NOx IFGR is one of the main air pollutants emitted by combustion processes and environmental regulations are the only factor that forces the industry to install NOx control systems. In the Houston Galveston area alone, there are more than 6,000 point sources that would need to be controlled to comply with the new Statewide Implementation Plan adopted by the TNRCC. Many in the industry are in the process of evaluating control techniques to comply with the new regulations.
Before evaluating NOx control strategies, it is important to establish current baseline emission levels and understand the mechanism of NOx formation. Most of the NOx formed during gas and light oil combustion comes from the high-temperature oxidation (or “fixation”) of atmospheric nitrogen and is referred to as thermal NOx. NO is the main component of thermal NOx and its formation can be modeled using the Zeldovich equation:
[NO] = k 1 exp (-k2/T) [N2] ·[O2]1/2 ton
where, [ ] = molar fraction, k’s = constants, T = temperature and t = residence time. The equation indicates that NOx formation is an exponential function of temperature and a square root function of oxygen concentration. Therefore, by manipulating the temperature or oxygen concentration, the formation of thermal NOx can be controlled. Systems that manipulate oxygen concentration are called stoichiometry-based combustion control techniques (e.g., low NOx or LNB burners) and those that reduce temperature are called stoichiometry-based combustion control techniques. dilution (eg, flue gas recirculation or FGR). LNBs control NOx emissions by providing air stages to create an initial fuel-rich zone (partial combustion zone) followed by an air-rich zone to complete the combustion process.
Some burner designs incorporate fuel stages that result in lower NOx levels. Since NOx formation is a function of the square root of oxygen concentration, the reduction capacity of stoichiometry-based technologies is limited. According to theory, the formation of NOx should increase with the concentration of oxygen or with the amount of excess air. In practice, however, increasing the amount of air reduces NOx formation due to reduced flame temperature. Typical NOx reduction as a function of actual air to theoretical air ratio is shown in Figure 1. For comparison, typical NOx reduction as a function of flue gas recirculation rate.
NOx reduction due to fuel staging or varying oxygen levels can be as high as 40%. NOx reduction due to dilution with flue gas can reach 80%. Newer LNB designs such as ULNBs attempt to capture the concept of dilution by incorporating internal recirculation for lower NOx levels. Unfortunately, to obtain the desired levels of internal recirculation, a higher fuel gas pressure and a larger burner throat are required. In retrofit applications, larger burners create the need to rebuild the furnace floor. Furnace floor rebuilding is expensive and time consuming; And it exponentially increases the cost of installing the LNB! ULNB performance is also very sensitive to several factors, including hydrogen in the fuel gas, air preheating, particulates, liquid droplets, and air leaks. Hydrogen-containing fuel gas, air leakage in the furnace, and air preheating result in increased NOx emissions from ULNB. Changing the fuel gas system to remove hydrogen, particulates and droplets from the fuel gas and sealing the furnace to ensure no air leaks also significantly increases the cost of installing ULNB. Eliminating air preheating for ULNB application not only increases installation costs, but also increases operating costs in terms of additional fuel costs.
Most combustion experts agree that once the air preheater is removed, the unit never returns to original operation! Also, at low loads, the internal recirculation rate is reduced due to the lower fuel flow. This results in limiting the rotation rate of newer burner designs. To avoid the limitations of the new burner design but to capture the concept of lower NOx reduction, some burner suppliers offer external FGR along with LNB, but at a cost equivalent to SCR! Since the total installed cost of burner systems is an order of magnitude higher than FGR; And since most of the NOx reduction in newer burners is due to FGR, the most cost-effective alternative is proven stand-alone FGR technology.
In a typical FGR application, about 5-25% of the flue gas is recycled back to the combustion zone, resulting in up to 80% NOx reduction. Therefore, the NOx levels that can be achieved with newer burners can also be achieved by external FGR at significantly lower cost and without limiting the unit’s turndown capability. Elimination of air preheater operation to reduce NOx is not necessary with FGR. The higher cost of traditional FGR technology is due to an additional hot gas fan requirement to transport the flue gas.
To further reduce cost, a very cost-effective technology has been developed that eliminates the need for a separate FGR fan and windbox mixing devices. The patented Induced FGR (IFGR) technology is based on using the existing forced draft fan to induce combustion gases into the combustion air at the fan inlet. The FDF capacity required for IFGR is equivalent to operating the fan with an additional 1 to 4% O2 at the flue. IFGR technology (patent pending) reduces NOx emissions by up to 80% and generally improves combustion efficiency and performance. IFGR technology requires very little modification and can be installed in less than a week. IFGR has relatively little or no impact on performance and operation.
For units with limited fans, there are several low cost debottle options to accommodate the IFGR stream. For natural draft unit operators planning to install SCR technology, there is a modified technology called Slip Stream FGR technology (patent pending), in which a downward flow from the SCR fan is recirculated into the flame zone for high NOx reduction levels. A combination of FGR based systems with afterburning SCR technology is more cost effective compared to applying SCR technology alone. This is because when SCR is used in combination with FGR systems, the costs associated with catalyst and ammonia handling systems are significantly reduced due to the lower NOx concentration. In certain situations, the reduction in ammonia use alone pays for the technology in less than 6 months. BCCA estimates that about $8 billion will be spent in the Houston-Galveston area to comply with the rules. It is believed that the approach presented here can reduce costs by more than 35%.