Flameless Oxidation Technology

Transcription

1 Flameless Oxidation Technology A.Milani, J.G.Wünning WS Wärmeprozesstechnik Dornierstr Germany WS Abstract Flameless combustion is the most significant recent advancement in high temperature combustion technology and has been applied to industrial furnaces with well proven very low NOx performance and high energy savings. This experience has produced spin-offs in power generating equipment, from innovative gas turbine combustors to small reformers for decentralized H 2 production, and R&TD of flameless oxidation techniques is quite promising for new advanced process design. 1. Introduction The flameless combustion technology applied to high temperature industrial processes stems from systematic investigations carried out at laboratory scale and from application to large plants in the steel industry. Results are very satisfactory both for abatement of NOx emissions and for energy savings; spin-off and on-going R&TD in the field of power generation is very promising. All this started from looking again at basic principles. A conventional flame is based upon a mechanism as old as fire discovered in nature many centuries ago: a stable flame develops from a stationary flame front, that is a few mm thick layer. Burner design is primarily concerned with the problem of stabilizing the flame front by means of fluid dynamic devices. Typically a bluff body drives back hot reacting products that heat up the fresh fuel-air mixture thereby triggering a stable chain reaction. High gradients of temperature and species concentration in a confined space are required to obtain a stationary flame. 2. Flameless Combustion Figure 1 NOx emissions from steel furnaces In a burner stabilized flame most reactions occur within the flame front, where local temperature approaches adiabatic temperature. In a flameless burner, the flame front is deliberately avoided and combustion reactions occur as fuel and air mix together with entrained recirculated combustion products. For the process to occur, combustion products must be above self-ignition temperature (> 850 C for safety). The reaction rate is determined by the mixing pattern between three partners: fuel, air and combustion products entrained before combustion. In the flameless I2.1

2 F 29th Meeting on Combustion mode temperature profile is determined by the mixing pattern with the recirculated combustion products and cannot depart much from the temperature of these entrained combustion products. In the flame mode, the temperature profile peaks in the flame front close to the burner: this is conducive to enhanced thermal NO formation [1]. To abate temperature peaks means to abate thermal NO and flameless combustion does abate NOx emissions by one order of magnitude. Figure 1 reports accumulated data relevant to many natural gas fired furnaces in the steel industry: the advantage of the flameless technology for temperatures > 850 C with respect to the best low-nox burners designs is quite clear. Figure 2 shows how flame and flameless mode are implemented in high velocity burners, that are common in heat treatment furnaces for steel products. Flue,air,gas,flameless The domain of flameless combustion has been investigated on a test furnace as a function of the recirculation ratio K v defined as ratio of recirculated mass flow of combustion products (before reaction) with respect to the driving flow rate of reactants [1,2] K v = M rec / (M air + M fuel ) Results are schematized in Figure 3: for temperatures > ~ 850 C, above self-ignition, a domain of stable reaction region without flame front can be established, corresponding to large K v values (order of K v > ~3), that are obtained with high momentum of the injected fluids. This domain has been called flameless Figure 2 Flame and flameless combustion oxidation or with the trademark FLOX. It is not possible to establish a conventional flame front for K v values ~ > and the intermediate region is typical of lifted flames and of unstable combustion. Below ignition temperature, burner stabilized flame mode only is admissible. Flameless oxidation does not produce a visible flame and furthermore this combustion mode is almost silent and abatement in combustion noise (~ 15 dba) is at least as impressive, as disappearance of a visible flame, proving that the turbulent flame front accounts for most of the typical combustion roar of high velocity burners [3]. Flameless oxidation has been thoroughly investigated by WS [3,4]. FLOX has been shown to work for rich, near stoichiometric and for very lean combustion conditions; it works with and without air or fuel preheat. It also works for diffusion, partial premixed and premixed combustion. A well known advantage concerns low-nox burners operated at very high air preheat: unlike burner stabilized flames ~ ctemperature unstable lifted flames self-ignition temperature no stable flame explosion risk stable flameless combustion or FLOX K v Figure 3 Domain of FLOX vs K v factor I2.2 2 v

