NOx formation

Although the gasifier product itself has low levels of NOx, the total systems emissions of this product must be carefully scrutinized. When clean biogas is eventually burned, NOx will be produced, as it is in most combustion systems with all fuels. The use of biogas rather than solid biomass fuels provides the opportunity to better control the combustion process, which can potentially result in lower NOx emissions. As such, gasification offers potential environmental emissions advantages over combustion alternatives. However, NOx may still occur as the gas is burned, and appropriate NOx control technologies may be needed. 

Most biomass feedstocks contain low percentages of sulfur. In contrast, this is a major concern for coal gasification. 

Sulfur in the biomass feedstock can be converted to hydrogen sulfide or sulfur oxides during gasification. Wood typically contains less than 0.1% sulfur by weight, and herbaceous crops may contain 0.3 to 0.4%. Some feedstocks such as refuse-derived fuel (RDF) may contain 1% or more, approximately the same as bituminous coal. As a result of the low levels of sulfur in the biomass, the concentrations of H2S and SOx levels in the product gases are below those requiring cleanup in most applications. 

The low concentration of sulfur in biomass offers potential advantages for some applications. In cofiring applications, for example, the cleaner combustion gases from biomass dilute those from coal, and the overall concentrations of sulfur per unit of combustion gas are reduced. In most applications 

Sulfur is a potential problem even at low levels for synthesis gas systems using certain types of catalysts. The production of methanol from synthesis gas, for example, uses catalysts that are poisoned by sulfur. Some tar cracking catalysts are also sulfur sensitive. In those systems, thorough removal of sulfur will be required. Fuel cell systems are also sulfur sensitive. 

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Basis for NOx Prevention. Emissions from turbines are a funetion of temperature and thus a funetion of the F/A ratio. Figure 10-20 shows that as the temperature is inereased the amount of NOx emissions are inereased and the CO, and the unburnt hydroearbons are deereased. The prineipal meehanism for NOx formation is the oxidation of nitrogen in air when exposed to high temperatures in the eombustion proeess, the amount of NOx is thus dependent on the temperature of the eombustion gases and also, to a lesser amount on the time the nitrogen is exposed to these high temperatures. 

Benefit The use of LNB decreases the amount of NOx formation at the facility and therefore may help facilities meet state RACT or BA CT (40 CFR 52) requirements. Additionally, this technology may help facilities meet standards of performance for industrial-commercial-institutional steam generating units in 40 CFR 60, Subpart Db. A decrease in a facility s NOx emissions may decrease the possibility that a facility will meet the Nox emission threshold for an air permit under 40 CFR 70 and 71. 

It can be seen that excess air and humidity in the combustion air both act to reduce the theoretical flame temperature. However, the excess air has the more significant effect. In some combustion processes, steam is injected into the combustion process to decrease the flame temperature to decrease NOx formation. This will be discussed later in Chapter 25. 

For the prediction of NOx formation, the extended Zeldovich mechanism described by Heywood[603] was implemented. The soot emission modelis a modified version of previously published models for soot formation and oxidation. Details of the soot emission model have been discussed by Han et al.[604]... 

The present research has treated important parts of the modeling of combustion and NOx formation in a biomass grate furnace. All parts resulted in useful approaches. For all these approaches successful first steps were taken. Currently, more research is underway to obtain improved results NH3 production is measured in the grid reactor with the tunable diode laser, detailed kinetics will be attached to the front propagation model, including the measured NH3 release functionalities, and for the turbulent combustion model heat losses are taken into account. In addition, the fuel layer model has to be coupled to the turbulent combustion model in the furnace. 

Platinum, as well as other promotors, can cause problems such as NOx formation or other unfavorable side effects. The rare earths do not have these drawbacks so a good deal of attention has been given to their development for SOx removal. 

W.A. Hahn and J.O. L Wendt. NOx Formation in Flat, Laminar, Opposed Jet Methane Diffusion Flames. Proc. Combust. Inst., 18 121-128,1981. 

