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H2O modified atmosphere – Experimental and numerical study
Energy xxx (2015) 1e7
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Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Experimental and numerical study ˛ tkowski*, Ewa Marek Bartosz Swia wka 36, 02-981 Warsaw, Poland Department of Thermal Processes, Institute of Power Engineering, Augusto
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2014 Received in revised form 15 June 2015 Accepted 17 June 2015 Available online xxx
The article presents the results of a comprehensive research on the impact of O2/CO2/H2O atmosphere and coal milling quality on pulverized coal combustion process. In particular the content of combustibles in fly ash and heat transfer parameters were investigated. Experiments were conducted in two research stands: in a laboratory rig for single particle combustion investigations and in a semi-industrial 0.5 MW combustion chamber for pulverized fuel burner studies. It was shown that because of faster coal conversion in the oxy-fuel atmosphere it is possible to use lower quality coal grinding, which will save energy required for milling. Experimental results confirmed also the beneficial effect of the coarse coal milling that resulted in a decrease of necessary flue gas recirculation rate, which also means lowering the energy demand of flue gas fans. The impact of oxycombustion optimization on net efficiency of electric energy production was presented on the basis of 220 MW unit with OP-650 steam boiler. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Oxy-fuel combustion Coal combustion Numerical modelling Combustion optimization Efficiency improvement
1. Introduction Oxy-fuel combustion is a competitive alternative to other forms of CO2 capture. There are two important advantages to support this technology: the ability to adapt it also within existing boilers as well as cost-effectiveness in comparison to IGCC (Integrated Gasification Combined Cycle) and postcombustion, which was demonstrated in various economic analysis [1]. Nevertheless, oxyfuel combustion, as any CO2-capture method interrelates with a decrease in the efficiency of electricity production due to energy consumption of O2 production and CO2 liquefaction. Therefore, a lot of effort is devoted to minimize the loss in efficiency, which in the first concepts of oxy-fuel combustion installations was up to 12%. A technology breakthrough will be to reduce that loss by half. New concepts allow lower efficiency penalty through an extensive thermal integration of new equipment into existing power plant setups [2]. This usually complicates the installation and requires a use of large area heat exchangers which in turn increases CAPEX (capital expenditures). Although it is not the only method; oxy-fuel combustion itself may allow further optimization and additional efficiency loss reduction, practically without any significant investments. Combustion carried out at high O2, CO2 and H2O
* Corresponding author. Tel.: þ48 22 3451 441. ˛ tkowski). E-mail address: [email protected] (B. Swia
concentrations enables faster conversion of coal than the processes carried out in air. This is due to a higher combustion rate in mixtures with increased O2 concentration and because of further coal conversion with the participation of gasification reactions between fuel and CO2 and H2O. Higher concentration of O2 in the oxidant (usually about 27% O2) is needed in case of oxy-fuel combustion mainly to provide similar to air combustion flame heat parameters [3e5]. To obtain such concentration level oxygen must be first diluted by a stream of recycled flue gas corresponding to the RR (recirculation ratio) in the range of 70e76%. Under such conditions, the heat flux transferred to the combustion chamber will be comparable to the flux obtained during air combustion. Quicker coal conversion in oxy-fuel combustion can also result from the gasification reactions involving char and CO2 or H2O present in the combustion chamber [6,7]. The role of these reactions takes on importance in large scale boilers with typical oxidiser staging (usually applied in order to minimize NOx emissions) [8]. Within this kind of setup, an average coal particle - for about 90% of its residence time in a combustion chamber - is surrounded by a gaseous atmosphere with low (<10%) oxygen content (Fig. 1). At the same time both high temperature in the furnace and high concentrations of CO2 and H2O intensify the gasification reactions. Faster coal conversion in the atmosphere of O2/CO2/H2O may help optimize the combustion process by reducing the energy requirement for coal grinding. Low NOx combustion in
http://dx.doi.org/10.1016/j.energy.2015.06.064 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
B. Swia˛ tkowski, E. Marek / Energy xxx (2015) 1e7
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Nomenclature ASU CPU CAPEX CFD IGCC
air separation unite CO2 processing unite capital expenditure computational fluid dynamic Integrated Gasification Combined Cycle l oxidiserefuel equivalence ratio OFA overfire air PSI-CELL Particle-Source-In Cell RR recirculated flue gas ratio SPC single particle combustion TGA thermogravimetric analysis UBC unburned carbon in ash
conventional air atmosphere requires the use of finely grinded coal. On the one hand this is required to produce the reducing zone near the burner, on the other hand pulverization helps obtain a satisfactory degree of fuel burnout. Usually the low-NOx emission during oxy-fuel combustion creates a possibility to reorganize the combustion process also in terms of particle size distribution of the fuel. There are two possible benefits of this change. One is less energy demand for coal grinding, another one rely on the fact that bigger particles burn less intensively, with lower temperature and hence a lower radiative heat fluxes to the water-wall membrane is produced. This may allow decreasing the recirculated flue gas ratio (RR) and thus the energy consumption related to that process. Such changes in the combustion system can lower power-plant's energy demand, but can also cause changes in the operation of individual combustion devices, e.g. burners. To evaluate the described options for combustion optimization two sets of experiments has been carried out: in a laboratory rig for single particle combustion and in a 0.5 MW pilot-scale stand for pulverized fuel combustion. Laboratory studies were supported by numerical modelling in order to estimate the final effects of net plant efficiency improvement. 2. Experimental part The experimental studies were performed with the use of Polish bituminous coal Ziemowit (Table 1). 2.1. Single particle combustion Experiments were carried out in a laboratory rig designed for investigation of Single Particle Combustion (SPC stand). Detailed description of an experimental procedure and the SPC stand can be found elsewhere [9]. Single coal particles of a size less than 2 mm and weight about 4 mg were prepared manually. In each particle a small hole was drilled to prepare a place for a tip of a thermocouple. Particle placed on the thermocouple was introduced to a hot environment created within the SPC stand. At first the particle heated-up, then ignited and combusted, which was observed as a sequential burning of released volatile matter and then char residue. As a result particle temperature profile during combustion was obtained. Fig. 2 presents results obtained for particles combusted in different gas atmospheres. Based on the temperature profiles an overall time of particle combustion and the average particle temperature were determined (Table 2). Estimated uncertainty of combustion time determination was ±2.9%. The experimental error
Fig. 1. Typical O2 and CO2 mole fraction in the bulk surrounding of coal particle during residence in the combustion chamber of OP650 boiler.
