This paper investigates emissions of NO xfrom pulverized coal burning in O 2/CO 2en- vironments. Such environments are pertinent to oxy-coal combustion, a promising “clean-coal” technology. The replacement of the inert nitrogen gas in air with carbon dioxide, which has differ- ent physical properties, alters the combustion conditions in the furnace. Hence, the purpose of this work is to theoretically examine the effects of (a) the oxygen concentration in the O 2/CO 2gases, and (b) the resulting combustion temperatures, on the evolution of NO x. To achieve these goals a previously published kinetic model was used, which assumes that fuel-bound nitrogen is released along with the tars during coal devolatilization and converts mostly to hydrogen cyanide. A sizable fraction of hydrogen cyanide is then converted to NO. Flame simulations were performed using Can- tera to investigate the relative impacts of temperature and oxygen mole fraction, and to understand the causes of the observed trends. Keywords: Coal, NOx, Fuel-Nitrogen, Hydrogen Cyanide, Oxy-Combustion. 1. Introduction Combustion-generated pollutants, such as acid gases, particulates, and air toxics, as well as green- house gases such as carbon dioxide, are a serious concern. Technologies to capture the pollutants, with various degrees of efﬁciency, have been developed and implemented [1]. One promising approach is carbon capture and sequestration (CCS) or carbon capture and utilization (eg. for un- derground oil recovery)[2]. Oxy-coal combustion, where coal combustion occurs in neat oxygen provided by an air separation plant, can greatly facilitate CCS as, upon drying, ﬂue gases consist mainly of carbon dioxide. To reduce temperatures, ﬂue gases are recycled, so the oxidizer stream is essentially a mixture of O 2and CO 2. In addition to facilitating carbon dioxide capture, oxy-coal combustion can drastically reduce the generation of nitrogen oxides, NO x, which consist of mostly NO and little NO 2[3]. Since com- bustion takes place in the absence of atmospheric N 2no thermal NO xis generated; the detected amount of NO xoriginates from fuel nitrogen. It has been shown that increasing the concentration of O 2increases the NO xgeneration during coal oxy-combustion, but it remains unclear whether this is driven by the increased temperature, increased oxygen concentration, or both. This study aims to separate the effects of temperature and oxygen partial pressure. This separation was in- vestigated by ﬁrst assuming that fuel-bound nitrogen is released along with the tars during coal devolatilization and converts mostly to hydrogen cyanide, HCN. Then to assess the conversion 1 Sub Topic: Coal, Biomass, and New Devices of HCN to NO a published kinetic model for HCN oxidation was used. This model was devel- oped by Dagaut and Glarborg [4] and enhanced with reactions pertinent to CO 2by Glarborg and co-workers; see Alzueta et al.[5] and Hashemi et al. [6]. Hashemi et al.[6] have worked on developing the nitrogen chemistry of coal oxy-combustion along with a model for different reaction mechanisms. According to the model, differences in the NO yield between the oxy-fuel combustion and the conventional combustion of pulverized coal can mostly be attributed to the recycling of NO (re-burning effect) and to changes in the mixing patterns between fuel and oxygen. Alzueta et al.[5] formulated the model for calculating NO x concentration by combustion of HCN as intermediate. It was also found that the higher availability of oxygen increases the HCN conversion, due to the increased radical formation. The kinetic and stoichiometric data for modeling is collected from various research papers and theses on nitrogen chemistry and mechanisms in coal oxy-combustion [7–11]. 2. Methods 2.1 Kinetic Model Hashemi et al. [6] published a detailed kinetic model in CHEMKIN format but the transport data of species which is required to perform ﬂame simulations was not made available in their published model. Transport data was found by comparing the species names using RMG-models [12] database and the transport data for pseudo species in Hashemi model were found in Ergut et al [13]. 2.2 Thermogravimetric analysis of coal The volatilization temperature of an HV A bituminous black coal (Pittsburgh No.8 from Pennsyl- vania (PSOC-1451)) was determined by thermogravimetric analysis(TGA), using TA instruments’ Q500 Series thermogravimetric analyzer. A sample of bituminous coal was loaded on a platinum sample pan which was further loaded on a sensitive thermobalance in the analyzer. The volatiliza- tion temperature was determined by ramping the temperature to 900C at the rate of 20C=min while recording the weight change. The weight change data with increasing temperature was ana- lyzed using TA Universal Analysis software. 