Catalytic Gasification Of Lignite And Intermediates With Heterogeneous Catalysts In Supercritical Water

Lignite,regarded as a low-rank coal,accounts for 17%of total coal reserves in china.However,lignite contains high moisture and high volatiles,which restrain the consumption of lignite.Due to the higher H/C ratio and oxygen content than high-rank coal,lignite has intrinsic advantages as feedstock for liquid and gaseous fuels.Supercritical water gasification(SCWG)has been considered as a promising technology for using coal.The process of supercritical water gasification is efficient due to the unique physicochemical property of supercritical water(SCW,Pc= 22.1 MPa,Tc=374.3 ?)such as high diffusivity,high solvating ability for organic compounds and non-polar gases.The drying process for those feedstock with a high moisture can be also excluded for the SCWG.Moreover,the N,S and other elements in raw coal could deposit in supercritical water as inorganic salts during the SCWG.It usually needs adding catalyst to achieve a desired carbon gasification effiency(CGE)in the SCWG of lignite.The homogeneous catalyst,such as alkaline catalysts are usually used to decrease the reaction temperature.However,alkaline catalysts also have problems that it is difficult to be recovered and they aggravate the corrosion of the reactor.Therefore,it is necessary to exploit the application of heterogeneous catalysts.A series of Ru/CeO2-ZrO2 catalysts were prepared and employed to catalytic gasification of lignite in supercritical water with mini-batch reactors.The effects of Ru content,catalyst loading,reaction temperature,reaction time and lignite concentration on the gasification were systematically discussed.The results indicated that a CGE higher than 80%can be achieved at a moderate temperature 500 ? by adding Ru2/CeO2-ZrO2 catalyst.The kinetic plots in terms of CGE indicated that it is possible to use the pseudo-first-order reaction model to investigate the gasification of lignite and the activation energy(Ea)was 130±26 kJ/mol.The liquid intermediates formed during SCW of lignite under various operations were further characterized.The results showed that the liquid intermediates are very complex and can be principally grouped as alkenes,alkanes,aromatic hydrocarbons,phenols and oxygen containing compounds.These intermediates were stable and difficult to be gasified without Ru catalyst.While,the oxygen containing compounds and alkanes can be effectively gasified with Ru catalyst even at a relatively low temperature 500 ?.However,the phenol and aromatic hydrocarbons is difficult to be gasified completely,which can be seen as obstacles in the the SCWG of lignite.The stability and recycling of catalyst also studied.The XRD and XPS characterization of used catalyst showed that the Zr4-adding improved the stability of CeO,,but Ce(CO3)(OH)was also observed in the used catalyst,which contributed to some loss of catalytic efficiency.Ruthenium supported on graphitic carbon black(Ru/GCB)was prepared further for catalytic gasification of phenol(model liquid intermediate from SCWG of lignite)in SCW,with the expectation that the GCB would be stable in SCWG.It was found that a nearly complete gasification of phenol was achieved at a moderate temperature of 500 ? and 32 min.However,the lignite can be gasified under same conditions due to the formation of aromatic hydrocarbons.The catalyst recycling experiments showed that the CGE can be maintained at higher than 93%in 7 consecutive runs.The characterization of used catalyst indicated a mild aggregation of Ru particles occurred,which contributed to slight loss of catalytic activity.However,the XRD,N,physisorption,Raman spectroscopy and HR-TEM characterization of used catalyst suggested that GCB was very stable in SCWG,which can provide an option for the catalyst carries in SCWG.In order to overcome the disadvantage of the high price of noble metal,a series of Ni-M(M=Co or Zn)bimetallic alloy nanoparticles supported on graphitized carbon black were further synthesized and examined as catalysts for gasification of phenol.The results showed that a nearly complete gasification of phenol can be achieved even at a low temperature of 450 ? wth the noble-metal-free bimetallic alloy nanoparticle catalysts,indicating that the reaction temperature decreases by 50 ? for complete gasification of phenol compared with Ru/GCB.The NiCo alloy nanoparticles catalyst favored the production of H2 while NiZn alloy nanoparticles catalyst favored the CH4 production.The characterization of used catalysts suggested that the severe agglomeration of nanoparticles resulted in the great deterioration of their gasification activities.If the agglomeration can be overcomed in the future,the noble-metal-free bimetallic alloy nanoparticles with excellent activity are also expected to provide an alternative to precious metals catalyst.