Trapped-vortex approach for syngas combustion in gas turbines

A key issue in combustion research is the improvement of combustion efficiency to reduce fossil fuel consumption and carbon dioxide emission. Researchers are involved in the development of a combustion technology able to accomplish energy savings with low pollutant emissions.

The differences between syngas and natural gas combustion are mainly two. For the same power, fuel mass flow should be 4-8 times higher than natural gas, due to the lower calorific value. Premixed combustion of natural gas and air is one of the most commonly used methods for reducing NOx emissions, by maintaining a sufficiently low flame temperature. This technique poses some problems with syngas because of a significant presence of hydrogen and the consequent danger of flashback in the fuel injection systems. In the case of a non-premixed diffusion flame, diluents such as nitrogen, carbon dioxide and water, or other techniques, such as MILD combustion, can be employed to lower flame temperatures and hence NOx. The systems developed so far use combustion in cavities as pilot flames for premixed high speed flows. The goal is to design a device operating entirely on the principle of trapped vortices, which is able, based on its intrinsic nature of improving mixing of hot combustion gases and fresh mixture, that represents a prerequisite for a diluted combustion and at the most a MILD combustion regime. The trapped vortex technology offers several advantages for a gas turbine burner:

1. It is possible to burn a variety of fuels with medium and low calorific values.
2. NOx emissions reach extremely low levels without dilution or post-combustion treatments.
3. It provides extended flammability limits and improves flame stability.

MILD combustion is one of the promising techniques proposed to control pollutant emissions from combustion plants. It is characterized by high preheating of combustion air and massive recycle of burned gases before reaction. These factors lead to high combustion efficiency and good control of thermal peaks and hot spots, lowering NOx thermal emissions. Two important aspects are crucial for MILD combustion. First of all, the reactants have to be preheated above the self-ignition temperature. Secondly, the reaction region has to be entrained by a sufficient amount of combustion products. The first requirement ensures high thermal efficiency, whereas the latter allows flame dilution, reducing the final temperature well below the adiabatic flame temperature. In this way, reactions take place in a larger portion of the domain in absence of ignition and extinction phenomena, due to the small temperature difference between burnt and unburnt gases and as a result a flame front is no longer identifiable; this is why MILD combustion is often denoted as flameless combustion.

Another advantage is represented by the fact that the temperature homogeneity reduces materials deterioration. Because of the limited temperature, NOx emissions are greatly reduced and soot formation is also suppressed, thanks to the lean conditions in the combustion chamber, Trapped-vortex approach for syngas combustion in gas turbines Case History 6 - Newsletter EnginSoft Year 9 n°2 due to the large dilution level and the large CO2 concentration. The introduction of MILD technology in gas turbines is of great interest because it is potentially able to answer two main requirements:

1. A very low level of emissions.
2. An intrinsic thermo acoustic stability (humming).

The TVC project concerns gas turbines which use annular combustion chambers. The prototype to be realized, for simplicity of design and measurement, consists in a linearized sector of the annular chamber having a square section of 190x190mm (fig. 1).

The power density is about 15 MW/m3 bar. The most obvious technique to create a vortex in a combustion chamber volume is to set one or more tangential flows. In this case two flows promote the formation of the vortex, while other streams of air and fuel, distributed among the tangential ones, feed the "vortex heart". The air flows placed in the middle provide primary oxidant to combustion reaction, while the tangential ones provide the air excess, cool the walls and the combustion products, in analogy to what happens in the traditional combustion chambers, in which this process occurs in the axial direction. The primary and the global equivalence ratio (Fuel/Air/(Fuel/Air)stoic) were equal to 1.2 and 0.4, respectively. Given the characteristics of the available test rig, the prototype will be tested under atmospheric pressure conditions, but the combustion air will be at a temperature of 700 K, corresponding to a compression ratio of about 20 bar, in order to better simulate the real operating conditions. The syngas will have the following composition: 19% H2 - 31% CO - 50% N2 LHV 6 MJ/kg. The reference case boundary conditions are reported in table 1.

The simulations, performed with the ANSYS-FLUENT code, have been carried out according to steady RANS and LES approaches. The models adopted for chemical reactions and radiation are the EDC, in conjunction with a reduced CO/H2/O2 mechanism made up of 32 reactions and 9 species, and the P1, respectively. NOx have been calculated in post-processing. In order to save computational resources, the simulations have been conducted only on one sector (1/3) of the whole prototype reported in the figure 1, imposing a periodicity condition on side walls. A structured hexahedral grid, with a total number of about 2 million cells has been generated.

Fig. 4 - Maximum temperature vs compressor ratio

A big effort has been made to properly modulate flow rates, velocities, momentum and minimum size of the combustion chamber. It’s clear that inlets placement plays a key role in the formation of a energetic vortex which can be able to properly dilute the reactants and create sufficient residence time that favorites complete combustion. The resulting configuration establishes a perfect balance between the action of the tangential flows, which tend to generate the vortex and the action of the vertical flows that tend to destroy it (fig. 2).

