Lean Premixed Combustion
As a gas turbine fuel, hydrogen is attractive because it produces no carbon emissions. This is particularly important in light of growing concerns about continued global warming. In addition to carbon, many fossil fuels produce emissions of sulfur, unburned hydrocarbons, and various other harmful compounds. None of these are associated with hydrogen combustion. However, hydrogen-air combustion may produce one significant type of pollutant also associated with fossil fuels: oxides of nitrogen. Oxides of nitrogen, or NOx, are a source of smog and acid rain. NOx is produced primarily through the Zeldovich chemical mechanism when hydrogen is burned in air. Zeldovich kinetics are highly dependent on temperature and residence time, meaning that more NOx formation will occur during high residence times at high flame temperatures than during low residence times at lower flame temperatures. In fact, NOx production is highly nonlinear with flame temperature and can be reduced several orders of magnitude by burning at a sufficiently low temperature (below 1700 K).
While several methods exist for lowering the temperatures of hydrogen-air flames, Electric Jet, LLC,’s technology centers on lean premixed combustion. In lean premixed combustion, the fuel and oxidizer are mixed at a lean equivalence ratio prior to combustion. The excess air in the reaction acts as a thermal sink, decreasing the temperature of the flame and potentially limiting NOx to ultra low (<1ppm) levels. Hydrogen lends itself well to lean premixed combustion due to its wide flammability limits. Hydrogen is flammable at equivalence ratios ranging from 0.1 to 6.8, meaning that it can produce a stable flame under a wide range of conditions. This is a necessity in gas turbine combustion, since conditions vary considerably over the engine’s operating range.
The design of the hardware associated with lean premixed hydrogen combustion can be challenging. Premixing devices must assure adequate mixing of the fuel and oxidizer while preventing flashback, the potentially damaging upstream propagation of the flame into the premixer. Strong consideration must also be given to the characteristics of the engine itself to assure that the design of the combustor does not adversely affect operability. Electric Jet, LLC, has developed several patented/patent pending technologies to overcome these challenges, as well as a flexible design methodology that allows the application of lean premixed hydrogen combustion to a variety of gas turbine engines.