Energy Fuels 2010, 24, 1948–1957 Published on Web 03/04/2010
: DOI:10.1021/ef901534d
Experimental Investigation of Hydrogen Chloride Bonding with Calcium Hydroxide in the Furnace of a Stoker-Fired Boiler Szawomir Poskrobko,*,† Jan yach,‡ and Danuta Kr ol§ †
Biazystok Technical University, Wiejska 45C, 15-351 Biazystok, Poland, ‡Technical University of Radom, Krasickiego 54, 26-600 Radom, Poland, and §Silesian University of Technology, Konarskiego 18, 44-101 Gliwice, Poland Received December 14, 2009. Revised Manuscript Received February 10, 2010
The paper presents the results of experimental technical investigations to limit the mobility of chlorine released in the form of hydrogen chloride from the fuel in a stoker-fired boiler furnace. In the combustion process, hydrated lime was used as the bonding material. The background for the research was a lowchlorine-gram-fraction fuel, namely, extracted rapeseed meal supplemented with PVC recyclate granules to diversify the quantity of the quick-release chlorine. The research results indicate the tendency of changes of the hydrogen chloride concentration in the flue gas in relation to the amount of PVC added to the fuel, i.e., the gram fraction of chlorine and the amount of calcium hydroxide added to the fuel, as well as the Ca/Cl2 molar ratio.
which exert negative impact on the operation of the boiler (high-temperature corrosion) as the combustion of these species of biomass,1-3 just like in the case of waste fuels and waste,4-6 produces a gas;hydrogen chloride (HCl);and, besides chlorides of alkali metals, lower the ash melting point. In most waste fuels and waste-formed fuels, chlorine is found in organic bonding, very often in the form of PVC plastics that are present in wastes. On the other hand, in biomass chlorine appears in inorganic compounds, most often as salts (such as NaCl and KCl).3 In some species of biomass, the gram fraction of chlorine (such as, for instance, that in cereal straw or grass, as well as that in solid industrial and municipal wastes and fuels from waste) varies from ∼0.5% to 1.5%.1,7 Many years of experience in operating boilers in which the above-mentioned fuels are burnt has shown an accelerated wear of steel structural parts of shields, evaporators, steam heaters, etc., caused by corrosive activities of both chlorine and hydrogen chloride.8-11 The corrosion damage is mainly due to molecular chlorine, which is the oxidation product of hydrogen chloride present in the flue gas. The corrosion is also caused by the chlorides of alkali metals resulting from chemical reactions between chlorine and alkali elements such as
1. Introduction The production of renewable energy greatly depends on the species of solid biomass used for combustion. It could include wood chips and shredded waste, as well as remains (or byproduct) from forest production and the forest product processing industry: in other words, all the wood biomass that, because of either its size, its structure, or advancing decay processes has been classified as biomass waste. Moreover, the biomass used for combustion embraces the waste and remains of the agricultural production and processing industry, as well as other biodegradable waste. Furthermore, it includes the biomass derived from energy crops. It is desirable to increase the use of agricultural biomass, which may not only produce a greater variety of energy sources and make good use of local energy resources, but also, at the same time, results in limiting the scale of wasteful and careless exploitation of forest biomass. However, the regular use of large quantities of nonforest biomass is known to create many diverse and serious problems. All biomass species, whether in their natural or processed forms, such as wood chips, granulated matter, pellets, briquette or bales, are generally characterized by a variable moisture content, even amounting up to 60%. They also show changeable calorific value oscillating notably around 50% of that of coal. Frequently, biomass contains considerable amounts of alkaline elements and chlorine,
(5) Kobyashi, N.; Itaya, Y.; Piao, G.; Mori, S.; Kondo, M.; Hamai, M.; Yamaguchi, M. The behavior of flue gas from RDF combustion in a fluidized bed. Powder Technol. 2005, 151 (1-3), 87–95. (6) Hwang, I. H.; Matsuto, T.; Tanaka, N. Water-soluble characteristics of chlorine in char derived from municipal solid wastes. Waste Manage. 2006, 26 (6), 571–579. (7) Savolainen, K. Co-firing of biomass in coal-fired utility boilers. Appl. Energy 2003, 74 (3), 369–381. (8) Frandsen, F. J. Utilizing biomass and waste for power production;a decade of contributing to the understanding, interpretation and analysis of deposits and corrosion products. Fuel 2005, 84 (10), 1277– 1294. (9) Hansen, L. A.; Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Hoerlyck, S; Karlsson, A. Influence of deposit formation on corrosion at a straw-fired boiler. Fuel Process. Technol. 2000, 64 (1), 189–209. (10) Persson, K.; Brostr€ om, M.; Carlsson, J.; Nordin, A.; Backman, R. High temperature corrosion in a 65 MW waste to energy plant. Fuel Process. Technol. 2007, 88 (11-12), 1178–1182. (11) Nielsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Baxter, L. L. The implications of chlorine-associated corrosion on the operation of biomass-fired boilers. Prog. Energy Combust. Sci. 2000, 26 (3), 283–298.
