Formation of Nitrogen Oxides in Combustion of Pyrolysis Gases in Air Heavily Diluted with Recirculating Flue Gas

Abstract

Formation of nitrogen oxides in the combustion of pyrolysis gases from oil shale thermal processing (such as semicoke and generator ones) in recirculation flue gas containing air was investigated. The parameters that can considerably affect the formation of nitrogen oxides, such as lower and upper flammability limits, the minimum content of oxygen in the air-flue gas mixture at which ignition of a combustible gas can occur, fundamental burning velocity were determined, and the adiabatic flame temperature under the stoichiometry conditions were calculated. A design of a burner intended for reliable operation when the combustion air has a high content of recirculation flue gases is examined. For this purpose, 5–10% of combustible gas is burned in pure air in the central part of the burner, while the remaining gas is burned in the peripheral part of the burner where recirculation flue gases are fed together with combustion air. The results of full-scale experiments carried out in a type TP-101 boiler at the Estonian thermal power station and a Е-135-3.2-440DG VKG Energia OU boiler (in Kohtla-Jarve, Estonia) are presented. The effect of flue gas recirculation on the formation of nitrogen oxides during burning of semicoke gas only or a mixture of semicoke and generator gases is demonstrated. Data on the formation of nitrogen oxides in burning semicoke gas with a high excess air are presented. The effects of additional ballast matter, such as inert components of flue gases or air excess, which can be considered as an inert matter in calculation of the combustion product temperature, on nitrogen oxides formation are compared. The fact that the processes running in the flame front and the flame front temperature proper can affect nitrogen oxide formation is noted. The main result of the investigation is that quite low NOx concentrations (as low as 40–50 mg/Nm3) can be achieved by using flue gas recirculating technique only, without staged supply of air or fuel.