Two 800 kW containerized gas engines are the heart of the Géotexia Mené plant Credit: MWM

At Mené in Brittany, France, an imaginative energy production and waste disposal project featuring biogas-fired engines with ABB turbochargers is benefiting the region’s ecology and economy, reports Jonathan Walker.

With their high levels of total energy utilization, cogeneration plants based on spark ignited lean burn gas engines have long been at the forefront of the efficient, low emissions conversion of fossil fuels into heat and electrical power. The electrical power can be fed to the grid or an industrial process, and where there is a use for the heat from the engine, over 90% of the energy released from the fuel can be put to good use.

The foundations of this success are the very high efficiencies achievable by this type of engine, the lean burn principle and the use of clean burning methane. The result is efficient power combined with extremely low emiss-ions of both the pollutant NOx (oxides of nitrogen) due to relatively low combustion temp-eratures and minimized emissions of the greenhouse gas carbon dioxide (CO2) due to the simplicity of the methane molecule with its four atoms of hydrogen to one of carbon.

Each type TCG 2016 V16 C gas engine produces 800 kW of electrical power and roughly the same in thermal energy for maintaining fermentation and drying processes Credit: MWM

ABB Turbocharging’s essential role in this is providing high pressure, high efficiency turbochargers – from the TPS range on many modern high-speed gas engines – to supply the right amount of air and ensure the effective combustion of the fuel. This implies that maximum power has been extracted from the fuel and that a minimum of pollutants and greenhouse gases are left behind. Most recently, ABB’s Power2 two stage turbocharging has extended this beneficial role by enabling the stronger Miller Cycles that facilitate further rises in gas engine efficiency, power density, load acceptance and insensitivity to ambient conditions.

Going bio

In recent times, another element has been added to the lean burn gas engine’s already favourable fuel efficiency and emissions balance sheet – one that puts gas engine cogeneration plants right at the forefront low carbon energy production. By burning methane derived from renewable biological sources, broad carbon neutrality is achieved, as about the same amount of CO2 is returned to the atmosphere during combustion of biofuels as was taken out during the growth of the organisms supplying them. And there is a further greenhouse gas bonus, since the methane which would be released during the natural decomposition of flora and fauna is eliminated, and methane can absorb around 20 times more infra-red radiation than CO2, which is the major factor in global warming.

The Géotexia Mené plant has been designed to deliver substantial environmental benefits along with renewable heat and power Credit: MWM

Hence, in Europe, the US and elsewhere, cogeneration plants with engines from many of ABB Turbocharging’s gas engine customers have long been indispensable at larger sewage works or waste disposal sites, where they burn the methane in the gases released during the decay of the waste products to produce heat and electrical power.

Organic waste from both plants and animals is converted through fermentation in digesters into biofuels including biogases. In the best cases, as well as producing heat and electricity, the process can convert substances that could harm the environment into useful residues such as fertilizers or solid fuels for industrial processes.

Innovative and imaginative

Many projects involved in producing and exploiting biogases in cogeneration plants are highly inventive. High-efficiency spark ignited lean burn gas engines from German builder MWM feature in the Géotexia Mené plant near Mené in Brittany, France. This plant produces 700 m3 of biogas per hour from a combination of pig manure in the form of both slurry and sludge (40,000 tonnes per year), animal fats (35,000 tonnes per year) and other food processing waste, delivered from farms, abattoirs and the food processing industry in the Mené area. This has an important additional environmental benefit, since Brittany produces more than half of France’s pork, and the slurries and sludges can easily compromise drinking water.

Biogas from the plant’s large capacity digesters is burnt in a pair of containerized MWM type TCG 2016 V16 C gas engine gensets. Each produces 800 kW of electricity, which is sold to French national utility EDF.

Typical of cogeneration applications, the engines deliver a roughly equivalent output of thermal energy, which is used to maintain the fermentation of the manure and fats as well as for drying the residues. This leads to the complete recycling of the residues as dry and liquid fertilizers with chemicals that enhance their value as plant growth accelerators and make them a viable substitute for industrially produced products.

Finally, the waste water from the processes is cleaned by hydrolysis and inverse osmosis and then used for the irrigation of a plantation of fast growing willow trees. Including the plantation, the Géotexia Mené facility occupies a total of 13 hectares.

Passionate protagonist

The plant was commissioned in the winter of 2010 and Dominique Rocaboy reports that it has met expectations in full. As well as being a director of Géotexia Mené, he is a local farmer and a prime mover in the project, having lobbied for more than 10 years for its implementation. He points out that it is part of a plan to achieve important, interrelated ecological and economic goals in the region. These include improving the ‘bocage’ landscape of winding lanes between low embankments topped by hedges, a trademark feature of Brittany and Normandy.

Hence, as well as the production of renewable, carbon neutral energy at the cogeneration plant, the energy targets also increased the use of wood pellets in boilers for district heating – indeed pelletization is the ultimate fate of the willow tree plantation adjacent to the Géotexia Mené cogeneration plant.

A distinctive ‘bocage’ landscape, such as those that the Géotexia Mené scheme is aimed at protecting Credit: Matthieu Debailleul

Recreating the embankments and hedges of the bocage, removed by earlier generations of farmers to maximize crop yields, is primarily to reduce soil erosion, but has gone hand in hand with planting of trees on the basis of ‘short rotation forestry’. The plantation at the Géotexia Mené site is an example of this process, where fast growing saplings such as willows are planted in fields like any other crop and harvested after only about five years by self propelled machines especially developed for the purpose.

This article is reprinted from ABB’s charge! (Issue 1/2012).