Air staging

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. 

Vlanv oil biirnor.s arc do.SLgnod a.s cornbination ga.s/oil biirnor.s, An example of a modern low-N(), oil/ga.s forced-draft burner Ls. shown in Fig, 27-30, This is an air-staged design, with the air divided into pri-rnaiv, secondarv, and tertiarv streams. An air-staged natural draft process heater oil/gas burner is illustrated in Fig, 27-3L... 

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.)... 

Low-NO burners are designed to delay and control the mixing of coal and air in the main combustion zone. A typical low-NO air-staged burner is illustrated in Fig. 24-16. This combustion approach can reduce NO emissions from coal burning by 40 to 50 percent. Because of the reduced flame temperature and delayed mixing in a low-NO burner, unburned carbon emissions may increase in some apphcations and for some coals. Overfire air is another technique for... 

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. 

Primary stage residence time of 0.5 second Advanced low N0X firing systems which permit a high degree of air staging are capable of achieving this residence time. 

In the case of air staging, the conversion of fuel nitrogen in the fuel-rich zone follows a reaction path through the rapid initial formation of cyanides, their subsequent conversion to amines, and eventual conversion to molecular nitrogen (middle column in Fig. 2). The combustion is then completed in a fuel-lean zone by injecting additional amounts of combustion air. 

The goal of the project was the development of a low emission biomass burner for boilers in the range of 50 - 500 kW thermal output [5]. The main feature of the PDU burner design is the secondary combustion chamber creating a vortex to increase gas phase turbulence in order to maximise complete combustion. The main emphasis of the burner design is to reduce the release of particulate matter. The combination of vortex and air staging techniques is supposed to reduce also the NOx-emission level. The combustion system is designed to bum preferably wood chips. 

Experimental measurements of CO, O2, unbumed light hydrocarbons (UHC) and temperature are carried out under different operating conditions and over-fire secondary air staging. It is shown that the emissions of CO can be reduced through controlling the secondary air supply. Char formed in the bed is low in terms of its influence on the heat release, however it has significant influence on the CO distribution in the upper part of the furnace and at the outlet. 

The combustion process of wet wood chips and formation of pollutants in a biomass furnace have been investigated. Distributions of species CO, UHC, O2 where calculated numerically and compared to experimental data. It is shown that char, as flying particle, though in small amount has a significant influence on the CO emissions at the outlet. Numerical simulation indicates that half of the CO emission at the outlet is due to the combustion of flying char particles at the upper part of the furnace. Over-fire air staging has a significant influence on the residence time of panicles and gas species in the furnace, and thus the conversion of fuel and intermediate species to final products. 

Following the above mentioned results, an air staging con utational tool which controls the combustion air input in a way, that a mmimrnn of primary air input is given in order to minimize the NO emissions has been developed. The scheme of this control system is shown in Fig. 8. 

Nussbaumer, Th., Salzinann, R. (1996) Primary measures for NOx reduction in wood combustion by air staging and fuel staging. 1" European Conference on Small Burner Technology and Heating Equipment, Zuerich (CH), September 25-26 1996, pp. 165-174. 

Good, J., Nussbaumer, Th., Schaffher, H.P. (1998) NOx Reduction in Biomass Combustion by Combination of Air Staging and SNCR Technique. Biomass for Energy and Industry. 10 European Conference and Technology Exhibition, Wtirzburg (D) June 8 - 11 1998, pp, 1360 - 1361. 

The reduction of fuel-N to molecular nitrogen in air staging is favored in the fuel rich primary combustion zone ( Fig. 3 ). Investigations on fixed bed wood furnaces have shown that a minimum of the Total Fixed Nitrogen (TFN = HCN+NH3+NO+ NO2+2N2O emission from the primary combustion zone is reached for a stoichiometric ratio of 0.7 to 0.8 and a temperature of 1100 C to 1200 °C and providing a mean residence time of 0.5 s. After the reduction zone the combustion is completed in the burnout zone by injection of the excess air. 

Fi%. 8 Influence of stoichiometric ratio in the reduction zone in air staged combustion of wood chips and UF-chipboard. 

---