Dynamics of furnace processes in a CFB boiler

Abstract

With the aim of understanding the dynamics of combustion, this work examines simultaneous fluctuations in fluid dynamic parameters and gas composition measured in a CFB furnace operated with coal as a fuel. The fluid dynamic parameters investigated are pressure and air flow entering the furnace. Gas composition was recorded by a zirconia-cell probe and a gas suction probe connected to a mass spectrometer having a high time resolution (10 Hz). The principal fluctuations detected are around 1 Hz and below 0.3 Hz. The fluctuations below 0.3 Hz mostly originate from variations in the fuel-feed rate. These variations create periods of reducing conditions caused by a momentarily high fuel input accompanied by a pressure rise in the furnace and a reduction of the air feed, which occurs concurrent with the release of an enhanced quantity of volatiles. Modelled pressure fluctuations based on the relation between volatile release and pressure in the furnace give similar pressure fluctuations as the measured pressure fluctuations, with respect to amplitude and characteristic time scale of the fluctuations. There is also a correlation between reducing conditions and the concentration of hydrocarbons. ᭧

Consequences of Unwanted Local Reducing Conditions in Biomass-Fired Boilers on Chemistry and Operation: A Thermodynamic Evaluation

“…The main factors causing local reducing conditions have been presented earlier and can be mitigated by promoting fuel homogeneity by various methods, by designing efficient air injection systems or fluidized-bed bottoms (as recently performed in the Nyköping plant) and/or by better monitoring of oxygen levels in the furnace. The monitoring of pressure fluctuations could provide an innovative solution because local pressure increases have been associated with local reducing conditions . However, some factors, such as fuel intrinsic properties, are difficult to alter without the introduction of new materials (additives and secondary fuels) to modify the chemistry and/or physical processes taking place.…”

“…The consequences of local reducing conditions in furnaces have seldom been addressed because these conditions are often considered to be very limited in space and time. However, extensive experimental investigations − have shown that fluctuations between oxidizing and reducing conditions occur in industrial combustion systems and that they can actually be of considerable magnitude and duration, leading to higher than expected in-bed and/or superheater corrosion issues. The main factors that could cause local reducing conditions in thermal systems include: fuel properties, especially high volatility influencing therewith fuel/air mixing, fluctuations in fuel properties, particularly with heterogeneous fuels, such as municipal solid waste (MSW) causing unstable operation, and imperfections in the air-feed or fuel-feed system.…”

“…Other issues associated with reducing conditions (not all specific to reducing conditions) include: , fouling, slagging, agglomeration of bed material, and defluidization (for fluidized beds), ash sintering (heat exchangers and grate), ash properties, pressure fluctuations, and burn-up issues …”

Release Profile of Volatiles in Fluidised Bed Combustion of Biomass

“…has been applied [27]. Pyrolysis does not start immediately after feeding to the boiler and here a time lag t d due to drying and heating up (to about 473 K) is added to shift the zero point of the time scale.…”