Reference : Feasibility study of the diluted combustion in a domestic heating boiler
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Feasibility study of the diluted combustion in a domestic heating boiler
SEGGIO, Giovanni [ > > ]
Ngendakumana, Philippe mailto [Université de Liège - ULiège > Département d'aérospatiale et mécanique > Thermotechnique >]
PESENTI, Barbara [ > > ]
LYBAERT, Paul [ > > ]
4th European Combustion Meeting (ECM2009)
4th European Combustion Meeting (ECM2009)
du 14 avril 2009 au 17 avril 2009
[en] The diluted combustion (called also flameless oxidation) has been already applied in furnaces technology to get high process thermal efficiency with low NOx emissions. The principle of this type of combustion consists in providing a high level of dilution of the reactants with flue gases before combustion reaction occurs, to get a slower reaction in a much larger volume than in classical combustion. The resulting lower local heat release leads to a more homogeneous temperature field in the furnace, without peak values responsible of high thermal NOx formation. There are two requirements for working in diluted combustion: the dilution level of reactants by the flue gases has to be high enough and the temperature level in the combustion chamber has to be above a threshold (the auto-ignition temperature of the mixture).
The aim of this work is to assess the technical feasibility and highlight the specific problems of application of diluted combustion in a medium scale boiler. The main difficulty is due to the high geometrical confinement and heat losses of a typical boiler combustion chamber. That prevents from getting the minimum level of reactants dilution and temperature needed to reach diluted combustion regime. The idea is to use a natural gas jet-burner (to preheat the combustion chamber) along with a secondary gas injector in order to get a high entrainment of the flue gases by the reactant jets. In fact the air (through the jet-burner) and the gas (through the secondary gas injector) are injected separately in order to get a high dilution of the reactants and a mixture temperature above the auto-ignition threshold. Our test bench consists in a Viessmann hot water boiler whose nominal output power is 370 kW. The combustion chamber of this boiler is cylindrical (length = 1.41 m, diameter = 0.56 m) and is water-cooled.
A preliminary CFD study (Fluent ®) has first been performed to select on the market a jet burner and to determine the position of the air and gas injectors able to generate the requirements of diluted combustion. The influence of the operating conditions (firing rate and excess air) has also been studied numerically. The simulation results show the possibility to obtain a temperature field quite homogenous (typical of diluted combustion) but with a light increase of the CO level at the exit of the combustion chamber. These numerical results (obtained with a simple combustion model) have to be validated experimentally.
A first experimental study has been carried out in classical combustion on the boiler equipped with the selected jet-burner. The temperature field has been measured in the median plane of the combustion chamber for different excess air and firing rate. These measurements allowed us to verify that the preheating obtained with the jet-burner was important enough for getting a temperature level above the auto-ignition temperature near everywhere in the combustion chamber.
A second experimental study in diluted combustion on the combustion chamber equipped with the secondary gas injector will allow the validation the corresponding numerical results.
Région wallonne : Direction générale des Technologies, de la Recherche et de l'Energie - DGTRE
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