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.
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 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.
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 NOx, prompt NOx, and fuel NOx 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 NOx Reduction on Corner-Fired Unit
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.
The Nalco Mobotec Advanced Selective Non-Catalytic Reduction System
Rotamix Injectors on Front Wall
ROTAMIX® technology is Nalco Mobotec's next-generation, advanced Selective Non-Catalytic Reduction (SNCR) system. This sophisticated solution uses boosted air to carry beneficial urea or ammonia deep into the furnace cross flow, offering a viable alternative to selective catalytic reduction (SCR). The turbulent air injection and mixing provided by Rotamix enables higher chemical utilization when combined with Nalco Mobotec's ROFA® system. The benefit is a result of more effective mixing of chemical reagents with combustion products in the furnace.
The ROTAMIX System adapts to load and temperature changes in the furnace, and can preferentially introduce chemicals where the temperature is most favorable for pollution reduction. This is accomplished through advanced feedback-control algorithms and on-site tuning, using the controls on relevant equipment to adjust the quantities of chemicals added. The technique reduces chemical consumption considerably by increasing the efficiency of reactivity. In fact, ROTAMIX technology can decrease recurring chemical usage by up to 50 percent compared with other SNCR systems.
For larger units, urea is a preferable SNCR chemical to ammonia, since urea vaporizes more slowly than water. In SNCR design, the goal is to expose all urea to as much of the flue gas as possible at temperatures between 1600°F and 2100°F. Above this temperature range, urea actually reacts to form NOx; below this range, the HNCO (isocyanic acid) and/or NH3 (ammonia) will “slip” and can cause other operational problems.
NO concentration without (a) and with (b) urea injection
at the exit of the radiant furnace
The Chemical Reaction of Urea, Water and Pollutants
Urea, mixed with water and injected into the furnace, heats up quickly and vaporizes water. When most of the water is vaporized, the urea itself vaporizes (or sublimates) to a gas phase and almost immediately decomposes thermally into HNCO and NH3. Ammonia (NH3) is well known to reduce NOx, while Isocyanic Acid (HNCO) also reduces NOx, but at a slightly higher temperature. Relative to ammonia alone, urea is a superior SNCR chemical on large and hot units because the urea vaporizes only after most of the water has vaporized. This delays introduction of HNCO and NH3 and allows for more mixing (and flue gas cooling) before becoming chemically reactive. Note that urea can be injected into temperatures exceeding 2000º F due to the time delay for mixing. The result is efficient introduction of chemicals directly into a well distributed, rotating flue gas mixture. The cooling effect of the ROTAMIX high-velocity jets further extends the maximum temperature of SNCR injection.
A properly designed system that implements a high degree of mixing will inject urea well upstream of the appropriate temperature window. Sufficient water and humidification promote superior mixing, making urea chemically available to reduce NOx (as NH3 and HNCO) within that window.
At each ROTAMIX box, the ROTAMIX air is first humidified, and then injected with urea. This humidified ROTAMIX air transports urea into the furnace and mixes thoroughly with the combustion products before the urea itself fully vaporizes. The ROTAMIX-humidified and boosted air becomes an additional control parameter. It times the chemical availability of urea for NOx reduction in the ideal temperature window and thus increases chemical utilization and system flexibility. Nalco Mobotec models each system to minimize the number of injection locations while maintaining maximum NOx reduction. NOx can be further lowered by adding a small amount of induct catalyst, thereby reducing NOx to levels comparable with SCR systems.
Nalco Mobotec solutions offer modular technology deployment to meet current and future NOx requirements.
Dry Sorbent Desulfurization Systems
Dry sorbent desulfurization systems are used to remove sulfur dioxide (SO2) from flue gas at fossil fuel-fired electric generating stations. In a dry sorbent injection system, a powdered alkaline sorbent is injected into the boiler or into flue gas ahead of a particulate collector. SO2 reacts with the dry sorbent, and the dry reaction products are collected, along with fly ash, in the particulate collector—which is either a fabric filter (baghouse) or electrostatic precipitator (ESP). Fly ash containing SO2 reaction products and unused sorbent can be suitable for use as feedstock to cement plants or for stabilization of earthen structures.
