U. Greul

Basic effects on NOx emissions in air staging and reburning at a bench-scale test facility

Investigations on air staging and fuel staging were carried out with an electrically heated tube reactor. The effects of stoichiometry and residence time in the fuel-rich zone and the effects of temperature were evaluated for air staging with different coals and, in the case of reburning, for different gaseous reburn fuels. The results show the strong influence of residence time and stoichiometry, which may not be considered independently. The effects of temperature depend on the stoichiometric parameters. In air-deficient conditions, given by air staging and fuel staging, the NOx emission decreased with increasing temperature, whereas in oxygen-rich conditions the opposite trend occurred. In pulverized bituminous coal combustion, reburning is superior to air staging. While minimum NOx emissions of textasciitilde250 mg ms-3 could be achieved with air staging, reburning brought about values textasciitilde200 mg ms-3. For brown coal combustion, the minimum emissions far below 200 mg ms-3 with air staging could not be achieved with reburning.

Distribution of fuel nitrogen in pyrolysis products used for reburning

This report describes the nitrogen distribution in coal pyrolysis at different temperatures and oxygen partial pressures to allow discussion of the influence of different gas components on the DeNOx capability of the pyrolysis gases and tar species. Test runs with nitrogen-free reburn fuels and with coal pyrolysis gas showed that the nitrogen concentration, especially in the tar components of the pyrolysis gas, appears to have a positive effect on NOx reduction in the reburn zone of the combustion reactor.

Pyrolysis Gas of Biomass and Coal as a NOx-Reductive in a Coal Fired Test Facility

To lower NO x -emissions in pulverized coal combustion, pyrolysis gas from biomass and hard coal were used as reburn fuel. The results have been compared with synthetic reburn fuel mixtures. With pyrolysis gas as reburn fuel, minimum NO x -emissions of 200 mg/m n 3 (approx. 100 ppm) at 6% O 2 in the flue gas are possible. The main parameters are pyrolysis gas composition, stoichiometry and residence time in the reduction zone. Best minimizing results have been achieved with pyrolysis gas produced at about 800°C-900°C reactor temperature using coal as raw material. However, the pyrolysis temperature showed no distinct influence on NO x -reduction using biomass as feedstock. The nitrogen concentration, especially in the tar components of the pyrolysis gas, appears to have a positive effect on NO x -reduction in the reburn zone of the combustion reactor.

Low-NOx Combustion for Pulverized Coal - A Comparison of Air Staging and Reburning

The investigations on air staging and fuel staging were carried out at a 0.5 MW pulverized coal combustion chamber and an electrically heated tube reactor. In air-deficient conditions, given by air staging and fuel staging, the NOx emissions decreased with inreasing temperature. In pulverized bituminous coal combustion, reburning is superior to air staging. Whereas minimum NOx emissions of about 250 mg/Nm3 could be achieved with air staging, reburning brought about values around 200 mg/Nm3. The results obtained at the test facilities show the same tendency of dependence on the impact parameters and the same level of NOx emissions as the one attained at industrial-scale utilities.

Pyrolysis Gas from Biomass and Pulverized Biomass as Reburn Fuels in Staged Coal Combustion

The possibility of a combined application of hard coal and biomass using two different co-combustion technologies (combined combustion of pulverized fuels/pre-pyrolysis of biomass and use as a reburn fuel) was investigated in a 0.5 MWth test rig and in a 50 kWth small scale test facility. Reburn investigations with three pulverized biomasses in the 0.5 MWth facility resulted in NOx-emissions of approx. 300 mg/m3 (at 6% O2 in the flue gas). Besides the high DeNOx-efficiency, the test runs showed that the feedstock burnout attained approx. 99% (depending on the biomass particle size) and that no problems arose caused by CO-emissions.