Spread it around: Biogas in the grid

Tensions in the Middle East and the gas price conflict between the Ukraine and Russia have generated a great deal of unease in Europe over energy supply security. The answer to the problem, says Simon Ford, is home grown: biomass energy that can be distributed utilizing the existing gas pipeline network.

Spiralling gas prices and, in particular, concerns about security of supply have heightened interest in what was, until recently, a rather lethargic European gas market. Faced with dwindling production within Europe from a small number of countries (Norway, UK, the Netherlands and Germany) Europe is becoming increasingly dependent upon an equally small number of countries outside Europe which hold the world’s largest natural gas deposits, namely Russia and the Middle East. Europe, as a net importer of natural gas, is increasingly in competition with Japan, as well as emerging energy-hungry economies like China or India, for the same gas supplies. According to estimates by the International Energy Agency (IEA), there is a risk that Gazprom’s foreign customers will be faced with a supply shortage of a hundred billion cubic metres of gas by 2020. If gas prices are to be prevented from continuing to spiral out of control, alternatives must be found.

The thorny issues of security of supply and economic energy stocks can be addressed through the use of renewable energy resources, which conveniently also utilize existing infrastructure such as the gas or electricity distribution network. If biogas production is continuously expanded until 2020, then biogas alone could cover 10% of Germany’s natural gas consumption and substitute more than 30%–50% of gas imports from Russia, currently Germany’s largest supplier.

Feeding biogas into the natural gas grid

The use of biogas in cogeneration (CHP) for the decentralized production of power and heat is now a widely accepted technology. Electrical power has a more extensive transport network at its disposal than any other source of energy and the total efficiency of electricity generation from biogas lies between 30%, without the use of waste heat, and 68.5%, with 100% use of waste heat. However, the industrial use of biogas for electricity generation still very much depends on the ability to utilize heat on site at the biogas plant, and this has limited the potential development of biogas applications.

This aerial view of the biogas plant at Pliening clearly shows the biomass stacked up and ready for the digesters

Alternatively, feeding purified biogas (biomethane) into the natural gas grid offers access to a similarly well-developed transport network, but also opens the door to completely new fields of application.

Offering much more flexibility than with decentralized use – by taking advantage of the existing distribution infrastructure – the use of purified biogases does not depend on a specific location close to a biogas plant. Irrespective of where the production site is located, purified biogas can also be used as a renewable, net CO2-free fuel for cogeneration plants, vehicles or in applications such as stationary fuel cell CHP units.

Generating green gas

In principle, all agricultural biomass can be used as a basis for fermentation in biomethane plants. Today, maize, grain and grass are most commonly used. The feedstock is anaerobically digested in an enclosed and heated fermentation system. Various types of anaerobic micro-organisms are involved in the controlled biogas production process, whose ratio to each other is determined by the impact of the initial substances, pH value, and temperature during the course of fermentation.

Due to the adaptability of these micro-organisms to the process conditions, almost all types of organic substances can be cultivated for use during the fermentation process, although they yield varying quantities of energy. For example, 1 hectare of maize will produce approximately 2 kW of continuous supply of electricity, while 1 hectare of grain with produce around 1.5 kW.

This equipment is used to purify the raw biogas ready for injection into the grid

Biogas is a saturated gaseous mixture made up of 50%–60% methane, the rest being largely made up of carbon dioxide with trace amounts of hydrogen sulphide, nitrogen and oxygen. To be fed into the natural gas grid it has to be purified and upgraded to natural gas quality. The methane content must first be enriched through the removal of carbon dioxide, following desulphurization, and water removal. All other interfering substances are removed through low-level gas combustion.

After upgrading, the biomethane contains at least 96% methane: it is important to ensure that threshold limits for nitrogen, oxygen and hydrogen sulphide are not exceeded and the end-product must comply with the requirements and regulations of the German Gas and Waterworks Association (DVGW).

The economics behind the concept

Uncoupling the location of biogas production, which is determined by raw materials, and the location of energy production – determined in turn by the most economic location to convert this resource into electrical power and heat – substantially raises the energy yield. This, ultimately, explains why major power suppliers and municipal utilities are interested in this technology.

The concept was based on the legal framework now afforded by the German Energy Industry Act (EnWG), the Gas Network Access Regulations (GasNZV) and the law giving priority to the use of renewable energie (EEG). These legal frameworks allow the operator of the CHP plant to receive not only the revenue from the heat, but also subsidies for renewable electricity. Feed-in tariffs are received ‘virtually’ just as though the CHP unit, which can be located in any other part of the country, was directly associated with the biogas plant. The condition is that the amount of biogas energy fed into the grid is balanced with the amount taken out over time, to ensure network integrity. An additional advantage of the virtual trade is that the gas network capacity can distribute geographically but also balance out seasonal fluctuation in energy requirements, in particular for heat energy. The revenues from the subsidized electricity feed in tariffs and from the sale of heat, which is not regulated, minus costs from CHP operations and charges for the use of the natural gas grid then determine the market price for the biomethane at the feed-in point.

Click here to enlarge image This system flow diagram illustrates the overall process of converting organic materials into high quality natural gas and its usage

In the mid-term, government initiatives as well as local, regional and national utility companies’ self imposed quotas for renewable energy production are likely to push up demand for biomethane.

