Combustor emissions reduction

This chapter deals with the specific research and development issues related to the use of a catalyst at temperatures up to 1500 C in a catalytic combustor. A short survey of emission problems related to combustion and strategies for emission reduction is followed by a discussion of catalysis in combustion. Specific aspects of monolith combustion catalysts, such as material problems and combustor design, are then treated briefly. 

Thermal NOx is the dominating type of NOx produced in gas turbine combustors, which often use fuels with low nitrogen content. All efforts regarding emission reduction should thus aim for a reduction of thermal NOx- This is typically achieved by reducing the flame temperature and residence time. However, this results in increased CO and UHC levels, which causes many solutions to represent some sort of compromise. 

For a given combustion chamber, the flue gas volume reduction has the added benefit of reducing the average gas velocity by almost an order of magnitude. Lower gas velocities entrain fewer fine particles from the waste. This reduces particulate emissions. Other examples of particulate emission reductions are discussed in Chapter 2. Another potential advantage of the reduced velocity in the combustor chamber is the increased residence time. In an incinerator, increased residence time usually increases the level of destruction of undesired organic species in the off gases (see Chapter 8). 

Water or steam injection into the flame lowers the temperature and, hence, the NOy emissions are decreased. The reduction varies for each turbine and ranges from 60% to 94%. With low-NO combustors emissions of 25 ppm may be achieved. 

Noise emissions from gas-turbine engines, resulting from unsteady flow interactions with rigid and moving surfaces (compressor and diffuser noise) and from combustion (core noise), must also be reduced without sacrificing performance. Studies addressing simultaneous pollutant and noise emission reductions are rare. The current study may lead to a better understanding of the coupled acoustic and pollutant behavior of advanced combustor concepts. 

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. 

Chemical Volume Reduction Incineration has been the method commonly used to reduce the volume of wastes chemically. One of the most attractive features of the incineration process is that it can be used to reduce the original volume of combustible sohd wastes by 80 to 90 percent. The technology of incineration has advanced since 1960 with many mass burn facihties now have two or more combustors with capacities of 1000 tons per day of refuse per unit. However, regiila-tions of metal and dioxin emissions have resulted in higher costs and operating complexity. 

Hazardous waste combustors (HWCs) also are regulated under the Clean Air Act (CAA).6 The CAA protects human health and the environment from the harmful effects of air pollution by requiring significant reductions in the emissions of the most dangerous air pollutants. These pollutants are known or suspected to cause serious health problems such as cancer or birth defects, and are referred to as hazardous air pollutants (HAPs). 

A semi-industrial pilot plant has been developed in which air-borne ultrasound has been applied to the reduction of particle emissions in coal combustion fumes [62]. The installation basically consists of an acoustic agglomeration chamber with a rectangular cross-section, driven by four high-power and highly directional acoustic transducers operating at 10 and/or 20 kHz, and an electrostatic precipitator (ESP). In the experiments, a fluidised bed coal combustor was used as fume generator with fume flow rates up to about 2000 m /h, gas temperatures of about 150 °C. and mass concentrations in the range 1-5 gm. The acoustic filter reduced fine particle emissions by about 40 %. 

Regarding the emission levels, it is recommended to negotiate an emissions tolerance with the EPA. Reduction of the emission levels below the required EPA limit of 15 ppm would require another 40 plates. Even inserting a catalytic combustor after the absorption column would probably only reduce emissions to about 100 ppm. 

The rate of catalytic "NO" reduction by hydrogen and carbon monoxide over a char surface was measured and compared with the rate of noncatalytic "NO" reduction by char which has been previously reported to have a significant effect on "NO" emission control in a fluidized bed combustor of coal. 

The control of nitric oxide emission from a fluidized bed coal combustor has been extensively investigated and it was found that the level of nitric oxide emission was determined by the relative contribution of nitric oxide formation and reduction processes. (l.,2) There is a great need for quantitative information concerning the rate of these processes.(2)... 

It is especially rewarding that the solutions of practical engineering problems, such as the reduction of emissions of nitrogen and sulfur oxides and polycyclic aromatic compounds from boilers, furnaces, and combustors, are amenable to the application of chemical engineering fundamentals. Guidance for preferred temperature-concentration history of the fuel may be given by reaction pathways and chemical kinetics, and elements of combustion physics, i.e., mixing and heat transfer may be used as tools to achieve the preferred temperature-concentration history in practical combustion systems. 

Moritomi, H., Suzuki, Y., Kido, N., and Ogisu, Y. NO, Emission and Reduction from a Circulating Fluidized Bed Combustor, in Circulating Fluidized Bed Technology III (Basu, P., Horio, M and Hasatani, M.,eds.), pp. 399-404. Pergamon Press, Oxford (1991). 

Smoke emissions, also shown in Figure 11, followed a trend opposite to the combustion eflBciency, first decreasing to a minimum value and thereafter increasing. A net reduction in SAE smoke number (2) of 16% was measured for approximately 4% water addition. Thus, the data indicate that fuel emulsification can alter favorably the eflBciency of a practical gas turbine combustor without aflFecting adversely the turbine inlet temperature profile or the smoke emissions. It is speculated that part or all of this improvement may be attributed to improved atomization. 

The second approach for reduction of emissions from combustion processes is the improvement of the combustor itself. Improved fuel-air mixing, steam injection, advanced flow patterns, sophisticated injection systems, and flue gas recirculation have resulted in significant reduction of emissions of NO, CO, and UHC. However, continual development is needed to meet toughening future regulations. 

A partial combustion is achieved in a first zone with excess fuel, after which air is added under intense turbulence. The final combustion occurs under lean conditions in the second zone. The rich conditions in the first zone ensure low NOx levels and combustion stability, whereas the large air excess in the second zone, and hence the low temperature, avoids the formation of thermal NOx. This approach yields 50% reduction in NOx emissions today [27] as compared to conventional diffusion flame combustors. The problem with... 

Further reductions in NO levels can be achieved by removing NO from the turbine exhaust using selective catalytic reduction (SCR) with ammonia. SCR is an effective method for NO control that can reduce NO levels to about 10 ppm, usually in combination with water or steam injection or lean premix combustors. However, SCR is an expensive technology and the storage and handling of ammonia, a toxic chemical, also pose problems. Possible future restrictions on ammonia emissions may also limit the applieation of this technology. 

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