Effects Of Minerals On Carbon Retention And Carbon Stability During Biochar Formation

In recent years,there are wide concerns about using biomass wastes,such as crop residues and livestock excrements,to prepare biochar among domestic and foreign scholars.Biochar is an aromatization carbon-rich substance produced by biomass pyrolysis under anaerobic condition.Biochar has high stability of carbon,which is considered as carbon materials that can effectively solve the greenhouse effect,so there is an important significance for carbon sequestration to prepare high yield,carbon-rich and high stability of biochar.A small amount of metal minerals in biomass play a nonnegligible role in catalysis,and studies have proved that the addition of different kinds in minerals has a significant impact on the process of biomass pyrolysis carbonization,however,few studies about exploring the effect of biomass endogenous and addition of minerals on the biological carbon sequestration are reported.In this work,four different types of biomass,such as peanut shell,barley grass,cow manure and sludge,were chosen to prepared biochar by the methods of acid picking to minerals and additive minerals at 500?,and the effects of biomass endogenous minerals and additive minerals on the pyrolysis process,carbon retention and carbon stability in biochar are investigated.Research conclusions are as follows:Both peanut shell and barley grass in original biomass have KCl,and peanut shell and barley grass have CaCO₃ and NaCl,respectively;cow manure contains more CaCO₃,Ca?COO?₂ and KCl;sludge has more SiO₂as well as CaCO₃ and Ca?COO?₂.Endogenous minerals in peanut shell,barley grass and cow manure have a catalytic role in biomass thermal decomposition,the decomposition temperature of carbon skeleton decreases from 250⁴00?to 200³50?,and they can decrease the activation energy of biomass pyrolysis;But the above results are not observed in sludge due to the complex mineral composition and low carbon content.In peanut shell and barley grass,the typical mineral is KCl;In cow manure,the content of Ca is higher,and Ca exists in the form of CaCl₂ and in other forms;Both KCl and CaCl₂ have a catalytic role in biomass thermal decomposition,and the catalytic performance of CaCl₂ is better than KCl.The decomposition temperature of the biomass after demineralization is lag,but the decomposition temperature increases significantly.The mechanical mixing has no catalytic role,but the mixed impregnation of biomass and mineral solution has a catalytic role in biomass pyrolysis,the catalytic effect of Ca₃?PO₄?₂ is not obvious.Minerals have an effect on the formation of functional groups in the biomass pyrolysis process.The C=C bond strength on the benzene ring in biochar prepared by peanut shell,barley grass and cow manure increases,but the vibrating peaks attributed to CO₃^2-and C-O at 1400 cm^-1 become weak obviously;P-O is eliminated due to acid picking to minerals;The addition of KCl in peanut shell and barley grass after demineralization makes the C=C bond strength on the benzene ring become weak and the vibration of CO₃^2-become strong slightly,.After the addition of CaCl₂ in cow manure after demineralization,the vibration peaks of phenolic hydroxyl become strong obviously and have significant shift,as well as C=C on the benzene ring,CO₃^2-and C-O-C.The two critical factors affecting carbon sequestration of biochar are carbon retention in the pyrolysis process and carbon stability of biochar.The results indicated that endogenous minerals had a negative effect on carbon retention in the pyrolysis process.Though the carbon retention of demineralized biomass tended to increase while no significant changes were observed.With regard to carbon stability,an obvious increase was btained in biochar produced from demineralized biomass except sludge,with the disordering index of I_D/I_G being decreased from 1.28,1.30,and1.48 to 1.27,1.23,and 1.29 for peanut shell,barley grass,and cow manure biochar respectively.At the same time,the oxidized carbon loss of biochar produced from demineralized biomass was decreased significantly respectively,indicating endogenous minerals of biomass decreased carbon stability of biochar.Adding exogenous KCl to demineralized peanut shell and barley grass enhanced carbon retention in the pyrolysis process and carbon stability of biochar,with the oxidized carbon loss being decreased by 44.7%and 27.4%for peanut shell and barley grass biochar.The I_D/I_G of peanut shell and barley grass biochar was increased as well.The carbon retention in the pyrolysis process was remarkably increased when exogenous CaCl₂ was added into demineralized cow manure.However,the oxidized carbon loss of cow manure biochar was also increased,probably due to the dramatically increase in the disorder degree of biochar.The addition of exogenous Ca₃?PO₄?₂ to demineralized cow manure had no obvious effect on carbon retention in the pyrolysis process.The oxidized carbon loss was reduced by 22.4%and the I_D/I_G was decreased from 1.29to 1.18,indicating that exogenous Ca₃?PO₄?₂ enhanced carbon stability of cow manure biochar.The different results obtained from exogenous CaCl₂and Ca₃?PO₄?₂ showed that the species of minerals affected the carbon retention and carbon stability of biochar inevitably.It may be the reason why varied results were observed in the influence of endogenous and exogenous minerals on carbon sequestration of biochar.Overall,the endogenous minerals of biomass catalyzed the pyrolysis process,making the carbon structure more disorderly and less stable.The effect of endogenous minerals on carbon retention depended on mineral content and the pyrolysis conditions.The results indicated that the effect of exogenous minerals on carbon retention and stability of biochar was different,probably resulting from various species of minerals.Minerals containing potassium or calcium promoted carbon retention in the pyrolysis process.Minerals containing phosphorus tended to increase carbon retention and stability of biochar,which might be utilized to regulate the pyrolysis process and improve carbon sequestration of biochar.