*Author to whom correspondence should be addressed. Tel.: þ48857469205, þ48857469200. E-mail:
[email protected]. (1) Demirbas, A. Potential applications of renewable energy sources, biomass combustion problems in boiler power systems and combustion related environmental issues. Prog. Energy Combust. Sci. 2005, 31 (2), 171–192. (2) Kuramochi, H.; Wu, W.; Kawamoto, K. Prediction of the behaviors of H2S and HCl during gasification of selected residual biomass fuels by equilibrium calculation. Fuel 2005, 84 (4), 377–387. (3) Khan, A. A.; de Jong, W.; Jansens, P. J.; Spliethoff, H. Biomass combustion in fluidized bed boilers: Potential problems and remedies. Fuel Process. Technol. 2009, 90 (1), 21–50. (4) Vassilev, S. V.; Colette Braekman-Danheux, C.; Laurent, P. Characterization of refuse-derived char from municipal solid waste;1. Phase-mineral and chemical composition. Fuel Process. Technol. 1999, 59 (2), 95–134. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 1948–1957
: DOI:10.1021/ef901534d
Poskrobko et al.
Ca(OH)2 that were, in turn, mixed with PVC. The investigations performed via a thermogravimetry (TG) method, in association with infrared spectroscopy and Fourier transform infrared (FT-IR) spectroscopy (TG-FTIR), showed that chlorine is most effectively bonded using a Ca(OH)2 lime sorbent mixed with fuel (i.e., PVC). The high effectiveness of calcium hydroxide (Ca(OH)2), or what is commonly referred to as hydrated lime, was confirmed by the results of an experiment in which the compound was used to bond chlorine released in the combustion of fuels formed from municipal wastes (i.e., refuse-derived fuels (RDF)20). At the same time, the research has shown that mixing the sorbent with fuel yields better results than injecting the sorbent directly into the combustion chamber, which is essential from the practical point of view of the boiler operating system. As far as the bonding efficiency of chlorine in thermal processes is concerned, interesting results can be found in ref 21, where similar to ref 19, investigations were conducted using the TG method; however, the research concerned the thermal decomposition of PVC with sorbent additions in the form of CaCO3 and dolomite minerals. The positive effect of limiting chlorine mobility was noted during the pyrolysis of PVC when dolomite was added. Note that dolomite is commonly used as an addition to coals with high sulfur content. The use of dolomite to limit HCl emission could be linked to lower costs, when compared to the more-effective Ca(OH)2. The available literature on the subject clearly emphasizes destructive chlorine activities in combustions of fuels in which it is bonded both organically and inorganically. Considering the negative effects of chlorine activities in both operational as well ecological aspects, the present work shows the results of experimental investigations that were intended to limit chlorine migration in the furnace of a water-underfeed stoker-fired boiler. It should be stressed here that the results, because of the power of the boiler used in the investigations (i.e., 225 kW) are already of a semitechnical nature. The model or basic fuel used (i.e., extracted rapeseed meal) was the biomass waste obtained from the production of rapeseed oil. The actual choice of the fuel is justified by the fact that its elemental composition is characterized by the lowest gram fraction of chlorine among the fuels available and, also, as for a biomass, a relatively low content of alkali metals such as sodium, potassium, and calcium. On the other hand, the biofuel is rich in elemental sulfur that will, in the first place, bond alkali metals, thus raising the ash softening point, which will facilitate the conduction of the process at 750-850 °C. Keeping this temperature ensures relatively high and stable lower heating values (LHVs) of the fuel as well as its stable moisture content. In the following tests, the chlorine fraction in the fuel was consecutively increased by PVC addition, because organically bonded chlorine can be easily released at high temperature.18 Supplementing the basic fuel with PVC plastics resulted in the formation of a new type of fuel characterized by a higher content of chlorine in the combustible substance, whereas the sorbent added to the fuel to bond the released chlorine was hydrated calcium (Ca(OH)2). Four different PVC-extracted rapeseed meal mixtures with an increasing mass fraction of
sodium, potassium, or magnesium that are largely present in biofuels. These compounds are characterized by relatively low melting temperatures (i.e.,
