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A Fluidized Bed Combustion Model with Discretized Population Balance. 2. Experimental Studies and Model Validation Atif A. Khan,* Wiebren De Jong, and Hartmut Spliethoff Department of Process & Energy, Section Energy Technology, Faculty 3ME, Delft UniVersity of Technology, Leeghwaterstraat 44, NL-2628 CA, Delft, The Netherlands ReceiVed August 10, 2006. ReVised Manuscript ReceiVed June 27, 2007
The fluidized bed combustion behavior of a Dutch fuel, demolition wood, is of practical interest because of its commercial use for heat generation purposes and has been analyzed by experimental techniques. Experiments with a steady state bubbling fluidized bed combustion boiler at a thermal input scale of 1 MWth were carried out. The effect of different operating parameters (fluidization velocity, bed temperature, etc.) and air-staging (secondary air injection) on gaseous emissions (CO and NO) has been studied. The intention is to increase the use of this type of boiler especially for greenhouse heating, making use of the available residues. One of the unique features of this boiler is a moveable internal heat exchanger, which makes it possible to run it at variable thermal outputs. Experimental data was further analyzed and compared with the simulation results carried out with the fluidized bed model especially developed for solid fuel feedings. The model takes into account devolatilization, fragmentation, and attrition of the solid phase together with heterogeneous and homogenous reactions. A particle size distribution model is also included to calculate the elutriation losses of fine char particles. Experimental and model results indicate that optimization of operating parameters and air-staging ratios can significantly reduce the gaseous emissions. KEYWORDS: biomass; combustion; fluidized bed combustor; attrition; particle size distribution; gaseous emissions
1. Introduction Biomass as an alternative energy source is getting a lot of attention worldwide because of the environmental and cost incentives it offers. In the European Union and especially in The Netherlands, renewable energy policies are driven by the well-recognized need for a sustainable society, in which both the environment and the resources will be available for generations to come. The Dutch government aims in its white paper on energy (1995) at a simultaneous approach of continuous energy savings, efficiency improvement (33% in 2020), and the introduction of renewable energy (10% in 2020). This target for renewable energy is almost 5-fold of the present 53 PJ to 270 PJ in 2020. From this target, 40% (120 PJ) could be realized with energy from waste and biomass1 Biomass offers a number of advantages in comparison to other fuels. Biomass is considered as a CO2-neutral fuel. The thermal utilization of biomass can contribute to reducing CO2 emissions since the same amount of CO2 is extracted from the air during the growth period of the plants as it is released by combustion (CO2 balance). An evaluation of the CO2 balance shows that compared with the combustion of hard coal, the CO2 emissions can be reduced by 93%.2 Furthermore, most biomass fuels have lower sulfur and chlorine content than fossil fuel leading to * Corresponding author. Telephone: +31 15 2786987. Fax: +31 15 2782460. E-mail:
[email protected]. (1) Jansen, J. P.; Koppejan, J.; Meulman, P. D. M. Perspectives for reduction of NOx and dust emissions in small-scale energy production from clean wood. In 12th European conference on biomass for energy, industry and climate protection, Amsterdam, The Netherlands, 2002; pp 452–455. (2) Spliethoff, H.; Hein, K. R. G. Effect of co-combustion of biomass on emissions in pulverized fuel furnaces. Fuel Process. Technol. 1998, 54, 189–205.
lower SO2 and HCl emissions.3–9 However, biomass has its own problems, from availability to fuel characteristics. Biomass fuels comprise of a wide variety of materials characterized by their high level of heterogeneity. Other factors which handicap the utilization of biomass as a combustion fuel include low density, fibrous structure, high ash and alkali contents, and sometimes high fuel nitrogen content. Gaseous emissions form an integral part of every combustion process. The emission rate of so-called unburned pollutants (such as hyrdrocarbon (HC), tar, polycyclic aromatic HC, etc), in the case of biomass, is expected to be at insignificant levels with sufficient combustion air. The major harmful emissions from biomass combustion are NOx (generally NO) and CO. CO emissions are important in terms of combustion efficiency and (3) Baxter, L. L.; Miles, T. R.; Miles, T. R.; Jenkins, B. M.; Milne, T.; Dayton, D.; Bryers, R. W.; Oden, L. L. The behavior of inorganic material in biomass-fired power boilers: field and laboratory experiences. Fuel Process. Technol. 1998, 54 (1–3), 47–78. (4) Grass, S. W.; Jenkins, B. M. Biomass Fueled Fluidized-Bed Combustion - Atmospheric Emissions, Emission Control Devices and Environmental-Regulations. Biomass Bioenergy 1994, 6 (4), 243–260. (5) Jenkins, B. M.; Bakker, R. R.; Gilmer, J.; Wei, J. B. In Combustion characteristics of leached biomass, Thermochemical Biomass ConVersion; Banff: Canada, 1996. (6) Jenkins, B. M.; Baxter, L. L.; Miles, T. R., Jr.; Miles, T. R. Combustion properties of biomass. Fuel Process. Technol. 1998, 54, 17– 46. (7) Jenkins, B. M.; Bakker, R. R.; Wei, J. B. On the properties of washed straw. Biomass Bioenergy 1996, 10 (4), 177–200. (8) Miles, T. R. J.; Miles, T. R. Alkali deposits found in biomass power plants; National Renewable Energy Laboratory (NREL): USA, 1994; pp 1–72. (9) Miles, T. R.; Miles, T. R. J.; Baxter, L. L.; Bryers, R. W.; Jenkins, B. M.; Oden, L. L. Boiler deposits from firing biomass fuels. Biomass Bioenergy 1996, 10, 125–38.
