Case Study:

Rice Hulls to Electrical Energy & Process Steam

Project Location:

Stuttgart, Arkansas

Photograph of Jobsite Construction
Commissioned in December 1996

The technology that Primenergy employs to convert rice hulls into energy is called gasification.  In the process of gasification, or the conversion of a solid into a gas, there is an insufficient amount of oxygen to completely burn the hull. The controlled atmosphere within the gasifier prohibits the formation of molten silica from the hull. 

The Stuttgart Project

Primenergy provided a Model R-318 gasifier system, which is comprised of three gasifiers operating in parallel.  The conceptual design of the process incorporated three design cases; maximum electricity production, maximum steam production and normal electrical power and concurrent process steam production.  To accommodate the three design criteria, an extraction turbine with a controlled extraction lobe was selected.

The rice hulls are delivered from the rice milling operation via bottom unloading trucks.  Each truck is capable of delivering eighteen tons of hulls per load.  The delivery trucks dump the hulls through a metal grate constructed of square metal tubing.  The hulls are transferred from the unloader into one of two storage silos.  Each silo can contain twenty-four hours of hulls at full design feed rate.  Within each silo is specially designed anti-bridging recovery device.  Hulls are transferred from storage into the gasifiers’ metering bin.  The capacity of the metering bin is about fifteen minutes at full design feed rate.  From the metering bin, rice hulls are transferred into the three gasifiers.  The gasifiers can be operated at independent rates or can be automatically operated in parallel.

Hot synthesis gases are generated in the gasification process and are burned in stages, for nitrogen oxide control, with final oxidation occurring within two waste heat recovery boilers.  For compliance with particulate matter in vent stacks, the products of combustion exiting the boiler are filtered of fly ash in a conventional baghouse.  An induced draft fan, which maintains a slight negative pressure through out the entire process, is located after the baghouse.  The induced draft fan discharges into a vent stack. 

High pressure, superheated, turbine quality steam from the boilers is piped to a fifteen megawatt, 13,800 volt, reconditioned turbine/generator.  Located immediately beneath the turbine is a steam condenser, which operates at near full vacuum conditions.  In the maximum electrical power generation mode, all of the steam directed to the turbine is condensed and returned to the boiler via boiler feedwater pumps.  The turbine selected for this application does have an extractive lobe which allows the extraction of up to 100,000 pounds per hour of medium pressure steam.  The extracted steam is piped to a re-boiler, which generates utility steam for consumption in the adjoining food processing plant.  By incorporating a re-boiler into the design, turbine quality feedwater is segregated from process plant quality water and avoids the possible contamination resulting from use in food processing.

The entire system, from the receipt of the rice hulls to the generation of electricity is controlled by a human machine interface located in a control room.  The control room adjoins the turbine generator building and crew quarters.  From the central CTR’s each function of the overall process can be individually adjusted, or the overall process can operate in a fully automatic mode.

All fabricated equipment was manufactured in Tulsa by Primenergy’s sister company, Heater Specialists, L.L.C. and major equipment was pre-assembled prior to shipment to Arkansas.  The project duration from final contact signing until initial commissioning was less that one calendar year.