EFEECT OF HYDROGEN ADDITION ON METHANE COMBUSTION IN A CAN TYPE COMBUSTOR

Transcription

1 EFEECT OF HYDROGEN ADDITION ON METHANE COMBUSTION IN A CAN TYPE COMBUSTOR Ramesh E (Roll No ) Abstract The effects of hydrogen addition on lean methane combustion was numerically investigated in a can type combustor using the CFD commercial code FLUENT. Fuel mixtures containing a volumetric fraction of hydrogen ranging from 0 to 100 percentage were simulated at ambient pressure condition. The simulations are carried out for both isothermal and reactive flow for constant energy input to the combustor. The isothermal flow analyses were carried out with air as fluid. The reactive analysis is performed with mixtures of methane and hydrogen as fuel and oxidizer as air. The reactive flow analysis indicates significant change in flow structure compared to that of isothermal analysis. This change is due to release of energy as a result of combustion of fuel mixture and air. The combustion characteristics like static temperature and pollutant emission levels (CO NOX) are significantly increased as the hydrogen content is increased in the fuel mixture. This attributes to the shorter reaction zone in the vicinity of the combustor axis. Key words: Gas turbine combustion chamber, Hydrogen, CFD analysis, FLUENT. 1 Introduction In Gas turbine combustor the flow analysis involves momentum, mass and heat transfer and highly turbulent flow. The aim of present work is to study the physical and chemical processes occurring in a combustor by numerical simulation. Numerical solutions are obtained by solving continuity equations, momentum equations and equations pertaining to specific turbulence model considered in the analysis. The Geometric modeling and grid generation has done using GAMBIT and the CFD commercial code FLUENT [2] is used for both isothermal and reacting flow simulations. 1.1 Geometry of the Combustor A simple can type gas turbine combustion chamber has selected from the work of P. Koutmos and J. J. McGuirk [1]. It consists of hemispherical head attached to a cylindrical barrel, which terminated in a circular to rectangular contraction nozzle as shown in figure 1. The exit nozzle of circular to rectangular section has exit area 100mm X 25 mm at the outlet of the combustor. Figure 1: Geometry of the Combustor 1

2 1.2 Boundary Conditions The total inlet mass flow rate is obtained by keeping constant energy input to the combustor at a fixed actual air to fuel ratio 40. The simulation has been performed with 10percenge of swirl flow split. 2 Numerical results and Discussion 2.1 Comparison of Overall flow distribution The fig.2 shows the streamlines distribution for isothermal and reacting flow. It is observed that there are two recirculation zones formed in dome portion. One is near the dome wall i.e. Corner Recirculation zone (CRZ) and second is near axis i.e. Toriodal Recirculation Zone (TRZ). For isothermal flow the effect of the recirculation zones are wider compared to the reacting flow. In the reactive flow, these recirculation zones are responsible for mixing of fuel and air and subsequently the fuel mixture will get heated up during the recirculation journey from injector to the walls of the dome. Thus, the mixture will burn and releases the heat. This heat accelerates the flow in all directions and it expands along the combustor axis and finally moves towards the exit of the combustor. Figure 2: Overall flow distribution 2.2 Effect of hydrogen addition on methane combustion Static temperature contours: The fig.3 shows the temperature contours on vertical and horizontal planes different mixtures of fuel. While increasing the percentage of the hydrogen in the fuel, a considerable changing of the reaction zone could be noticed. The numerical results were showing that the reaction zone is more compact, shorter and temperatures were higher for pure hydrogen feeding. The values of the highest temperature while burning pure hydrogen were about K and while using pure methane about K. This is due to the higher Lower heating value (LHV), burning velocity and fuel jet penetrations for hydrogen than the pure methane[3]. It is also observed that the hottest temperature zone is at walls of the combustor, while burning the pure methane and it moves towards the axis of the combustor zone while increasing the percentage of the hydrogen in the fuel mixture. Pollutant Emission: The emissions of NOX and CO [4], measured at the exit of the combustor as shown in fig.3, while using different fuel mixtures. Main reason for the increased NOX emission is the poor mixing quality, particularly using pure hydrogen, where the fuel jet penetration is much greater than that of pure methane. When the hydrogen content 2

3 Figure 3: Temperature contours of reactive flow in the fuel mixture reached 75percentage, maximum emission levels of CO attained (551.3ppm) as shown in fig. At pure hydrogen content the CO fraction in the exhaust gases decreased to zero value. This is due to the decrease of carbon content in the fuel. The increased CO levels are probably related to a quenching of CO oxidation. The 3

4 quenching is due to both reduction zone (reduction residence time) and the reduction of total mass flow rate of air, both of them increase when the hydrogen percentage increases. In fact, a maximum in CO emission profile suggests that at this low thermal power (i.e MJ) the hot gases generated by the fuel combustion are quickly cooled downstream of the reaction zone by secondary air; the rapid cooling prevents the CO from being further oxidized to CO2. It is also observed that a small reduction of the CO levels for 50/50 hydrogen/methane mixture where jet penetration reaches maximum. Furthermore the OH and O radicals contributes to the pyrolysis of the carbon species. The higher OH radicals are likely to promote completion of CO oxidation to CO2 via the OH radicals. Figure 4: Pollutant Emission 3 Conclusions The isothermal flow simulation shows the overall physics involved in the combustor by studying the flow regimes. The reactive flow simulation shows that the flow filed variation is significant in the combustor due to the combustion. The main outcomes of the present work were as follows: The flow accelerates in all directions of the combustor due to the combustion. This accelerated flow compresses recirculation zone in primary zone and it creates the two additional recirculation zones. For isothermal flow the effect of the recirculation zones are wider compared to the reacting flow. The highest temperature is at walls of the combustor, while burning the pure methane. It moves towards the axis of the combustor, when increasing the percentage of the hydrogen in the fuel. Pollutant emission (NOX and CO) levels are minimum for pure methane. Those levels are increased as the addition of hydrogen to the methane/air mixture is increased. The increased in CO levels probably related to a quenching of CO oxidation due to the reduction in the size of the reaction zone (reduction of residence time) while the increase in the NOX level is due to the higher temperatures and shorter reaction zone, when hydrogen is added to the fuel mixture. 4

5 References [1] P. Koutmos and J. J. McGuirk, Investigation of swirler/dilution jet flow split on primary zone flow patterns in water model gas turbine combustor. ASME Journal of Engg. For Gas Turbines and Power, Vol. 111, pp , [2] H-J Tomczak, G Benelli, L Carrai and D Cecchini, Investigation of gas turbine combustion system fired with mixers of natural gas and pure hydrogen,. IFRF Combustion Journal Number , December [3] Geir J. Rortveit, Exprimental investigation on lean premixed fuel mixtures of CH4 and H2 in four different low-nox burners Combustion Journal, [4] Arthur H. Lefebvre, Gas turbine combustion