NREL Biogas Digester

April 9, 1996

A unique VP-6 screw press is currently being manufactured for the National Renewable Energy Laboratory. This press will be used at a demonstration pilot plant to be erected in Stanton, California.

The pilot plant will demonstrate a High-Solids Digester being used to convert wastes into biogas and compost. The unique characteristic of the High-Solids Digester is that it produces up to ten times as much gas per unit of tank volume as compared to conventional "wet" or low solids reactors. Conventional reactors cannot handle more than 10% solids, whereas the High-Solids Digester works at solids concentrations close to 35%.

This high capacity characteristic will greatly reduce the capital investment required in biogas production. In addition operating costs will be lower because the system generates greatly reduced quantities of effluent wastewater.

The biogas produced is about 55% methane (the main component of natural gas) and 45% carbon dioxide. As such, it is a valuable fuel suitable for electricity generation.

The sludge left over after the fermentation process will be pressed in the Vincent machine. Earlier testing has reportedly resulted in press cake with greater than 50% solids content. The press cake will be used as a mulch material, while the press liquor will have use as a liquid fertilizer in an ornamental foliage farm. (Because of contaminants in municipal trash, it cannot be used on food plants.)

Most of the waste to be digested will come from municipal trash. Because of the high paper content, this shredded material will enter the digester at 70% solids. The high solids content contributes to a rapid digestion process.

Some oily or fatty wastes, which have only 15% to 20% solids, will be added from food (fish) processing facilities. The microorganisms that occur naturally in these FOG (fat, oil, grease) waste materials accelerate the fermentation process. Also, they have been found to increase methane concentration by 10%.

Most of the cellulose paper fiber entering the digester is converted to sugar. However, fibers left over after digestion play a vital role as a press aid. Normally digester sludges are made up of particles way too small to be captured in a screw press. They can have a strong tendency to blind press screens.

The VP-6 press that will be used at the demonstration facility has several optional features. Polyurethane wipers will be used to minimize screen blinding. The rotating cone feature will prevent channeling through the press. A frequency inverter, in conjunction with an inverter duty motor, will allow testing over a wide range of screw speeds. Commercial operations are anticipated to use much larger presses.

Issue 41