Biomass on the boil: A new wood-fuelled combined heat and power plant in Simmering, Austria

A brand new biomass-fired combined heat and power station near Vienna in Austria is using a state-of the art combustion system to produce high efficiency energy with ultra-low emissions. By David Appleyard.

The Simmering energy complex on the outskirts of Vienna is home to Austria’s largest, and most modern, biomass-fired combined heat and power (CHP) station. The 66 MWth plant, which was officially commissioned in October 2006, first began development in May 2001 with a feasibility study conducted by the City of Vienna. The study covered the development of a biomass-fired CHP facility on the site of an existing power plant, focusing, where possible, on utilizing the infrastructure of the earlier development.

Straight away this approach began to reduce costs as access to both heat and electrical transmission systems, as well as maintenance and operating personnel, were readily available. The new plant also uses the existing chimney cladding along with additional infrastructure such as the machine house and cooling water systems which use water from the Danube. In addition, existing rail and road infrastructure simplifies delivery of biomass to the facility, again minimizing costs. Furthermore, by keeping the development on an existing site, planning and other permissions were more easily obtained.

Plant development

With a population of more than 1.6 million people, the city of Vienna covers an area of 415 km2. Its district heating network is some 800 km in length and supplies around 240,000 dwellings and 5000 industrial customers with energy for space and water heating purposes. Operated by Fernwärme Wien, energy from waste and cogeneration accounts for 97% of the heat output on what is one of Europe’s largest district heating networks, through the operation of ten interconnected heating and combined heat and power (CHP) facilities, including the new biomass plant at Simmering.

The development was conducted by a special purpose organization, Wien Energie Bundesforste Biomasse Kraftwerk, which was created in May 2004 by local utility company Wienstrom and OBf Beteiligungs, a wood chip supply company which owns a one-third stake.

Wien Energie Bundesforste issued a call for tenders in early 2004 with planning approval and other requisite permits in place by July of that year. By August 2004 legally binding approval had been granted without any objections.

The €52 million contract for the plant was awarded to Siemens on a turnkey basis in September 2004 and, following the November demolition of the earlier boiler island, plant construction began in January 2005. Additional supply contracts added some €5 million to the €52 million prime development cost.

The 66 MWth biomass heat and power plant in Simmering, Vienna

With Siemens as the lead contractor, Foster Wheeler Energie was employed to construct the boiler island using its CFB technology with intermediate superheating.

The circulating fluidized bed system uses a bed of bubbling sand which has hot air blown through it as a combustion platform, rather than vibrating grates or similar, which reduces the risk of corrosion and caking problems (See boxed text on page 92).

The technology was also selected on the basis of its improved environmental performance when compared with other technologies, despite the higher costs.

Biomass supply

The fuel for the power plant – wood chips with a maximum of 10% bark – is currently transported from a wood handling site on the banks of the Danube some 5 km away. Currently 80% of the wood supplied comes from less than 100 km away, with the bulk of supplies from a maximum of 70 km from the facility. However, while there are additional emissions associated with the road transport aspects of fuel delivery, there is nonetheless a net saving of carbon dioxide and wood transported from larger distances is delivered via barge or rail. An equivalent oil-fired facility would produce some 144,000 tonnes of CO2 annually.

The wood, which has a number of specific parameters such as a maximum of 50% water content by mass, is currently delivered five days per week. The material is delivered into a hopper which uses a pushing floor system to transport the fuel to the boiler via conveyor belt.

There is a much larger potential for biomass within Austria, which has some 4 million hectares of forest amounting to around 47% of the country’s surface area. However wood prices have already doubled in recent years as demands from larger biomass facilities and elsewhere have increased. Furthermore, because prices for primary resources such as oil and gas have almost doubled over the past two years generators have shifted generation.

Delivering woodchip to the plant

The Austrian biomass association has suggested that Austrian biomass production for electricity generation will have to steadily increase to meet mounting demand.

Existing infrastructure may be expanded if the market dictates, but there is a maximum of 5%-6% of the country’s total electrical capacity is available from natural wood on a sustainable basis, although fast growing plants such as miscanthus are another possibility.

