Numerical Study of Single Coal Particle Combustion in O2/n2 and O2/co2 Atmospheres

This paper reports a modelling work on the existing experimental data of single coal particles combustion process [1,2]. Two kinds of coal particles (a high-volatile bituminous coal / a lignite coal) are injected in a drop-tube furnace, ignited and burned in quiescent O2/N2 and O2/CO2 atmospheres with oxygen mole fraction varying from 20% to 100%. Ignition delay time, particle life time and particle surface temperature are calculated to analyse the combustion behaviour of the single coal particle. Transient simulations are carried out based on Discrete Phase Model (DPM). The predicted results are in good agreement with the experimental data and give the details of combustion process. Further, predictions of NOx emissions are made to understand the influence of different gas atmosphere and oxygen mole fraction. This includes both thermal-NOx and fuel-NOx formation mechanisms. It is argued that the NOx emissions in both O2/N2 and O2/CO2 atmospheres are the result of competitions between the maximum particle temperature and the burning time. INTRODUCTION Coal is widely used for energy production across the world and pulverised combustion is perhaps the most common technology for utilising coal energy. Despite serious emissions caused by coal combustion, it is still and, most probably, will continue to be amongst the main resources for power generation [3,4]. Coal combustion which involves devolatilization, char combustion and gas phase reactions is a multi-phase, multi-scale and multi-component process [2]. Types of coal and the operation condition of reactors are also the key factors in its combustion behaviour. Further, the complex chemical processes involved in coal combustion need to be explored. Due to the increasing concerns about global warming, carbon dioxide emission by coal combustion is now a serious issue. Different methods have been developed for carbon capture and storage (CCS) to reduce the CO2 emission from coal-fired power plants. Pre-combustion capture, postcombustion capture, oxy-fuel combustion and chemical looping are four main CCS technologies [5]. Among these, the oxy-fuel combustion is a near-zero emission technology and the most promising one for power station utilisation [5]. The replacement of air with O2/CO2 leads to modified distributions NOMENCLATURE K [kmol/ms] Reaction rate k Reaction rate constant c [kmol/m] Concentration A Pre-exponential factor E [J/kmol] Activation energy T [K] Temperature R Universal gas constant X Mole fraction a Oxygen reaction order b Temperature exponent d Reaction order e Reaction order Special characters α Distribution coefficient Subscripts s Solid x Composition of chemical element H y Composition of chemical element O R Reactant ox Oxidant of temperature and species, as well as radiation flux, resulting from the property differences between N2 and CO2. This also results in changes in NOx formation and reduction in the flame temperature [6]. Nitrogen oxides, including nitric oxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O) are mostly produced through the oxidation of the molecular nitrogen in air (thermal NOx) and the organically bound nitrogen in the coal (fuel NOx) [7]. NOx formed from molecular nitrogen and hydrocarbon fragments resulting from the devolatilization process near the reaction zone of flame (prompt NOx) accounts for less than 5% of the total NOx formed [8], and is usually neglected in modelling. In a combustion system, including pulverized combustion, the formation and destruction of NOx emissions are influenced by several factors such as fuel properties and, combustion conditions e.g. temperature of reaction and the fuel–air ratio [9]. Replacing the background gas N2 by CO2 can limit the NOx emissions formed at high temperatures from nitrogen of the combustion air, and then the formation of NOx is mainly due to the conversion of fuel-N, partially or totally into nitrogen oxides [10]. The effects of O2/N2 and O2/CO2 gas atmosphere on coal combustion and NOx emissions have been investigated by a number of authors. Experiments conducted by Rathnam et al. [11] showed that the devolatilization rate was similar in either N2 or CO2 at temperatures lower than 1000 K, while it was higher in CO2 than in N2 when temperatures exceeded 1000K. These results were confirmed by Al-Makhadmeh et al. [12]. On the other side, different studies show a minor or even a negative effect of CO2 atmosphere on coal reactivity [13,14]. Experimental studies on NOx emission characteristics in coalfired O2/CO2 combustion have been performed in the laboratory [15-18]. Okazaki and Ando [15] concluded that the influence of CO2 on NO emission is negligible despite its high concentration. Furthermore, an effect of the interaction of fuelN and recycled NO was detected. Moreover, reduction of NO to molecular N2 due to chemical reaction in the combustion zone is taken as the main reason for the overall decrease (5080%) in NO during recycling. Hu et al. [16] evaluated three types of coals for a wide range of stoichiometries with the recycling ratio ranging from 0 to 0.4 and concluded that the reduction ratio (RR) increased with the fuel equivalence ratio and recycling