Oxidation flameless

There are two basic processes for thermal tfeatment of sludge. One, wet air oxidation, is the flameless oxidation of sludge at temperatures of 450 to 550° F and pressures of about 1,200 psig. The other type, heat treatment, is similar but canied out at temperatures of 350 to 400° F and pressures of 150 to 300 psig. Wet air oxidation (WAO) reduces the sludge to an ash and heat treatment improves the dev/aterability of the sludge. The lower temperature and pressure heat treatment is more widely used than the oxidation process. 

In 2000, cost estimates for the NoVOCs system were calculated based on a U.S. Environmental Protection Agency (EPA), Superfund Innovative Technology Evaluation (SITE) demonstration at North Island Naval Air Station in California. One-time capital costs for the NoVOCs system were estimated to be 190,000. Operational costs were an additional 160,000 for the first year and 150,000 for each additional year. Factoring in a 4% annual inflation rate, total costs are as follows 350,000 for 1 year, 670,000 for 3 years, 1,000,000 for 5 years, and 2,000,000 for 10 years of operation. At the North Island site, a Thermatrix flameless oxidation system (T0795) was also used as part of the treatment train. The Thermatrix system cost an additional 989,000 (D21594L, pp. ES-8, ES-9). 

Improvements are needed in combustion technologies to improve combustion stability and to reduce the intact of transients, especially on emissions. Greater effort is needed to take advantage of new developments such as flameless oxidation and other advances. 

In the concluding remarks, features of flameless oxidation and perspectives for widespread use in several applications are summarized. 

The flameless oxidation was discovered in 1989 during trials aimed at NO reduction in combustion with highly preheated air [5,6]. No flame could be detected or heard in the combustion chamber and the NO abatement was above all expectations. Figure 23.2 shows pictures of these trials. On the left-hand side, the burner is shown in flame mode the stable high velocity flame is pale blue and the high temperature at the burner nozzle causes a local bright luminosity. On the right-hand side, the same burner is shown in flameless oxidation mode. The hot zone at the burner nozzle has disappeared and no flame is visible or any combustion roar is audible. 

During the following years, the phenomena of the flameless oxidation has been investigated from a basic point of view, but also practical solutions have been developed for transferring the principle into practice. As stated in the introduction, there are many ways of flameless combustion the flameless oxidation can be defined as "stable combustion without flame with a defined recirculation rate of hot combustion products." With experiments, but also with computer simulations, it could be demonstrated that in flameless oxidation not only the flame is not visible, but also other typical flame features are not present [1,11]. 

Some definitions in the literature can be misleading because they are often referred to as hot preheated air, which is in fact the most widely used technical application until now. Therefore it should be noted here that with flameless oxidation ... 

In contrast to the combustion in stabilized flames, flameless oxidation is mixture and temperature controlled and is achieved by specific flow and temperature conditions. A prerequisite for a stable flame front is a balance between flow and flame velocity. This is true in premixed and in diffusion flames and stability depends on species concentrations, flow velocity, flow field, temperature, pressure, and other parameters. Creating flow conditions for flame stabilization is an essential burner design criterion. Swirl or bluff body are most often used to create stagnation points or areas of low velocity for stabilization. The species concentration also plays an important role. Air, with an oxygen content of 21% can create a flammable mixture with... 

To describe patterns of combustion processes, ideal models are often introduced such as piston flow or a perfectly stirred reactor as well as an ideal stirred boiler. For describing the flameless oxidation the model of the loop reactor is appropriate. Figure 23.6 shows different combinations of loop reactors. Here the piston flow (k, = 0) and the well-stirred reactor (k, = °°) can be considered as limiting cases of loop reactors. 

Based on a model of a reactor loop for better understanding, Figure 23.7 shows an ideal flameless oxidation... 

As a conclusion, the initial fundamental tests with the experimental furnace described in Section 23.5.1, backed up by the modeling computations, have demonstrated that the fundamental picture of flameless oxidation as a well-defined flameless firing technique is basically correct. 

Figure 23.15 shows the potential of NO reduction of the flameless oxidation. The values are in a logarithmic scale as the emissions are very much temperature dependent. As reference values, the emissions from air-staged high velocity burners are reported. The NO, values from burners without NO countermeasures and with preheated air are clearly much higher. 

The obtainable NO, emissions depend not only on the burner, but are also affected by the particular application. For instance, in a small radiant tube it is clearly more difficult to obtain an efficient recirculation of flue gases as required by flameless oxidation (Figure 23.17). The lowest NO values down to 1- digit ppm could be reached with burners, for which the combustion chamber had been optimized on purpose for flameless oxidation (combustor). In applications of direct firing in industrial furnaces with air preheating in the range... 

The most challenging design has concerned a combustor for gas turbines [9,22]. In gas turbines, the adiabatic temperature is moderated by large excess air and the combustion intensity is in order of magnitudes higher than in furnaces, because of size constraints and, of course, because of pressure. Flameless oxidation has been sometimes described as volume combustion in contrast to surface combustion that hints to a turbulent flame front, where volumetric reaction rate is certainly much higher than in the former case. The basic question was then will a flameless oxidation system cope with the required... 

Furthermore, Chapters 12 and 13 of this book are devoted to advanced diagnostic techniques that have almost superseded traditional intrusive techniques (water-cooled probes inserted into the combustion chamber for measuring local temperature, gas composition, etc.). Also flameless oxidation is better studied by nonin-trusive techniques as the insertion of probes disturbs the local conditions and may cause the onset of a local flame front. Fluorescence (like LIF techniques) as described in other chapters, or even simpler, a UV picture of the reaction zone are very suitable to describe flameless oxidation... 

A peculiar feature of flameless oxidation is that it requires recirculation of flue gases above self-ignition threshold (850°C for safety). Below threshold, a burner stabilized flame must be provided (Figure 23.9) and this is carried out by a system capable of selecting between flame mode (the only possible mode below 850°C) and FLOX mode (above threshold both modes are possible). Below threshold, a flame detector (ionization or UV principle) is required if FLOX is selected above threshold, then the flame detector must be neutralized (because there is steady combustion without flame). Dedicated control units have been developed for this purpose [1] the overall operation of the control system should be carefully checked in operating conditions. 

The flameless oxidation can be adopted where a suifable flue-gas recirculation af high temperature of flue gases can be carried out These conditions can be fulfilled by numerous heaf process planfs wifhouf modifications of the plant That is, the existing burners can often be modified and adapted to flameless oxidation. In ofher processes, especially fhose af low femperafure, the whole plant has been adapted on purpose, which is worth doing in green field construction only. 

Beside the very low nitric oxide emissions that can be obtained even with high air preheating, the flameless oxidation features other interesting aspects such as ... 

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