Energy & Fuels 2005, 19, 1889-1897
1889
Alkali Metals in Circulating Fluidized Bed Combustion of Biomass and Coal: Measurements and Chemical Equilibrium Analysis Michal P. Glazer,*, † Nafees A. Khan,† Wiebren de Jong,† Hartmut Spliethoff,† Heiko Schu¨rmann,‡ and Penelope Monkhouse§ Section Energy Technology, Delft University of Technology, Mekelweg 2, NL-2628 CD Delft, The Netherlands, Institute of Process Engineering and Power Plant Technology, University of Stuttgart, Pfaffenwaldring 23, D-70569 Stuttgart, Germany, and Institute of Physical Chemistry, University of Heidelberg Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany Received January 31, 2005. Revised Manuscript Received June 10, 2005
Combustion and co-combustion experiments with four kinds of straw, specially selected for their different alkali, Cl, and Si contents, and Columbian black coal were carried out in a circulating fluidized bed (CFB) reactor at Delft University of Technology. The influence of operating conditions and fuel composition on the release of the alkali compounds to the gas phase was investigated. The amount of the total gas-phase sodium and potassium compounds in the flue gases was measured with excimer laser induced fluorescence (ELIF). The results show that the release of gaseous alkali species depends on fuel composition, in particular the K/Cl and K/Si ratios in the fuel. The fuels with high K and Cl values show higher concentrations of the gaseous alkalis. A synergetic effect of the co-combustion with coal was observed, which led to a strong decrease in gaseous alkali concentrations. Together with experiments, chemical equilibrium modeling was performed to help in interpreting the experimental data. The calculations confirmed that the equilibrium is very strongly influenced by the composition of the fuel blend. Moreover, the simulations provided more information on sequestering of alkali species.
Introduction Herbaceous biofuels such as straw seem to be promising for utilization. Every year, 300 Mtons of biofuels such as straw, also called high-alkali (HIAL) biofuels, are available on the EU common market and can be used for small, decentralized combined heat and power (CHP) plants. According to an EU directive, the combustion of straw alone and co-combustion with coal should be promoted in order to reach the target of 8% of the current primary energy supplied from biosources in 2010 1 and to help reduce CO2 emissions by up to 366 Mtons/year (EU study funded by the ALTENER (Alternative Energy) Program in 1998/1999 in both existing and newly built power plants. Moreover, co-combustion of coal with low-sulfur biofuels reduces SO2 emissions as well. This is partly because it is believed that part of the sulfur present in high sulfur coals can be bound by alkalis present in straw. On the other hand, HIAL biofuels such as straw are characterized by extremely high alkali content, which in combination with certain * Corresponding author. E-mail:
[email protected]. † Section Energy Technology, Delft University of Technology, Mekelweg 2, NL-2628 CD Delft, The Netherlands. ‡ Institute of Process Engineering and Power Plant Technology, University of Stuttgart, Pfaffenwaldring 23, D-70569 Stuttgart, Germany. § Institute of Physical Chemistry, University of Heidelberg Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany. (1) Spliethoff, H.; Unterberger, S.; Hein, K. R. G. Status of Cocombustion of coal and biomass in Europe. In Proceedings of the Sixth International Conference on Technologies and Combustion for a Clean Environment, Instituto Superior Tecnico: Portugal, 2001; pp 575-584.