Since desulfurization is all-dry, no water is consumed, and no waste water is produced. The flue gas is not cooled nor saturated with water, so reheating of desulfurized flue gas is not required. No gas-sorbent contacting vessel is required to be installed.
Furnace Sorbent Injection
Furnace sorbent injection using the ROFA®/ROTAMIX® system is one type of dry sorbent desulfurization system that is available from Nalco Mobotec. In furnace sorbent injection, a calcium-based dry sorbent, including limestone (CaCO3) or hydrated lime (Ca(OH)2 ), is injected into the upper portion of a coal-fired boiler where combustion gas temperature is optimal for SO2 capture. SO2 reacts rapidly with the calcium-based sorbent to form stable calcium sulfate solid.
Where higher SO2 removal efficiency is required, special sorbents designed by Nalco Mobotec are used in place of normal limestone or lime. Calcium sulfate and unused sorbent are carried by the gas through the boiler backpass air preheater and are removed from flue gas, along with fly ash in the particulate collector. The particulate collector is preferably a fabric filter (baghouse), but an ESP is suitable with modification.
Fly ash from furnace sorbent injection can be suitable for use a feedstock to a cement plant or for stabilization of earthen structures.
Nalco Mobotec's furnace sorbent injection (FSI) system is most effectively applied in combination with Nalco Mobotec's ROFA/ROTAMIX systems, which provides highly efficient mixing of the sorbent and flue gas at the optimum gas temperature for SO2 capture. Efficient mixing allows sorbent to react with the maximum possible amount of SO2, which minimizes the amount of sorbent required.
Post-Furnace Sorbent Injection
Post-furnace sorbent injection is another type of dry sorbent desulfurization system available from Nalco Mobotec. In post-furnace injection, a sodium-based dry alkaline sorbent, including powdered sodium bicarbonate (NaHCO3) or sodium sesquicarbonate (NaHCO3•Na2CO3•2H2O), is injected into the flue gas ductwork immediately after a coal-fired boiler or after an air preheater and ahead of a particulate collector. Sodium sulfate, other dry reaction products, and unused sorbent are carried by the gas and removed from flue gas, along with fly ash in the particulate collector. Where the particulate collector is a fabric filter (baghouse), substantial additional SO2 capture occurs as flue gas passes through fly ash and sorbent collected on the filter surface.
The primary advantages of FSI and post-FSI sorbent injection using the ROFA/ROTAMIX systems, compared with typical desulfurization systems, are very low installed equipment cost due to few processing steps, no water consumption, no waste water treatment, and no reheating of flue gas. Dry sorbent injection systems are very easy to retrofit to existing power plants. Power consumption is low—less than 0.5% of generating capacity when applied with the ROFA/ROTAMIX systems.
ROFA/ROTAMIX FSI and post-FSI systems are most applicable to generating units 500 MW or smaller, which burn low-sulfur coals (less than 1.5% sulfur) and have a fabric filter (baghouse) for particulate collection. It can be applied to some units with cold-side electrostatic precipitators (ESP) where a fly ash conditioning system is installed ahead of the ESP.
Comparison of Dry Sorbent Injection with Wet FGD
Coal Sulfur Content for Best Application
Power Consumption, % of electric generation
Gypsum solid or MgSO4 solution for use or disposal
Alkaline Reagent or Sorbent Consumption, kg/kg SO2 in flue gas
Water consumption, m3/hr/MW
Wastewater treatment required?
Flue gas reheating required?
