Case Study:

Rice Hulls to Process Heat and Steam

Project Location:

Jonesboro, Arkansas

Photograph of Completed Installation
January 1997

Summary Description

Gasification of 175 US tons per day of rice hulls to provide process heat, for rice boiling, and 600º F heated air for drying.

Background Description

Rice that is delivered to a rice mill has not had the outer covering of the hull or husk removed.  Between the husk and the rice kernel is a layer of bran.  Parboiled rice is rice that has been steamed in order to dissolve the nutrients from the bran (which will be removed) and then pressurized to deposit the nutrients into the kernel.  Much of rice for the consumer market in the United States has been parboiled for this reason.  However, this process is very energy intensive.  First, the rice must be boiled and secondly, it must be dried prior to the removal of the outer hull or husk.  Although the hull has an energy value about one half that of coal, silica contained in the hull melts into an agglomerating glass when burned in a conventional industrial method.  Consequently, the hulls were not used as an energy source.  After removal, the hull was disposed in landfill, ground into filler for animal feed, hauled away as animal bedding or stockpiled.  Most methods of disposal were an operational expense.

Primenergy employs a commercially proven technology that converts rice hulls through the process of 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.  Unlike conventional furnaces, the controlled atmosphere within the gasifier prohibits the formation of molten silica from the hull.  This controlled process provides continuous operation, converting the expense of disposal of hulls into an energy asset.

The Jonesboro Project

At a rice mill in Jonesboro, Arkansas, rice is parboiled, dried and milled.  The energy for the production of steam for boiling and heat for drying had been provided by natural gas.  The removed rice husks were discarded.

Primenergy designed, manufactured and installed a model R-18 gasifier with a steam generating boiler and a hot air heater exchanger.  The integrated system is designed to operate on rice hulls as the exclusive energy source.  The hulls, that have been separated from the kernels of rice, require no subsequent processing or sizing and are feed directly to the gasifier from interim storage.  The design feed rate of the hulls is just over seven US tons per hour.  The equipment availability is over ninety-two percent.

Click here to see a simplified block illustration of the process.

Rice hulls are metered into the gasifier in amounts proportionate to energy demand.  In this installation, the demand signal is steam pressure.  Gases exiting the gasifier are oxidized in a vertically oriented, refractory lined, steel combustion vessel at a design temperature of 1800º F.   Temperature in the combustion vessel is maintained by the modulation of an ambient air fan, which provides injected combustion air. 

Exiting the combustion vessel, the high temperature gases split into two streams, one directed to a hot air exchanger, the other to a heat recovery boiler. The gases directed to the hot air exchanger are cooled by heat exchanging with ambient air.  Ambient air exits the heat exchanger at 650º F and is sent to the rice hull rotary dryers.  The cooled combustion gases exiting the hot air exchanger are re-mixed with the combustion gases entering into the heat recovery boiler.  Saturated steam is generated in the boiler at 125 psig.  This steam is used to boil rice.

Process control for this equipment is maintained by a computer controlled, human machine interface displaying real time process variables.  The CTR’s for this process were integrated into an existing control room.  No additional operators were required.

From the date of order to initial commissioning, less than one year was required.  All major process equipment was manufactured in Tulsa by Primenergy’s manufacturing sister company, Heater Specialists, LLC, and shipped via truck to the Jonesboro jobsite.  Construction services were provided Mohawk Field Services, Inc. under the supervision of Primenergy’s field construction manager.