ratios of Cl and Si leads to corrosion and deposit formation and, in the case of fluidized bed technology, also to defluidization problems. High-temperature corrosion associated with biomass combustion is often reported at power plants using biofuels, especially highchlorine and -alkaline straw.2 Locally high concentrations of chlorine from chloride deposits on heat exchangers were observed to substantially increase the corrosion rates of the heat exchanging surfaces.3 Deposit formation on relatively cold heat exchanging surfaces is another widely recognized problem. Therefore extensive research is needed to reduce the operational costs and improve the reliability of the existing and newly built power plants. To prevent these operational problems, a clear understanding of the complex behavior of alkali metals during combustion is needed. Many factors still remain unknown. To fulfill these goals, effective monitoring of alkali species within combustions systems is needed. Currently several modern techniques exist which allow direct measurement of alkali compounds in the flue gases on-line. In recent years, three such methods have been employed increasingly, namely, excimer laser induced fluorescence (ELIF), surface ionization (SI), and plasma excited alkali resonance line (2) Baxter, L. L.; Miles, T. R.; Miles, T. R., Jr.; 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, 47-78. (3) John, R. C. High-temperature condensation of deposits based on Na, Cl, S, Fe and O and the corrosion of Fe. In High-Temperature Corrosion in Energy Systems; Rothman, M. F., Ed.; American Institute of Mining, Metallurgical, and Petroleum Engineers: New York, 1984.
10.1021/ef0500336 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/17/2005
1890
Energy & Fuels, Vol. 19, No. 5, 2005
spectroscopy (PEARLS). The ELIF method is based on excimer laser induced fragmentation fluorescence and (for low laser energy densities) is sensitive only to gasphase species of sodium and potassium.4-6 If an optical access (window) is available, ELIF can be operated in the in-situ mode (i.e. avoiding sampling). PEARLS is based on dissociation of alkali compounds by mixing sampled process gas with a nitrogen plasma jet generated with a nontransferred dc plasma torch.7 The SI alkali detector is based on the ionization of alkali metals upon desorption from a hot Pt surface.6,8 SI detects alkali both in the gas phase and on aerosol particles. PEARLS, apart from measuring gaseous alkalis, can also detect particles below 10 µm. To make the description of the combustion system more complete, parallel with the experiments, a set of chemical equilibrium model calculations was performed using the FACTSage computer program. The program incorporates the Chemsage engine for free energy minimization combined with the FACT (Facility for Analysis of Chemical Thermodynamics) database of chemical compounds.9 There is great potential in chemical equilibrium calculations when combined with the proper experimental data. The proper thermodynamic database can be developed then over a whole range of temperatures and applications. The research community is involved in the process of developing the comprehensive models describing biomass combustion. By implementing the chemical equilibrium models for straw combustion, formation of molten silicates and the very high volatilization of alkali compounds were already predicted.10,11 Although the modeling work available covers the broad range of applications, the calculations still suffer many weaknesses.12 In this paper, emphasis was put on a high-alkali biomass-coal system, especially on the influence of the coal composition on the alkali species originating from biomass. Moreover we tried to better predict the behavior of the combustion (4) Gottwald, U.; Monkhouse, P.; Bonn, B. Dependence of alkali emissions in PFB combustion on coal composition. Fuel 2001, 80, 1893-1899. (5) Gottwald, U.; Monkhouse, P.; Wulgaris, N.; Bonn, B. In-situ study of the effect of operating conditions and additives on alkali emissions in fluidised bed combustion. Fuel Process. Technol. 2002, 75, 215-226. (6) Monkhouse, P. B.; Gottwald, U. A.; Davidsson, K. O.; Lo¨nn, B.; Engvall, K.; Pettersson, J. B. C. Phase discrimination of alkali species in PCFB combustion flue gas using simultaneous monitoring by surface ionisation and photofragmentation fluorescence. Fuel 2003, 82, 365371. (7) Ha¨yrinen, V.; Hernberg, R.; Aho, M. Demonstration of plasma excited atomic resonance line spectroscopy for on-line measurements of alkali metals in a 20 kW bubbling fluidized bed. Fuel 2004, 83, 791797. (8) Tran, K.; Iisa, K.; Hagstro¨m, M.; Steenari, B.; Lindqvist, O.; Pettersson, J. B. C. On the application of surface ionisation detector for the study of alkali capture by kaolin in a fixed bed reactor. Fuel 2003, 83, 169-175. (9) Eriksson, G.; Hack, K. Chemsage-A computer program for the calculation of complex chemical equilibrium. Metall. Trans. B 1990, 21, 1013. (10) Blander, M.; Pelton, A. D. The inorganic chemistry of the combustion of wheat straw. Biomass Bioenergy 1997, 12 (4), 295-298. (11) Blander, M.; Milne, T. A.; Dayton, D. C.; Backaman, R.; Blake, D.; Ku¨hnel, V.; Linak, W.; Nordin, A.; Ljung, A. Equilibrium chemistry of biomass combustion: A round-robin set of calculations using available computer programs and databases. Energy Fuels 2001, 15, 344-349. (12) Backman, R.; Nordin, A. High-temperature equilibrium calculations of ash forming elements in biomass combustion/gasification systems-State-of-the-art, possibilities and applications. In Proceedings of the International Biomass Ash Workshop, Obernberger, I., Ed.; Graz University of Technology: Austria, 1998.
