Energy & Fuels 2008, 22, 2033–2042
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Fine Particle and Gas Emissions from the Combustion of Agricultural Fuels Fired in a 20 kW Burner Jarkko Tissari,*,† Olli Sippula,† Jyrki Kouki,‡ Kari Vuorio,‡ and Jorma Jokiniemi†,§ Fine Particle and Aerosol Technology Laboratory, UniVersity of Kuopio, P.O. Box 1627, FI-70211 Kuopio, Finland, TTS Research, P.O. Box 5 FI-05201 Rajama¨ki, Finland, and VTT, Technical Research Centre of Finland, Fine Particles, P.O. Box 1000, FI-02044 VTT, Espoo, Finland ReceiVed December 19, 2007. ReVised Manuscript ReceiVed February 19, 2008
Emissions from the combustion of agricultural biomasses have not been studied extensively. In this study, the effects of different biomasses and mixed fuels on fine particle (PM1) and gas emissions from a residential cereal burner were investigated. The cereal seeds of oat and rape, rape bark pellets, and wood pellets were the main fuels. In addition, oat was mixed with peat and kaolin and wood with kaolin. The gas emissions of NOx, SO2, and HCl were clearly higher from cereal or mixed-cereal fuels than from pure wood fuel. The emissions of carbon monoxide (CO), organic gaseous carbon (OGC), PM1 and the particle numbers from the cereal fuels did not differ significantly from the emissions of wood fuels, although the fuel ash contents were substantially higher. The release of alkali metals varied substantially between different fuels, probably due to large differences in ash chemical compositions. In contrast to wood fuels, phosphate contributed significantly to the formation of fine particles in the cereal fuels. In rape fuels, probably due to high S/Cl and S/K ratios, all of the fuel chlorine was released in the gas phase and was not enriched in the fine particles. At least partly due to this, the PM1 and alkali metal emissions from the combustion of rape seeds were low, considering the high ash content (4.4%) and high alkali metal contents in the fuel. The addition of 5 wt % of kaolin to oat grains seemed to decrease the alkali metal emissions but slightly increased the emissions of incomplete combustion. It seems that the formation of chlorides (e.g., KCl) affects significantly the emission of fine ash particles. Moreover, sulfation of alkali metals seems to decrease the emission of fine alkali metal particles.
1. Introduction The use of renewable energy sources, especially biomass, will increase in the near future, as part of the attempt to decrease the emissions of greenhouse gases. At the same time, the combustion of biomass is an important source of both gaseous and particulate pollutants such as fine particles (PM2.5: particulate mass, i.e., 2.5 µm or smaller in aerodynamic size), polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), and carbon monoxide (CO).1–4 In Finland, biomass fuels consist mainly of different wood materials. In recent years the use of logging residues has increased in large-scale combustion units, whereas the potential of field energy is almost entirely unutilized. Reed-canary grass and straws are the most commonly used field crops in heat and power production. The stalks of oil and fiber plants, cereal grains, and energy willow are also possible fuels for energy production in Finland. In addition to rape oil,5 the residue from * To whom correspondence should be addressed. Tel: +358-17-163285. Fax: +358-17-163-229. E-mail:
[email protected]. † University of Kuopio. ‡ TTS Research. § VTT, Technical Research Centre of Finland, Fine Particles. (1) Tissari, J.; Hyto¨nen, K.; Lyyra¨nen, J.; Jokiniemi, J. Atmos. EnViron. 2007, 41, 8330–8344. (2) Sippula, O.; Hyto¨nen, K.; Tissari, J.; Raunemaa, T.; Jokiniemi, J. Energy Fuels 2007, 21, 1151–1160. (3) Christensen, K. A.; Stenholm, M.; Livjberg, H. J. Aerosol Sci. 1998, 29, 421–444. (4) Olsson, J. G.; Ja¨glig, U.; Pettersson, J. B. C. Energy Fuels 1997, 11, 779–784. (5) Vaitilingom, G.; Perilhon, C.; Liennard, A.; Gandon, M. Ind. Crops Prod. 1998, 7, 273–279.