10.1021/ef060369v CCC: $37.00 2007 American Chemical Society Published on Web 09/25/2007
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Figure 1. Schematic picture of the boiler.
can be effectively controlled with operating parameters (combustion air, temperature, combustor load, etc.) while NO emissions show weak dependence on operating parameters and are a strong function of fuel N content.10,11 Fluidized bed combustion technology is reported to be the most efficient, and suitable combustion of wood and agricultural residues are among the available proven combustion technologies (grate-fired, suspension-fired, etc.)12–14. Particularly, the capability of the fluidized bed technology of combusting almost any type of biomass and residues with low emissions (low NOx and SO2) has made it a favorable system over others. This article deals with the experimental and mathematical studies of combustion of demolition wood (DW, also called B quality wood (BQW)) in a 1 MWth bubbling fluidized bed boiler. The major objectives were • to study the effect of different operating parameters on the formation and reduction of gaseous emissions (CO and NO) • to study the effect of air-staging • to predict the effect of operating variables on char particle size distribution (PSD) • to validate the model developed by Khan et al.15 2. Experimental 2.1. Experimental Setup. Experiments were carried out using a state of the art bubbling fluidized bed boiler, engineered and constructed by the Dutch company Crone. It is a 1 MWthmax boiler designed to run on solid fuels; see Figure 1. The vertical fluidized bed section consists of a vessel with a square geometry existing of two parts: the bed and the freeboard zone. (10) Khan, A. A.; Jong, W. de; Spliethoff, H. Biomass combustion in fluidized bed boiler; Bioenergy for Wood Industry: Jyväskylä, Finland, 2005; pp 365–370. (11) Jong, W.; Khan, A. A.; Spliethoff, H. Biomass blend combustion in a 1 MWth bubbling fluidized bed boiler. In Clean Air 2005. 8th International conference on energy for a clean environment, Calouste Gulbekian Foundation; Lisbon, Portugal, June 27–30, 2005. (12) Van-der-Broek, R.; Faaij, A.; Wijk, A. v. Biomass combustion for power generation. Biomass Bioenergy 1996, 11 (4), 271–281. (13) Werther, J.; Saenger, M.; Hartge, E. U.; Ogada, T.; Siagi, Z. Combustion of agricultural residues. Progr. Energy Combust. Sci. 2000, 26 (1), 1–27. (14) Kouprianov, V. I.; Permchart, W. Emissions from a conical FBC fired with a biomass fuel. Appl. Energy 2003, 74, 383–392. (15) Khan, A. A.; Jong, W. de; Gort, D. R.; Spliethoff, H. A fluidized bed combustion model with discretized population balance. 1. Sensitivity Analysis. Energy Fuels 2007, 21 (4), 2346–2356.
Table 1. Chemical Analysis of B Quality Wooda calorific value HHV (MJ/kg) LHV (MJ/kg) moisture percent ash percent C% S% H% N% Cl% O% volatile percent fixed C%
Ar
wf
waf
18.2 ( 1.5 16.6 ( 1 9.1 ( 2.0 1.7 ( 0.05 45.7 ( 1.0 0.0b 6.3 ( 1.0 0.9 ( 0.1 0.1 ( 0.02 36.2 ( 2.0 69.6 ( 5.0 19.7 ( 1.0
20.0 ( 1.5 18.5 ( 1
20.4 ( 1.5 18.9 ( 1
1.8 ( 0.05 50.3 ( 1.0 0.0b 6.9 ( 1.0 1.0 ( 0.1 0.1 ( 0.02 39.9 ( 2.0 76.5 ( 5.0 21.6 ( 1.0
51.2 ( 1.0 0.0b 7.0 ( 1.0 1.0 ( 0.1 0.1 ( 0.02 40.6 ( 2.0 78.0 ( 5.0 22.0 ( 1.0
a Ar, as received; wf, water free; waf, water and ash free. than the detection limit Vt). Second and more effectively, it increases the attrition rate of char particles which in turn results in the generation of more fine particles and therefore the total elutriation rate increases. Almost a similar effect can be seen in Figure 8, which shows the char particle size distribution for different fluidization velocities. As shown in this figure, the average char particle
size decreases with increasing fluidization velocity. As explained above, higher fluidization velocity means higher attrition rates which not only reduces the mother particle size but also only generates more fines and results in the decrease of the average char particle size in the bed. Figure 9 shows the ratio of particle shrinkage due to attrition and combustion at two fluidization velocities (0.8 and 1.4 m/s). It is evident from the figure that, in comparison to lower fluidization velocities, particle shrinkage due to attrition contributes a major share at higher fluidization velocities (around 80% at 1.4 m/s and 50% at 0.8 m/s). 4.3. Effect of Air-Staging on Gaseous Emissions. Figure 10 shows the effect of air-staging on gaseous emissions. It should be pointed out that all staging experiments were carried out at approximately the same boiler loadings (∼1 MWth). Since the system automatically controls the bed temperature, the only way possible to achieve this was by bringing the moveable heat exchanger close to the bed zone (see Table 4). This in turns effects the temperature profile in the splashing zone and freeboard, but the change in temperature profile is not only due to the movement of the heat exchanger as decrease in primary air flow or in other words decrease in fluidization velocity also contributes to the lowering of temperatures in these two sections. NO emissions generally decrease with increasing air-staging while CO emissions remain almost constant. Upon reaching the higher degree of air-staging (>40 %), both CO and NO
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Figure 9. Ratio of particle shrinkage due to attrition and combustion for two different fluidization velocities.