2.3 Counter current diffusion ﬂame simulations Upon injection of a bituminous coal particle in a furnace, it heats up and devolatilizes. Upon ignition of the volatiles, an envelope diffusion ﬂame appears surrounding the particle, as de- picted in Fig. 1. This ﬂame was simulated herein as a one-dimensional counter-current diffusion ﬂame. Simulations were carried out using the open-source software Cantera[14] via the Python interface. The solutions are converged on approximately 120 grid points with convergence cri- teria set_refine_criteria( ratio=3.0,slope=0.1,curve=0.1,prune=0.04) , without ra- diation, and a domain width of 0.02 m. The simulations are carried out for bituminous coal by varying oxidizer inlet temperature and oxidizer composition. The inlet fuel composition is as- sumed to be composed of the coal volatile matter composition determined by Hashemi et al. Table 1 shows the molar composition of bituminous coal. 2 Sub Topic: Coal, Biomass, and New Devices Figure 1: Schematic of simulated counter-current diffusion ﬂame Species SOOT H 2 CO2CO CH 4C2H4H2O HCN Molar composition 28.36 38.52 3.26 8.53 3.76 1.76 14.56 1.25 Table 1: Estimated molar composition of volatiles for bituminous coal 3. Results 3.1 Thermogravimetric analysis 431.89 452.51 Figure 2: Weight change proﬁles for bituminous coal As shown in Figure 2, during the thermogravimetric analysis, the weight of coal decreases with temperature. Initially the weight drops due to evaporation of water in both coals. The weight of bituminous coal sample starts to decrease sharply between 430C and 450C. Such behavior can be attributed to volatilization of gases from this coal. Hence we assumed the devolatilization temperature of the bituminous coal to be 442C. This determines the fuel feed temperature in the ﬂame simulations. 3 Sub Topic: Coal, Biomass, and New Devices 3.2 Effect of inlet oxidizer temperature on NO concentration in ﬂame Counter-current diffusion ﬂame simulations were carried out by changing the oxidizer inlet tem- perature, which is the furnace gas temperature, but keeping the oxidizer composition constant. This was done to quantify the effect of furnace gas temperature on NO generation and particle ﬂame temperature. The oxidizer composition was kept constant at O2:0.20,CO 2:0.80 while the inlet temperatures of the oxidizer gas were set at [1300, 1400, 1500] in K. Figure 3.a, shows the effect of oxidizer inlet temperature on the NO concentration in the diffusion ﬂame. Figure 3.b shows the effect of the oxidizer inlet temperature on the ﬂame temperature across the width of ﬂame and the peak temperatures in the ﬂame are reported. 0.000 0.005 0.010 0.015 0.020 Width (m)0.00000.00010.00020.00030.0004NO Concentration 1300K 1400K 1500K (a) NO concentration proﬁle 0.000 0.005 0.010 0.015 0.020 Width (m)8001000120014001600180020002200Temperature(K) 2037K2089K2141K1300K 1400K 1500K(b) Flame temperature proﬁle Figure 3: NO species and ﬂame temperature proﬁles by varying oxidizer inlet temperature 3.3 Effect of oxidizer inlet composition on NO concentration in ﬂame Counter-current diffusion ﬂame simulations were carried out by changing the oxidizer inlet com- position but keeping the oxidizer inlet temperature, which is the furnace gas temperature, constant to quantify the effect of oxidizer composition on NO generation and ﬂame temperature. The ox- idizer inlet temperature is kept constant at 1400 K while the O 2and CO 2concentrations at inlet were set at [0.20,0.30,0.40] ,[0.80,0.70,0.60] respectively. Figure 4.a shows the effect of the oxidizer inlet temperature on the NO concentration in the ﬂame. Figures 4.b shows the effect of the oxidizer inlet composition along the width of the ﬂame and the peak temperatures in the ﬂame are reported. The difference between the peak temperatures in the ﬂames is considerable when the oxidizer composition is changed as shown in 4.b . 4 Sub Topic: Coal, Biomass, and New Devices 0.000 0.005 0.010 0.015 0.020 Width (m)0.00000.00010.00020.00030.00040.0005NO Concentration 20% O2 30% O2 40% O2 (a) NO concentration proﬁle 0.000 0.005 0.010 0.015 0.020 Width (m)7501000125015001750200022502500Temperature(K) 2089K2406K2630K20% O2 30% O2 40% O2 (b) Flame temperature proﬁle Figure 4: NO species and ﬂame temperature proﬁles by varying oxidizer inlet composition A comparison of the results shown in Figures 3 and 4 shows that the change in oxygen mole fraction in the range of [0.20-0.40] is more inﬂuential on both the ﬂame temperature and on the NO formation than the gas temperature variation in the range of 1300-1500 K. 4. Conclusions From the theoretical investigation of the NOx concentrations in a diffusion ﬂame and of the temper- ature proﬁle of this ﬂame, it can be concluded that the oxidizer inlet temperature has a signiﬁcant effect on the peak NO concentration during oxy-combustion of coal at a ﬁxed oxidizer inlet com- position as shown in ﬁgure 3, whereas the oxidizer inlet composition has a more signiﬁcant effect on ﬂame temperatures as shown in 4. These conclusions are valid for the ranges of the two pa- rameters (gas temperature and oxygen concentration) input to this model. Further theoretical and experimental work is needed to determine what type of reactions and at what temperatures are producing more NOx and how to minimize the production on NO while also retaining combustion efﬁciency. Acknowledgments Support is gratefully acknowledged from the Departments of Chemical Engineering and of Me- chanical and Industrial Engineering at Northeastern University.