pressing rape seeds is usable. In general, a large source of field energy is available, in the form of the surplus from the production of food crops and plant residues and energy crops. The hygienic quality of Finnish cereal grains is good, but the grain size is small. The precleaning and classification of grains produce agricultural residues which can be used for energy production. However, the volumes of the residues are small for large-scale combustion units, and it is cost-effective to combust these residues locally in small-scale combustion units. This enhances the cost efficiency of the whole agricultural production. Grains of low quality due to poor weather conditions during the growing season, for example, are also used for heat production. Cereals can also be produced specially for energy production, in which case the cultivation techniques can be suited for the production of fuels. For example, it is possible to use fertilizers which are cheaper and not suitable for nutritional production of cereals. In addition, the cereals can be artificially ripened and the need of drying is reduced, which also enhances the efficiency of energy production. Agricultural fuels are combusted mainly in grate-fired boilers or cofired in pulverized coal combustion of from a few megawatts up to 1000 MW. In Denmark, for example, the annual biomass such as straw has been combusted for more than a decade.6 However, there are technical problems in the use of agricultural biomass due to relatively high concentrations of potassium, chlorine, and sulfur in the fuels. The other most abundant ash-forming elements are Ca, Mg, P, and Si.6 (6) Knudsen, J. N.; Jensen, P. A.; Dam-Johansen, K. Energy Fuels 2004, 18, 1385–1399.
10.1021/ef700766y CCC: $40.75 2008 American Chemical Society Published on Web 04/05/2008
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The presence of alkali metals in biomass fuels is problematic because they form compounds which are volatile and have relatively low melting temperatures. The volatilization of alkali metals from the fuel leads to the formation of fine fly ash particles. This leads to operational problems such as fouling, slagging, and corrosion of heat transfer surfaces in boilers, which reduces efficiency and may even lead to costly shutdowns and repairs.7–9 Further, alkali metals typically form the dominant fraction of fine particle emissions from good biomass combustion. In wood combustion, the fly ash is mainly composed of potassium compounds such as potassium sulfates (K2SO4), potassium chlorides (KCl), potassium hydroxides (KOH), and potassium carbonates (K2CO3).2 K2SO4 and KCl were the most common from the combustion of wheat and barley straws.3 The release of ash-forming species during the combustion of biomass has been studied since the early 1990s. In addition to the fuel ash concentration and fuel temperature, the composition of ash also has an influence on the release of species.2,4,6,9 The release of alkali metals is influenced mainly by the fuel chlorine, sulfur, and different sorbent mineral concentrations. A high chlorine content has been found to enhance the release of alkali metals due to the formation of volatile alkali metal chlorides.4,6 Knudsen et al.6 observed that the ratio of molar concentration of K/Si and Cl/K is important for alkali emission. If there are silicates present, the aluminum and silicon compound can react with potassium, forming more stable compounds.9,10 Thus, the low ratio of K/Si has been observed to limit the release of potassium. The high ratio of Cl/K increases the release of alkali metals, since the chlorine prevents potassium from combining with silicates.6,11 In contrast, a sufficient amount of sulfur in the fuel may inhibit the effect of chlorine through a sulfation reaction, in which the alkali metal chloride is converted to less volatile alkali metal sulfate (eq 1).12 Further, sulfation of other alkali metal species such as hydroxides (eq 2) may decrease the release of alkali metals. Equations 1 and 2 present the global sulfation reactions for potassium. 2KCl + H2O + SO2 + 1/2O2 T K2SO4 + 2HCl
(1)
2KOH + SO2+ 1/2O2 T K2SO4+H2O
(2)
The sorbent minerals (in this context) are nonvolatile compounds that capture the alkali metals either throughout a chemical reaction or by physisorption. Hence, these kinds of minerals decrease the release of alkali metals, and they may be used to solve ash-related problems in combustion units. For example, kaolin (Al2Si2O5(OH)4) and dolomite (CaMg(CO3)2 have been used as fuel additives to give the ash a higher melting point and to reduce the emission of alkali compounds.13–15 The addition of lime (CaO) has been found to reduce the formation (7) Dayton, D. C.; French, R. J.; Milne, T. A. Energy Fuels 1995, 9, 855–865. (8) Blander, M.; Pelton, A. D. Biomass Bioenergy 1997, 12, 295–298. (9) Davidsson, K. O.; Stojkova, B. J.; Pettersson, J. B. C. Energy Fuels 2002, 16, 1033–1039. (10) Jensen, P. A.; Frandsen, F. J.; Dam.Johansen, K.; Sander, B. Energy Fuels 2000, 14, 1280–1285. (11) Dayton, D. C.; Jenkins, B. M.; Turn, S. Q.; Bakker, R. R.; Williams, R. B.; Belle-Oudry, D.; Hill, L. M. Energy Fuels 1999, 13, 860–870. (12) Sippula, O. M. J.; Lind, T.; Jokiniemi, J. Effects of chlorine and sulphur on particle formation in wood combustion performed in a laboratory scale reactor. Manuscript submitted for publication. (13) Davidsson, K. O.; Steenari, B.-M.; Eskilsson, D. Energy Fuels 2007, 21, 1959–1966. (14) Tran, K.; Iisa, K.; Steenari, B.; Lindqvist, O. Fuel 2005, 84, 169– 175. (15) Steenari, B.-M.; Lindqvist, O. Biomass Bioenergy 1998, 14, 67– 76.
