Projects

Projects: Projects for Investigator

• To investigate the performance of pulverised fuel meters to measure the relative mass flow rate in two parallel pneumatic conveyors.
• To demonstrate, on a small-scale, control of the distribution (split) of pulverised coal between two parallel pneumatic conveyors, fed from a source via a bifurcator, using pulverised fuel meters to measure the mass flow rate The meters providing signals used to actuate a flow control device.

To improve the combustion efficiency of pulverised fuel (p.f.)- fired furnaces in electrical power generation, it is desirable to measure, optimise and then, ideally, control the p.f. flow velocity and the solids distribution (split) between pneumatic conveyors leading to burners in the furnace.

On leaving the pulverising mill, the solids-air mixture is split into several different pipes each feeding an individual burner. One mill can feed as many as eight burners. Usually the fuel and air are metered before the mill where the ratio can be accurately set. Differences in routing of the lines injecting p.f. into the furnaces, and phase maldistribution at the splitting points, results in an uneven feed to the burners. Consequently, the combustion stoichiometry at the burners is disturbed. This leads to increased fuel costs, higher levels of carbon in the ash and excessive specific emissions in the flue gas. If the mass flow rate of the fuel can be measured, and the flowsp li tter controlled, then, in principle, the air-fuel ratio can be accurately set and the combustion stoichiometry optimised.

There are several possible ways in which the split can be varied. One proposal has been to use an active riffle box, another is the use of compressed air to deflect the solids at the pipe junctions. The ideal is to have a perfectly dispersed solids phase in the approach to the flow splitter. In small diameter pipes, the simplest method, used in these experiments,is to restrict one or both pipes using a variable orifice or valve so that the back pressure deflects the solids from one pipe to the other. This method will also affect the velocity and total mass flow rate but these can now be measured on-line using instruments developed by ABB Ltd. Automatic control was demonstrated, balancing pf mass flowrates and velocities from initial open loop conditions, where significant maldistributions were present.