Combustion modification, reduction

TTte most cost-effective methods of reducing emissions of NO are the use of low-NO burners and the use of low nitrogen fuels such as natural gas. Natural gas has the added advantage of emitting almost no particulate matter or sulfur dioxide when used as fuel. Other cost-effective approaches to emissions control include combustion modifications. These can reduce NO emissions by up to 50% at reasonable cost. Flue gas treatment systems can achieve greater emissions reductions, but at a much higher cost. 

Combustion modifications and postcombustion processes are the two major compliance options for NO., emissions available to utilities using coal-fircd boilers. Combustion modifications include low-NO burners (LNBs), overfire air (OFA), reburning, flue gas recirculation (FGR), and operational modifications. Postcombustion processes include selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR). The CCT program has demonstrated innovative technologies in both of these major categories. Combustion modifications offer a less-expensive appiroach. 

Many of the combustion modification methods attempt to reduce the temperature of the flame to lower NOx emissions. In many cases, this may result in a reduction of the combustion efficiency.6 For example, if water is injected into the flame to lower NOx, the water absorbs heat from the flame and carries most of that energy out with the exhaust gases and does not transfer much of that energy to the load. Combustion modification methods are usually less capital intensive than most posttreatment methods. In many cases, there is a limit to how much NOx reduction can be achieved using these combustion modification methods. 

The removal of nitrogen oxides from combustion and industrial exhaust remains an important problem which has been studied extensively. Combustional modifications (controlling burner stoichiometry and lowering flame temperature) have led to methods that are both cost-effective and energy-efficient, however, these methods by themselves cannot achieve the reduction of NOx to levels required in new regulations [1], The wet methods (and the adsorption techniques) for NOx control, which can be used for stationary source emissions only, are expensive to operate and additionally, these methods have serious problems with adsorbent treatment and disposal [2],... 

NOx emissions may be controlled by primary or secondary measures. Primary measures are aimed at reducing the formation of NOx. Examples of primary measures include fuel switching (e.g., moving from coal to oil or to gas) and in-combustion modifications. Examples of in-combustion modification such as Lx)w NOx Burners (LNB), Over-Fired Air (OFA), Bumers-Out-Of-Service (BOOS), Flue Gas Recirculation (FGR), etc. have been reviewed in the literature, e.g., see [10]. Secondary measures reduce NOx after it is formed. An example. Selective Non-Catalytic Reduction (SNCR), reduces NOx via ammonia injection at temperatures of between 1500 and 1700°F [11]. The use of reductants other than ammonia, such as urea [12] and cyanuric acid [13], has also been discussed. 

The most obvious approach for NOx control is to minimize its formation. There is considerable research activity on NOx reduction via combustion modification. One such approach, is the low NOx burner developed by Beer and coworkers, in which gradual mixing of burner air with the centrally injected fuel is achieved by radial stratification of the flame. This stratification is brought about by a combination of swirling air flow and strong positive radial density gradients in the flame. The NOx emissions ( 3% O2) of 70 ppm without FGR and 15 ppm with 32% FGR have been achieved for a 1 MW-size burner [22]. 

Table 10-11 indicates projects intended to demonstrate some of these combustion modification technologies for commercial feasibility or demonstrate applicability where they have not been used previously. The degree of NO, reduction achievable while maintaining acceptable boiler performance is an important element of these test programs. As of mid-1994, all of the programs had been completed except the TVA micronized coal rebuming program. 

The formation of NOx oxygen partial pressure, temperature and coal properties such as nitrogen content and volatile content. Measures can be taken to modify the combustion conditions so that they are less favourable for NOx formation. There are a number of options for combustion modification measures such as reduction of combustion temperature, reduction of residence time in high temperature zones and reduction of excess air. 

The results of different combustion measures varies with the specific conditions. In general 25-40% NOx reduction has been reached by combustion modification measures in the FRG, reaching 600-800 mg NOx/m at 6% O2 on dry bottom boilers and 900-1200 mg N02/m at 6% O2 on wet bottom boilers, using low NOx burners in combination with air staging in the furnace. Advanced low NOx burners have recently been installed at some plants, where about 50% NOx reduction has been achieved. NOx emissions in the range of 300-600 mg N02/m are reported from Japan with a combination of combustion measures. 

Improving the cetane number by additives results in better engine behavior, as would be predicted by the combustion mechanisms in the diesel engine (noise reduction, better operating characteristics, particularly when cold). Nevertheless, concerning certain items such as pollution emissions, it may be better to obtain a higher cetane number rather by modification of the... 

These ideas form the basis of most approaches to NO control with N-containing fuels. In principal, they are readily appHcable to the modification of certain combustors in which the desired divisions in the combustion process exist for other reasons. Although such improvements have been demonstrated, it is difficult in practice to make the required revisions in the air and fuel distribution without adverse effects on other emissions or on performance. It has also been shown that when steam is used to reduce thermal NO production, the formation of NO from fuel N is enhanced, or the reduction is less than otherwise expected. 

Selective catalytic reduction (SCR) and selective noncatalytic reduction processes (SNCR) are widely employed in large industrial and utility boiler plants, as well as in municipal waste incineration plants and other combustion processes. They are used to complement mechanical improvements (such as low NOx burners and furnace design modifications) as an aid to reducing the emission levels of NOx, S02, and other noxious gases into the atmosphere. 

The control of NO from stationary sources includes techniques of modification of the combustion stage (primary measures) and treatment of the effluent gases (secondary measures). The use oflow-temperature NO,.burners, over fire air (OFA), fiue gas recirculation, fuel reburning, staged combustion and water or steam injection are examples of primary measures they are preliminarily attempted, extensively applied and guarantee NO reduction levels of the order of 50% and more. However, they typically do not fit the most stringent emission standards so that secondary measures or flue gas treatment methods must also be applied. 

In 1995, IT Corporation published an economic estimate of an HTTS using oxygen rather than air for combustion. Results indicate that the modification could reduce the time required to remediate the site. The vendor stated that for every month s reduction in the length of the project, 500,000 would be saved from project costs. Therefore the cost of steady-state operation of an HTTS is estimated to be 500,000 per month (D106172, p. 61). 

The NSR technology has been also applied to diesel engines, and is most reliable and attractive method for lean-burn combustion vehicles. Diesel particulate-NOx reduction system (DPNR) method is used to realize the simultaneous and continuous reduction of particulate and NOx is also recommended. This catalyst system is DPF combined with NSR catalyst. Soot on catalyst is removed during NOx reduction by occasional rich engine modification. Many other catalyst systems with NSR catalyst have been also developed. With decreasing S content in fuel and successive development of... 

III. Reduction of Combustion-Generated Pollution by Combustion Process Modification... 

A World Wide Web site [239] has been set up to provide software for the investigation and reduction of combustion mechanisms. The programs available include MECHMOD, a code for the automatic modification of CHEMKIN format combustion mechanisms, and KINALC, which is an almost automatic program for the investigation and reduction of gas-phase reaction mechanisms. KINALC is a postprocessor to CHEMKIN-based simulation packages SENKIN, PREMIX, OPPDIF, RUNIDL, PSR, SHOCK and EQLIB. 

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