July 5, 2010
ISSUE # 223

FUEL CELL

Vincent screw presses are in service with numerous biogas digesters. These digesters typically are run on dairy manure, producing methane biogas which is either burned to generate hot water, or used as fuel for internal combustion engines driving electric generators. The press is used either to separate fibrous solids, preventing them from entering the digester, or to dewater sludge solids leaving the digester.

A biogas digester installation at Gills Onions in Oxnard, California is unique. Instead of burning the digester gas, it is converted directly into electric current in a fuel cell. The fuel cell was supplied by FuelCell Energy, Inc. of Danbury, Connecticut, with overall engineering by HDR Inc. The system cost $9,500,000, and it qualified for $ 5,700,000 in
grants.

Gills Onions is a major processor of onions. Running 400 tons a day of onions, the plant produces 100 to 150 tons per day of waste peel and culls. Pressing this generates 30,000 to 40,000 gallons a day of 7 Brix press liquor which is fed to the digester.

Gills’ system starts with dosing the onion peel with hydrated lime. This breaks down the pectin, opening up the cells for greater juice yield. The reacted peel is fed to a Vincent VCS-18 shredder.

The shredded peel goes to a Model KP-24 screw press. It runs at an unusually low speed,3 rpm, with 80 psi cone pressure. The rotating cone feature is used to improve cake discharge. Still, the press cake comes out surprisingly wet.

The press cake from the KP-24 falls into a Corenco angle shredder. From there it is conveyed to a pair of Model KP-16 presses. These operate in parallel, running at only 4 rpm. A Vincent Cone Timer is used on certain varieties of peel which do not want to exit these screw presses.

The final press cake is sold as animal feed. Gills now sells two 40′ trailer loads a day, whereas they used to have two trucks running continuously, hauling peel for land-spreading.

The press liquor is allowed to acidify, rapidly taking the pH down to 4.4. This is a fermentation reaction which happens on its own accord. Then this press liquor is heated to 98 F using waste heat from an existing 850 hp natural gas engine which drives an air compressor. Once heated, the flow goes into the digester. Methane biogas is produced in this anaerobic 145,000 gallon digester.

They have to strip the hydrogen sulfide and organic sulfur from the biogas, going from 5,000 down to 1 ppm. And they must dehydrate the biogas. This is done to protect the fuel cells.

The gas BTU content is high for biogas, 700 to 800 BTU per cubic foot. It is piped, at 30 psi, to the FuelCell Energy equipment. Here a pair of fuel cells each produce 300 kW (400 hp) of DC electric current.

There are three major components in the FuelCell equipment. The first module receives the methane and breaks it down into elemental hydrogen and carbon. Heat is supplied to this unit in the form of waste heat from the fuel cell. Temperatures are elevated to 1,100 Fahrenheit.

The second module is the fuel cell itself. Actually it consists of multiple fuel cells operating in parallel. Here the hydrogen and carbon are used in a battery type process, producing direct current electricity. The fuel is combined with oxygen, releasing heat required for the process.

The third module inverts the direct current into alternating current at a frequency and voltage suitable for the grid. The power generated supplies from 20% to 40% of the power needs of the Gills facility.

During a recent visit it was noted that the screw presses are being operated at low throughput capacity in order to maximize juice yield. This yield could be improved by increasing the dosage of hydrated lime. However there is concern that the lime could affect the fuel cell.

As an alternate for increasing juice yield, we have recommended steam injection. To test this, the press’ resistor teeth will be drilled with injection holes, and a small boiler rented for a week. If it works, a permanent steam line will be run.

Another suggestion was to try a field modification of the screw of the KP-24, converting it to a step shaft design. Reducing the cross sectional area in this manner will push peel against the screen and shorten the press liquor migration distance. This modification can be done without removing the screw from the press. The resistor teeth will have to be shortened because of the increased diameter of the screw shaft.

A final suggestion made was to add wing feeders to the tips of the screws. This may eliminate the need of the Cone Timers.