NOx formation occurs by a complex reaction network of over 100 free-radical reactions, and is highly dependent on the form of nitrogen in the waste. Nitro-compounds form N02 first, and then NO, approaching equilibrium from the oxidized side. Amines form cyano intermediates on their way to NO, approaching equilibrium from the reduced side. Using air as the oxidant, NO also forms from N2 and 02. This last is known as thermal NO. ... 

Drost et al. [129] developed an evaporator combined with a micro scale combustion chamber for homogeneous combustion of hydrocarbons (Figure 2.82). The main focus of the work was to maintain a stable combustion of the fuel avoiding NOx formation. Evaporation tests were carried out under isothermal conditions. Fifty-four parallel channels, 270 pm wide and 1 000 pm deep with a length of20.52 mm, were cut into a copper substrate with a diamond saw. 

PYROLYSIS SOOT FORMATION SOOT OXIDATION GAS PHASE OXIDATION NOx FORMATION... 

The antioxidants butylated hydroxyanisole (BHA), butylated hydroxytolu-ene (BHT), tm-butylhydroquinone (TBHQ), propyl gallate, ascorbyl palmitate and citric acid were evaluated for potential to reduce NOx emissions from a single cylinder, direct-injection, air-cooled, naturally aspirated Yanmar engine (Hess et al, 2005). BHA and BHT reduced NOx emissions by 4.4 and 2.9%, respectively, but the other antioxidants evaluated did not exhibit any beneficial effects (see Table 1.2). Antioxidants may impede NOx formation by inhibiting the formation of combustion-derived radicals. 

NOx formation, that have led to its virtual elimination in the United States.13... 

The liquid ammonia of the high-pressure separator is flashed to about 20 bar, whereby the majority of the dissolved gases are released in the let-down vessel. This gas is normally used as a fuel, preferably after removal of ammonia vapor to avoid NOx formation in the combustion furnace. 

There are many methods that can be suggested for reduced NOx formation in IGCC, and measures can be taken at different stages of the overall process. These methods can be categorised as follows ... 

Figures 6 and 7 show the experimental PFBG results with respect to fuel nitrogen to Ammonia and Hydrogen Cyanide. These species are known precursors for NOx formation under e.g. gas turbine combustion conditions, which is a problem when dry, high temperature gas cleaning is applied, see e.g. Hoppesteyn [9], From the results it can be concluded that a major part of the fuel-bound nitrogen is converted to Ammonia. This has also been indicated before, in the description of the pressurised fluidised bed pinewood gasification using the DWSA test rig, although somewhat lower conversion values are observed in the PFBG tests.

One major topic of the work is an extension of the CFD code with a post-processor for the fuel nitrogen to NOx conversion, A reduced kinetic scheme has been obtained which describes quite well the combustion emission behaviour with respect to NOx formation. Figure 9 shows a comparison between experimentally observed NH conversion to NO and modeling with this modified post processor. The agreement is reasonably good for experiments at 0.5 and 0.7 Mpa. Significant deviation between model and experiment is seen for an experiment at 0.4 MPa. This is attributed to a measurement error at that pressure, A recommendation is to perform more experiments at this or lower pressure. 

The influence of the maximum temperature in the combustor on the composition of the flue gases is shown on Figures 3 and 4, The temperature increase leads to better and faster combustion of the char and CO (Figure 3), It is clear that the temperature in the combustor cannot be too high to avoid ash melting and increase in the NOx formation. The concentration of NO is usually within 100 - 200 ppm limits (Figure 4) in reasonable agreement with experimental observations. 

Chemistry of NOx Formation from Fuel Nitrogen Conversion... 

Residual NHj and HCN are decomposed by afterburning under atmosphere of low oxygen concentration, but if the structure of the combustion chamber is unsuitable, a large amount of NOx will be formed here. Table V indicates that an amount of NOx is formed by afterburning in each run. Further experiments for decreasing NOx formation at afterburning of exhaust gas has been conducted... 

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