CHARACTERIZATION OF A BIOMASS MILLING PILOT ... - circe

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used to convey the particles from mill outlet to other equipments of handling, storing, treatmeant or directly to consume milled biomass. Usually, mills are composed by rotating elements inside the grinding chamber and at high revolutions The material circulates around the mill along paths parallel to the screen surface making the openings are only partially effective [13]. This rotating flow produces an oblique particle trajectory to the screen (e.g. Fig. 2). Therefore the probability of particle evacuation through screen openings decreases. Higher angle between normal to the screen and particle trajectory causes higher probability of impact against the screen and bounce back into the impact zone where non necessary impacts may happen. Working under negative pressure in the mill exit modifies the air stream and the trajectory of fine particles against the internal screen, increasing the particle evacuation probability of grinding chamber. Inertial effect predominates on coarser particles due to their higher mass and high velocity after impact. Figure.2: Particle impact trajectory against the internal screen. On the other side, high negative pressure may produce screen blinding since the particles leave so far the impact zone blocking the screen. After the impact, high negative pressure hinder the bounce particle, remaining against the screen and decreasing the screen effective surface. Under the most unfavorables conditions, biomass can totally blind the screen and cause the stop of mill drive and the stop of the process. Negative pressure is controlled by means of variable speed drive installed in fans. These electronic devices vary the rotary revolutions of fan blades, increasing or decreasing the suction in the mill outlet. To assess the influence of this parameter in the properties of milled material and milling energy consumption it has been necessary to install pressure transducers in both mills outlets. Negative pressure is obtained, registred and saved for data processing. The parameter study allows to determine: Upper limit of negative pressure without blinding Optimal pressure to decrease the milling energy requirement Their influence in the pulverized biomass properties 3.1.2 Speed of rotation Speed of rotation influences residence time in two aspects: Released energy in impacts Rotating air-particles flow inside the grinding chamber 3.1.2.1 Impact energy Residence time depends on breaking velocity, i.e. the velocity at which the particle is fractured until it achieves a lower size than the screen opening size. In the case of hammer mill, the fragmentation (creation of new surfaces) is influenced by the energy released in impact. The impact energy is entirely in the form of the relative kinetic energy between the particle and the point of hammer where the impact happens. 2 r I = mν / 2 = m ⋅ ( ν −ν ) / 2 = m ⋅ ( r ⋅ω − v m p m p ) / 2 Where m is the particle mass, vr the relative lineal velocity between the particle velocity (vp), and velocity of the impact point in hammer (vm),r radius from mill axis to impact point in hammer and ωm the hammers angular velocity. Lower angular velocity ωm implies lower impact energy and lower particle fragmentation. Therefore, the original particle needs a higher number of impacts until the suitable size is achieved, increasing the residence time of particle in grinding chamber. 3.1.2.2 Rotating air-particle flow inside the grinding chamber Higher angular velocity of internal elements produces higher rotating air-particle flow effect inside the chamber. As it is explained in section 3.1.1, this effect increases the angle between the particle trajectory and normal to the screen and decreases the probability of particle evacuation of the chamber. 3.2 Drying effect during milling Grinding process causes significant secondary effects in final milled product. One of those is the mositure content reduction. Drying effect is caused by the heat released through two processes: Global drying: particles are in air-particles turbulent atmosphere receiving constantly heat flows from friction process between the mobile elements and between this elements and the airparticle flow. Local drying: It is caused by the evaporation of water content in particle due to released heat in impact. As the particle strikes and compresses, some of the kinetic energy is converted to strain energy in the compressed particle, with the sum of strain energy and kinetic energy equal to the impact energy. When the strain energy reaches that required to cause fracture, some of the stored strain energy is used to create new surfaces and the rest is converted to heat by relaxation of the fragments of breakage [14]. The released heat as from friction process as from particle relaxation after impact is difficult to quantify. For this reason, a global energy balance in mill is raised. The objective is to determinate quantitevily the disiped and inverted energy in drying effect by means of the numerical value determination of implicit variables regarding as air as biomass: Input and output temperature Input and output air humidity and biomass moisture content Biomass feed rate and air mass flow 3.2.1 Input and output temperature In practice, air-particle flow temperature inside the grinding chamber is increased during the milling.
Input and output temperature measurement of air and biomass allow to assess the heat flow spent in temperature increase. 3.2.2 Input and output air humidity and biomass moisture content Moisture content is analyzed by means of initial and final biomass representative sampling [15]. Input and output air humidity are also instrumented. Water evaporation flow per mass unity is calculated considering the difference between input and output moisture content of biomass. On the other side, it is necessary to know the initial air humidity to assess the heat flow to increase the water vapour temperature. 3.2.3 Biomass feed rate and air mass flow An average biomass feed rate is calculated as ratio between total raw biomass weigtht and the measured feeding time. Air volumetric flow is instrumented with an annubar flowmeter located in a pipe stretch in ausence of particles. 4 MILLED <strong>BIOMASS</strong> SAMPLING Milling process characterization includes the analysis and influence of several variables and operational parameters in the final properties of milled biomass. First of all, different sampling methodologies to obtain a representative sample of the total comminuted biomass in milling experimental pilot plant are reviewed. 4.1 Coning and quartering procedure. Once the total volume is milled, a representative sample can be obtained using the coning and quartering procedure. It is a progressive process, from the bulk volume (A) two quarters are taken (C and D) and they are divided into quarters again. The final sample is taken from the last one (G). The process is shown in figure 3. Figure 3: Coning and quartering procedure [16] It is a static sampling method where powder is confined or heaped. In this situation, size segregation occurs. Fine particles tend to remain at the center of the heap and coarse ones congregate at the periphery and the particle size analyse of the sample is disturbed. 4.2 Gravity discharge sampling. Sampling from flowing powder such as discharging flow by gravity, the entire stream of powder should be sampled by traversing the stream, and the sampling should continue for a long series of short time intervals. The sampling point is located between cyclone outlet and screen classifier (e.g. Fig. 6, GS.1). Particles from grinders are conveyed until cyclone where are collected and precipitated to screen classifier. Two-way distributor (e.g. Fig. 4) has been introduced between both equipments to change the direction flow on discharge to the sampling outlet. This sampling method is simple, economic and allows taking a representative sample of collected biomass in cyclone. However, finer particles not collected by cyclone are not represented in this sample. Figure 4: Two-way distributor 4.3 Isokinetic sampling The principal standard for direct measurement of local particle mass flux in most gas-solid flows is provided by the isokinetic sampling system. The isokinetic sampling principle requires that the sampling probe which is aligned with the flow (isoaxial) extracts airborne particulates at the sampling velocity matching the original undisturbed local flow velocity. In practice, the isokinetic sampling is closely approached but almost impossible to be rigorously realized. Several problems as the determination of flow velocity in the presence of significant amount of particles, the elimination of intrusive effect of the sampling probe, the interactions between particles and carrying fluid, the loss of particles to the wall deposition and particle bounce/reentrainment in the sampling tube are frequently observed. By these reasons, three types of sampling (e.g. Fig. 5) can be carried out: isokinetic (sampling velocity=stream velocity) or two kind of anisokinetic sampling: over-sucking (sampling velocity>stream velocity), under-sucking (sampling velocity< stream velocity). Figure 5: Isokinetic and non-isokinetic sampling [17]
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Page 3: Belt conveyor B.1 A.1.Configuration
Page 7 and 8: 4.4 Characterization of sampling po

CHARACTERIZATION OF A BIOMASS MILLING PILOT ... - circe

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