivity of the char from acid-washing or acid-re-washing coal decreased as the pyrolysis temperature increased from 500 to 900 °C,while the char from pyrolysis of Na2SO4-loaded coal at 900 °C showed the highest reactivity,and the char from pyrolysis of Na-form coal at 700 °C showed the highest reactivity.This is because that,catalytic capacity of Na depends on both content of Na and its occurrence modes in char,and the insoluble sodium covered by the char matrix showed a higher catalytic activity.Effects of Na and secondary reaction on the volatile and tar yields were studied over a temperature range of 500–900 °C in the horizontal fixed bed.Results show that,removals of alkali and alkaline earth metals increased the volatile yield from pyrolysis of coal below 700 °C,and the tar yield from pyrolysis of coal above 700 °C.Na could inhibit the release of tar precursors(mainly polycondensed aromatic rings)and promote their transformation by connecting them with char matrix.Increases in both interparticle and gaseous secondary reaction time had little effects on volatile yields,but decreased the tar(expecially polycondensed aromatic rings)yield.A lowtemperature two-stage gasification based on the fixed-bed pyrolysis was proposed.