CHARACTERIZATION OF A BIOMASS MILLING PILOT ... - circe

More documents

Recommendations

Info

milled biomass characteristics analysis: particle size distribution, moisture content, bulk density, particle shape factor and chemical composition as function particle size. The properties variations permit to assess the influence of the parameters mentioned in final milled material. 2 BIOMASS MILLING PILOT PLANT Experimental facility characterization should analyse the design and operational parameters needed to satisfy the particle size requeriment with the lowest milling energy consumption. The size requirements are fixed by the final energetic conversion technology and can be expressed by range of particle sizes, or by more specific granulometric regulations. Biomass shows a high heterogeneity in connection with inherent mechanical and physical properties, and in reception conditions (moisture content, raw particle size…). Flexibility in experimental comminution facility permit to carry out several tests varying: Type of mill Equipment combinations Operational parameters Biomass mechanical properties represent a relevant parameter to select the adequate type of mill. Biomass is mainly composed of non-brittle materials [11] in contrast with other energetic resources as coal. Several studies show three types of mills as the most appropriate for biomass milling: hammer mills, knife mills and also disc mills in specific conditions [11]. Studied facility is composed of two mills: a hammer mill and duplex mill [8]. Hammer mill with horizontal axis and a drive of 11kW comprises 6 hammers arranged on 3 radial lines. Duplex mill with a drive of 4kW consist of three hammers in revolution that carry out two different milling actions: impact breakage of hammers against particles and breakage by shearing stress between the hammer, particles and a fixed element inside the milling chamber. Grinders are the main equipment in any milling facility. However, classifying equipments are incorporated to separate the particles as function their size. The classifier has operational flexibility which increases the number of possible mills configuration allowing the most suitable solid treatment. Classifying systems select those particles whose size satisfies the final requirement. Classifier can be used as a previous stage to separate the particles which satisfy the size requirement, as a final stage to guarantee the particle size of final product or as an intermediate stage to convey the coarse particle again to mill (recirculation flow). When the upper limit size is fixed, coarse particle can be eliminated or can be passed by the mill a number of times necessary to obtain the desired size. In case of lower size limit, the fine particles are eliminated of the final milled product. Milling facility also incorporates auxiliary solid handling equipment to convey the particles between the main equipments. At CIRCE milling biomass facility (e.g. Fig. 1), three different systems are installed to convey solids. First of all, a belt conveyor is used to feed the biomass to the mills. Subsequently, pneumatic conveying, composed of two fans to generate the air flow, and two equipments (cyclone and bag filter) to separate the air-particles flow, carries the pulverized biomass from mills to the final bin. Finally, coarse particle recirculation is conveyed to mills using an screw conveyor. The different combinations between hammer mill, duplex mills and screen classifier provide five milling configurations: 1. Configuration A.1: hammer mill 2. Configuration A.2: hammermill and screen classifier 3. Configuración B.1: duplex mill 4. Configuración B.2: duplex mill and screen classifier 5. Configuración C: hammer mill, screen classifier and duplex mill. Experimental tests in A.1 and A.2 configurations are focused on the hammer mill analysis: A.1. Hammer mill. Particle size is fixed by the internal screen opening size. Particles pass trough the hammer mill and are directly carried by pneumatic conveying to the bin. . A.2. Hammer mill with recirculation to the same mill. Milled biomass particles in hammer mill are conveyed to the screen classifier where fines are separated from the coarse ones. Fine particles are directly transported to the bin, whereas coarse particles are conveyed again to the hammer mill until a suitable particle size is achieved. In this configuration, the goal particle size is fixed by the screen opening size in the classifier. Third and fourth configurations allow the study to be focused on duplex mill: B.1. Duplex mill. As in the A.1 configuration, particles pass trough the duplex mill before they are being transported by pneumatic conveying to the bin. Internal screen determines the final particle size. B.2. Duplex mill with recirculation to the same mill. This configuration is similar to A.2. In case finer particle sizes are needed, this configuration