Thermal recycling of spent grains – residue becomes resources


Courtesy of Flottweg SE

Introduction – the History of Beer

The origin of beer was a happy coincidence: In the fourth millennium BC, the Sumarians discovered the fermentation process.

The country of the Sumarians, called Mesopotamia, was located between the Euphrates and the Tigris. The Sumarians wanted to help an ill person swallow some bread, so and they soaked a piece in water. They simply forgot the bread in the pitcher for a few days and it began to ferment.

An intoxicating mash was generated and dispensed to the patient. Since the man was not a very heavy drinker, the mixture turned out to be very efficient. After his quick convalescence, he began to reproduce the drink – for medical purposes only, of course.

Quickly the recipe spread and the Sumarians, as the first civilized people, were beer brewers. They had invented a really divine beverage.

In the Middle Ages, monasteries, manors, and independent citizens got interested in the production of beer. Slowly small brew houses began to develop. Communities also began to found breweries in order to cover their expenses by selling beer.

The Industrial Revolution in the 19th century lead to further innovations: for the first time, it was possible to brew beer in a bottom-fermented way. Modern technologies were developed to produce beer at an industrial scale.

In every brewery, a large quantity of spent grains is generated as a by-product of the brewing process. Approximately 20 kg of wet spent grains are generated per hectolitrer of beer (1.7 lb/gal). The moisture content is about 80%, which means that the dry substance content is only 20%. Due to this moisture content the spent grains are not biologically stable. In order to prevent biological degradation they have to be disposed of or further treated quickly and continuously in a reliable manner.

The Current Major Problem – How to Get Rid of Brewery Residues?

Breweries used to sell spent grains to farmers as cattle food because of their high protein content. Now, as heads of cattle decline, the demand for cattle food is also dropping.

Other possibilities to treat spent grains (e.g. the production of bricks, faceplates, or biogas) were examined during the last several years, but were determines not to be economical.

The Göss brewery in Leoben, Austria, contacted the University of Leoben in order to find a solution for the reliable and continuous utilization of spent grains. Was there a possibility to close the internal resource cycle in a brewery with spent grains? 'Combustion of spent grains was not possible because of their high water content,“ said the President of the Institute, Dr. Werner Kepplinger. So the scientists at the institute for process engineering had the idea to apply mechanical pre-dewatering. With the help of a modified filter belt press, they managed to reduce the water content to less than 60%, a level which enables economical combustion. The process patent was awarded to the Austrian Brewing Group Brau Union Österreich in 1998. Utilization of the process is subject to a license agreement.

This kind of energy-saving brewing has already piqued international interest.

The first installation worldwide for the thermal recycling of spent grains was commissioned in December 2002 at the Göss brewery in Leoben, Austria. It was designed for a capacity of 2-3 tons of wet spent grains per hour. This complies with a saturated vapour performance of approximately 2-3 megawatts at an operating pressure of 8 bars (116 psi).

A second installation was put into operation in Nigeria.

Thermal Recycling of Spent Grains – How it Works

1. Dewatering

The wet spent grains generated during production are conveyed from an initial silo to a continuously operating belt press. There, the spent grains are being dewatered mechanically to a dry matter content of about 42 percent making them biologically stable to a large extent. The material can be stored approximately 2-3 days in an intermediary silo in order to bridge dead-time phases of the installation. The press water can either be purified by anaerobic water treatment or recycled into the production process. During anaerobic water treatment, gas containing methane is produced, which can be used as additional fuel in the incineration process. The press water can also be recycled for the production process. Trials consistently show no beer quality deterioration.

2. Biomass combustion

After the pressing procedure, the spent grains are treated in such a manner that they can be utilized as a fuel by special biomass incineration. The energy produced is recycled during the production as saturated steam or hot water. This installation can provide about 90 percent of the primary energy. The combustion plant with fully automated kettle can be adjusted to the current energy demand. The ash generated during the combustion process is suitable as fertilizer additive due to its specific composition. It can easily be given away or sold. When spent grains are found to accumulate at mash filters, there has to be an additional belt dryer before combustion. The dryer works using hot combustion fumes.

3. Spent grains incineration plant

The spent grains incineration plant consists of two main components – the belt press and the biomass combustion. The belt press, used for mechanical dewatering of the wet spent grains, was especially developed in order to allow for the special requirements of the spent grains. The product is distributed evenly between two filter belts and led under pressure across a roller system. Only through the special arrangement of dewatering rollers and press rollers with different diameters is a remaining spent grains moisture content of 58 percent achieved, due to high press forces and shear forces. Mechanical dewatering is much more efficient than thermal drying. Each percent less of residual spent grains moisture content after the belt press increases the gross yield of energy production above average. With a belt width of 2.5 m (8.2 ft), up to 7 tons (7.7 U.S. tons) of wet spent grains can be processed per hour.

For the furnace, an air-cooled feeder grate is used that is especially suited for moist combustibles. The used incineration technology was especially optimized for the utilization of spent grains. Using fire-proof brick lining inside the combustion chamber, the spent grains are further dried via radiant heat, whereas the heating value increases. In doing so, a very constant combustion performance is achieved and moisture content variations of the spent grains are counterbalanced. By zoning the furnace in different combustion zones in combination with filter equipment, low emission combustion is achieved. The thermal capacity of the used boiler construction ranges between 0.5 and 15 megawatts.

Thermal Recycling of Spent Grains – Advantages and Benefits at a Glance
  • Reduction of brewery primary energy consumption of the breweries to up to 60%
  • Secured and sophisticated utilization of spent grains– independent from external routes
  • No disposal costs
  • Further utilization of press water
  • Independent of energy prices
  • Additional combustion of further residual materials such as used labels
  • Reduction of CO2-emissions
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