Spent grain burns into clean energy

17 June 2008
Spent grain burns into clean energy

Based on extensive development work, Wärtsilä biopower plants burn now a mixture of wood chips and spent grain into clean energy. In 2009, the company will complete two combined heat and power plants for the international brewing company Scottish and Newcastle. As far as is known, this is the first time spent grain is used in a full factory scale for energy production.

The spent grain is a side stream product of the brewing process. It consists primarily of grain, i.e. cellulose, and other residual compounds, which are not converted to fermentable sugars by the mashing process.

On the other hand brewery process is consuming electricity and steam produced from fossil fuels. Thus, there is growing interest of improving the energy balance and sustainability of the process using its own, renewable raw materials.

Scottish and Newcastle (S&N) is one the world´s leading beer-led beverage companies. It offers such top beer brands as Baltika, Foster´s and Kronenbourg 1664 known all over Europe, and also several national beer brands.

The two new BioPower plants will be located on the premises of S&N's breweries in Manchester and Tadcaster, UK. Their electrical power outputs will be 3,1 MWe, and a thermal outputs 7,4 MWth. The electrical outputs in condensing operation are 4,7 MWe.

The plants will be taken into operation in the first and second quarters of 2009 and start producing steam for the breweries´ manufacturing processes and providing electricity to the UK national grid.

Profitable business from side stream

For the Scottish and Newcastle (S&N), burning spent grain as a fuel in energy production allows a low value sidestream product to be used for economically profitable and sustainable business, says Tauno Kuitunen, Regional Director, Wärtsilä.

“This, for its part, complies with the national demand for renewable energy sources in the UK,” Kuitunen notes.

Before its application for energy production, spent grain has been used for animal feed or simply dumped.

Spent grain and same type of grain based materials are scattered in several European countries.

“Plenty of the breweries and distilleries are at the capacity level, which would suit to our technology, especially when using with co-fuel like wood chips;” Kuitunen estimates.

Technology tested thoroughly

Grain originated fuels differ in the chemical compositions and in the detailed fuel properties. Partly differences are related to processes, partly to growing of the grains – soil properties and fertilizers.

According to Juha Huotari, General Manager, Biopower R&D, Wärtsilä, all the grain materials are known of difficult combustion properties. Typically ash left on the grate may contain compounds, which cause ash deformation, softening and melting at too low combustion temperatures.

"It is not possible to calculate in advance the detailed information for the dimensioning without any pre-studies with the materials," Huotari says.

To respond to these challenges, Wärtsilä has carried out an extensive fuel and combustion research program to get detailed knowledge for the process dimensioning in order to minimizing the risks.

The program ranged from detailed laboratory studies at VTT Technical Research Centre of Finland up to the container tests at the client’s brewery in Manchester.

“After running a great number of grate firing test and chemical analysis on spent grain in our test container, we convinced our customer that our combustion technology is well suited for burning spent grain,“ Huotari describes.

Demanding fuel burns with high efficiency

Mixture of sidestream product of the brewing process and wood chips is burning efficiently on a rotating grate system.

When the spent grain is delivered from the brew house, its moisture content is approximately 80%. After the moisture content is reduced to 58-60% in a belt press, the spent grain is suitable for the grate without additional drying.

”Even if the moisture content is high and the heat value is low the design is based on not to use a separate thermal drying unit in a Biopower plant,” Huotari stresses.

Compounds volatilizing on the grate and flowing in the flue gases on the heat transfer surfaces of the boiler may cause fouling and slagging and eventually leading to corrosive processes on the metal surfaces.

”Negative influences are minimized by mixing fuels and by maximizing ash remaining on the grate,” he explains.

Emissions also in control

Due to rather low combustion temperature of biomass fuel the NOx –emissions are practically all coming from fuel-nitrogen.

Spent grain has very high nitrogen content, around 3,6 weight-%, db. This is 5 - 20 times higher compared to conventional biomass like wood.

"NOx emission reduction methods based on combustion air staging are not sufficient, and therefore a more effective method, additive injection urea into the combustion chamber of the boiler, was selected," Huotari notes.

The power plants are locating inside the densely populated area, and need extremely efficient particulate cleaning of the flue gases. A bag filter was chosen for a cleaning device. Flue gas cleaning from particulates is in the scope of the client.

Modularity shortens delivery time

Wärtsilä´s BioPower plants are modular and based on well-proven standardized components and can, because of that, be delivered and installed in a short time.

A special emphasis has been put on modules, which can be constructed and pre-tested at the factory.

Their high automation level allows unmanned operations.