Ease of retrofit to existing power station
Fireside Chemical Treatment
Nalco Mobotec Fireside Chemical Treatment Programs: How They Work
Nalco Mobotec's Fireside Chemical Treatment (FCT) programs are applicable to any combustion system whose availability and efficiency are diminished by slag or fouling deposits, whether boilers, kilns, incinerators, or others. These slag and fouling deposits result from the combustion of ash-producing fuels such as coal, heavy oil, wood products, and other biomass and waste materials. Nalco Mobotec FCT employs a variety of proprietary slag-inhibiting or combustion-enhancing chemistries. These products (when combined with Nalco Mobotec's unique application technologies) are designed to maximize program effectiveness and minimize customer cost. FCT programs also include the consultative and mechanical maintenance services of Nalco Mobotec professionals to help ensure that desired results are achieved.
Chemicals injected into the furnace through Nalco Mobotec FCT programs actually change the molecular composition of slag deposits as they form. Not only are the deposits themselves dramatically decreased, but the resilience of remaining deposits is lessened, so that soot blowers and other slag removal devices can clean surfaces more thoroughly and efficiently.
Technological and Financial Benefits of Nalco Mobotec FCT Programs
Certain FCT formulations offer the added ability to enhance combustion efficiency and reduce unburned carbon that may be part of ash material. Nalco Mobotec uses a custom design process to develop an application strategy that maximizes the efficiency of each system's applied chemistry. The benefits of this approach include:
Reducing the fuel required to keep a system running at peak performance by as much as 5 percent
Maximizing production capability while a system is online
Extending run times between offline cleanings by up to 100 percent
Minimizing costs for maintenance (de-slagging / de-fouling) and equipment repairs, such as those necessitated by falling slag
A typical Nalco Mobotec FCT program begins with a complete system survey that may involve computerized modeling of the combustion system. From this, a custom solution is developed that best enables a high return for the customer. Nalco Mobotec provides all application equipment and chemicals necessary to achieve desired results. In addition, Nalco Mobotec offers unique monitoring and laboratory analysis to diagnose slag, fouling, and combustion-related problems; and to help ensure that successful solutions are implemented at the customer site. The goal of every program is to optimize efficiency while lowering the customer's total cost of operation.
Application Design Sample Model of Chemical
Distribution with ROFA-FCT for Uniform Furnace Distribution
Sample Model of ROTAMIX-FCT Injection to
Control Burners Eyebrows
Developed for installation as a retrofit to pulverizers at coal-fired power plants, the MagMillâ„¢ system offers the opportunity to lower levels of pollutants before they are brought to the combustion chamber.
The MagMill system is a dry magnetic separation technology that combines the existing pulverizer with a magnetic separator. This method removes mineral contaminants from coal more efficiently and economically than conventional wet cleaning at the prep plant and has effective iron and pyrite separation providing a coal quality improvement.
The MagMill system has the ability to reduce levels of the following pollutants and particles—each of which can have distinct negative effects on plant operations and profit margins:
The primary upside of the MagMill system is the reduction of major pollutants, making it easier for suppliers to meet regulatory requirements and allowing more fuel flexibility. Cleaner fuel enables both enhanced generation capacity and significant reduction of operational and maintenance problems.
The refuse from the MagMill system is non-hazardous and can be disposed of or sold. MagMill system benefits include:
Reduced wear on the pulverizer and associated piping and equipment
Lowered pulverizer power draw
Delivery of a better quality fuel to the furnace
Increased pulverizer output
Less slagging in furnace and superheater
Pre-combustion iron and pyrite separation from the coal
Minimized creation of both fly ash and bottom ash
The MagMill system is very effective in removing both ferromagnetic materials (iron-containing compounds pulled into a magnetic field with great force) and diamagnetic materials (compounds pushed out of a magnetic field) for effective iron and pyrite separation.
Every coal is different, and client needs vary greatly, so the MagMill system must be customized for every application of coal quality improvement. To this end, a comprehensive evaluation of the facility is conducted for each potential installation. In determining the responsiveness of a client's fuel to the MagMill system's dry separation techniques, the Nalco Mobotec technical team conducts a series of magnetic responsiveness tests on the fuel. A thorough lab analysis of the fuel is also conducted. The results of all evaluations allow the MagMill team to provide a custom solution for each application.
