Study on Torrefaction Kinetics and Gasification Characteristic of Typical Agricultural and Forest Residues and Sewage Sludge

Author:Huang You Wang

Supervisor:chen mei zuo


Degree Year:2019





Developing an efficient pretreatment equipment of biomass and its subsequent energy utilization system is an important guarantee for its conversion to green low-carbon energy and chemicals.The low quality of biomass feedstock limit its low-cost and large-scale applications,and can also lead to inefficiencies in subsequent processes and equipment operations.Torrefaction,also known as mild pyrolysis,can greatly improve its fuel quality,making it easy for transportation,storage and subsequently diversive applications.Torrefaction kinetics is an important basis for optimal design and operation of the torrefaction reactor.In addition,innovative application of torrefied biomass is also an important aspect for its efficient use.Therefore,it is crucial to systematically reveal the torrefaction kinetics of biomass and to develop high-quality utilization schemes for torrefied biomass.This work will comprehensively study the isothermal/non-isothermal torrefaction mechanism of bioamss/sludge,and explore new ways to efficiently utilize torrefied biomass in order to conduct full-chain research on biomass processing and reuse.The main contents of this study include non-isothermal torrefaction kinetics of biomass,isothermal torrefaction kinetics of biomass,neural network model to evaluate biomass torrefaction kinetic parameters,and co-gasification scheme and system design of biomass and sludge.Based on complex reaction kinetics theory and thermogravimetric experiments,nonisothermal and isothermal torrefaction kinetics of biomass were studied.Lignocellulosic biomass(cotton straw and eucalyptus bark)and non-lignocellulosic biomass(sludge)experienced three stages of dehydration,coexistence and devolatilization during torrefaction.Considering the mass yield and energy yield,the effective torrefaction temperature range of the sludge was evaluated.From the perspective of kinetic model,the model-free distributed activation energy model based on differential equation was further developed.Parametric research method was used to reveal the interaction among three parameters in the Model-fitting distributed activation energy model.The results show that there is a compensation effect between the average activation energy and the pre-exponential factor,and the activation energy standard deviation is independent of the average activation energy and the pre-exponential factor.By using the mathematical statistics method,an advanced kinetic parameter evaluation scheme was proposed.This scheme can simultaneously ensure the prediction ability of the model and obtain the real kinetic parameters,which has been successfully applied to the biomass torrefaction kinetics.In terms of isothermal torrefaction,a new isothermal kinetic model was developed based on the multi-step reaction kinetics mechanism.This model can not only predict the torrefaction reaction process well,but also predict the final conversion degree of the reaction.In order to reveal the sensitivity of the torrefaction reaction with the torrefaction temperature,the reaction degree index was proposed.This index was successfully used to identify that the reaction progress of lignocellulosic biomass at region of high torrefaction temperature and long torrefaction time was greatly affected by reaction temperature,and sewage was greatly affected by reaction temperature at low torrefaction temperatures.In addition,a generalized regression neural network model was used to simultaneously determine thermal-induced solid-state reaction kinetic parameters(activation energy and pre-exponential factor)and mechanism model.The non-quantitative mechanism model was numbered by using a five order 0-1 code.Together with the activation energy and the pre-exponential factor,it was used as the output data of the neural network model.The reaction temperatures at different conversion rates are input data.The results show that the activation energy,pre-exponential factor and mechanism model can be obtained by using the nonisothermal experimental data at less than three heating rates.This method has been successfullly to determine the kinetic parameters and mechanism model of biomass torrefaction.Considering the intrinsic synergistic effect during co-gasification of torrefied biomass and wet sludge,a scheme for co-gasification of sludge and torrefied biomass was proposed to produce hydrogen-rich syngas.Thermodynamic equilibrium model was used to theoretically explore the effect of torrefaction extent,mixing ratio and gasification temperature on carbon precipitation boundary,carbon conversion rate,dry syngas composition and hydrogen production.The results show that when the gasification temperature is higher than 900K,the torrefaction extent has little effect on the maximum hydrogen yield,but mixing ratio of sludge corresponding to the maximum hydrogen yield becomes larger.From the perspective of system design,an autothermal drying-gasification system of the torrefied biomass and sludge was established.The energy balance and thermoeconomic of the system were evaluated.Exergy destruction of the entire system occurs mainly in dryers,gasifiers and burners.As the gasification temperature increases,the unit exergy costs of both the fuel(input)stream and the product(output)stream of each component increase.In addition,an extended exergy cost allocation method based on energy quality was developed,which can be used in the cascade untilization process of coupling physical enthalpy and chemical exergy.The unit exergy cost of the high-temperature syngas cascade utilization was calculated by means of this method,satisfiying the principle of high quality-high price.This study can not only provide basic data and feasible solution for the design and optimization of biomass torrefaction reactor and its gasification utilization,but also provide theoretical support for solid state chemical reaction kinetic analysis.