In this sense, it is worth pointing out that it is especially the tangential flow further from the outlet which is the most effective. The negative effects on vortex location and size resulting from a reduction of its strength, compared with the other inlets, have been evident. Even the distance between the two vertical hole rows has been properly adjusted. In fact, the upper tangential stream flows between the two vertical rows on the opposed side and, if the available space is insufficient, it doesn’t remain adherent to the wall and the vortex is destroyed. In principle, a significant presence a very reactive hydrogen, can produce elevated temperatures and fast reaction, especially near fuel inlets. For this reason, inlet velocities are sufficiently high to generate a fast rotating vortex and then a rapid mixing. Further increase in fuel injection velocity has a negative influence on vortex temperature peaks. The aim of the LES simulation has been to analyze the unsteadiness of the system. The vortex appears very stable in the cavity, i.e. trapped. No vortex shedding has been noted. Compared to axial combustors, the minimum space required for the vortex results in an increase in volume and a reduction in power density. On the other hand, carbon monoxide content, with its slow chemical kinetic rate compared to natural gas, requires longer residence time and a bigger volume.

Fig. 5 - EICO (CO emission index) vs compressor ratio

It can be observed that high temperature zones (fig. 2) are concentrated in the vortex heart. If the hydrogen content is rapidly consumed, the carbon monoxide lasts longer and accumulates given that the amount of primary air is less than the stoichiometric value. The mean LES and the steady RANS fields have provided very similar results. It is interesting to evaluate the residence time inside the chamber. In the central part of the chamber, the residence time ranges from 0.02 to 0.04 sec. The establishment of a MILD combustion regime depends especially on a sufficient internal exhaust gas recirculation, which associated to a good degree of mixing with reactants, represents a necessary condition for MILD combustion. In order to quantify the degree of mixing, the following variable has been mapped:

MIX=|H2-H2mean|+|H2OH2Omean|+| CO2-CO2mean|+|O2- O2mean|+|CO-COmean|+|N2-N2mean|

If all the species involved were perfectly mixed, MIX should be zero everywhere. In practice, the more MIX tends to zero, the more reactants and products are well mixed. If the zones immediately downstream the inlets are neglected, MIX assumes very low values in the chamber. This supports the fact that the vortex is able to produce the expected results. The recirculation factor, i.e. the ratio between exhaust recirculated and fresh mixture introduced, is about 0.87, while exhaust composition is: 0.19% CO2, 0.05% H2O, 0.005% CO, 0.039% O2, 0.713% N2.

In order to determine where reactions are concentrated, it is useful to analyze radical species distribution, such as OH. The fact that radicals are not concentrated in a thin flame front, but well distributed in the volume (fig. 3), represents an evidence of a volumetric reaction regime.

Fig. 6 - EINOx (NOx emission index) vs compressor ratio

A sensitivity analysis has been conducted varying the boundary conditions around the references previousy reported in table 1. The quality of the different cases was judged in terms of pollutants emission indices (g pollutant/kg fuel), in particular for CO and NOx. CO is an indicator for incomplete combustion. Its presence in the exhaust is favored by low temperature and lack of oxygen. On the contrary, NOx are favored by high temperature and oxygen abundance. A 30% increment of the tangential flow velocity causes a faster rotation of the vortex, but the recirculation ratio increases only to 0.92. A 30% decrement of the equivalence ratio (leaner combustion) causes a reduction in temperature and a subsequent increment in unburnt species, especially CO. NOx emissions are in general unimportant for all the cases mentioned.

If the operating pressure is augmented from 1 bar to 10-20 bar at constant geometry and inlet velocities, the mass flow rates and then the burner power increases by about 10-20 times, even if the specific power density (MW/m3 bar) remains constant. The temperature and pollutants trends for those cases are reported in figures 4-6. The maximum and average temperatures in the chamber increase almost linearly, while EICO increases and EINOx decreases as pressure increases. For each operating pressure, it is then possible to identify an optimal equivalence ratio condition, at the intersection of the two curves in figure 7, where the major pollutants are kept down at the same time.

One of the aims of this work has been to verify if when varying the prototype dimensions, their behavior would remain unchanged or not, in terms of fluid dynamics and chemistry. The scaling method based on constant velocities has been chosen for this purpose, because all the physics of the system are based on a fluid dynamic equilibrium, if one refers to the above discussion. If one imagines to halve all sizes, the volume will be reduced by a factor of 0.5x0.5x0.5=0.125, while the areas will be reduced by a factor of 0.5x0.5=0.25. Then mass flowrates will be scaled by a factor of 0.25. Therefore, the power density (power/volume) will increase by a factor of 0.25/0.125=2. The simulations show that the behavior of the burner remains unchanged, in terms of velocity, temperature, species, etc. fields. It can be concluded that, if the overall size of the burner is reduced, the power density increases accordingly to Power0.5.