But for large-scale biogas plants, the future lies in favourable feed-in tariffs and in the substitution of costly imported gas. An additional factor that may well influence the market is mobility. Natural-gas-powered transport still only plays a small role within the German market (60,000 vehicles to date) although popularity is rising, particularly among local transport authorities. While in the short-term there is potential for biomethane as a fuel, a significant role could be played in the mid-term by biomethane-to-liquid by using biomethane as the input for the Gas-to-Liquid process.

Carbotech already has a total of 20 reference projects in five European countries, and many new biomethane feed-in projects are set to begin or are currently under construction. Furthermore, other methods of gas purification are currently being investigated in pilot projects, based on membrane technology and amine wash.

Stacking up the benefits

The use of processed biomass in the national gas distribution grid yields a number of distinct benefits. For instance, cultivated biomass is so-called ‘storable energy’ in that – in contrast to solar and wind power for instance – biomethane can be continuously produced and temporarily stored in response to demand. As a result, such technologies are particularly suitable for ensuring a sustained power supply which contributes to grid stability. The production of biomethane may also become part of a closed agricultural cycle with renewable raw materials from farms in the region used to produce biomethane and the odourless fermentation residue returned to the fields as fertilizer. In addition, crop rotation of energy plants enables various types of energy crop to be harvested each year, for example maize harvested in summer, and rye in winter. This enables the EU’s stricter requirements on the agricultural sector to be optimally complied with, but does not impact on plant operations.

However, perhaps the key advantage concerns the economics of biomethane production. At around 8 Eurocents/kWh, the cost of producing biomethane is already very close to the consumer price of natural gas. When biogas is conditioned to be fed into the natural gas grid, the plant’s own electricity consumption, including heating, is around 15%. A further 5% is lost through processing. Yet this still leaves an impressive net efficiency level of 80% for the gas transmitted via the gas grid to CHP plants.

With advances in technology and anticipated rises in natural gas prices, the sector will gain a further boost through the expansion of the range of biogas applications.

Ulrich Schmack, managing director of Schmack Biogas AG, expects to see the gas produced by his plants compete with imported gas on price in five to seven years time. Current price trends on the energy markets suggest his prediction may yet come to pass as political crises or the gas price conflict drive commodity prices.

With just two million hectares, approximately 12% of the agricultural area in Germany, as much as 50% of Russian gas imports could be substituted by biogas.

Biogas engineering and the gas purification technology is only in the initial stages of development and technical advances will lead to further cost reductions in biogas production. This fact, along with rising energy prices and finite fossil fuel reserves, will improve the competitive position of alternative energy sources. Furthermore, the ability to feed biogas directly into the natural gas grid opens up completely new dimensions to the gas market and brings Germany one step closer to its goal of increasingly meeting natural gas demand with ‘home-grown’ gas.

Simon Ford works with Schmack/Aufwind in Germany
e-mail: [email protected]

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Biomethane plant in Pliening

The biomethane plant in Pliening, north of Munich, in Germany, utilizes maize silage, maize-corn cob mix (MCC), whole plant silage (WPS) and grain. Biogas produced at the facility is fed directly into the natural gas grid.

Schmack Biogas AG launched the Pliening project in collaboration with Renewable Energy Systems (RES) and Aufwind SchmackBetriebs GmbH & Co. KG. The technology used for purifying and processing the gas was developed by a subsidiary of Schmack Biogas, CarboTech Engineering GmbH, based in Essen in the heart of Germany’s industrial and mining region, the Ruhr valley.

The three digesters at the Pliening site

The plant has a feed-in capacity of 40 GWh per year and once the raw biogas has been upgraded to natural gas quality this amounts to around 3.9 million cubic metres of biomethane per year, generated using the pressure swing adsorption (PSA) process.

The process used to upgrade raw biogas to natural gas quality is an established technology that has been in use for many years. The biogas is cooled to remove water, while any hydrogen sulphide that hasn’t already been removed via biological desulphurization in the fermenter is extracted via an activated charcoal filter. The biogas is then compressed into a pressurized column containing specially treated activated charcoal. Pressure swing adsorption makes use of the different adsorption properties of the component gases present in the biogas.

Click here to enlarge image

The activated charcoal, acting as a molecular sieve, adsorbs and selectively removes the unwanted gases, while the methane becomes enriched and is removed in the decompression phase. Six pressurized containers act in series, going through the ‘pressure swing’ process in turn to produce a continuous output stream.

To ensure that the fermenters are supplied with sufficient heat, a stream of raw biogas is combusted in a CHP unit, the electricity produced being fed into the public grid.

Click here to enlarge image This diagram of the biomass plant at Pliening shows the major components of the system

The purified gas produced is then fed into the gas grid of Stadtwerke München (Munich City Utilities). The power supplier, E.On Bayern Wärme GmbH, is a project partner and buys the biomethane. The project was financed by Aufwind Schmack, which not only owns the plant but operates it with a team of four employees from the local area, sharing the work in shifts.

The project in Pliening is of considerable strategic importance, being the first time this technology has been used to upgrade biogas to natural gas quality on an industrial scale.