| The substrate utilization of livestock manure is an effective treatment plan and comprehensive utilization technology for realizing the effective use of livestock manure.On the other hand,plastic trays are widely used in factory nursery,which are not well recycled and cause serious environmental pollution.If the substrate is molded into a shaped block,the use of the seedling tray can be reduced,and the production cost and environmental pollution risk can be effectively reduced.Composting is a new type of composting method for livestock and poultry manure.After the treatment of livestock and poultry manure by the abdomen,the obtained manure substrate is reduced in heavy metal content compared with livestock manure,and the physical and chemical properties are relatively good and relatively stable,which is an ideal seedling substrate.The use of the vermicompost substrate(VS)as a molding block can not only realize the comprehensive utilization of livestock manure,but also reduce the dependence on the peat in the seedling raising,reduce the use of the plastic seedling tray,and also generate certain economic value.In this study,the formation characteristics of VS were studied,and substrate molding technology and molding equipment were studied.The discrete element model(DEM)of VS was established for the key process of molding,and relevant simulation analysis was carried out to prepare VS and other seedlings.The molding and utilization of VS provides a reference and basis.The main research contents and conclusions are as follows:(1)Physicochemical properties and molding pre-test of VS.The physical properties such as water content,bulk density,profile shape,particle size,grain size and the chemical properties of EC,pH,organic matter content,TOC,TN,TP,TK,metal ion content,etc.were measured.The test results show that the main physical and chemical indicators of VS are in compliance with the relevant national standards and are suitable for use as a substrate.VS has a low ventilator porosity and it may be considered to add certain excipients to improve the pore characteristics.VS still contains certain cellulose,hemicellulose and lignin.In order to avoid the pore characteristics caused by the clogging of the micropores in the substrate,VS molding should adopt the cold press molding process.(2)Experimental study on the effect of substrate block morphology on the compression of VS.In order to determine the influence of the shape parameters such as shape,size and molding compression ratio on the molding effect of the substrate block to meet the design needs of VS molding equipment,the influence of the substrate block shape on the molding effect was studied.The test first studied the relaxation rate,drop breakage rate and saturated water absorption of cylindrical and truncated-shaped blocks of 40 mm height at different molding densities and different(equivalent)diameters.Experimental results show that there are significant differences in relaxation rate,drop breakage rate and saturated water absorption ratio of cylindrical and round-shaped two-shaped substrate blocks at different target densities and different target(equivalent)diameters.Relaxation rate of truncated block is lower than that of the cylindrical substrate block,and target density is too large or too small in both shapes to increase relaxation rate.Average relaxation rate of the low cylindrical shape was 144.84%,and average relaxation rate of truncated cone shape was131.30%.The drop damage of the cylindrical block is better than that of truncated block.Under the three different target density conditions,average drop breakage rate of cylindrical block is 89.23%,68.08%and 76.05%,respectively,and drop of truncated substrate block the breakage rates were91.01%,76.08%and 72.55%,respectively,and drop breakage rate increased with the increase of the size of blocks.Saturated water absorption of cylindrical block is also higher than that of truncated block(average saturated water absorption is 53.88%and 47.58%,respectively),and average saturated water absorption of the two shows a large difference with the change of diameter;It can be seen that difference in water loss between the two shape substrate blocks in the first 72h is not obvious,but with the extension of time,water loss of cylindrical block is slightly higher than that of truncated shape.Considering the manufacturing complexity and cost of the molding mold,it is reasonable to determine that shape of the substrate block is cylindrical.On the basis of this,cylindrical substrate block was taken as basic shape,and diameter,height and density of block are taken as experimental factors.Relaxation rate,drop breakage rate and saturated water absorption rate are the main evaluation indexes.The 3-factor and 3-level orthogonal tests are carried out.The test results show that the height of the substrate block has the most significant effect on the molding effect,followed by the diameter of the substrate block,and the influence of the molding density is relatively small.The optimal parameters are the target height of the substrate block is 30mm,the target diameter is 30mm,and the target density is 1.5g/cm~3.(3)Design and analysis of mechanical substrate compression molding machine.Based on the understanding of the physical and chemical properties and molding properties of VS,a mechanical substrate molding machine was designed.The 2-way power transmission scheme is adopted,and the power transmission method and arrangement of each part and the transmission ratio of each stage are designed.The molding system uses a biased crank slider mechanism with a stroke speed variation factor of 1.10.The motion law of the molding punch was simulated and analyzed.The results show that the design can effectively improve the molding time in the molding cycle,shorten the idle stroke time,and make the molding punch compress rapidly at a large rate in the initial stage of molding,and slowly compress at a small rate in the later stage of molding,which is beneficial to the compression of VS.The rest time of the sheave and the molding die(accounting for 66.67%of the molding cycle)is greater than the time during which the molding punch is located in the molding die(45.67%of the molding cycle),and there is no motion interference between the two.In order to ensure the smooth feeding of mechanical substrate molding machine feeding system,the simulation of the mixing feeding effect of the machine under different stirring feeder speed and blade distribution form was carried out.Both distribution pattern of the blades and the rotational speed of the agitating feeder have a significant effect on the particle agitating feed motion.Mixing motion of the particles showed a significant difference in different blade distribution patterns and agitating feeder rotation speeds.The agitator feeder speed has a greater effect on the substrate centrifugation rate than the blade profile.The blades near the agitator shaft have little effect on the overall substrate motion.There is no significant difference in the rate of centrifugation of the substrate particles and the angular velocity of the particles near the agitator shaft.The difference in motion characteristics of substrates with different particle sizes is significant.Small particle size substrate particles are more sensitive to changes in agitation conditions than large particles;under the same agitation conditions,particles with smaller particle size are more active,showing a higher rate of centrifugation and a greater change in angular velocity.Through the analysis of the stirring feed verification test,the numerical simulation