Low NOx emissions from fuel-bound nitrogen in gas turbine combustors

Biomass-derived LCV (Low Calorific Value) gas represents one of the best alternatives for fossil fuels. It is very attractive, because it is CO2 neutral as biomass consumes an amount of CO2 when growing and releases almost the same amount when combusted. However, the raw gasifier producer gas contains a high content of fuel-bound nitrogen (FBN), which results in high NOx emissions after its combustion. The NOx emissions compromise the neutral aspect of the biomass-derived LCV gas. Reducing the conversion of FBN to NOx has been one of the main challenges for researchers working in the field of LCV gas combustion. There are indeed three main ways to reduce the emission of NOx; upstream of the combustor by scrubbing, downstream the combustor by SNCR (Selected Non Catalytic Reduction), or SCR (Selected Catalytic Reduction), or inside the combustor system by optimizing the combustion process to result in the lowest conversion of FBN to NOx, also called "primary measures". For this research work the third approach was adopted, i.e. reducing the conversion of FBN to NOx by primary measures. A new combustor has been developed within the framework of this thesis, the newly designed combustor was named "Winnox-TUD". The Winnox-TUD combustor has been developed after a series of first experiments using an available Winnox combustor. The Winnox combustor was designed for a Rover micro-gas turbine. It is composed of two stages, with an inserted depletion plate between the first and second stage. The Winnox combustor was fueled with natural gas doped with ammonia to simulate the FBN in the gas. Those first experiments consisted of investigating the effect of stoichiometry in the diferent stages on the conversion of ammonia to NOx. Finding out the main factors controlling the conversion of FBN to NOx and op- timizing those factors to result in the lowest possible conversion of FBN is the main goal throughout this research work. To achieve the defined goal, the Winnox-TUD combustor was the subject of extensive experimental and modeling investigations. The combustor was tested experimentally to define the effect of stoichiometry, power, FBN concentration, gas composition, heating value and primary air temperature on the conversion of FBN to NOx. It was found that all those parameters affect the conversion of FBN to different extent, however, stoichiometry in the first stage, FBN content in the LCV gas in addition to natural gas (especially CH4) are the main factors controlling the conversion of FBN. Optimizing those factors can result in a very important reduction of FBN conversion to NOx thus a reduction in NOx emissions. In this thesis experiments are described, the modeling of the reacting flow field using CFD (Fluent) and the modeling of the chemical kinetics (Chemkin). The experiments were divided into two categories: experiments using natural gas diluted with nitrogen and experiments using LCV gas from a mixing station, where the main components of the real biomass-derived LCV gas were mixed to produce a synthetic LCV gas. Ammonia was added to the gas to simulate the presence of the fuel bound nitrogen in the fuel gas. Furthermore, one of the other goals set for this research work, was to define clearly the limits of such primary measures in reducing the conversion of FBN to NOx, and the reduction of the total NOx emitted from the combustion of biomass-derived LCV gas in gas turbines, gas engines or boilers.