| Efficient utilization of biomass resources is crucial for providing renewable energy and mitigating the risk of environmental pollution caused by crop straw burning in China.But the majority of these resources comprise smaller particles in loose form,which makes them unsuitable for direct combustion and long distance transportation.Densification of these loose and smaller biomass resources is an attractive option,and the need for this transformation has been widely justified.Straw rotary extrusion forming is a convenient densification technology to make the low density biomass into densified solid biofuels;however,the high energy consumption and low lifetime of ring-die of this process are still challenges.To improve the production efficiency and product quality,mechanical property of compacted material,modeling of energy consumption,wear mechanism of ring-die and pretreatment of raw material were studied through theoretical analysis and experimental investigation,and the influencing rules of each factor to the densification process were also explored.The main research works in this dissertation are as follows:Basic physical and chemical property parameters of rice straw,such as composition,bulk density,friction coefficient,combustion heat value and thermal decomposition characteristics were obtained firstly.In addition,the basic principle of biomass rotary extrusion forming and bonding mechanism were also studied.The straw rotary extrusion process was divided into a compression deformation stage and an extrusion forming stage,and the rheological process of straw material in these two areas were discussed.Based on the requirements of biomass densification experiment,an extrusion testing platform was designed and established.Then the rules and effects of moisture content,granularity,forming pressure,temperature,forming speed and ring-die length to diameter ratio on product density,tensile strength,energy consumption and extrusion force were analyzed.Mechanical models describing the compacting process in compression deformation stage were established both for smashed and chopped materials.Based on the RSM experimental results,the mechanical model describing extrusion force in extrusion forming stage and quality evaluation model describing density and tensile strength were also generated.Experimental verification results showed that the prediction error of extrusion force model,product density and tensile strength model were 4.66%,1.77%and 3.25%respectivly.According to the grab condition of rice straw,the productivity model was built up and the related factors were also analyzed.When the accurate partition of compacting process was determined,the stress of roller was calculated and the corresponding energy consumption model was established.Mechanical modeling based on quadratic polynomial and exponential model were considered for the calculation of pressure in the torque model.A calculation case was then conducted according to the theoretical models,and actual productivity and torque testing were performed for validation purposes.Analysis of the results showed that smaller diameter ratio,larger friction angle,higher material density and larger mould size corresponded to higher production efficiency.Results indicated that this deduced productivity model is successful because the deviation is 4.52%and torque model determined by the calculation model based on exponential model had a better accuracy with a deviation of 3.01%.The wear behavior of ring-die was studied based on experimental testing and theoretical analysis.It was found that abrasion mainly occured in the middle part and lower side of the die,and abrasion gradually weakened along the feed direction.Hard abrasive wear played a leading role in the wear process.According to the testing results,a calculation model of this abrasive wear was deduced based on fractal theory,it was indicated that an optimum fractal dimension could minimize the wear rate.Finite element simulation results showed that die hardness played a more important role than forming speed to the die wear.The optimum heat treatment hardness of 45 steel ring-die was 47.9 HRC.When wear resistance of the ring-die was evaluated by means of maximum wear depth,surface contour curve,surface roughness and fractal dimension,testing results showed that the material wear-resisting performance order was 9SiCr>65Mn>42CrMo>T10A>45 steel.Considering wear-resisting performance and production cost,42CrMo is an ideal ring-die material.A new kind of compound modification method for rice straw was presented in this dissertation:the biomass was pre-treated by two different thermal ways,torrefaction and hydrothermal carbonization respectively,followed by adding spent fuel and binder.Then the energy consumption and product quality were studied through deeply analysis and engineer testing.Results showed that energy consumption of the compound modification material significantly decreased,and the calorific value increased markedly;product density and tensile strength increased with the increasing additive proportion of colza cake and decreased with the increasing additive proportion of paraffin,crude glycerine and waste engine oil.These pretreatment ways of raw material might affect the tensile strength to some extent.Energy consumption of the hydrothermal compound modification pretreated rice straw was reduced by 51.35%compared to the unpretreated materials,while the product density increased by 5.52%,calorific value increased by 70.85%and tensile strength meet the usage requirement.It also was obtained that the emergy yield rate of torrefaction compound modification treatment was 2.848,while the single modification treatment was 2.480,and hydrothermal carbonization compound modification treatment was only 1.475.The emergy conversion rate of torrefaction compound modification treatment was 4.566 set/J,which was very close to the coal energy conversion rate 4 set/J.It is indicated this treatment solution has higher ratio of performance to price,the densification fuels produced in this way will be a good substitute for coal. |
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