ISFTA is one of the six Institutes of CERTH, which is the largest research center in Northern Greece. The mission of CERTH/ISFTA is to carry out fundamental and applied research emphasizing on development of novel products and services for solid fuels and their residues.

The Institute is located at Ptolemais, while also maintaining offices at the cities of Thessaloniki and Athens. CERTH / ISFTA is the leading research organization in Greece for solid fuels. Among its activities is the CFD simulation of coal fired boilers. The main technologies investigated, using the Fluent platform provided by SIMTEC, are the Pulverized Fuel (PF) combustion and the Circulating Fluidized Bed Combustion (CFBC).

Regarding PF modeling CERTH/ISFTA has developed custom built subroutines (User Defined Functions – UDFs) for two and three step intrinsic char combustion model, drag coefficient for non –spherical particles and radiation models that where integrated to Fluent platform. Moreover, as concerns the challenging CFB modeling, CERTH / ISFTA has developed Energy Minimization Multi-Scale scheme (EMMS) for the simulation of the drag coefficient inside fluidized beds.

SIMTEC is a valuable partner for CERTH / ISFTA since it provides consulting and guidance especially for proper integration of new theories in the Fluent platform.

Partial oxy – fuel in PF boilers

CERTH / ISFTA within ECOSCRUB /RFCR-CT-2007-00009 EU funded project, simulated the industrial PF boiler of MELITI 330 MWe PPC power plant. Three operating principles were investigated. The current, reference case (case A), partial oxy – fuel (Case B) and full oxyfuel (Case C) combustion modes were simulated. In order to simulate the effect of elevated oxygen partial pressure a three step intrinsic combustion model was coded and integrated to Fluent platform. Moreover, due to the elevated H2O and CO2 flue gas content the custom built Exponential Wide Band Model (EWBM) radiation model was incorporated.

This numerical investigation concluded that the implementation of oxy-firing conditions in large scale boilers is quite sensitive as far as its efficient operation is concerned. Phenomena such as char burnout percentage, exit temperature, heat transfer, hopper losses and NOx emissions were efficiently and accurately simulated.

Image 1, 2 and 3 depict the prediction for wall fluxes, NOX concentration in different boiler heights and temperature profiles respectively for cases A to C. The CFD model was validated for the operating conditions of case A with experimental data for exit temperature, NOx and oxygen concentration.

Co – combustion of biogenic fuels (cardoon, SRF)

CERTH / ISFTA has numerically investigated the co – combustion mode for biogenic fuels like cardoon and Solid Recovered Fuels (SRF) in large-scale utility boilers.

Within FP7 DEBCO (Grant nr. 218968) the simulation of cardoon co – combustion scenario was undertaken for a 300 MWe pulverized-fuel, tangentially fired PPC boiler (Kardia), operating with low quality lignite. The CFD model takes into account the non-spherical form of the biomass particle, which influences the drag coefficient and its devolatilization and combustion mechanisms. Simulations under different co-firing rations and biomass particle sizes were performed. Validation of the simulations was performed using operating data for the reference case. The numerical results provided useful conclusions regarding the maximum allowable biomass particle size and substitution ratio for an efficient boiler operation.

The co – combustion of SRF with brown coal was also numerically investigated within the project RECOFUEL (TREN/04/FP6EN/S07.32813/503184). In order to overcome the difficulty of the complex, inhomogeneous nature of waste recovered fuels, SRF was modeled as a mixture of two different fractions, the biogenic and the plastic one. The proposed combustion mechanism of the plastic fraction is incorporated in a commercial CFD code and validated against available experimental data. Based on the numerical results, the optimum co-firing concepts regarding the more efficient operation of the boiler was identified.

Simulation of isothermal CFB flow

Fluidized bed technology boilers are characterized by an elevated particle loading that necessitates the Eulerian description of the particulate phase. However, conventional drag model such as Gidaspow’s assume homogeneous conditions in each control volume and induces significant error. ISFTA developed a sophisticated sub – grid Energy Minimization Multi – Scale model (EMMS) that was introduced in fluent platform through UDF. The new drag model was tested and validated in the isothermal simulation of a 1.2 MWth CFBC pilot unit within EU funded project CFB800 RFCR-CT-2005-00009. The isothermal CFD model was also implemented in a 3D full loop CFD isothermal simulation of a transparent plexi – glass CFB carbonator. Moreover, in this simulation, the re – circulation system of the unit i.e. cyclone, down – comer and a pneumatic valve (Loop Seal, working as flow regulator), was included in the simulation. The Pitman – Schaffer – Gray – Stiles yield criterion was used in the pneumatic valve, for the formulation of the granular material stress tensor, to address the modeling of high frictional forces within the Loop – Seal. Simulation results (pressure profile and re – circulation flux) were compared with the corresponding experimental data. The error in the re – circulation rate is less than 2% while in pressure prediction less than 10%.