of temperature measurement was 0.7 K, while error for particle mass determination was on the order of ±6.5%. Particle temperature in a 21% O2/CO2 atmosphere was lower than in air combustion. To achieve comparable temperatures oxygen concentration in oxy-fuel atmosphere had to be increased. It was found that combustion in 27% O2/CO2 atmosphere resulted in particle temperature close to one obtained in air. It is in general agreement with other research, although some other results indicate that the concentration of O2 may need to be increased even up to 30e35% [3,19]. In this study the increase of O2 concentration resulted also in much quicker particle combustion. Complete burnout time decreased from 29 s in the 21% O2/CO2 atmosphere to 18÷20 s in 27%O2/CO2 atmosphere. This was also true in comparison to a burnout time determined for air experiment (particle mass: 4.23 mg). Both the particle of similar mass (4.07 mg) as well as a slightly bigger particle (4.63 mg) burned quicker in the 27% O2/CO2 atmosphere than the particle tested in air. Based on that comparison it can be concluded that under the typical oxy-fuel conditions (O2 concentration above 21%) the combustion of bigger particles should not adversely affect an overall burnout efficiency of coal. Similar studies but for pulverized coal combustion in oxy-fuel conditions can be found in the literature. Rathnam et al. investigated the char burnout and reactivity using a drop tube furnace and TGA (thermogravimetric analysis) [6]. They found that in a case of quick particle heat up e the burnout is higher in oxy-fuel atmosphere, regardless the oxygen concentration. They attributed the observed burnout enhancement to char-CO2 gasification reaction. Second study performed in a TGA with a heating rate of 25 K/min shown similar results but this time the dependency on the oxygen concentration was visible. The char reactivity was much higher when oxygen concentration was increased and at the same time was very similar for air and oxy-fuel modes. This means that in a typical oxy-fuel atmosphere the expected burnout efficiency should be higher than in air due to increased O2 concentration (27% O2/CO2). Similar results were found by Liu et al. with the average increase in the coal burnout between 0.5 and 2% [10]. 2.2. Semi-industrial pulverized coal combustion The experiments were carried out in a semi-industrial laboratory stand. The setup consisted of 15 m long horizontal combustion furnace with one 0.5 MW front burner. The furnace was equipped with several ports for oxidizer staging and suction pyrometers for gas analysis. The scheme of the measuring system is presented in the Fig. 3. The setup was supplied with mixtures of O2 and CO2 provided from tanks and mixed in different proportion respectively to the various RR. The combustion tests were focused on the influence of coal grinding and flue gas recirculation on UBC
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Table 1 Properties of coal used in the experiments. Proximate analysis (as received)
Ziemowit a
Ultimate analysis (on dry basis)
M, %wt.
A, %wt.
VM, %wt.
FC, %
HCV, kJ/kg
LCV, kJ/kg
C, %
H, %
N, %
S, %
Oa, %
6.1
8.5
33.2
52.2
27,545
26416
72.03
4.75
1.16
1.36
12.15
By diff.
Fig. 2. Single particle temperature during combustion in different atmospheres.
(unburned carbon in ash), profiles of combustion temperature, NOx emission and heat fluxes. The coal was prepared in laboratory mills from its original size to the three portions of polydisperse dust with a size classification defined by RossineRamler parameters in the Table 3. The range of a grinding quality was selected on the basis of the characteristic of static classifier of the MKM-25 ball-ring mill. Both the classifier characteristic and power demand for the
preparation of one ton of coal was taken from measurements performed in one of polish power stations. The new construction of a high-performance swirl burner for oxy combustion, developed at Institute of Power Engineering, was used to ensure a stable flame in a wide range of RRs and fuel quality changes. The burner was equipped in a bluff-body flame holder and specially designed secondary and tertiary oxidizer jets with dumpers for regulation of flow division to hold the momentum ratio in spite of decrease of the mass due to higher density of CO2 and hence less amount of volumetric flow with respect to the air combustion. The flame stability was estimated visually by a direct flame observation through the quartz glass windows mounted along the left side of the combustion chamber. To estimate the stability of the flame the anchored and lifted flames distinguish was applied similar to [11]. The temperature profile in the combustion chamber was measured by several S type thermocouples (Fig. 3). To measure the radiative heat flux in near burner zone an elliptic radiometer was inserted into the combustion chamber through the front wall. Except of oxidiser staging in the burner, three sections of OFA (overfire air) ports were implemented to fulfil a comparable over fired conditions at furnace exit for various levels of substoichiometric in burner zone. The fly ash for UBC determination was removed from the bottom of cyclone at the end of flue gas duct after 30 min of each combustion test. The experimental error for coal burnout determinations was on the order of ±1%.
Table 2 Combustion time and temperature of a single particle in different atmospheres. Experimental atmosphere
Particle mass, mg
Average temperature, K
Combustion time, s
AIR 21%O2e79%CO2 27%O2e73%CO2 27%O2e73%CO2
4.23 4.00 4.07 4.63
1429 1380 1456 1461
27 29 18 20
Fig. 3. Scheme of the measuring system in the 0.5 MW semi-industrial stand where: Tk0, Tk1, Tk2, TK3, Tk4 represent thermocouples; burner, cone, outlet represent gas suction pyrometer.
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Table 3 The quality of pulverised coal prepared for combustion investigations.
Sieve residue e Mesh Size 90 mm, % Sieve residue e Mesh Size 200 mm, % Mean diameter, mm Spread parameter Power demand, kW/tona a
Fine grinding
Medium grinding
Coarse grinding
15.70 2.30 45.09 0.89 34.21
23.55 4.15 61.95 0.91 32.43
40.50 16.10 100.94 0.88 28.30
From the measurements in power plant.