ratio. Recently, Dhungel et al. [17] examined NO emission behaviour during oxy coal combustion in a 20 kW electrically heated furnace and concluded that the pathways of NO reduction in oxy-fuel were fundamentally similar to those in air combustion. Mackrory et al. [18] investigated oxy-fuel combustion in a multi-fuel flow reactor (MFR) and found that oxy-fuel combustion can result in lower NOx emissions than air combustion independent of the reduction of recycled NOx, which was different from previous studies. Computational fluid dynamics (CFD), which can effectively integrate turbulent mixing, combustion chemistry, heat transfer and their interactions together has become a very powerful method to understand and simulate the combustion of coal. Kuffa et al. [19] successfully modelled coal gasification in a fluidized bed reactor. Oevermann et al. [20] established a model for simulating wood gasification in a fluidised bed reactor by using the Euler–Euler approach. Useful CFD analyses of oxy-fuel combustion have been successfully performed and reported in literature as well. CFD simulations of lab-scale oxy-coal combustion were carried out by Chui et al. [21] to assist with future pilot scale oxy-fuel combustion experiments and burner scale-up. Wang et al. [22] conducted a comprehensive CFD simulation of a propane-fueled, oxygenenriched, turbulent, non-premixed jet flame. Zhou and Moyeda [23] performed CFD simulations for process evaluation of oxyfuel combustion with flue gas recycle in a conventional utility boiler. However, fewer research focused on the whole combustion process of single coal particles, e.g. ignition characteristics, particle temperature, NOx emissions. Moreover, attention on coal combustion under varied gas atmosphere need to be paid. This paper aims at developing a combustion model considering the different gas–solid behaviours, heat transfer and thermal conversion processes for single coal combustion by using Discrete Phase Model. The work is also concerned with formation of NOx emissions when burning different kinds of coal under varying oxygen concentration with both N2 and CO2 as the background gas. FUEL PROPERTIES The proximate and ultimate analysis of the two kinds of coal is shown in Table 1 [1]. The diameter of particles is defined as 80 μm when carrying out numerical simulations as the size of 75-90 μm was used in the experiments [1]. Table 1 Coal Analysis Data Fuel PSOC-1451 Bituminous DECS-11 Lignite Proximate analysis ( received) Moisture (%) 2.5 13.2 Volatile (%) 33.6 48.6 Fixed Carbon (%) 50.6 29.8 Ash (%) 13.3 8.4 Ultimate analysis (dry basis) C 71.9 66.2 H 4.7 4.0 O 6.9 18.6 N 1.4 0.9 S 1.4 0.7 Ash 13.7 9.6 Heating value dry fuel (MJ/kg) 31.5 25.7 COMBUSTION MECHANISMS Coal Combustion Four chemical processes were considered during coal particles combustion: devolatilization, volatile (represented by CHxOy) combustion, char (represented by C(s)) oxidation and other gas phase reactions. The particles are heated up and consequently they release moisture (drying process) at first, and then releasing volatiles rapidly (devolatilization process). In this study, volatiles release is described by the single rate model. It assumes that the rate of devolatilization is first-order dependent on the amount of volatiles remaining in the particle and employs global kinetics. The reaction and its rate constant are: R1: coal → α volatile + (1-α) char kd = A exp(-E/RT) Char formed during devolatilization process is consumed by heterogeneous processes of combustion and gasification and its combustion yields carbon monoxide (CO) and carbon dioxide (CO2). Heterogeneous reactions can also include the char-H2O reaction. R2: C(s) + O2 → CO2 R3: C(s) + 0.5 O2 → CO R4: C(s) + CO2 → 2 CO R5: C(s) + H2O → H2 +CO The combustion rate of char is assumed to be limited by the chemical kinetics because the only reactive species that is included in the gas phase is O2. Then, the reaction rate K (kmol/ms) is defined as K = ka cc(s) co2 and ka is the reaction rate constant given by the Arrhenius type relation: ka = AT exp(-E/RT) In the gas phase reactions, as the detailed chemical species in the volatile are not completely understood due to the complexity of the chemical structure of coal, for simplicity in this study, it was generally treated as a single species which varies depending on the type of coal and comprising carbon, hydrogen and oxygen (CHxOy) in a ratio determined from the ultimate analysis of coal. For the two kinds of coal (PSOC1451, DECS-11) used in this study, the volatile gas species are represented as CH2.99O0.24 and CH1.48O0.43 separately. The homogeneous reaction related to the volatile is as followed: R6: CHxOy + m O2 → CO2 + n H2O Thus, equating the numbers of atoms of each element in the reactants to the number in the products gives: m = 1 + x/4 – y/2 n = x/2 And the carbon monoxide (CO) is oxidised to CO2 according to the following homogeneous reaction: R7: CO + 0.5 O2 → CO2 The reaction rates are given as K= AT exp(-E/RT) [cR][ cox] Six chemical reactions are considered totally and a summary of the kinetics data used in the present combustion modelling and the values of d and e are provided in Table 2. Table 2 Kinetic constants Reaction NO. Kinetic parameters d e Ref.