| The greenhouse gases emitted during the exploitation and utilization of fossil energy cause a series of climate problems.The utilization of renewable energy is one of the main ways to reduce carbon emissions.Biomass is a kind of zero-carbon,clean renewable energy,and its large-scale utilization will play an important role in the future low-carbon economy.Biochar produced by biomass pyrolysis can be widely used in soil remediation,gas purification,wastewater treatment,product synthesis,etc.In the pyrolysis process,the organic matter in biomass has pyrolysis,polycondensation and other reactions,while inorganic minerals also undergo a complex process of ash formation.In the thermal conversion process,alkali metals and alkaline earth metals with high content not only have their own mineral evolution process,but also participate in the pyrolysis reaction of organic matter.Therefore,inorganic minerals in biomass can affect the properties of final products through both effects.The catalytic cracking effects of alkali metal/alkali earth metal on lignin,cellulose and hemicellulost were mainly observed before 600 ℃.The mineral evolution process of biomass itself exists in both low and high temperature conditions,especially the melting behavior of minerals at high temperature,which has a significant effect on the structure of biochar products.In this paper,the difference of melting behavior of biomass ash was investigated by basic water leaching method,and the influence of melting property difference of water leaching ash on the physicochemical structure of biochar and the application of biochar adsorption property was investigated.The main research conclusions are as follows:(1)Water immersion can effectively remove alkali metals and Cl from biomass.The results of ICP showed that after water leaching,the potassium escape from corn stalk(CS)at 800-900 °C decreased,and that from rice stalk(RS)at 900-1000 °C decreased.Water leaching can increase the ash melting temperature,because there are more insoluble alkali earth metals in CS ash,the effect of water leaching on the DT of CS is not as good as that of rice straw(RS).Although the melting temperature of ash was increased by water leaching,SEM characterization showed that the CS and RS ash after leaching still started sintering at about 700 °C,because of the presence of residual potassium,the low temperature eutectic temperature caused by K-Ca-Mg/ K-silicate remained unchanged.The change of dynamic melting behavior shows that water immersion effectively reduces the risk of RS deposition and slagging.The melting process of RS conforms to the typical characteristics of the "melt-dissolution" mechanism,which means serious deposition and slagging trend.After water immersion,RS presents a typical "softening-melting" mechanism,which means that the deposition risk is low.The change of melting behavior of CS after water leaching is not obvious,because the insoluble alkali earth metals Ca and Mg in CS dominate the effect on the silicate structure.(2)Water leaching reduced the inhibition of ash melting coating on pyrolysis,and the biomass pyrolysis was more thorough after high temperature water leaching,so the yield of biochar decreased significantly after water leaching.Due to the reduction of alkaline component content in the biomass after water leaching weakens the catalytic cracking effect of alkaline component on the organic structure of the biomass,and more and higher concentration of functional groups(C-O-C,aromatic C-H)are retained in the straw biochar after water leaching.The specific surface area of biochar increased by 28%-46% at 800℃,the pore structure became more abundant,and the biochar structure tended to be more orderly,because the sintering phenomenon was reduced by water immersion.(3)By comparing rice straw biochar pyrolysis at 800 ℃ for 10 minutes before and after water leaching,it was found that under acidic conditions,RS carbon and WRS carbon had good adsorption and binding force on phosphorus due to the electrostatic attraction between metal oxides or hydroxyl groups and phosphate in solution after protonation.Due to the more developed pore structure and larger specific surface area of biochar after water leaching,it can provide more active adsorption sites and accelerate the internal diffusion of phosphate.Therefore,the biochar after water leaching uses less carbon to achieve the same adsorption effect,and the time to reach the adsorption equilibrium is shorter,which saves the adsorption cost and improves the adsorption efficiency.When the adsorption takes place under the conditions of higher phosphate concentration and higher adsorption temperature,due to the driving force of high concentration gradient and the effect of high temperature to accelerate ion movement and improve ion activation energy,the phosphate adsorption capacity of biochar is significantly improved and the adsorption effect is better. |
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