Gao Zicheng, Li Jiping, Yan Yonglin, Chen Xilong, Tan Yaohui
Abstract: As a fuel after biomass molding, it is one of the development directions of renewable energy. Biomass raw material has a high moisture content before being made into granules. It is necessary to dry the particles after the first comminution, so that the moisture content reaches the granulation requirements. , Easy to enter the granulation process after the second crushing. Venturi dryer is a new type of dryer, which has the advantages of small volume, low airflow resistance, high drying efficiency, suitable for large particle size and wide particle size distribution. The particles are dried. Taking the venturi biomass particle dryer with a drying capacity of 500 kg/h as an example, the principle and method of drying the Venturi biomass are discussed.
The use of biomass pellets as fuel is one of the development orientations of renewable energy. China has established a medium and long-term development goal for biomass energy, and biomass energy has developed rapidly in recent years. However, biomass particles are dried before The moisture content is generally higher, but the high moisture content is very disadvantageous to its subsequent conversion and utilization (granulation, briquetting, etc. The current drying methods mainly use air-drying, the equipment mainly includes a drum dryer, the equipment covers a large area, and is dry For a long time, other drying methods mainly include natural drying. This drying method not only takes a long time, but is greatly affected by weather changes, which severely limits the large-scale utilization of biomass.
Venturi dryer is a new type of drying equipment. It has the characteristics of small volume, low air flow resistance, suitable for large particle size of dry material and wide particle size distribution. As a particle of biomass energy, it can also be dried using Venturi dryer. Based on the drying requirements of biomass particles and the principle of Venturi dryer, the author discusses the design of a venturi particle drying machine with a drying capacity of 500 kg/h.
1 Venturi particle dryer structure and principle
For the needs of the Venturi Dryer for drying granules, the basic structure of a Venturi granule dryer is shown in Figure 1. The main structure includes a primary feed screw 2, feed shutoff fan 3, secondary feed screw 4, once The air inlet duct 5, the hot air distributor 7, the secondary air inlet duct 8, the venturi tube 9, the dryer shell 12 and the like. The working principle of the venturi particle dryer is that the particles to be dried enter a level from the hopper The screw conveyor 2 is fed into the upper inlet of the blower fan. Under the action of the blower fan, the particles are fed into the inlet of the secondary screw conveyor, and then the particles are transported to the dryer cylinder under the action of the secondary screw conveyor. , Venturi tube and the outer side of the nozzle reducer, the air from the high pressure centrifugal fan is heated by the hot air furnace and sent to the inlet of the hot air distributor 7. The hot air distributor divides the hot air into two parts by a certain proportion, and part of the hot air passes through once. The hot air pipe 5 enters the lower nozzle of the dryer body, and another part of the hot air is blown into the dryer body along the tangential direction at the upper part of the dryer body through the secondary hot air pipe 8.
The hot air distributor controls the proportion of hot air entering the dryer body. The nozzle of the primary hot air duct 5 in the dryer cylinder and the simplified internal venturi 9 constitute a jet conveying device, and a relative vacuum is generated in the lower part of the reducing tube. The particles between the tapered tubes are sucked in and mixed well with the hot air at the throat of the venturi. The particles exchange heat with the hot air inside the venturi. The moisture in the particles is released. The mixture of the air and particles passes through the venturi tube. The gradually expanding tube reaches the upper part of the dryer body. The classifier 10 is arranged at the center of the upper cone surface of the dryer body, and the classifier outlet and the discharging tube 11 are connected. After reaching the drying requirements, the particles reach the discharging tube 11 after entering the cyclone separator after passing through the classifier. , It is separated by cyclone separator under the effect of cyclone separator and induced draft fan. The particles that do not reach the drying requirement cannot enter the outlet pipe through the classifier, and the secondary hot air blows in from the tangential direction along the inside of the dryer cylinder. The direction of blowing is l4° downward, so that the particles that do not reach the drying requirements enter the surrounding area of the secondary air during the fall of gravity, and the secondary hot air Under the effect, the mixture of gas and particles is spirally moved up and down along the gap between the shell in the dryer body and the venturi tube, reaches the lower part of the dryer, and reaches the lower tapered nozzle of the venturi tube. The nozzle is once again sucked into the jet nozzle by the action of a hot air, and is circulated and dried. When the drying requirement is reached, the discharge pipe 11 is entered from the classifier outlet at the upper part of the dryer body and then separated by a cyclone separator.
2 Design of Key Parts of Biomass Dryer
2.1 Technical Parameters of Biomass Dryer
The technical parameters of the biomass particle dryer designed in this paper are shown in Table 1.
2.2 Technical parameters required in the design process
2.2.1 Churi Dryer Design
In the design process, relevant parameters such as water, dry air and particles are needed. See Table 2 for details.
