NO_x REDUCTION IN BIOMASS GRATE FURNACES BY PRIMARY MEASURES - EVALUATION BY MEANS OF LAB-SCALE EXPERIMENTS AND CHEMICAL KINETIC SIMULATION COMPARED WITH EXPERIMENTAL RESULTS AND CFD CALCULATIONS OF PILOT-SCALE PLANTS

摘要

Reliable boundary conditions concerning the inlet concentrations of NO_x precursors (HCN, NH_3, NO, NO_2, N_2O) are necessary for modelling the optimisation of NO_x reduction in biomass furnaces by primary measures. In the context of the work presented, a method for reducing NO_x emissions by primary measures is evaluated by combining test runs for fibre-board residues as a nitrogen rich biomass fuel in a lab-scale reactor with the simulation of chemical kinetics of NO_x formation in the gas phase. Moreover, the simulation results and effects discovered at the laboratory test facility are discussed in comparison with experimental results from a pilot-scale furnace (horizontally moving grate) with a nominal boiler capacity of 440 kW_(th) and CFD simulations (Computational Fluid Dynamics) of gas phase combustion performed for this pilot scale plant. The experimental results for the fibreboard fuel investigated showed NH_3 to be the main species released from the fixed bed. NO is of minor importance, HCN is almost negligible. A maximum release of TFN (total fixed nitrogen) can be detected by an inlet superficial velocity (air flow at inlet/cross section area of the fixed bed at inlet) of 0.06 m/s. The subsequent kinetic simulations used the release behaviour of gaseous compounds from the fuel, which had been investigated within various test runs, as boundary conditions and applied the ideal reaction models plug flow (PFR) and perfectly stirred reactor (PSR) of the software code CHEMKIN 3.5~(~R). By means of these simulations, the optimum combustion conditions for NO_x reduction by primary measures were investigated. Simulations of the reaction kinetics based on the measurement results of the lab-scale experiments showed a strong influence of the oxygen concentration in the reacting gas phase on the TFN and NO_x reduction potential, and, for the PSR model, also an influence of temperature. Both mechanisms predicted the same trends for these parameters but with diverging TFN reduction rates. A comparison with results from test runs at the pilot scale furnace and with CFD simulations validated the influence of temperature and oxygen concentration on NO_x reduction. The presented approach proved to be sufficient for the definition of operating conditions for Low-NO_x combustion in biomass grate furnaces. However, the a priori prediction of NO_x emissions as well as additional reduction potential by optimisation measures has not been possible so far. This is due to a strong sensitivity of the predicted NO_x reduction rates to the reaction mechanism applied as well as to reaction conditions, inlet concentrations of N species and CH_4. These tasks must be investigated in more detail in future in order to further improve the accuracy of NO_x prediction.