Energy & Fuels 2008, 22, 2955–2962
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Suspension Combustion of Wood: Influence of Pyrolysis Conditions on Char Yield, Morphology, and Reactivity Michelangelo Dall’Ora, Peter Arendt Jensen,* and Anker Degn Jensen Department of Chemical and Biochemical Engineering, Denmark Technical UniVersity, Søltofts Plads Bygning 229, Kgs. Lyngby 2800, Denmark ReceiVed February 22, 2008. ReVised Manuscript ReceiVed May 8, 2008
Chars from pine and beech wood were produced by fast pyrolysis in an entrained flow reactor and by slow pyrolysis in a thermogravimetric analyzer. The influence of pyrolysis temperature, heating rate and particle size on char yield and morphology was investigated. The applied pyrolysis temperature varied in the range 673-1673 K for slow pyrolysis and between 873 and 1573 K for fast pyrolysis. The chars were oxidized in a thermogravimetric analyzer and the mass loss data were used to determine char oxidation reactivity. Char yield from fast pyrolysis (104-105 K/s) was as low as 1 to 6% on a dry ash free basis, whereas it was about 15-17% for slow pyrolysis (10-20 K/min); char yield decreased as pyrolysis temperature increased. During fast pyrolysis wood particles underwent melting, yet to different extents for the two investigated fuels: pine wood produced chars of porous spherical particles, whereas beech sawdust chars showed a somewhat less drastic change of morphology with respect to the parent fuel. Char produced by low heating rate pyrolysis fully retained the original fibrous structure of wood. Fast pyrolysis chars were significantly more reactive than slow pyrolysis chars; moreover, char oxidation reactivity decreased as pyrolysis temperature increased. The amount and composition of the ash forming matter of the wood fuels seems to play an important role in determining the differences in char yield, morphology and reactivity.
Introduction The large availability of wood, neutrality with respect to CO2 emissions when used as fuel, and the fact that it is a renewable source make it an attractive power plant fuel. A variety of wood species are burned nowadays in power plants, depending on the location of the plant, the wood-related activities in the area (pulp and paper industry, sawmills, etc.) and other economical reasons; wood from conventional forestry, residues from manufacturing of wood based products such as bark, sawdust and off-cuts from sawmills are some of the sources of wood fuel. The most common ways of burning wood in power plants are grate firing and pulverized wood combustion. Suspension firing of pulverized fuel has been used for decades to burn coal; environmental concern and legislation have contributed to the conversion of some of those plants to wood combustion and to the building of new pulverized wood power plants. In suspension firing, wood particles are heated up fast to high temperatures as they enter the furnace and thereby pyrolyze and leave a solid residue called char. The subsequent oxidation of the char is the slowest step in the conversion of wood and thus determines the degree of burnout of the fuel as well as the heat release profile in the boiler, affecting the operation and efficiency of the plant. The overall reaction rate between char and oxygen can be controlled by different phenomena depending on the conditions at which oxidation takes place, char characteristics and degree of conversion. Three different situations may occur that are commonly referred to as regime I (or zone I), regime II, and regime III:1,2 in regime I, the overall burning rate is controlled * Corresponding author. E-mail:
[email protected]. (1) Smith, I. W. Proc. Combust. Inst. 1982, 19, 1045–1065.
by the chemical heterogeneous reaction between O2 and the carbon of the char particle; in regime II, the rate is determined by the chemical reaction as well as by internal diffusion of O2 in the char pores; finally, in regime III, external diffusion of O2 from the bulk phase to the particle surface controls the burning rate. In suspension fired boilers it is likely that pulverized wood particles burn under regimes II and III at the beginning of the conversion, whereas they may burn under regime I at higher conversion, when both the particle size and temperature have decreased. Therefore, in order to describe the burning of wood char in such plants it is necessary to investigate parameters that affect transport phenomena such as char porosity and particle size as well as char density and the intrinsic reactivity of char.2 It is well-known that both the yield of char and its properties, including size, morphology, composition, and reactivity depend strongly on the pyrolysis conditions in which the char is formed.2 The literature available about characterization of wood char produced at suspension boiler conditions is not extensive, although some recent works address the topic.3–11 (2) Hurt, R. H. Proc. Combust. Inst. 1998, 27, 2887–2904. (3) Kang, B.; Lee, K. H.; Park, H. J.; Park, Y.; Kim, J. J. Anal. Appl. Pyrolysis 2006, 76, 32–37. (4) Zhang, Y.; Kajitani, S.; Ashizawa, M.; Miura, K. Energy Fuels 2006, 20, 2705–2712. (5) Cetin, E.; Moghtaderi, B.; Gupta, R.; Wall, T. F. Fuel 2004, 83, 2139–2150. (6) Biagini, E.; Fantozzi, C.; Tognotti, L. Combust. Sci. Technol. 2004, 176, 685–703. (7) Janse, M. C. A.; de Jonge, H. G.; Prins, W.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 1998, 37, 3909–3918. (8) Mermoud, F.; Salvador, S.; Van de Steene, L.; Glofier, F. Fuel 2006, 85, 1473–1482. (9) Guerrero, M.; Ruiz, M. P.; Alzueta, M. U.; Bilbao, R.; Millera, A. J. Anal. Appl. Pyrolysis 2005, 74, 307–314. (10) Biagini, E.; Narducci, P.; Tognotti, L. Fuel 2008, 87, 177–186.
10.1021/ef800136b CCC: $40.75 2008 American Chemical Society Published on Web 07/09/2008
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Dall’Ora et al.
Table 1. Proximate Analysis and Chemical Composition of the Fuels pine
beech sawdust
proximate analysis 7.1 moisture (wt %, a.d.a) volatiles (wt %, d.b.b) 85 ash (wt %, d.b.) 0.5 fixed C (by difference, wt %, d.b.) 14.5
7.4 84.2 0.9 14.9
ultimate analysis C (wt %, d.b.) 49.7 H (wt %, d.b.) 6.3 N (wt %, d.b.) 0.1 Cl (wt %, d.b.) 0.011 S (wt %, d.b.) 0.022 O (by difference) (wt %, d.b.) 43.5 Ca (wt%, d.b.) 0.125 K (wt%, d.b.) 0.033 Mg (wt%, d.b.) 0.027 Si (wt%, d.b.) 0.065
49.5 6.1 0.13
