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CHARACTERIZATION OF COAL COMBUSTION IN HOT AND DILUTED ENVIRONMENT USING A SURFACE STABILIZED GAS NATURAL FLAME Pedro N. Alvarado, Luis F. Cardona, Alexander Santamaria, Andres A. Amell, and Wilson Ruiz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03255 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017
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CHARACTERIZATION OF COAL COMBUSTION IN HOT AND DILUTED ENVIRONMENT USING A SURFACE STABILIZED GAS NATURAL FLAME Pedro N. Alvarado1,3, Luis F. Cardona1, Alexander Santamaria3*, Andres A. Amell2, Wilson. Ruiz3. 1
Advanced materials and energy group, Faculty of Engineering, Instituto Tecnológico Metropolitano, Campus Fraternidad, Calle 4ª No 30-1, Medellín, Colombia.
2
Grupo de Ciencia y Tecnología del Gas y Uso Eficiente y Racional de la Energía, Facultad de Ingeniería, Universidad de Antioquia (UdeA) Calle 70 No. 52-21, 1226 Medellín, Antioquia, Colombia 3
Química de Recursos Energéticos y Medio Ambiente, Instituto de Química, Facultad de
Ciencias Exactas y Naturales, Universidad de Antioquia (UdeA) Calle 70 No. 52-21, 1226 Medellín, Antioquia, Colombia
*Corresponding Author Telephone: + 57 4 2196654 E mail:
[email protected],
[email protected] ACS Paragon Plus Environment
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ABSTRACT. In this study a surface stabilized flat flame burner has been evaluated for burning pulverized coal samples. The aim of this research was to emulate operating conditions similar to those usually found in MILD combustion of coal, including high temperature and low oxygen environment. The experimental setup decouples chemical kinetics from complex processes as aerodynamics phenomena. Additionally, it does not require oxygen-nitrogen mixture preparation or electrical heaters. In a first stage, both simulations and experiments were done with natural gas, testing several lean equivalence ratios and unburned gas velocities in the burner to obtain a hot and diluted environment. Then, in a second stage the coal was suspended at a fixed distance from the burner surface in 4 and 8 % v/v oxygen levels. The obtained results show quantitative evolution profiles for CO, CO2, and O2 and NO, as well as, N and C releases during the combustion of coal emulating the MILD combustion conditions. This approach is an option for tracking combustion products and physicochemical changes of the solid fuel during reactions, such as those that take place in coal MILD combustion.
KEYWORDS. Coal MILD combustion, hot and diluted environment, Cantera, nitrogen release.
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INTRODUCTION. The development of advanced combustion systems, that allow reductions in pollutant emissions and increases in efficiency of energy production and use, is one of the main scientific and technological challenges in combustion science. It is continuingly motivated by uncertainties of the fossil fuel prices and promotion of policies of clean energy production with minimum environmental impact. A significant research in this area has been driven towards the development of burning systems that recover heat from combustion exhaust gases, that is then used for preheating of reactants or air involved in the process1. Originally called excess-enthalpy combustion, the preheating of reagents (fuel and oxidizer) by means of heat released from combustion products has important effects on reaction speed, flammability limits, flame temperature and thermal efficiency2. The amount of preheating of the fresh mixture, controls the increase of temperature of the process. This technique, initially developed for fuels with low-calorific value, was later described more generally as heat recirculation combustion3. However, most significant drawback of this technique is the increase in NOx emissions due to thermal mechanism proposed by Zeldovich, which is related to temperature rise4. In order to mitigate high NOx emissions, several techniques have been developed allowing high dilution of oxygen into air by means of combustion gas recirculation into main reaction zone
5–12
. Principles of heat recirculation and oxygen dilution in air by mean of combustion
products recirculation apply in industrial furnaces, as well as, in combustion engines, using techniques such as Exhaust Gas Recirculation (EGR). In furnaces, there are several terms that describe this technology; some are focused in the furnace design, such as Fuel Direct Injection (FDI)13, or Low NOx injection (LNI)14, while others are focused in the process description
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and/or characteristics, such as Flameless Oxidation (FLOX®)5, High Temperature Air Combustion (HiTAC)15, highly preheated lean combustion16, flameless combustion17,18 and Moderate or Intense Low Oxygen Dilution (MILD) combustion19. In MILD combustion, the temperature of the reagents injection is higher than the mixture selfignition. At the same time, “the maximum allowable temperature increase with respect to inlet temperature during combustion is lower that mixture self-ignition temperature”
19
. There are
