The mill throat, also called the port ring, vane wheel, nozzle ring or throat, is the component on which everything else in the mill depends. Of the many factors influencing mill performance, the one that matters most is the interface between the primary air, and the ground particles, which occurs immediately above the throat. Get the air wrong here and every process above it inherits the problem. EnFloTech's design strength lies in mastering how that air meets and conveys those particles.

What goes wrong with conventional throats

Where air enters the grinding zone turbulent an unevenly, the flow recirculates and induces suspended particle clouds inside the mill. Resistance rises, good combustible material is rejected alongside the pyrites, internals wear prematurely and the mill consumes more power to deliver less to the boiler. As delivered coal quality declines, the effect compounds.

The EnFloTech design approach

The HPRT straightens the velocity vector into the mill, eliminating the recirculating flow that causes suspended particle clouds. A Venturi ratio and skirted design then blanket the non-uniform flow underneath, so that air arrives at a uniform velocity around the entire periphery of the grinding zone. This is aerodynamic conditioning at the inlet rather than compensation further up the mill.

Impact on mill performance

• Reduced system resistance, which can be taken as increased mill throughput or used to adjust the classifier for a finer particle size distribution (PSD) at the same throughput
• Reduced mill power consumption
• Optimised rejection of non-value contributing particles
• Reduced wear of mill internals
• Optimised primary air and pulverised fuel (PA/PF) distribution between fuel pipes
• Elimination of particle roping effects
• Elimination of particle settling inside the fuel pipes and large particle carry-over

The current range

All three versions allow for straightforward gearbox replacement, and each carries a guaranteed 10% mill improvement.

Version 4 — camber angle of 30 and 45 degrees. Mill types: Babcock E and EL. Fuels: coal, biomass and cement.

Version 5 — straight vector design, superseding the earlier Versions 1 and 3. Mill types: MPS, Loesche, MBF, Foster Wheeler, XRP, CE, HP, Raymond bowl, Lopulco, MB. Fuels: coal, biomass and cement.

Version 6 — straight vector design, engineered for large feed and resistant to tramp iron. Mill types: MPS, Loesche, MBF, Foster Wheeler, XRP, CE, HP, Raymond bowl, Lopulco, MB. Fuels: coal and cement.

Installation and guarantee

The rotating throat is a modular component that fits the existing footprint of the mill and requires no major modifications. Turnaround for a conversion is no more than 56 working hours. When converting OEM mills to EnFloTech HPRTs, EnFloTech guarantees a minimum 10% improvement in mill performance, backed by a 3-year guarantee.

Field reference: Kogan Creek Power Station, Australia

A Version 5 vane wheel was installed in an MPS 255 vertical roller mill at Kogan Creek, part of CS Energy's Queensland fleet. The converted mill has run at an average differential pressure approximately 20% below adjacent mills still fitted with OEM nozzle rings, correlating closely with the Computational Fluid Dynamics study conducted at design stage. Reject quality improved in parallel, the mill rejecting predominantly larger grey rock with visually negligible pulverised fuel.

Working with the classifier

Conditioning the air at the inlet is what allows the classifier above to work on a clean, evenly distributed input rather than correcting turbulence generated below it. Fitted alongside an EnFloTech Ultra-High-Performance Static Classifier, the HPRT completes the treatment of the mill's aerodynamics from inlet to outlet.

Why it matters for the region

Where coal quality is falling but boiler output must still be met, the usual alternative is investment in new mills and new primary air fans. A throat conversion recovers that capacity from the plant already installed, within a normal outage window and at a fraction of the capital cost.