Particulate and CO Emissions from a Moving-Grate Boiler Fired with

Oct 2, 2007 - units with a thermal capacity of less than 5 MW; larger boilers, however ..... K atoms may form 87 g of K2SO4, 74 g of KCl, 69 g of K2CO...
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Energy & Fuels 2007, 21, 3653–3659

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Particulate and CO Emissions from a Moving-Grate Boiler Fired with Sulfur-Doped Woody Fuel Michael Strand* Department of and Technology and Design, DiVision of Bioenergy, Växjö UniVersity, SE-351 95 Växjö, Sweden ReceiVed May 16, 2007. ReVised Manuscript ReceiVed August 3, 2007

Particulate and gaseous emissions were studied in a 7 MW moving-grate boiler fired with moist sawmill residues together with varying admixtures of elementary sulfur. The particle number concentration decreased by approximately 25% and the sub-micrometer particle mass concentration increased by approximately 100% as 0.17% (by mass, dry substance) of elementary sulfur was added to the fuel. Sulfur addition also resulted in a shift towards larger particle mean diameters. Elementary analysis indicated that the amount of particlebound sulfur increased significantly with sulfur addition, probably by favoring the formation of alkali sulfates. CO emissions were reduced by approximately 50% in the measurement period when the sulfur admixture with the fuel was 0.17% or higher. The SO2 concentration measured downstream from the flue gas condenser was below the detection limit when no sulfur was admixed and was approximately 30 ppm when 0.12% elementary sulfur was admixed. Sulfur admixture had no significant effect on the NOx concentration.

Introduction Biomass is regarded as a renewable and CO2-neutral fuel with great potential in efforts to reduce global warming. The use of biomass for heat and power production also represents an efficient use of locally produced fuels and residues from agriculture and forestry. In Sweden, biofuels, including peat, are major energy sources, accounting for approximately 18% (112 TW h) of the total energy supply in 2005,1 and a growing number of medium-sized boilers (0.5–10 MW) are using wood-based biofuels. Boilers in this size range are typically of the grate type and often lack gascleaning equipment, for example, for reducing nitrous oxide (NOx) emissions and efficiently removing fine (1 µm) particles, whereas fine particles are not collected and are emitted to the ambient air. In Sweden, using cyclones alone is often sufficient to meet current particulate mass restrictions for units with a thermal capacity of less than 5 MW; larger boilers, however, generally require an additional separation unit, such as an electrostatic precipitator. Fine particles have been associated with adverse health effects,7,8 and there is ongoing debate as to whether current total particle mass concentration restrictions adequately address potential health risks. Epidemiological studies suggest that to assess potential health effects, other particle characteristics than simply mass concentration should also be considered, such as particle number concentration, particle morphology, and detailed chemical composition. Enhancing the sulfur content of biomass fuels has been used to decrease the formation of ash deposits and high-temperature corrosion in power units by transforming sticky fly ash components, such as alkali chlorides, into alkali sulfates.9 The sulfur content of a fuel may be enhanced in several ways, including by mixing the biomass with high-sulfur fuels such as coal or peat or by admixing various sulfur-containing chemicals, such as elementary sulfur, ammonium sulfate, or iron sulfate. There are also commercial systems on the market in which sulfur-containing species are sprayed into the postcombustion (6) Obernberger, I.; Brunner, T.; Jöller, M. In Characterisation and formation of aerosols and fly-ashes from fixed-bed biomass combustion, Proceedings of the IEA seminar Aerosols from Biomass Combustion, Zürich, Switzerland, June 27, 2001; Nussbaumer, T., Ed.; Verenum: Zürich, 2001; pp 69–74. (7) Dockery, D. W.; Pope, C. A.; Xu, X.; Spengler, J. D.; Ware, J. H.; Fay, M.; Ferris, B. G.; Speizer, F. E. N. Engl. J. Med. 1993, 329, 1753– 1759. (8) Lighty, J. S.; Veranth, J. M.; Sarofim, A. F. J. Air Waste Manage. Assoc. 2000, 50, 1565–1618. (9) Skrifvars, B.-J.; Lauren, T.; Hupa, M.; Korbee, R.; Ljung, P. Fuel 2004, 83, 1371–1379.