2.2.2 Dry Air Density Calculation
Its formula is:
2.3 Dryer Thermal Balance Calculation
Since the moisture content of the particles entering and exiting the dryer is different, the design of the dimensions of the various body parts of the dryer should be based on the relevant parameters of the drying process and the drying capacity, because the quality of the absolutely dry particles does not change during the drying process, corresponding to There are the following calculations:
2.4 Venturi design
2.4.1 Determination of Primary Hot Air Jet Diameter
2.4.2 Determination of maximum sedimentation velocity of particles
According to the settlement velocity calculation method, the mixture of particles and hot air in the venturi tube expander tube movement is turbulent, located in the venturi throat and gradually expanding tube, particles and air have been fully mixed and exchanged heat, The dried moisture in the particles at the outlet of the progressive tube has been vaporized. At this point, the particles have approached the drying requirement. The net density of the dry particles is:
2.4.3 Determination of Main Venturi Size
Figure 2 shows a schematic diagram of Venturi tube. The dimensions of each part are marked as shown in the figure.
According to the principle of the Venturi dryer, the secondary air goes down spirally along the barrel to the tapered lower end of the Venturi tube and enters the Venturi tube along with the primary air and particles. It is assumed that the moisture in the particles is in the Venturi tube. Completely gasified, expands the tube outlet at Venturi, and the mixture of air, water vapor and dry particles flows out.
(1) Calculation of the diameter of the expanding tube outlet
Due to the gas temperature during the drying process, both the velocity and the pressure need to change. When calculating the size of the main part of the venturi, the conservation of mass is used to calculate.
2.5 simplified design
The flow of secondary hot air exists between the venturi tube and the simplified one. Therefore, the space between the outer cylinders of the Venturi dryer needs enough space to facilitate the spiral movement of the secondary air. The diameter of the outer cylinder of the dryer is 1150 mm. the distance between the primary air nozzle and the venturi tube 50 ~ 150mm tapered, for adjusting the spacing between the primary air nozzle and the venturi tube through the spout tapered flange, the tapered cone apex angle of 150 ° dryer Simplified , the outlet of the dryer upper cone classifier, classifier made various specifications to meet needs of a variety of particles and drying different water contents. lower cone apex angle of cone 60 °, with the secondary air in order to facilitate falling The particles enter the inlet of the venturi tube and enter the secondary drying.
The position and shape diameter of the secondary air vent affect whether the secondary air can make a descending spiral movement along the outer cylinder of the dryer, and the speed and angle of the spiral movement, combined with the position of the upper cone top cover of the dryer and the gradual expansion of the venturi tube position of the tube, in the present design, rectangular secondary air along a tangential access simplified dryer, to guarantee that the secondary air is injected into the English do fall along a helical motion along the cylinder center line of the secondary air nozzle The body axis is downwardly inclined by 11°, the secondary air outlet has a rectangular size of 180mm×240mm, and the secondary air nozzle is installed at 100mm below the outlet of the expander tube of the venturi tube. This ensures that the particles that cannot reach the classifier cannot reach the drying requirements. Into the simplified inner along the cylinder to make a downward spiral movement, and continue to dry heat exchange, thereby increasing the drying efficiency.
In addition to the first stage feed screw mechanism and hopper, the entire Venturi particle dryer is made of stainless steel 1Cr17. The venturi tube and Simplified and all parts of the trachea are made of stainless steel. Considering that the biomass particles are fibrous, Wenqiu The inside of the inside of the dryer must be polished smooth, no sharp edges, burrs, etc., to prevent the particles from being caught in the drying process, affecting the drying air flow and reducing the drying efficiency.
2.6 Design of hot air distributor
In order for the Venturi dryer to dry particles of different types and moisture contents, the proportional relationship between the secondary air and the secondary air needs to be adjusted. Therefore, a hot air distributor is designed. As shown in FIG. 3, the distributor has a rectangular cross section and the distributor The blades are located in the center and two outlets are opened behind the blades. They are connected to a hot air duct and a secondary hot air duct respectively. The top of the distributor is equipped with a worm gear reducer. The worm gear reducer is connected to the vane shaft and reduces the speed by rotating the worm and worm. The input shaft of the instrument changes the position of the blades to change the flow distribution ratio of the secondary air and the secondary air, so that the dryer adapts to the particles with different particle sizes and densities to dry.
2.7 Design of feed mechanism device
Due to the complex shape of biomass particles and large variation in particle size, some fibers are longer. In the drying process, the pressure of a single hot air nozzle reaches 4000Pa, and the pressure of secondary air is around 4000Pa. Therefore, the dryer feeding mechanism adopts two stages. In the form of a screw conveyor, a shutoff fan with a ring cutter blade is installed between the first and second stage screw conveyors, and the ring cutter blade is used to shut off the excessively long particle fibers at the inlet of the blower fan. Closed-pitch screw conveyor with a pitch of 100 mm at the entrance and a pitch of 160 mm in the rest. To avoid the screw conveyor being blocked by particles, the simplified unilateral gap between the screw and the conveyor is 10 mm. The first screw conveyor The frequency conversion is used for feeding, and the second-stage screw conveyor adopts high-speed rotary feeding. First, the secondary screw conveyor motor power is 2.2kW, and the fan motor power is 2.2kW.
3 Conclusion
In this paper, the working principle of Venturi Dryer is discussed in the light of actual needs. The design and calculation process and method of Venturi Dryer for drying biomass particles are discussed in detail.
