Flue gas recirculation

Flue gas recirculation. Recirculation of part of the flue gas as shown in Fig. 11.4 lowers the peak flame temperature, thus reducing formation. There is clearly a limit to how much flue gas can be recirculated without affecting the stability of the flame. 

NO reductions on the order of 40 percent are possible by flue gas recirculation. 

Assessments of control, operabiHty and part load performance of MHD—steam plants are discussed elsewhere (rl44 and rl45). Analyses have shown that relatively high plant efficiency can be maintained at part load, by reduction of fuel input, mass flow, and MHD combustor pressure. In order to achieve efficient part load operation the steam temperature to the turbine must be maintained. This is accompHshed by the use of flue gas recirculation in the heat recovery furnace at load conditions less than about 75% of fiiU load. 

NO Emission Control It is preferable to minimize NO formation through control of the mixing, combustion, and heat-transfer processes rather than through postcombustion techniques such as selective catalytic reduction. Four techniques for doing so, illustrated in Fig. 27-15, are air staging, fuel staging, flue-gas recirculation, and lean premixing. 

Flue Gas Recirculation Flue gas recirculation, alone or in combination with other modifications, can significantly reduce thermal NO. Recirculated flue gas is a diluent that reduces flame temperatures. External and internal recirculation paths have been applied internal... 

FIG. 27-35 Low-NOj burner with air-staging and flue-gas recirculation for use in bigh-temperature furnaces. (Hauck Manufactuiing Company. Developed and patented by the Gas Research Institute.)... 

Flue gas recirculation (FGR) is the rerouting of some of the flue gases back to the furnace. By using the flue gas from the economizer outlet, both the furnace air temperature and the furnace oxygen concentration can be reduced. However, in retrofits FGR can be very expensive. Flue gas recirculation is typically applied to oil- and gas-fired boilers and reduces NO, emissions by 20 to 50%. Modifications to the boiler in the form of ducting and an energy efficiency loss due to the power requirements of the recirculation fans can make the cost of this option higher. 

Techniques involving low-excess-air firing staged-combustion, and flue gas recirculation are effective in controlling both fuel NO, and thermal NO,. The techniques of reduced air preheat and reduced firing rates (from normal operation) and water or steam injection are effective only in controlling thermal NO,. These will therefore not be as effective for coal-fired units since about 80% of the NO, emitted from these units is fuel NO,. 

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. 

Future legislation will stimulate burner development in the areas of carbon monoxide, NOx and particulate generation. Techniques will include flue-gas recirculation, staged combustion, and additives to reduce the NOx and more sophisticated controls. Controls over the sulfur generated do not affect burner design greatly since the sulfur dioxide is a natural product of combustion and can only be reduced by lower fuel sulfur contents or sulfur removal from the exhaust gases. 

Flue gas recirculation. Recirculating part of the flue gas, as shown in the Figure 25.28, also reduces the peak flame temperature and reduces the thermal NO formation. Usually 10 to 20% of the combustion air is recirculated. 

Flue gas recirculation Flue gas recirculation, alone or in combination with other modifications, can significantly reduce thermal NO,. Recirculated flue gas is a diluent that reduces flame temperatures. External and internal recirculation paths have been applied internal recirculation can be accomplished by jet entrainment using either combustion air or fuel jet energy external recirculation requires a fan or a jet pump (driven by the combustion air). When combined with staged-air or staged-fuel methods, NO emissions from gas-fired burners can be reduced by 50 to 90 percent. In some applications, external flue-gas recirculation can decrease thermal efficiency. Condensation in the recirculation loop can cause operating problems and increase maintenance requirements. 

Air-staged burners Low-NO air-staged burners for firing gas (or oil) are shown in Fig. 24-28. A high-performance, low-NO, burner for high-temperature furnaces is shown in Fig. 24-32. In this design, both air-staging and external flue-gas recirculation are used to achieve extremely low levels of NO emissions (approximately 90 percent lower than conventional burners). The flue gas is recirculated by a jet-pump driven by the primary combustion air. 

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. 

At the left hand side a flue gas recirculation zone is formed over the whole height of the burnout zone. 

Salzmann, R. Good, J. Nussbaumer, T, Leiser, O. (1998) Temperature reduction by flue gas recirculation in biomass combustion with air staging modeling and experimental results. In l(f European Conference cS Technology Exhibition, Wurzburg (Germany). 

At present, no completely satisfactory and universally applicable control technique is available, but several approaches (e.g., flue gas recirculation, low excess air flring, and staged combustion) appear very promising. A better understanding of the chemistry of pollutant formation should lead ultimately to optimization of these and other new techniques for total control of NO emissions. 

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