allows the particle flow to pass several times trough the mill until the suitable size is reached. The last configuration is focused on the combination between both mills with a screening intermediate stage: C. Hammer mill with recirculation to the duplex mill. In this configuration (e.g. Fig. 1) the milling and classifying process are the same as in A.2 configuration, except for the coarse recirculation. In this case, the coarse particles are redirected to the duplex mill. The facility allows both recirculation types. Configuration changes can be made by means of removable pneumatic conveying ducts. The combinations between configurations and internal screens allow to obtain the suitable particle size in the process. To control the equipment of the facility, an control automatic system and data acquisition have been implemented. The main elements are: Programable logic Control (PLC): it is the main control unit. It collets all plant information, and it take the action on the plant. Distributed input/output units: they are remote device, where signals from sensor an signals to actuators are connect to the PLC Variable speed drivers: It is the main actuator in the mill plant. With this we can regulate the angular speed and the power supply to mill, fan and others devices.
Belt conveyor B.1 A.1.Configuration A.2 Configuration B.1 Configuration B.2 Configuration C Configuration Duplex mill C Coarse recirculation Hammer mill l Hammer mill l A.2 Fig 1: Milling configurations of experimental pilot plant Comunication network: It is used to communicate the PLC with the others devices in the network. The industrial fielbus used in as plant are CANOpen and Ethernet. Supervisor Control And Data Acquisition (SCADA): It is the Human Machine Interface (HMI) and offers the function of monitoring and control the operation in the plant. Several control loops have been implemented to regulate and optimize the equipment electrical consumption. Used control loops are: Biomass feed rate control: this control loops optimize the mills electrical consumption regulating the biomass feed rate by means to control the feeding belt conveyor velocity. It exits an optimal input biomass feed rate for which milling energy requirement (kWh/t) decreases [12]. Pneumatic conveying control. Control is carried out varying the angular speed of fans blades. By means of this control, the pressure at mill outlet can be modified to observe the influence in the particle residence time in grinding chamber (section 3.1.1). Finally, operational parameters should be obtained, registered and analyzed to study their influence in the process and in the milled product. A study of the instrumentation has been carried out to obtain these variables. 3 OPERATIONAL PARAMETERS Pilot plant characterization requires the inherent processes identification during the milling to determinate the variables that have significant influence in the pulverized biomass properties or/and in the milling A.1 B.2 Cyclone + Screen classifier Biomass bin Bag filter Primary air Secondary air Screw feeder energy requirement. Two influential processes inside the milling chamber are analyzed: Residence time of the particles Drying effect during milling 3.1 Residence time Describes the amount of time a particle could spend inside the grinding chamber. Consequently, the particle can be impacted and fractured by the mobile elements inside the mill. Usually, the final maximum particle size in the milling chamber is fixed by a metal screen that encloses the chamber, or by pneumatic system. The final goal consists in determining the optimal operational conditions for which the particle evacuates the chamber as far as possible when its size is smaller as desired size. Residence time increase involves higher possibility of new breaking actions which entails milling energy consumption increment in creating new particle surfaces. This over-consumption would not be necessary because the particle would have had a suitable size. Obviously, the screen openings sizes and mechanical properties of biomass are fundamental factors in connection with residence time. Others aspects with no direct dependency have been analyzed to check his influence on residence time. 3.1.1 Mill outlet pressure Particle removal can be by gravity or pneumatic discharge. Gravity discharge takes place when other pneumatic system is not connected to mill outlet. The mill fan effect generates a slight overpressure in the outlet region. Rotating hammers in the grinding mill act as fan and built up air pressure against the screen and mill outlet. On the other side, pneumatic discharge is commonly
Page 1: CHARACTERIZATION OF A BIOMASS MILLI
Page 5 and 6: Input and output temperature measur
Page 7 and 8: 4.4 Characterization of sampling po

CHARACTERIZATION OF A BIOMASS MILLING PILOT ... - circe

Create successful ePaper yourself

Delete template?

Save as template?