ATAC i system - Energimyndigheten

cycles, repowering of old steam units into hybrid ones and combined cycles with internal
combustion engines.
The state-of-art report is a separate chapter within the literature study report.
Descriptions of existing hybrid combined cycle installations with biofuel-fired bottoming
cycle in Sweden and surrounding countries are compiled in it. The presentation shows that
hybrid combined cycles are a standard technology in many respects. Almost all described
installations have been repowered units, where an old steam cycle (or a hot-water boiler) has
been topped by a gas turbine (or by internal combustion engines in one example) and these
specific configurations have been chosen as the most rewarding ones out of various
alternatives. They have proved their advantages in commercial operation.
Parallel with the literature study, computer modeling of different cycle configurations
started in September 2001. The first step was simulation of various hybrid cycles with a gas
turbine as topping engine and a steam bottoming cycle fired with municipal solid waste and
wet wood chips. Simulations will continue with diversifying the configuration range by
including internal combustion engines as topping cycles (possibly also evaporative gas
turbines and gas turbines with reheat) and air turbines as bottoming cycles (or other approach
to externally-fired types of arrangements), as well as steam cycles with very modest steam
parameters (in very small scales).
The first cluster of results provoked an investigation of how to define the advantages or
disadvantages of hybrid combined cycle configurations in general. An approach was
developed, based on the overview of previously published materials relevant to the subject
and further extended to respond well to the actual cases. Further on, the exergy concept will
be applied with a purpose to derive a generalized and easy-to-use model for comparison
among different cycle configurations in steady state. This work is under way at the moment.
Simulations will continue also with a deeper insight into part-load features of the
various calculated configurations at varying topping-to-bottoming fuel energy input ratio.
A M.Sc. thesis work (ex-jobb) was conducted within the project during November 2001
to March 2002 by Cecilia Ekblad, a student graduating from KTH/Energiprocesser. The thesis
focused on the impact of moisture content of wet biofuels on their energy utilization and
possible incorporation of a fuel dryer into a steam power cycle. The first part of the thesis is a
Literature Study on drying methods and technologies for biofuels. The second part involves
simulations of steam power cycles firing wet wood chips. Impact of fuel moisture content on
the electric efficiency in particular was studied and assessed, including fuel drying with heat
supplied by exhaust gases or by steam extraction.
Slutsatser. Utfall i relation till ursprunglig målsättning (Conclusions):
The literature study and the state-of-art report proved that hybrid combined cycles are a
well-developed technology based on standard components and showing promising
performance. Reliability, load flexibility, fuel flexibility and adaptivity to various areas of
application at various power-to-heat ratios of such configurations are unrivaled. Moreover, it
has been observed that interest in hybrid combined cycles worldwide is steadily rising. There
is a need for further investigations especially in the small-scale range, relevant to biofuel
utilization with standard components at affordable costs.
The literature study within the M.Sc. thesis arrived at the conclusion that various drying
methods for biofuels exist and are commercially available. Their application however is sitespecific
and strongly dependent on prices. Installation costs for fuel dryers are quite high and
rule out their application in power generating units, unless the production of a certain highvalue
product requires the use of a dryer.
The simulation of steam cycles fired with wet biomass within the M.Sc. thesis showed
the extreme controversy between defining electric efficiency according to low (net) heating