Entropy Technology & Environmental Consultants, Inc. (ETEC)

- NOx Control for Air Preheat Systems



New advancements in combustion control technologies, such as flue gas recirculation (FGR) and low NOx burners (LNB), have made it possible to achieve ultra low NOx levels without using expensive post-combustion flue gas clean-up technology. For some natural draft units it may be possible to meet the most stringent of the NOx regulations with cost-effective combustion control technologies. For forced draft (mechanical draft) units it may not be possible to achieve efficient NOx control with LNB. This is because most forced draft systems have air preheat that results in a higher baseline NOx emissions. Increased air preheat temperature results in higher adiabatic flame temperatures and LNB technology is ineffective in reducing flame temperatures.

The most cost-effective approach for reducing NOx from air-preheated systems is by flue gas recirculation (FGR). FGR systems have been extensively used since early 1970's and have proven to be very effective in reducing NOx emissions. In the FGR NOx control process, a portion of the exhaust flue gas is recycled back into the combustion air stream. Recycling the flue gas helps in reducing NOx emissions by up to 90%, depending on the recirculation rate. Typical NOx reduction as a function of FGR rate from an air preheated system is shown in Figure 2. Clearly, FGR systems are very efficient means of reducing NOx from air-preheated units.

Traditional FGR systems use a separate fan to move the flue gas from the air preheater exit to the combustion air stream downstream of the forced draft fan. The additional fan requirement adds to the cost of traditional FGR. ETEC has developed a more cost-effective process referred to as Induced Flue Gas Recirculation (IFGR). ETEC's IFGR technology (patent pending) uses the existing forced draft fan to induce flue gas into the combustion air at the fan inlet. Elimination of the fan requirement has made the IFGR system the most cost-effective demonstrated NOx control technology available in the market today! A major advantage of ETEC's IFGR system over the economizer option is that it not only maintains high-energy efficiency of the system but also reduces NOx to levels approaching SCR reduction (see Figure 2). For solving NOx issues from air-preheated systems, IFGR is the most ideal solution. Another major advantage of the IFGR technology is that it can be used in combination with other control technologies to lower NOx emissions even further.

In a typical IFGR application, about 5 - 25 % of flue gas (equivalent to operating the fan with extra 1 - 4 % O2 in flue) is recycled back into the combustion air. ETEC has demonstrated NOx reduction as high as 80%, or even greater reductions for higher uncontrolled emitters. Thus, NOx levels that can be achieved from 'newer' burners can also be achieved by IFGR, but at a significantly lower cost and without limiting the turndown capability of the unit! For air preheat systems, IFGR performance is superior to that of LNB. The installed cost of IFGR in retrofit application is a fraction of the cost of low NOx burners. This makes IFGR technology the most cost-effective technology available in the market today. ETEC's IFGR technology has several advantages: minimum downtime for installation, very minor modification, relatively little or no impact on performance and operation, and, eliminates existing flame impingement problems. Unlike other combustion control systems, ETEC's IFGR system improves fuel and air mixing inadequacies and in certain cases improves the energy efficiency. For fan limited units, ETEC offers several low cost de-bottlenecking options to accommodate IFGR flow. ETEC's IFGR designs are now being used in over 80% of working IFGR systems.

For operators of natural draft units planning on installing SCR technology, ETEC provides a modified IFGR technology called Slip Stream FGR technology (patent pending), where a slipstream from downstream of SCR fan is recirculated back into the flame zone to obtain high levels of NOx reduction. A combination of ETEC's FGR based systems with post-combustion SCR technology is more cost-effective compared to application of only SCR technology (see Chemical Engineering, Feb 2001). This is because when SCR is used in combination with ETEC's FGR systems, the costs associated with catalyst requirements (such as: equipment size, structural steel, etc.) and ammonia-handling systems are significantly reduced due to lower NOx concentration. In certain situations, reduction in ammonia usage alone payback ETEC technology cost in less than 6 months.

For units with multiple burners, ETEC also offers hybrid of IFGR and C-Mods technologies that achieves SCR NOx levels for a fraction of cost. C-Mods technology is similar to Burners-Out-Of-Service and replicates the performance of LNB and Over Fire Air. When compared to competing NOx control technologies like SCR, ETEC's IFGR+C-Mods hybrid technology is expected to result in cost savings of 5 to 20 million dollars depending on the size of the unit.

In the HGA area, several companies are retiring boilers and planning to import steam from a third party operated cogen facility. Depending on a third party for plant utilities not only limits operating flexibility, but eliminates the option of burning waste gas stream that were burned in the boilers. Cost-effectiveness of ETEC's systems make it possible to continue operating the boilers while achieving significant NOx reduction with minimal cost implications.

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