The compositions of exhaust gases were measured by set of Siemens Ultramat 23 analysers (O2, CO, CO2, SO2, and NO). The experimental error of gas composition and temperature measurement were respectively on the order of ±1% and ±1.2%. The error of radiative heat flux measurement was on the order of ±2%. To obtain similar conditions as in the large scale boiler, the primary agent was heated up to 100 C while the temperature of the secondary agent was around 300 C. The rate of the primary agent was set constant e equal 50 mN3/h with about 21% of O2 in all cases whilst the secondary agent was widely changed in the range of 60e180 mN3/h and the oxygen content from 34 to 100%. Experiments confirmed that the flue gas recirculation ratio RR is a major factor that influences the heat emanation from the flame. Decreasing of RR resulted in a higher flame temperature and increased the radiative heat flux to the wall (Fig. 4). It was noticed that for the experiments in air and oxy-fuel atmosphere with the use of fine grinded coal the similar heat flux has been achieved for RR equal to 74%. However for coarse grinded coal slight decrease in flame temperature measured by thermocouple Tk0 was noticed. Also the heat flux was lower by about 10% than for the fine grinded coal and the level as for air was achieved for the RR equal to 68%. An influence of particle size on the combustion process in different atmosphere compositions finds also explanation in other research study e.g. Ref. [12]. They found that the oxy-fuel combustion of big coal particles (75÷90 mm) results in lower particle temperature and longer combustion time than for smaller particles (45e53 mm). Also Brix et al. [13] measured the particle temperature in oxy- and air-modes in regards to a particle diameter and found that the temperature is only slightly lower for bigger particles in both tested atmospheres. Influence of RR on NOx and CO revealed that for both gas species they emissions slightly lowered with decrease of recirculation ratio (Fig. 5). Emission of NOx in the oxy fired atmosphere was around 20% lower than in air within a typical for low-NOx combustion substoichiometric conditions (0.85< l < 0.95) (Fig. 6). However for a lower l the NO emission was similar to the one observed in air. What is worth noticing for the coarse grinded coal fired in l < 0.9 the NOx emission was lower than for the fine grinded one. A reverse tendency occurred for higher l > 0.9. This could be
explained by the fact that bigger coal particles burn longer so NOx precursors (NHi, HCN) are not completely reduced in substoichiometric zone but are oxidised to NO in OFA region. The results for l less than 0.9 are not so obvious and definitely do not correspond to the phenomena observed for air, where fine particles are required to better fulfil low-emission combustion methodology. This could be caused by a higher participation of CO in NO reduction [14]. Experiment investigating the influence of a coal grinding on the unburnt carbon in fly ash confirmed the results from the single particle combustion stand and also investigations of other researchers. Fig. 7 clearly shows that even for the coarse grinding, the fuel conversion in oxy-fuel with RR ¼ 74% is higher than for the fine one but fired in the air atmosphere. It could be noticed that also for sub-stoichiometric condition the coarse grinded coal that was fired in oxy-mode caused a lower UBC in fly ash than the result obtained for air stoichiometric combustion. The measured UBC in fly ash was lower by 30e50% depending on the l in a near burner region. Similar results were also reported by Riaza and Alvarez [3,19]. This observation gives opportunities for further process optimisation. 3. Computational part 3.1. Optimisation of oxy-fuel combustion in 0.5 MW semi-industrial stand Experimental results revealed some potential for oxy-fuel combustion optimisation. It was based on the fact of faster coal
Fig. 5. Influence of RR on gas emissions at burner sub-stoichiometric (l ¼ 0,8).
Fig. 4. Influence of RR and coal grinding on radiative heat flux.
Fig. 6. Influence of l and coal grinding on NOx emission.
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Table 5 Gas phase reactions included during simulations. volatile combustion: Volatiles þ 2.567O2 / 1.556CO2 þ 2.785H2O þ 0.095N2 other homogeneous reactions: CO þ ½O2 / CO2 H2 þ ½O2 / H2O
Fig. 7. Influence of gas atmosphere and coal grinding on C in fly ash.
conversion during combustion at elevated initial O2 molar fraction (around 0.36) in reference to air. Higher O2 concentration results from the optimal RR level in the range of 70÷76%, which is necessary for keeping the same heat exchange condition. The elevated O2 fraction in oxidiser is not the only factor responsible for faster coal conversion. Also the increased contribution of gasification reaction as well as different physical parameters of OXY/RFG atmosphere has been considered as important. For a better estimation of the potential effect of oxy-fuel combustion optimisation, the CFD (Computational Fluid Dynamics) modelling with the use of Ansys Fluent software has been involved. To realise this task a numerical model has been created mapping to the computational domain each individual feature of a combustion chamber of semi industrial laboratory stand. It was especially concerned to detailed consideration of burner, OFA ports, combustion chamber and secondary pass. The Euler-Lagrange approach was chosen for multiphase modelling with heat and mass exchange according to PSI-CELL (Particle-Source-In Cell) method of [15]. The standard k-ε; model was chosen for turbulence prediction. Gas properties were calculated according to the mixing-law with polynomial temperature dependence. Discrete ordinates model was used for the calculations of radiative heat transfer exchange [16]. For better prediction of coal burnout, four surface reactions were involved with kinetic data listed in Table 4. In gas phase, three homogeneous reactions were involved as it is presented in Table 5. The first assessment of model quality was positive. The wall heat fluxes were slightly overestimated (around 14% in reference to data from a radiometer) but the overall trends, which reflected different RR and coal qualities changes were adequate. Also kinetic data used for coal burnout prediction and obtained from references listed in the Table 4 - properly followed the various experimental conditions, however predicted UBC was three times higher than in the experiments. Therefore the pre-exponential factor of the first oxidising reaction was fitted to meet an acceptable UBC prediction (Fig. 8). Referring to the experimental data calculation error was ±8%. Analysis of CFD results revealed that two factors are especially responsible for lower unburnt carbon in ash in the oxy fired atmosphere. One of them is related to the longer residence time of burning particles which occurred in the oxycombustion atmosphere. It was found that the residence time in oxy fired
Fig. 8. Predicted and measured UBC at the end of second pass of 0.5 MW stand.
Fig. 9. Comparison of residence time of coal particle in oxy and air fired atmosphere.
atmosphere with RR equal to 74% is longer than in the air case by about30e50% depending on the size of coal particle (Fig. 9). The second factor is related to a higher coal conversion caused by the gasification reactions that plays significant role in the substoichiometric reduction zone between burner and OFA ports (Fig. 10). Relatively high flame temperature about 1400 C, and five times higher CO2 partial pressure than in the air case enhance coal gasification reaction and increase carbon consumption rate. According to CFD calculations, with the assumed kinetic data, the prediction of coal conversion of 100 mm particle is almost completed in the reduction zone and is caused mainly by the gasification reactions whereas the calculation performed without gasification reactions clearly shows only a partial coal conversion at near burner region until O2 introduced by the burner has been consummated. Even if CFD model (due to its simplification) does not perfectly reflect real process, the presented in the Fig. 9 results clearly justify the role of gasification reactions. Better understanding of faster coal conversion in oxy fired atmosphere confirms the assumption that the controlled deterioration of coal grinding, in the range possible to achieve in industrial mills, is possible.
Table 4 Surface reactions included during simulations. Char reactions: O2 / CO2 ½O2 / CO CO2 / 2CO H2O / CO þ H2
C C C C
þ þ þ þ
a
Own fitting to experimental results.
A [kg/(s m2 Pa)]
Ea [kJ/mol]
Diffusion-limited rate constant
Ref.
0.0447->(0.06a) 0.0570 0.1482 0.1482
74.9 74.8 130 130
2.18e-12 4.36e-12 5e-12 5e-12
[17] [18] [18] [18]
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
B. Swia˛ tkowski, E. Marek / Energy xxx (2015) 1e7
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Fig. 10. Predicted mass consumption of burned 100 mm particle with/without gasification reactions.
Fig. 11. Predicted optimal RR for coarse grinded coal application.