10.1021/ef700248f CCC: $37.00  2007 American Chemical Society Published on Web 10/02/2007

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zone of the furnace.10 In some cases it has also been recognized that sulfur addition has a beneficial effect on CO and TOC emissions.11,12 This has led to an increasing interest in using sulfur addition to reduce the emissions from medium-sized grate boilers fired with biomass fuels, especially in units that have difficulties achieving the legislated CO and NOx emission limits. However, there has also been some concern that sulfur addition may promote the formation and emission of fine particles. One report estimated the concentration of sub-micrometer particles in a bubbling fluidized bed (BFB) boiler fired with moist forest residues by analyzing the current–voltage data from an ESP installed for emission control.12 The authors estimated that an addition of 39 mg elementary sulfur/MJ fuel would raise the concentration of fine particles in the gas entering the ESP by approximately 50 mg/m3. In another report, the particle mass size distribution was measured in a 110 MW BFB incorporating the injection of ammonium sulfate in the zone between the tertiary air inlets and the superheaters.11 In this case, the submicrometer particle mass concentration increased by approximately 50% as ammonium sulfate was injected; there was also a shift in the particle mass size distribution towards slightly larger particles. The commercial units in Sweden where sulfur admixture is presently used are generally equipped with a flue gas condenser. In the flue gas condenser, the condensate is recirculated and sprayed into the flue gas. Gases such as SO2 and HCl are to a great extent absorbed by the condensate, and therefore, the emissions of these gases can be kept at a low level also when extraneous sulfur is added to the combustion process. The mechanisms causing the reduced CO and TOC emissions during sulfur addition to biomass fuels have not been determined. Usually SO2 is regarded as a radical recombination agent that will inhibit oxidation of fuels; however, the promotion of oxidation of CO by SO2 in a narrow range of operating conditions close to stoichiometric conditions has been reported.13 The proposed mechanism was a chain branching sequence that promotes the formation of OH and O radicals. The effect of alkali components on fuel oxidation has also been investigated. Experiments on gas-phase potassium as an inhibitor on CO oxidation under wet, reducing conditions have been reported.14 The authors interpretation of the results was that gaseous alkali will consume OH radicals and thereby inhibit CO oxidation. A heterogeneous mechanism has also been suggested, whereby sulfur addition promotes the formation of fine alkali particles in the combustion zone, particles that will act as heterogeneous catalysts to oxidize CO.12 This is similar to a mechanism that has been proposed for heterogeneous oxidation of CO on ashes from spruce wood in fluidized bed combustors.15 The inorganic matter of the fine mode particles from the combustion of woody fuels is formed mainly from K, Na, S, and Cl compounds volatilized during fuel combustion.16–18 During combustion of biomass, volatilized inorganic compo(10) Henderson, P.; Kassman, H.; Andersson, C. In The use of ammonium sulphate in waste-fired boilers to reduce superheater fouling and corrosion and NOx emissions, Proceedings of Swedish-Finnish Flame Days 2005, Borås, Sweden, October 18–19, 2005; pp 68–78. (11) Kassman, H.; Andersson, C.; Carlsson, J.; Bjorklund, U.; Strömberg, B. Technical Report 908, Värmeforsk: Stockholm, 2005, http://www. varmeforsk.se. (12) Lindau, L.; Skog, E. Technical Report 812, Värmeforsk: Stockholm, 2003, http://www.varmeforsk.se. (13) Alzueta, M. U.; Bilbao, R.; Glarborg, P. Comb. Flame 2001, 127, 2234–2251. (14) Hindiyarti, L.; Frandsen, F.; Livbjerg, H.; Glarborg, P. Fuel 2006, 85, 978–988. (15) Löffler, G.; Wargadalam, V. J.; Winter, F. Fuel 2002, 81, 711– 717.

