I

Uncontrolled Partial Control Controlled Highly Controlled

Uncontrolled

Low NOx Burner

0 20,000 40,000 60,000 80,000 100,000 120,000 140,000 160,000 180,000 200,000

Boiler Capacity (PPH)

Fig. 17-15 Typical NOx Emissions Rates for Gas-Fired Industrial Boilers with Various Control Technologies.

burner throat. Staged air burners can be used for all fuel types.

Figure 17-16 shows a packaged industrial watertube boiler featuring a cyclonic burner and two-staged combustion. In the first stage, a fuel-rich mixture is injected tangentially at high velocity into the cylindrical combustion chamber. The swirling flow pattern adds a significant convective component to the radiant heat transfer from the flame and limits flame temperature. In the second stage, additional air is introduced to complete combustion. A steam-injection system further reduces NOX without significantly affecting the unit's fuel efficiency. NOX emissions levels of 30 ppm and CO emissions levels of 50 ppm have been reported while maintaining a fuel efficiency of 84% or greater.

Staged fuel burners also use two-stage combustion, but mix a portion of the fuel and all of the air in the primary combustion zone. The high level of excess air greatly lowers the peak flame temperature achieved in the primary combustion zone. The secondary fuel is injected through nozzles, which are positioned around the perimeter of the burner. Because of its high velocity, the fuel gas entrains furnace gases and mixes them with first-stage combustion products, simulating flue gas recirculation and cooling the flame. The staged fuel burner can be operated

Fig. 17-16 Packaged, Industrial Watertube Boiler Featuring Cyclonic Burner and Two-Staged Combustion. Source: Donlee Technologies

with lower excess air than the staged air burner due to the mixing caused by high-pressure second-stage fuel injection. An additional advantage of the staged fuel burner is its compact flame. Unlike staged air burners, staged fuel burners are only designed for gas firing.

Burner spacing is a significant design element in NOx control. The interaction between closely spaced burners, especially in the center of multiple-burner installations, increases flame temperature at these locations. Therefore, in most new utility boiler designs, vertical and horizontal burner spacing has been widened to provide more cooling of the burner zone area.

Thermal NOx formation generally increases as the heat release rate or combustion intensity increases. Load reduction, or derating, as a NOx control option is applicable to all fuel types in existing units. In new installations, furnace enclosures are built to allow sufficient time for complete combustion with slower and more complete heat release rates.

Reduced firing rates can lead to several operational problems. The reduced mass flow can cause improper fuel-air mixing during combustion, creating CO and soot emissions and reducing fuel efficiency. Increased excess air levels may improve mixing, but can further reduce efficiency and increase fuel NOx generation. When the combustion unit is designed for a reduced heat release rate, the problems associated with derating are largely avoided.

Catalytic combustion is combustion occurring close to a solid surface that has a special catalyst coating. The catalyst accelerates the rate of a chemical reaction and can itself be recovered unchanged at the end of the reaction. As a result, reactions occur faster and with lower energy requirement. Catalytic combustion can be effective in reducing NOx emissions, as well as emissions of CO and unburned hydrocarbons, by allowing combustion to occur at lower temperatures. However, at present, this control option has limited applicability due to catalyst degradation at temperatures above 1,830°F (1,000°C).

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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