Calculation of the heat emanating from the flame confirmed the experimental results leading to the same conclusion that for the coarse particles combustion the heat fluxes are slightly lower than for the fine ones. However, according to the CFD calculations, possible RR decreasing to 72% (Fig. 11) is not as much noticeable as results from measurements by a radiometer, which indicated the necessity of RR reduction to 68% (Fig. 4). The possible explanation of
this discrepancy can rely on the fact that the total heat flux to the entire wall surface of combustion chamber was integrated in CFD model while the radiometer could measure only the heat flux from near burner zone. The recommended (on the basis of CFD) decrease of RR: from 74 % to 72% looks reasonable especially that both calculated and measured temperatures revealed the same trend of lower flame temperature at near burner zone and slightly higher exit furnace temperature while the coarse grinded coal was used. To summarize the CFD results, the recommended burning of coarse grinded coal poses no risk of increase in the UBC but to keep the similar heat exchange a slight decrease of RR from 74 to 72% is necessary. 3.2. The assessment of oxy-combustion optimisation on electric power efficiency For the evaluation of possible improvement of power generation efficiency related to the appropriate deterioration of coal grinding combined with the decrease of exhaust gas recirculation ratio, ASPEN software was used. Evaluation was performed taking into account medium flows that occur in OP-650 boiler connected with steam turbine, which drives a power generator with a capacity of 220 MW. The set of main medium flows of the boiler is presented in Table 6. Results of calculations, presented in Table 7, demonstrate that power saving due to the fact of less energy usage for coarse grinding of coal improves the overall net power efficiency by 0.09%. Decrease in the amount of recycled exhaust gas from RR 74 to 72% gives further efficiency improvement of about 0.023%, which is not as remarkable as for coal preparation. Higher improvement up to0.15% can be possible if more significant RR reduction (RR ¼ 68%) is applied. 4. Conclusions Results of the research revealed that oxy-combustion of pulverised coal has still some potential for an optimisation. It is possible because UBC under oxygen modified atmosphere is significantly lower due to longer residence time of coal particles (of about 30 ÷
Table 6 Medium flows of the OP-650 boiler. AIR Coal rate Steam rate Oxidiser rate Recycled exhaust gas rate Exhaust gas rate
t/h t/h t/h m3/h t/h t/h m3/h
78 650 791 668,000 e e 883 10,61,500
OXY RR ¼ 74%
OXY RR ¼ 72%
OXY RR ¼ 68%
78 650 182 120,000 665 396,000 897 798,360
78 650 182 120000 596 355,300 828 730,500
78 650 182 120000 499 297,100 730 636,430
Table 7 Power plant electricity production and internal loads in the oxy fired OP-650 boiler for considered options of optimisation.
Gross generation, MWe ASU power use, MWe CPU power use, MWe Coal grinding, MWe Induced draft fans, MWe Forced draft fans, MWe Other power use, MWe Net power, MWe Net efficiency, %
AIR-fired fine grinding
OXY-fired RR ¼ 74% fine grinding
OXY-fired RR ¼ 74% rough grinding
OXY-fired RR ¼ 72% rough grinding
OXY-fired RR ¼ 68% rough grinding
215 e e 2.67 1.16 0.76 9.29 201.12 39.500
215 30.89 18.35 2.67 0.88 0.44 9.29 152.48 29.947
215 30.89 18.35 2.21 0.88 0.44 9.29 152.94 30.038
215 30.89 18.35 2.21 0.81 0.39 9.29 153.06 30.061
215 30.89 18.35 2.21 0.70 0.33 9.29 153.23 30.094
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
B. Swia˛ tkowski, E. Marek / Energy xxx (2015) 1e7
50%) and because of higher activity of the gasification reactions especially noticeable in sub-stoichiometric reduction zone between burners and OFA ports. The faster coal conversion in the O2 enriched atmosphere was confirmed by both experimental stands: by the study of single particle combustion and by the semi industrial stand of 0.5 MW thermal capacity. Idea outlined and tested in this study presents a possibility for the use of coarse grinded coal, which will enable some energy savings from coal preparation process. For a more precisely study a real data from MKM-25 ballring mill has been analysed in order to obtain a possible range of particle size distribution as well as the energy usage for coal preparation. According to the experiment carried out in the semiindustrial stand, UBC obtained in oxy coal combustion was lower by 30e50 % than in the air fired case. At the same time, none negative impact on the flame stability was observed during coarse grinded coal oxy-combustion. Referring to the impact of the oxy coal combustion on the emissions, it was found that measured level of NOx was around 20% lower than in air, but still the oxidiser staging within substoichiometric reduction zone is required to avoid an unnecessary exceed of NO concentration in the flue gases after the combustion chamber. Both experimental and calculated results confirmed that the flue gas recirculation ratio has a great impact on the adjustment of heat exchange. It was also revealed that firing of coarse grinded coal results in a lower flame temperature in the near burner region and therefore a slight decrease of RR is necessary to keep the comparable heat flux. These observations gave the opportunities for the overall optimisation of combustion process, which was evaluated in the ASPEN calculations of decrease internal load of power-plant unit with an electric capacity of 220 MW. The possible improvement of power generation efficiency due to the use of coarse grinded coal and slightly decreased RR (of around 2%) is in the range of 0.1÷0.15%. It is worth noticing that the proposed optimization improves the efficiency practically without any significant investments. The efficiency improvement seems to be not the only one benefit of the proposed oxy-combustion optimisation. Other effects such as better availability and longer lifetime of auxiliary devices engaged in coal preparation, are possible and require further analysis. Acknowledgements Research was founded by the National Centre for Research and Development, within the confines of Research and Development
7
Strategic Program Advanced Technologies for Energy Generation project No 2: Oxyecombustion technology for PC and FBC boilers with CO2 capture Grant No SP/E/2/666420/1066420/10.
