Coal Combustion model
Solved within our cooperation with company Vítkovice ÚAM.
Energy engineering is one of the most important branches of industry. Classic engineering is based on combustion of fossil fuels like black or brown coal, lignite natural gas etc. Research is focussed on reduction of CO and NOx emission and increase of combustion efficiency. Design of low emission burners and combustion method of NOx and CO emission like zonal combustion, OFA etc. can be realised with the aid of numerical CFD simulation. CFD including chemical reaction can be used to simulate fuel combustion as well as production of pollutants like CO, NOX SO2, SO3 etc. It is combination of Lagrange and Euler multiphase model. Lagrange particle model solves trajectories of the fuel particles and Euler model solves flow of gas mixture which represents combustion air and flue gas, respectively.
Fuel combustion process is divided into some steps. First water vapour and volatile matter is evaporated whereas the volatile matter is mixture of CH4, H2, CO and H2O. Evaporation of volatile matter is defined by multiphase reaction, because parent of chemical reaction is raw fuel and children are chare and volatile matter. Next the char is oxidised and product of this reaction is CO and CO2 whereas CO/CO2 ratio is defined by multiphase reaction. Other reactions specified in reaction scheme are defined by Arrhenius law. These reactions define process of combustion of volatile matter and CO. NOx and SOx model can be added to Basic fuel combustion model. Both models add to reaction schemes new radicals that are related to production of NOx and SOx pollutants.
The NOx model includes three sources of NOx: 1) Fuel , 2) Prompt , 3) Thermal .
Production of thermal NOx is strictly dependent on local temperature and it is defined in Zeldovich reaction scheme. Reaction rates have been defined and verified on the base of many research works. Since the chemical reactions are reversible the reaction rates have to be defined as forward and backward.
Prompt is produced only in area where hydrocarbons are combusted. Thus production of prompt NOx is dependent on local concentration of hydrocarbons, which produces HCN radical.
Production of fuel NOX is dependent on mass flow of Nitrogen which is chemically bonded in compound contained in fuel. Nitrogen radical in fuel can be NH3 or HCN eventually both can be used. Since the first step is to release nitrogen substances from fuel, the reaction have to be defined as multiphase reaction.
The SOx model is defined by chemical reaction scheme which has been proposed by Kramlich. Chemical reaction scheme includes 20 reversible reactions and 12 chemical compounds. This scheme has been reduced by reason of practical usability. Chemical radicals included in fuel are SO and H2S. These radicals are released from fuel, thus these process have to be defined by multiphase reactions. The radicals react with other gaseous compounds in accordance with Kramlich reaction scheme.