Figure 10. Effect of air-staging on gaseous emission (dry basis, experimental and simulated results).
Figure 11. Implemented temperature profile according to the temperature measured in different air-staging runs.
emissions increase. The model predictions for the air-staging simulations are also quite close to experimental results except for CO emissions. Both experimental and modelling results show that the air-staging is beneficial up to a certain degree of airstaging. In comparison to the nonstaging experiments where most of the combustion occurs in the bed, significant combustion also occurs in the splashing and freeboard region in staging experiments as more oxygen becomes available due to the injection of secondary air. This effect is shown by the comparatively higher temperatures in the splashing zone and freeboard in Figure 11. This advantage of air-staging is limited
to 30%, and upon further increasing the degree of air-staging, there is not only not enough residence time to convert all unburned CO but also the large volume of secondary air decreases the temperature (see Figure 11) in the splashing zone and freeboard and results in high CO emissions. The comparatively higher freeboard temperatures at a higher degree of airstaging also indicate that the CO conversion to CO2 is still ongoing. NO emissions, on the other hand, show a minimum at a primary air to fuel ratio of 0.8. The overall excess air factor (lambda 1, also called air to fuel ratio), bed excess air factor (lambda 2), expanded bed height, and calculated fluidization
Fluidized Bed Combustion Model
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Figure 12. Calculated parameters for air-staging experiments.
velocities for air-staging experiments are plotted in Figure 12. With increasing air-staging, the bed excess air factor has to be decreased in order to run the system at the same boiler loadings. At such a low bed air to fuel ratio, the system behaves as a combination of a gasifier and a combustor. Strong reducing conditions exist in the bed, while the splashing zone and freeboard have strong oxidizing conditions due to the availability of O2 by secondary air injection and this helps in the reduction of NO emissions.2,39–41 5. Conclusions Combustion experiments in a bubbling fluidized bed boiler and simulations with B quality wood were carried out. These studies suggest that both fluidization velocity and bed temperature are important parameters for fluidized bed combustion. Bed temperature can not be increased beyond a certain limit (max 850 °C) because of the presence of lower melting point alkali compounds in most fuels which at higher temperatures results in the formation of sand agglomerates, therefore disrupting the total fluidization process. Velocity variation experiments and simulations indicate that running the system at lower velocities delivers better results in (39) Grubor, B. D.; Oka, S. N.; Ilic, M. S.; Dakic, D. V.; Arsic, B. T. Biomass FBC combustion-bed agglomeration problems. Proceedings of 13th International Conference on Fluidized Bed Combustion, ASME: New York, 1995; pp 515–522. (40) Spliethoff, H.; Rudiger, U.; Hein, K. R. G. Basic effects of NOx emissions in air staging and reburning at a bench scale test facility. Fuel 1996, 5, 560–564. (41) Lu, Y.; Hippinen, I.; Jahkola, A. Control of NOx and N2O in pressurized fluidized-bed combustion. Fuel 1995, 74 (3), 317–322.
terms of gaseous emissions, particle attrition, and elutriation rates of unburned char particles, if the high temperature profile can be maintained in the splashing zone and freeboard (e.g., by removing the internal heat exchanger). However, at lower fluidization velocities, the boiler loading will be lower and some degree of mixing efficiency has to be compromised for a certain particle size range (inert material). Air-staging experiments show promising results for both CO and NO emissions. It also has the advantages that the boiler loadings do not need to be compromised, and it leads to lower attrition and elutriation rates because of the lower fluidization velocities. However, air-staging is only beneficial to a certain degree and that is determined by the fluidization velocity and the temperature profile in the splashing zone and freeboard. The fluidized bed model reasonably predicts the experimental results, and therefore, it can be concluded that the shrinking particle model represents the particle combustion behaviour of the BQW. In order to improve the prediction of the CO emissions in broad ranges of bed temperature and velocity, the char combustion kinetics for each fuel should be determined experimentally and used. Acknowledgment. The European Union is acknowledged for funding the research in the framework of NNE5 (Project No. E52001-00601) via the project “Safe co-combustion and extended use of biomass and biowaste in FB plants with accepted emissions” (contract ENK5-CT2002-00638, FBCOBIOW). EF060369V