Strand

nents are subjected to homogenous chemical reactions that may form vapors with low saturation vapor pressure, leading to gasto-particle conversion at high temperatures. The particle inception is then followed by condensation of more volatile components. During straw combustion, for example, the main constituents of the sub-micrometer particle fraction were K2SO4 and KCl.19 The theoretical analysis indicated that the gas-toparticle conversion occurred during the cooling of the flue gas; the conversion occurred via the homogeneous nucleation of K2SO4 particles, which act as condensation nuclei for the subsequent condensation of KCl. The formation of K2SO4 is initiated by the oxidation of SO2 to SO3, known to be kinetically limited at the lower temperatures at which the reaction is thermodynamically favorable.20 The suggested formation mechanism has been verified by experimental studies of particle formation at high temperatures in flue gases from biofuels combustion.16,21–24 The formation of sulfates from gaseous alkali compounds such as KCl and KOH may depend on several parameters, such as the temperature profile and residence time in the furnace, as well as the concentration of O2 and SO2 in the gas. Adding extraneous sulfur may thus affect both particle composition and the temperature at which sulfate precipitation occurs. During the combustion of pulverized olive residues in a laboratory-scale, down-fired, entrained flow reactor, it was demonstrated that higher SO2 and O2 concentrations in the combustion gases shifted the particle composition toward a higher S:Cl ratio in the fine particles.25 The sub-micrometer particulate matter may also contain unburned carbon in the form of soot and condensed hydrocarbons. In larger units where the combustion is well-controlled, the fraction of soot and hydrocarbons in the fine particles is generally below 10%.4 This is in contrast to domestic boilers and stoves where in some cases very high concentrations of unburned particulate matter may be formed due to poor combustion conditions. The inorganic fraction of the coarse particles (>1 µm) present in the flue gas from the fixed-bed combustion of woody fuels are formed mainly from solid ash and inorganic fuel inclusions entrained from the fuel bed; they consequently contain a higher concentration of refractory ash components, such as Ca, Mg, and Mn, than the fine particle fraction.17,24,26 The flue gas in a movinggrate boiler may also contain a high concentration of unburned fuel and char particles. These particles are generally large enough to be separated in the multicyclone and do not contribute to the emitted sub-micrometer particulate matter. The present study examined the effect of adding elementary sulfur to the fuel in a 7 MW moving-grate boiler fired with moist sawmill residues; the focus of the study was the effect (16) Valmari, T.; Lind, T. M.; Kauppinen, E. I.; Sfiris, G.; Nilsson, K.; Maenhaut, W. Energy Fuels 1999, 13, 379–389. (17) Strand, M.; Pagels, J.; Szpila, A.; Gudmundsson, A.; Swietlicki, E.; Bohgard, M.; Sanati, M. Energy Fuels 2002, 16, 1499–1506. (18) Boman, C.; Nordin, A.; Boström, D.; Öhman, M. Energy Fuels 2004, 18, 338–348. (19) Christensen, K. A.; Stenholm, M.; Livbjerg, H. J. Aerosol Sci. 1998, 29, 421–444. (20) Livbjerg, H. In Aerosol formation from straw combustion–Danish experiences, Proceedings of the IEA seminar Aerosols from Biomass Combustion, Zürich, Switzerland, June 27, 2001; Nussbaumer, T., Ed.; Verenum: Zürish, 2001; pp 29–30. (21) Valmari, T.; Kauppinen, E. I.; Kurkela, J.; Jokiniemi, J. K.; Sfiris, G.; Revitzer, H. J. Aerosol Sci. 1998, 29, 445–459. (22) Strand, M.; Bohgard, M.; Swietlicki, E.; Gharibi, A.; Sanati, M. Aerosol Sci. Technol. 2004, 38, 757–765. (23) Jimenez, S.; Ballester, J. Aerosol Sci. Technol. 2004, 38, 707–721. (24) Wiinikka, H.; Gebart, R.; Boman, C.; Bostrom, D.; Nordin, A.; Ohman, M. Comb. Flame 2006, 147, 278–293. (25) Jiménez, S.; Ballester, J. Comb. Flame 2005, 140, 346–358. (26) Jöller, M.; Brunner, T.; Obernberger, I. Energy Fuels 2005, 19, 311–323.

