Intense interest in technologies facilitating the sequestration of CO2 has led to the concept of combining oxygen obtained from an air separation unit with recycled exhaust flue gas containing primarily CO2 to replace air as the oxidizer in coal fired power plants. Cycle analysis suggests this is the most economic process for producing a sequestration-ready stream of CO2 from coal combustion with the primary capital and operating cost coming from the air separation unit. While pursuing this technology in pilot scale studies, a substantial and somewhat unexpected reduction in NOx was observed. Additionally, oxygen assisted combustion, where oxygen was introduced at selected locations in the burner, has produced both NOx reductions and NOx increases. The objective of the work is to determine the cause of NOx reduction in oxy-fuel combustion and thereby understand how NOx reduction might be maximized. Axial measurements comparing gas species of NOx, CO, O2, and SO2 for air-fired and oxy-fired combustion have been obtained in an entrained-flow, pulverized coal reactor modified to run under staged oxy-fuel combustion conditions. This process is being modeled with a series of plug flow reactors using the GRI 30 mechanism combined with a Coal Percolation and Devolatilization (CPD) model.

Combustion measurements were obtained in a down-fired, premixed, pulverized-coal reactor with a nominal coal flow rate of 1 kg/hr or approximately 8.3 kWth. Fuel, air and methane are introduced into the reactor at the top through and inverted cone and then pass through a honeycomb geometry, water-cooled burner where a premixed methane flame is stabilized and begins the devolatilization process for the coal. The coal proceeds to drop through a 152.4 mm tube for a distance of 2m. The reactor is unheated except for that evolved from coal and methane combustion. Sample ports along the wall allowed access for species sampling approximately every 0.6m. Gas samples were evaluated using a Horiba, five gas analyzer which measures NOx using chemiluminescence, CO and CO2 using NDIR and O2 using a paramagnetic cell.

Data were obtained using three different oxidizer mixtures and two different stoichiometries for a total of six operating conditions using an Illinois #6, bituminous coal.

The first stoichiometry was not staged and overall lean (stoichiometric ratio of 1.05). The oxygen mass flow rate for air at this condition was held constant and air was replaced with a mixture of CO2 and O2 in order to simulate oxy-fuel combustion at the same stoichiometric ratio. Two oxy-fuel conditions were run, 25% and 30% O2 (by mass) in CO2. Thus, the amount of CO2 or diluent was changed not the fuel or oxygen flow rates. The intent of this experiment was to provide an environment which was always oxidizing in order to study NOx formation rates while temperature and oxygen concentrations were varied. The oxidizer mixtures of 25% O2 in CO2 and air produce similar adiabatic flame temperatures but molar concentrations are considerably higher in the oxy-fuel case (32 vs 21%). The 30% O2 in CO2 produced both an elevated temperature and higher molar concentrations (37% vs 21%) in the.

The second stoichiometry was staged using a fuel rich zone (S.R. = 0.76) followed by a fuel lean burnout zone (S.R. = 1.06). This staged combustion used the same oxidizer set as the non staged: air, 25% O2 in CO2, and 30% O2 in CO2. The staged combustion actual produced three zones which are common to low NOx burners. 1. A premixed zone where primary air (oxidizer) reacts with the fuel and is consumed. 2. A fuel rich zone where volitiles continue to be released from the coal under reducing conditions and oxygen availability is low. 3. A burnout zone where oxidizer is added to the fuel rich products completing burnout of the volatiles and char.

Modeling of the process was completed using a combination of a comprehensive mechanism for nitrogen (GRI 30) combined with a Coal Percolation Devolatilization (CPD) model. The two models are coupled in a Matlab program which uses Cantera function calls to integrate the set of ordinary differential equations produced by the kinetic mechanism. The CPD model produces light gases and nitrogen containing species as a source term, adding species between the plug flow reactors.

Under non-staged combustion, the 30% O2 oxidizer converts the highest fraction of fuel Nitrogen to NO. This is consistent with higher temperatures and higher oxygen concentrations both increasing the formation of NO. Both the 25%O2 and air oxidizers produced NO at the same rate consistent with the similar flame temperatures dominating NO formation rates, but the air case produced NO over a longer distance in the reactor and eventually reached the same nitrogen conversion rate as 30% O2 in air. All three oxidizer mixtures produced a gradual decline in NO prior to exiting the reactor which is not completely understood at this time.

Under staged conditions, the conversion rate of fuel N to NO was more rapid with the 30% O2 having the highest initial N-conversion (0.65), followed by 25%O2 (0.58), and air (0.49). This formation was followed by dramatic reductions to values below (0.1) for both oxy-fuel cases (25% and 30% O2). NOx reduction in air was significant dropping from 0.49 to 0.37. When burnout oxidizer was added, NOx increased to 0.15 and 0.18 in the oxy-fuel cases. The data suggest that the oxy-fuel cases are devolatilizing more rapidly and producing a richer environment in which NOx is reduced more significantly. It is also possible that the lower total mass flow rates of diluent (CO2) produce a longer residence time for reburning reactions in the oxy-fuel cases. The final fuel nitrogen conversion rate was highest in air and lowest for 30%O2 in CO2.

It can be concluded that staged oxy-fuel combustion can reduce overall NO formation rate by increasing the devolatilization rate and a subsequent increase in NO reduction in the fuel rich reduction zone. The higher reduction rates may be caused in part by the longer residence times created in the reducing zones by smaller amounts of diluent. The data show that reburning of recirculated NO is not required to produce NO reduction.