3004
Energy & Fuels 2007, 21, 3004-3009
Melting Behavior of Ashes from the Co-combustion of Coal and Straw S. Arvelakis* and F. J. Frandsen CHEC Research Centre, Department of Chemical Engineering, Technical UniVersity of Denmark (DTU), Lyngby 2800, Denmark ReceiVed January 25, 2007. ReVised Manuscript ReceiVed May 16, 2007
Straw may be used today as a substitute fuel to lower the greenhouse gas emissions from traditional coalfired power plants and provide green-based electricity. It may also provide an alternative source of income to the local farmers helping the developed countries to support sustainable development. The use of straw as a co-firing feedstock in traditional coal-fired plants is associated with operational problems, such as deposition, agglomeration, and/or corrosion, mainly because of the higher amounts of alkali metals and chlorine in straw compared to coal. This may lead to unscheduled shutdowns and costly repairs, increasing the operational costs and the cost of the produced power. In this paper, the melting characteristics of several ash fractions sampled from different parts of a pilot-scale pulverized fuel (PF) boiler operating with different coal/straw mixtures is determined by measuring the ash viscosity using a high-temperature rotational viscometer. The produced data provide information on the melting of the ash material, its flow characteristics, and the rates of crystallization and recrystallization, as a function of the temperature. This information may be used to modify the temperature profile in the different parts of the boiler to reduce the deposition of the ash material. The results show that the straw in the co-combustion mixture changes the viscosity characteristics of the produced ash fractions. The viscosity of the different ash fractions is lowered, as the percentage of straw in the cocombustion mixture increases, and leads to higher stickiness of the produced ash particles at lower temperatures.
1. Introduction An increased use of renewable energy sources for energy production is anticipated in the coming years for the European Union (EU) to meet its obligations for 8% reduction in CO2 emissions by 2012, according to the Kyoto protocol. The cofiring of fossil fuels with CO2-neutral fuels is an attractive alternative to decrease CO2 emissions from energy production. Because all biomass-based materials are considered CO2-neutral, several new fuels, such as straw, herbaceous biomass, and agricultural/agro-industrial wastes, are being introduced into the marketplace together with more traditional biomass fuels, such as wood and wood waste. Straw is considered to be one of the most important agricultural byproducts worldwide, and approximately 800 megatons of straw is actually usable for energy production in the EU and North America yearly.1-4 Several technologies, such as pulverized fuel (PF) combustion, fluidizedbed combustion (FBC), and grate firing technology have been shown to meet the technical needs of such co-firing well and have also been successfully demonstrated at a large scale, although with limited biomass share in the fuel blend for PF, normally below 10% in a thermal basis, and FBC. The grate combustion technology is considered to be a mature technology for the use of straw for energy production, and in countries such * To whom correspondence should be addressed. Telephone: +4545252835. Fax: +45-45882258. E-mail:
[email protected]. (1) Wolf, K. J.; Smeda, A.; Muller, M.; Hilpert, K. Investigations on the influence of additives for SO2 reduction during high alkaline biomass combustion. Energy Fuels 2005, 19, 820-824. (2) Easterly, J. L.; Βurnham, M. Overview of biomass and waste fuel resources for power production. Biomass Bioenergy 1996, 10, 79-92. (3) Nielsen, C. Conversion of straw and similar agricultural wastes. Biomass Bioenergy 1992, 2, 331-339. (4) Sander, B. Properties of Danish biofuels and the requirements for power production. Biomass Bioenergy 1997, 12, 177-183.
as Denmark, several power plants are operating with straw as the combustion feedstock. However, the high corrosion problems experienced with the straw fuels require the use of lower flue gas temperatures to reduce corrosion mainly in the superheater area of the plants. Straw contains more or less alkali and alkaline earth metallic species that are associated with oxygen-containing functional groups of the organic substrate or are present in the form of inorganic salts in the cells. They are released from char and/or ash during pyrolysis and subsequent combustion or gasification processes. This relatively high content of alkali material in combination with high amounts of chlorine, average amounts of sulfur, and with certain amounts of silica is wellknown to cause severe problems when used in boilers/gasifiers for heat and power production. The formation of large amounts of aerosols and the high deposition rates of potentially corrosive components on heat-transfer surfaces are among the created problems. In the case of fluidized bed technology, agglomeration and defluidization problems are experienced.5-10 High-temperature corrosion is often reported at power plants using high(5) Natarajan, E.; Ohman, M.; Gabra, M.; Nordin, A.; Liliedahl, T.; Rao, A. N. Experimental determination of bed agglomeration tendencies of some common agricultural residues in fluidized bed combustion and gasification. Biomass Bioenergy 1998, 15, 163-169. (6) Gomez-Barea, A.; Arjona, R.; Ollero, P. Pilot-plant gasification of olive stone: A technical assessment. Energy Fuels 2005, 19, 598-605. (7) Brus, E.; Ohman, M.; Nordin, A. Mechanisms of bed agglomeration during fluidized-bed combustion of biomass fuels. Energy Fuels 2005, 19, 825-832. (8) Ohman, M.; Pommer, L.; Nordin, A. Bed agglomeration characteristics and mechanisms during gasification and combustion of biomass fuels. Energy Fuels 2005, 19, 1742-1748. (9) 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.