Emissions from a MoVing-Grate Boiler

Figure 1. Schematic view of the moving-grate boiler including the sulfur-dosing system.

on CO concentration as well as on the concentration, size distribution, and elementary composition of the fine particles in the flue gas. Data regarding SO2 and NOx emissions are also reported. Experimental Plant Description. Measurements of gases and particles were made in a 7 MW moving-grate boiler for producing heat, power, and process steam. (Figure 1). The fuel was fed into the boiler using two parallel screw feeders. The fuel was dried, pyrolyzed, and combusted as it traveled down two parallel moving grates. The moving grates were step-type grates where the fuel was continuously pushed down the grate by the relative sliding of the grate sections. Approximately 70% of the combustion air was supplied as primary air from below the grates. Secondary and tertiary air was supplied from ports located on the walls of the boiler. Steam at 30 bar was produced in vertical tubes at the boiler walls. The gas temperature upstream the secondary air ports was approximately 1000 °C. The boiler included an economizer where the gas temperature was reduced from approximately 700 to 160 °C. In the multicyclones, coarse ash particles and unburned fuel particles were removed. Downstream of the multicyclones, the flue gas passed though the flue gas condenser where the temperature was reduced to 55 °C. The unit included a flue gas duct that could be used to bypass the condenser. Particle and gas samples were extracted from the flue gas channel close to the entrance of the stack, and measurements were made both with and without bypassing the condenser. The flue gas temperature at the sampling position was 55 °C when the condenser was in use and 160 °C when it was bypassed. The operational parameters were kept constant during the measurement period. The thermal output of the boiler was close to 6 MW, and the thermal output of the condenser was 0.9–1.3 MW when it was not bypassed. The O2 concentration in the flue gas as measured downstream of the condenser was close to 6% (dry basis) during all measurements. The fuel used was a moist sawmill residue consisting of wood chips and bark with a calorimetric heating value of 20.5 MJ/kg (dry substance) and a moisture content of 50–55%. The ash content by mass of dry substance (ds) was 1%, and the sulfur content was 0.01%. The chlorine content was below the detection limit of the analytical method used (12).

of K2SO4 in preference to any of the other potassium compound will increase the mass concentration of fine particles, assuming that the same amount of alkali is available for fine particle formation. Effects of Flue Gas Condenser. As shown in Table 1, the particle number concentration increased from 4.0 × 107 to 4.5 × 107 particles/cm3 as the flue gas condenser was bypassed in step 1, indicating that 11% of the particles were separated from the gas in the condenser when no sulfur was admixed. The corresponding separation was 14% when using 0.17% sulfur admixture in step 6. The SMPS results presented in Table 1 also indicate that there was a shift towards larger particle diameters as the condenser was bypassed. This effect is also clearly revealed by the change of the volume GMD shown in Figure 6; as the flue gas condenser was bypassed at 18:10, the mass GMD increased from approximately 125 to 145 nm in 20 min. There was a similar effect when the condenser was bypassed during step 6. In that case, the GMD increased from approximately 150 to 160 nm in 10 min. The shift may partly have been an effect of size-dependent particle separation in the condenser, coarse particles being more effectively collected. However, it was more likely caused by the densification of aggregated particles in the humid conditions prevailing in the condenser, as the highly soluble particle aggregates absorbed water and collapsed. These results are consistent with previously reported results, in which 100 nm particles from grate combustion of woody fuels were shown to shrink by ∼10% when exposed to a relative humidity of 75% in a hygroscopic tandem differential mobility analyzer.28 Effects of Sulfur Admixture on Particle Composition. The elementary compositions of the filter samples as analyzed by means of ICP-AES and ICP-MS are presented in Table 2. The same table presents the added elementary masses as analyzed by means of PIXE for particles collected on the DLPI stages corresponding to aerodynamic particle diameters between 0.03 and 1.0 µm. The results indicate that the lowest sulfur addition used (0.06%) was sufficient to increase the concentration of S and decrease the concentration of Cl in the fine particles. Further addition did not change the S concentration, but the Cl concentration was further reduced. These results are consistent with those of Jiménez et al., who found that moderate additions of SO2 to postcombustion gases decreased the amount of Cl in the fine particles and increased the amount of particle-bound S.25 Sulfur admixture did not significantly affect the concentra(28) Rissler, J.; Pagels, J.; Swietlicki, E.; Wierzbicka, A.; Strand, M.; Lillieblad, L.; Sanati, M.; Bohgard, M. Aerosol Sci. Technol. 2005, 39, 919– 930.