complex interactions among fluid dynamic, chemistry, mass and heat transport phenomena in this regime, being highly sensible to changes in temperature, gas composition, turbulence intensity and dilution, making it difficult to perform fundamental studies in the reaction zone, even more for fuels with complex thermochemical properties like coal. Therefore, study of advanced combustion systems in burners allowing control of such parameters, while keeping phenomena of interest, can provide valuable information about combustion technology. The effect of hot gases recirculation in the combustion characteristics has been studied in burners that allow the decoupling of chemical kinetics and complex processes typical of the MILD regime. For example, jet in hot coflow (JFC) burner20–22 consists of a central fuel jet surrounded by an annular hot combustion products stream that comes from a secondary premixed flame. Oxygen dilution level and combustion products temperature in coflow can be varied independently, allowing the emulation of MILD combustion conditions. In this experimental system, Medwell and Dally21 found that reaction zone structure is independent when gaseous fuel is changed, suggesting that an great variety of fuels can be used for obtaining MILD combustion regime. Using a similar configuration Arrieta and Amell23 performed experiments burning methane in a hot and diluted environment. Axial temperature profiles above
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burner surface were very homogeneous and luminosity was very low. These results are typical of MILD combustion systems. Experiments focused on coal particle burning characteristics have also been performed; many of them allowing conditions usually found at MILD combustion regime. In a work by Ponzio and coworkers, coal pellets made from coal powder and binder were burned in a small-scale batch reactor24. Pellets had 15 mm diameter and 35 mm height. A premixed propane-air flame was used to preheat a ceramic porous honeycomb within the reactor, in a way such, once desired temperature was reached, only oxidizer was supplied to coal sample. Oxygen and nitrogen composition were manually adjusted to different dilution levels and then they were preheated when passed through the honeycomb. The ignition behavior reported was classified into sparking, flaming and glowing-surface mechanisms, depending on oxidizer temperature and oxygen concentration. Another study by the same authors25 was focused on the nitrogen release during the combustion of coal pellets at several temperatures and oxygen concentrations. Results obtained show that an increase in oxygen concentration, from 5% to 21% at 1273K, raises the fuel nitrogen release, mainly during the devolatilisation stage. Ponzio and coworkers25 are among the few researchers that have reported the nitrogen release dependence on diluted oxygen concentrations, during coal combustion from a fundamental point of view. On the other hand, many studies have investigated the factors influencing NO emissions and formation mechanisms during coal MILD combustion26–31. For example, Weber and coworkers32 obtained very low NOx emissions, in the range of 160–175 ppm (at 3% O2) indicating the high NO reduction potential of the technology for pulverized coal. Several authors, using numerical approach33–35, reveal that the fuel NO dominates the NO emissions under coal MILD combustion, while the contributions from the prompt NO and thermal NO accounts for a little
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percent. Results of those simulations highlighted the NOx reduction potential of the technology. The volatiles, that are leaving the particles, react in a high sub-stoichiometric environment; therefore, the N2 formation is favored trough reduction reactions of the N species rather than the NO formation. In addition, there is a strong NO-reburning mechanism that enhances the NO reduction when the fuel and the air merge. However, there are still few studies that correlate NOx formation with the release of nitrogen from the fuel from an experimental point of view. This is due to the difficulties inherent in coal particle sampling, to obtain significant sample quantities at established burnout levels. Therefore, adequate experimental methodologies that allow fundamental studies of the coal solid particles, when they burn at conditions similar to the MILD regime, are needed. This work is an extension on our published work on reactivity and structural changes of pulverized coal particles in a hot and diluted environment36. In this paper, we show details of the design and operation of a surface stabilized flame which can be modulated to a wide range of conditions to investigate burning of pulverized coal and emissions. The experimental setup does not require oxygen-nitrogen mixture preparation or electrical heaters, as it is the case in other studies. Evolution profiles of major species such as O2, CO and CO2 during the combustion of coal in a hot and diluted environment conditions were measured and reported. The main contributions of this work are (1) to corroborate that proposed burner and experimental setup can provide adequate conditions to emulate coal MILD combustion; and (2) to further study the nitrogen release during all the stages of coal combustion.
