BIOMASS LOOKING FOR EFFICIENT UTILIZATION –THE REHEAT CONCEPT

Through the Kyoto protocol and its country-specific targets, power producers are strongly driven towards CO2-reducing solutions such as biomass energy production. Germany encourages this through new energy legislation, EEG (Renewable Energy Law), which gives higher subsidies for high-efficiency, CO2-reducing power generation. This has led to increased investment in biomass plants. The impact of the high efficiency requirements has enabled new technology to enter this market segment.
Before the new energy legislation was implemented, only wood shavings/straw etc. was economical for use in biomass energy production; today the more expensive fresh wood is used as well. A typical boiler fired with fresh wood generates high steam parameters, mainly high temperatures (480-540°C ) where reheat solutions are used to further increase the efficiency of the power plant. The reheat concept is based on live steam run through a high-pressure (HP) turbine. Before entering the low pressure (LP) turbine, the steam is returned to the steam generator to increase its temperature to the live steam parameters (pressure remains as is). Because of the high temperatures necessary for this process, Siemens PG has developed a standard dual-casing reheat concept to fit this new market requirement. The advantage to the power producer is that plant efficiency can thus be increased by as much as five per cent.
At the Bischofferode plant in Thüringen (Germany), Stadtwerke Leipzig uses this concept for a new 20MW greenfield power plant together with a fluidized bed boiler using fresh wood fuel.
The high live steam parameters of 128bara/532degC and the reheat parameters enable an efficiency otherwise unattainable. The solution also maximizes the subsidies set by German legislation
Siemens continues to develop solutions for this market segment. This paper will present the new technical requirements as well as their solution and our experience.