Study On The Quality Promotion And Bonding Mechanism Of Agricultural Residues Pellets

Due to its low energy density,low bulk density,high moisture content,and irregular shape and size,biomass is not conducive to storage and transportation,which limits the development of biomass utilization technology.One of the most effective methods to solve the above problems is to densify the biomass raw materials into pellets or briquettes.However,due to the influence of biomass types and pelletizing parameters,the physical properties of the pellets prepared from agricultural straw biomass are poor,and the research on their improvement is not obvious and comprehensive.Therefore,in order to obtain high-quality biomass pellets,first,this paper studied the influences of the pelletizing parameters on the physical properties of biomass pellets,and revealed the relevant mechanisms.Then,the hydrothermal pretreatment method was used to explore the evolution of physicochemical structures for hydrochar pellets and the bonding mechanisms under the hydrothermal conditions.In addition,new evergy-efficient methods aiming to prepare high-quality biomass pellets were explored by adding additives and mixing different biomasses into the raw materials.Finally,the effects of hydrothermal carbonization conditions and additives on the combustion characteristics and kinetics of pellets were studied,which will provide scientific basis for the quality optimization of biomass pellets.Firstly,a universal materials testing machine was used to investigate the effects of different feedstock and pelletizing process parameters(e.g.particle size,water content and pelletizing temperature)on the quality of biomass pellet.Then,the key parameters of pelletizing process are summarised and optimised.Moreover,the changes in the microphysical structures of biomass pellet were studied to explore the bonding mechanism of the biomass pellets.It was found that the water content of 10-15% eased the pelletizing process.At the same pelletizing pressure,the better quality of pellets was achieved with smaller particle size(<0.6mm)and higher pelletizing temperature(170?).In addition,due to the differences in chemical composition and structures for various biomass feedstocks,the pellet quality also varied broadly.The biomass pellets from cotton stalk with high lignin content had the best quality among all feedstock in our study,while those from rape straw rich in extractives had the lowest quality.Generally,the biomass pellets densified at room temperature showed lower quality than those densified at high temperature.To reduce the impact of aliphatic hydrocarbons in biomass on pellet properties,the biomass feedstock was pre-treated with carbonisation using hydrothermal reactors.The physical properties of hydrochar pellet from different hydrothermal temperature and holding time were evaluated.The effects of hydrothermal pre-treatment on biomass pellet quality and bonding mechanisms were investigated.It was found that the hydrothermal pre-treatment increased the energy density and hydrophobicity of the pellets due to the reduction of extract content and hydrophilic functional groups in biomass.Moreover,the increase in the number of polar functional groups on particle surface formed the liquid bridge between the particles and strengthened the adhesion force,thereby improving the quality of pellets.In addition,when the hydrothermal temperature is 240? and the hydrothermal holding time is 2h,the hydro-char pellet has the best quality characteristics.However,further increasing hydrothermal temperature and hydrothermal time will lead to the increase of void size and pore volume,resulting in the decrease of the mechanical properties of hydro-char pellets.Also,the surface roughness of the biomass increased the friction between particles after hydrothermal treatment,resulting in increased compression energy consumption of pellets.As the hydrothermal pretreatment would increase the energy consumption of hydro-char pellets,the effects of different additives,additives content and biomass mixture on the bonding characteristics of biomass pellets were invesitigated under the pellitizing process at room temperature The comprehensive evaluation index of the properties of biomass pellets characteristics was also established by the method of principal component analysis.It was found that adding 2% of the modified cellulose was the best to enhance the mechanical property of cotton stalk pellets.The effects of adding bentonite on improving the durability of the cotton stalk pellets were insignificant,while adding lignin would increase the energy consumption for densified cotton stalk pellets.However,lignin had the most significant effects on the quality of wheat straw pellets,of which the best quality was achieved with 2% lignin added.The wheat straw pellets had lower quality with adding bentonite and the lowest quality with adding modified cellulose.However,adding single additives was only effective to improve a part of the mechanical properties for straw pellets.Due to the solid-bridge formed through the cross-linking between long-chain fiber of wheat straw and internal particles of cotton stalk,the biomass pellets from wheat straw-cotton stalk mixture reached superior physical characteristics with wheat straw content of 20%~30% compared with sawdust pellets.In addition,composite additives were added into biomass feedstock to investigate the synergistic effects between different compounds in the composite additives on the physical characteristics of biomass pellets.It was found that the mechanical property of cotton stalk and wheat straw pellets was enhanced with increasing content of modified cellulose and bentonite in the composite additives.This is mainly due to the formation of hydrogen bond between OH in cellulose molecule,water molecule and OH.Also,the effect of ion exchange between water molecule in cotton stalk and bentonite enhance the bonding strength.And the weakly bounded cation of bentonite in the spatial structure promotes the adsorption of modified cellulose on the surface of bentonite and strengthens the force between the two substances.It was observed that the mechanical property of wheat straw pellets was improved negligibly with composite additive A3(cellulose,bentonite,alumina ratio 3/3/4)compared to that with additive A2(2/3/4).Combined with the comprehensive evaluation index of biomass pellets characteristics,the addition of compound additives can achieve a compromise between the improvement of the quality characteristics and the low energy consumption of the biomass pellets,which even exceeds that of wooden pellets.The pellets with the better physical and mechanical properties were selected to investigate the effects of hydrothermal pretreatment and additives on their combustion characteristics.The kinetic mechanism of biomass pellets combustion was also analysed by thermokinetic model.The results show that the weight loss peak of hydrothermal carbon pellets moved to high temperature zone with increasing hydrothermal temperature and hydrothermal holding time during combustion.The reduction of burning rate at low temperature and the increase of burnout temperature both indicated more stable,more complete,higher combustion efficiency,and lower activation energy at low temperature for the hydrothermal carbon pellet than that without hydrothermal treatment.The combustion kinetics study indicates that the combustion of biomass powder followed first-order and second-order reaction at low and high temperature stage,respectively.For the biomass pellets without pretreatment,its kinetic model was mainly diffusion model;for the hydrochar pellets,they followed first-order reaction at low temperature stage and diffusion model at high temperature stage.The volatilization stage overlapped with the fixed carbon combustion stage for biomass pellets when they were burned at low temperature,resulting in an increase in the activation energy.The addition of additives decreased the ignition temperature and burnout temperature of the pellets,resulting in more intense combustion and shorter burnout time.The additives also significantly increased the combustibility index,combustion stability and combustion index,and reduced the activation energy of the pellets at low temperature.However,the composite additive reduced the combustion characteristics of the pellets compared to a single additive.The cotton/wheat mixed pellets showed the best combustion characteristics with when the mixing ratio of cotton stalk and straw is 4/1.