With global demand for electricity predicted to rise 70 % by 2020, and fossil fuel pollution becoming a headline political issue, all forms of power generation are likely to be exploited even without the impetus of climate change. One of these is biomass firing.

Benefiting from approximately 4 million hectares of forest accounting for some 47 % of the country’s land area, Austria is host to one of Europe’s highest forest densities. Consequently biomass is also becoming increasingly important in Austria, and is seen as a key factor in meeting the county’s renewables targets. The current target is that by 2012, 8 % of national electricity production must be from renewables. The plant described here meets 7.6 % of that target.

In Vienna, WEBBK (Wien Energie Bundesforste Biomasse Kraftwerk GmbH, owned in equal parts by Fernwarme Wien Gelleschaft, Wienstrom and OBf Beteiligungs) has constructed the largest forest-biomass-fired power plant in Europe. It is designed to burn up to 200 000 metric tons of forest biomass per annum in the form of forest residue – chipped wood, branches and green cuttings. For this plant, trees do not need to be felled for fuel.

Simmering biomass plant is a 24.5 MWe wood burning CHP plant. It is housed in an existing building constructed in the 1960s for a 110 MW plant and cost r52 million to build. A key financial driver is the electricity feed-in price, which has been set at r102/MWh and has been guaranteed until 2019. Simmering’s owners say that this is high enough to produce a small profit margin.

Running at maximum heat output it is capable of feeding 37 MW of heat into the city’s district heating network, supplying approximately 12000 households. At maximum heat extraction the fuel efficiency of the plant is more than 80 %. If this energy total were generated in a conventional power plant, some 44 000 tons of heavy oil equivalent would need to be used, equivalent to an additional annual CO2 burden of approximately 144 000 tons.

Power generation plant

The power plant functions like a conventional steam plant, via a conventional boiler, steam turbine and generator, although it does have one unusual design feature in that the steam is not cooled directly after passing through the turbine but is fed back into the boiler and heated a second time in a so-called mixed pressure process. In this way heat extracted from the fuel is utilised more effectively and the overall efficiency is increased. Cooling water is supplied by the Danube.

Siemens AG Austria acted as general contractor for the project. Its engineers were involved in the turnkey project right up until it was incorporated into the existing power plant infrastructure at the Simmering location. In this way, Siemens PG was able to meet its own target of 100% fulfillment of the strict cost-efficiency criteria required in the fiercely competitive Austrian biomass market. Groundbreaking took place in April 2005, with cold commissioning in January 2006, followed by hot commissioning in April 2006. The first wood firing took place on May 10, 2006, with trial operation beginning at the end of July 2006. Acceptance test measurements for the plant were also carried out during this period. Since September, Wienstrom has been operating the plant in accordance with the Austrian Okostromgesetz 2002 (Green Electricity Act).

Siemens delivered the turbogenerator plant complete with downstream block transformer and all electrical engineering equipment. The components for the power plant’s own electricity generation system such as transformers, switch gear, frequency converter, cabling and infrastructure equipment such as lighting and fire detection system also came from Siemens.

The turbine set was built as a compact block (turbine, gearing and generator on a common base frame) with integrated oil system and installed at ground level.

A control system based on Siemens’ Teleperm XP product runs the site, which can be managed by a permanent staff of six operators. The biomass plant’s automation system and human-machine interface have been merged with the existing instrumentation and control, and it is monitored and operated from the central control room.

Steam circuit

The steam circuit consists of the feed water tank, the low pressure preheater, the high pressure preheater, the district heating heat exchanger, and various condensers. Feed water pumps and the main water cooling pumps as well as the condensation system (with the condenser, the main condensate system with proportioning of chemicals) were supplied by Siemens.

Foster Wheeler delivered the combustion system, a circulating fluidised bed boiler with intermediate superheating and flue gas cleaning system using fabric filters and a de-nitrification system. An FCB was chosen instead of a less expensive stationary bed because of its advantages in thermal efficiency, boiler temperature control, lower risk of caking and emissions performance. The latter is more important in Austria than elsewhere because of its very stringent emissions legislation. Flue gas cleaning plant was supplied by Foster Wheeler and Balcke Dürr.

Fuel and emissions

Wood fuel is steadily becoming more expensive in Austria, not because of availability of wood but because of the limited capacity of the industry to process it. The current price is considered to be near the limit of what the power industry can bear.

The radius of operation is fixed at 200 km, although 80% of the plant’s fuel is planned to come from within a 100 km radius. Currently the maximum distance is 70 km. For distances over 100 km, rail or water transport would have to be employed. Beyond that the area needed for supply means that the emissions created by transport make a significant difference to the emissions total.

The basic fuel of forest waste goes through a chipper before being fed onto ‘pushing floors’ which transport the fuel to the boiler via conveyor belts. The wood chip fuel is specified at a 100 mm maximum size, with a water content of 41% to a maximum of 50%, and sulphur content of no more than 0.2 % by dry weight.

Mass flow

At 24.5 MWe operation, biomass fuel is entering the combustor at 24 t/h, with 0.3 t/h of sand. 44 kg/h of limestone and 1.1 kg/h of ammonia salts together with a small amount of activated carbon are injected into the flue section. 0.29 t/h of bed ash and sand with 0.37 t/h of filter ash and dry sorption product are extracted from the combustor bed and flue stack. Flue gas emission amounts to 105.8 Nm3/h.

The Simmering site. The biomass plant is to the right of the picture. Product of the chipper. Simmering bio-plant during construction. Simmering bomass energy plant flow chart.