Effect Of Torrefaction Pretreatment On The Pyrolysis Characteristics Of Nitrogen-rich Biomass And Its Phosphoric Acid Activation For The Production Of Carbon Materials

Fast pyrolysis of nitrogen-rich biomass can be used to produce bio-oil and nitrogen-doped carbon materials.The nitrogen containing substances in the pyrolysis bio-oil are important precursors of some fine chemicals,The heteroatoms in nitrogen-doped carbon materials can significantly change its surface active sites and electrochemical properties.The nitrogen-doped carbon materials can thus be widely used as catalytic materials and electrochemical energy-storage materials.At present,the direct pyrolysis of biomass usually provides bio-oil with low quality and low char yield.Torrefaction can be used as an effective pretreatment method prior to pyrolysis of biomass for increasing the char yield and improving the quality of bio-oil,which is beneficial to the further efficient application of the pyrolysis products.In this study,two representative nitrogen-rich biomasses:nitrogen-rich wood waste and chlorella sp.were selected and first subjected to torrefaction pretreatment in a fixed bed reactor.The effects of torrefaction severity on the physicochemical properties of biomass were explored by using elemental analyzer,oxygen bomb calorimeter,and Fourier transform infrared spectrometer.The influences of torrefaction severity on the pyrolysis characteristics and the release of volatile products of biomass were systematically investigated by thermogravimetry-Fourier transform infrared spectroscopy(TG-FTIR)and pyrolysis gas-chromatography/mass spectrometry(Py-GC/MS).The pyrolysis kinetic parameters were calculated by using the distributed activation energy model(DAEM).Finally,the carbon material(activated carbon)was prepared through chemical activation of torrefied biomass with phosphoric acid.X-ray photoelectron spectroscopy(XPS)and a fully automatic specific surface area and pore size analyzer were employed to investigate the surface functional group changes and pore size distribution of activated carbon.The results show that torrefaction pretreatment of nitrogen-rich wood waste and chlorella sp.mainly proceed dehydration reactions to decrease their oxygen content and increase their relative energy density,thus resulting in the reduction in the formation of water during subsequent pyrolysis.Pyrolysis kinetic analysis demonstrate that torrefaction pretreatment lowered the pyrolysis activation energy of nitrogen-rich wood waste,and the torrefied nitrogen-rich wood waste obtained at 250°C exhibited the lowest pyrolysis activation energy,while torrefaction pretreatment increased the pyrolysis activation energy of chlorella sp.In addition,torrefaction pretreatment effectively reduced the generation of NO_x precursors during the pyrolysis of nitrogen-rich wood waste,whereas torrefaction pretreatment increased the formation of NO_x precursors during the pyrolysis of chlorella sp.Torrefaction is beneficial to increase the formation of levoglucosan within bio-oil obtained from fast pyrolysis of nitrogen-rich wood waste.As the pyrolysis temperature of torrefied nitrogen-rich wood waste increased from 500°C to 900°C,the bio-oil is prone to undergo secondary cracking reactions,such as ring opening and deoxygenation,to form a large amount of acetaldehyde,acetic acid,acetone,hydroxyacetone,alkenes,benzene,toluene and cyclopentanones,etc.Bio-oil obtained from fast pyrolysis of chlorella sp.contained more nitrogen-containing compounds,e.g.,indoles,pyrroles,and quinolines.As the torrefaction temperature increased,the proportion of heterocyclic compounds within bio-oil increased.When torrefied chlorella sp.was pyrolyzed at 900°C,it mainly produced light aromatics and olefins,and almost no oxygen-containing compounds were observed.It is found that torrefaction of nitrogen-rich wood waste cannot effectively increase the specific surface area of its derived char,but torrefaction can reduce the average pore diameter of char.After torrefied nitrogen-rich wood waste was chemically activated by using phosphoric acid,the total pore volume and specific surface area of carbon material increased,and the average pore diameter decreased.When the impregnation ratio of phosphoric acid to the torrefied nitrogen-rich wood waste was 2.5,the resulting carbon material had the best performance,and its total pore volume,specific surface area,and the average pore size were448.60 mm~3/g,761.611 m~2/g,and 2.36 nm,respectively.Torrefaction of chlorella sp.at 250°C for 90min combined with subsequent pyrolysis at 500°C for 2 hours without chemical activation can provide the carbon material with excellent textural properties,its total pore volume,specific surface area,and average pore diameter are 550.30 mm~3/g,974.990 m~2/g and2.26 nm,respectively.On the contrary,the activation of torrefied chlorella sp.with phosphoric acid decreased the total pore volume,specific surface area of carbon material,and increased its average pore diameter.