Policy measures

A large proportion of Austria’s electricity is already produced from renewable energy sources, mainly hydropower, at almost 68% of the total in 1999. Some 50% of electricity demand is met by hydro projects on the Danube alone but the current demand-driven capacity growth in Austria is expected to require the equivalent of another set of Danube dams within a decade. With some 100 MW of additional thermal capacity required annually, the current demand-led boom is expected to last for at least another three or four years. However, while gas and coal still account for about 23% of production, Austria has set out its ambition to generate 78.1% of electricity from renewable sources by 2008 through the 2002 Green Electricity Act or Ökostromgesetz. This policy, which supersedes the Elektrizitätswirtschafts- und organizationsgesetz 1 and 2 acts (ElWOG 1 and ElWOG 2), proposes measures to generate 9% of electricity from small-scale hydroelectric plants and 4% from other renewable energy sources, the rest being generated from large-scale hydro. This is to be achieved by providing federally decreed aid in the form of supply tariffs guaranteed for 13 years until 2015 and in July 2006 the European Commission retrospectively approved the feed-in tariffs for electricity from renewable sources.

Through the Ökostromgesetz, in force since 2003, fees which had varied from state to state are replaced with a uniform fee for power generated by combined heat and power plants, renewable sources and small hydro power plants. The targets first set by ElWOG functioned as minimum quota for the nine provinces, which set the feed-in tariffs independently, and which resulted in confusion between provinces for developers. For example, tariffs for solid biomass ranged from €0.8-0.12 per kWh in Burgenland depending on time and season and €0.067-0.1745 per kWh in Carinthia depending on quality of fuel and size of installation.

Under the Ökostromgesetz, for wood chip biomass the current feed-in tariff is €0.102/kWh to €0.16/kWh depending on the size of the facility. This compares with wind at €0.078/kWh for new plants.

The Simmering plant receives the lower €102/MWh rate and Ludwig Gockner of Wienstrom suggests that the company makes a margin of ‘a few percent’ at this level.

With both the feed-in tariffs and the wood chip supply contracts due to run for thirteen years the plant may use other fuels in future, once the feed-in tariff programme comes to an end. Consequently the plant is fitted with the limestone injection capability to reduce potential emissions from any future fossil component. Nonetheless, the Simmering CHP plant currently contributes some 7.6% of the current national renewable energy target.

Biomass CHP – as clean as a whistle

Combined heat and power installations already hold a well deserved reputation for high efficiencies of 80%-90%, comparing favourably with the most advanced combined cycle gas-fired units which are reaching a maximum 60% overall thermal efficiency. However, while efficiency is the modern watchword in today’s cost and carbon conscious world, the combustion of carbon neutral biomass adds a new dimension to the Simmering plant. With policy support boosting the use of renewable forest resources, all but doubling prices over the last year or so, the Austrian biomass association has called for an end to the construction of power-only biomass plants, which have efficiencies of say 25%-35%, in favour of CHP units with their impressive returns. With developments such as the Simmering biomass CHP held up as an example, it is hard to argue otherwise.

David Appleyard is a freelance energy journalist

Technical details

Air blowers
Balcke Durr was contracted to supply the bag filtration system for particulates from the flue gases which are stripped of NOx using a selective catalytic reduction process, also supplied by Foster Wheeler. Limestone injection systems have also been installed for SOx reduction, but the system is not currently used as the wood biomass produces very low levels of sulphur. In addition, activated carbon is injected to reduce emissions, although the plant operates at far below the current maximum emissions limits.

Operating parameters
The plant, which has a maximum fuel capacity of 66 MW thermal, produces up to 24.5 MW electrical and a maximum 37 MW of thermal energy for district heating, and can use a total of 200,000 tonnes of biomass annually at a rate of around 24 t/h or 75 m3/h.

With a maximum thermal efficiency of 80% at full heat capacity the plant produces some 16.2 MW of electrical capacity, 37 MW of thermal capacity and can supply around 12,000 households with heat during winter. During summer months, when there is no heating demand, efficiency drops to 36% representing 24.5 MW of electrical production only. The plant operates for around 8000 hours annually with a maximum fuel capacity of 520 GWh.

The plant’s high-pressure, low-pressure turbine is highly efficient, achieving 80% thermal conversion

Bed ash and sand is produced at a rate of 290 kg/h together with 370 kg/h of filter ash and dry sorption products. The plant uses 300 kg/h of sand for the fluidized bed.

The steam circuit comprises the feed water tank, the low and high pressure pre-heater, district heating heat exchanger and condensing circuit. Steam exhausted from the turbine does not pass directly to the condensing circuit but is fed back into the boiler in a mixed pressure process in order to maximize efficiency. The steam cycle operates at some 520ºC and 120 bar through a two stage steam turbine.

The twin air blowers used in the fluidized bed combustor

The feed water pumps, main water cooling pumps, condensing system were supplied by Siemens along with the steam turbine and generator system. Siemens also supplied the electrical engineering and the block transformer together with the instrumentation and control system.

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