References [1] Davidson RM, Santos S. Oxyfuel combustion of pulverised coal. London: IEA Clean Coal Centre; 2010. [2] Zheng L, Tan Y. Overview of oxy-fuel combustion technology for CO2 capture. Cornerstone Mag 2013;1:48e52. [3] Riaza J, Gil MV, Alvarez L, Pevida C, Pis JJ, Rubiera F. Oxy-fuel combustion of coal and biomass blends. Energy 2012;2010(41):429e35. 23rd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems, ECOS. [4] Smart JP, O'Nions P, Riley GS. Radiation and convective heat transfer, and burnout in oxy-coal combustion. Fuel 2010;89(9):2468e76. [5] Khare SP, Wall TF. Retrofitting of a conventional coal fired plant to OXY-firing: heat transfer impacts for the furnace and associated oxygen levels. In: 5th Asia-Pacific conference on combustion; July 2005. p. 17e20. [6] Rathnam RK, Elliott LK, Wall TF, Liu Y, Moghtaderi B. Differences in reactivity of pulverised coal in air (O2/N2) and oxy-fuel (O2/CO2) conditions. Fuel Process Technol 2009;90:797e802. [7] Li Q, Zhao C, Chen X, Wu W, Li Y. Comparison of pulverized coal combustion in air and in O2/CO2 mixtures by thermo-gravimetric analysis. J Anal Appl Pyrolysis 2009;85:521e8. [8] Blume M, Bohn J-P, Goanta A, Spliethoff H. Reduction of the flue gas recirculation rate in oxycoal processes by means of non-stoichiometric burner operation. Energy 2012;2011(45):117e24. The 24th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy, ECOS. ˛ tkowski B. Experimental studies of single particle combustion in [9] Marek E, Swia air and different oxy-fuel atmospheres. Appl Therm Eng 2014;66:35e42. [10] Liu H, Zailani R, Gibbs BM. Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle. Fuel 2005;84:2109e15. [11] Lockwood FC, Mahmud T. Short communication. On the prediction of pulverized coal flame stability. Combust. Sci. And Tech. 1989;66:319e28. [12] Bejarano P, Levendis Y. Single-coal-particle combustion in O2/N2 and O2/CO2 environments. Combust Flame 2008;153:270e87. [13] Brix J, Clausen S, Jensen AD, Toftegaard MB. Advanced diagnostics in oxy-fuel combustion processes (DK), T.U. of D.D.C.E., Kgs Lyngby. 2012. [14] Van der Lans RP, Glarborg P, Dam-Johansen K. Influence of process parameters on nitrogen oxide formation in pulverized coal burners. Prog Energy Combust Sci 1997;23(4):349e77. [15] Crowe CT, Sharma MP, Stock DE. The particle-source-in cell(PSI-CELL) model for gas-droplet flows. J Fluids Eng Trans ASME 1977;99(2):325e32. [16] Murthy JY, Mathur SR. Finite volume method for radiative heat transfer using unstructured meshes. J Thermophys Heat Transf 1998;12(3):313e21. [17] Price R. Modeling three reacting flow systems with modern computational fluid dynamics. Brigham Young University; 2007. [18] Lu Hong. Experimental and modelling investigations of biomass particle combustion. Brigham Young University; 2006. [19] Alvarez L, Yin C, Riaza J, Pevida C, Pis JJ, Rubiera F. Oxy-coal combustion in an entrained flow reactor: application of specific char and volatile combustion and radiation models for oxy-firing conditions. Energy 2013;62:255e68. http://doi.org/10.1016/j.energy.2013.08.063.
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Experimental and numerical study ˛ tkowski*, Ewa Marek Bartosz Swia wka 36, 02-981 Warsaw, Poland Department of Thermal Processes, Institute of Power Engineering, Augusto
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 November 2014 Received in revised form 15 June 2015 Accepted 17 June 2015 Available online xxx
The article presents the results of a comprehensive research on the impact of O2/CO2/H2O atmosphere and coal milling quality on pulverized coal combustion process. In particular the content of combustibles in fly ash and heat transfer parameters were investigated. Experiments were conducted in two research stands: in a laboratory rig for single particle combustion investigations and in a semi-industrial 0.5 MW combustion chamber for pulverized fuel burner studies. It was shown that because of faster coal conversion in the oxy-fuel atmosphere it is possible to use lower quality coal grinding, which will save energy required for milling. Experimental results confirmed also the beneficial effect of the coarse coal milling that resulted in a decrease of necessary flue gas recirculation rate, which also means lowering the energy demand of flue gas fans. The impact of oxycombustion optimization on net efficiency of electric energy production was presented on the basis of 220 MW unit with OP-650 steam boiler. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Oxy-fuel combustion Coal combustion Numerical modelling Combustion optimization Efficiency improvement
1. Introduction Oxy-fuel combustion is a competitive alternative to other forms of CO2 capture. There are two important advantages to support this technology: the ability to adapt it also within existing boilers as well as cost-effectiveness in comparison to IGCC (Integrated Gasification Combined Cycle) and postcombustion, which was demonstrated in various economic analysis [1]. Nevertheless, oxyfuel combustion, as any CO2-capture method interrelates with a decrease in the efficiency of electricity production due to energy consumption of O2 production and CO2 liquefaction. Therefore, a lot of effort is devoted to minimize the loss in efficiency, which in the first concepts of oxy-fuel combustion installations was up to 12%. A technology breakthrough will be to reduce that loss by half. New concepts allow lower efficiency penalty through an extensive thermal integration of new equipment into existing power plant setups [2]. This usually complicates the installation and requires a use of large area heat exchangers which in turn increases CAPEX (capital expenditures). Although it is not the only method; oxy-fuel combustion itself may allow further optimization and additional efficiency loss reduction, practically without any significant investments. Combustion carried out at high O2, CO2 and H2O
* Corresponding author. Tel.: þ48 22 3451 441. ˛ tkowski). E-mail address: [email protected] (B. Swia
concentrations enables faster conversion of coal than the processes carried out in air. This is due to a higher combustion rate in mixtures with increased O2 concentration and because of further coal conversion with the participation of gasification reactions between fuel and CO2 and H2O. Higher concentration of O2 in the oxidant (usually about 27% O2) is needed in case of oxy-fuel combustion mainly to provide similar to air combustion flame heat parameters [3e5]. To obtain such concentration level oxygen must be first diluted by a stream of recycled flue gas corresponding to the RR (recirculation ratio) in the range of 70e76%. Under such conditions, the heat flux transferred to the combustion chamber will be comparable to the flux obtained during air combustion. Quicker coal conversion in oxy-fuel combustion can also result from the gasification reactions involving char and CO2 or H2O present in the combustion chamber [6,7]. The role of these reactions takes on importance in large scale boilers with typical oxidiser staging (usually applied in order to minimize NOx emissions) [8]. Within this kind of setup, an average coal particle - for about 90% of its residence time in a combustion chamber - is surrounded by a gaseous atmosphere with low (<10%) oxygen content (Fig. 1). At the same time both high temperature in the furnace and high concentrations of CO2 and H2O intensify the gasification reactions. Faster coal conversion in the atmosphere of O2/CO2/H2O may help optimize the combustion process by reducing the energy requirement for coal grinding. Low NOx combustion in
http://dx.doi.org/10.1016/j.energy.2015.06.064 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Nomenclature ASU CPU CAPEX CFD IGCC
air separation unite CO2 processing unite capital expenditure computational fluid dynamic Integrated Gasification Combined Cycle l oxidiserefuel equivalence ratio OFA overfire air PSI-CELL Particle-Source-In Cell RR recirculated flue gas ratio SPC single particle combustion TGA thermogravimetric analysis UBC unburned carbon in ash
conventional air atmosphere requires the use of finely grinded coal. On the one hand this is required to produce the reducing zone near the burner, on the other hand pulverization helps obtain a satisfactory degree of fuel burnout. Usually the low-NOx emission during oxy-fuel combustion creates a possibility to reorganize the combustion process also in terms of particle size distribution of the fuel. There are two possible benefits of this change. One is less energy demand for coal grinding, another one rely on the fact that bigger particles burn less intensively, with lower temperature and hence a lower radiative heat fluxes to the water-wall membrane is produced. This may allow decreasing the recirculated flue gas ratio (RR) and thus the energy consumption related to that process. Such changes in the combustion system can lower power-plant's energy demand, but can also cause changes in the operation of individual combustion devices, e.g. burners. To evaluate the described options for combustion optimization two sets of experiments has been carried out: in a laboratory rig for single particle combustion and in a 0.5 MW pilot-scale stand for pulverized fuel combustion. Laboratory studies were supported by numerical modelling in order to estimate the final effects of net plant efficiency improvement. 2. Experimental part The experimental studies were performed with the use of Polish bituminous coal Ziemowit (Table 1). 2.1. Single particle combustion Experiments were carried out in a laboratory rig designed for investigation of Single Particle Combustion (SPC stand). Detailed description of an experimental procedure and the SPC stand can be found elsewhere [9]. Single coal particles of a size less than 2 mm and weight about 4 mg were prepared manually. In each particle a small hole was drilled to prepare a place for a tip of a thermocouple. Particle placed on the thermocouple was introduced to a hot environment created within the SPC stand. At first the particle heated-up, then ignited and combusted, which was observed as a sequential burning of released volatile matter and then char residue. As a result particle temperature profile during combustion was obtained. Fig. 2 presents results obtained for particles combusted in different gas atmospheres. Based on the temperature profiles an overall time of particle combustion and the average particle temperature were determined (Table 2). Estimated uncertainty of combustion time determination was ±2.9%. The experimental error
Fig. 1. Typical O2 and CO2 mole fraction in the bulk surrounding of coal particle during residence in the combustion chamber of OP650 boiler.
