The power generation industry is constantly challenged with the need to improve the emissions and heat rate performance of their assets. In the case of pulverized coal fired units, poor fuel balance at the burners is a relatively common issue that impedes efforts for combustion optimisation. The problem of fuel imbalance is particularly acute on units that employ splitting devices between the pulverizers and the burners. Ontario Power Generation’s Nanticoke Generating Station employs quadrafurcators (one-to-four splitters) on many of its units. These splitters boxes are arranged in the vertical, typically immediately downstream of a 90° elbow which results in exceptionally poor fuel balance - baseline measurements indicate values in excess of +/- 50% (RMS) from the mean. This translates to a number of problems including reduced flame stability and carbon conversion. The generally poor combustion conditions also require increased levels of excess air with the expected negative impact on NOx emissions, capacity and heat rate. In 2006, Nanticoke GS installed a Variable Area Rope Breaker (VARB®) system supplied by Greenbank Energy Solutions on a single mill serving Unit 2. This technology was developed in the United Kingdom and has since been successfully installed on a number of large (500 MWe) coal-fired units configured with 3-way and 4-way splitters. The trial at Nanticoke represents the first application of the technology in North America. Initial testing of the system indicates a significant improvement in fuel distribution, reducing the imbalance to approximately +/- 15% (RMS) from the mean. The commissioning and testing of the VARB® system are described in this paper.
Nanticoke GS, operated by Ontario Power Generation, is located on the north shore of Lake Erie, approximately one hour from Hamilton, Ontario, Canada. The station is comprised of eight units, each with a nominal rating of 500 MWe. The boilers are of the opposed-fired design and are equipped with five 10E10 ball-race pulverizers per unit. The original design of the firing system employs two large (28”) outlet pipes on each mill. Each of these outlet pipes is routed to the centre-line of the boiler where a quadrafurcator is used to split each pipe into four smaller burner lines (14.25”). The resulting eight burners per mill are arranged in three rows on the front wall and two rows on the rear. A typical Nanticoke piping configuration is shown in Figure 1. The original designers of the piping obviously expected to have problems with a strong rope formation on the outer radius of the elbow. A venturi has been used in the original design with the intention of centering the rope on the splitter box as an aid to distribution. However, in practice this venturi is simply not aggressive enough to disperse the rope. Physical and computational models as well as field measurements confirm that the rope quickly reforms on the outer wall. This results in two of the four outlet pipes being heavily loaded with pulverized fuel. Table 1 shows the results of physical modeling for one of the groups of row “A. Pipes 3 and 4 in this example are located above the outer radius of the inlet pipe elbow.