Benefits of MagMill System to the Plant
Some clients are interested in increasing pulverizer throughput, while others are more concerned with eliminating pollutants such as sulfur and mercury prior to combustion. Still others wish to remove inorganic materials to reduce slagging and abrasiveness in the mill and its associated piping and furnace. The MagMill system can be tuned to achieve any of those objectives.
The system also employs dry screening for density differences and size fractions, in addition to magnetism, to separate unwanted material from the coal.
The MagMill concept shown here is applied to a B&W MPS pulverizer, but can be used with any vertical mill.
Effect of MagMill on Pulverizer Output and Power Draw
The graph below shows the effects on output and on power draw after removal of hard and abrasive minerals before they are overground. Using the MagMill system, the output of the pulverizer can be increased of that of the unmodified mill, while the power draw is simultaneously reduced by using an Upper Freeport (UF) raw coal. The UF is a bituminous steam coal with about 20 wt.% ash and HGI of 53.
MagMill and Lower Kittanning Raw Coal (MagMill Screening)
A Material Balance from a MagMill screening of Lower Kittanning (a common eastern bituminous coal) coal is presented below.
Note the beneficial impact of the MagMill system on this commonly used coal from the eastern United States:
Mill capacity increases by 24% due to eliminating hard-to-grind material
Sulfur Dioxide (SO2) reduced by 38%
Mercury (Hg) reduced by 45%
Ash to the burner reduced by 32%
94% of the heating value (by BTU) is recovered in just 87% of the original weight of fuel. Much of the carbon in this portion is in very difficult-to-burn fuel that remains in the furnace as unburned carbon.
These measurements illustrate the MagMill system's significant improvements in the quality and quantity of coal to burner when directly coupled to a pulverizer.
The MagMill system can increase mill feed and production rates while simultaneously lowering ash, sulfur and trace metal quantities to the burner. The MagMill system does this by rejecting problematic, hard-to-grind materials.
Mineral Reductions and Btu Recovery
Grade and recovery curves show trade-offs can be made between mineral reductions and Btu recovery for this particular coal. The symbols on the graph represent measured values for four different runs. To evaluate the impact of a MagMill system, a Btu Recovery curve is created for every potential coal.
ROFA® Technology—Nalco Mobotec's Approach to Reducing Harmful Pollutants
Rotating Opposed Fired Air (ROFA-CFB) is Nalco Mobotec's state-of-the-art, patented system for staging the CFB furnace, reducing NOx and SOx, and optimizing combustion.
ROTAMIX® Technology is Nalco Mobotec's patented, advanced Selective Non-Catalytic Reduction (SNCR) solution. In this process, the bulk flow upward through the furnace is set in rotation, using custom-designed, asymmetrically-placed air nozzles. This action brings about increased turbulent mixing and bulk rotation throughout the entire furnace—improving gas temperature and species distribution, heat absorption, CO oxidation, and particle burnout in the upper furnace.
ROFA-CFB mixing and rotation technology prevent bulk laminar flow and raise efficiency substantially, allowing more effective use of the entire furnace volume for the combustion and sorbent mixing processes. The ROFA-CFB technology swirl reduces maximum reaction temperature, reduces CFB emissions, burns out carbon more efficiently, and increases convective heat absorption. These improvements in combination can boost overall boiler efficiency significantly.
By mixing combustion air more effectively, the ROFA system also reduces the need for surplus air. Less unused combustion air means that less cooling takes place in the furnace—further raising the efficiency of heat absorption. The ROFA system has been proven to enhance CFB limestone utilization and SNCR chemical utilization, resulting in lower SOx and NOx.
Strategically placed along the furnace walls, ROFA boxes optimize reduction of SOx and NOx while raising combustion intensity. Ultimately, the performance and functionality of the entire ROFA system works in conjunction with, and is dependent on overall furnace geometry and operation. Operators realize significant savings by improving limestone utilization and SNCR performance while achieving the targeted reductions in CFB emissions.
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