Preparation And Study Elmental Mercury Adsorption Characteristics Of High Surface Area Biomass Chars

Mercury emissions from coal-fired power plants are the main sources of anthropogenic mercury emissions.Its emission reduction technology is another hotspot after SO2 and NO emission controlling.Activated carbon injection technology is a more mature mercury removal technology.But the use of activated carbon is limited by its mercury adsorption capacity and cost.Therefore,research and development of high adsorption capacity,low cost mercury removal adsorbent is the key point for the technology applied in practice.Biomass resource is widely available and the cost is low.Its main component is carbon,so it is a kind of raw material of mercury removal adsorbent with great potential.Biomass pyrolysis chars,KOH activating chars,and halogen modified chars were prepared by pyrolysis-activation experimental facility,respectively,using four kinds of biomass which are walnut shell,wheat straw,corn stalk and corncob.The structure,internal functional groups and elemental mercury adsorption properties of these biomass chars were studied by means of fixed bed mercury adsorption test bed,specific surface area analyzer,scanning electron microscope(SEM),Fourier transform infrared spectroscopy(FTIR)and X-ray diffraction analyzer(XRD)The results of characterization data are that:(1)After activation,the specific surface area of the chars increased significantly.With the increase of activation temperature,activation time and the ratio of alkali-carbon,the specific surface area of activated chars increases first and then decreases.The optimum preparation conditions are activation temperature,800 ?,activation time,60 min,the ratioof alkali-carbon is 3: 1.The specific surface area of activated chars after halogen modification will decrease.(2)The results of SEM show that the surface of biomass chars are more smooth after activation,and the modification of halogen impregnation does not destroy its surface structure.(3)The biomass chars contains O-H functional groups and C = C functional groups from the carboxyl and phenolic hydroxyl groups,and C-O-C functional groups from the lactone groups,as well as possibly C-N functional groups.The content of activated chars functional groups after halogen modification is increased.(4)XRD results showed that the irregular graphite microcrystals in the biomass chars were regularized and the degree of graphitization increased after activation.While the internal structure of the activated chars after halogen modification is re-broken,reducing the degree of graphitization.Fixed bed mercury removal experiments show that:(1)The mercury removal efficiency decreased with the increase of the adsorption temperature.However with the increase of inlet mercury concentration,the mercury removal efficiency of adsorbents decreased,but the mercury adsorption capacity of the adsorbents per increased gradually.(2)The adsorption capacity of biomass pyrolysis chars is the smallest,the maximum only 5.42?g/g.The mercury adsorption capacity of activated chars prepared by the best preparation condition was 109.34?g/g.The capacity of mercury removal is in the order of wheat straw activated chars> corn stalk activated coke> walnut shell activated coke> corncob activation coke.Some of the metal elements in the ash contained in the biomass or the remaining element K may act as a catalyst for the mercury removal.(3)The mercury removal ability of activated chars after halogen modification is further improved.The ability of modifier is NH4Br> KBr> KCl.The maximum adsorption capacity of activated chars after modification with NH4 Br is 235.28?g/g for two hours experiment.(4)Temperature-programmed desorption experiments show that the desorption temperature of activated chars is between 180 and 700 ?,which is mainly chemical adsorption.Among them,the mercury adsorption site of walnut shell activated char may be lactone groups,and the mercury adsorption site of activated chars of wheat straw and corncob could be carbonylgroups.After modification with NH4 Br,the Br-in halogen binds to the C atoms in the different functional groups in the charsmay be the resean of mercury desorption temperature increasing.