10.1021/ef070045m CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007
Melting BehaVior of Ashes
chlorine and high-alkali biomass, such as straw.11-14 High concentrations of chlorine from chloride deposits on heat exchangers were observed to substantially increase the corrosion rates of the heat exchanger surfaces.15 Deposit formation on relatively cold heat exchanger 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 co-fired and/or biomass-fired power plants. An important ash property that can be used to provide useful information regarding the melting behavior and flow characteristics of ash slags to reduce the ash-related problems and deposits is viscosity. Viscosity is a nonequilibrium property,16 a measure of the resistance of a fluid toward motion. Isaac Newton first defined dynamic or absolute Viscosity (N = s/m2) as the ratio of shear stress, ϑ (kg/s2 = m), to shear rate γ (s-1).17,18 Viscosity depends upon composition and temperature. It has been shown that the strength of ash-fouling deposits is inversely proportional to the viscosity of the liquid phases present on them. Several publications have pointed out the importance of measuring and predicting viscosities of coal ash slags and related melt phases to ensure a trouble-free operation of combustors and gasifiers. Furthermore, the development of new technologies, such as slagging gasifiers, where maintaining the slag flow is critical to the operation, as well as the use of biomass fuels alone or in combination with coal require an accurate prediction of slag properties, such as viscosity and the temperature of the critical viscosity to control the ash flow and minimize slagging, fouling, agglomeration, and corrosion problems. The modeling and prediction of viscosity is also important in the development of comprehensive models of ash behavior in boilers. Coal ash slags are complex mixtures of mainly aluminosilicates, silicates, and oxides. Straw ash is a reach on alkali metals and chlorine and average in sulfur. The co-firing of coal and straw produces ashes rich in aluminosilicates that are also substantially enriched in alkali metals and dependent upon the boiler section in chlorine and sulfur. This leads to higher amounts of molten phases in the ash deposits at substantially lower temperatures and increased ash-related problems compared to the coal combustion/ gasification.19-23 This paper studies the viscosity characteristics of several ash fractions produced from the PF combustion of various wheat straw/coal blends in a 0.5 MWth PF combustor. The results (10) Skrifvars, B-J.; Yrjas, P.; Kinni, J.; Siefen, P.; Hupa, M. The fouling behavior of rice husk ash in fluidized-bed combustion. 1. Fuel characteristics. Energy Fuels 2005, 19, 1503-1511. (11) Jensen, P. A.; Frandsen, F. J.; Dam-Johansen, K. HCl and SO2 emissions from full-scale biomass-fired boilers. In Power Production in the 21st Century: Impacts of Fuel Quality and Operations; United Engineering Foundation: New York, 2001. (12) Glazer, M. P.; Khan, N. A.; de Jong, W.; Spliethoff, H.; Schurmann, H.; Monkhouse, P. Alkali metals in circulating fluidized bed combustion of biomass and coal. Measurements and chemical equilibrium analysis. Energy Fuels 2005, 19, 1889-1897. (13) Michelsen, H. P.; Frandsen, F. J.; Dam-Johansen, K.; Larsen, O. H. Fuel Process. Technol. 1998, 54, 95-108. (14) Sander, B.; Henriksen, N.; Larsen, O. H.; Skriver, A.; RamsgaardNielsen, C.; Jensen, J. N.; Stærkind, K.; Livbjerg, H.; Thellefsen, M.; DamJohansen, K.; Frandsen, F. J.; van der Lans, R.; Hansen, J. Emissions, corrosion and alkali chemistry in straw-fired combined heat and power plants. The 1st World Conference on Biomass for Energy and Industry, June 2000, Sevilla, Spain. (15) 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. (16) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. In The Properties of Gases and Liquids; McGraw-Hill: Singapore, 1988; pp 388-490. (17) De Jong, B. H. W. S. In Ullmann’s Encyclopedia of Industrial Chemistry; Wu¨rzburg: Memphis, TN, 1990; pp 365-432.
