Yinka S. Sanusi

Experimental Study on the Effect of Hydrogen Enrichment of Methane on the Stability and Emission of Nonpremixed Swirl Stabilized Combustor

An ultra lean mixture (ϕ ≤ 0.5) of methane–hydrogen–air was experimentally investigated to explore the effect of fuel flexibility on the flame stability and emission of a nonpremixed swirl stabilized combustor. In order to isolate the effect of hydrogen addition to methane, experiments were carried out at fixed fuel energy input to the combustor while increasing the hydrogen content from 0% up to 50% in the methane–hydrogen mixture on volume basis. The combustor fuel energy was then increased up to the range of typical gas turbine combustors. Equivalence ratio sweep was carried out to determine the lean stability limit of the combustor. Results show that the hydrogen content in the fuel mixture and fuel energy input have a coupled effect on the combustor lean blow off velocity (LBV), temperature and emissions. The LBV increases by ∼103% with the addition of 30% H2. On the other hand, the LBV increases by ∼20% as the fuel energy increases from 1.83 MW/m3 to 2.75 MW/m3. Burning under ultra lean condition serves two purposes. (1) The excess air supplied reduces the overall combustor temperature with its ensuing effect on low NOx formation. (2) It increases the overall combustor volume flow rate which reduces the residence time for NOx formation. The axial temperature profile presented along with the emission data can serve as basis for the validation of numerical models. This would give more insight onto the effect of hydrogen on the turbulence level and how it would improve the localized extinction of methane in a cost-effective way.

Evaluation of the Accuracy of Selected Syngas Chemical Mechanisms

The implementation of reduced syngas combustion mechanisms in numerical combustion studies has become inevitable in order to reduce the computational cost without compromising the predictions’ accuracy. In this regard, the present study evaluates the predictive capabilities of selected detailed, reduced and global syngas chemical mechanisms by comparing the numerical results with experimental laminar flame speed values of lean premixed syngas flames. The comparisons are carried out at varying equivalence ratios, syngas compositions, operating pressures, and preheat temperatures to represent a range of operating conditions of modern fuel flexible combustion systems. NOX emissions predicted by the detailed mechanism, GRI-Mech. 3.0, are also used to study the accuracy of the selected mechanisms under these operating conditions. Moreover, the selected mechanisms’ accuracy in predicting the laminar flame thickness, species concentrations of the reactants, and OH profiles at different equivalence ratios and syngas compositions are investigated as well. The laminar flame speed is generally observed to increase with increasing equivalence ratio, hydrogen content in the syngas, and preheat temperature, while it is decreased with increasing operating pressure. This trend is followed by all mechanisms understudy. The global mechanisms of Watanabe-Otaka and Jones-Lindstedt for syngas are consistently observed to over-predict and under-predict the laminar flame speed up to an average of 60% and 80%, respectively. The reduced mechanism of Slavinskaya has an average error of less than 20% which is comparable to the average error of the GRI-Mech. 3.0. It however overpredicts the flame thickness by up to 30% when compared to GRI-Mech. 3.0. The NO prediction by Li mechanism and the reduced mechanisms are observed to be within 10% prediction range of the GRI-Mech. 3.0 at intermediate equivalence ratio (φ = 0.7) up to stoichiometry. Moving towards more lean conditions, there is significant difference between the GRI-Mech. 3.0 NO prediction and those of the reduced mechanisms due to relative importance of the prompt NOX at lower temperature compared to thermal NOX that is only accounted for by the GRI-Mech. 3.0.

Transition From a Single to a Double Flame Structure in Swirling Reacting Flows: Mechanism, Dynamics, and Effect of Thermal Boundary Conditions

We examine experimentally the transition from a single flame stabilized along the inner shear layer (ISL) to a double flame stabilized along both the inner and the outer shear layers (OSL) and spreading over the outside recirculation zone (ORZ) in a fully premixed swirl-stabilized combustor. This work is mainly driven by previous studies demonstrating the link between this transition in the flame macrostructure and the onset of thermo-acoustic instabilities [1, 2]. Here, we examine the transition mechanism under thermo-acoustically stable conditions as well as the dominant flow and flame dynamics associated with it. In addition, we explore the role of changing the thermal boundary conditions around the ORZ and its effect on the presence or absence of the flame there. We start by analyzing the two flames bounding the transition, namely the single conical flame stabilized along the ISL (flame III) and the double conical flames with reactions taking place in the ORZ (flame IV). A dual chemiluminescence approach — using two cameras with a narrow field of view focused on the ORZ — is undertaken to track the progression of the flame as it reaches the ORZ. During the transition, the flame front, initially stabilized along the ISL, is entrained by OSL vortices close to where the turbulent jet impinges on the wall, leading to the ignition of the reactants in the ORZ and the ultimately the stabilization of the flame along the outer shear layer (OSL). This ORZ flame is also subject to extinction when the equivalence ratio (ϕ) is between values corresponding to flames III and IV. For ϕ lower than the critical transitional value, the flame kernel originating from the ISL-stabilized flame is shown to reach the ORZ but fails to grow and quickly disappears. For ϕ higher than the critical value, the flame kernel expands as it is advected by the ORZ flow and ultimately ignites the reactants recirculating in the ORZ. Sudden and extreme peak-to-peak values of the overall heat release rate are found to be concomitant with the ignition and extinction of the ORZ reactants. Finally, Different thermal boundary conditions are tested by modifying the heat flux through the combustion chamber boundary, particularly around the ORZ. We find that the transition is affected in different ways: while the transition from flame III to IV (i.e. as ϕ increases) is insensitive to these changes; flame IV persists at lower ϕ as its value is reduced when heat losses through the boundaries are diminished.Copyright

PerformanceAnalysis of Integrated Solar Combined Cycle Power Plant for Dhahran, Saudi Arabia

Saudi Arabia is blessed with abundant solar energywhichcan be use to meet its ever increasing power requirement. In this regard, the energy analysis and plant performance of integrated solar combined cycle (ISCC) plant with direct steam generation (DSG) was carried out for Dhahran, Saudi Arabia using four representative months of March, June, September and December. The plant consists of 180MW conventional gas turbine plant and two steam turbines of 80MW and 60MW powered by the solar field and gas turbine exhaust. With high insolation during the summer month of June the plant can achieve up to 25% of solar fraction with ISCC plant efficiency of 45% as compared to gas turbine base of 38%.This can however be improved by increasing the number of collectors or/and the use of auxiliary heater .