tion of any other elements analyzed, since all changes were within what could be expected from variations in the inorganic content of the fuel. The concentration of the Ca, Al, Si, Fe, and Mn were also analyzed by means of PIXE. The concentration of Ca was below 3%, and the concentrations of Al, Si, Fe, and Mn were below 1% in all samples. The content of elementary and organic carbon in the particle samples was not analyzed. The piles of particles deposited on the impactor substrates appeared white or yellowish white, and the Teflon filters remained bright white after sampling, regardless of the amount of sulfur admixture used. It may therefore be assumed that the fraction of unburned matter in all particle samples was low. However, it can not be excluded that the small changes in particle size distribution and number concentration that appeared when sulfur was admixed to the fuel are somehow related to the presence of unburned carbon, i.e. soot or condensed hydrocarbons. The molar ratio of (K + Na)/(2S + Cl) can be used to investigate the amount of alkali metals bound as sulfates or chlorides. The molar ratios are presented in Table 2. Na was not analyzed by means of PIXE, and Cl was not analyzed by ICP. To obtain the molar ratios for the PIXE results, the Na concentration was assumed to be 1.5%, as based on the ICP results. Correspondingly, the Cl concentrations for the ICP results were estimated from the PIXE results; a concentration of 2.7% Cl was assumed when no sulfur was added, and 0.5% Cl was assumed when 0.12% and 0.17% sulfur were admixed with the fuel. An ion ratio of approximately 1 would indicate that all alkali is bound as sulfates and chlorides, whereas a higher ratio would indicate that other alkali compounds, such as oxides, hydroxides, or carbonates, are present in the sample. In the case of no sulfur addition, the ion ratio was approximately 2, indicating that half of the alkali was not present in the form of sulfates and chlorides. As sulfur was added, the ratio was approximately 1, indicating that the alkali metals and sulfur were present mainly as alkali sulfates. Figure 8 presents the composition profiles of K, S, Cl, and Zn for the impactor stages corresponding to aerodynamic diameters of 0.4–1.3 µm. The fractions presented represent the mass ratio of each element to the total elementary mass as analyzed by means of PIXE (Z > 12). The fraction of potassium was similar on all stages. There was a slight drop of K content in particles larger than 1 µm in diameter, due to the presence Ca probably originating from coarse refractory fly ash particles.

Emissions from a MoVing-Grate Boiler

The higher fraction of K in the case of no sulfur admixture was due to the lower sulfur content when no sulfur was added to the fuel. Conclusions Elementary sulfur was admixed with the fuel in a 7 MW moving-grate boiler fired with woody sawmill residues, and the resulting effects on gaseous and particulate emissions were studied. As a sulfur admixture comprising 0.17% ds of elementary sulfur was admixed with the fuel, the CO concentration in the flue gas gradually decreased by approximately 50% of the concentration measured when no sulfur was admixed. The CO response was slow when the sulfur admixture was increased. This stands in contrast to the response when the admixture was decreased, in which case the CO concentration started to increase in less than 1 h. The concentration of SO2 measured downstream from the flue gas condenser was below the detection limit when no sulfur was admixed and was approximately 30 ppm when 0.12% elementary sulfur was admixed. Sulfur admixture had no significant effect on NOx concentration.

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Sulfur admixture shifted the particle size distribution toward larger and fewer particles. It had no significant effect on the sub-micrometer particle volume concentration as measured with the SMPS system, though the mass concentration increased by 100% when 0.17% S was admixed, indicating a change in the effective density of the fine particles. Elemental analysis of the particle samples indicated that the amount of particle-bound sulfur increased significantly during sulfur addition, probably by favoring the formation of alkali sulfates. As the flue gas was passed through the flue gas condenser, both the particle number concentration and particle mobility diameter decreased. The effect on particle concentration most likely occurred through particle collection inside the condenser, while the change in particle diameter was caused by coalescence of agglomerated particles due to the high relative humidity in the condenser. Acknowledgment. I would like to thank Mr. Mats Åbjörnsson, E.ON Värme Syd and Fredrik Axby, and Carl Bro AB for their support. EF700248F