04/19/2012 | Author / Editor: Eddie McGee * / Marcel Dröttboom
A plug forming approach using a screw feeder is often preferred when feeding material into environments where there is a significant temperature or pressure differential. The plug is a controlled build-up of material created at the point where the screw enters the process, protecting the feed equipment and prior material from early exposure to high temperature and preventing ingress of vapours from the process into the screw and feed hopper.
The screw end is designed to generate maximum end force, and a balanced compacting force. The plug forms in the barrel section after the flights end and is replenished by fresh incoming material (note it does not self clear though may degrade with process conditions). A weighted flap valve can also be used to generate resistance and encourage plug formation. The screw pushes material into plug forming zone where a diverging wall generates good compression and ensures positive advance of the material.
Knowing the optimum plug length to achieve a steady flow is based on the measured flow properties of the feed material, experience and often trials. Changes in either the biomass material formulation or environmental conditions, can affect the plug performance, leading to poor screw feeder discharge and an increased risk of gas ingress through the feeder. To address these issues, and make the equipment flexible enough for dealing with a range of material and process conditions, the operator is able to adjust the length of the "plug" to match or exceed the back pressure on the plug, thus ensuring consistent discharge from the screw feeder. In so doing it overcomes the inflexibility encountered with fixed length plugs that limits there use in the processing of highly variable, biomass materials.
The same plug screw technique can be applied to ash removal. In biomass gasification plant, ash is produced during the gasification which has to be removed from the gasifier. Maintaining an oxygen-free gasifier is essential. Here the plug screw feeder densifies the ash as it leaves the screw to create an impermeable barrier, allowing the ash to be removed without oxygen entering the system.
The spider diagram flow characteristics analysis incorporating features of shear strength (Ts) and particle size (dp) for ash and char extracted from the gasifier is as shown in Fig. 5 The red line for the charred, part combusted materials shows it has low shear strength and coarse particle size indicating that flowability is good despite its low bulk density. Also in Fig. 5, the blue line for the ash shows it has high shear strength, while the wall friction values confirm poor flowability. However the high Hausner Ratio (compressibility) and modest permeability mean the ash's plug forming aspects are good.
The main issues in the handling of biomass materials are not necessarily in the pyrolysis techniques or the calorific science of the energy conversion processes used in biomass power plants, but the avoidance of the problems that commonly arise from the tendency of bulk materials to cause blockages or flow erratically.
In the processing of biomass materials there is a need for extremely tolerant design, yet careful attention to the fine detail to ensure reliable flow. Designs should be based on relevant bulk flow property measured values. Spider diagrams are good at encompassing many of the handling relevant attributes, and providing an insight into equipment design for these difficult to handle materials.
Plug-screw feeder technoloy has emerged as a proprietary technique that allows biomass energy companies to successfully overcome the challenge of both feeding pyrolysis and gasifier processes and extracting char and ash after combustion, while providing a very good thermal barrier and inhibiting air ingress. n
* Dr. Eddie McGee is Technical Director of Ajax Equipment, United Kingdom, Tel. +44 (0)1204/3867-23, E-Mail: email@example.com
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