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METHODOLOGY Experimental setup. Experiments were carried out in a natural gas premixed burner for lean equivalence ratios from 0.6 to 1 at intervals of 0.05. Flame was stabilized on the surface of a cylindrical honeycomb porous ceramic material, of 16 cm height and 8,4 cm diameter, Figure 1. Concentric to honeycomb, there were two concentric stainless steel tubes of 10 cm and 16 cm diameter. In the inner annular section, water was pumped to avoid excessive heating of porous material and dangerous flashback. In the outer annular section, Argon stream was injected in coflow to avoid air diffusion into reaction zone. Additionally, burner was enclosed by mounting a glass tube of 16 cm diameter and 35 cm height, for isolation of flue gases from surrounding air. Furthermore, this cylindrical glass had 10 cm height and 1,5 cm width window slit, next to burner surface, for gas sample probe insertion. This slit was also useful for rapid insertion of a Rtype thermocouple for measuring temperature at probe location.
Figure 1. Schematic diagram of the experimental setup
The burner has a cylindrical mixing zone of 20 cm height and 8.4 cm diameter. Fuel was supplied through a single injection in the lower part of the mixing chamber, while air was injected through four tangential inlets. This kind of arrangement promotes air-fuel mixing before entering ceramic porous material where the flow becomes laminar. Flue gas emissions released during combustion were measured using a water-refrigerated sampling probe and on-line gas analyzers. The probe consists of two concentric stainless steel tubes: sampled gases were suctioned through the inner tube while refrigeration water flows through the outer annular space. The aim of water circulation was to cool hot gases within inner tube and therefore to stop chemical oxidation reactions from CO to CO2. The O2, CO and CO2
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concentrations were measured with a SICK MAIHAK S710 gas analyzer, with accuracies of 3%, 6% and 3% of the measured value respectively. The NO concentration was measured with a Thermo Model 42iHL using a chemiluminiscence technique, with accuracy of ± 5ppmv. A Colombian coal extracted from Amaga, Antioquia region, was selected for this study36,37. We have chosen this coal due to it is widely used in electricity generation and local industries such as textile manufacturing or cement factories, in Antioquia region. In this work, coal sample proximate analysis was determined using the standard method ASTM D7582-15 while ultimate analysis was determined using the standard method ASTM D3176-15. The oxygen content was calculated by difference and gross calorific value was determined using the standard method ASTM D5865. This information is presented in Table 1.
Table 1. Proximate and ultimate analyses of the coal
After optimization of flame conditions, the coal samples introduced into the flame react with combustion products of natural gas at high temperature and low oxygen concentration (See Table 2). In each case, 300 mg of pulverized coal were suspended in a holder made of stainless steel mesh (Tyler series reference 200) at heights of 2.5 cm and 3.0 cm above the surface of the burner. The coal samples, with particle diameters between 75 and 150 µm, react with natural gas combustion products at high temperature and low oxygen concentration. The selection of particle size was based on previous combustion experiments at low oxygen concentration in TGA. The experiments indicated that particle sizes
