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Using Wood Dust as Feedstock

Q: I am designing a pilot plant that will use wood dust as its feedstock. I am having difficultly finding a method for moving the wood dust from a hopper into a reactor that is operating at 25-40 psig. My initial idea was to pressurize the hopper at 25-40 psig and use a screw feeder that would meter the flow rate, but all the manufactures are telling me that the feeder cannot operate with an internal pressure. I have also looked at using an airlock after the screw feeder and operating the hopper at atm. Has feeding wood dust into a 40 psig vessel been done before? If so, how was it accomplished?

A: There are a couple of companies that specialize in rotary airlocks for feeding into high pressure systems. Several other companies have tried to make valves for this service, but I've never had any experience with them. It is a very demanding service, but the rate can be controlled within 2%-3% of a set point. The origin of the technology is in feeding coal to pressurized boilers operating at even higher pressures. In some cases I've used two in series for very high pressures. This improves the feed accuracy. An alternative that may work for the 25 psig range is a double flapper valve, but rate control is not that good. I've used these for high temperature service (over 800 F). The vent gases need to be treated with either valve. A last alternative is to use a pressurized blow tank, but this won't meter the wood dust. However, a loss-in-weight feeder could be used to feed the blow tank.

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