of temperature measurement was 0.7 K, while error for particle mass determination was on the order of ±6.5%. Particle temperature in a 21% O2/CO2 atmosphere was lower than in air combustion. To achieve comparable temperatures oxygen concentration in oxy-fuel atmosphere had to be increased. It was found that combustion in 27% O2/CO2 atmosphere resulted in particle temperature close to one obtained in air. It is in general agreement with other research, although some other results indicate that the concentration of O2 may need to be increased even up to 30e35% [3,19]. In this study the increase of O2 concentration resulted also in much quicker particle combustion. Complete burnout time decreased from 29 s in the 21% O2/CO2 atmosphere to 18÷20 s in 27%O2/CO2 atmosphere. This was also true in comparison to a burnout time determined for air experiment (particle mass: 4.23 mg). Both the particle of similar mass (4.07 mg) as well as a slightly bigger particle (4.63 mg) burned quicker in the 27% O2/CO2 atmosphere than the particle tested in air. Based on that comparison it can be concluded that under the typical oxy-fuel conditions (O2 concentration above 21%) the combustion of bigger particles should not adversely affect an overall burnout efficiency of coal. Similar studies but for pulverized coal combustion in oxy-fuel conditions can be found in the literature. Rathnam et al. investigated the char burnout and reactivity using a drop tube furnace and TGA (thermogravimetric analysis) [6]. They found that in a case of quick particle heat up e the burnout is higher in oxy-fuel atmosphere, regardless the oxygen concentration. They attributed the observed burnout enhancement to char-CO2 gasification reaction. Second study performed in a TGA with a heating rate of 25 K/min shown similar results but this time the dependency on the oxygen concentration was visible. The char reactivity was much higher when oxygen concentration was increased and at the same time was very similar for air and oxy-fuel modes. This means that in a typical oxy-fuel atmosphere the expected burnout efficiency should be higher than in air due to increased O2 concentration (27% O2/CO2). Similar results were found by Liu et al. with the average increase in the coal burnout between 0.5 and 2% [10]. 2.2. Semi-industrial pulverized coal combustion The experiments were carried out in a semi-industrial laboratory stand. The setup consisted of 15 m long horizontal combustion furnace with one 0.5 MW front burner. The furnace was equipped with several ports for oxidizer staging and suction pyrometers for gas analysis. The scheme of the measuring system is presented in the Fig. 3. The setup was supplied with mixtures of O2 and CO2 provided from tanks and mixed in different proportion respectively to the various RR. The combustion tests were focused on the influence of coal grinding and flue gas recirculation on UBC
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Table 1 Properties of coal used in the experiments. Proximate analysis (as received)
Ziemowit a
Ultimate analysis (on dry basis)
M, %wt.
A, %wt.
VM, %wt.
FC, %
HCV, kJ/kg
LCV, kJ/kg
C, %
H, %
N, %
S, %
Oa, %
6.1
8.5
33.2
52.2
27,545
26416
72.03
4.75
1.16
1.36
12.15
By diff.
Fig. 2. Single particle temperature during combustion in different atmospheres.
(unburned carbon in ash), profiles of combustion temperature, NOx emission and heat fluxes. The coal was prepared in laboratory mills from its original size to the three portions of polydisperse dust with a size classification defined by RossineRamler parameters in the Table 3. The range of a grinding quality was selected on the basis of the characteristic of static classifier of the MKM-25 ball-ring mill. Both the classifier characteristic and power demand for the
preparation of one ton of coal was taken from measurements performed in one of polish power stations. The new construction of a high-performance swirl burner for oxy combustion, developed at Institute of Power Engineering, was used to ensure a stable flame in a wide range of RRs and fuel quality changes. The burner was equipped in a bluff-body flame holder and specially designed secondary and tertiary oxidizer jets with dumpers for regulation of flow division to hold the momentum ratio in spite of decrease of the mass due to higher density of CO2 and hence less amount of volumetric flow with respect to the air combustion. The flame stability was estimated visually by a direct flame observation through the quartz glass windows mounted along the left side of the combustion chamber. To estimate the stability of the flame the anchored and lifted flames distinguish was applied similar to [11]. The temperature profile in the combustion chamber was measured by several S type thermocouples (Fig. 3). To measure the radiative heat flux in near burner zone an elliptic radiometer was inserted into the combustion chamber through the front wall. Except of oxidiser staging in the burner, three sections of OFA (overfire air) ports were implemented to fulfil a comparable over fired conditions at furnace exit for various levels of substoichiometric in burner zone. The fly ash for UBC determination was removed from the bottom of cyclone at the end of flue gas duct after 30 min of each combustion test. The experimental error for coal burnout determinations was on the order of ±1%.
Table 2 Combustion time and temperature of a single particle in different atmospheres. Experimental atmosphere
Particle mass, mg
Average temperature, K
Combustion time, s
AIR 21%O2e79%CO2 27%O2e73%CO2 27%O2e73%CO2
4.23 4.00 4.07 4.63
1429 1380 1456 1461
27 29 18 20
Fig. 3. Scheme of the measuring system in the 0.5 MW semi-industrial stand where: Tk0, Tk1, Tk2, TK3, Tk4 represent thermocouples; burner, cone, outlet represent gas suction pyrometer.
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Table 3 The quality of pulverised coal prepared for combustion investigations.
Sieve residue e Mesh Size 90 mm, % Sieve residue e Mesh Size 200 mm, % Mean diameter, mm Spread parameter Power demand, kW/tona a
Fine grinding
Medium grinding
Coarse grinding
15.70 2.30 45.09 0.89 34.21
23.55 4.15 61.95 0.91 32.43
40.50 16.10 100.94 0.88 28.30
From the measurements in power plant.