Glazer et al.
Figure 1. Circulating fluidized bed combustor used for the experiments.
system, expanding it with the compounds not directly measured with ELIF. This is necessary for a detailed description of the system. The objective of this work was to investigate the influence of fuel composition and combustion conditions on the release of the alkali compounds to the gas phase during combustion and co-combustion of high-alkali straw with coal at different ratios on an energy basis in a circulating fluidized bed (CFB) combustor. The ELIF technique has been used for the measurements on the facility. Moreover, to better explain the relationships between reacting components, the chemical equilibrium modeling was applied. Combustion Facility The CFB test rig (Figure 1) available within the university is 5 m high with an inner riser diameter of 80 mm. The thermal output for the combustion experiments was about 25 kW and is operated atmospherically. The installation is started with an electrical preheating; the temperature within the system can be controlled. The average operational temperature is between 750 and 850 °C with a maximum level of 900 °C. The reactor operates with standard silica sand as a bed material, with particle diameters between 0.3 and 0.6 mm. The installation is equipped with a screw-based feeding system that consists of three independently controlled screw feeders with variable feeding rates for different fuel/additive mixtures (upper part) and a main feeder that transports the mixture to the reactor (lower part). The installation is equipped with sampling ports at different heights of the riser and downcomer. Combustion experiments can be performed with variable fuel composition, feeding rates, and feeding position. Further downstream, after the cyclone but before a hot gas filtering unit, the installation has been equipped with an optical access point/optical port for ELIF measurements. The experiments were performed at 850 °C as a mean temperature in the reactor and approximately 750 °C at the ELIF port. Downstream of the optical port, the reactor is equipped with the hot gas filter installation based on four ceramic textile BWF candles and operating at 450 °C on average.
ELIF The ELIF method uses pulsed, ArF excimer laser light at 193 nm to photodissociate alkali compounds and simultaneously excite electronically the alkali atoms formed. Fluo-
Akali Metals in CFB of Biomass and Coal
Energy & Fuels, Vol. 19, No. 5, 2005 1891 Table 1. Fuel Composition (Oxygen by Difference) for given fuel HIAL 3 HIAL 4 HIAL 7 HIAL 9
coal
LHV(exp) (mJ/kg)
16.3
16.3
16.7
16.4
26.02
composn moisture (wt %) ash (wt % dry) C (wt % dry) H (wt % dry) N (wt % dry) S (wt % dry) Cl (wt % dry)
3.70 5.70 46.00 6.10 0.28 0.06 0.27
6.83 5.00 46.00 6.10 0.46 0.10 0.03
6.57 5.10 45.00 6.00 1.10 0.28 0.06
6.80 7.10 45.00 5.90 0.60 0.07 0.64
4.24 12.06 69.90 4.41 1.29 0.52 0.03
Table 2. Calculated Ash Composition of Some Elements in HIAL Fuels and Coal % ash for given fuel SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O
Figure 2. ELIF system. rescence from the excited Na(32P) or K(42P) states can easily be detected in the visible region. Since the laser energy densities used are only a few mJ/cm2, only gas-phase alkali is monitored. Also, because of the fixed excitation wavelength of 193 nm and the low energy used, only chloride and hydroxide can be detected with the present system. To detect sulfates, either a shorter wavelength (
