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Energy & Fuels 2007, 21, 3240–3248
Influence of Coal Composition and Operating Conditions on the Release of Alkali Species During Combustion of Hard Coal Holger Oleschko and Michael Müller* Forschungszentrum Jülich GmbH, IEF-2, 52425 Jülich, Germany ReceiVed July 12, 2007. ReVised Manuscript ReceiVed September 5, 2007
In solid fuel conversion systems, the release of alkali species can cause severe problems such as fouling, slagging, corrosion, and erosion. Especially in future coal-fired combined cycle processes such as pressurized pulverized coal combustion (PPCC), alkali species can cause corrosion of the gas turbine blading. In order to solve these problems, detailed information about the alkali release is required. In this work, the influence of coal composition and operating conditions on the release of alkali species is investigated. For this reason, laboratory combustion experiments with six different hard coals were conducted at temperatures of 800 and 1200 °C and absolute pressures of 1, 3, and 9 bar. High pressure mass spectrometry (HPMS) was used for online analysis of combustion products such as HCl, NaCl, KCl, SO2, and Na2SO4. In addition, two of the coals were subjected to a leaching procedure. Thermodynamic equilibrium calculations with the software package FactSage 5.4 and the FACT database were conducted for comparison with the experimental findings.
Introduction During the combustion of coal, part of the alkali in the coal is released into the gas phase. This often leads to problems in power plants as far as fouling, slagging, erosion, and corrosion are concerned. Sodium is frequently found to be the most common cause for fouling.1 During combustion, organically associated sodium is easily liberated and reacts with other gasphase species to form chlorides, hydroxides, and oxides. These species react with SO3 and form sodium sulfate, which can condense on heat transfer tubes in the cooler section of the boiler.2 Alkali metals can also be harmful for future coal-fired combined cycle processes such as pressurized pulverized coal combustion (PPCC) as alkali-laden gas is fed directly to a gas turbine.3 Together with sulfur compounds present in the gas phase, alkali species can react to form Na2SO4 which can condense on the gas turbine blading causing hot corrosion. In the case of PPCC, a hot gas cleanup using getter materials has been devised to reduce alkali concentrations. Kaolin and silicaenriched bauxite have shown the best ability to sufficiently remove the alkalis which are bonded in a melt/glass phase formed during alkali sorption.4 On the other hand, it was found that coal ash and slag have a high potential of capturing alkali * Corresponding author. Tel.: +49-2461-616812. Fax: +49-2461-613699. E-mail address:
[email protected]. (1) Osborn, G. A. Review of Sulfur and Chlorine Retention in CoalFired Boiler Deposits. Fuel 1992, 71 (2), 131–142. (2) Radhakrishnan, P.; Zakkay, V.; Agnone, A. Alkali and Gas Emissions from PFB Combustion of Lignite. Combustion Sci. Technol. 1986, 50, 271– 281. (3) Förster, M. E. C.; Neumann, F.; Telöken, R.; Pavone, D. Druckkohlenstaubfeuerung: aktueller Fortschritt des kohlebasierten GuD-Prozesses in Richtung Gasturbinenverträglichkeit und Stand des Entwicklungsprojektes. VGB PowerTech 2005, 85 (6), 38–41. (4) Escobar, I.; Müller, M. Alkali Removal at about 1400 °C for the Pressurized Pulverized Coal Combustion Combined Cycle. 2. Sorbents and Sorption Mechanisms. Energy Fuels 2007, 21 (2), 735–743.
metals released during combustion.5–7 As the installation and operation of an alkali removal unit would increase both investment and operational costs of a future PPCC power plant, it is desirable to capture most of the alkalis with the coal ash during combustion. Taking all this into consideration, it should be clear that an increased understanding of the alkali release during coal combustion is necessary, both for the optimization of existing power plants and the development of future coal-fired combined cycle processes. Krishnan et al.8 used a molecular-beam mass spectrometer to directly sample gas from the freeboard region of a fluidizedbed coal combustor and gasifier in the temperature range from 797 to 947 °C. The two coals they used were Beulah Zap lignite and Illinois No. 6 subbituminous coal. Beulah Zap was high in Na (0.32 mass %) but low in K (0.004 mass %), Cl (
