Regulation Mechanism Of Biochar Production By The Combination Of Oxidative Torrefaction And Pyrolysis Of Rice Straw

The heavy metals can be spread into farmlands through water runoff and atmospheric sedimentation as mining and smelt,and then contaminate the soil,resulting in the accumulation in plant as the growth of crops.Recently,the Cd-contaminated rice has attracted many attentions.However,those attentions are always focused on the transition resistance of Cd into rice,which are lack of disposal of heavy-metal content rice straw.In the form of increasing attentions to environmental hygiene and public safety,the synergistic control of product quality and safety in the process of resource utilization of heavy-metal-content rice straw should also be paid attentions to.In view of the actual demand of soil amendment for heavy metal pollution site remediation project in mining area of Hunan province,the pyrolysis of heavy metal straw to prepare biochar can not only be used as the main formula of cheap soil improver,but also can"return"the source of heavy metal straw,which is a potential resource and safe disposal technology for heavy-metal-content rice straw.However,how to establish the synergistic regulation between the quality improvement and pollution resistance of heavy metal straw is the key scientific issue to ensure the economy and safety of the technology.In this study,heavy-metal-content rice straw was collected as raw material to investigate the influence mechanism of oxidative torrefaction coupled pyrolysis process on the physicochemical properties and quality of the prepared biochar.The mechanism of quality improvement and pollution resistance control in the preparation of biochar by oxidative torrefaction coupled pyrolysis of heavy metal straw was revealed.The main results are described as following:The torrefaction temperature is the main factor affecting the physicochemical properties of torrefied solid products during atmospheric-pressure（AP）torrefaction,while increasing oxygen content in carrier gas can slightly enhance the weight loss,higher heating value（HHV）and surface color during torrefaction.The torrefied solid product obtained from the torrefaction with oxygen content of 9%in carrier gas is significantly different from the torrefied solid product under pure nitrogen atmosphere in physicochemical properties,resulting in lower solid yield,energy yield,O content and hemicellulose relative content,as well as higher energy density,C content,HHV、hydrophobicity,surface color（darker）.The composition of liquid products from AP torrefaction is complex as alcohols,phenols and esters,which is more complex with the increase of torrefaction temperature.During gas-pressurized（GP）torrefaction,secondary reactions occur between the solid product in the reactor and the volatiles formed during torrefaction,which obtains higher weight loss of feedstock at low temperature.Therefore,the effects of temperature and carrier gas on the properties of the products are weak.Meanwhile,compared with AP torrefaction,the solid yield,energy yield,volatile matter,H content and O content of solid phase products from GP torrefaction are lower,while the energy density,fixed carbon,ash content,C content,HHV and packing density are higher.The composition of liquid products from GP torrefaction is mainly phenolic compounds,accounting for more than 70%of them.The L^*、a^*and b^*of surface color of AP and GP torrefied solid products show a good correlation with the yield of torrefied solid products,HHV,bulk density,combustion characteristics（ignition point,burnout temperature,flammability index and combustion characteristic factor）,which provides a theoretical basis for rapid prediction of quality of torrefied solid product using CIELAB color changes in industrial applications.The increase of AP and GP torrefaction intensity can significantly improve the activation energy（E）.The E of torrefied solid product from GP torrefaction is always higher than that from AP torrefaction under the same reaction conditions.After AP and GP torrefaction,the ignition point and burnout temperature of solid products is increased,while the combustion characteristic factor and flammability index is decreased.The combustion parameters can also establish a good correlation with the L^*,a^*and b^*of the surface color of the torrefied solid product.It is also feasible to assess the combustion performance of the torrefied solid product by their surface color.Torrefaction pretreatment does not significantly change the types of functional groups on the surface of biochar obtained from pyrolysis.However,it can improve its pore structure,increase the specific surface area and pore volume,and thus provide more effective sites for adsorption.Furthermore,straw biochar takes SiO₂ as the skeleton,of which the mineralization process is not affected by torrefaction pretreatment.The adsorption capacity of direct pyrolysis biochar（B1）,AP torrefaction coupled pyrolysis biochar（B2）and GP torrefaction coupled pyrolysis biochar（B3）is shown as following B2>B1>B3.The adsorption capacity of those three biochars for Pb²⁺was higher than Cd²⁺.The adsorption capacity in a single system is higher than that in a mixed system.It is shown by the adsorption kinetics and isotherm test that the absorption process is in accordance with the pseudo-second-order kinetic model and Langmuir isotherm model,indicating that adsorption involves monolayer coverage in which chemisorption dominates the adsorption process.It is also shown by the XRD、FTIR and XPS analysis that ion exchange,complexation,precipitation and cation-πinteraction are the main adsorption mechanisms.The maximum adsorption capacity of B2 to Pb²⁺and Cd²⁺is 152.42 mg/g and 50.03mg/g.,respectively,indicating that adsorption performance of biochar from oxidative AP torrefaction coupled pyrolysis is better than that of biochar from direct pyrolysis.Therefore,oxidative AP torrefaction coupled pyrolysis is a promising process for biochar materials.It can be shown by pot cultivation experiments showed that Cd²⁺had a remarked stress effect on germination and growth of Robinia pseudoacacia seedlings.The plant height,root length,dry fresh weight and root growth were less than 50%of the control group,and the root system was especially seriously poisoned.Meanwhile,the content of malondialdehyde（MDA）in plants is increased rapidly,while the Cd content in plant and root surface iron membrane was much higher than that in other groups.With the additive of biochar,the toxic effect of Robinia pseudoacacia is reduced gradually,which made the plant height,root length,dry fresh weight and root growth increase significantly.The MDA content is decreased,while the Cd content in the above ground and underground parts of the plant is also decreased significantly.The growth index of the treated group with the largest additive of biochar exceed that of the Cd stress group,indicating that after absorption of Cd²⁺by the biochar,the free Cd²⁺in the system can induce Robinia pseudoacacia for the promotion of growth.As the enhancement of torrefaction intensity,heavy metals in straws are accumulated in torrefied solid products,while the aromatization of GP torrefied products further promotes the immobilization of heavy metals.The bioavailability of Cd,Cr,Cu,Zn and Pb are decreased significantly.The main fractions are transferred from F1 and F2 to F3 and F4.TCLP toxic leaching and the amount of heavy metal leaching under different pH conditions are decreased.The torrefaction process enables to converse Zn of extremely high risk,Cd of high risk and Cu and Pb of moderate risk in the feedstock to Zn and Cd of moderate risk,Cu of low risk,and Cr of no risk.The potential environmental risk index（RI）is decreased gradually,and decreased to low potential environmental risk in the GP-9-270 of the highest torrefaction intensity.Therefore,torrefaction treatment of heavy-metal-content rice straw can fix heavy metals,which is of great significance to reduce its potential environmental risk.