Torrefaction is considered as a kind of mild pyrolysis that is carried out in inert atmosphere (usually nitrogen). During this process, the moisture of the initial fuel and part of volatiles are removed from the biomass particles into the inert atmosphere resulting in advanced biomass properties as high energy density, hydrophobicity and durability. The most beneficial result of torrefaction process is that biomass feedstock logistics cost can be reduced, as less tones of biomass are required for a given amount of energy input. The development of a process model examining basic parameters as reaction temperature and residence time can provide useful information, which can be used for the more efficient design of a torrefaction reactor. This study presents such a process model for a straw torrefaction pilot plant. This model is based on the thermodynamic calculation of a single and/or a two batch reactor, built in the commercial software ASPEN Plus. The calculation of required flow rates of inert gas, cooling medium for a specific biomass feedstock value, is based on relevant results found in the literature. 

Abstract: The application of lignite pre-drying technologies in next generation of lignite power plants by utilizing low pressure steam as a drying medium instead of hot recirculated flue gas – combined with thermal utilization of the vaporized coal moisture – is expected to bring efficiency increase of 2-4 percentage points in future lignite power plants compared with today’s state-of-the-art. The pre-drying concept is of particular importance in Greek boilers firing lignite with a high water and ash content. The combustion of Greek pre-dried lignite has been investigated experimentally and via numerical simulations in our previous research. This study focuses on the potential integration of a lignite pre-drying system in an existing Greek power plant with dry lignite co-firing thermal share of up to 30%. The radiative and convective heat fluxes to the boiler and the over-all boiler heat balance is calculated for reference and dry lignite co-firing condi-tions by an in-house calculation code. The overall plant’s thermal cycle is then simulated using commercial thermal cycle calculation software. The net plant ef-ficiency is in this way determined for reference and dry coal co-firing conditions. According to the simulation results the integration of a pre-drying system and the implementation of dry lignite co-firing may bring an efficiency increase of about 1.5 percentage points in existing Greek boilers. It is therefore considered as an important measure towards improving plant efficiency and reducing specific CO2 emissions in existing plants.

Abstract: The pre-drying technology is expected to play an important role in the next generation of brown coal power plants. The integration of the fluidised bed drying system with internal heat utilization (WTA technology) is estimated to bring an efficiency increase of up to 6 percentage points in future brown coal power plants compared to the current state of the art. Furthermore, brown coal pre-drying is considered as an indispensable part in future oxy-fuel brown coal fired boilers. In the present work the potential utilisation of brown coal pre-drying is evaluated based on a realistic reference case scenario. Thermal cycle calculations and economic feasibility studies are carried out for a modern Greek power plant, taken as reference case and for two dry lignite firing cases. In the first co-firing scenario investigated a co-firing share of dry lignite to a thermal share of 25% is regarded. Furthermore, the configuration of a brown coal boiler fired 100% with pre-dried lignite is examined. For all cases, the main economic parameters are calculated including investment costs, cost of electricity, payback period, return on investment. The effect of the CO2 allowances price on the economic feasibility of the considered scenarios is also evaluated.