The compositions of exhaust gases were measured by set of Siemens Ultramat 23 analysers (O2, CO, CO2, SO2, and NO). The experimental error of gas composition and temperature measurement were respectively on the order of ±1% and ±1.2%. The error of radiative heat flux measurement was on the order of ±2%. To obtain similar conditions as in the large scale boiler, the primary agent was heated up to 100 C while the temperature of the secondary agent was around 300 C. The rate of the primary agent was set constant e equal 50 mN3/h with about 21% of O2 in all cases whilst the secondary agent was widely changed in the range of 60e180 mN3/h and the oxygen content from 34 to 100%. Experiments confirmed that the flue gas recirculation ratio RR is a major factor that influences the heat emanation from the flame. Decreasing of RR resulted in a higher flame temperature and increased the radiative heat flux to the wall (Fig. 4). It was noticed that for the experiments in air and oxy-fuel atmosphere with the use of fine grinded coal the similar heat flux has been achieved for RR equal to 74%. However for coarse grinded coal slight decrease in flame temperature measured by thermocouple Tk0 was noticed. Also the heat flux was lower by about 10% than for the fine grinded coal and the level as for air was achieved for the RR equal to 68%. An influence of particle size on the combustion process in different atmosphere compositions finds also explanation in other research study e.g. Ref. [12]. They found that the oxy-fuel combustion of big coal particles (75÷90 mm) results in lower particle temperature and longer combustion time than for smaller particles (45e53 mm). Also Brix et al. [13] measured the particle temperature in oxy- and air-modes in regards to a particle diameter and found that the temperature is only slightly lower for bigger particles in both tested atmospheres. Influence of RR on NOx and CO revealed that for both gas species they emissions slightly lowered with decrease of recirculation ratio (Fig. 5). Emission of NOx in the oxy fired atmosphere was around 20% lower than in air within a typical for low-NOx combustion substoichiometric conditions (0.85< l < 0.95) (Fig. 6). However for a lower l the NO emission was similar to the one observed in air. What is worth noticing for the coarse grinded coal fired in l < 0.9 the NOx emission was lower than for the fine grinded one. A reverse tendency occurred for higher l > 0.9. This could be
explained by the fact that bigger coal particles burn longer so NOx precursors (NHi, HCN) are not completely reduced in substoichiometric zone but are oxidised to NO in OFA region. The results for l less than 0.9 are not so obvious and definitely do not correspond to the phenomena observed for air, where fine particles are required to better fulfil low-emission combustion methodology. This could be caused by a higher participation of CO in NO reduction [14]. Experiment investigating the influence of a coal grinding on the unburnt carbon in fly ash confirmed the results from the single particle combustion stand and also investigations of other researchers. Fig. 7 clearly shows that even for the coarse grinding, the fuel conversion in oxy-fuel with RR ¼ 74% is higher than for the fine one but fired in the air atmosphere. It could be noticed that also for sub-stoichiometric condition the coarse grinded coal that was fired in oxy-mode caused a lower UBC in fly ash than the result obtained for air stoichiometric combustion. The measured UBC in fly ash was lower by 30e50% depending on the l in a near burner region. Similar results were also reported by Riaza and Alvarez [3,19]. This observation gives opportunities for further process optimisation. 3. Computational part 3.1. Optimisation of oxy-fuel combustion in 0.5 MW semi-industrial stand Experimental results revealed some potential for oxy-fuel combustion optimisation. It was based on the fact of faster coal
Fig. 5. Influence of RR on gas emissions at burner sub-stoichiometric (l ¼ 0,8).
Fig. 4. Influence of RR and coal grinding on radiative heat flux.
Fig. 6. Influence of l and coal grinding on NOx emission.
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Table 5 Gas phase reactions included during simulations. volatile combustion: Volatiles þ 2.567O2 / 1.556CO2 þ 2.785H2O þ 0.095N2 other homogeneous reactions: CO þ ½O2 / CO2 H2 þ ½O2 / H2O
Fig. 7. Influence of gas atmosphere and coal grinding on C in fly ash.
conversion during combustion at elevated initial O2 molar fraction (around 0.36) in reference to air. Higher O2 concentration results from the optimal RR level in the range of 70÷76%, which is necessary for keeping the same heat exchange condition. The elevated O2 fraction in oxidiser is not the only factor responsible for faster coal conversion. Also the increased contribution of gasification reaction as well as different physical parameters of OXY/RFG atmosphere has been considered as important. For a better estimation of the potential effect of oxy-fuel combustion optimisation, the CFD (Computational Fluid Dynamics) modelling with the use of Ansys Fluent software has been involved. To realise this task a numerical model has been created mapping to the computational domain each individual feature of a combustion chamber of semi industrial laboratory stand. It was especially concerned to detailed consideration of burner, OFA ports, combustion chamber and secondary pass. The Euler-Lagrange approach was chosen for multiphase modelling with heat and mass exchange according to PSI-CELL (Particle-Source-In Cell) method of [15]. The standard k-ε; model was chosen for turbulence prediction. Gas properties were calculated according to the mixing-law with polynomial temperature dependence. Discrete ordinates model was used for the calculations of radiative heat transfer exchange [16]. For better prediction of coal burnout, four surface reactions were involved with kinetic data listed in Table 4. In gas phase, three homogeneous reactions were involved as it is presented in Table 5. The first assessment of model quality was positive. The wall heat fluxes were slightly overestimated (around 14% in reference to data from a radiometer) but the overall trends, which reflected different RR and coal qualities changes were adequate. Also kinetic data used for coal burnout prediction and obtained from references listed in the Table 4 - properly followed the various experimental conditions, however predicted UBC was three times higher than in the experiments. Therefore the pre-exponential factor of the first oxidising reaction was fitted to meet an acceptable UBC prediction (Fig. 8). Referring to the experimental data calculation error was ±8%. Analysis of CFD results revealed that two factors are especially responsible for lower unburnt carbon in ash in the oxy fired atmosphere. One of them is related to the longer residence time of burning particles which occurred in the oxycombustion atmosphere. It was found that the residence time in oxy fired
Fig. 8. Predicted and measured UBC at the end of second pass of 0.5 MW stand.
Fig. 9. Comparison of residence time of coal particle in oxy and air fired atmosphere.
atmosphere with RR equal to 74% is longer than in the air case by about30e50% depending on the size of coal particle (Fig. 9). The second factor is related to a higher coal conversion caused by the gasification reactions that plays significant role in the substoichiometric reduction zone between burner and OFA ports (Fig. 10). Relatively high flame temperature about 1400 C, and five times higher CO2 partial pressure than in the air case enhance coal gasification reaction and increase carbon consumption rate. According to CFD calculations, with the assumed kinetic data, the prediction of coal conversion of 100 mm particle is almost completed in the reduction zone and is caused mainly by the gasification reactions whereas the calculation performed without gasification reactions clearly shows only a partial coal conversion at near burner region until O2 introduced by the burner has been consummated. Even if CFD model (due to its simplification) does not perfectly reflect real process, the presented in the Fig. 9 results clearly justify the role of gasification reactions. Better understanding of faster coal conversion in oxy fired atmosphere confirms the assumption that the controlled deterioration of coal grinding, in the range possible to achieve in industrial mills, is possible.
