Projects

Projects: Summary of Projects by Region

Projects in Region Scotland involving University of Edinburgh : DTI/CC/133

• To determine the optimum burner size relative to furnace dimensions
• To determine the minimum acceptable pitch between adjacent burners and between the wing burners and furnace side walls
• To investigate the effect of interleaving the burner pitches in opposed wall fired plant in order to increase furnace utilisation and improve the turndown characteristics of the plant
• To investigate whether biasing of fuel or air across a burner row offers significant improvements in NO_x levels without adversely affecting carbon burnout

Retrofit installations of low NO_x systems are often constrained to some extent by the configuration of the existing plant. These practical constraints can be avoided in the design process for new plant. Factors such as the size, number and pitching of burners are selected to optimise furnace performance in terms of heat input, residence time, corrosion, pollutant formation and economics. The identification of optimum burner size and pitch with particular regard to NO_xemissions and carbon burnout is
of significant interest.

The complex process of coal combustion can be simulated with some degree of accuracy by means of Computational Fluid Dynamics (CFD) modelling. This represents a valuable technique for the analysis of the effect of various factors on combustion performance and pollutant (NO_x) formation.

A review of existing front and opposed wall furnace designs found that for furnace sizes of 300 and 500 MWe, the typical thermal ratings of the burners employed are between 40 and 60 MWth. A non-dimensionalised horizontal, vertical and wall clearance pitch value of around 2.75d (2.75 x throat diameter) was representative of the units examined, with a number of units featuring burners having considerably tighter pitches (around 2d). The review also allowed the specification of the dimensions of generic furnaces for modelling purposes.

Physical modelling of utility furnaces can be undertaken to obtain accurateflow measurement data. Data was obtained from a 1/10th scale perspex model of a generic front wall fired furnace. The measured data was used to validate the use of CFD techniques to investigate the effect of changes to furnace configuration.

The effect of the variation of the number and size of burners was investigated in a front and an opposed wall fired furnace. Results established that fewer large burners are preferable to a larger number of smaller burners in terms of lower NO_x (fuel and thermal) and plant economics, without adversely affecting carbon in ash (CIA). The predictions were consistent with previous studies of the effect of scaling criteria. Air biasing or off-stoichiometric firing was also investigated. NO_x reductions predicted were poor, in line with plant experience.

As pitch was reduced in a generic opposed wall fired furnace, flames became narrower, with a merging of tertiary air streams, a lessening in strength of the internal recirculation zone and a flaring out of flow to occupy available volume. Although fuel NO_x showed little variation with reducing pitch, thermal NO_x and CIA increased. The minimum acceptable pitch was identified. An additional study showed the sensitivity of fuel NO_x to excess air level, and predictions from the CFD model were consistent with plant experience.

The novel technique of burner interleaving in opposed wall fired furnaces was e valuated.Significant NO_x reductions were gained through vertical interleaving but at the expense of higher CIA. The analysis of the design of the OFA systems applied to the front and opposed wall fired furnaces gave reductions in NO_x comparable with expectations. Little or no NO_x reduction benefit can be expected to accrue from the use of interleaving on opposed wall fired furnaces with deeply staged OFA systems, as staging becomes the limiting factor for NO_x generation.