Effects of fuel properties, temperature and pressure on fuel reactivity, formation and destruction of nitrogen oxides and release of alkalis

This study assists in the development of advanced combustion technologies (PFBC, IGCC) with high efficiency of electricity production from solid fuels (η = 47-50%) and in minimizing emissions of nitrogen oxides in atmospheric and pressurised FB combustion. In addition to the work done within the LIEKKI 2 programme, research work has been carried out inside the Joule 2 programme of EU. The research work may be divided into three parts: (1) Study of NxOy formation and destruction, (2) Study of fuel reactivity at elevated pressures, and (3) Study on alkali release from different coals. Experimental work was carried out utilizing a novel pressurized entrained flow reactor (PEFR) completed in VTT Energy in the autumn 1992. The device was unique in the world between 1992 and 1995. The effects of fuel properties on the formation of N2O and NO at conditions typical to FB combustion were studied for a large number of fuels including different coals, coal-derived char, peat, and bark. This work started before 1993 and was completed in 1995. FTIR technology was utilized for on-line gas analysis of N2O, NO, and NO2. The ratio fuel-O/fuel-N was found to be the most important fuel factor determining the formation of N2O and NO from volatile fuel-N. Only a small part of N2O is formed from char-N. The effect of pressure (0.2-2.0 MPa) on the formation of N2O, NO, and NO2, and destruction of NO with ammonia (Thermal DeNOx, experiments at 0.2, 0.5, and 1.5 MPa) and urea (NOxOut, experiments at 0.5 MPa) were studied in cooperation with Aabo Akademi University (AaAU). VTT performed the experimental work and AaAU the kinetic modelling. A part of these results are presented in the report by AaAU. Increase of pressure decreases NO formation and increases NO2 formation. The behaviour of N2O is more complex. Both destruction processes for NO seem to operate well at elevated pressure, although clear effects of pressure on the temperature window of Thermal DeNOx process were found. Dominating reaction mechanisms were found to illustrate the reasons for the observed trends (by AaAU). Fuel reactivity study at elevated pressures (0.2-1.0 MPa) was carried out in co-operation with VTT and Tampere University of Technology (TUT). VTT performed the experiments with PEFR. TUT developed a novel pyrometric method to measure temperature and size distribution of burning fuel particles through the observation ports of the PEFR. This method was successfully utilized. Experimental work was combined to single particle combustion modelling showing the reasons of the observed phenomena. The results of modelling are presented in another report of VTT. Pressure increased strongest the burning rate of less reactive coal (anthracite), which is difficult to burn completely in atmospheric PF and FB boilers. The effect of pressure strengthened with decreasing particle size. Usually the effect was observed between 0.1 and 0.5 MPa. Further increase of pressure did not contribute to the burnout. Pressure transferred the mechanism from chemical control to diffusion control. To conclude, pressurized combustion produces higher efficiency and more complete burnout of high-rank coals. Sodium and potassium in vapours and aerosols cause fouling on heat transfer surfaces of the furnaces and high-temperature corrosion of gas turbine blades. The study of vaporisation of these elements from different coals was carried out in co- operation with VTT and TUT. VTT performed the experiments with PEFR. TUT completed the development of an on-line Plasma-assisted measurement method for Na and K in vapour phase and aerosols (PEARLS) and connected it to PEFR for several sets of experiments. The pyrometric method for particle temperature and size measurement was used simultaneously to combine the degree of vaporisation to fuel temperature. These results, measured simultaneously at well controlled pressurized conditions, were unique in the world in 1995. The pressure used in measurement was 1.0 MPa. The fuel temperature (1000-2600 °C) covered the range of fluidized bed and pulverized fuel combustion. Large differences in alkali vaporisation were found between different coals. It is well known that increase of chlorine content in the fuel increases the degree of alkali vaporisation. The quantitative effect of Cl was measured as a function of fuel temperature. On the basis of these results, the flow of gaseous and aerosolic alkalis towards the cleaning devices can be calculated for the studied fuels. This helps to determine the type of the gas cleaning problem and assists in selecting the optimal cleaning procedure. All the measured alkali concentrations were far too high for the blades of gas turbines, without gas cleaning