Energy & Fuels, Vol. 21, No. 5, 2007 3005
Figure 1. Experimental setup: (a) high-temperature viscometer and (b) principle of a rotational viscometer.
provide information on the melting characteristics of the various ash fractions as a function of the temperature and may be used for the development of models for the prediction of the ash behavior of wheat straw/coal mixtures in boilers. 2. Experimental Section The viscosity of the ash melts was determined using a hightemperature viscometer, with a maximum temperature of 1700 °C. The viscometer is a Rotovisco RV20 model equipped with a Rheocontroler RC20 and a high-temperature furnace ME 1700 made by the German firm Haake. RV20 is a rotational viscometer. The liquid is contained in an outer cylinder, a cup, and a concentric inner cylinder, and a spindle is rotated at a steady speed in the liquid, as seen in Figure 1. The ME 1700 furnace is basically a vertical tube furnace with a maximum temperature of 1700 °C. The furnace is equipped with a protective inner ceramic tube that enables the establishment of an inert atmosphere. This ceramic tube has the side-effect of lowering the maximal test temperature to 1650 °C. The whole setup is shown schematically in Figure 1. The measuring cup is placed on a pedestal that is raised and lowered simultaneously with the viscometer on the top of the setup. The arm that holds the viscometer is water-cooled to 5 °C to prevent overheating of the viscometer. Temperature is measured in a hole just under the measuring cup, and to ensure isothermal conditions in the measurement area, the cup is placed both vertically and horizontally centralized in the furnace. This demands a 40 cm long spindle to reach from the viscometer to the cup. The vertical steering of the cup has been rebuilt because the original construction was not sufficiently stable to maintain the pedestal in a vertical position when fitted with a heavy measuring cup. The bottom plate that holds the pedestal was fitted with a three-legged steering aid. The viscosity measurements are performed at an air temperature, and an 80:20 Pt/Rh alloy (w/w) is used for the cup and spindle arrangement (the sensor), for precision measurements at temperatures up to 1650 °C. The ash material is first heated in a muffle furnace at 900 °C for at least 12 h in a porcelain crucible to burn out any residual carbon left in the ash that might causes problems later on during (18) Clark, S. P., Jr. In Handbook of Physical Constants; The Geological Society of America: Boulder, CO, 1966; pp 291-300. (19) Benson, S. A. Ash formation and behavior in utility boilers. www.microbeam.com (accessed 2004). (20) Vargas, S.; Frandsen, F. J.; Dam-Johansen, K. Prog. Energy Combust. Sci. 2001, 27, 237-429. (21) Hurst, H. J.; Novak, F.; Patterson, J. H. Viscosity measurements and empirical predictions for fluxed Australian bituminous coal ashes. Fuel 1999, 78, 1831. (22) Folkedahl, B. C.; Schobert, H. H. Effects of atmosphere on viscosity of selected bituminous and low-rank coal ash slags. Energy Fuels 2005, 19, 208-215. (23) Vuthaluru, H. B.; Domazetis, G.; Wall, T. F.; Vleeskens, J. M. Reducing fly ash deposition by pretreatment of brown coal: Effect of aluminium on ash character. Fuel Process. Technol. 1996, 46, 117132.
3006 Energy & Fuels, Vol. 21, No. 5, 2007
ArVelakis and Frandsen
Table 1. Feedstock Used in the Co-combustion Tests reference
fossil fuel
biomass
thermal input share of biomass (%)
OP18 OP19 OP20
brown coal RBF brown coal RBF bituminous coal Go¨ttelborn
wheat straw wheat straw wheat straw
25 50 25
the viscosity measurements. Residual carbon can react with iron oxide, producing free iron that can form low-melting alloys with the Pt/Rh material, leading to a destruction of the crucible. Then, the ash is premelted in a Pt-Rh dish to reduce its volume and determine the actual working temperature of the viscometer. After the premelting of the sample, the resulting slag, glassy or nonglassy, is used to fill in the Pt-Rh crucible that is used to study the viscous behavior of the sample as a function of the temperature in an air atmosphere.20 The ash samples used in the viscosity measurements were produced from the co-firing of coal and wheat straw at the 0.5 MWth PFC rig at the IVD research institute as a part of the EU Joule III research project: Operational Problems, Trace Emissions and ByProduct Management for Industrial Biomass Co-Combustion (OPTEB) (1996-1998), under contract JOR3-CT95-0057. Table 1 presents the co-combustion mixtures used in the tests performed in the 0.5 MWth PF combustor. In all cases, the straw feed is Danish wheat straw characterized by a high content of chlorine. In two runs, the German brown coal RBF was co-fired with wheat straw (sample ID: OPTEB 18 and 19), and in the last run, the bituminous coal Go¨ttelborn was used as co-feed (sample ID: OPTEB 20). The ash samples have been sampled from the ash hopper (OP19), the cyclone (OP18, OP19, and OP20), and the filter (OP19 and OP20) of the PF combustor. The ash hopper is placed in the bottom of the PF combustor and collects the largest ash fraction as the ash falls down during the combustion process. The temperature in the cyclone is 350 °C, and in the filter, the temperature is 200 °C. Table 2 presents the elemental analysis of the ash samples studied in this paper. The samples were analyzed by using the atomic absorption spectroscopy (AAS) technique. Table 2 shows that the cyclone (OP18, OP19, and OP20) ash samples have similar compositions. They are rich in alkali metals, sulfur and chlorine, and average to calcium, while the concentrations of the other inorganic constituents are seen to be very low, with the exception of OP20 that is rich in silica and aluminum. OP19 shows the highest content of alkali metals, sulfur, and chlorine because of the higher proportion of straw into the specific fuel mixture compared to the other co-combustion mixtures. The high concentrations of silica and aluminum as well as the low concentrations of calcium and sulfur in the case of the OP20 cyclone ash sample are attributed to the composition of the bituminous coal Go¨ttelborn used in the specific combustion test compared to the brown coal RBF used in the other tests. The ash hopper samples (OP19 and OP20) also have similar compositions. They are seen to have mainly an aluminosilicate structure, while they contain low amounts of calcium, magnesium, phosphorus, sulfur, and chlorine and rather low amounts of alkali metals. The filter ash (OP19 and OP20) fractions are seen to be rich in alkali metals, sulfur, and especially chlorine. However, the concentration of chlorine is seen to be higher now in the case of the OP20 filter sample, while the amount of straw is lower in the co-firing mixture. This might be due to the different types of coal used in the co-firing mixtures during the two tests and/or the different temperature conditions prevailing in the filter section during the tests. The silica content is seen to increase now (30%, w/w) as in the case of the cyclone OP20 sample because of the high silica content of the ash from the bituminous coal used in the co-combustion mixture. The very low temperature in the filter section leads to a substantial condensation of alkali and chlorine vapors that have not been condensed previously in the cyclone section and to the enrichment of the fly ash particles now especially in chlorine. The composition of the ash fractions is consistent with the composition of the individual
Figure 2. Viscosity characteristics of the cyclone ash fractions: (a) OP18 cyclone ash, (b) OP19 cyclone ash, and (c) OP20 cyclone ash.
fuels (coal and straw) used to form the co-firing mixtures. The very low amounts of alkali metals observed in the ash hopper ash fractions is due to the low amount of these elements to the coal ash as well as the relatively high temperatures (1600 °C) prevailing into the main body of the PF combustor that do not favor the condensation of the volatile alkali metals and chlorine. On the other hand, the low to average amounts of aluminosilicates in the cyclone and filter fractions are attributed to the low amounts in the ash of the brown coal, the capture of the majority of the large ash particles in previous boiler sections (air preheater) as well as the limited share of straw in the co-combustion mixtures and the design of the boilers. Table 3 presents the results from the premelting of the ash samples in the high-temperature furnace to determine their melting temperature and use the produced slag to fill in the crucible for the viscosity measurements. The ash samples with a low content of aluminosilicates, such as the cyclone OP18 and OP19, as well as the OP19 and OP20 filter ash, melt at a rather low temperature compared to the samples with
Melting BehaVior of Ashes
Energy & Fuels, Vol. 21, No. 5, 2007 3007 Table 2. AAS Elemental Analysis of the Various Ash Samples and Slags (%, w/w)
reference
K2O
Na2O
CaO
MgO
SiO2
Al2O3
Fe2O3
TiO2
P2O5
SO2
Cl
brown coal RBF bituminous coal straw OP18 cyclone OP18 cyclone slag OP19 cyclone OP19 ash hopper OP20 cyclone OP20 ash hopper OP19 filter OP20 filter
0.63 1.00 12.0 25.3 22.1 32.9 7.6 16.13 5.6 19.24 21.37
2.73 0.3 0.8 14.9 12.93 9.8 0.6 5.48 0.3 5.25 2.85
37.95 1.3 12.5 11.2 16.4 6.5 7.6 4.12 5.6 9.53 2.76
15.51 0.5 2.1 0.8 1.16 0.4 2.3 1.4 1.8 3.38 2.85
6.99 70.2 59.6 1.2 1.41 3.4 76.2 42.14 67.7 30.12 30.8
4.29 17.1 0.5 0.18 0.31 0.43 0.8 6.9 10.8 4.18 5.8
14.52 6.7 0.4 0.0 0.0 0.7 1.87 2.17 6.6 5.25 3.2
0.23 0.7 0.1 0.0 0.0 0.01 0.045 0.93 0.4 0.26 0.58
0.03 nda 3.5 0.68 0.87 0.97 1.9 1.86 1.2 3.92 2.69
15.28 1.00 2.1 40.5 44.4 35.7 0.44 11.9 1.6 11.22 9.8
0.46 nd 3.88 4.9 0.43 7.2
