Combustion modifications

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. 

Because NO, formation is a function of the temperature, fuel-air mixture, and fluid dynamics in the furnace, the goal of a combustion modification is to mix fuel and air more gradually to reduce the flame temperature (lower thermal NO, production), and to stage combustion, initially using a richer fuel-air mixture, thus reducing oxidation of the nitrogen in the fuel. LNBs sewe the role of staged combustion. 

Turner, D. W., R. L. Andrews, and C. W. Siegmund, "Influence of combustion modification and fuel nitrogen content on nitrogen oxides emissions from fuel oil combustion," presented at 64th Annual AIChE Meeting, San Francisco, Calif., American Institute of Chemical Engineers, New York, November 1971. 

A very small fraction of the nitrogen in the coal and/or air is converted to gaseous NO, during combustion. These are pollutants that must also be removed from the flue gas or controlled bv combustion modification, but that technology is not within the scope of this case study. 

By an incomplete combustion (modification m) the diamond combines with 16 grammes of oxygen, forming tiie mixture CO+0, brought to 0 , which is the state 1 it is desired to know the quantity of heat q set free by this reaction this cannot be done directly, because it is impossible to regulate the combustion so that the product corresponds exactly to the preceding formula. 

The quantity of heat O set free by the combustion (modification AfO of 12 grammes of carbon in 88 grammes of nitrous oxide the intern passes from the state 0 to the state 2. 

The Chemistry of Nitrogen Oxides and Control through Combustion Modifications... 

As Table I indicates, almost half of the total national NOa emissions result from stationary fuel combustion. In principle, NOa emissions from fossil fuel-combustion systems can be reduced by three methods fuel cleaning (removal of the fuel nitrogen), combustion modification, and flue gas treatment. Combustion modification appears to be by far the easiest and most economical of the three. Therefore, EPA has assigned to the Control Systems Divisions, Combustion Research Section (CRS) the responsibility for assisting industry in developing technology to re-... 

Relationship of Pollutant Chemistry to Existing Combustion Modification Control Technology... 

Japan was the first country to do this, starting with SCR for the removal of NOx from gas, coal and oil-fired power plants [63]. Utility companies in Japan introduced the catalytic SCR process as early as 1972. At present, many countries utilize SCR with Germany and Japan being the leading countries. Although priority is currently given to combustion modifications for NOx control in European countries, extended operation of catalytic SCR is expected in the near future. 

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],... 

Cato, G. A. Muzio, L. I., and Shore, D. E. (1976) Field testing application of combustion modifications to control pollutant emissions from industrial boilers—phase II, EPA-600/2-76-086a, U.S. Environmental Protection Agency, Research Triangle Park, NC. 

We have previously demonstrated (].) our ability to make time-resolved mass spectrometer sampling measurements of a flame propagating through a combustion bomb. Our interest in making these kinds of measurements is to study the chemistry of ignition by various ignition methods and to determine the value of these different methods for combustion modification. This report presents our first results on the chemistry of spark ignition. 

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]. 

The Combustion Modification Group. Serving the Plastics Industry with Quality Flame Retardant Products and Technology. Product sheet of Dow. Form No. 101-234-85 (1985)... 

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