The Nalco Mobotec ROFA^® (Rotating Opposed Fire Air) System is a state-of-the-art combustion staging and NO_x reduction solution. Formation of NO_x, a harmful pollutant, is strongly dependent on fuel/air stoichiometric ratios. In a furnace, achieving a near- or sub-stoichiometric combustion ratio is key to creating a NO_x-reducing environment.

To achieve sub-stoichiometric conditions, part of the combustion air must be introduced downstream of the burner. This allows the primary combustion zone in the lower furnace to operate in a “reducing environment” that partially starves the lower furnace of oxygen.

On a typical ROFA System installation, 25 to 40 percent of the total furnace air is injected into the upper furnace through special asymmetrically-placed air nozzles. This creates a sub-stoichiometric condition at the burner area, which significantly decreases NO_x formations (primarily fuel-bound, as opposed to thermal NO_x). Compared to a conventional Over Fire Air (OFA) system, which is usually combined with Low NO_x Burners (LNB), the ROFA Systems enables the lower furnace to stage much deeper—achieving significantly greater NO_x reduction while maintaining good combustion and eliminating the need for Low NO_x Burners.

At a molecular level, oxidation of volatile nitrogen takes place during the initial stages of combustion, with subsequent formation of intermediate compounds. These compounds, under conventional combustion, form harmful NO_x, but the ROFA System reduces them to N₂—standard, harmless nitrogen gas.

ROFA System Ducting and Boxes

The Nalco Mobotec high-velocity ROFA System jets also enhance turbulent mixing significantly compared to Over Fire Air and form a rotating bulk flow in the entire furnace. In the upper furnace, the ROFA System creates strong mixing. This action greatly improves gas temperature distribution and species distribution, which increases overall heat absorption, reduces CO (carbon monoxide) emissions, and increases char burnout.

The ROFA System also creates a secondary zone of combustion in the downstream area that increases char burnout and further reduces NO_x. Thermal NO_x formation declines as peak temperatures decrease in a reducing environment. Typical NO_x reductions achieved with ROFA technology alone range from 45 to 65 percent without chemical addition.

Example of EPA data showing NOx reduction with ROFA technology

Nalco Mobotec employs Computational Fluid Dynamics (CFD) to model ROFA technology projects. This model breaks the furnace into millions of tiny squares set on a detailed three-dimensional grid. The CFD model provides the capability to simulate thermal NO_x, prompt NO_x, and fuel NO_x formation. We are able to reflect such factors as: hardware/furnace specifications (dimensions, material, and age); turbulence flow regimes; heat transfer; radiation data; and fuel data (chemical makeup, density, etc.). For more information, refer to the CFD Modeling page.

Information obtained from CFD modeling and verified by field High Velocity Temperature (HVT) testing and pyrometer readings provides insight into the optimal placement of high-velocity ROFA jets. Unit-specific information also points out possible operational inefficiencies in combustion, such as skewed combustion due to excessive burner velocities.

Here is an example of a corner-fired unit.

CFD Model of NO_x Reduction on Corner-Fired Unit

More than a NO_x control system and superior to OFA alone, ROFA technology is a comprehensive combustion improvement solution that offers the advantages of decreased NO_x formation, increased thermal efficiency and, in many cases, reduced operating costs. Going beyond emissions control, Nalco Mobotec’s evaluation of every unit’s operational parameters yields solutions to address combustion inefficiency while lowering NO_x emissions. These improvements reduce costs for our clients while simultaneously benefiting the environment.

The turbulent furnace environment created by ROFA jets poses an ideal opportunity to introduce sorbents for SO_x and Hg, as well as to enhance NO_x control through urea and ammonia. This mixing works as the perfect accent to Nalco Mobotec’s ROTAMIX^® (SNCR), FSI, and FCT systems. ROFA thus becomes a powerful core tool that enables the use of other technologies to achieve maximum chemical utilization—assuring clients that they receive optimal benefits at minimal operating cost.