In this paper different gas-steam combined-cycles fueled by syngas produced in a local downdraft gasifier, are analyzed. At first, the downdraft gasifier model is briefly described, where waste biomass is transformed into syngas, which can be used more efficiently than the original solid biomass to generate useful power, and can be transported much more easily. The gasifier model is able to estimate, with good approximation, the composition of the produced syngas, taking separately into account the biomass drying and the pyrolysis, oxidation and reduction processes. The gasifier operates at ambient pressure using air as gasification agent and biomass as input. Among others, pomace has been considered, since, in Italy (where the plant is supposed to be located) there are many regions, like Apulia, where this biomass is largely available. Three different plant configurations have been proposed and compared in terms of overall performance. The first two, named REXC (Regenerative cycle with EXternal Combustor) and CR (Conventional Regenerative cycle), burn the syngas in an internal combustor, whereas the third one, named SyEXC (Syngas External Combustion), considers an externally fired configuration for the syngas combustion.
In the REXC cycle, a secondary external combustion system, fed by cellulosic biomass, is connected to a heat exchanger in order to increase the air temperature, as in a regenerative cycle. The combustion products pass through a primary heat exchanger placed in the external combustion system, heating the compressed air, which flows into the primary internal combustion chamber, where a defined quantity of syngas reacts with the compressed air. The turbine exhaust gas (TEG), before going back into the external combustion, partially transfers its enthalpy content to a Heat Recovery Steam Generator (HRSG) of a bottoming Rankine cycle.
The second plant configuration (CR cycle) is characterized by a main internal combustor fueled by the produced syngas and implements a conventional regenerative process. The TEG is reheated by an externally fired combustion before flowing in the HRSG of a bottoming Rankine cycle.
The last plant configuration (SyEXC) differs from the CR one only for the main externally fired gas turbine fueled by syngas, thus avoiding any costly cleanup operation and cooling.