Development of an emission-free gas turbine utilizing micromixer-combustion technology

Combustion of fossil fuels emits one of the primary greenhouse gases, namely carbon dioxide (CO2). While the use of renewable energies (e.g., wind, solar, geothermal, etc.) reduces such emissions, most of those energies are not yet economically feasible, especially in developing countries. Thus, the world is still forced to use fossil fuels at the current stage; strict policies have, however, been enforced to govern greenhouse-gas emissions. Fossil fuels are expected to remain the primary source of energy in Saudi Arabia for a long time. On the utility scale in particular, the Kingdom is the largest world user of gas turbines (GTs) for power generation. All these GTs utilize the diffusion-combustion technology with air as oxidizer. High-temperature zones exist within the diffusion flame resulting in excessive NOx emissions. Lean-Premixed (LPM) combustion technology, on the other hand, is based on full premixing of fuel and oxidizer in fuel-lean proportions before admitting the mixture into the flame. This eliminates the creation of high-temperature zones within the flame, which significantly reduces NOx emissions. However, unlike diffusion flames, premixed ones require an efficient means of flames stabilization, as the combustor is operated close to its lean blowout limit. Micromixer stabilization is a successful emerging LPM combustion technology that is already implemented on the utility scale (e.g., General Electrics F-class gas turbines), and it has a technology-readiness level of 9. The use of air as oxidizer is, however, still the key source of NOx generation whether in diffusion or premixed flames. Moreover, unreacted nitrogen from air forms the primary constituent of GT exhaust, which reduces CO2 concentration, complicates the CO2 separation process, and increases the associated costs of CO2 capture. This study, thus, proposes to experimentally and numerically examine the conversion of a micromixer-based GT to implement oxy-combustion (pure oxygen) instead of air-combustion. In the absence of air-based nitrogen, NOx emissions will inherently be eliminated. Moreover, GT exhaust will consist mainly of CO2 and H2O, so once the latter is condensed out, the former can readily be captured at the lowest cost. To achieve this objective, two different approaches will be taken considering a gas-turbine combustor fueled by natural gas. In the first approach, LPM air-combustion will be investigated. The operability window between blowout and GT-safe maximum flame temperature will be baselined with air, and NOx emissions will be quantified. Oxygen-enriched air will then be examined, because oxygen enrichment increases flame temperature and boosts carbon oxidation rates, which is expected to broaden the operability window by allowing the combustor to run leaner prior to blowout. NOx emissions are expected to decrease with oxygen enrichment as a result of the declining N2 concentration. In the second approach, the benefits of oxy-combustion will be combined with exhaust-gas recirculation (EGR) by diluting oxygen with CO2, in order to control flame temperature and ensure safe GT operation. Oxy-CO2 performance will be compared to that of oxygen-enriched air to highlight the effects of CO2 vs. N2 on flame stability and structure. GT performance and emissions will be evaluated at different oxygen fractions targeting proper application in the energy sector.