results of DEM for the motion law of substrate particles are similar to the experimental results.It is feasible to use the DEM to study the motion law of substrate particles.The molding density,drop breakage rate and water holding characteristics were the main evaluation indexes,with compression ratio(2:1 and 2.5:1)and molding rate(4.85 times/min,9.70times/min,14.55 times/min and 19.40 times/min).Experimental study on the molding effect of mechanical substrate molding machine for experimental factors.The test results show that the molding rate has no obvious influence on the molding density of the substrate,and the compression ratio has a great influence on the substrate molding density.When adjusting the molding density,the molding compression ratio should be considered.Both the molding rate and the compression ratio have a great influence on the breakage rate of the substrate block.At a compression ratio of 2.5:1,the molding rate is 14.55 times/min and below;or at a compression ratio of 2:1,the molding rate is 4.85 times/min and below,the substrate block can be broken and the damage rate is less than 5%;in production,if saturated water absorption is required to be larger,the compression ratio can be appropriately reduced.Adjusting the compression ratio has little effect on the water loss performance of the substrate block.(4)Parameter calibration of VS by DEM.It can be seen from the test results of the mechanical molding machine that the machine can meet the needs of small users,but the quality of the substrate block is greatly affected by the characteristics of the raw materials and the technical performance of the molding machine,and the impact is also complicated.It is difficult to analyze by simple experimental research.The key to the problem is to analyze the molding mechanism and key influencing factors of VS by using DEM for simulation analysis.To accurately establish the DEM model of VS,the model parameters need to be calibrated first.Parameters of VS DEM model were calibrated by angle of repose(AoR)method.A method for obtaining AoR of granular material was proposed.Contour of pile was extracted,contour curve was fitted with normal distribution to fit curve inflection point.The angle of inclination was taken as AoR.Tests show that this method has a good fitting effect and provides a treatment idea for determination of AoR of bulk.The JKR bond model was applied to VS particles,and parameters of VS particles were calibrated to screen out parameters that have significant influence on AoR(ie,inter-particle static friction factor,interparticle rolling friction factor,JKR surface energy).Based on quadratic polynomial regression analysis of AoR test data,a regression model between AoR and three significant parameters was established.Experimental results show that model results are in good agreement with experimental results.AoR of VS particles at different moisture contents was determined,and the exponential function between the water content and AoR was established.The regression model between the function and AoR and the three significant parameters can be used to determine the moisture content of the manure substrate,predict AoR,and then reasonably estimate three significant parameters and other parameters.Based on this,a reliable DEM model of VS is established.(5)Mathematical model and simulation analysis of compression of VS.Mathematical model of compression of VS was established.Internal pressure and compressibility of VS were theoretically analyzed,and contact model of VS particles was analyzed from microscopic point of view.Based on the Janssen model,the stress distribution under different initial pressures during molding process of VS was theoretically analyzed.Theoretical relationship between the Janssen model and the internal pressure calculation formula was proved.Theoretical compression limit of VS was determined.Based on the DEM model of VS,the change of the compression force of the punch,the relationship between the compression force of the punch and the support force of the base and the variation curve of the molding resistance were studied.The results show that during the molding process of VS,the molding resistance of the substrate away from the molding punch is reduced;VS is similar to other bulk materials in the compression process,and there is a“granary effect”.The effect of the seed effect should be selected according to the characteristics of the roots of the plant seedlings.The JKR contact model can accurately describe the contact between VS particles,and simulation results of molding process are close to experimental results,which can be used to simulate process of molding VS;molding process of VS block is mainly divided into two stages,namely,the stage of discharging the air(and moisture)between substrate particles(aeration pores are compressed)and the stage of rearranging VS particles,in determining the compression of substrate.After the compression ratio of the substrate block,the initial amount of the substrate can be determined according to the displacement of VS into the second stage.The substrate particles are rearranged during the compression molding process,and the particles are redistributed along the wall of the mold,thereby affecting the subsequent broken mode of the substrate block.(6)Design and test of hydraulic VS compression molding machine.According to the theoretical analysis and simulation results of DEM,a hydraulic VS molding machine was designed.The molding machine was equipped with a side cylinder as feeding device on the basis of three-beam four-column hydraulic press machine,and the prototype was tested.The test result shows that the machine can meet expected design requirements.In order to further determine the substrate formulation suitable for the molding machine,three kinds of binders such as sodium carboxymethyl cellulose(CMC),agar(Agar)and Xanthan gum(XG)were selected to carry out binder germination.The toxicity test showed that the three kinds of binders did not show the effect of inhibiting the germination of the seeds within the range of the amount of the test,but had a certain promoting effect.The molding performance of the hydraulic substrate molding machine and the appropriate substrate compression were tested.The substrate block molding and seedling test were carried out with the type of binder,the amount of binder added and the molding pressure as test factors.The compressive strength,drop breakage rate and seedling helth index were selected as evaluation indexes.The test results show that the optimum molding effect can be obtained by using XG binder with an addition amount of 1.5%and a molding pressure of 15 MPa.The molding pressure has little effect on the compressive strength and drop breakage rate of the substrate block.On the whole,the three kinds of binders XG have the best effect,followed by CMC;in the production,the molding pressure reaches10 MPa to meet the molding requirements;in the range of the test binder addition amount,the slightly higher addition amount is beneficial to the substrate block.FTIR analysis of VS under different formulations and molding conditions showed that some macromolecule substances were decomposed and digested,lignin and cellulose were degraded,and nitrogen compounds were completely mineralized after the cow excrement was treated by earthworms through abdomen.There was no obvious chemical reaction between VS and XG in the range of test pressure,which was formed in the test results.Pressure had no significant effect on the formation and seedling raising of XG binder. |
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