A brand new combined heat and power station near Vienna in Austria uses biomass in a fluidized bed combustion system to produce high efficiency energy with ultra-low emissions.

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

Figure 1: The plant and chimney stack along with part of the biomass conveyor system.
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Straight away costs were minimized as access to both heat and electrical transmission systems, and maintenance and operating personnel were readily available. The 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 on 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 and its district heating network is some 800 km in total 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 per cent of the heat output on what is one of Europ’s largest district heating networks, through the operation of ten interconnected heating and combined heat and power (CHP) facilities, including Simmering.

Figure 2: The twin air blowers used in the fluidized bed combustor
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The development was conducted by Wien Energie Bundesforste Biomasse Kraftwerk, a venture created in May 2004 by local utility company Wienstrom and OBf Beteiligungs, the wood chip supply company which owns a one-third stake. The €52 million contract for the plant was awarded to Siemens on a turnkey basis in September 2004 and construction began in January 2005. Additional supply contracts added some €5 million to the €52 million prime development cost.

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.

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.

Figure 3: The steam turbine stages
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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 per cent 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 little heating demand, efficiency drops to 36 per cent for 24.5 MW of electrical production only. The plant operates for around 8,000 hours annually with a maximum fuel capacity of 520 GWh.

Bed ash and sand is produced at a rate of 290kg/h together with 370kg/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 520 oC and 120 bar through a two stage steam turbine.

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.

The fuel (wood chips with a maximum of ten per cent bark) is currently transported from a wood handling site on the banks of the Danube some five kilometres away. Currently 80 per cent 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.

Despite additional emissions associated with the road transport aspects of fuel delivery, there is nonetheless a net saving of carbon dioxide (CO2), with wood transported from larger distances delivered via barge or rail. An equivalent oil-fired facility would produce some 144,000 tonnes of CO2 annually.

Policy measures

Within Austria a large proportion of generation is already produced from renewable energy sources, mainly hydropower with around 68 per cent. Some 50 per cent 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 Danube 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 per cent of production, Austria has set out its ambition to generate 78.1 per cent of electricity from renewable sources by 2008 through the 2002 Green Electricity Act. This is to be achieved by providing Federally decreed aid in the form of supply tariffs guaranteed for 13 years until 2015, and this July the European Commission retrospectively approved the feed-in tariffs for electricity from renewable sources.

Figure 4: The biomass delivery by truck into the hopper.
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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. With both the feed-in tariffs and the wood chip supply contracts due to run for 13 years the plant may use other fuels in future, once the scheme comes to an end. Consequently the plant has been fitted with limestone injection capability to reduce potential emissions from any future fossil component. Nonetheless, the Simmering CHP plant currently contributes some 7.6 percent 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 or even 90 per cent, comparing favourably with the most advanced combined–cycle gas fired units which are reaching up to 60 per cent 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 CHP. 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 per cent, in favour of CHP units with their impressive returns. With developments such as the Simmering biomass CHP held up as an example, it is increasingly hard to argue otherwise.