Table 4 Surface reactions included during simulations. Char reactions: O2 / CO2 ½O2 / CO CO2 / 2CO H2O / CO þ H2
C C C C
þ þ þ þ
a
Own fitting to experimental results.
A [kg/(s m2 Pa)]
Ea [kJ/mol]
Diffusion-limited rate constant
Ref.
0.0447->(0.06a) 0.0570 0.1482 0.1482
74.9 74.8 130 130
2.18e-12 4.36e-12 5e-12 5e-12
[17] [18] [18] [18]
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
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Fig. 10. Predicted mass consumption of burned 100 mm particle with/without gasification reactions.
Fig. 11. Predicted optimal RR for coarse grinded coal application.
Calculation of the heat emanating from the flame confirmed the experimental results leading to the same conclusion that for the coarse particles combustion the heat fluxes are slightly lower than for the fine ones. However, according to the CFD calculations, possible RR decreasing to 72% (Fig. 11) is not as much noticeable as results from measurements by a radiometer, which indicated the necessity of RR reduction to 68% (Fig. 4). The possible explanation of
this discrepancy can rely on the fact that the total heat flux to the entire wall surface of combustion chamber was integrated in CFD model while the radiometer could measure only the heat flux from near burner zone. The recommended (on the basis of CFD) decrease of RR: from 74 % to 72% looks reasonable especially that both calculated and measured temperatures revealed the same trend of lower flame temperature at near burner zone and slightly higher exit furnace temperature while the coarse grinded coal was used. To summarize the CFD results, the recommended burning of coarse grinded coal poses no risk of increase in the UBC but to keep the similar heat exchange a slight decrease of RR from 74 to 72% is necessary. 3.2. The assessment of oxy-combustion optimisation on electric power efficiency For the evaluation of possible improvement of power generation efficiency related to the appropriate deterioration of coal grinding combined with the decrease of exhaust gas recirculation ratio, ASPEN software was used. Evaluation was performed taking into account medium flows that occur in OP-650 boiler connected with steam turbine, which drives a power generator with a capacity of 220 MW. The set of main medium flows of the boiler is presented in Table 6. Results of calculations, presented in Table 7, demonstrate that power saving due to the fact of less energy usage for coarse grinding of coal improves the overall net power efficiency by 0.09%. Decrease in the amount of recycled exhaust gas from RR 74 to 72% gives further efficiency improvement of about 0.023%, which is not as remarkable as for coal preparation. Higher improvement up to0.15% can be possible if more significant RR reduction (RR ¼ 68%) is applied. 4. Conclusions Results of the research revealed that oxy-combustion of pulverised coal has still some potential for an optimisation. It is possible because UBC under oxygen modified atmosphere is significantly lower due to longer residence time of coal particles (of about 30 ÷
Table 6 Medium flows of the OP-650 boiler. AIR Coal rate Steam rate Oxidiser rate Recycled exhaust gas rate Exhaust gas rate
t/h t/h t/h m3/h t/h t/h m3/h
78 650 791 668,000 e e 883 10,61,500
OXY RR ¼ 74%
OXY RR ¼ 72%
OXY RR ¼ 68%
78 650 182 120,000 665 396,000 897 798,360
78 650 182 120000 596 355,300 828 730,500
78 650 182 120000 499 297,100 730 636,430
Table 7 Power plant electricity production and internal loads in the oxy fired OP-650 boiler for considered options of optimisation.
Gross generation, MWe ASU power use, MWe CPU power use, MWe Coal grinding, MWe Induced draft fans, MWe Forced draft fans, MWe Other power use, MWe Net power, MWe Net efficiency, %
AIR-fired fine grinding
OXY-fired RR ¼ 74% fine grinding
OXY-fired RR ¼ 74% rough grinding
OXY-fired RR ¼ 72% rough grinding
OXY-fired RR ¼ 68% rough grinding
215 e e 2.67 1.16 0.76 9.29 201.12 39.500
215 30.89 18.35 2.67 0.88 0.44 9.29 152.48 29.947
215 30.89 18.35 2.21 0.88 0.44 9.29 152.94 30.038
215 30.89 18.35 2.21 0.81 0.39 9.29 153.06 30.061
215 30.89 18.35 2.21 0.70 0.33 9.29 153.23 30.094
˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064
B. Swia˛ tkowski, E. Marek / Energy xxx (2015) 1e7
50%) and because of higher activity of the gasification reactions especially noticeable in sub-stoichiometric reduction zone between burners and OFA ports. The faster coal conversion in the O2 enriched atmosphere was confirmed by both experimental stands: by the study of single particle combustion and by the semi industrial stand of 0.5 MW thermal capacity. Idea outlined and tested in this study presents a possibility for the use of coarse grinded coal, which will enable some energy savings from coal preparation process. For a more precisely study a real data from MKM-25 ballring mill has been analysed in order to obtain a possible range of particle size distribution as well as the energy usage for coal preparation. According to the experiment carried out in the semiindustrial stand, UBC obtained in oxy coal combustion was lower by 30e50 % than in the air fired case. At the same time, none negative impact on the flame stability was observed during coarse grinded coal oxy-combustion. Referring to the impact of the oxy coal combustion on the emissions, it was found that measured level of NOx was around 20% lower than in air, but still the oxidiser staging within substoichiometric reduction zone is required to avoid an unnecessary exceed of NO concentration in the flue gases after the combustion chamber. Both experimental and calculated results confirmed that the flue gas recirculation ratio has a great impact on the adjustment of heat exchange. It was also revealed that firing of coarse grinded coal results in a lower flame temperature in the near burner region and therefore a slight decrease of RR is necessary to keep the comparable heat flux. These observations gave the opportunities for the overall optimisation of combustion process, which was evaluated in the ASPEN calculations of decrease internal load of power-plant unit with an electric capacity of 220 MW. The possible improvement of power generation efficiency due to the use of coarse grinded coal and slightly decreased RR (of around 2%) is in the range of 0.1÷0.15%. It is worth noticing that the proposed optimization improves the efficiency practically without any significant investments. The efficiency improvement seems to be not the only one benefit of the proposed oxy-combustion optimisation. Other effects such as better availability and longer lifetime of auxiliary devices engaged in coal preparation, are possible and require further analysis. Acknowledgements Research was founded by the National Centre for Research and Development, within the confines of Research and Development
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Strategic Program Advanced Technologies for Energy Generation project No 2: Oxyecombustion technology for PC and FBC boilers with CO2 capture Grant No SP/E/2/666420/1066420/10.
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˛ tkowski B, Marek E, Optimisation of pulverized coal combustion in O2/CO2/H2O modified atmosphere e Please cite this article in press as: Swia Experimental and numerical study, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.06.064