Study On Production Of Biochar In Fluidized Bed Reactor And Adsorption Performance

Biomass energy ranks fourth in the world’s total energy consumption and occupies an important position in the entire energy system,especially the energy utilization of agricultural and forestry wastes,which has important strategic significance for ensuring the sustainable development of my country’s economy,society,and environment.Biomass pyrolysis technology for carbon production is an important biomass thermochemical conversion technology,but there are still problems such as discontinuous production,dependence on external energy sources,and serious secondary pollution.This thesis used a bench-scale fluidized bed to study the properties of pyrolysis production by agricultural and forestry wastes;and to investigate the adsorption performance of modified,activated,or original biochar as adsorbents for different adsorbates in different adsorption environments;and to explore the adsorption mechanism of adsorbents qualitatively and quantitatively.The machine learning method was used to rank the relative importance of the influencing factors,and further determine the impact of key factors on Pb²⁺adsorption performance.The above studies could provide reliable operating parameters for biochar production by fluidized bed reactor and follow-up application guidance of biochar products.Corn stalks,rice stalks,rice husks,and sawdust were used as raw materials to conduct pyrolysis in a bench-scale fluidized bed reactor to produce biochar.The types of raw materials have an important influence on the yield of biochar from biomass pyrolysis.Among them,the yield of corn stalks biochar（CSBs）is the highest.With the increase of pyrolysis temperature,the H/C and O/C ratio decreased significantly,and the intensity of aromaticγ-CH peak increased,indicating that the degree of biomass carbonization and aromatic carbon structure continuously increased.The participation of O₂ in a low-oxygen atmosphere caused the transfer from the solid phase and liquid phase products to the gas phase products;the increase of the oxygen-containing compounds contents and polymerization degree of tar;the loss of alkyl group on the aromatic structure and the strengthen degree of aromatization of biochar.With the increase of O₂ concentration,the total energy yield of the three-phase products and the energy yield of tar showed a downward trend,while CSBs and rice husk biochar（RHBs）reached the highest char energy yield when the O₂content was 4%,accounting for 45.03%and 41.67%to the total energy yield,respectively.Based on the study of bench-scale fluidized bed pyrolysis process,when the electric heating method is replaced by the external jacket heating method using circulating flue gas,a combined internal and external heating fluidized bed system is obtained.When system running continuously for 24 hours at 4%O₂ concentration with of feeding rate of 1040 kg/h,the internal and external heating can save 6321.10 k W·h and 5279.15k W·h of power consumption,respectively.The biochar produced was modified with NH₄Br to investigate its removal performance of Hg⁰ in flue gas.The contents of C=O groups and COOH groups on the surface of the Br-modified biochar increased,showing a better affinity for Hg⁰.The increase of pyrolysis temperature and oxygen content of pyrolysis atmosphere was beneficial to the removal of Hg⁰,and the increase of oxygen concentration promoted the increase of the covalent halide functional group C-Br peak area ratio.In the process of removing Hg⁰ by Br-modified biochar,Hg⁰ formed Hg Br₂ under the action of C-Br oxidation and C=O groups on the adsorbent surface would oxidize Hg⁰ to Hg O.The modified biochar is more suitable for the pseudo-second-order kinetic model,and its activation energy is-41.52 k J/mol,indicating that the chemical adsorption is the dominant process.The Hg⁰ adsorption reaction is a spontaneous endothermic reaction.O₂ in the flue gas could promote the formation of carbon-oxygen complex compound and supplement the consumed surface oxygen（O*）,which improved mercury removal performance;SO₂ competed with Hg⁰ to form sulfate or H₂SO₄,and form Br₂O_S with Br to destroy bromine-containing functional groups and inhibited the adsorption of Hg⁰;NO generated active NO₂ and NO⁺,which promotes the adsorption of Hg⁰.KOH and KOH-Na₂EDTA were used to activate biochar,and the adsorption performance of biochar before and after activation to phenol was studied.The pore structure of rice straw activated biochar（ACRSBs）were the best,and the optimum mass ratio of RSB500:KOH:Na₂EDTA is 1:2:2.The adsorption of phenol on the activated biochar is dominated by chemical adsorption（π-πEDA）,partial physical adsorption（hydrogen bond andπ-πdispersion）and solvent effect.KOH activation significantly increased the contents of C-OH groups and C-O groups,which is conducive toπ-πEDA effect and hydrogen bonding effect.The participation of Na₂EDTA can not only promote KOH activation to increase the content of oxygen-containing functional groups（OFGs）on the surface of the adsorbents,but also introduce N-containing functional groups on the surface to promoteπ-πdispersion.The Pb²⁺adsorption performance of the original biochar was studied,and the rice stalk biochar（RSBs）shown the largest adsorption capacity.The best Pb²⁺adsorption conditions were obtained:initial Pb²⁺concentration of 100 mg/L,adsorption contact time of 12 h,adsorption temperature of 25℃,Na⁺ion concentration of 0 mol/L,initial p H of 5.5 and biochar dosage of2 g/L.In the Pb²⁺adsorption process of biochar,a single layer of adsorption occurs on the surface of the biochar and is mainly affected by chemical adsorption.As the pyrolysis temperature increased,the contribution ratio of the complexation with OFGs decreased due to the drop of total acidity in biochar.Under both inert and low-oxygen atmosphere,the role of inorganic minerals including ion exchange and mineral precipitation were shown as the main adsorption mechanism（relative contribution of 66.40%-90.99%）.When the degree of oxidation increased,the contribution of ion exchange by CSBs and RHBs shown completely different trends due to their different metal,silicon content and morphological structure.Compared with commercial activated carbon or modified biochar obtained by the preparation methods,the above three kinds of biochar,whether NH₄Br modified biochar for Hg⁰ removal in flue gas,KOH and KOH-Na₂EDTA activated biochar for phenol removal or original biochar for Pb²⁺removal,all exhibited superior adsorption performance.In particular,the untreated raw biochar showed superior adsorption performance of Pb²⁺,standing out among the three types of biochar due to its advantages of simple operation,high efficiency,and low cost,and showing great potential for practical industrial application.Finally,three types of machine learning methods:Artificial Neural Network（ANN）,SVR（Support Vector Regression）and RF（Random Forest）,were used to predict the Pb²⁺adsorption capacity on the biochar produced by the fluidized bed.Compared with the ANN and SVR models,the RF model showed better prediction performance（R²=0.984）.Furthermore,the RF model showed superior generalization ability to the ANN model on datasets with higher adsorption capacity.The results indicated the adsorption conditions provided about 90%of the contribution for adsorption capacity,especially biochar dosages accounting for 80.09%,which were the most affecting features based on the RF model.The element compositions were determined as the most important factors in affecting the biochar properties on adsorption performance,accounting for 66.20%of importance.