3 Italian Section of the Combustion Institute conventional flame mode, the flameless mode is insensitive to air preheat temperature as far as NOx is concerned, and this is very important for application to high temperature industrial processes or furnaces. 3. Energy savings in steel furnaces The thermal efficiency of high temperature furnaces can be increased very much by means of efficient heat recovery by means of air preheating: high efficiency is equivalent to reduction in fuel consumption and to a corresponding saving in greenhouse gas emissions. High preheat, like air at C, is only technically feasible if special combustion techniques are adopted in order to prevent unacceptable NOx emissions and local overheating. Flameless oxidation fits perfectly this requirement and can be considered a prerequisite for such applications. ~cold flue gases hot flue gases nat. gas cold air furnace wall combustion chamber Figure 4 Burner-integrated heat recovery A preferred burner design for high air preheating is based on burner integrated heat recovery (Figure 4): flue gases are extracted through the burner itself and combustion air is preheated in counter-current while cooling the flue gases. This is a convenient solution for furnaces equipped with several burners: cold combustion air is distributed to the burners while almost cold flue gases are extracted from a common manifold. This design offers effective preheating efficiency. Centralized heat recovery allows thermal efficiency ~ 60 % (~ 40% for no preheating at all), burner integrated recovery scores ~ 75-85%, which is a good step forward, corresponding to a fuel saving 15-25% with respect the to state of the art (centralized heat recovery). Thousands of FLOX burners have been installed in continuous industrial plants and perform satisfactorily. Also regenerative burners firing in FLOX mode have been adopted in several large annealing lines for stainless steel strips and in batch furnaces. Regenerative air preheating is certainly most efficient and allows energy savings in the order of 30-50% [5]. The radiant tube is a device used in large heat treatment furnaces for steel products: it radiates to the stock without permitting contact with the flue gases and combustion is developed inside a long tubular chamber, which makes combustion control difficult. Experience has demonstrated that internal recirculation of combustion products is the key to good performance: re-circulating geometries allow low-nox performance and uniform temperature of the radiant tube thanks to flameless oxidation. Temperature uniformity has beneficial consequences on the strength of the radiant tube and on the average allowable heat flux, which implies a better exploitation of the radiating surface: in other words, a saving in I2.3 3

4 29th Meeting on Combustion installation costs. A good example are the annealing furnaces equipped with ceramic singleend tubes in SiSiC: the cost of ceramic radiant tubes has been largely overridden by excellent performance. Figure 5 shows pictures of large plants equipped with several hundreds FLOX burners. Figure 5 Continuous steel furnaces equipped with FLOX burners 4. Power generating equipment The FLOX principle is not limited to steel furnaces and can be applied to several high temperature processes. Examples are the Stirling engines, where heat is made available at high temperature with high efficiency, with the purpose of providing combined heat and power in small power generating units. A very promising application of flameless combustion to combustors for gas turbines is being presently developed and successfully tested: a specially designed FLOX prototype burner (Figure 7) ensures very low-nox, emissions and overcomes the nasty problem of fluctuations or humming that affects premix-based GT combustors, where the flame front stabilization is a critical issue. R&TD is ongoing with the participation of several academic and industrial partners in Europe. The inherent temperature uniformity obtained with flameless combustion finds an ideal application in steam reformers for hydrogen production: reforming reactions take place inside vertical tubes filled with a catalyst and reheated from the outside. The uniform temperature distribution is essential for high productivity, reduced stress on the reaction tubes and better control. The experience of the W S company in steel process furnaces has been used to found a daughter company specialised in minireformers for producing Figure 6 CFD computations of the prototype GT combustor small amounts of hydrogen I2.4 4

5 Italian Section of the Combustion Institute (order of Nm 3 /h) for decentralised fuelling stations for future H 2 powered vehicles. Figure 8 shows the scheme of the WS minireformer: such plants have been installed in the airports of Munich and of Madrid to provide the H 2 used by local buses for passenger service. Flameless oxidation has been investigated with gaseous fuels and in particular with natural gas. However, the basic principle holds good for any fuel, at least any fuel as soon as it is made available in fluid form (like evaporation of liquid droplets or release of volatile matter from pulverised solid fuel). Trials are being carried out together with German universities to test effects of flameless oxidation (Figure 8): flameless mode occurs with any fuel and consistent NOx reduction has been observed. Encouraging tests have been carried out under pressure, which might be applicable to envisaged future processes aiming at CO 2 sequestration. 5. Conclusions Referring to the case of high temperature furnaces, the industrial application has Figure 7 The FLOX based demonstrated that flameless technology can minireformer greatly renew and improve the design and the performance of traditional plants / processes; advantages like downsizing (reduction of the furnace length), NOx minimization, temperature uniformity, better control and improved product quality make investment for revamping old plants advantageous. Similar arguments hold true for the R&TD applications to power generating devices as quoted in 4 above. We can conclude that the principle of flameless oxidation has still a large potential for further development in many equipment where combustion plays the important role. Figure 9 FLOX performance with different fuels I2.5 5

6 29th Meeting on Combustion The tendency is to tighten regulations concerning pollutant emissions and to limit specific emissions of greenhouse gases, which implies reducing specific fossil fuel consumption. This is based upon steady grounds: not the available or future fossil fuel resources put an effective limit to economic and abundant energy, but the available clean air. Clean air for combustion is a limited global resource that cannot be wasted or corrupted beyond a sustainable threshold. In former times California had promoted use of catalytic converters and had thereby stimulated the competitive production of cleaner engines. A similar, virtuous pattern should be followed in other domains related to fossil energy conversion as awareness of the worldwide environmental challenge proceeds. References [1] Wünning, J.A. Flammenlose Oxidation von Brennstoff mit hochvorgewärmter Luft, Chem.- Ing.Tech. 63, No.12, p , 1991 [2] Wünning J.A., Wünning J.G. (1997): Flameless Oxidation to reduce thermal NO formation, Prog. Energy Combust. Sci., 23, 81-94, 1997 [3] Wünning J.G.(2005): Flameless Oxidation, 6 th int Symposium HTACG Essen, October, 2005 [4] WS Patents EP and EP (1990) [5] Milani A., Wünning J.G. (2002): Design concepts for radiant tubes - Millennium Steel 2002 I2.6 6