of slag in barley combustion and to totally eliminate it in rye and wheat grain combustion in a 20 kW residential cereal burner.16 The lime reacts with potassium forming calcium/ magnesium potassium phosphates, for example, which have a high melting point. Thus, monocalcium phosphate additive, for example, has been found to capture alkali metals in straw combustion.17 In Finland, peat is an important fuel which is used in combined heat and power production, and the Finnish peatland areas and production volumes are among the largest in the world.18 Moreover, the use of peat in small-scale wood combustion will increase. In large power plants, peat is used to reduce the high-temperature corrosion caused by potassium chloride. The sulfur and silicates in peat affect the reactions of alkali metals, which leads to the formation of gaseous HCl (e.g., eq 1), and thus, less corrosive alkali metal compounds are formed.19,20 Extensive detailed modeling and tests on alkali metal behavior in field biomass combustion have been done in laboratory reactors. In small combustion units, the local atmosphere and temperature vary considerably depending on the grate and burner, so emission measurements from real appliances are needed to get reliable results from the combustion of agricultural biomass. However, no such measurements are available. The aim of this study was to determine the effect of different biomass and mixed fuels on fine particle and gas emissions from a residential cereal burner. The effects of different factors on the release of ash species are discussed. The cereal seeds of oat and rape, rape bark pellets, and wood pellets were the main fuels. In addition, oat was mixed with peat and kaolin and wood with kaolin. Experiments The experiments were conducted in the emission test laboratory at the TTS Institute (formerly Finnish Work Efficiency Institute; Rajama¨ki, Finland). A commercially available 20 kW cereal and/ or pellet burner (Farmjet 20, Ariterm Ltd.) connected with a 20 ¨ SPI-PELLETTI 20, Kaukora Ltd.) was used in the kW boiler (JA experiments. The fuels in 10 measured cycles were oat grains, oat grains with 5 wt % kaolin powder, oat grains with 20 wt % peat pellets, wood pellets, wood pellets with 1 wt % kaolin powder, rape seeds, and pellets made from the bark of rape seeds. Kaolin powder and peat pellets were manually mixed before combustion with the pure fuels. In addition to nominal load, the oat, wood pellets, and oat-peat were also combusted at partial load, in order to get a picture of emissions under typical burner operation out of the heating season. Table 1 shows the properties of the used fuels, analyzed at the Finnish Forest Research Institute. A schematic drawing of the experimental setup is presented in Figure 1. The feeding of pellets from the fuel tank to the burner was controlled by a fully automatic feeding system connected to the burner automatics. In the burner, the dispensing of pellets was done first with a separate feeding screw through the airtight rotary feeder and thereafter by a burner screw to the burner head. This enables a fire safe operation (Figure 2). The different fuels were combusted with similar burner combustion conditions. In all the cycles/experiments, the power of the air blower ¨ hman, M. Energy (16) Lindstro¨m, E.; Sandstro¨m, M.; Bostro¨m, D.; O Fuels 2007, 21, 710–717. (17) Tobiasen, L.; Skytte, R.; Pedersen, L. S.; Pedersen, S. T.; Lindberg, M. A. Fuel Process. Technol. 2007, 88, 1108–1117. (18) Tissari, J. M.; Yli-Tuomi, T.; Raunemaa, T. M.; Tiitta, P. T.; Nuutinen, J. P.; Willman, P. K.; Lehtinen, K. E. J.; Jokiniemi, J. K. Boreal EnViron. Res. 2006, 11, 283–293. ¨ hman, M.; Bostro¨m, D. Energy Fuels (19) Lundholm, K.; Nordin, A.; O 2005, 19, 2273–2278. (20) Lind, T.; Hokkinen, J.; Jokiniemi, J.; Aurela, M.; Hillamo, R. EnViron. Sci. Technol. 2003, 37, 2842–2846.
Emissions from Combustion of Agricultural Fuels
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Table 1. Properties of the Fuels Useda oat moisture (%) ash (%) NHV (mg MJ-1) Al B Ca Cd Cr Cu Fe K Mg Mn Mo Na Ni P Pb S Zn Cl ratios S/Cl S/K K/Cl
wood oat-peatb rape seed rape seed bark
11.36 2.39 18.26 24.39 1.52 640
