The Sensing of Unbalanced Pulverised Coal Feed Rates at the Exit of Riffle Boxes in Coal Fired Power Station Fuel Distribution Systems

To achieve maximum efficiency in large coal fired boiler furnaces it is essential that the air fuel ratio should be maintained at each burner, and this means that the fuel flow to each burner should be the same. In practice this desirable condition is rarely met because it is very difficult to achieve an equal division of pulverised fuel PF at the various pipe bifurcations in fuel delivery systems due to roping and also due to erosive wear on the riffle boxes which can be considerable. The loss in thermal efficiency occurs because the majority of the combustion air is supplied separately to the furnace and is equally divided between the burners, consequently stoichiometric conditions will not be met at each burner if the fuel feeds are unbalanced. Improvements in efficiency will be possible by the individual control of the secondary air to the burners if the relative mass flow rates of the fuel at the burners are known. The paper includes a discussion on the level of thermal losses that will occur for specified unbalanced PF splitting in a typical large scale power station furnace and then reports on the techniques being developed at the Wolfson Centre for Bulk Solids Handling Technology at the University of Greenwich London UK to directly measure the fuel flow rates at the exit ports of the final stage riffles in a PF distribution system. If these measurements can be effectively made then the possibility exists of adjusting the secondary air at the burners so as to achieve stoichiometric conditions at each burner.