ClearStack’s Stage3-Cyclone™ is a staged-combustion technique for cyclone boilers. The technology, like the Ashworth Combustor™, is based on the use of three stages of combustion air, the third of which is overfire air (OFA). Limestone is added with the coal to maintain good slag fluidity. This staged combustion technique will effectively reduce NOX emissions from cyclone units at very low capital and operating costs.
The Stage3-Cyclone™ is similar to the Ashworth Combustor™ 3-stage oxidation technique, with the exception that the existing cyclone barrels are used for the first combustion stage, using a stoichiometric air to fuel ratio of 0.6 to 0.7 to begin the combustion of the coal. As the fuel gas enters the furnace, air is added to bring the stoichiometry up to 0.9, still slightly reducing to minimize thermal NOX production. As the furnace gases cool to 2400°F-2500°F, air is added to bring the overall stoichiometric air to fuel ratio up to 1.10 to 1.15. Excess oxygen at this temperature level also minimizes thermal NOX production. The typical feed coal size to a cyclone (minus ¼ inch) is used and limestone is added to flux the coal ash to maintain a fluid slag under the cooler barrel conditions. The rate of limestone, relative to a specific coal rate, will be determined using a T10 viscosity versus silica ratio (SiO2 /(SiO2 + Fe2O3 + MgO + CaO) correlation for the coal ash/limestone mix. The British Gas Council developed this predictive slag viscosity technique. Ultra-low NOX emissions are expected but significant sulfur capture is not, due to the way the air is introduced into the cyclone barrels and the fact that the slag sees the more oxidizing condition of the cyclone furnace. A Stage3-Cyclone™ retrofit to an existing cyclone-fired unit will require the following additions/modifications:
- Addition of limestone bin with feeders (one feed line per barrel)
- Furnace tube-wall penetrations
- Second-stage air
- Over-fire air
- Installation of second-stage and over-fire air ductwork
- Air flow
- Air flow sensing and control devices on all air lines
- Tie-in to existing boiler master control system
This technology is protected under by U.S. Patents #6,085,674 and #6,325,002.