The Nalco Mobotec ROFA® (Rotating Opposed Fire Air) System is a state-of-the-art combustion staging and NOx reduction solution. Formation of NOx, 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 NOx-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 NOx formations (primarily fuel-bound, as opposed to thermal NOx). Compared to a conventional Over Fire Air (OFA) system, which is usually combined with Low NOx Burners (LNB), the ROFA Systems enables the lower furnace to stage much deeper—achieving significantly greater NOx reduction while maintaining good combustion and eliminating the need for Low NOx 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 NOx, but the ROFA System reduces them to N2—standard, harmless nitrogen gas.
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 NOx. Thermal NOx formation declines as peak temperatures decrease in a reducing environment. Typical NOx reductions achieved with ROFA technology alone range from 45 to 65 percent without chemical addition. More than a NOx control system and superior to OFA alone, ROFA technology is a comprehensive combustion improvement solution that offers the advantages of decreased NOx 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 NOx 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 SOx and Hg, as well as to enhance NOx 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.
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On the right is a 3D rendering of a typical Nalco Mobotec Patented ROFA (Rotating Opposed Fire Air) system (in green) on a pulverized coal boiler. The ROFA system consists of a ROFA fan, modulating dampers, and ROFA ports.
Nalco Mobotec employs Computational Fluid Dynamics (CFD) to model ROFA technology projects. 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.
ROFA technology has been referenced by the EPA and EPRI as capable of achieving enhanced combustion and higher NOx reduction, while maintaining or further reducing CO and unburned carbon. The ROFA system has been established as a best available retrofit technology (BART) in several US states specifically for regional haze NOx reduction.
According to EPRI, the ROFA system achieved a higher degree of mixing with the flue gas as evidenced by the lower fly ash unburned carbon levels and zero measured CO values at the precipitator inlet. The standard OFA system (non Nalco Mobotec system) exhibited greater stratification, slightly higher fly ash unburned carbon levels (5.2% versus 2.9% and peak double digit CO values.
The EPA Federal Register stated that "the affected CAIR sources also have the option to use an advanced OFA system with the potential to achieve high NOx reduction levels, with no increases in CO and unburned carbon levels. This technology utilizes rotating opposed fire air (ROFA) and has been installed or demonstrated at several plants worldwide." Furthermore, the EPA states that with ROFA, "the addition of a combustion air fan to inejct high velociry air that breaks up the fireball within the firebox smoothes out temperature profiles and oxygen levels and typically does not result in increases in UBC or CO emissions. An improvement in boiler heat rate may actually occur."
Nevada Division of Environmental Protection (NDEP) considers ROFA a BART technology for several units in Nevada.